Nutrition: photosynthesis, minerals and digestion
Nutrition
This domain tests photosynthesis, leaf structure, limiting factors, plant mineral requirements, human diet, digestion and absorption.
Core rules
- Photosynthesis: carbon dioxide + water → glucose + oxygen, using light energy and chlorophyll. Light intensity, CO2 concentration and temperature can limit the rate.
- Leaf adaptations: large surface area, thin, stomata for gas exchange, palisade mesophyll with chloroplasts, xylem and phloem in veins.
- Minerals: nitrate for proteins and growth; phosphate for roots and energy transfer; potassium for enzymes and water balance; magnesium for chlorophyll.
- A balanced human diet needs carbohydrates, proteins, lipids, vitamins, minerals, fibre and water. Deficiency examples: vitamin C and scurvy; iron and anaemia; calcium and weak bones.
- Digestion breaks large insoluble molecules into small soluble ones: amylase starch to maltose; protease proteins to amino acids; lipase fats to fatty acids and glycerol. Absorption occurs in the small intestine through villi.
Worked example
A graph shows photosynthesis rate rising with light intensity then levelling off. At the plateau, increasing light no longer increases rate, so light is not limiting; another factor such as CO2 concentration or temperature is limiting. If raising CO2 increases the plateau, CO2 was limiting at that point. Reasoning: the limiting factor is the one in shortest supply, not simply the variable on the x-axis.
Traps
- Writing the photosynthesis equation with oxygen as a reactant.
- Saying plants respire only at night.
- Confusing nitrate with magnesium deficiency.
- Naming the stomach as the main site of absorption.
- Forgetting bile emulsifies fats but does not digest them chemically.