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

Question 1

Simple diffusion is an example of ———-

Answer 1

Simple diffusion is an example of passive transport.

Question 2

What is simple diffusion?

Answer 2

It is the movement of molecules or ions from a region where they are in high concentration to a region of lower concentration, i.e. down a concentration gradient, until they are equally distributed.

Question 3

Describe the movement of ions and molecules in simple diffusion.

Answer 3

Ions and molecules are always in a state of random movement but if they are highly concentrated in one area there will be net movement away from that area, until there is a uniform distribution.

Question 4

What are the main factors that affect the rate of diffusion?

Answer 4

The rate of diffusion is affected by:

1. The concentration gradient
2. The thickness of the exchange surface or distance of travel over which diffusion takes place.
3. The surface area of the membrane

Question 5

How does the concentration gradient affect the rate of diffusion?

Answer 5

The greater the difference in the concentration of molecules in two areas, the more molecules diffuse in a given time.

Question 6

How does the thickness of the exchange surface or distance of travel over which diffusion takes place affect the rate of diffusion?

Answer 6

The thinner the membrane or the shorter the distance, the more molecules diffuse in a given time.

Question 7

How does the surface area of the membrane affect the rate of diffusion?

Answer 7

The larger the area, the more molecules have room to diffuse across in a given time.

Question 8

Describe the main factors affecting the rate of diffusion as an equation.

Answer 8

Question 9

The diffusion equation is ——————to the ———– but ———–

Answer 9

The diffusion equation is a good general guide to the rate of diffusion, but other factors may also affect the rate.

Question 10

What are the other factors that affect the rate of diffusion?

Answer 10

1. The size of the diffusing molecule
2. The nature of the diffusing molecules
3. Temperature
4. Chemical composition of the membrane
5. Number of pores in the membrane

Question 11

How does the size of the diffusing molecule affect the rate of diffusion?

Answer 11

Smaller molecules diffuse faster than larger molecules.

Question 12

How does the nature of the diffusing molecules affect the rate of diffusion?

Answer 12

Fat-soluble molecules diffuse faster than water- soluble molecules.

Non-polar molecules diffuse faster than polar ones.

Question 13

How does temperature affect the rate of diffusion?

Answer 13

Increased temperature increases the rate, as the molecules or ions have more kinetic energy.

Question 14

What is the effect of the concentration gradient on the rate of uptake of oxygen around a root?

Answer 14

As the external concentration of oxygen around a root increases, the concentration gradient across its cell-surface membrane becomes steeper.

This graph shows that the rate of oxygen uptake increases in direct proportion to the increase in the oxygen concentration and, therefore, to the increase in the concentration gradient.

Question 15

Diffusion (Key-Term)

Answer 15

The passive movement of a molecule or ion down a
concentration gradient, from a region
of high concentration to a region of low concentration.

Question 16

Passive (Key-Term)

Answer 16

Not requiring energy provided by the cell.

Question 17

Where can diffusion happen?

Answer 17

Diffusion can happen across membranes, such as the cell membrane.

It can also happen in solution, such as within the cytoplasm