How plants make food, move water and control their growth
Photosynthesis has two stages. In the light reactions on the thylakoid membranes, chlorophyll absorbs light, water is split, oxygen is released, and ATP and NADPH are formed. In the Calvin cycle in the stroma, these are used to fix CO₂ into sugars, with RuBisCO as the key enzyme. C₄ plants such as maize and sugarcane avoid wasteful photorespiration through a special leaf anatomy. Cellular respiration then breaks glucose down through glycolysis, the Krebs cycle and the electron transport chain to make ATP.
Water enters roots by osmosis and moves through the apoplast and symplast pathways to the xylem. It rises mainly through transpiration pull, explained by the cohesion–tension theory, while minerals are absorbed partly by active transport. Food moves in the phloem from sources to sinks by the pressure flow hypothesis. Growth is controlled by plant growth regulators: auxins promote root initiation and apical dominance, gibberellins elongate stems, cytokinins promote cell division, ethylene ripens fruit and abscisic acid causes dormancy and stomatal closure. Photoperiodism and vernalisation control flowering.
Why it matters
Plant physiology explains crop yields, why farmers use fertilisers containing nitrogen, phosphorus and potassium, and how growers ripen fruit with ethylene or produce seedless fruits with hormones. Understanding photosynthesis matters for food security and climate, since plants absorb carbon dioxide. It also guides irrigation and greenhouse farming.
Key terms
- Calvin cycle
- The light-independent reactions that use ATP and NADPH to fix carbon dioxide into sugar.
- Transpiration pull
- The upward force on water in xylem created by evaporation of water from leaves.
- Plant growth regulator
- A chemical, such as auxin or ethylene, that controls plant growth and development.
- Photoperiodism
- The response of plants to the relative length of day and night, especially for flowering.