Unit 5 - Plasma Membranes

Question 1

What components make up cell membranes?

Answer 1

Phospholipids arranged in a bilayer, cholesterol and protein molecules.

Question 2

What are extrinsic proteins?

Answer 2

Proteins attached to only the inner and outer surface of the membrane.

Question 3

What are intrinsic proteins?

Answer 3

Proteins which span the whole width of the membrane.

Question 4

What are the functions of intrinsic proteins?

Answer 4

As carrier/channel proteins (for transport), receptors and enzymes.

Question 5

How do channel proteins help transport substances across the cell membrane?

Answer 5

Channel proteins provide a hydrophilic channel allowing passive movement of polar molecules and ions down a concentration gradient across the membrane.

Question 6

How do carrier proteins help transport substances across the cell membrane?

Answer 6

Carrier proteins play a role in both passive and active transport, often involving the protein’s shape changing.

Question 7

How are channel proteins held in position?

Answer 7

Channel proteins are held in position by interactions between the hydrophobic core of the membrane and the hydrophobic R groups on the outside of the proteins.

Question 8

What are glycoproteins?

Answer 8

Glycoproteins are intrinsic proteins embedded in the membrane with attached carbohydrate chains.

Question 9

What are the functions of glycoproteins?

Answer 9

• Cell adhesion
• As receptors for chemical signals (cell signalling)

Question 10

Give an example of a glycoprotein involved in cell signalling.

Answer 10

One of:
- receptors for neurotransmitters
- receptors for peptide hormones (insulin and glucagon)
- receptors which bind to drugs

Question 11

What are glycolipids?

Answer 11

Lipids embedded in the cell membrane with attached carbohydrate chains.

Question 12

What is the function of glycolipids?

Answer 12

As cell markers/antigens so the immune system can recognise the cell as self or non-self.

Question 13

How does cholesterol add stability to the membrane?

Answer 13

The hydrophobic and hydrophilic ends interact with the phospholipids, pulling them closer.

Question 14

How does cholesterol increase membrane fluidity and prevent the membrane from becoming too rigid?

Answer 14

The cholesterol molecules are positioned between the phospholipids, preventing them from packing too closely and crystallising.

Question 15

Will small non-polar molecules diffuse across a plasma membrane?

Answer 15

Yes - they will diffuse rapidly.

Question 16

Will small polar molecules diffuse across a plasma membrane?

Answer 16

Yes - they will diffuse slowly (compared to non-polar molecules).

Question 17

Will charged particles diffuse across a plasma membrane?

Answer 17

No - they are unlikely to diffuse across.

Question 18

Why is membrane fluidity important?

Answer 18

• So substances can diffuse across
• So membranes can fuse, e.g. in exocytosis
• So cells can move and change shape

Question 19

At temperatures below freezing, how permeable is a plasma membrane?

Answer 19

The membrane typically has low permeability as the phospholipids are packed rigidly and have low energy. However, if ice crystals pierce the membrane, the permeability will increase significantly.

Question 20

As the temperature increases (before 45 degrees), how does membrane structure and permeability change?

Answer 20

The kinetic energy of the phospholipids increases and they move more, creating gaps in the bilayer. This increases the permeability of the membrane.

Question 21

At temperatures above 45 degrees, how does membrane structure and permeability increase?

Answer 21

The permeability will increase. This is because firstly, the phospholipid bilayer may start to melt, and secondly, channel/carrier proteins may denature.

Question 22

Why is water important for membrane stability?

Answer 22

Because it interacts with the hydrophilic heads of the phospholipid bilayer.

Question 23

What effect will organic solvents have on the membrane?

Answer 23

Organic solvents will disrupt/dissolve the membrane. Non-polar alcohols can enter the cell membrane but will disrupt the membrane, increasing permeability. Strong organic solvents will dissolve membranes.

Question 24

Why are pure or very strong alcohol solutions toxic?

Answer 24

They dissolve cell-surface membranes, destroying cells.