Diffusion and factors affecting its rate

Particles do not all travel in the same direction. Some move towards the higher-concentration region, but more move away from it because there are more particles there to begin with. The overall result is called net movement. The difference in concentration between two regions is the concentration gradient, and diffusion produces net movement down this gradient.

Eventually, in a closed system, the particles become evenly spread. They still move randomly, but equal numbers move in opposite directions, so there is no net movement.

Diffusion across cell membranes

The cell membrane separates the inside of a cell from its surroundings. Some substances can diffuse across it, allowing materials to enter or leave the cell. The membrane is partially permeable: it allows some substances through but not others.

For a substance that can cross the membrane, the concentration gradient determines the direction of net movement. If its concentration is higher outside the cell, it diffuses into the cell. If its concentration is higher inside, it diffuses out. A change in the gradient can therefore reverse the direction of net movement.

Diffusion is a passive process, so the cell does not have to supply energy for it. The moving particles already have kinetic energy—the energy of movement.

Oxygen, carbon dioxide and urea

Cells need oxygen for aerobic respiration. When oxygen concentration is higher outside a cell than inside, oxygen diffuses into it. Using oxygen in respiration helps maintain this concentration difference. Carbon dioxide produced by respiration can build up inside the cell and diffuse out into surroundings with a lower carbon dioxide concentration.

Diffusion also allows gas exchange in the lungs. Oxygen diffuses from the air in the alveoli—the small air sacs in the lungs—into the blood. Carbon dioxide diffuses in the opposite direction, from the blood into the alveoli, and is then breathed out. Each gas follows its own concentration gradient: oxygen and carbon dioxide do not have to move in the same direction.

Urea is a waste product made in liver cells when excess amino acids are broken down. It diffuses from these cells into the blood plasma, the liquid part of the blood. The blood carries it to the kidneys for excretion. Diffusion gets urea out of the cells; transport in the bloodstream takes it to the organs that remove it from the body.

What makes diffusion faster?

The rate of diffusion describes how quickly particles spread out or cross a surface. To understand the effect of one factor, compare situations in which the other factors remain the same.

A steeper concentration gradient does not make particles deliberately move towards the less concentrated region: their movement remains random. It increases the overall imbalance between movement in the two directions.

Temperature changes how quickly the particles move. Membrane surface area has a different effect: a larger area provides more space for particles to cross at the same time. Both can increase diffusion rate, but for different reasons.

• Diffusion is the spreading out of particles in a solution or gas, with net movement from higher to lower concentration.
• Particles move randomly in both directions.
• Diffusion is passive: no energy is supplied by the cell.
• Substances that can cross a cell membrane diffuse into or out of cells according to their concentration gradients.

Substances to recall

• Oxygen: alveoli → blood; surroundings → respiring cells.
• Carbon dioxide: respiring cells → surroundings; blood → alveoli to be breathed out.
• Urea: liver cells → blood plasma → kidneys for excretion. The first step is diffusion; the blood then transports it.

Faster diffusion

• Steeper concentration gradient: greater net movement from higher to lower concentration.
• Higher temperature: more kinetic energy; faster particle movement.
• Greater membrane surface area: more particles can cross in a given time.

These comparisons assume other factors remain the same.