Photoionisation, Chemiosmosis, Photolysis & The Calvin Cycle (RuBP, GP, TP)
Photosynthesis in eukaryotic plants occurs across two distinct compartments of the chloroplast: the thylakoid membranes (stacked into grana) where the light-dependent reaction synthesises ATP and reduced NADP (NADPH), and the fluid-filled stroma where the light-independent Calvin cycle fixes CO₂ into organic hexose sugars. During the light-dependent stage, photons excite electrons in chlorophyll (photoionisation), driving an electron transport chain that pumps protons (H⁺) into the thylakoid lumen to power ATP synthase via chemiosmosis. In the stroma, the enzyme RuBisCO carboxylates 5-carbon ribulose bisphosphate (RuBP) into two molecules of 3-carbon glycerate 3-phosphate (GP), which are subsequently reduced to triose phosphate (TP) using the ATP and NADPH generated on the thylakoids. Examiners routinely test what happens to relative concentrations of RuBP, GP, and TP when light is switched off or CO₂ concentration falls.
- Chloroplast Ultrastructure & Adaptation
- Double-membrane organelle containing: 1. Thylakoid membranes stacked into grana that provide a large surface area for photosynthetic pigments (chlorophyll a, b, carotene) arranged in photosystems, electron carrier proteins, and ATP synthase enzymes. 2. A small internal thylakoid lumen volume so pumped H⁺ ions rapidly build up a steep proton gradient. 3. Fluid stroma containing RuBisCO and Calvin cycle enzymes surrounding the grana so ATP and reduced NADP have a short diffusion distance.
- Photoionisation & Electron Transport Chain
- Light energy is absorbed by chlorophyll molecules in Photosystem II (and Photosystem I), exciting a pair of electrons to a higher energy level so they leave the chlorophyll molecule (photoionisation; chlorophyll is oxidised). The excited electrons pass along a series of electron carrier proteins embedded in the thylakoid membrane in a sequence of redox reactions, losing energy at each step.
- Chemiosmotic Theory & Photophosphorylation
- Energy released by electrons passing down the thylakoid electron transport chain is used to actively pump protons (H⁺) from the stroma across the thylakoid membrane into the thylakoid space (lumen). This creates an electrochemical proton gradient (high [H⁺] inside thylakoid lumen). H⁺ ions diffuse back into the stroma only through channels associated with ATP synthase enzymes, causing the rotor to spin and catalyse ADP + Pᵢ → ATP (photophosphorylation). At the end of the chain, electrons and H⁺ combine with NADP⁺ to form reduced NADP (NADPH).
Photoionisation, Chemiosmosis, Photolysis & The Calvin Cycle (RuBP, GP, TP): Enzyme Collision, Active Site & Rate Curve Lab
Drag the temperature, pH, and substrate sliders to watch how kinetic collisions, active-site binding, and denaturation change the reaction rate in real time.
Key Concepts & Exam Definitions (6)
- Chloroplast Ultrastructure & Adaptation
- Double-membrane organelle containing: 1. Thylakoid membranes stacked into grana that provide a large surface area for photosynthetic pigments (chlorophyll a, b, carotene) arranged in photosystems, electron carrier proteins, and ATP synthase enzymes. 2. A small internal thylakoid lumen volume so pumped H⁺ ions rapidly build up a steep proton gradient. 3. Fluid stroma containing RuBisCO and Calvin cycle enzymes surrounding the grana so ATP and reduced NADP have a short diffusion distance.
- Photoionisation & Electron Transport Chain
- Light energy is absorbed by chlorophyll molecules in Photosystem II (and Photosystem I), exciting a pair of electrons to a higher energy level so they leave the chlorophyll molecule (photoionisation; chlorophyll is oxidised). The excited electrons pass along a series of electron carrier proteins embedded in the thylakoid membrane in a sequence of redox reactions, losing energy at each step.
- Chemiosmotic Theory & Photophosphorylation
- Energy released by electrons passing down the thylakoid electron transport chain is used to actively pump protons (H⁺) from the stroma across the thylakoid membrane into the thylakoid space (lumen). This creates an electrochemical proton gradient (high [H⁺] inside thylakoid lumen). H⁺ ions diffuse back into the stroma only through channels associated with ATP synthase enzymes, causing the rotor to spin and catalyse ADP + Pᵢ → ATP (photophosphorylation). At the end of the chain, electrons and H⁺ combine with NADP⁺ to form reduced NADP (NADPH).
- Photolysis of Water
- Light-driven splitting of water in Photosystem II: 2H₂O → 4H⁺ + 4e⁻ + O₂. The electrons (e⁻) replace those lost from oxidised chlorophyll during photoionisation; the protons (H⁺) contribute to the thylakoid proton gradient and pass through ATP synthase to reduce NADP⁺ in the stroma; oxygen (O₂) is released as a by-product or used in mitochondrial aerobic respiration.
- The Calvin Cycle (Light-Independent Reaction)
- Occurs in the stroma: 1. Carbon Fixation: CO₂ diffuses into the stroma and reacts with 5-carbon Ribulose Bisphosphate (RuBP), catalysed by the enzyme RuBisCO, forming an unstable 6C intermediate that immediately splits into two molecules of 3-carbon Glycerate 3-Phosphate (GP). 2. Reduction: Each GP (3C) is reduced to Triose Phosphate (TP, 3C) using energy from ATP hydrolysis and H⁺/electrons from reduced NADP (oxidising it back to NADP⁺). 3. Regeneration: 5 out of every 6 TP molecules are used to regenerate 3 molecules of RuBP (5C) using additional ATP; 1 out of 6 TP molecules goes toward synthesising hexose sugars (glucose, 6C requires 2 TP / 6 turns of the cycle), glycerol, or amino acids.
- Effects of Light & CO₂ Deprivation on RuBP, GP, and TP
- If light is switched off (dark): light-dependent reaction stops, so no ATP or reduced NADP are produced. GP cannot be reduced to TP, nor can TP regenerate RuBP; meanwhile, RuBP continues to fix CO₂ into GP. Therefore, [GP] rises and [RuBP] & [TP] fall. If [CO₂] drops to near 0%: RuBP has nothing to react with so cannot be converted into GP, while existing GP and TP continue to regenerate RuBP using light-dependent ATP/NADPH. Therefore, [RuBP] rises and [GP] & [TP] fall.
Quick-Fire Flashcards (6)
Click any card to check the answer, then mark it as Known to track your progress.
Define photoionisation and state where it occurs in the chloroplast.
Write the equation for the photolysis of water and state the fate of its three products.
How many carbon atoms are in RuBP, GP, and TP, and what enzyme catalyses the reaction between RuBP and CO₂?
Why does the Calvin cycle stop within seconds if a plant is placed in the dark, even though the cycle does not directly use light?
What happens to the relative concentrations of RuBP and GP if the light is suddenly turned off?
How many turns of the Calvin cycle are required to produce one net molecule of glucose (C₆H₁₂O₆)?
Exam-Style Questions & Mark Schemes (4)
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- Q1
Describe how ATP and reduced NADP are synthesised in the light-dependent reaction of photosynthesis. (6 marks)
Click “Check Mark Scheme” to compare your answer, 1. Light energy is absorbed by chlorophyll in the thylakoid membrane, exciting electrons to a higher energy level so they are released from chlorophyll (photoionisation) [1 mark]. 2. Excited electrons are taken up by an electron carrier and passed along an electron transport chain via a series of oxidation-reduction reactions [1 mark]. 3. Energy released by the electrons as they move down the chain is used to actively pump protons (H⁺) from the stroma across the thylakoid membrane into the thylakoid lumen [1 mark]. 4. Photolysis of water (2H₂O → 4H⁺ + 4e⁻ + O₂) replaces the electrons lost from chlorophyll and adds further H⁺ ions to the thylakoid lumen, maintaining a steep electrochemical proton gradient [1 mark]. 5. Protons diffuse down their gradient back into the stroma through ATP synthase channels (chemiosmosis), driving the synthesis of ATP from ADP and Pᵢ [1 mark]. 6. In the stroma, electrons from the end of the electron transport chain and H⁺ ions combine with NADP to form reduced NADP [1 mark].
- Q2
Describe how carbon dioxide in the air outside a leaf is converted into organic hexose sugars in the stroma during the light-independent reaction. (5 marks)
Click “Check Mark Scheme” to compare your answer, 1. Carbon dioxide diffuses through open stomata, through the leaf air spaces, and into the chloroplast stroma where it combines with 5-carbon ribulose bisphosphate (RuBP) [1 mark]. 2. This reaction is catalysed by the enzyme RuBisCO and produces two molecules of 3-carbon glycerate 3-phosphate (GP) [1 mark]. 3. GP is reduced into 3-carbon triose phosphate (TP) using hydrogen/electrons from reduced NADP (which returns to NADP) and energy from the hydrolysis of ATP (to ADP + Pᵢ) [1 mark]. 4. Two out of every twelve TP molecules formed (1 in 6) combine to synthesise a 6-carbon hexose sugar such as glucose [1 mark]. 5. The remaining five-sixths (10 out of 12) of the TP molecules are phosphorylated using ATP to regenerate RuBP so the cycle can continue [1 mark].
- Q3
An experiment measured the concentrations of RuBP and GP in Chlorella algae when the carbon dioxide concentration was suddenly reduced from 1.0% to 0.003% at constant light intensity. Explain the changes in the concentrations of GP and RuBP. (4 marks)
Click “Check Mark Scheme” to compare your answer, 1. The concentration of GP decreases rapidly [1 mark]. 2. Because less CO₂ is available to react with RuBP (less carbon fixation), less GP is formed while existing GP continues to be reduced to TP using ATP and reduced NADP from the ongoing light-dependent reaction [1 mark]. 3. The concentration of RuBP increases and then plateaus at a higher level [1 mark]. 4. RuBP continues to be regenerated from TP (using ATP from the light-dependent reaction), but is not being used up by reacting with CO₂ [1 mark].
- Q4
The weedkiller atrazine binds to electron carrier proteins in the thylakoid membrane, preventing electrons from passing along the electron transport chain. Explain how atrazine kills weeds. (4 marks)
Click “Check Mark Scheme” to compare your answer, 1. Because electrons cannot pass along the thylakoid electron transport chain, no energy is released to pump H⁺ ions into the thylakoid lumen, so no proton gradient is established [1 mark]. 2. Without chemiosmosis through ATP synthase and electron transfer to NADP, the light-dependent reaction cannot produce ATP or reduced NADP [1 mark]. 3. In the light-independent reaction (Calvin cycle), GP cannot be reduced to triose phosphate (TP) and RuBP cannot be regenerated [1 mark]. 4. Consequently, the plant cannot synthesise hexose sugars (glucose for respiration/cellulose) or amino acids, halting growth and ATP production so the plant dies [1 mark].
Common Exam Questions (4)
What is the difference between non-cyclic and cyclic photophosphorylation?
In non-cyclic photophosphorylation, electrons flow linearly from water through PSII and PSI to NADP⁺, producing ATP, reduced NADP, and O₂. In cyclic photophosphorylation, excited electrons from PSI are recycled back to the cytochrome proton pump complex instead of reducing NADP⁺, generating extra ATP (needed because the Calvin cycle uses 3 ATP for every 2 NADPH) without producing reduced NADP or O₂.
How is DCPIP used as a redox indicator to investigate the light-dependent reaction in isolated chloroplasts (the Hill reaction)?
Oxidised DCPIP is blue and acts as an artificial electron acceptor in place of NADP⁺. When illuminated chloroplasts undergo photoionisation and photolysis, excited electrons reduce blue DCPIP to colourless DCPIP-H₂; the rate of decolourisation (measured with a colorimeter) indicates the rate of the light-dependent reaction.
Why do commercial glasshouse growers enrich CO₂ to ~0.1% and raise temperature to ~25°C, and why would raising temperature to 40°C reduce yield?
Atmospheric CO₂ (~0.04%) and cool temperatures are the primary limiting factors of the enzyme-controlled Calvin cycle in bright light. Raising temperature above ~30-35°C increases transpiration water loss (causing stomatal closure), denatures photosynthetic enzymes, and increases photorespiration (where RuBisCO binds O₂ instead of CO₂) as well as respiratory glucose consumption.
Why do leaves contain multiple different photosynthetic pigments (chlorophyll a, chlorophyll b, xanthophylls, carotenes) as seen in TLC chromatography?
Each accessory pigment absorbs a slightly different wavelength of visible light (particularly across the blue-violet and red regions of the spectrum) and funnels that excitation energy to the primary reaction centre chlorophyll a, maximising the total light energy captured for photosynthesis.
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