Nutrition, Respiration and Gas Exchange
Photosynthesis and plant nutrition
Photosynthesis uses light energy trapped by chlorophyll to convert carbon dioxide and water into glucose and oxygen: 6CO2 + 6H2O → C6H12O6 + 6O2. Light intensity, carbon dioxide concentration and temperature can limit the rate. Leaves are broad and thin, with palisade cells rich in chloroplasts, stomata for gas exchange, xylem for water and phloem for sugars. Mineral ions are needed: nitrate for proteins, phosphate for roots, magnesium for chlorophyll.
Human nutrition and digestion
A balanced diet supplies carbohydrates, proteins, lipids, vitamins, minerals, fibre and water. Enzymes break food down: amylase converts starch to maltose, protease converts proteins to amino acids, and lipase converts lipids to glycerol and fatty acids. Bile emulsifies fats and neutralises stomach acid. Villi in the small intestine increase surface area for absorption.
Respiration
Aerobic respiration: glucose + oxygen → carbon dioxide + water, releasing energy. Anaerobic respiration in muscles produces lactic acid; in yeast it produces ethanol and carbon dioxide. Respiration is a chemical process in cells, not breathing.
Gas exchange
Alveoli have thin walls, a large total surface area, a moist lining and a rich blood supply. In plants, stomata open and close through guard cells, and lenticels allow gas exchange in stems.
Worked example
During exercise, muscles respire faster, so they need more oxygen and produce more carbon dioxide. Breathing rate and depth increase to supply oxygen and remove carbon dioxide, and heart rate rises to transport these gases faster.
Traps that cost marks
Plants respire all the time, not only at night. Anaerobic respiration in yeast and muscle gives different products. Do not confuse breathing with respiration, and always balance the photosynthesis equation.