Membrane Structure and Function

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

(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]

(3) Phospholipids (amphipathic)

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

(b) Recall “clothespin” representation

(c) See Figure 5.12, The structure of a phospholipid

(d) See Figure 5.13, Two structures formed by self-assembly of phospholipids in aqueous environments

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

(4) 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

(d) See Figure 8.1, 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 phosopholipid, C is one of the aqueous solutions surrounding the lipid bilayer, and that the big black object represents an integral membrane protein:

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

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

(5) 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 8.2b, Two generations of membrane models (note that Figure 8.2a is an obsolete model)

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

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

(a) Lipid bilayers serve as two-dimensional fluids

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

(d) See Figure 8.4a, 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 8.4b, The fluidity of membranes

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

(8) 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 8.4c, 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]

(9) Membrane proteins

(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

(10) Integral membrane proteins

(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

(d) See Figure 8.6, The detailed structure of an animal cell’s plasma membrane, in cross section

(e) See Figure 8.7, 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

(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 7.30, Intercellular junctions in animals)

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

(vi) Attachment to the cytoskeleton and extracellular matrix

(b) See Figure 8.9, 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

(d) See Figure 8.6, 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 8.8, Sidedness of the plasma membrane

(f) See Figure 8.4a, The fluidity of membranes (movement of phohspholipids)

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

(15) Oligosaccharides (glycoproteins)

(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 8.6, 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 8.8, Sidedness of the plasma membrane

(f) Oligosaccharides play important roles in cell-cell recognition (i.e., oligosacherides 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]

(16) 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)

(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 euakaryotic 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:

(f) [transport across membranes (Google Search)] [membrane transport mechanisms (MIT Biology Hypertextbook)] [transport across the plasma membrane (Portrait of a Eucaryotic Cell)] [index]

(18) Simple 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:

(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:

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

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

(19) 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

(20) Down the concentration gradient (diffusion)

(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 8.10, 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]

(21) Solvents moving down concentration gradients

(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 8.11, 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

(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)

(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 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 unpure 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 semipermiable 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 semipermiable 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)] [osmotic pressure (see chime if graphics won’t load) (Chemistry Learning Center)] [index]

(23) Tonicity (isotonic, hypertonic, hypotonic)

(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

(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 8.12, The water balance of living cells

(g)

(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

(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 8.12, The water balance of living cells

(25) Turgidity

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

(b) See Figure 8.12, The water balance of living cells

(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

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

(b) See Figure 8.12, The water balance of living cells

(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

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

(b) Instead they display flaccidity

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

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

(28) Transport proteins

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

(b)

(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

(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 8.14, 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) [faciliated diffusion (Google Search)] [facilitated diffusion through a channel (follow steps)] [index]

(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 8.15, Review: Passive and active transport compared

(32) 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)

(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]

(33) 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 à

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 8.15, The sodium-potassium pump: A specific case of active transport

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

(34) 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

(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

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

(36) 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 8.17, 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

(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)

(d) See Figure 8.18, 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]

(38) 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 8.19, 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

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

(b) See Figure 8.19, The three types of endocytosis in animal cells (a) Phagocytosis

(c) See Figure 7.14, The formation and function of lysosome, respectively

(d)

(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

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

(b) See Figure 8.19, 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

(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 8.19, The three types of endocytosis in animal cells (c) Receptor-mediated endocytosis

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

(42) 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