Cambridge IGCSE Biology 3 Movement into and out of Cells Exam Questions

About Cambridge IGCSE Biology 3 Movement into and out of Cells

Movement into and out of cells explains how substances cross membranes, which you will apply later in gas exchange, absorption in the small intestine, water uptake by roots and the kidney. Cambridge IGCSE Biology splits it into diffusion, osmosis and active transport. Core candidates describe each process and the factors affecting diffusion; Extended candidates explain osmosis using water potential, describe turgor pressure and plasmolysis, and link active transport to protein carriers. Practical questions are common, especially investigations using potato cylinders or visking tubing, where you calculate percentage change in mass and identify variables. Examiners expect precise language: down or against a concentration gradient, and partially permeable membrane.

What you need to know

• 3.1 Diffusion The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random movement. It is passive and needs no energy from respiration.
• 3.1 Factors affecting diffusion Rate increases with a larger surface area, a higher temperature, a steeper concentration gradient and a shorter distance. Investigate these with agar cubes containing indicator, or with dyes in water at different temperatures.
• 3.2 Osmosis The net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane. Water enters cells by osmosis through the cell membrane.
• 3.2 Plant cells and water Water entering a plant cell creates turgor pressure against the cell wall, keeping the cell turgid and supporting the plant. In a concentrated solution, water leaves, the cell becomes flaccid and the membrane pulls away from the wall (plasmolysis). Animal cells have no wall, so they can burst or shrink.
• 3.2 Percentage change in mass Percentage change = (final mass - initial mass) / initial mass x 100. The concentration where the line crosses 0% on a graph is where the tissue's water potential equals that of the solution.
• 3.3 Active transport The movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration, against a concentration gradient, using energy from respiration. Protein carriers in the membrane move the molecules, for example ion uptake by root hair cells and glucose absorption in the gut.

Common mistakes in 3 Movement into and out of Cells

• Saying water moves from high to low concentration in osmosis without saying of what. Use water potential, or say water moves from a dilute solution to a concentrated solution.
• Saying plant cells burst in pure water. The cell wall prevents bursting; the cell becomes turgid.
• Describing active transport as 'using energy' without saying the energy comes from respiration. Examiners credit 'energy from respiration'.
• Forgetting to calculate percentage change and comparing raw mass changes. Pieces start at different masses, so percentage change allows a fair comparison.
• Writing that diffusion stops at equilibrium. Particles keep moving randomly; there is just no net movement.

Try three 3 Movement into and out of Cells exam-style questions