Diffusion, osmosis and active transport compared

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, as a result of their random movement. Examples:

• Oxygen diffuses from the alveoli into the blood; carbon dioxide diffuses the other way.
• Carbon dioxide diffuses into leaves through the stomata.
• Urea diffuses from cells into the blood plasma.

Osmosis is the diffusion of water across a partially permeable membrane, from a dilute solution (more water) to a more concentrated solution (less water). Cambridge IGCSE Extended describes this as movement from a region of higher water potential to lower water potential.

• Plant cells in pure water take in water and become turgid, which keeps plants upright.
• In a concentrated solution, they lose water and become flaccid; badly affected cells are plasmolysed.
• Animal cells have no cell wall, so in pure water they can burst.

Active transport moves substances from a lower to a higher concentration, against the gradient, using energy from respiration and carrier proteins in the membrane. Examples:

• Root hair cells absorb mineral ions from the soil, where the concentration is lower than in the root.
• Cells in the small intestine absorb glucose when its concentration in the gut is lower than in the blood.

• Concentration gradient: steeper is faster.
• Temperature: higher is faster, because particles move faster.
• Surface area: larger is faster. Exchange surfaces like alveoli, villi and root hairs are adapted with large surface areas, thin walls and good blood supplies.

Cut equal potato cylinders, record their masses, and place them in sugar or salt solutions of different concentrations. After a set time, blot them dry and reweigh. Calculate the percentage change in mass: potato gains mass in dilute solutions and loses it in concentrated ones. Where the line crosses zero on a graph estimates the concentration inside the potato cells.

1. Oxygen moves from the alveoli into the blood. Which process is this?
2. A potato cylinder loses mass in a concentrated sugar solution. Explain why.
3. Root hair cells absorb mineral ions from soil with a lower concentration of ions. Which process do they use?

Answers: diffusion; water leaves the potato cells by osmosis, from the dilute solution inside the cells to the more concentrated solution outside; active transport.