5.1 the structure and function of membranes (a,b)

Question 1

What is compartmentalisation and why is it vital to the cell?

Answer 1

• compartmentalisation is the formation of separate membrane-bound areas in a cell
• vital to a cell because it ensures incompatible reactions stay separated; specific conditions for cellular reactions are maintained and it protects cell components

Question 2

What are the roles of membranes?

Answer 2

• partially permeable barriers between the cell and its external environment, between organelles and the cytoplasm and within organelles
• the sites of chemical reactions, e.g. they must be in particular positions for chemical reactions to occur
• the sites of cell communication/cell signalling - when the chemical binds to the receptor, causing a response from the cell, direct response or a cascade of events inside the cell

Question 3

What is the fluid mosaic model?

Answer 3

• phospholipid bilayer forms the plasma membrane
• phospholipids are free to move within the layer relative to each other (they are fluid), giving the membrane flexibility + proteins are embedded, varying in shape, size and position (like a mosaic)

Question 4

Describe intrinsic proteins

Answer 4

• transmembrane proteins embedded through both layers of the membrane, e.g. channel proteins + carrier proteins
• have amino acids with hydrophobic R-groups on their external surfaces, which interact with the hydrophobic core of the membrane, keeping them in place

Question 5

Describe glycoproteins

Answer 5

• intrinsic proteins - embedded in the cell-surface membrane with attached carbohydrate chains
• play a role in cell adhesion (when cells join together to form tight junctions in certain tissues) + as receptors in cell signalling (the site where drugs and hormones can bind)

Question 6

Describe glycolipids

Answer 6

• lipids with attached carbohydrate chains

- cells markers or antigens and can be recognised by cells of the immune system as self or non-self

Question 7

Describe extrinsic

Answer 7

• present in one side of the bilayer
• normally have hydrophilic R-groups on outer surfaces + interact with the polar heads of the phospholipids or with intrinsic proteins

Question 8

Describe cholesterol

Answer 8

• regulates fluidity of the membrane - positioned between phospholipids in a membrane bilayer, with the hydrophilic end interacting with the heads and the hydrophobic end interacting with the tails, pulling them together
• thus adds stability without making them too rigid
• prevents membranes becoming too solid by stopping phospholipid molecules from grouping too closely + crystallising