57 WJEC Biology AS Level - Marianne Izen - 2nd Edition (1.3 Cell Membranes And Transport)

Question 1

Why ions and molecules such as glucose cannot pass through the cell membrane?

Answer 1

lons and molecules such as glucose cannot pass through the cell membrane because they are relatively insoluble in the phospholipid bilayer.

Question 2

What allows movement of ions and molecules such as glucose through the cell membrane ?

Answer 2

Facilitated diffusion is a special form of diffusion that allows movement of these molecules across a membrane.

Question 3

What does the word “facilitated” mean?

Answer 3

‘Facilitated’ means ‘made easier

Question 4

Facilitated diffusion is a ————- and happens ————

Answer 4

Facilitated diffusion is a passive process and happens down a concentration gradient.

Question 5

Where does facilitated diffusion occur?

Answer 5

It occurs at specific sites on a membrane where there are transport protein molecules.

Question 6

What limits the rate of facilitated diffusion?

Answer 6

The number and availability of transport protein molecules limit the rate of facilitated diffusion.

Question 7

What are the types of transport proteins?

Answer 7

The transport proteins are of two types:

1. Channel proteins
2. Carrier proteins

Question 8

What are channel proteins?

Answer 8

Channel proteins are molecules with pores lined with polar groups.

Question 9

How do channel proteins help with facilitated diffusion?

Answer 9

The channels open and close according to the needs of the cell.

As the channels are hydrophilic, ions, being water-soluble, can pass through.

Question 10

What do carrier proteins do?

Answer 10

Carrier proteins allow diffusion of larger polar molecules, such as sugars and amino acids, across the membrane.

Question 11

How does the carrier protein perform facilitated diffusion?

Answer 11

A molecule attaches to its binding site, on the carrier protein.

The carrier protein changes shape and releases the molecule on the other side of the membrane before changing back to its original shape.

Question 12

What does this graph show?

Answer 12

The graph shows that amino acid uptake into cells is by facilitated diffusion.

The rate of uptake is directly proportional to amino acid concentration between points A and B but reaches a plateau at C.

Beyond C there is no increase in rate of uptake because all the carriers are already occupied and their number has become limiting.

Question 13

Draw a diagram to explain facilitated diffusion by channel proteins and carrier proteins.

Answer 13

Question 14

Compare channel proteins and carrier proteins

Answer 14

Question 15

Facilitated diffusion (Key-Term)

Answer 15

The passive transfer of molecules or tons down a concentration gradient, across a membrane, by protein carrier molecules in the membrane.

Question 16

Why diffusion and facilitated diffusion can only go down a concentration gradient?

Answer 16

The cell does not put energy into passive processes.

Hence diffusion and facilitated diffusion can only go in the direction that is energetically possible, i.e. down a concentration gradient.

The molecules or ions move because they have kinetic energy.

Question 17

Can channel proteins help in the movement of all ions always?

Answer 17

No.

Many channel proteins are specific for one type of ion, such as the calcium channels at the nerve synapse.

Question 18

What do channel proteins and carrier proteins do to the rate of diffusion?

Answer 18

Carrier proteins and channel proteins increase the rate of diffusion down a concentration gradient.

This does not require energy in the form of ATP from respiration.