Cell Membrane Flashcards
What are the general functions of biological membranes?
- Continuous, highly selective permeable barrier
- Control of the enclosed chemical environment
- Communication
- Recognition: signalling molecules, adhesion proteins, immune surveillance.
- Signal generation in response to stimuli (electrical, chemical)
What is the general composition of the membrane?
It varies with the source of the membrane but generally, the dry weight of the membrane contain approximately:
40% lipid
60% protein
1-10% carbohydrate
Although, membranes are hydrated structures so 20% of total weight is water.
What is the name for a molecule that contains but a hydrophilic and a hydrophobic moiety?
Amphipathic
Talk about phospholipids.
They have a range of polar head groups. Eg choline, amines, amino acids, sugars.
The fatty acid chain varies in length between C14-C24 (although C16 and C18 are most prevalent).
The cis double bind induces a kink.
Phospholipids get their name from from head group. Eg phosphatidylcholine.
What is inositol?
A phospholipid head group. It is a substrate for signalling molecules in response to message. It releases molecules into the cell.
It plays a role in lipid signalling, cell signalling and membrane trafficking.
What is a glycolipid?
Glycolipids are lipids with a carbohydrate attached by a glycosidic bond. Their role is to maintain stability of the membrane and facilitate cellular recognition.
(The phosphate head has been replaced with a sugar)
What are the two types of glycolipid?
Cerebroside - the head group is a sugar monomer
Ganglioside - the head group is a oligosaccharide (sugar multimers)
Name the formations that phospholipids get into
Micelles
Liposome
Lipid bilayer
Name the motions or phospholipids (4)
Flexion
Rotation
Lateral diffusion
Flip flop
What effect does a double bond in a phospholipid have on the membrane?
A cis double bond induces a kink in the chain. This reduces phospholipid packing and increases the possibility for movement.
You need unsaturated fatty acids in the diet to keep the membrane dynamic.
What is the structure of cholesterol?
It has a polar head group;a rigid, planar steroid ring structure. (It has 4 rings); and a non-polar hydrocarbon tail.
Why is cholesterol described as having a ‘paradoxical’ effect?
Because it prevents phospholipids forming crystals at low temperatures and prevents lots of movement at high temperatures.
It homogenises phase transition from crystalline to fluid by preserving a fluid environment.
It does both of these things by forming hydrogen bonds with the carbonyl of the phospholipids.
What is the evidence for membrane proteins?
Functional:
Facilitated diffusion, ion gradients, specificity of cel responses.
Biochemical: membrane fractionation and gel electrophoresis, freeze fractures.
Freeze fracture
Freeze cells then put a knife on them and push. If lucky, it will fracture through the middle of the bilayer. Some proteins will stay in the membrane and others will leave a whole where they should be.
You then preserve with Osmium and view under an electron microscope.
How do membrane proteins move?
Conformational change (flicker between stable conformations),
Rotational,
Lateral ,
NO FLIP FLOP
(Think of it like ships in a sea of lipids)
How can membrane protein movement be restricted?
Aggregation (when one protein joins with another to form a raft),
Tethering (adhesion proteins p- Use proteins to join cells to basement membrane or to anchor it to a structure in the cell),
Interactions with other cells (this fixes proteins because they are used to join the two cells together),
Lipid mediated effects,
Membrane protein associations,
Association with extra-membranous proteins (peripheral proteins like the cytoskeleton)
Where do proteins tend to go?
Cholesterol poor regions
What types of proteins do not go to cholesterol poor regions?
Signal proteins
What are the two types of membrane protein?
Peripheral and integral
Where are peripheral proteins located and how can they be removed?
Peripheral proteins are not in the bilayer but are attached to the cell surface.
They are attached by electrostatic and hydrogen bind interactions so can easily be removed by changes in pH or ionic strength (salt)
Where are integral proteins located in the membrane and how can they be removed?
Integral proteins interact extensively with hydrophobic domains of the lipid bilayer. This means that they cannot be removed by manipulation of pH or ionic strength (salt).
This means that you have to use agents this compete for non polar interactions and so dissolve the bilayer. For detergents and organic solvents.
Summarise how a transmembrane protein might fold
Transmembrane domains are often alpha helical and the R groups of the amino acid residues are largely hydrophobic.
The hydrophobic amino acids are usually in the centre of the me,brand while the polar ones end up on the outside. The small amino acids (eg glycine) have very little effect on the membrane shape.
What parts of the protein are usually on the outside of the cell?
Disulphide bonds and carbohydrates always face outwards (NOT into cytoplasm).
The C terminal is usually on the outside (and N terminal on the inside) but not always!
What is spectin?
A large protein found in the membrane of erythrocytes
They are rod like molecules that join end to end through the inside surface of the erythrocyte membrane
