IGCSE Biology: Movement Into and Out of Cells Practice Questions

Osmosis is the net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a concentrated solution), through a partially permeable membrane.

Mark schemeWater molecules (1). Higher to lower water potential, or dilute to concentrated (1). Through a partially permeable membrane (1).

Common mistakeWriting that solute or particles move. Osmosis is the movement of water only.

Exam tipAlways name the barrier as a partially permeable membrane. Leaving it out costs a guaranteed mark.

The salt solution has a lower water potential than the potato cells. Water moves out of the cells by osmosis across the partially permeable cell membrane. The cells lose water and become flaccid, so the potato is softer and its mass decreases.

Mark schemeSolution has lower water potential than cells (1). Water leaves cells by osmosis (1). Named membrane, partially permeable (1). Cells flaccid, so mass and firmness fall (1).

Common mistakeReversing the direction of water movement, or saying the salt enters the potato.

Exam tipLink the direction of water movement directly to the change in mass. State cause then effect.

Diffusion is passive and moves particles down a concentration gradient, needing no energy. Active transport moves substances against the concentration gradient and requires energy from respiration, using carrier proteins in the membrane.

Mark schemeDiffusion down gradient or passive (1). Active transport against gradient (1). Active transport needs energy from respiration, or uses carrier proteins (1).

Common mistakeSaying active transport needs energy but not stating that the energy comes from respiration.

Exam tipThe phrase energy from respiration is worth a mark on its own. Write it in full.