OCR GCSE Biology B B3.2 How do producers get the substances they need? Exam Questions

About OCR GCSE Biology B B3.2 How do producers get the substances they need?

B3.2 explains how plants obtain carbon dioxide, water and mineral ions, and move substances around their bodies. You need to describe diffusion, osmosis and active transport, relate the structure of root hair cells, xylem, phloem and stomata to their functions, and explain how transpiration pulls water up the plant. Factors affecting the rate of transpiration and use of a potometer are commonly tested. This topic follows photosynthesis in B3.1 and connects to exchange surfaces in B5.1. Examiners often ask candidates to explain results from osmosis practicals or potometer data.

What you need to know

• Diffusion Net movement of particles from a higher to a lower concentration. Carbon dioxide diffuses into leaves through stomata and oxygen diffuses out.
• Osmosis Net movement of water across a partially permeable membrane from a dilute solution to a more concentrated solution. Root hair cells take in water from soil by osmosis.
• Active transport Movement of substances against a concentration gradient using energy from respiration. Root hair cells absorb mineral ions such as nitrate this way, so they contain many mitochondria.
• Xylem and phloem Xylem is made of dead, hollow cells strengthened with lignin and carries water and minerals upward. Phloem is living tissue that carries sugars both up and down the plant (translocation).
• Transpiration Water evaporates from leaf cells and diffuses out through stomata, pulling a continuous column of water up the xylem. Rate increases with higher temperature, light intensity and air movement and lower humidity.
• Stomata and guard cells Guard cells open stomata in the light to let carbon dioxide in, and close them when water is short to reduce water loss.
• Potometer A potometer measures water uptake as an estimate of transpiration rate by timing how far an air bubble moves along a capillary tube.

Common mistakes in B3.2 How do producers get the substances they need?

• Describing osmosis as movement of water from a high to low concentration solution. Water moves from a dilute solution (high water potential) to a more concentrated one.
• Saying minerals are absorbed by osmosis or diffusion. Mineral ions are absorbed mainly by active transport.
• Writing that phloem carries water. Xylem carries water; phloem carries dissolved sugars.
• Stating a potometer measures transpiration directly. It measures water uptake, some of which is used in photosynthesis.
• Saying stomata close at night to stop photosynthesis. They close in the dark because no carbon dioxide is needed for photosynthesis, which reduces water loss.

Try three B3.2 How do producers get the substances they need? exam-style questions