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Summary of Membranes
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MembranesFluid mosaic model:
- Shows proteins embedded in a lipid bilayer
- Globular proteins are inserted into the lipid bilayers
- Their non-polar segments in contact with nonpolar interior of bilayer
- Their polar portions protruding out from membrane surface
Membrane proteins
1. Integral proteins – embedded in the membrane
2. Peripheral proteins – associated with surface of the membrane
Membranes consists of 4 groups:
1. Phospholipid bilayer
- provides a flexible matrix
- imposes a barrier to permeability
2. Transmembrane proteins
- collection of proteins that float in the lipid bilayer
- functions:
transport
communication across membrane
- many integral membrane proteins aren’t fixed to position, they can move about, just like
phospholipid molecules
3. Interior protein network
- membranes are structurally supported by intracellular proteins that reinforce the
membranes shape
- membranes use networks of other proteins to control the lateral movement of some key
membrane proteins, anchoring them to specific sites
4. Cell-surface markers
- the ER adds chains of sugar molecules to membrane proteins and lipids to make
glycoproteins and glycolipids
- different cell types exhibit different varieties of these glycoproteins and glycolipids on
their surface acts as cell identity markers
Electron microscopy has provided structural evidence:
Allows biologists to examine
the delicate, filmy structure of
a cell membrane
, - Both TEM and SEM are used to examine membranes.
- Freeze – fracturing visualizes the inside of membranes
1. A cell frozen in medium is cracked with a knife blade
2. The cell often fractures through the interior, hydrophobic area of the lipid bilayer, splitting
the plasma membrane into two layers
3. The plasma membrane separates such that proteins and other embedded membrane
structures remain within one or the other layers of the membrane
4. The exposed membrane is coated with platinum, which forms a replica of the membrane.
The underlying membrane is dissolved away, and the replica is then viewed with electron
microscopy.
Phospholipids
Spontaneously form bilayers this is driven by the tendency of water molecules to form
the max number of hydrogen bonds
Because of their amphipathic structure
Polar head = hydrophilic
Nonpolar tail = hydrophobic
- A lipid bilayer is fluid but stable because waters affinity for hydrogen bonding never stops
- Hydrogen bonding of water holds a membrane together
Membrane fluidity varies with lipid composition:
- Triglycerides can be solid/liquid at room temperature
o Depends on their fatty acid composition
- Membrane fluidity can be altered by changing the membranes lipid composition.
- Glycerol phospholipids that are saturated, or mono cis-unsaturated tend to make the
membrane less fluid, as they pack well
- Sphingolipids, which are usually saturated, also make the membrane less fluid
o Increasing temperature = membrane more fluid
o Decreasing temperature = less fluid
- Bacteria have evolved mechanisms to maintain a constant membrane fluidity despite
fluctuating temperatures
o Some bacteria contain enzymes (fatty acid desaturases) that cam introduce double
bonds into fatty acids in membranes
o The action of these enzymes confers cold tolerance
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