1.2.3 Diffusion

1.2.3 Diffusion: Define diffusion, explain why it needs no energy, and describe how concentration gradient and membrane transport proteins affect it.

Particles of liquids and gases are constantly moving randomly, which gradually spreads them out.

Definition

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration.

Concentration gradient

The difference in concentration between two regions; particles move down this gradient until they are evenly spread.

Passive process

Diffusion does not need energy from respiration because it relies on the random movement of the particles themselves.

Equilibrium

Movement continues in both directions, but there is no net change once the concentration is the same everywhere.

Diffusion Across Membranes

Small molecules

Oxygen, carbon dioxide and water move across the phospholipid layers directly down their concentration gradients.

Larger molecules

Glucose can diffuse into cells only through transport proteins, but this is still passive if it moves down its gradient.

Gas exchange

Oxygen diffuses from the air into the blood in the lungs, and carbon dioxide diffuses the opposite way.

Cells

Respiring cells use up oxygen, which keeps its concentration low inside and so keeps the gradient steep.

A steeper concentration gradient means a greater difference between the two regions, so particles move across faster.

Higher temperature gives particles more kinetic energy, so they move faster and diffusion speeds up.

A larger surface area of membrane gives more space for particles to cross at the same time.

A shorter distance for particles to travel allows them to reach the other side more quickly.

1.2.3 – Key Points for the Exam

Diffusion: The net movement of particles from higher to lower concentration.
Concentration gradient: A difference in concentration between two regions.
Passive: Needing no energy from the cell.
Net movement: The overall direction in which particles are moving.

Common Mistakes

Saying that particles stop moving at equilibrium; they keep moving but there is no net change.
Stating that diffusion needs energy from respiration; it is a passive process.
Writing that particles move 'from low to high' because they want to even out; movement is random but the net result is high to low.
Mixing up diffusion with osmosis; osmosis refers only to water.