5.1- Structure And Function Of Membranes

Question 1

What is compartmentalisation and why is it necessary?

Answer 1

The formation of separate membrane bound areas in a cell.

Vital because metabolism has different and incompatible reactions so separate parts allow specific conditions to be maintained.

Question 2

What is the plasma membrane and outline its structure.

Answer 2

Plasma membrane= the cell surface membrane that separates a cell from its external environment.
It is formed from a phospholipid bilayer:
- hydrophobic tails form a hydrophobic core inside the membrane.
- hydrophilic phosphate heads form the inner and outer surface of the membrane. Can interact with water.

Question 3

What is the fluid mosaic model and why is it given this name?

Answer 3

It is a model of the structure of a plasma membrane.
Called fluid mosaic because:
- phospholipids are free to move within the layer (are fluid), giving the membrane flexibility.
-proteins embedded in the bilayer vary in shape/size (like tiles of a mosaic).

Question 4

What are intrinsic proteins?

Answer 4

They are transmembrane proteins embedded through both layers of a membrane.
- contain amino acids with hydrophobic R groups on their external surfaces which interact with the hydrophobic core and keep them in place.

Question 5

What are channel and carrier proteins?

Answer 5

Intrinsic proteins involved in transport.

1. Channel proteins:
   - provide a hydrophilic channel that allows the passive movement (diffusion) of polar molecules and ions down a concentration gradient through membranes.
2. Carrier proteins:
   - have an important role in both passive and active transport into cells. This often involves the shape of proteins changing.

Question 6

What are glycoproteins?

Answer 6

(Intrinsic)
-proteins with a carbohydrate attached.
- they are embedded in the cell surface membrane.
- play a role in cell adhesion and as receptors for chemical signals.
The chemical binds to the receptor, it elicits a response which may causes a direct response inside the cell. = called cell signalling.
Eg. They act as receptors for neurotransmitters and hormones (insulin)

Question 7

What are glycolipids?

Answer 7

Lipids with attached carbohydrate chains.

• act as cell recognition sites (detect pathogens).
• are cell markers/ antigens. Recognised by immune system as self/ non-self.

Question 8

What are extrinsic proteins?

Answer 8

• present in one side of the bilayer.
• have hydrophilic R groups on outer surfaces and interact with polar heads of phospholipids/ intrinsic proteins.
• can be present in either layer, some move between layers.

Question 9

What is cholesterol?

Answer 9

• a lipid which regulates fluidity of membranes. Has a hydrophilic end and hydrophobic end.
• positioned between phospholipids in a bilayer.
• binds to the hydrophobic tails of lipids which causes them to pack closely= Therefore adds stability and some rigidity.
• stops membranes being solid by preventing phospholipid molecules getting close and crystallising.