Important words and concepts from Chapter 8, Campbell & Reece, 2002 (1/14/2005):

by Stephen T. Abedon (abedon.1@osu.edu) for Biology 113 at the Ohio State University

 

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(1) Chapter title: Membrane Structure and Function

(a)                    The "ability of the cell to discriminate in its chemical exchanges with the environment is fundamental to life, and it is the plasma membrane that makes this selectivity possible."

(b)                    [cell membranes (Google Search)] [transport in and out of cells (Online Biology Book)] [index]

(2) Membrane (see also membrane and cell membrane)

(a)                    The membranes that are found within cells (plus the plasma membrane surrounding cells) consist of phospholipids (and other lipids plus membrane proteins) arrayed by hydrophobic exclusion into two-dimensional fluids known as lipid bilayers

(b)                    [membranes (Google Search)] [an introduction to the structure of biological membranes (author unknown)] [index]

 

LIPIDS MAKING UP MEMBRANES

 

(3) Phospholipids (amphipathic) (see also phospholipid)

(a)                    Phospholipids are amphipathic molecules meaning that they have both a hydrophobic and a hydrophilic end

(b)                    Recall "clothespin" representation

(c)                    See Figure, The structure of a phospholipid

(d)                   See Figure, Two structures formed by self-assembly of phospholipids in aqueous environments

(e)                    [phospholipids, amphiphatic (Google Search)] [index]

(4) Lipid bilayer (see also lipid bilayer)

(a)                    Phospholipids can exist as bilayers in aqueous solutions

(b)                    The hydrophobic portion of the phospholipid is shielded in middle of these bilayers

(c)                    The hydrophilic portion is exposed on both sides to water

(d)                   See Figure, Artificial membranes (cross section)

(e)                    Note in the following that A is a hydrocarbon tail of a phospholipid, B is the hydrophilic head of a phospholipid, C is one of the aqueous solutions surrounding the lipid bilayer, and that the big black object represents an integral membrane protein: 001mbrn

(f)                     Lipid bilayers are held together mainly by hydrophobic interactions (including hydrophobic exclusion)

(g)                    [lipid bilayer (Google Search)] [index]

(5) Fluid mosaic model (see also fluid mosaic model)

(a)                    The plasma membrane contains proteins, sugars, and other lipids in addition to the phospholipids

(b)                    The model that describes the arrangement of these substances in and about lipid bilayers is called the fluid mosaic model

(c)                    Basically, membrane proteins are suspended within a two-dimensional fluid that in turn is made up mostly of phospholipids

(d)                   See Figure, Two generations of membrane models (note that Figure is an obsolete model)

(e)                    [fluid mosaic (Google Search)] [index]

(6) Two-dimensional fluid (flip-flopping) (see also flip-flopping)

(a)                    Lipid bilayers serve as two-dimensional fluids

(b)                    Lipids are capable of rapid diffusion within their layer

(c)                    However, "flip-flopping" from one layer to the other is rare

(d)                   See Figure, The fluidity of membranes

(e)                    It is the lack of flip-flopping that maintains the asymmetry of membranes (e.g., different components are present in different layers)

(f)                     Integral membrane proteins are especially resistant to flip-flopping

(g)                    ["two dimensional fluid" membrane, flipping leaflet membrane, flip-flopping membrane (Google Search)] [model of lipid bilayer (see chime if graphics won't load) (Antony Crofts)] [index]

(7) Temperature-dependence of fluidity

(a)                    To function, a lipid bilayer must maintain its fluidity

(b)                    The fluidity of a cell membrane typically is considered to be about equivalent to the fluidity of salad oil

(c)                    To maintain fluidity at lower temperatures, organisms use phospholipids containing increasing degrees of unsaturation in their fatty acids

(d)                   See Figure, The fluidity of membranes

(e)                    [membrane fluidity (Google Search)] [membrane fluidity (Physiological Ecology)] [index]

(8) Cholesterol (see also cholesterol)

(a)                    Cholesterol, a kind of steroid, is an amphipathic lipid that is found in lipid bilayers that serves as a temperature-stability buffer

(b)                    At higher temperatures cholesterol serves to impede phospholipid fluidity

(c)                    At lower temperatures cholesterol interferes with solidification of membranes (e.g., cholesterol functions similarly, in the latter case, to the effect of unsaturated fatty acids on lipid-bilayer fluidity)

(d)                   See Figure, The fluidity of membranes

(e)                    Cholesterol is found particularly in animal cell membranes

(f)                     [membrane cholesterol lipid bilayer (Google Search)] [cholesterol (Molecules of Life)] [index]

 

PROTEINS ASSOCIATED WITH MEMBRANES

 

(9) Membrane proteins (see also membrane)

(a)                    Proteins are typically associated with cell membranes

(b)                    These proteins have numerous functions, but may be divided structurally into two types: Integral membrane proteins and peripheral membrane proteins

(c)                    [membrane proteins (Google Search)] [index]

(10) Integral membrane proteins (see also integral membrane protein)

(a)                    Membrane proteins differ in the degree to which they span lipid bilayers

(b)                    Integral membrane proteins span the lipid bilayer at least a little

(c)                    Some (probably many or most) integral membrane proteins completely span the lipid bilayer

(d)                   See Figure, The detailed structure of an animal cell's plasma membrane, in cross section

(e)                    See Figure, The structure of a transmembrane protein

(f)                     Integral membrane proteins are typically hydrophobic where they interact with the hydrophobic portion of the membrane

(g)                    Integral membrane proteins are typically hydrophilic where they interact with the hydrophilic portion of the membrane and overlying (and underlying) H2O

(h)                    [integral membrane proteins (Google Search)] [index]

(11) Peripheral membrane proteins (see also peripheral membrane protein)

(a)                    Contrasting with integral membrane proteins, peripheral membrane proteins do not enter the lipid bilayer

(b)                    Instead, peripheral proteins are attached to the outside of the membrane

(c)                    Typically this attachment is via attachment to portions of integral membrane proteins jutting out of the membrane interior

(d)                   [peripheral membrane proteins (Google Search)] [index]

(12) Functions of membrane proteins

(a)                    Functions of membrane proteins include:

(i)                     Transport of substances across membranes

(ii)                   Enzymatic activity (e.g., smooth endoplasmic reticulum)

(iii)                 Signal transduction (e.g., cell communication)

(iv)                 Intracellular joining (See Figure, Intercellular junctions in animals)

(v)                   Cell-cell recognition (e.g., cell communication)

(vi)                 Attachment to the cytoskeleton and extracellular matrix

(b)                   See Figure, Some functions of membrane proteins

(c)                    [function or functions "of membrane proteins" (Google Search)] [functions of plasma membrane proteins (graphic) (Access Excellence)] [index]

(13) Fluidity of membrane proteins

(a)                    Many membrane proteins are capable of diffusing within the membrane

(b)                    This diffusion is similar to that of phospholipids within membranes, though not as rapid

(c)                    Other membrane proteins are tied in place by attachment to the cytoskeleton or the extracellular matrix

(d)                   See Figure, The detailed structure of an animal cell's plasma membrane, in cross section

(14) Membrane asymmetry (leaflet)

(a)                    It is important when thinking about membranes to keep in mind that a typical cell membrane tends to have a different composition on one side (a.k.a., leaflet; say, the inside, or inner leaflet) than on the other (the outside, or outer leaflet)

(b)                    Differences between leaflets tend to include different ratios or types of amphipathic lipid-based molecules found in each leaflet, different kinds of proteins facing in or facing out, or fixed orientations of proteins spanning the membrane

(c)                    This asymmetry allows the cell to automatically differ its intracellular environment from that existing extracellularly

(d)                   As might therefore be expected, asymmetries tend to be rigidly maintained via minimal flip-flopping

(e)                    See Figure, Sidedness of the plasma membrane

(f)                     See Figure, The fluidity of membranes (movement of phospholipids)

(g)                    [membrane asymmetry, membrane leaflet (Google Search)] [functions of plasma membrane proteins (graphic) (Access Excellence)] [membrane sidedness (BSC Courseware)] [index]

(15) Oligosaccharides (glycoproteins) (see also oligosaccharide and glycoprotein)

(a)                    Many eukaryotic membrane proteins are glycoproteins, proteins to which carbohydrate molecules of intermediate length (oligosaccharides) have been covalently attached

(b)                    The attached oligosaccharides are always found on the extracellular side of the plasma membrane

(c)                    See Figure, The detailed structure of an animal cell's plasma membrane, in cross section

(d)                   The extracellular placement of oligosaccharides on membrane proteins makes intuitive sense since the oligosaccharides are added to these proteins within the lumen of the endomembrane system

(e)                    See Figure, Sidedness of the plasma membrane

(f)                     Oligosaccharides play important roles in cell-cell recognition (i.e., oligosaccharides of specific monomer sequence and branching pattern are recognized by other cells)

(g)                    [membrane oligosaccharide or carbohydrate or sugar or oligosaccharides or carbohydrates (Google Search)] [index]

 

MEMBRANE PERMEABILITY

 

(16) Selective permeability (see also selective permeability)

(a)                    Lipid bilayers display selective permeability

(b)                    In general, intact lipid bilayers are permeable to:

(i)                     Hydrophobic molecules (including many gasses)

(ii)                   Small, not-ionized molecules (e.g., H2O, CO2)

(c)                    Simultaneously, lipid bilayers are NOT permeable to:

(i)                     Larger, polar molecules (e.g., sugars)

(ii)                   Ions, regardless of size

(d)                   Thus, lipid bilayers are selectively permeable barriers that allow the entry of small or hydrophobic molecules while blocking the entry of larger polar or even small charged substances

(e)                    [selective permeability (Google Search)] [index]

(17) Transport across membranes

(a)                    Given the selective permeability of lipid bilayers, a number of mechanisms exist by which substances are moved across lipid bilayers (movement across membranes is important, for instance as a means of removing wastes from a cell or bringing food into a cell)

(b)                    Categories of substance transport across membranes include:

(i)                     Passive transport

(ii)                   Facilitated diffusion

(iii)                 Active transport (including cotransport)

(c)                    Endocytosis, phagocytosis, and exocytosis, also considered below, technically are not mechanisms of movement of substances across lipid bilayers (though these do represent movements of substances into and out of cells; to be movement across the eukaryotic cell membrane, a substance must actually pass through an endomembrane lipid bilayer)

(d)                   Note that in considering transport across membranes we will once again confront the concept of movement away from or towards equilibrium, i.e., endergonic and exergonic processes

(e)                    There are three basic types of movement across membranes: simple diffusion, passive transport, and active transport: Fig_12

(f)                     [transport across membranes (Google Search)] [index]

(18) Simple diffusion (see also passive diffusion)

(a)                    Simple diffusion is the movement of substances across lipid bilayers without the aid of membrane proteins

(b)                    This image (below) shows how substances move through membranes, regardless of net direction and concentration gradients: 038diffu

(c)                    This image (below) shows how substances net move through membranes in the direction of their concentrations gradients (i.e., with their concentration gradients)--note that regardless of how net movement is accomplished, all simple diffusion across membranes occurs in the manner illustrated above, i.e., it is a process that is driven by the random movement of molecules: simple         

(d)                   This figure (below) indicates the kinds of molecules that are capable of moving across membranes via simple diffusion: 054semip

(e)                    [simple diffusion membrane (Google Search)] [cells and diffusion (Online Biology Book)] [index]

(19) Passive transport (see also passive transport)

(a)                    Passive transport is the term used to describe the diffusion (as well as what is termed facilitated diffusion, below) of substances across lipid bilayers

(b)                    Passive transport is a consequence of movement through the lipid bilayer (whether by diffusion through the membrane or with movement across facilitated by an integral membrane protein) with (down) a concentration gradient thereby contrasting with active transport

(c)                    [passive transport (Google Search)] [cells and diffusion (Online Biology Book)] [index]

(20) Down the concentration gradient (diffusion) (see also down a concentration gradient and up a concentration gradient)

(a)                    Diffusion is a random process that tends to result in the net movement of substances from areas of high concentration to areas of low concentration

(b)                    This includes movement from one side of a permeable lipid bilayer to the other from the higher concentration side to the lower concentration side (i.e., passive transport)

(c)                    See Figure, The diffusion of solutes across membranes

(d)                   Movement from high to low concentration areas is described as going "down its concentration gradient."

(e)                    The direction of movement of substances across lipid bilayers by passive transport is controlled by concentration gradients

(f)                     Note that this movement represents movement toward equilibrium (i.e., it is an exergonic process)

(g)                    [down the concentration gradient membrane (Google Search)] [index]

 

OSMOSIS

 

(21) Solvents moving down concentration gradients (see also down a concentration gradient and osmosis)

(a)                    Even solvents can display concentration gradients

(b)                    Given two otherwise identical solutions:

(i)                     One has a higher solute concentration so has a lower solvent concentration

(ii)                   The other has a lower solute concentration has a higher solvent concentration

(c)                    That is, the more solute you add to a solution, the less solvent you will have per unit volume of solution (i.e., lower solvent concentration)

(d)                   Water will tend to flow (net) from the side of a selectively permeable membrane (permeable to water but not to the solute) that has less solute (higher water concentration) to the side of the membrane that has more solute (lower water concentration); that is, water will tend to flow down its concentration gradient from regions of high water concentration to regions of low water concentration (exergonic process)

(e)                    See Figure, Osmosis

(f)                     Note: the number of solute particles dissolved tends to be more important in determining which direction water will flow than the chemical nature of the solute

(g)                    Note also that in complex, real-world solutions different solutes may be controlling the water concentration on different sides of the membrane (i.e., the water solutions will not be identical except for solute concentration)

(h)                    [water and solvent movement (Online Biology Book)] [index]

(22) Osmosis (see also osmosis)

(a)                    Movement of water across selectively permeable membranes down the water concentration gradient is called osmosis

(b)                    Note that this is movement toward equilibrium (exergonic process)

(c)                    See Figure, Osmosis

(d)                   FAQ: I am a bit confused on the osmolarity thing. Can you explain? Osmosis is the movement of water across a membrane. It turns out that water can readily cross lipid bilayers (despite water's being a polar molecule and lipid bilayers being relatively impermeable to polar molecules). Consequently, osmosis is very relevant to biological systems. Given that water can cross membranes and that movement across membranes typically occurs going from a state of disequilibrium (i.e., unequal concentrations of water on either side of the membrane) to equilibrium (i.e., equal concentrations of water on either side of the membrane), then one would expect a net movement of water across membranes when concentrations on either side of the membrane are unequal. Specifically, one would expect a net movement of water from the side that has more water on it to the side that has less water on it. Up to now, this concept is exactly analogous to the movement by diffusion of anything across a membrane. Those dye molecules we looked at represent the net movement of a solute from a region of high concentration, across a membrane, to a region of low concentration. In other words, movement with (i.e., down) the concentration gradient (a.k.a., toward equilibrium). Water does the same thing, it moves down its concentration gradient. The big difference, though, is that water is the solvent rather than the solute. Consequently, it is not quite as straightforward to calculate water's concentration in a solution as it is to calculate a solute's concentration. To do the calculation for water, lets start with the idea that pure water contains some maximal concentration of water molecules, call this 100%. If pure water is maximum, then not pure water must have a lower than maximal concentration of water molecules, i.e., <100%. What kinds of things can lower the concentration of water in a solution? Just about anything that can go into solution. In other words, holding volume constant, the more of something other than water you have, the less water you have. If you have pure water on one side of a membrane and not pure water on the other side, then the not pure side will have a lower concentration of water. Water will move down its concentration gradient from the pure side to the not pure side, i.e., down its concentration gradient. We call this movement osmosis. Osmosis can occur with or without pure water on one side of the membrane, too, so long as the water concentration on either side of the membrane is different. (otherwise you will have equilibrium and no net movement of water will occur). That is, with a 10%-solute solution on one side of a membrane and a 20%-solute solution on the other side, there will be a net movement of water from the 10% side to the 20% side. This movement will continue, barring the application of any external forces, until the solute concentration on either side of the membrane is the same. Note that this equalization cannot occur if pure water is on one side of the membrane (i.e., there is no solute in pure water to equilibrate). It turns out that solute concentration units are best expressed in terms of solute particle number when calculating osmotic movement. This is because it is the number of solute particles that determines the direction of water's movement (indeed, it is the relative number of solute particles that is important) rather than the molecular weight, mass, etc. associated with the solute particle (that is, osmolarity, a solute's concentration measured in terms of its impact on osmosis, is considered a "colligative" property of solute particles, one that varies as a function of solute particle number rather than other solute particle properties such as size or shape). Given this understanding we can then derive just how it is that osmosis works. For a water molecule to cross a semipermeable membrane, one that is only permeable to water, the water molecule must first collide with the membrane. As you might expect, this happens quite often in a water solution. Depending on how many holes the membrane has in it, the colliding molecule will either bounce off of the membrane, or pass through it. Thus, the rate of passage of a water molecule through a semipermeable membrane is proportional to the number of water molecule collisions and the number of holes in the membrane (as well as the hole size). From here on out we'll assume that both the number and the size of holes in the membrane are held constant. If there is a higher concentration of water molecules on one side of the membrane than on the other, it is obvious that there will be more collisions on the more-water-molecules side than there will be on the less-water-molecules side. All else held constant, therefore, there should be a net movement of water from the more-water-molecules side to the less-water-molecules side. That is, down water's concentration gradient (i.e., toward equilibrium). Now, lets assume that the number of collisions of anything with the membrane is the same on both sides of the membrane. If you have pure water on one side of the membrane, then all collisions on that side of the membrane will be made by water molecules. However, if there is a solute present, then some of those collisions will involve a solute molecule rather than a water molecule. In fact, the fraction of collisions made by solute molecules is proportional to the number of solute molecules present, with the more present the more solute-molecule collisions with the membrane. If a solute molecule collides with the membrane, then a water molecule cannot simultaneously collide at the same point, at the same time. Hence, there will be fewer collisions by water molecules on that side, hence there will be less movement of water from that side across the membrane. Consequently, water will tend to move across a semipermeable membrane from a region of low solute concentration to a region of high solute concentration, and this movement we call osmosis.

(e)                    [osmosis membrane not reverse (Google Search)] [osmosis (MicroDude)] [index]

(23) Tonicity (isotonic, hypertonic, hypotonic) (see also isotonic solution, hypertonic solution, and hypotonic solution)

(a)                    Picture a membrane separating two solutions, one side with a higher solute concentration than the other

(b)                    The side with the higher solute concentration is said to be hypertonic

(c)                    The side with the lower solute concentration is said to be hypotonic

(d)                   (I keep track of the difference by recalling that a hypodermic syringe is so named because the tip of the needle is placed "beneath" the dermis, i.e., under the skin; a hypotonic solution has a solute concentration that is beneath, i.e., lower than that of the reference solution)

(e)                    If both sides have the same solute concentration, they are said to be isotonic

(f)                     See Figure, The water balance of living cells

(g)                    010plasm

(h)                    FAQ: A girl is getting ready to go out. As she is getting ready to put her contacts in, she realizes that she is out of contact solution. Her mother has a bottle of distilled water that she uses for ironing sitting cloths. The girl picks up the bottle and uses the distilled water on her contacts. Her eyes get red and irritated and her contacts pop out. Why does this happen? I do not understand if it is hypertonic or hypotonic. It is hypotonic. Her eyes are getting red because water is flowing into her cells and damaging and/or lysing those cells. Ideally contact lens solution is isotonic, i.e., a salt solution (NaCl) with the same osmolarity as is found in her extracellular tissues as well as inside of her cells. The contrasting situation is placing too concentrated a salt solution in your eyes (sea water, for example). In this case the eye's cells are damaged by too much water flowing out of them, into the seawater. With that example the solution (the seawater) is hypertonic. There is too much salt. The underlying mechanism of osmosis is the flow of water from a region of high water concentration to a region of low water concentration. However, the prefixes hyper and hypo refer to the relative salt (solute) concentrations. The hypertonic solution has too much solute (more than the reference solution) while the hypotonic solution has too little solute (less than the reference solution). Finally, I keep track of hyper and hypo by thinking of "hypo"dermic needles. These are employed to deliver stuff to "beneath" the surface of the skin. The term "hypo"tonic, in turn, refers to a solution which has a salt concentration which is "beneath" that of the reference solution, e.g., the inside of a cell.

(i)                      [tonicity, isotonic membrane, hypertonic membrane, hypertonic saline, hypotonic membrane, hypotonic (Google Search)] [the osmotic environment (Physiological Ecology)] [index]

(24) Animal cells and tonicity (see also isotonic solution, turgidity, and plasmolysis)

(a)                    Normally animal cells are bathed in an isotonic solution

(b)                    Placement of an animal cell in a hypertonic solution causes the cell to shrink (i.e., water is lost from the cell by osmosis)

(c)                    Placement of an animal cell in a hypotonic solution causes it to take on water then burst (lyse, i.e., die) (water is gained by the cell, lost from the environment bathing the cell, both by osmosis)

(d)                   See Figure, The water balance of living cells

(25) Turgidity (see also turgidity and hypotonic solution)

(a)                    Normally a plant cell exists in a hypotonic environment

(b)                    See Figure, The water balance of living cells

(c)                    The hypotonicity causes the plant cytoplasm to expand

(d)                   However the plant cell does not lyse and this is due to the presence of its cell wall

(e)                    This conditions is known as turgidity (i.e., the pressing of the plant plasma membrane up against its cell wall)

(f)                     Plant cells prefer to display turgidity

(g)                    [turgidity (Google Search)] [index]

(26) Plasmolysis (see also plasmolysis and hypertonic solution)

(a)                    A plant or bacterial cell placed in a hypertonic environment will show a shrinkage of its cytoplasm

(b)                    See Figure, The water balance of living cells

(c)                    This shrinkage is called plasmolysis

(d)                   At the very least plasmolysis will inhibit growth

(e)                    Often plasmolysis will lead to cell death

(f)                     This is the principle upon which foods are preserved in highly osmotic solutions (e.g., salt or sugar); such solutions impede most microbial growth

(g)                    [plasmolysis (Google Search)] [index]

(27) Flaccidity (see also flaccidity and isotonic solution)

(a)                    Plant cells bathed in isotonic solutions will fail to display turgidity

(b)                    Instead they display flaccidity

(c)                    See Figure, The water balance of living cells

(d)                   At a whole-organismal level, flaccidity is otherwise known as wilting

(e)                    [flaccidity plants (Google Search)] [index]

 

FACILITATED MOVEMENT ACROSS MEMBRANES

 

(28) Transport proteins (see also transport protein)

(a)                    Substances (e.g., sugars) that are not permeable through lipid bilayers may still cross via membrane-spanning transport proteins

(b)                    002mbrn

(c)                    [transport proteins (Google Search)] [index]

(29) Expands permeability but still selectively permeable

(a)                    Generally, transport proteins are as selective in what they allow to cross membranes as enzymes are selective in what substrates they will act upon

(b)                    In fact, the parallels between the properties of transport proteins and enzymes are fairly extensive, to the point where one may consider a transport protein simply as an enzyme-like protein that "catalyzes" the physical process of movement from one side of a membrane to another

(30) Facilitated diffusion (see also facilitated diffusion)

(a)                    Facilitated diffusion is the movement of a substance across a membrane via the employment of a transport protein, where net movement can only occur with the concentration gradient, is called facilitated diffusion

(b)                   See Figure, Two models for facilitated diffusion

(c)                    The key thing to keep in mind is that facilitated diffusion, in contrast to other mechanisms of transport-protein-mediated membrane crossing, does not require any input of energy beyond that necessary to place the protein in the membrane in the first place (i.e., facilitated diffusion is an exergonic process)

(d)                   Note that this movement of substances across membranes via facilitated diffusion is movement towards equilibrium

(e)                    [facilitated diffusion (Google Search)] [facilitated diffusion through a channel (follow steps)] [index]

 

ACTIVE TRANSPORT ACROSS MEMBRANES

 

(31) Passive versus active transport

(a)                    Two general categories of transport across membranes exist:

(i)                     Those that don't require an input of energy (passive transport, simple diffusion, facilitated diffusion)

(ii)                   Those that do require an input of energy (active transport)

 

Passive Transport

Active Transport

Concentration gradient

With (Down)

Against (Up)

Without Integral Protein

Yes (Simple Diffusion)

No

With Integral Protein

Yes (Facilitated Diffusion)

Yes

Examples

Small or Hydrophobic Substances, Osmosis (by simple diffusion) or Not-Small or Charged Substances (by facilitated diffusion)

Cotransport, Proton Pump, Sodium-Potassium Pump

(b)                     

(c)                    See Figure, Review: Passive and active transport compared

(32) Active transport (see also active transport)

(a)                    Active transport is the movement of substances across membranes against their concentration gradients

(b)                    Moving things against their concentration gradients requires an expenditure of energy (i.e., it is an endergonic process)

(c)                    This energy can be in the form of ATP (e.g., sodium-potassium pump)

(d)                   This energy can also be in the form of electrochemical gradients (i.e., cotransport)

(e)                    Note that the movement of substances by active transport is in a direction that is away from equilibrium

(f)                     [active transport (Google Search)] [active transport (follow steps)] [group translocation (follow steps)] [index]

 

TRANSPORT OF IONS ACROSS MEMBRANES

 

(33) Sodium-potassium pump (see also sodium potassium pump)

(a)                    One means by which cells actively transport substances across membranes is via the sodium-potassium pump

(b)                    The sodium-potassium pump is important especially in animal cells, and is the means by which the sodium-potassium electrochemical gradient is established by these cells

(c)                    Once established, the sodium-potassium electrochemical gradient may be tapped to perform additional mechanisms of active transport, though ones that are powered by the sodium-potassium electrochemical gradient rather than directly by ATP (i.e., via cotransport)

(d)                   Though without question physiologically important, the sodium-potassium pump also serves as an excellent, visually intuitive example of an enzyme-like catalyzed reaction (though to a large extent a physical reaction, i.e., transport across a membrane, rather than a solely a chemical reaction)

(e)                    The sodium-potassium pump pumps sodium out of cells and potassium into cells against a concentration gradient in a manner stoichiometrically balanced as follows:

 

3Na (intracellular) + 2K (extracellular) + ATP + H2O -->

3Na (extracellular) + 2K (intracellular) + ADP + Pi

 

(f)                     This is a thumb-nail sketch of how the sodium-potassium pump functions:

(i)                     Intracellularly the pump presumably has a relatively low affinity for potassium ions but high affinity for sodium ions

(ii)                   Sodium and potassium ions move to or into the pump via diffusion but only sodium ions can bind

(iii)                 Sodium ion binding stimulates ATP hydrolysis

(iv)                 This ATP hydrolysis drives a pump conformational change

(v)                    As a result of this conformational change, sodium ions, as well as the section of protein bound to these ions, is presented extracellularly

(vi)                 The pump thus is no longer shaped in a manner that will allow attachment of intracellular sodium atoms

(vii)               Pump conformational change with ATP hydrolysis also results in a change in pump affinity (affinity is lowered) for sodium ions

(viii)             Sodium ions are consequently free to diffuse away from the pump

(ix)                 Since the sodium ions are now presented extracellularly, they diffuse into the extracellular environment (thus raising the concentration of sodium ions in the extracellular environment)

(x)                   Pump conformational change with ATP hydrolysis also results in a increased pump affinity for potassium ions; recall that the region of the pump capable of binding sodium or potassium ions is now found extracellularly

(xi)                 Extracellular potassium and sodium ions diffuse to or into the pump but only potassium ions bind

(xii)               Potassium ion binding stimulates a relaxation of the previous (above) ATP-hydrolysis-induced conformational change

(xiii)             Upon this second, relaxing conformational change, bound potassium ions are carried across the membrane and thus presented intracellularly (i.e., as were the sodium ions prior to ATP hydrolysis)

(xiv)             Relaxation of conformational change-driven change in pump affinity results in lowered potassium affinity and raised sodium affinity

(xv)               Potassium ions are free to diffuse into the intracellular environment (thus raising the concentration of potassium ions in the intracellular environment)

(xvi)             At this point the pump has essentially returned to the state it was when we began this sequence

(g)                    Thus, with ATP hydrolysis coupled to sodium- and potassium-ion pumping a cell may maintain the following:

(i)                     high intracellular concentration of potassium ions

(ii)                   lower extracellular concentration of potassium ions

(iii)                 higher extracellular concentration of sodium ions

(iv)                 low intracellular concentration of sodium ions

(h)                    Note that the continued existence of these gradients demands that the cell membrane remain intact

(i)                      A great deal of energy is cumulatively expended by the sodium-potassium pump working throughout, for example, your body, but the maintenance of the above-noted concentration gradients is key to a number of processes including:

(i)                     nerve function

(ii)                   muscle function

(iii)                 active transport of many additional substances (i.e., cotransport).

(j)                     See Figure, The sodium-potassium pump: A specific case of active transport

(k)                    [sodium potassium pump (Google Search)] [index]

(34) Electrochemical gradient (see also electrochemical gradient)

(a)                    In addition to pumping against a concentration gradient, the sodium-potassium pump pumps against an electrochemical gradient

(b)                    This occurs because the pump exchanges two potassium ions for three sodium ions

(c)                    This results in a net loss of positive charge from the cytoplasm (i.e., the cytoplasm becomes negatively charged relative to the outside of the cell)

(d)                   The amount of charge lost from the cytoplasm increases as more sodium and potassium ions are pumped

(e)                    This creates an electrochemical gradient because not only is there a chemical concentration gradient (e.g., sodium ions going from outside to inside of the cell) but there is also an electrical charge gradient (positive on the outside, negative on the inside)

(f)                     Electrochemical gradients may be harnessed to do work

(g)                    Electrochemical gradients are analogous to waterfalls in which an overabundance of ions on one side of a membrane are equivalent to the water at the top of the falls, transport proteins within the membrane are equivalent to turbines that convert kinetic energy to other forms of physical or chemical energy, and the ion that has passed through the membrane into the cell is equivalent to water that is now found at the bottom of the falls

(h)                    [electrochemical gradient (Google Search)] [index]

(35) Membrane potential (see also membrane potential)

(a)                    The charge differential between the outside and inside of a cell is known as a membrane potential

(b)                    This membrane potential serves as the "electro" portion of the electrochemical gradient

(c)                    Membrane potentials serve cells, essentially, as batteries, i.e., stored energy

(d)                   [membrane potential (Google Search)] [index]

(36) Proton pump (see also proton pump)

(a)                    The sodium-potassium pump is the means by which animal cells generate membrane potentials

(b)                    In bacteria, plants, and fungi, proton (H+) pumps play the same role

(c)                    The proton pump is simply ATP-driven active transport in which the substance pumped across the membrane is a hydrogen ion

(d)                   See Figure, An electrogenic pump

(e)                    Consistent with the idea that mitochondria and chloroplasts are bacteria, we will return to proton pumps when we consider cellular respiration and photosynthesis

(f)                     [proton pump, proton pump chemiosmosis (Google Search)] [index]

(37) Cotransport (see also cotransport)

(a)                    Much of the active transport accomplished by a cell isn't directly powered by ATP

(b)                    Instead, much active transport is powered by membrane potentials (i.e., electrochemical gradients)

(c)                    Such electrochemical-gradient-driven active transport is called cotransport

(d)                   See Figure, Cotransport

(e)                    In cotransport, one substance, such as a sugar, is driven up its concentration gradient while a second substance, e.g., sodium ions or protons, are allowed to fall down their electrochemical gradient; the energy gained from the latter is employed to power the former (i.e., energy coupling)

(f)                     FAQ: Could you explain cotransport? Active transport involves the expenditure of energy to pump something across a membrane up its concentration gradient. That energy may be derived from ATP but that is not the only possible source. Another source is membrane potentials. That is, by pumping ions, a cell can set it up so that (typically) the interior of the cell has a net negative charge while the exterior has a net positive charge. This arrangement essentially represents a battery, i.e., it is a storage of potential energy. Allowing ions to cross the membrane by heading toward the side containing the net opposite charge allows the system to return to equilibrium. Movement toward equilibrium is exergonic, i.e., energy is liberated. This energy can be used to do work, such as the transport of other substances up their concentration gradient. The coupling of these two reactions is termed cotransport. Another way of looking at this is that the ions waiting to cross the membrane are equivalent to water found at the top of a waterfall. As they cross the membrane they are equivalent to water going over a waterfall. When they reach the other side of the membrane they are equivalent to water found at the bottom of the waterfall. During movement over the waterfall, potential energy is converted to kinetic energy (by gravity in the waterfall; with membrane potentials this occurs via the attraction between opposite charges), which may be harnessed to do work. In the case of cotransport the work done is the movement of the cotransported substance across the membrane against its concentration gradient.

(g)                    [cotransport (Google Search)] [index]

 

MOVEMENT INTO/OUT OF CELLS WITHOUT CROSSING MEMBRANES

 

(38) Endocytosis (see also endocytosis)

(a)                    Endocytosis is a general category of mechanisms that move substances from outside of the cell to inside of the cell, but neither across a membrane (technically) nor into the cytoplasm (again, technically speaking)

(b)                    Instead, substances are moved from outside of the cell and into the lumens of endomembrane system members

(c)                    To enter the cytoplasm an endocytosed substance must still be moved across the membrane of the endomembrane system, e.g., following their digestion (typically hydrolysis) to smaller molecules

(d)                   See Figure, The three types of endocytosis in animal cells

(e)                    Examples include: phagocytosis, pinocytosis, and receptor-mediated endocytosis

(f)                     [endocytosis, endocytosis not receptor (Google Search)] [vesicle-mediated transport (Online Biology Book)] [index]

(39) Phagocytosis (see also phagocytosis)

(a)                    Phagocytosis is the engulfing of extracellular particles and is achieved by wrapping pseudopodia around the particles, thus internalizing the particles into vacuoles

(b)                   See Figure, The three types of endocytosis in animal cells (a) Phagocytosis

(c)                    See Figure, The formation and function of lysosome, respectively

(d)                   039phago

(e)                    Amoebas employ phagocytosis to "eat"

(f)                     Most protozoa obtain their food by engulfing, i.e., via some form of endocytosis

(g)                    The advantage of endocytosis as a mechanism of food gathering has to do with minimizing the volume within which digestive enzymes must work in order to digest food, i.e., the engulfed food particle

(h)                    Cells in our own bodies, called phagocytes and macrophages employ phagocytosis to engulf (and then destroy) debris floating around our bodies as well as to engulf and destroy invading bacteria

(i)                      [phagocytosis (Google Search)] [index]

(40) Pinocytosis (see also pinocytosis)

(a)                    Pinocytosis is the engulfing of liquid surrounding a cell

(b)                   See Figure, The three types of endocytosis in animal cells (b) Pinocytosis

(c)                    This is how developing ova obtain nutrients from their surrounding nurse cells (ova are very large cells so have surface-to-volume problems--pinocytosis solves the problem of nutrient acquisition by allowing nutrients to be obtained across many internal membranes rather than being limited to crossing the plasma membrane)

(d)                   [pinocytosis (Google Search)] [index]

(41) Receptor-mediated endocytosis (see also receptor-mediated endocytosis)

(a)                    Receptor-mediated endocytosis involves the binding of extracellular substances to membrane-associated receptors, which in turn induces the formation of a vesicles

(b)                   See Figure, The three types of endocytosis in animal cells (c) Receptor-mediated endocytosis

(c)                    Receptor-mediated endocytosis is how your cells take up blood-transported cholesterol

(d)                   [receptor-mediated endocytosis (Google Search)] [index]

(42) Exocytosis (see also exocytosis)

(a)                    Exocytosis is more or less the mechanistic opposite of endocytosis

(b)                    Exocytosis is the delivery of vesicles to the plasma membrane whereupon fusion occurs and lumen contents are deposited outside of the cell

(c)                    Think secretion of the protein insulin or antibodies into the blood

(d)                   Figure, Sidedness of the plasma membrane

(e)                    [exocytosis (Google Search)] [index]

 

VOCABULARY

 

(43) Vocabulary [index]

(a)                    Active transport

(b)                    Animal cells and tonicity

(c)                    Cholesterol

(d)                   Cotransport

(e)                    Down the concentration gradient

(f)                     Electrochemical gradient

(g)                    Endocytosis

(h)                    Exocytosis

(i)                      Expands permeability but still selectively permeable

(j)                      Facilitated diffusion

(k)                    Fluid mosaic model

(l)                      Fluidity of membrane proteins

(m)                  Functions of membrane proteins

(n)                    Glycoproteins

(o)                    Hypertonic

(p)                    Hypotonic

(q)                    Integral membrane proteins

(r)                     Isotonic

(s)                     Flaccidity

(t)                     Flip-flopping

(u)                    Lipid bilayer

(v)                    Membrane

(w)                  Membrane asymmetry

(x)                    Membrane potential

(y)                    Membrane proteins

(z)                    Oligosaccharides

(aa)                 Osmosis

(bb)                Passive transport

(cc)                 Passive versus active transport

(dd)               Peripheral membrane proteins

(ee)                 Phagocytosis

(ff)                  Phospholipids

(gg)                Pinocytosis

(hh)                Plasmolysis

(ii)                    Proton pump

(jj)                    Receptor-mediated endocytosis

(kk)                Selective permeability

(ll)                    Simple diffusion

(mm)            Sodium-potassium pump

(nn)                Solvents moving down concentration gradients

(oo)                Temperature-dependence of fluidity

(pp)                Tonicity

(qq)                Transport across membranes

(rr)                   Transport proteins

(ss)                  Turgidity

(tt)                   Two dimensional fluid

(44) Practice questions [index]

(a)                    What component of the membranes of mammals (such as ourselves) contributes to their membrane's ability to maintain relatively low temperature fluidity, despite our relatively high normal body temperature?

(i)                     saturated fatty acids

(ii)                   peripheral proteins

(iii)                 cholesterol

(iv)                 phospholipids

(v)                   membrane flipping

(vi)                 cytoskeleton

(b)                    Describe the relative hydrophobicity of amino acid R groups associated with the various regions (i.e., more than one) of an integral membrane protein that spans the membrane.

(c)                    The term (or phrase) "__________" describes the state of being of a plant cell suspended in a hypertonic environment.

(d)                   Describe the water concentration inside of a cell relative to the environment outside of a cell if there occurs a net movement of water from outside of the cell to inside the cell. Assume that the cell has an intact plasma membrane that is impermeable to all solutes present inside or outside of the cell, but is permeable to water.

(e)                    Contrast pinocytosis and phagocytosis.

(f)                     Aside from generating chemical gradients, explain how the sodium-potassium pump generates conditions that support subsequent cotransport. (note that I am not looking for a complete description of the sodium-potassium pump's mechanism as an answer to this question, but instead am looking for answer which is an additional consequence of the pump's action)

(g)                    Analogous to the sodium-potassium pump employed by animal cells supporting cotransport, what sort of pump do bacteria, plants, and fungi employ?

(h)                    Under what conditions would you expect that a plant cell would be flaccid?

(i)                      Describe cotransport including a description of those conditions necessary for it to occur and what is the consequence of its occurrence. Limit your discussion to topics other than (i) how the conditions necessary to power cotransport arise and (ii) specific details of interaction between proteins and "substrates" (e.g., treat proteins as black boxes into which "substrates" and out of which "products" flow; additionally, assume that conditions necessary for the occurrence of cotransport are already present).

(j)                      Integral membrane proteins of eukaryotes are synthesized by what?

(k)                    Name two different types of membrane molecules that allow membranes to remain fluid at lower temperatures.

(l)                      What aspect of eukaryote membranes is always found on the non-cytoplasmic side (e.g., the surface of the cell)?

(m)                  Movement of water across a semipermeable membrane and down its concentration gradient is called what?

(n)                    Describe the tonicity of the extracellular solution either at the time the given description is appropriate (for intact cells) or while the cell was still intact for lysed cells.

A turgid plant cell

 

A plasmolyzed bacterial cell

 

A lysed red blood cell

 

An intact red blood cell suspended in blood

 

A bacterial cell in salt-preserved meat

 

(o)                    Animal cells establish their plasma-membrane potential by employing what specific membrane component? Note that establishment is the not the same as maintenance. The membrane potential is maintained via a lack of lipid-bilayer permeability to the substances whose charges are responsible for the membrane potential.

(p)                    What is responsible for the facilitation in the facilitated diffusion that differs this mechanism from passive diffusion? Be specific.

(q)                    What general category of active transport is powered by the electrochemical gradient established by a cell?

(r)                     In receptor-mediated endocytosis, assume that the receptor is an integral membrane glycoprotein that becomes incorporated into the resulting vesicle. Draw the resulting vesicle with all appropriate detail, indicating the existence of the lipid bilayer using adjacent lines (i.e., ========).

(s)                     The mechanistic opposite of endocytosis is termed __________ and involves the transport of vesicles from __________.

(t)                     Name an organism or type of cell that employs phagocytosis either in the course of obtaining food or in the course of performing its physiological function.

(u)                    Other than a cell wall, what is found in bacteria, plants, and fungi, but is functionally replaced by the sodium-potassium pump in the plasma membrane of animals?

(v)                    What component of a typical eukaryotic plasma membrane is not terribly fluid at normal temperatures (e.g., an animal at body temperature) and why isn't it?

(w)                  What role does cholesterol play in animal membranes?

(x)                    What does the term "flipping" mean within the context of membranes?

(y)                    In what kind of organisms is cholesterol typically found?

(z)                    What is a peripheral membrane protein?

(aa)                 The interaction of a membrane protein with what would you predict would result in an absence of fluidity within a membrane (i.e., no significant transverse movement)?

(bb)                Why would you expect that the oligosaccharides of an integral membrane glycoprotein would project into the extracellular environment rather than into a cell's cytoplasm?

(cc)                 Proteins are costly for a cell to produce and if a protein is responsible for catalyzing an endergonic physical or chemical process, then proteins are also costly for a cell to operate. Given both of these costs, arrange the following processes in order going from most-costly to a cell to least-costly to a cell: active transport, facilitated diffusion, and passive transport. Briefly justify your answer.

(dd)               __________ is an exergonic process by which ions may be transported across a cell membrane.

(i)                     Active transport

(ii)                   Exocytosis

(iii)                 Facilitated diffusion

(iv)                 Passive diffusion

(v)                   Pinocytosis

(ee)                 What is osmosis?

(ff)                  Scan in Figure and have label the different environments as hypertonic, hypotonic and isotonic. Include no words, change order, and show only shriveled animal cell, flaccid plant cell, and plasmolyzed plant cell.

(gg)                In terms of tonicity, how do animal cells get away with not having cell walls?

(hh)                What is turgidity?

(ii)                    Why is it that the existence of transport proteins expands the permeability of a lipid bilayer, but nevertheless such layers remain selectively permeable?

(jj)                    Other than ATP, what commonly is employed by cells to directly power active transport (if you use a specific example, make sure that you include the type of organism in which you would expect to find this specific example employed)?

(kk)                What is the stoichiometrically balanced equation describing the action of the sodium-potassium pump found in animal cells. Don't forget to indicate the intracellular versus extracellular locations of the various "substrates" and "products".

(ll)                    Describe the mechanism of the sodium-potassium pump beginning just after the release and diffusion of the sodium ions out of the pump and ending with the diffusion of the sodium ions back into the pump. Be sure to use correct stoichiometries and please do not describe the pump in any more detail than I've asked for (i.e., if you include descriptions of the rest of the pump's mechanism, e.g., after sodium ion diffusion into the pump, then I will assume that you don't understand the process well enough to not talk about these steps so consequently will dock points for this extra effort).

(mm)            An electrical charge differential between the inside of a cell and the outside of a cell (i.e., across the plasma membrane) is known as a/the __________. Note that I am not asking for the name of the larger phenomenon that includes both the electrical charge differential and a chemical gradient.

(nn)                Surface-to-volume problems require developing ova to obtain their nutrients other than (or in addition to) directly transporting those nutrients across the plasma membrane. What is this other, endocytotic means that these very large cells employ to take up nutrients from their environment.

(oo)                By what mechanisms do animal cells secrete proteins such as insulin into the extracellular environment? (looking for a one-word answer)

(pp)                In the fluid mosaic model the "fluid" portion consists mainly of _________, a type of lipid.

(qq)                In addition to (or instead of) employing cholesterol as a temperature buffer, how do organisms (such as cold-water fish) maintain the fluidity of their lipid bilayers, particularly at lower temperatures?

(rr)                   Which would you expect would more likely impede the fluidity of a membrane?

(i)                     Higher temperatures

(ii)                   Integral membrane proteins

(iii)                 Isotonic environments

(iv)                 Peripheral membrane proteins (particularly ones not connected to an extracellular matrix or cytoskeleton)

(v)                   Polyunsaturated fatty acids

(ss)                  Draw a peripheral membrane protein, in situ (i.e., associated with a membrane), including the structure of the phospholipid bilayer representing the bulk of the membrane and how the protein interacts with (attaches to) this membrane.

(tt)                   Name a function of membrane proteins that is in addition to those listed below:

(i)                     Attachment to the cytoskeleton and extracellular matrix

(ii)                   Cell-cell recognition

(iii)                 Intracellular joining

(iv)                 Signal transduction

(v)                   Transport of substances

(uu)                In animals, what substance - covalently bound to integral membrane proteins - do we find only oriented towards the extracellular environment, even with cells that do not interact extensively with an extracellular matrix? ("extracellular side of proteins" is not correct answer)

(vv)                Which is the energy-requiring process?

(i)                     Diffusion of water across lipid bilayers

(ii)                   Electrochemical gradient establishment

(iii)                 Facilitated diffusion of monosaccharides

(iv)                 Osmosis within a hypertonic environment

(v)                   Passive transport of calcium ions

(ww)            In osmosis, which is most important?

(i)                     Peripheral membrane proteins

(ii)                   The chemical nature of a solute

(iii)                 The dissolved concentration of solute

(iv)                 The pH of the solution

(v)                   The size of a solute ion

(xx)                Describe the probable aqueous, extracellular environment in which a plant cell (not displaying plasmolysis) is bathed given that the overall plant is wilted.

(yy)                In the sodium-potassium pump, which specific step stimulates pump phosphorylation by ATP?

(zz)                 In the sodium-potassium pump, what does potassium binding stimulate?

(aaa)             Name three specific structures, proteins, or systems employed by cells for the generation of electrochemical gradients (not all cells will employ all three mechanisms) [NOTE, YOU WILL BE IN A GOOD POSITION TO NAME THE THIRD ONLY UPON STUDYING THE FOLLOWING CHAPTER OF YOUR TEXT]

(bbb)            Which of the following does not (necessarily) involve endomembrane system membranes other than the plasma membrane?

(i)                     Cotransport

(ii)                   Endocytosis

(iii)                 Exocytosis

(iv)                 Phagocytosis

(v)                   Pinocytosis

(ccc)             The majority of active transport utilized by a cell is directly powered by what physical phenomenon? (note: ATP hydrolysis is a chemical phenomenon)

(ddd)          One solution to the surface-to-volume problem associated with nourishing ova is the employment of __________, a type of endocytosis.

(eee)             Draw a lipid bilayer associated with two proteins, one an integral membrane protein and the other a peripheral membrane protein. Make sure you label your drawing.

(fff)               Cholesterol serves as a temperature-stability buffer in the membranes of animals: At higher temperatures cholesterol interferes with the movement of phospholipids, thereby bestowing higher temperature stability to membranes. What is the mechanism by which cholesterol bestows lower-temperature functionality to membranes?

(ggg)            Membrane proteins play numerous roles in the physiology of cells as well as their interactions with their environments. Name four functions of membrane proteins.

(hhh)            Indicate why oligosaccharides attached to integral plasma membrane proteins tend to project into the extracellular environments of eukaryotic cells.

(iii)                  What are two basic properties of a substance that would tend to impede its ability to cross a lipid bilayer via simple diffusion?

(jjj)                  Classify the following in terms of movement of solutes "with" or "against" a concentration gradient:

(i)                     Active transport

(ii)                   Facilitated diffusion

(iii)                 Simple diffusion

(kkk)            The direction of movement of substances across lipid bilayers by passive transport is controlled by __________.

(lll)                  Movement of water across selectively permeable membranes __________ the water concentration gradient is called osmosis.

(mmm)      Describe a red blood cell suspended in a hypertonic solution.

(nnn)            What is turgidity, and why is it important?

(ooo)            What is plasmolysis, and why is it important?

(ppp)            Facilitated diffusion, in contrast to other mechanisms of transport-protein-mediated membrane crossing, does not require any input of __________ beyond that necessary to place the protein in the membrane in the first place (i.e., facilitated diffusion is an exergonic process).

(qqq)            Name two energy sources that may be tapped by integral membrane proteins to effect active transport (note that any given transport protein need not necessarily be able to tap both energy sources--just what sources, generally, are employed by at least some of the active transport proteins found in a cell membrane).

(rrr)                In addition to pumping against a concentration gradient, the sodium-potassium pump pumps against a(n) __________ gradient.

(sss)               The sodium-potassium pump is the means by which animal cells generate membrane potentials. In bacteria, plants, and fungi __________ pumps play the same role.

(ttt)                What is cotransport?

(uuu)            Which of the following does not involve movement of substances into the lumen of an endomembrane system member?

(i)                     Endocytosis

(ii)                   Exocytosis

(iii)                 Phagocytosis

(iv)                 Pinocytosis

(v)                   Receptor-mediated endocytosis

(vvv)            The engulfing by a cell (e.g., by a developing ovum) of nutrient-rich liquid is termed __________

(www)      Following endocytosis, the movement of the products of hydrolytic digestion into a cell's cytoplasm requires movement across a(n) __________.

(xxx)            Relate the term Flip-Flop to the idea of membrane asymmetry.

(yyy)            Cholesterol...

(i)                     Increases the fluidity of lipid bilayers

(ii)                   Decreases the freezing (solidification) point of lipid bilayers

(iii)                 Impedes phospholipid movement

(iv)                 Is an example of an amphipathic molecule

(v)                   All of the above

(vi)                 None of the above

(zzz)             Integral membrane proteins can be quite fluid, i.e., capable of laterally diffusing within lipid bilayers. Some membrane proteins, however, are locked in place. What causes these latter proteins to be locked in a specific location in, for example, plasma membranes?

(aaaa)          Glycoproteins spanning lipid bilayers have a consistent asymmetry. That is, the carbohydrates attached to integral plasma-membrane proteins are always found outside of the hydrophobic interiors of lipid bilayers and are also always found in what orientation?

(bbbb)        Substances with what two properties are lipid bilayers particularly permeable to?

(cccc)          Passive transport occurs either by simple diffusion or is __________.

(dddd)      Can solvent molecules display concentration gradients? Why or why not?

(eeee)          Water will tend to move across a semipermeable membrane from a region of low solute concentration to a region of high solute concentration, and this movement we call __________.

(ffff)            Matching: (a) Hypertonic, (b) Hypotonic, (c) Isotonic, (d) not applicable. (may use more than once)

(i)                     Flaccid: __________

(ii)                   Inhibition of bacterial growth: __________

(iii)                 Plasmolysis: __________

(iv)                 Solution bathing the cells of a well-watered plant: __________

(v)                   Turgid: __________

(gggg)        True or False, facilitated diffusion requires no input of energy by the cell--other than the costs of making and inserting the facilitating proteins--because substances are being moved up their concentration gradients.

(hhhh)        Give an example of a specific, non-gaseous substance (i.e., other than O2 and CO2) that crosses lipid bilayers by simple diffusion.

(iiii)                Why does movement of substances against concentration gradients require energy?

(jjjj)                Describe the mechanism of the sodium-potassium pump beginning just after the release and diffusion of the potassium ions out of the pump and ending with the diffusion of the potassium ions back into the pump. Be sure to use correct stoichiometries and please do not describe the pump in any more detail than I've asked for (i.e., if you include descriptions of the rest of the pump's mechanism, e.g., after potassium ion diffusion into the pump, then I will assume that you don't understand the process well enough to not talk about these steps so consequently will dock points for this extra effort).

(kkkk)        How does an electrochemical gradient differ from a concentration gradient?

(llll)                What is a membrane potential?

(i)                     A charge differential between the inside and outside of a membrane

(ii)                   A concentration gradient across a membrane

(iii)                 A concentration gradient of ions across a membrane

(iv)                 An indication of the selective permeability of a membrane

(v)                   The ability of a membrane to pump potassium and sodium

(mmmm)                        Whereas animals pump potassium and sodium ions, bacteria, plants, and fungi pump __________ to establish electrochemical gradients.

(nnnn)        Beside the fact that it involves the transportation across a lipid bilayer of two different substances, what is cotransport?

(oooo)        What form of endocytosis primarily involves the engulfing of water solution surrounding a cell.

(pppp)        What form of endocytosis primarily involves the engulfing of relatively large particles of food.

(qqqq)        What is osmosis?

(rrrr)              The fluid mosaic model describes what aspect of cells?

(ssss)            What molecule both impedes phospholipids fluidity and interferes with membrane solidification?

(tttt)              In contrast to peripheral membrane proteins, proteins that span into lipid bilayers we describe as __________.

(uuuu)        What word do we use to describe proteins that are found attached to the outside of membranes but do not enter into the hydrophobic region of lipid bilayers?

(vvvv)        Other than transport, name one function of a membrane protein.

(wwww)                        Describe the importance of cell membranes to cell functioning from the perspective of their asymmetry.

(xxxx)        If lipid bilayers are selectively permeable, what kind of substances are they permeable to and what kind of substances are they not permeable to?

(yyyy)        Describe simple diffusion across membranes.

(zzzz)          Distinguish passive transport from simple diffusion.

(aaaaa)      What is osmosis?

(bbbbb)    Normally a plant cell exists in a __________ environment.

(ccccc)      Distinguish passive and active transport in terms of movement towards equilibrium.

(ddddd) Fill in the missing information and otherwise describe what is going on. Yes, stoichiometry matters.

fig8_15modified

(eeeee)      What is an electrochemical gradient?

(fffff)         Describe the electrochemical gradient commonly found in bacteria, plants, and fungi.

(ggggg)    The three types (categories) of endocytosis include _________, __________, and __________.

(45) Practice question answers [index]

(a)                    iii, cholesterol.

(b)                    Hydrophilic on the cytoplasmic and extracellular sides, hydrophobic over the spanning region.

(c)                    undergoing plasmolysis.

(d)                   the water concentration outside of the cell is higher than that inside of the cell; in terms of tonicity, the environment is hypotonic.

(e)                    pinocytosis involves an engulfing of primarily solution while phagocytosis involves an engulfing of particle of significant heft, along with the surrounding solution.

(f)                     The sodium-potassium pump generates a membrane potential by pumping out three sodium ions for every two potassium ions pumped in.

(g)                    Proton pump.

(h)                    When the cell is suspended in an isotonic solution, i.e., where the water concentration inside of cells is the same as the water concentration outside of cells.

(i)                      The necessary players are (i) an electrochemical potential, (ii) a transport protein, (iii) something that the cell wants to pump against a concentration gradient, and, of course, (iv) and an intact membrane. In cotransport, the substance generating the electrochemical gradient (e.g., hydrogen ions) is moved from one side of the membrane to the other, down its electrochemical gradient via protein facilitation. The same or a coupled protein employs the energy gained by this movement toward equilibrium to drive the second substance up its concentration gradient, across the cell membrane.

(j)                      bound ribosomes; rough endoplasmic reticulum

(k)                    unsaturated phospholipids (unsaturated fatty acids); cholesterol

(l)                      attached sugars, i.e., oligosaccharides

(m)                  Osmosis

(n)                     

A turgid plant cell

Hypotonic

A plasmolyzed bacterial cell

Hypertonic

A lysed red blood cell

Hypotonic

An intact red blood cell suspended in blood

Isotonic

A bacterial cell in salt-preserved meat

Hypertonic

(o)                    the sodium-potassium pump

(p)                    integral membrane protein or transport protein

(q)                    Cotransport

(r)                     This should be a circle in which at least one protein is embedded with the oligosaccharide facing inward

(s)                     exocytosis and the Golgi apparatus

(t)                     Amoeba, protozoa, phagocyte, macrophage

(u)                    proton pump

(v)                    integral membrane protein linked to cytoskeleton member; because it is not free to mover around in the membrane as unbound proteins and phospholipids, etc. are

(w)                  buffers temperature stability, i.e., allows membranes to be fluid at lower temperatures than they otherwise might be and stable at higher temperatures than they otherwise might be

(x)                    Flipping is a change in the orientation (facing in versus facing out) of integral membrane proteins or a shift of a phospholipid from one leaflet of a lipid bilayer (e.g., the inner leaflet the plasma membrane that is in contact with the cell cytoplasm) to the other (e.g., the outer leaflet that in the same example is in contact with the extracellular environment)

(y)                    Cholesterol is typically found in the membranes of animals (e.g., not in plants)

(z)                    A peripheral membrane protein is a protein that is associated with a membrane but which does not span the lipid bilayer (i.e., is attached to the outside of the membrane, on one side or the other)

(aa)                 Cytoskeleton or extracellular matrix (membrane or membrane proteins is not a correct answer)

(bb)                Oligosaccharides are added to proteins within the lumen of the endomembrane system, which is equivalent to the extracellular environment in terms of membrane asymmetries; given minimal flipping of integral membrane glycoproteins, one would predict minimal presence of the oligosaccharide component of glycoproteins projecting into the cytoplasm

(cc)                 Active transport (requires proteins and is costly to run), facilitated diffusion (requires proteins but is not costly to run), and passive transport (does not require proteins and it not costly to run)

(dd)               (iii) Facilitated diffusion; exergonic meaning occurs spontaneously without an input of energy (once membrane protein is in place; this contrasts with active transport which involves an input of energy) and capable of transporting ions (passive diffusion is not); pinocytosis and exocytosis are both complex, energy-requiring processes

(ee)                 Osmosis is the net movement of water across a selectively permeable barrier in a direction that is with water's concentration gradient; Osmosis is also the movement of water from regions of low solute concentration to regions of high solute concentration

(ff)                  Sdf

(gg)                Animal cells are bathed in an isotonic solution so, under normal circumstances, have no need to guard against the bursting that occurs when exposed to a hypotonic environment

(hh)                Turgidity is the firmed-up state of a plant cell that results from bathing a plant cell in a hypotonic solution

(ii)                    Transport protein selectivity, in terms of what substrates they are able to bind and then translocate, explains why membranes remain selectively permeable despite the presence of permeability-expanding integral membrane proteins

(jj)                    Electrochemical gradients are employed to power cotransport; specific examples include sodium or potassium gradients in animal cells or hydrogen ion gradients in plant, bacteria, and fungal cells; note that a failure to employ the term gradient (or its equivalent) is necessary to answer this question correctly

(kk)                3Na (intracellular) + 2K (extracellular) + ATP --> 3Na (extracellular) + 2K (intracellular) + ADP + Pi

(ll)                    Two potassium ions diffuse into the pump from the extracellular environment; their binding triggers a relaxation of the conformation of the protein to the shape that was present prior to ATP hydrolysis; the completion of this conformational relaxation finds the potassium ions translocated to the intracellular side of the membrane, upon which they are released from the pump and they diffuse away into the cytoplasm

(mm)            Membrane potential

(nn)                Pinocytosis

(oo)                Exocytosis

(pp)                Phospholipids

(qq)                Additional double bonds in their fatty acids; decreased saturation of the phospholipids making up their lipid bilayers

(rr)                   (ii) Integral membrane proteins (since they are found directly in membranes yet do not mover around as quickly-i.e., are less fluid than-phospholipids)

(ss)                  Note that the peripheral membrane protein should be found on the surface of the lipid bilayer and attached to an integral membrane protein

(tt)                   Catalysis (i.e., enzymatic activity)

(uu)                Carbohydrates; oligosaccharides

(vv)                (ii) Electrochemical gradient establishment

(ww)            (iii) The dissolved concentration of solute

(xx)                Isotonic

(yy)                Sodium-ion binding

(zz)                 Relaxation of the protein to its pre-ATP hydrolysis conformation (thereby dragging the potassium ions across the membrane)

(aaa)             Sodium-potassium pump; proton pump; electron transport system

(bbb)            (i) Cotransport; the rest involve vesicles

(ccc)             An electrochemical gradient

(ddd)          Pinocytosis

(eee)             The integral membrane protein should be at least partially within the lipid bilayer while the peripheral membrane protein should be attached but not within the lipid bilayer

(fff)               Cholesterol impedes the crystallization (freezing) of lipid bilayers by interfering with the interactions between the hydrophobic tails of otherwise adjacent phospholipids

(ggg)            Transport, Enzymes, Signal transduction, Joining of cells, Cell-cell recognition, Attachment (to cytoskeleton and extracellular matrix)

(hhh)            The oligosaccharides are added to membrane proteins within the lumen of the endomembrane system, which is equivalent to the extracellular environment

(iii)                  Large size and charges, especially full charges

(jjj)                  (i) against, (ii) with, (iii) with

(kkk)            Concentration gradients

(lll)                  With

(mmm)      It is shriveled

(nnn)            Turgidity is the pressing of the plasma membrane of the cell wall and it is important for the support of plant tissue

(ooo)            Plasmolysis is the retraction of the plasma membrane away from the cell wall and it impedes bacteria replication

(ppp)            Energy

(qqq)            ATP and electrochemical gradients

(rrr)                Electrochemical

(sss)               Proton

(ttt)                Cotransport is the simultaneous transport of a substance with its concentration gradient with the energy derived from that exergonic process is used to pump a second substance against its concentration gradient

(uuu)            (ii) Exocytosis

(vvv)            Pinocytosis

(www)      Membrane

(xxx)            To function properly the membranes of a cell must retain the asymmetry with which they are manufactured. That is, the orientation of proteins and the types of lipids found on the inside versus outside of membrane must not change to any great degree, for otherwise cells  would not be able to distinguish between their external and internal environments. Key to the maintenance of this asymmetry is the relative rarity of flip-flopping, which is defined as the tendency of lipids to switch leaflets (e.g., inner to outer) or membrane proteins to reverse orientation or leaflet

(yyy)            (v) All of the above

(zzz)             Attachment to cytoskeleton or extracellular matrix

(aaaa)          Outside of the cell; oriented extracellularly; within the lumen of the endomembrane system

(bbbb)        Small and hydrophobic

(cccc)          Mediated by proteins; something that requires integral membrane proteins

(dddd)      Sure, that's the basis of osmosis. This occurs because substances dissolved in solvents displace solvent molecules. Consequently, if more solute is found at one end of a gradient than the other, forming a solute concentration gradient, then there can also be a solvent concentration gradient (can be since it is conceivable that in a two-solute system there still could be concentration gradients but ones that completely balance thereby keeping the solvent concentration constant throughout the solution)

(eeee)          Osmosis

(ffff)            (i) isotonic, (ii) Hypertonic, (iii) Hypertonic, (iv) Hypotonic, (v) Hypotonic

(gggg)        False (down their concentration gradients)

(hhhh)        Water

(iiii)                Movement up a concentration gradient is an exergonic process; movement up a concentration gradient involves decreasing entropy, which is equivalent to increasing the free energy of the system, which requires a net input of energy

(jjjj)                Three sodium ions diffuse into the pump from the intracellular environment; their binding triggers ATP hydrolysis and a resulting phosphorylation of the pump; phosphate binding causes the pump to change conformation resulting in the sodium ions being translocated to the extracellular side of the membrane, upon which they are released from the pump, and they diffuse away into the extracellular environment

(kkkk)        An electrochemical gradient involves not just concentration gradients but concentration gradients involving charged substances

(llll)                (i) A charge differential between the inside and outside of a membrane

(mmmm)                        hydrogen ions

(nnnn)        Cotransport is the active transport of one substance powered by the movement of a second substance with its concentration or electrochemical gradient

(oooo)        Pinocytosis

(pppp)        Phagocytosis

(qqqq)        Osmosis is the movement of water across membranes from regions of high water concentration to regions of low water concentration

(rrrr)              The dynamic structure of membranes

(ssss)            Cholesterol

(tttt)              Integral

(uuuu)        Peripheral

(vvvv)        Enzymatic activity, signal transduction, intracellular joining, cell-cell recognition, attachment to the cytoskeleton and extracellular matrix

(wwww)                        The asymmetric nature of membranes allow cells to distinguish the compartments on either side of the membrane, or the cytoplasm from the extracellular environment; this distinguishing occurs in terms of what is and is not transported across the membrane, which direction transportation occurs (particularly with regard to endergonic transport mechanisms), and what the membrane attaches to and how (e.g., cytoskeleton or extracellular matrix); in short, through their asymmetry, membranes can significantly contribute to defining the chemistry and function of the compartments they surround

(xxxx)        They are permeable to small or hydrophobic molecules and impermeable to large or hydrophilic molecules

(yyyy)        Molecules collide with the membrane which can result in entrance; within the lipid bilayer movement occurs via diffusion as a dissolved substance which may or may not result in collision with the opposite face of the lipid bilayer, which may or may not result win exit from the bilayer; for small, hydrophobic substances we may expect a concentration build up within these hydrophobic layers at some point resulting in an equilibrium established between the tendency of the membranes to dissolve these substances and the much lower tendency of these substances to re-dissolve in water

(zzzz)          Simple diffusion may or may not be included under the heading of passive transport, but regardless, passive transport is the movement of substance across lipid bilayers with their concentration gradient while simple diffusion is a specific kind of movement across lipid bilayers with the concentration gradient that does not require the employment of proteins to facilitate that movement

(aaaaa)      Osmosis is the movement of water across a semi-permeable membrane with water's concentration gradient (i.e., from high water concentration to low water concentration); alternatively osmosis may be described as movement of water from a region of low salt (solute) concentration to a region of high salt (solute) concentration

(bbbbb)    Hypoosmotic

(ccccc)      Generally passive transport involves movement of substances towards equilibrium (with concentration gradient) while active transport is movement away from equilibrium (against concentration gradient)

(ddddd) Three sodium ions diffuse into the sodium-potassium pump from the cytosol. This causes the protein to be phosphorylated at the expense of one ATP. The phosphorylation changes the conformation of the pump resulting in the movement of the sodium ions to an orientation outside of the cell whereupon they diffuse into the extracellular environment. Meanwhile two sites for potassium-ion binding are opened up and two potassium ions bind. This causes the protein to dephosphorylate, and relaxing back to its ground state. The potassium ions diffuse into the cytosol and the pump is once again available to bind sodium ions.

fig8_15

(eeeee)      And electrochemical gradient is an imbalance across a membrane in terms of both concentration and electrical charge

(fffff)         It is a proton motive force, i.e., a hydrogen ion electrochemical gradient

(ggggg)    Phagocytosis, pinocytosis, and receptor-mediated endocytosis

 

Chapter 8, Bio 113 questions:

 

(#) Secretion from cells of proteins synthesized into the lumen of the endoplasmic reticulum occurs via a phenomenon known as __________, which is mediated by secretory vesicle fusion with the plasma membrane.

 

A: exocytosis

 

(#) What is pinocytosis?

 

A: pinocytosis is the endocytosis of liquids such as nutrient-rich liquids

 

(#) What is pinocytosis?

(i)                     Endocytosis involving receptor binding

(ii)                   Endocytosis of "chunks"

(iii)                 Endocytosis of liquids

(iv)                 Exocytosis involving receptor binding

(v)                   Exocytosis of "chunks"

(vi)                 Exocytosis of liquids

 

A: (iii) Endocytosis involving liquids

 

(#) True or False, cotransport is a form of protein-facilitated active transport which, like the sodium-potassium pump, is directly powered by ATP.

 

A: False

 

(#) How is it that the sodium-potassium pump is able to generate an electrochemical gradient?

 

A: Because three potassium ions are pumped for every two sodium ions.

 

(#) What stimulates the "relaxation" of the sodium-potassium pump back to its pre-ATP hydrolysis conformation?

 

A: potassium binding.

 

(#) Tell me what facilitated diffusion is.

 

A: Facilitated diffusion is the movement of a substance across a membrane via the employment of a transport protein, where net movement can only occur with the concentration gradient, is called facilitated diffusion.

 

(#) In a plant cell, the opposite of turgidity is called _________.

 

A: Flaccidity

 

(#) What happens when you place an animal cell, such as a red blood cell, into a hypotonic solution?

 

A: The cell will take in water and then burst (lyse)

 

(#) True or False, because it can only take place in systems perturbed from equilibrium (i.e., in which concentration gradients exist), osmosis is an example of an endergonic physical process.

 

A: False, it's exergonic

 

(#) The direction of net movement of substances directly across lipid bilayers is controlled by what?

 

A: the substance's concentration gradient

 

(#) What properties of water allows it to cross lipid bilayers?

 

A: It is a small and uncharged molecule.

 

(#) Plasma membrane __________ allows a cell to automatically differ its intracellular environment from that existing extracellularly.

 

A: asymmetry

 

(#) Integral membrane proteins are typically _________ where they interact with the _________ portion of the membrane. Use same word twice.

 

A: hydrophobic.