Session 1 Flashcards
(10 cards)
Outline the composition of the plasma membrane
40% lipid
60% protein
1-10% carbohydrate
(Dry weight)
20%water when hydrated
What are the different lipids present in a phospholipid bilayer?
Phospholipids (predominant) - a range of polar heads (choline, amines, sugars) and fatty acid chains (mostly C16&C18) are employed
Sphingolipids - sphingomyelin and glycolipids. Sphingomyelin is the only phospholipid not based on glycerol, conformation in the membrane resembles other phospholipids. Glycolipids have sugar groups in place of phosphocholine.
Cholesterol - makes up 45% of total membrane lipid.
What is the function of cholesterol in plasma membranes?
Stabilises the plasma membrane by H bonding to the fatty acid chains. The rigid steroid ring structure phospholipid chain motion, reducing fluidity. The non polar hydrocarbon chain reduces phospholipid packing, increasing fluidity. This allows cholesterol to act as a buffer, membrane is neither too frozen or mobile.
What are the 4 permitted modes of mobility of lipid and protein molecules in a lipid bilayer?
Lipid:
Intra chain motion - kink formation in fatty acyl chains
Fast axial rotation
Fast lateral diffusion within the plane of the bilayer
Flip-flop
Protein: conformational change, rotation and lateral diffusion (no flip flop due to hydrophilic parts)
How are peripheral and integral membrane proteins bound and removed?
Peripheral proteins bound to the surface by electrostatic and hydrogen bonds. Removed by changes in pH or ionic strength
Integral proteins interact with the hydrophobic region of the bilayer. Removed by agents (detergents, organic solvents) that compete for the non-polar interactions.
What is the evidence for proteins in the membrane?
Functional - facilitated diffusion, ion gradients, specificity of cell responses
Biochemical - membrane fractionation and gel electrophoresis, freeze fracture
How do membrane proteins in erythrocytes contribute to the cytoskeleton?
The network of spectrin and actin binds to adapter proteins on the membrane.
Describe two haemolytic anaemias associated with a disrupted erythrocytes cytoskeleton
Hereditary spherocytosis - depleted spectrin levels means cells round up and become less resistant to lysis in the capillaries and are cleared by the spleen
Hereditary elliptocytosis - spectrin molecules are unable to form heterotetramers resulting in fragile elliptoid cells
What is the difference between the synthesis of a membrane and a secreted protein?
The passage of the membrane protein through the ER membrane is arrested. A hydrophobic stop transfer signal (18-22aa) forms the transmembranous region of the protein. The ribosome then detaches from the ER and synthesis continues into the cytoplasm.
What can be used to see how many transmembrane domains a protein has?
Hydropathy plot
