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BaConCell Lecture Notes
Membrane structure
Membrane structure
Reading: Molecular Biology of the Cell, Chapter 10 (p 583-614)
1. The plasma membrane encloses the cell. Inside the cell, membranes of the
endoplasmic reticulum, Golgi, mitochondria, and other membrane-bound organelles in
eucaryotes maintain the characteristic differences between the contents of each
organelle and the cytosol.
2. All membranes have a common general structure. A lipid bilayer of about 5nm thick
that serves as a relatively impermeable barrier to most water soluble molecules, and
within the bilayer, proteins that have specific functions, such as ATP synthesis or the
transport of molecules across the membrane.
3. The lipid molecules are amphipathic molecules with hydrophilic and hydrophobic ends.
The most abundant are the phospholipids.
4. There are four major phospholipids in mammalian plasma membranes: phosphatidylethanolamine, phosphatidylserine, phosphatidylcholine and sphingomyelin. Because
phosphatidylserine is found mostly in the inner part of the membrane and because it is
also negatively-charged, the membrane is asymmetric. The difference in charge is
important in certain membrane functions, such as signal transduction.
5. The bilayer also consists of cholesterol and glycolipids. Cholesterol can be as much as
50% of the total, and it enhances the permeability-barrier properties of the membrane.
It also helps prevent phase changes (such as freezing).
6. Glycolipids are found on the outer surfaces of all plasma membranes (ie the noncytoplasmic half of the membrane). They tend to aggregate. They are thought to have
various possible functions. For example the ganglioside GM1 in the intestinal epithelium
probably functions as a receptor for normal extracellular molecules. However, it also
acts as a cell-surface receptor for the cholera toxin. Binding of the toxin leads to an
increase in internal cyclic AMP which in turn leads to a large efflux of Na+ and water
into the intestine, and thus you get diarrhea.
7. Membrane proteins can pass completely through the membrane (transmembrane
proteins) as single or multiple alpha helices, or be attached covalently, either directly or
indirectly by attachment to a fatty acid or prenyl group, or attached by non-covalent
interactions with other proteins.
8. Many membrane proteins diffuse throughout the membrane (lateral diffusion), like tiny
ships on a miniature sea. They can also rotate (rotational diffusion), but very rarely flip
inside out (flip-flop). Proteins can cluster together and can be located in certain areas
of the membrane, called domains. E.g., the three domains in the plasma membrane of
guinea pig sperm that are defined by monoclonal antibodies and shown in Fig 10-42.
9. Red blood cell (rbc) membranes have been studied more than any other membrane.
Most of the protein molecules in the rbc cell membrane are on the cytoplasmic side.
The most abundant of these is spectrin, which is a heterodimer consisting of two
antiparallel chains helically intertwined. Spectrin is the principal component of the
protein network that underlies the membrane, which maintains the structural integrity
of the cell. Spectrin interacts with actin and tropomyosin (two components of the
cytoskeleton to be presented in Cell Dynamics in Autumn term) and is bound to various
proteins on the inner surface of the cell membrane. For example band 3, which is a
trans-membrane protein to which is attached ankyrin, which binds spectrin; a
covalently-bound membrane protein called band 4.1 binds spectrin in a complex with
adducin, actin and tropomyosin. A major component of the rbc cell membrane is
glycophorin, which is a single-pass trans-membrane protein.
10. The cell surface is not necessarily naked. In fact many cells have a sugary coating
called the glycocalyx. These sugar residues are attached to protruding membrane
Ridge
BaConCell Lecture Notes
Membrane structure
proteins and appear to be involved in many cell-cell interactions, including recognition
(by highly specific binding to lectins) and cell-cell adhesion.