Diffusion: Some of the substances transported in and out of cells by diffusion are oxygen and carbon dioxide in gas exchange, and of the waste product urea from cells into the blood plasma for excretion in the kidney.

The idea

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. It needs no energy from the cell. Oxygen enters cells and carbon dioxide leaves them because each gas moves down its own gradient. In gas exchange, oxygen diffuses from the air in the alveoli into the blood, while carbon dioxide diffuses from the blood into the alveoli to be breathed out. Urea is made in the liver from the breakdown of excess amino acids. It is a waste product, so it diffuses from liver cells into the blood plasma. The blood carries urea to the kidneys, where it is removed from the plasma and excreted in urine.

What earns the marks

• Diffusion is the net movement of particles down a concentration gradient, from higher to lower concentration.
• Oxygen diffuses into cells and carbon dioxide diffuses out of cells during gas exchange because each gas moves down its own concentration gradient.
• In the lungs, oxygen diffuses from the alveoli into the blood, and carbon dioxide diffuses from the blood into the alveoli.
• Urea is a waste product made in the liver from excess amino acids and diffuses from cells into the blood plasma.
• The blood plasma transports urea to the kidney, where it is removed from the blood and excreted in urine.
• Diffusion is a passive process and does not require energy from respiration.

Common mistakes and how to put them right

• Thinking that oxygen and carbon dioxide swap places in a single exchange.

  Put it right by stating that each gas diffuses down its own gradient, oxygen inwards and carbon dioxide outwards.

• Believing that diffusion needs energy from the cell.

  Put it right by explaining that diffusion is passive and relies on the random motion of particles.

• Confusing urea with faeces or saying urea is excreted by the lungs.

  Put it right by stating that urea is made in the liver, carried in blood plasma and excreted by the kidneys in urine.

Exam tips

• Name the substance, its direction of movement and the concentration gradient in each example you give.
• Use the phrase 'down a concentration gradient' rather than 'from high to low' alone, so the direction is clear.
• Link each example to a named exchange surface or organ, such as alveoli for gas exchange or kidneys for urea excretion.