Photosynthesis uses light energy to synthesise organic molecules — Eduqas A-Level Biology

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in organic molecules. This process is fundamental to life on Earth, as it provides the primary source of organic matter and oxygen. In the WJEC A-Level Biology specification, you need to understand that photosynthesis occurs in chloroplasts, specifically in the thylakoid membranes and stroma. The overall equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, but this masks the complexity of the light-dependent and light-independent reactions.

The light-dependent reactions capture light energy using chlorophyll and other pigments, producing ATP and reduced NADP (NADPH). These products are then used in the Calvin cycle (light-independent reactions) to fix carbon dioxide into organic molecules like glucose. Understanding the roles of photosystems, electron transport chains, and chemiosmosis is crucial. This topic links to respiration, as both involve ATP synthesis via electron transport chains, and to ecology, as photosynthesis drives energy flow through ecosystems.

Mastering photosynthesis is essential for understanding how plants grow and produce biomass, which has implications for agriculture, climate change, and biofuel production. In exams, you must be able to describe the stages in detail, explain how factors like light intensity, CO₂ concentration, and temperature affect the rate, and interpret graphs of limiting factors. Practical skills, such as using a redox indicator like DCPIP to measure the rate of the light-dependent reaction, are also assessed.

• →Light-dependent reactions: Occur in thylakoid membranes; involve photosystem II (PSII) and photosystem I (PSI); produce ATP via photophosphorylation and reduce NADP⁺ to NADPH using electrons from photolysis of water.
• →Calvin cycle (light-independent reactions): Occurs in the stroma; uses ATP and NADPH to fix CO₂ into glycerate-3-phosphate (GP), which is reduced to triose phosphate (TP); some TP is used to regenerate RuBP, and some is converted to glucose or other organic molecules.
• →Limiting factors: Light intensity, CO₂ concentration, and temperature; the law of limiting factors states that the rate is limited by the factor in shortest supply; graphs show plateau when another factor becomes limiting.
• →Chloroplast structure: Thylakoids (grana) for light-dependent reactions; stroma for Calvin cycle; pigments (chlorophyll a, chlorophyll b, carotenoids) absorb different wavelengths; action spectrum shows rate vs wavelength, absorption spectrum shows absorbance vs wavelength.

• Distribution of chloroplasts for light trapping
• Chloroplasts as transducers converting light photons to chemical energy (ATP)
• Light harvesting and energy transfer to reaction centres
• Basic features of Photosystems I and II
• Cyclic and non-cyclic photophosphorylation
• Photolysis as a source of electrons for Photosystem II
• Reduction of NADP in the stroma
• Light-independent stage: Rubisco-catalysed uptake of CO2 by ribulose bisphosphate to form glycerate-3-phosphate
• Reduction of glycerate-3-phosphate to triose phosphate and regeneration of ribulose bisphosphate
• Production of carbohydrates, lipids, and amino acids from triose phosphate
• Concept of limiting factors in photosynthesis
• Role of inorganic nutrients (nitrogen and magnesium) in plant metabolism

• 💡Ensure you can distinguish between cyclic and non-cyclic photophosphorylation.
• 💡Be prepared to interpret graphs related to limiting factors and calculate rates of change.
• 💡Understand the role of Rubisco in the Calvin cycle.
• 💡Be able to link the light-dependent and light-independent stages through ATP and reduced NADP.
• 💡When describing the light-dependent reactions, use specific terms: 'non-cyclic photophosphorylation' (produces ATP and NADPH) and 'cyclic photophosphorylation' (produces only ATP). Mention that photolysis of water provides electrons to replace those lost from PSII and releases protons for the proton gradient.
• 💡In the Calvin cycle, remember that for every three CO₂ molecules fixed, six molecules of GP are produced, which are reduced to six TP. Five of these TP are used to regenerate three RuBP, and one TP is used to make a hexose sugar. Show this stoichiometry in your answers.
• 💡When interpreting graphs of limiting factors, always state which factor is limiting at each part of the curve. For example, at low light intensity, light is limiting; as light increases, CO₂ or temperature may become limiting. Use the phrase 'rate is limited by the factor in shortest supply'.

• Misconception: Photosynthesis only occurs in leaves. Correction: Photosynthesis occurs in any green part of the plant, including stems and unripe fruit, wherever chloroplasts are present.
• Misconception: The oxygen produced comes from carbon dioxide. Correction: Oxygen comes from the photolysis of water; the oxygen in CO₂ is incorporated into organic molecules.
• Misconception: The light-dependent reactions only happen in the light, and the Calvin cycle only happens in the dark. Correction: The Calvin cycle requires ATP and NADPH from the light-dependent reactions, so it stops in the dark when these run out; it is not a 'dark reaction' that occurs at night.