Photosynthesis in Higher Plants

The One Thing

Photosynthesis converts light energy into chemical energy: light reactions produce ATP and NADPH, which drive carbon fixation in the Calvin cycle to form carbohydrates from carbon dioxide and water. Its rate depends on pigment organisation, photosynthetic pathway, and environmental limiting factors.

Reactions, Processes and Experiments

Key Terms

• Photosynthesis: An anabolic process in which light energy is converted into chemical energy and carbon dioxide is reduced to form carbohydrates.
• Photosynthetic equation: A simplified representation is 6CO2 + 12H2O + light energy → C6H12O6 + 6O2 + 6H2O.
• Chloroplast: The organelle where photosynthesis occurs; its grana contain thylakoids for light reactions and its stroma contains enzymes for carbon fixation.
• Photosynthetic pigments: Light-absorbing molecules, mainly chlorophylls and carotenoids, located in the thylakoid membranes.
• Chlorophyll a: The primary photosynthetic pigment present in all oxygenic photosynthetic organisms; it directly participates in the reaction centre.
• Chlorophyll b: An accessory pigment that absorbs additional wavelengths of light and transfers the energy to chlorophyll a.
• Carotenoids: Accessory pigments, including carotenes and xanthophylls, that broaden light absorption and protect chlorophyll from photo-oxidation.
• Absorption spectrum: A graph showing the wavelengths of light absorbed by a pigment.
• Action spectrum: A graph showing the relative effectiveness of different wavelengths in driving photosynthesis.
• Photosystem: A functional unit of pigments and proteins in the thylakoid membrane that includes an antenna complex and a reaction centre.
• Photosystem II: The photosystem with reaction-centre chlorophyll P680; it absorbs light, splits water, releases oxygen, and supplies electrons to the electron transport chain.
• Photosystem I: The photosystem with reaction-centre chlorophyll P700; it helps reduce NADP+ to NADPH.
• Antenna complex: A group of pigment molecules that absorbs light energy and transfers it to the reaction-centre chlorophyll.
• Photolysis of water: Light-driven splitting of water associated with Photosystem II, producing electrons, protons, and molecular oxygen: 2H2O → 4H+ + 4e− + O2.
• Electron transport chain: A series of electron carriers that transfers electrons and helps create a proton gradient across the thylakoid membrane.
• Photophosphorylation: The formation of ATP using light energy during the light reactions.
• Non-cyclic photophosphorylation: A pathway involving both Photosystems II and I that produces ATP, NADPH, and oxygen; electrons do not return to the original chlorophyll.
• Cyclic photophosphorylation: A pathway involving Photosystem I in which electrons return to the same photosystem and produce ATP but not NADPH or oxygen.
• Chemiosmosis: ATP formation driven by the movement of protons through ATP synthase down their electrochemical gradient.
• Light reactions: Reactions occurring in the thylakoid membranes that convert light energy into ATP and NADPH and release oxygen.
• Carbon fixation: The incorporation of inorganic carbon dioxide into an organic molecule, usually catalysed by RuBisCO.
• Calvin cycle: The cyclic pathway in the chloroplast stroma that uses ATP and NADPH to convert carbon dioxide into carbohydrate.
• Carboxylation: The first step of the Calvin cycle in which CO2 combines with RuBP to form two molecules of 3-phosphoglycerate.
• RuBisCO: Ribulose-1,5-bisphosphate carboxylase-oxygenase, the enzyme that catalyses carbon fixation but can also bind oxygen.
• Reduction: The Calvin-cycle stage in which 3-phosphoglycerate is converted into triose phosphate using ATP and NADPH.
• Regeneration: The Calvin-cycle stage in which some triose phosphate is rearranged, using ATP, to regenerate RuBP.
• C3 pathway: The carbon fixation pathway in which the first stable product is the three-carbon compound 3-phosphoglycerate.
• Photorespiration: A light-dependent process initiated when RuBisCO acts as an oxygenase; it consumes oxygen and releases carbon dioxide without producing useful sugar.
• C4 pathway: A carbon-concentrating mechanism in which CO2 is first fixed into a four-carbon compound in mesophyll cells and later released in bundle sheath cells.
• PEP carboxylase: The enzyme that initially fixes CO2 in C4 plants; it has a high affinity for CO2 and is not inhibited by oxygen.
• Kranz anatomy: The specialised arrangement in C4 leaves in which vascular bundles are surrounded by prominent bundle sheath cells containing chloroplasts.
• CAM pathway: A water-saving pathway in which plants take in CO2 mainly at night, store it as organic acids, and use it for photosynthesis during the day.
• Limiting factor: A factor present in insufficient amount that restricts the rate of photosynthesis even when other factors are favourable.
• Compensation point: The light intensity or carbon dioxide concentration at which the rate of photosynthesis equals the rate of respiration.
• Gross primary productivity: The total rate at which plants capture energy and produce organic matter through photosynthesis.
• Net primary productivity: The organic matter remaining after plant respiration; NPP = GPP − respiration.

Easily Confused

• Absorption spectrum vs action spectrum: An absorption spectrum shows wavelengths absorbed by a pigment, whereas an action spectrum shows the relative effectiveness of wavelengths in driving photosynthesis.
• Chlorophyll a vs chlorophyll b: Chlorophyll a directly participates in the reaction centre; chlorophyll b is an accessory pigment that transfers absorbed energy to chlorophyll a.
• Photosystem II vs Photosystem I: Photosystem II contains P680 and is associated with water splitting and oxygen release; Photosystem I contains P700 and helps reduce NADP+ to NADPH.
• Non-cyclic vs cyclic photophosphorylation: Non-cyclic photophosphorylation uses Photosystems II and I and produces ATP, NADPH, and oxygen; cyclic photophosphorylation uses Photosystem I and produces ATP only.
• Light reactions vs Calvin cycle: Light reactions occur in thylakoid membranes and produce ATP and NADPH; the Calvin cycle occurs in the stroma and uses them for carbon fixation.
• C3 vs C4 pathways: The first stable C3 product is 3-phosphoglycerate, whereas the first stable C4 product is a four-carbon compound such as oxaloacetic acid.
• C4 vs CAM pathways: C4 plants separate initial fixation and the Calvin cycle spatially between mesophyll and bundle sheath cells; CAM plants separate them mainly by time, fixing CO2 at night and using it during the day.
• RuBisCO carboxylase vs oxygenase activity: Carboxylase activity fixes CO2 and supports sugar formation; oxygenase activity initiates photorespiration and causes carbon and energy loss.
• Gross primary productivity vs net primary productivity: GPP is total photosynthetic production, whereas NPP is the remainder after respiration; NPP = GPP − respiration.
• Oxygen source in photosynthesis vs carbon source: Released oxygen comes from water, while carbon in carbohydrate is derived from carbon dioxide.

What Gets Asked

• Questions may require the overall photosynthetic equation and identification of the oxidation-reduction changes. The mark-losing error is stating that released oxygen comes from carbon dioxide rather than water.
• Questions may ask for the sequence and products of non-cyclic electron flow. The required sequence is water → Photosystem II → electron carriers → Photosystem I → NADP+, producing ATP, NADPH, and O2.
• Questions may compare cyclic and non-cyclic photophosphorylation. The key distinction is that cyclic photophosphorylation uses only Photosystem I and produces ATP without NADPH or oxygen.
• Questions may require the three stages of the Calvin cycle and ATP/NADPH requirements. For three CO2, the values are 3 RuBP, one net triose phosphate, 9 ATP, and 6 NADPH; for one glucose, 18 ATP and 12 NADPH are required.
• Questions may compare C3, C4, and CAM photosynthesis. The specific distinctions concern the first stable product, the spatial separation in C4 plants, and the nocturnal carbon fixation of CAM plants.
• Questions may present changes in light, carbon dioxide, temperature, or water availability. The relevant error is ignoring limiting factors, saturation points, stomatal closure, or the higher temperature optimum generally found in C4 plants.