CCEA GCSE Biology 2.1 Osmosis and Plant Transport Exam Questions

About CCEA GCSE Biology 2.1 Osmosis and Plant Transport

Osmosis and Plant Transport opens Unit 2 (Body Systems, Genetics, Microorganisms and Health) of CCEA GCSE Biology. It builds on diffusion from Unit 1 and explains how water moves into and out of cells and through a plant. You need a precise definition of osmosis, the effects of different solutions on plant cells, and how transpiration moves water from roots to leaves. There are two prescribed practicals: osmosis using Visking tubing, and investigating water uptake and loss using a potometer and the washing line method. Examiners often test the wording of the osmosis definition, rate calculations from potometer readings and explanations of how environmental factors change transpiration.

What you need to know

• Osmosis Osmosis is the diffusion of water molecules from a dilute solution (high water concentration) to a more concentrated solution (lower water concentration) through a selectively permeable membrane. It is passive, so it needs no energy.
• Prescribed practical: Visking tubing Fill Visking tubing with a concentrated sugar solution, tie it and place it in water. Water enters by osmosis, so the tubing gains mass or the liquid rises up an attached capillary tube. Visking tubing acts as a model of a selectively permeable membrane.
• Plant cells in solutions In water, plant cells take in water by osmosis and become turgid; the cell wall stops them bursting. In a concentrated solution they lose water, become flaccid and eventually plasmolysed, with the membrane pulled away from the cell wall.
• Transpiration Transpiration is the evaporation of water from leaves, mainly through the stomata. It pulls water up the xylem from the roots. It is faster when it is warmer, windier, less humid or brighter (more stomata open).
• Prescribed practical: potometer and washing line A potometer measures water uptake: rate = distance moved by the air bubble / time. In the washing line method, leaves are hung up with different surfaces covered in petroleum jelly and their mass loss is recorded, showing most water is lost from the lower surface where most stomata are.
• Uses of water in plants Water is a raw material for photosynthesis, keeps cells turgid to support the plant, carries dissolved minerals and cools the leaves as it evaporates.

Common mistakes in 2.1 Osmosis and Plant Transport

• Defining osmosis without mentioning water or the selectively permeable membrane. Both are essential for the mark.
• Saying plant cells burst in pure water. The cell wall prevents bursting; the cells become turgid.
• Claiming a potometer measures transpiration directly. It measures water uptake, which is nearly but not exactly equal to water loss.
• Saying that more humidity increases transpiration. Humid air reduces the concentration gradient, so transpiration slows.
• Describing water moving from a high to a low concentration of solution. Water moves from a dilute solution to a concentrated solution.

Try three 2.1 Osmosis and Plant Transport exam-style questions