Transport across cell membranes

Question 1

What is a cell surface membrane?

Answer 1

The plasma membrane that surrounds cells and forms a boundary between the cytoplasm and the environment.

Question 2

What does the cell surface membrane allow for?

Answer 2

It allows different conditions to be established inside and outside of the cell.

Question 3

What does the cell surface membrane control?

Answer 3

It controls the movement of substances in and out of cells.

Question 4

What makes up a cell membrane?

Answer 4

A phospholipid bilayer

Question 5

Why is the phospholipid bilayer an important component of cell membranes?

Answer 5

Because: - the hydrophilic heads of both phospholipid layers point to the outside of the cell-surface membrane attracted by water on both sides and - the hydrophobic tails of both layers point into the centre of the cell membrane, repelled by water on both sides. Therefore, the membrane has a non-polar core which prevents the passage of polar molecules across it, e.g. water, and allows relatively free diffusion of non-polar molecules.

Question 6

How do lipid-soluble material move through the membrane?

Answer 6

Via the phospholipid portion.

Question 7

Name 3 functions of phospholipids in the membrane

Answer 7

• Allow lipid-soluble substances to enter and leave the cell. - Prevent water-soluble substance entering and leaving the cell. - Make the membrane flexible and self-sealing.

Question 8

How are proteins embedded in the cell-surface membrane?

Answer 8

Some proteins occur only in the surface of the bilayer whereas others completely span the phospholipid bilayer from one side to the other.

Question 9

What is the function of proteins which only occur in the surface of the phospholipid bilayer?

Answer 9

The act either to give mechanical support to the membrane or, in conjunction with glycolipids, as cell receptors for molecules e.g. hormones.

Question 10

Give 2 examples of proteins that completely span the phospholipid bilayer.

Answer 10

Carrier proteins, channel proteins.

Question 11

What do carrier proteins do?

Answer 11

They bind to ions or molecules like glucose or amino acids, then change shape in order to move these molecules across the membrane.

Question 12

What do protein channels / channel proteins do?

Answer 12

They form water-filled tubes to allow water-soluble ions to diffuse across the membrane.

Question 13

Name 6 functions of the proteins in membranes.

Answer 13

• Provide structural support. - Act as channels transporting water-soluble substances across the membrane. - Allow active transport across the membrane through carrier proteins. - Form cell-surface receptors across the membrane for identifying cells. - Act as receptors e.g. for hormones. - Help cells stick/adhere together.

Question 14

Where do cholesterol molecules occur?

Answer 14

Within the phospholipid bilayer.

Question 15

What do cholesterol molecules add to the membrane?

Answer 15

Strength.

Question 16

Are cholesterol molecules very hydrophobic or hydrophilic? What does this mean for the cell?

Answer 16

Hydrophobic. Therefore plays an important role in preventing loss of water and dissolved ions from the cell

Question 17

What do cholesterol molecules pull together? What does this achieve?

Answer 17

They pull together the fatty acid / hydrophobic tails of the phospholipid molecules. This limits their movement and that of other molecules but doesn’t make the membrane too rigid.

Question 18

Name the 3 overarching functions of cholesterol.

Answer 18

• Reduce lateral movement of other molecules including phospholipids. - Make the membranes less fluid at high temperatures. - Prevent leakage of water and dissolved ions from the cell.

Question 19

What are glycolipids made up of?

Answer 19

A carbohydrate covalently bonded with a lipid

Question 20

What does the carbohydrate portion of a glycolipid act as?

Answer 20

It acts as a cell-surface receptor for specific chemicals.

Question 21

Name 3 functions of glycolipids in the membrane.

Answer 21

• Acts as recognition sites. - Helps maintain the stability of the membrane. - Helps cells to attach to one another and therefore form tissues.

Question 22

Where are glycoproteins found?

Answer 22

On the outer surface of the cell membrane.

Question 23

What are glycoproteins made up of?

Answer 23

Carbohydrate chains attached to many extrinsic proteins.

Question 24

What are the functions of glycoproteins in the membrane?

Answer 24

• Act as recognition sites (allows cells to recognise one another - lymphocytes can recognise an organism’s own cells). - Help cells attach to one another and form tissues. - Act as cell-surface receptors, more specifically for hormones and neurotransmitters

Question 25

What controls the movement of substances in and out of cells?

Answer 25

The cell surface membrane.

Question 26

Why does a molecule not being soluble in lipids affect its movement across a membrane?

Answer 26

Because if it is not soluble in lipids, it can’t pass through the phospholipid bilayer.

Question 27

Why does a molecule being large affect its movement across a membrane?

Answer 27

Because it is too large to pass through protein channels in the membrane.

Question 28

Why does a molecule having the same charge as the protein channel affect its movement across the membrane?

Answer 28

Because like charges repel one another.

Question 29

Why do polar molecules, e.g. water, have difficulty passing through the membrane?

Answer 29

Because the membrane has a non-polar core (from the hydrophobic tails) which prevents the passage of polar molecules across it.

Question 30

Why is the model of a cell surface membrane described as fluid?

Answer 30

Because the individual phospholipid molecules can move relative to one another. This gives the molecule a flexible structure that is constantly changing in shape.

Question 31

Why is the model of a cell surface membrane described as a mosaic?

Answer 31

Because the proteins embedded in the bilayer vary in size, shape and pattern.