Photosynthesis is the process plants use to make glucose from carbon dioxide and water, using light energy absorbed by chlorophyll. It is one of the most-examined topics in GCSE and A-Level Biology because it underpins ecosystems, food chains and the carbon cycle.
At GCSE (AQA, Edexcel, OCR) you need the word and symbol equations, the factors that affect the rate, and the required practical investigating light intensity. At A-Level you go further into the light-dependent and light-independent (Calvin cycle) reactions, including the role of NADP, ATP and RuBisCO.
Photosynthesis is the process by which plants, algae and some bacteria convert light energy into chemical energy stored in glucose. It takes place in chloroplasts, mainly in the palisade mesophyll cells of leaves.
Chlorophyll, the green pigment in the thylakoid membranes, absorbs red and blue light and reflects green, which is why leaves appear green. The absorbed light energy drives the splitting of water and the production of ATP and reduced NADP, which are then used to fix carbon dioxide into glucose.
At GCSE you must recall both equations. The word equation is carbon dioxide + water produces glucose + oxygen, with light and chlorophyll.
The balanced symbol equation is 6CO2 + 6H2O produces C6H12O6 + 6O2, with light energy absorbed by chlorophyll driving the reaction.
The light-dependent reactions take place in the thylakoid membranes of the chloroplast. Light energy is absorbed by photosystems, clusters of chlorophyll molecules embedded in the membrane, exciting electrons to a higher energy level.
In non-cyclic photophosphorylation, electrons from Photosystem II pass along an electron transport chain to Photosystem I, losing energy at each transfer. This energy pumps protons from the stroma into the thylakoid space, creating a proton gradient. Protons diffuse back through ATP synthase, driving ATP synthesis. This is chemiosmosis, the same principle as in respiration.
Water is split by light (photolysis) at Photosystem II, providing replacement electrons and releasing oxygen as a by-product. At Photosystem I, electrons reduce NADP to form reduced NADP, which carries hydrogen to the Calvin cycle.
The Calvin cycle takes place in the stroma of the chloroplast. It does not directly require light, but it depends on the ATP and reduced NADP produced by the light-dependent reactions.
Carbon dioxide combines with ribulose bisphosphate (RuBP), a five-carbon compound, in a reaction catalysed by the enzyme RuBisCO. The unstable six-carbon product immediately splits into two molecules of glycerate 3-phosphate (GP), a three-carbon compound.
GP is reduced to triose phosphate (TP) using ATP and reduced NADP from the light-dependent reactions. Some TP is used to synthesise glucose, while the rest is used to regenerate RuBP, allowing the cycle to continue. For one glucose molecule, the cycle turns six times, fixing six molecules of CO2.
The rate of photosynthesis is limited by whichever factor is in shortest supply: light intensity, carbon dioxide concentration, or temperature. This is the principle of limiting factors.
The GCSE required practical investigates the effect of light intensity on the rate of photosynthesis by varying the distance of a lamp from pondweed (Cabomba or Elodea) and counting the oxygen bubbles produced per minute. The inverse-square law applies: doubling the distance quarters the light intensity.
At A-Level, you may be asked to explain graphs showing the interaction of limiting factors. At high light intensity, increasing CO2 concentration may increase the rate further, but above a certain temperature the enzymes of the Calvin cycle denature and the rate falls sharply.
Photosynthesis and respiration are near-opposite processes. Photosynthesis builds glucose from CO2 and water using light energy, storing energy. Respiration breaks down glucose using oxygen, releasing energy as ATP.
Photosynthesis occurs in chloroplasts and needs light; respiration occurs in the cytoplasm and mitochondria and happens continuously, day and night. The net gas exchange of a plant depends on the balance between the two.
At GCSE you must recall the word and symbol equations, know the role of chlorophyll, and use the required practical (light intensity vs distance of a lamp from pondweed) to apply the inverse-square law. Common 6-mark questions ask you to explain how limiting factors interact.
At A-Level you need the two-stage mechanism in detail: the light-dependent reactions in the thylakoid membranes (photolysis of water, ATP via chemiosmosis, reduced NADP) and the Calvin cycle in the stroma (CO2 fixation onto RuBP by RuBisCO, reduction of GP to TP, regeneration of RuBP). Calculations on net primary productivity and questions on chromatography of plant pigments are common.
Writing the symbol equation with oxygen on the left.
Oxygen is a product, not a reactant. CO2 and water go in; glucose and oxygen come out.
Saying photosynthesis happens in the cytoplasm.
It happens in chloroplasts: the thylakoid membranes for light-dependent reactions and the stroma for the Calvin cycle.
Confusing the Calvin cycle with the Krebs cycle.
The Calvin cycle fixes CO2 in chloroplasts (photosynthesis); the Krebs cycle releases CO2 in mitochondria (respiration).
Forgetting that the Calvin cycle needs ATP and reduced NADP.
The Calvin cycle is light-independent but not energy-independent. It uses ATP and reduced NADP from the light-dependent reactions.
Saying the rate doubles when distance halves.
The inverse-square law means halving the distance quadruples the light intensity, not doubles it.
Claiming plants do not respire during the day.
Plants respire all the time. During bright light, photosynthesis exceeds respiration, but respiration never stops.
Original exam-style prompts. Answer out loud or in writing first, then open the answer to check your wording.
Word: carbon dioxide + water → glucose + oxygen (light, chlorophyll). Symbol: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
Light energy absorbed by chlorophyll in photosystem II excites electrons; water is split by photolysis providing replacement electrons and releasing oxygen; electrons pass along an electron transport chain, energy released pumps protons into the thylakoid space; protons diffuse back through ATP synthase making ATP (chemiosmosis); at photosystem I electrons reduce NADP to reduced NADP.
Thylakoid membranes provide a large surface area for chlorophyll and electron carriers; the stroma contains enzymes for the Calvin cycle; grana stack thylakoids to maximise light absorption; the double membrane controls entry and exit of materials.
Light intensity is inversely proportional to the square of distance; doubling the distance from 10 to 20 cm quarters the light intensity; less ATP and reduced NADP are made, slowing the Calvin cycle.
Another factor such as light or temperature becomes limiting; the enzymes of the Calvin cycle are working at their maximum rate; all active sites are saturated with substrate.
RuBisCO catalyses the fixation of CO2 onto ribulose bisphosphate (RuBP); the unstable six-carbon product splits into two GP molecules; this is the carbon fixation step that introduces carbon into the cycle.
The light intensity at which the rate of photosynthesis equals the rate of respiration; there is no net gas exchange.
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Q: Write the balanced symbol equation for photosynthesis.
A: 6CO2 + 6H2O → C6H12O6 + 6O2 (light energy, chlorophyll).
Q: Name three limiting factors of photosynthesis.
A: Light intensity, carbon dioxide concentration and temperature.
Q: Where in the chloroplast does the Calvin cycle take place?
A: In the stroma, the fluid-filled space surrounding the thylakoids.
Q: What does RuBisCO do?
A: It catalyses the fixation of CO2 onto ribulose bisphosphate (RuBP) in the Calvin cycle.
Light spreads out from a point source, so intensity falls with the square of distance (the inverse-square law). Doubling the distance quarters the light intensity reaching the leaf.
Light-dependent reactions occur in the thylakoid membranes, split water and produce ATP and NADPH. Light-independent reactions (Calvin cycle) occur in the stroma and use that ATP and NADPH to fix CO2 into glucose.
No, without light, the light-dependent reactions stop, so no ATP or NADPH is made and the Calvin cycle quickly runs out of substrate. Plants only respire at night.
Red (around 660 nm) and blue (around 450 nm), because these are absorbed most strongly by chlorophyll a and b. Green light is mostly reflected, which is why leaves look green.
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