Cambridge IGCSE Biology 6 Plant Nutrition Exam Questions

About Cambridge IGCSE Biology 6 Plant Nutrition

Plant nutrition covers photosynthesis and the structure of the leaf in Cambridge IGCSE Biology. You need to define photosynthesis, give the word equation (and the balanced symbol equation for Extended), describe how chlorophyll transfers light energy into chemical energy, and know what the plant uses its carbohydrates for. Practical work is central: testing leaves for starch, using variegated leaves to show chlorophyll is needed, and using hydrogencarbonate indicator or pondweed to investigate rate. Extended candidates also explain limiting factors and the importance of nitrate and magnesium ions. Leaf structure links photosynthesis to gas exchange and transport in plants.

What you need to know

• 6.1 Definition and equation Photosynthesis is the process by which plants synthesise carbohydrates from raw materials using energy from light. Word equation: carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll). Balanced: 6CO2 + 6H2O → C6H12O6 + 6O2.
• 6.1 Chlorophyll and uses of glucose Chlorophyll is a green pigment in chloroplasts that absorbs light energy and transfers it into chemical energy in glucose. Glucose is used for respiration, converted to starch for storage, cellulose for cell walls, sucrose for transport in phloem, and used to make amino acids, fats and oils, and nectar.
• 6.1 Mineral ions Nitrate ions are needed to make amino acids; deficiency causes stunted growth. Magnesium ions are needed to make chlorophyll; deficiency causes yellowing of leaves (chlorosis).
• 6.1 Investigations Destarch a plant in the dark, then test a leaf for starch: boil in water, then in ethanol to remove chlorophyll, rinse and add iodine. Investigate rate by counting bubbles from pondweed at different light intensities, or using hydrogencarbonate indicator, which turns yellow at high carbon dioxide and purple at low carbon dioxide.
• 6.1 Limiting factors (Extended) Light intensity, carbon dioxide concentration and temperature can each limit the rate of photosynthesis. The factor in shortest supply limits the rate; on a graph, where the line levels off, another factor is limiting.
• 6.2 Leaf structure Waxy cuticle and upper epidermis, palisade mesophyll (most chloroplasts), spongy mesophyll with air spaces for gas diffusion, vascular bundle with xylem and phloem, lower epidermis with guard cells and stomata. Leaves are broad and thin for a large surface area and short diffusion distance.

Common mistakes in 6 Plant Nutrition

• Saying plants get their food from the soil. Plants make carbohydrates by photosynthesis; the soil supplies water and mineral ions.
• Saying chlorophyll makes energy or converts light into glucose directly. Chlorophyll absorbs light energy and transfers it to chemical energy.
• Writing the balanced equation with incorrect numbers, such as C6H12O6 + O2. Learn 6CO2 + 6H2O → C6H12O6 + 6O2.
• Forgetting the safety point in the starch test. Ethanol is flammable, so heat it in a water bath with no naked flame.
• Saying the rate keeps increasing as light increases. Above a certain light intensity, another factor such as carbon dioxide or temperature becomes limiting.

Try three 6 Plant Nutrition exam-style questions