OxfordAQA International A Level Biology 3.1.4 Transport Into and Out of Cells Exam Questions

About OxfordAQA International A Level Biology 3.1.4 Transport Into and Out of Cells

Transport into and out of cells in Unit 1 covers how substances cross the plasma membrane. You need to describe the fluid-mosaic model, including the role of cholesterol and microvilli, and explain simple diffusion, facilitated diffusion through carrier and channel proteins, osmosis in terms of water potential, and active transport using carrier proteins and ATP. These ideas are used again in gas exchange, absorption in the ileum, cholera, nerve impulses and plant transport. Required practical 2, on the effect of solute concentration on water uptake or loss by plant tissue, belongs here. Examiners often ask you to interpret percentage change in mass graphs from that practical.

What you need to know

• Fluid-mosaic model The membrane is a phospholipid bilayer with proteins scattered through it (the mosaic) and molecules that can move (fluid). Glycoproteins and glycolipids carry carbohydrate chains. Cholesterol restricts the movement of other molecules in the membrane, and microvilli increase surface area for exchange.
• Simple diffusion Net movement of molecules from a higher to a lower concentration, down a concentration gradient, without ATP. The rate depends on surface area, the difference in concentration and the thickness of the exchange surface. Small non-polar molecules pass through the bilayer; the bilayer restricts larger and polar molecules.
• Facilitated diffusion Passive movement of molecules down a concentration gradient through channel proteins (for ions) or carrier proteins that change shape. The rate is limited by the number of proteins in the membrane.
• Osmosis Net movement of water from a higher water potential to a lower water potential across a partially permeable membrane. Pure water has a water potential of 0 kPa, and adding solute makes it more negative.
• Active transport Movement of substances against a concentration gradient using carrier proteins and energy from ATP. ATP is formed from ADP and inorganic phosphate and is the immediate source of energy for cells.
• Required practical 2 Plant tissue samples are placed in a range of solute concentrations and the percentage change in mass is calculated. The concentration where the line crosses zero change gives the solute concentration with the same water potential as the tissue.

Common mistakes in 3.1.4 Transport Into and Out of Cells

• Describing osmosis using water concentration. Use water potential and state that water moves from higher (less negative) to lower (more negative) water potential.
• Saying facilitated diffusion uses ATP. It is passive; only active transport uses ATP.
• Writing that cholesterol makes the membrane more fluid. In this specification cholesterol restricts the movement of other molecules, making the membrane less fluid.
• Using change in mass instead of percentage change in mass in Required practical 2. Samples start at different masses, so percentage change allows a fair comparison.

Try three 3.1.4 Transport Into and Out of Cells exam-style questions