Which of the following processes involves diffusion?

Analyze the question: We need the process that involves diffusion, which is the movement of particles from a region of higher concentration to a region of lower concentration, with no energy needed.

Concept: Diffusion is different from osmosis, which is the movement of water only, through a semi-permeable membrane. Active transport uses energy to move substances against a concentration gradient.

Checking each option:

• A. Opening and closing of stomatal pores: This is caused by changes in turgor pressure in the guard cells, which happen through osmosis. Water moves in or out, so the stomata open or close. This is mainly osmosis, not diffusion.
• B. Turgidity of herbaceous plants: Herbaceous plants have no woody tissue, so they stay upright because their cells are full of water. This is also due to osmosis.
• C. Absorption of water through the root hairs: Water moves from the soil, where it is dilute, into the root hair cell sap, where it is more concentrated. This is osmosis, which is a special case of diffusion for water, but the standard WASSCE answer treats it as osmosis.
• D. Absorption of digested food into the villi: Digested food such as glucose and amino acids is absorbed in the small intestine. When the concentration in the gut is higher than in the blood, the molecules move into the villi by diffusion. Active transport also helps with some nutrients, but diffusion is clearly involved.

Answer: D. Absorption of digested food into the villi.

Exam tip: When you see "water" and "membrane", think osmosis. When you see "digested food", "gas exchange" or "oxygen and carbon dioxide", think diffusion. Always check whether the question says diffusion, osmosis or active transport.