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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Course-external links are
in brackets Click [index] to access site index Click here to access
text’s website Vocabulary
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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]
(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]
(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]
(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 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]
(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
(c)
However, “flip-flopping” from one layer to the other is rare
(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]
(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]
(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)] [membrane protein introduction
(MIT Biology Hypertextbook)]
[index]
(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
(c)
Some (probably many or most) integral membrane proteins completely span
the lipid bilayer
(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
(c)
Other membrane proteins are tied in place by attachment to the cytoskeleton or the extracellular matrix
(d)
See Figure 8.6, The detailed
structure of an animal cell’s plasma membrane, in cross section
(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]
(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 bilyaers 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:
(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)] [tra