WJEC GCSE Biology 1.5 Plants and Photosynthesis Exam Questions

About WJEC GCSE Biology 1.5 Plants and Photosynthesis

Plants and photosynthesis is the plant biology topic of Unit 1. You need the word equation for photosynthesis, how temperature, carbon dioxide concentration and light intensity limit its rate, and how the plant uses the glucose it makes. You must label the internal structure of a leaf and explain how water is taken up by root hairs, carried in the xylem and lost by transpiration, while sucrose moves in the phloem. The topic also covers mineral ions and NPK fertilisers. It draws on osmosis and active transport from 1.1. There are two specified practicals, on photosynthesis and transpiration, so expect method, variable and graph questions.

What you need to know

• Photosynthesis carbon dioxide + water → glucose + oxygen, using light energy absorbed by chlorophyll in chloroplasts. Sodium hydroxide can be used to absorb carbon dioxide in experiments, and leaves can be tested for starch with iodine after decolourising in ethanol.
• Limiting factors Temperature, carbon dioxide concentration and light intensity can each limit the rate. On a graph, where the line levels off another factor has become limiting. Investigating factors affecting photosynthesis (e.g. counting bubbles from pondweed) is a specified practical.
• Uses of glucose Glucose is used in respiration, converted to starch for storage, used to make cellulose for cell walls, and converted into proteins (with nitrates) and oils.
• Leaf structure Label the cuticle, upper and lower epidermis, palisade layer, spongy layer, xylem, phloem, stomata and guard cells. The palisade layer has many chloroplasts; air spaces in the spongy layer allow gas diffusion; guard cells open and close stomata.
• Water and transport Root hairs increase surface area for water uptake by osmosis and mineral ion uptake by active transport. Xylem carries water and minerals up the plant, pulled by transpiration (evaporation of water from leaves through stomata). Phloem carries sucrose. Investigating factors affecting transpiration (e.g. with a potometer) is a specified practical.
• Minerals Nitrates are needed for proteins, phosphate and potassium for healthy growth. Deficiencies cause poor growth and symptoms such as yellow leaves; farmers add NPK fertilisers.

Common mistakes in 1.5 Plants and Photosynthesis

• Saying plants get their food from the soil. Plants make glucose by photosynthesis; the soil supplies water and mineral ions only.
• Writing that higher temperature always increases photosynthesis. Above the optimum, enzymes are denatured and the rate falls.
• Mixing up xylem and phloem. Xylem carries water and minerals upwards; phloem carries dissolved sugar (sucrose) both up and down the plant.
• Explaining transpiration as water being pumped up the stem. Water evaporates from the leaves and diffuses out through the stomata, which pulls a column of water up through the xylem.

Try three 1.5 Plants and Photosynthesis exam-style questions