Define diffusion and explain its importance in living organisms and the factors affecting its rate

What this dot point is asking

SEAB wants a precise definition of diffusion, an explanation of why it is essential for moving substances such as oxygen, carbon dioxide and dissolved food into and out of cells, and the factors that affect how fast it happens. You should be able to use the idea of a concentration gradient to predict the direction of movement.

The answer

What diffusion is

Diffusion is the net movement of particles (molecules or ions) from a region of higher concentration to a region of lower concentration, down a concentration gradient, until they are evenly spread. It happens because particles are always moving randomly; over time, more move away from the crowded region than back into it, so there is a net movement outward.

Diffusion is a passive process: it needs no energy from the cell, because the particles' own random movement drives it.

Why diffusion matters in organisms

Many vital substances enter and leave cells by diffusion:

• Oxygen diffuses from the air in the lungs (or water, in fish) into the blood, and from the blood into respiring cells.
• Carbon dioxide diffuses the opposite way, out of respiring cells and out of the lungs.
• Digested food such as glucose diffuses from the small intestine into the blood.

Without diffusion, cells could not get the materials they need or remove their waste.

Factors that affect the rate of diffusion

The rate of diffusion increases when:

• The concentration gradient is steeper. A bigger difference between the two regions means faster net movement.
• The surface area is larger. More area means more room for particles to cross at once.
• The diffusion distance is shorter. A thinner barrier is crossed more quickly.
• The temperature is higher. Particles move faster, so they spread more quickly.

How organisms use these factors

Exchange surfaces are adapted to make diffusion fast: alveoli and the small intestine have a very large surface area, thin walls (a short distance), and a good blood supply to keep the gradient steep.

Examples in context

Example 1. The small intestine. Glucose and amino acids from digested food are in high concentration in the gut and lower concentration in the blood, so they diffuse into the blood across the thin, folded intestinal wall, which has a huge surface area.

Example 2. A respiring muscle cell. A working muscle uses oxygen, lowering its concentration inside the cell. Oxygen therefore diffuses in from the blood, where it is more concentrated, supplying the respiration that powers the muscle.

Try this

Q1. Define diffusion. [2 marks]

• Cue. The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient.

Q2. State two factors that increase the rate of diffusion. [2 marks]

• Cue. Any two of: a steeper concentration gradient, a larger surface area, a shorter diffusion distance, a higher temperature.

Q3. Explain why a respiring cell takes in oxygen by diffusion. [2 marks]

• Cue. Respiration uses up oxygen inside the cell, lowering its concentration, so oxygen diffuses in from the blood where it is more concentrated, down the gradient.