Chapter 11. Membrane structure
The plasma membrane serves as a barrier to prevent the contents of the
cell from escaping, but it also must allow some things to pass, receives
information, and it has the capacity to move, expand and divide.
Fig. 11-2
Cellular membranes consist of a lipid bilayer and membrane
proteins, which allow the selective passage of molecules and signals across
the membrane.
Fig. 11-4
This is the called the “fluid-mosaic” model of membrane structure, because
proteins float (to some extent) in the lipid bilayer.
A. The lipid bilayer is a consequence of the way lipid molecules behave in a
watery (or aqueous) environment.
1) Membrane lipids are amphipathic; they have hydrophilic heads and
hydrophobic tails.
a. The most common one is phosphatidylcholine – a type of
phospholipid.
Fig. 11-6
In phospholipids, a
hydrophilic head is
linked to the rest of
the lipid through a
phosphate group.
b. Other types of membrane lipids are also amphipathic.
Fig. 11-7
2) Hydrophobic molecules like triacylglycerols cannot form bonds with
water molecules, which instead form a hydrogen-bonded network around
the hydrophobic molecule.
Fig. 11-9
Fig. 11-10
Hydrophilic
head group
would be
here in a
membrane
lipid.
3) By contrast, the hydrophilic head groups of amphipathic membrane lipids
can form noncovalent bonds with water, and so orient outwards towards the
water.
Fig. 11-11
Formation of a lipid bilayer allows the hydrophilic heads to face outwards
towards the water, while the hydrophobic tails are shielded from water on
the inside.
4) Phospholipid bilayers spontaneously form sealed compartments, so
that there is no exposure of hydrocarbon tails to water.
Fig. 11-12
a. The membrane bound compartments of cells are “self organizing.”
b. Tears in the membrane are spontaneously repaired because the membrane
reseals.
c. Hence, the cellular membrane compartments (cytosol, compartments of the
endomembrane system) result from the inherent properties of lipid molecules.
5) The bilayer membrane is a fluid because the lipid molecules can move around
in the plane of the membrane, but only rarely flip between the two bilayers of
the membrane.
Fig. 11-14
a. Just how fluid the membrane is depends on temperature (less fluidity at
lower temperature) and the degree of packing of the hydrocarbon tails
(close packing produces a less fluid bilayer). Why is fluidity important?
b. The structure of the hydrocarbon tail affects the “packing” of these tails.
-Shorter tails do not interact as well with each other and therefore
produce greater membrane fluidity.
-Unsaturated hydrocarbon tails containing one or more double bonds
between carbons (and therefore unsaturated in terms of the number of
possible C-H bonds) tend to have kinks, which reduce packing and increase
fluidity; most membrane lipids have one saturated, one unsaturated.
-In animal cells, cholesterol can be packed into the spaces formed by
these kinks, and thereby reduces the fluidity of membranes at high
concentrations and increases it a low concentrations.
Fig. 11-15
How will membrane lipid composition change to maintain a certain level of
fluidity, when temperature changes?
c. New lipids are synthesized in the cytosolic monolayer of the ER and remain there unless
moved to the lumenal monolayer by enzymes called “scramblases.”
Fig. 11-16
Flippases selectively
transfer lipids from
the exterior side of
the membrane to the
cytosolic side (in
Golgi, plasma
membrane),
contributing to an
asymmetry in lipid
composition; the two
monolayers have
different lipid
compositions.
d. Lipid vesicles bud off from the ER to transport lipids and membrane
proteins to other membranes in the cell. The original orientation of the
monolayers is preserved during fusion, so the asymmetry of membrane
lipids and proteins is preserved.
Fig. 11-17
Lumenal side of organelles becomes extracellular side of plasma membrane.
Sugars are added to lipids on the lumenal side of the
endomembrane system (e.g., the Golgi). Therefore, they are
principally found on the extracellular side of the plasma
membrane.
Plasma membrane
Fig. 11-18
B. Membrane Proteins give membranes their functional properties.
Fig. 11-19
Membranes in different parts of the cell, and in different cell types,
have different kinds of membrane proteins that allow them to
accomplish different functions.
1) Proteins can be associated with the lipid bilayer in several ways.
Fig. 11-20
Integral membrane proteins require disruption
of the lipid bilayer for removal (e.g., by
detergent).
Peripheral membrane
proteins require disruption of
protein/protein contacts
for removal.
2) The polypeptide backbone most often crosses the lipid bilayer as an ahelix.
a. When the polypeptide chain crosses the membrane just once,
hydrophobic amino acids are typically exposed on the outside of an a-helix.
Fig. 11-22
Atoms of the peptide bond
have a partial charge; this
can drive them to form ahelices in which they are
hydrogen-bonded.
b. Pores which allow passage of hydrophilic molecules can be formed by
multiple a-helices arranged in a cylinder, with hydrophobic side chains
exposed on the outside and hydrophilic side chains on the inside.
Fig. 11-23
Hydrophilic side chains on inside,
Hydrophobic side chains on outside
c. Large pores can be formed by b-sheets
that are wrapped into “b-barrels.”
Fig. 11-24
3) Detergents are amphipathic molecules which surround lipids and the
hydrophobic parts of membrane proteins, with their hydrophilic parts facing the
water. They solubilize membrane lipids and membrane proteins.
Fig. 11-25
Fig. 11-26
Because detergent molecules (only one hydrophobic tail) differ from membrane
lipid molecules in structure, they are wedged shaped rather than cylindrical and
form micelles rather than membrane-bound compartments.
4) The plasma membrane is reinforced by a cell cortex attached to the
cytosolic surface of the plasma membrane. This determines the shape of the
cell, its mechanical properties, and its capacity to move.
The distinctive shape of the red blood cell is determined by its cell
cortex, in which spectrin filaments are linked to membrane attachment
proteins.
Fig. 11-29
Fig. 11-28
5) The extracellular face of the plasma membrane has short (in
glycoproteins) and long (in proteoglycans) chains of sugars attached to many
of the proteins and lipids exposed on this side.
a. Protect the cells from chemical and physical damage
b. Absorb water to give the outside of the cell a slimy coat
(lubrication)
c. Because of diverse sequences and linkages, they are tags which
allow particular molecules, and other types of cells, to bind.
Fig. 11-33
6) Because the lipid bilayer is a fluid, proteins can diffuse throughout the
membrane – unless they are anchored in place.
a. Fluorescence microscopy and cell fusion experiments can be used to
show the diffusion of proteins.
Mouse and human proteins
gradually become
intermixed.
Fig. 11-30
b. The lateral diffusion of proteins can be restricted in several ways.
Fig. 11-31
Tethered to cortex
Tethered to extracellular matrix
11-32
Tethered to proteins on another cell
By diffusion barriers
c. Fluorescence Recovery After Photobleaching (FRAP) can be used to
determine how rapidly particular types of proteins diffuse in the plasma
membrane.
Fig. 11-34
The rate of fluorescence
recovery is proportional to the
rate of diffusion into the
bleached area from the
surrounding areas.
d. Single particle tracking of individual protein molecules
(e.g., tagged with gold particles/IgG) show a lot of variation
in diffusion rates and extents.
Purchase answer to see full
attachment
