OCR Specification focus:
‘Describe how light energy produces ATP and reduced NADP via electron carriers, and cyclic and non-cyclic photophosphorylation; include water’s role.’
The light-dependent reactions of photosynthesis convert radiant light energy into chemical energy, producing ATP and reduced NADP. These reactions occur on the thylakoid membranes within chloroplasts and drive the later light-independent stage.
The Site and Role of the Light-Dependent Reactions
The thylakoid membranes of chloroplasts contain photosystems, electron transport chains, and ATP synthase complexes, all essential for capturing and converting light energy. The process relies on chlorophyll and other pigments to absorb photons, exciting electrons to higher energy levels. This excitation sets in motion a flow of electrons through a series of redox reactions, resulting in the formation of ATP (adenosine triphosphate) and reduced NADP (nicotinamide adenine dinucleotide phosphate).
These two products provide the energy and reducing power required for the Calvin cycle in the stroma, where carbohydrates are synthesised from carbon dioxide.
Photosystems and Light Absorption
Two main photosystems participate in the light-dependent stage: Photosystem II (PSII) and Photosystem I (PSI). Each photosystem is a large protein-pigment complex that absorbs light at specific wavelengths.
PSII absorbs light most effectively at around 680 nm (known as P680).
PSI absorbs light most effectively at around 700 nm (known as P700).
Both photosystems contain light-harvesting complexes made up of accessory pigments such as carotenoids and chlorophyll b. These pigments absorb a range of wavelengths and funnel the energy efficiently to the reaction centre chlorophyll a molecule.
Photosystem: A protein-pigment complex in the thylakoid membrane that absorbs light energy to excite electrons for photosynthesis.
The Role of Water – Photolysis
At Photosystem II, absorbed light energy excites electrons which are transferred to the primary electron acceptor, leaving P680 oxidised. To replace these lost electrons, water molecules are split in a process called photolysis, catalysed by an enzyme complex associated with PSII.
EQUATION
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Photolysis of Water: 2H₂O → 4H⁺ + 4e⁻ + O₂
H₂O = Water molecule (substrate)
H⁺ = Protons contributing to the proton gradient inside the thylakoid lumen
e⁻ = Electrons replacing those lost from PSII
O₂ = Molecular oxygen, released as a by-product
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This reaction is the source of oxygen released during photosynthesis and provides protons for later ATP synthesis. The electrons from water replace those lost from PSII, maintaining the flow of charge through the system.
Non-Cyclic Photophosphorylation
Non-cyclic photophosphorylation involves both PSII and PSI acting together in sequence. It is the primary route through which light energy produces both ATP and reduced NADP.
The steps are as follows:
Light absorption at PSII excites electrons in P680 chlorophyll, which are transferred to an electron acceptor.
Photolysis of water replaces the lost electrons in PSII and generates protons and oxygen.
Electrons pass along an electron transport chain (ETC) through carriers such as plastoquinone (PQ), the cytochrome b₆f complex, and plastocyanin (PC).
As electrons move through the ETC, energy is released and used to pump protons from the stroma into the thylakoid lumen.
This establishes a proton gradient (electrochemical gradient) across the thylakoid membrane.
Protons flow back into the stroma via ATP synthase, driving ATP production by chemiosmosis.
Electrons arrive at Photosystem I, where they are re-excited by light absorbed by P700 chlorophyll.
The re-energised electrons are transferred to ferredoxin (Fd) and then to NADP⁺ reductase, which reduces NADP⁺ to NADPH (reduced NADP).
Schematic of the light-dependent reactions on the thylakoid membrane showing linear (non-cyclic) flow from water to NADP⁺ and the alternative cyclic route around PSI. Labels highlight PSII, PQ, cytochrome b₆f, PC, PSI, Fd, FNR, and ATP synthase. This directly illustrates where ATP and reduced NADP are generated. Source.
Chemiosmosis: The movement of ions (typically protons) across a semipermeable membrane down their electrochemical gradient, coupled to ATP synthesis.
Through non-cyclic photophosphorylation, ATP, reduced NADP, and oxygen are all produced. The electrons do not return to PSII; instead, they are transferred to NADP⁺, making this pathway “non-cyclic.”
Cyclic Photophosphorylation
In certain conditions, only Photosystem I operates independently in a process known as cyclic photophosphorylation. It generates ATP only, without producing reduced NADP or oxygen.
Cyclic electron flow around PSI generates a proton gradient used by ATP synthase but does not reduce NADP⁺ or split water. The diagram traces electron movement from PSI through carriers back to PSI. This visual reinforces the ATP-only outcome of the cyclic pathway. Source.
The process operates as follows:
Light excites electrons in PSI’s P700 chlorophyll.
Excited electrons are passed from ferredoxin back to the cytochrome b₆f complex instead of reducing NADP⁺.
As they flow through this complex and associated carriers, energy is released to pump protons into the thylakoid lumen.
These protons drive ATP synthase, generating ATP through chemiosmosis.
Electrons return to PSI, hence the term “cyclic.”
Cyclic photophosphorylation is thought to operate when the chloroplast’s demand for ATP exceeds its need for reduced NADP — for instance, under specific light conditions or during particular metabolic states of the chloroplast.
The Role of Electron Carriers and Proton Gradients
Electron carriers are proteins embedded in the thylakoid membrane that undergo redox reactions, transferring electrons from one molecule to another. Their sequential energy changes enable the pumping of protons into the thylakoid lumen, establishing a proton motive force. This gradient drives ATP synthesis via ATP synthase, a membrane-bound enzyme complex that uses the flow of protons to phosphorylate ADP.
Photophosphorylation: The synthesis of ATP from ADP and inorganic phosphate using light energy during photosynthesis.
The generated ATP and reduced NADP then move into the stroma, where they are utilised in the Calvin cycle to fix carbon dioxide into organic molecules.
The Z-scheme depicts electron energy changes from water at PSII (P680) through carriers to PSI (P700) and finally to NADP⁺. It clarifies why light must be absorbed twice and why non-cyclic flow yields reduced NADP. Extra detail: the vertical axis shows redox potential, which exceeds the OCR minimum but enhances conceptual understanding. Source.
Summary of Key Products
ATP: Provides energy for endergonic reactions in the Calvin cycle.
Reduced NADP: Supplies reducing power for the conversion of GP (glycerate-3-phosphate) to TP (triose phosphate).
Oxygen: Released as a by-product of water photolysis and diffuses out of the chloroplast.
In essence, the light-dependent reactions transform solar energy into forms usable by the cell, coupling the excitation of electrons with proton movement and redox chemistry to sustain the continuous flow of energy through the photosynthetic process.
FAQ
Photolysis refers to the splitting of water molecules using light energy at Photosystem II, producing protons, electrons, and oxygen.
Photoionisation, on the other hand, occurs when light energy excites electrons in chlorophyll, causing them to leave the molecule entirely and enter the electron transport chain.
Thus, photolysis supplies replacement electrons for chlorophyll after photoionisation and contributes to the proton gradient used for ATP synthesis.
Cyclic photophosphorylation involves only Photosystem I, where electrons are recycled through the same system instead of being transferred to NADP⁺.
Because water is not split to replace lost electrons (as in PSII), no oxygen is produced, and since the electrons return to PSI rather than reducing NADP⁺, no reduced NADP is formed.
The only product is ATP, generated through the proton gradient established by the cyclic electron flow.
The thylakoid membrane provides a large surface area for photosystems, electron carriers, and ATP synthase complexes.
It allows spatial separation between the stroma and thylakoid lumen, enabling a proton gradient to form.
Embedded proteins organise the electron transport chain, ensuring electrons flow efficiently between carriers and photosystems while maintaining the necessary proton movement for chemiosmosis.
The balance depends on the cell’s ATP and reduced NADP demand.
When the chloroplast requires more ATP than reduced NADP (for example, when the Calvin cycle slows), electrons flow cyclically through PSI.
When both ATP and reduced NADP are needed, non-cyclic photophosphorylation predominates.
The ratio can also shift due to light intensity, CO₂ availability, or the redox state of ferredoxin and NADP⁺.
ATP synthase complexes are embedded in the thylakoid membrane with their catalytic heads facing the stroma.
This orientation allows protons from the thylakoid lumen to pass through the enzyme, driving the phosphorylation of ADP to ATP in the stroma.
The produced ATP is then immediately available for the Calvin cycle, linking the two stages of photosynthesis efficiently.
Practice Questions
Question 1 (2 marks)
Explain the role of water in the light-dependent reactions of photosynthesis.
Mark Scheme:
1 mark: Water is split by photolysis to provide electrons that replace those lost from Photosystem II.
1 mark: The reaction also produces protons (H⁺) for the proton gradient and oxygen (O₂) as a by-product.
Question 2 (5 marks)
Describe and explain how light energy is converted into chemical energy during the light-dependent reactions of photosynthesis.
Mark Scheme:
1 mark: Light energy excites electrons in the chlorophyll molecules of Photosystem II.
1 mark: Excited electrons pass along an electron transport chain, releasing energy used to pump protons into the thylakoid lumen.
1 mark: The resulting proton gradient drives ATP synthesis by ATP synthase through chemiosmosis.
1 mark: Electrons reach Photosystem I, are re-excited by light, and used to reduce NADP⁺ to NADPH with the help of ferredoxin and NADP⁺ reductase.
1 mark: The products ATP and reduced NADP store chemical energy for the Calvin cycle.
(Allow alternative but equivalent wording as long as the described sequence of excitation, electron transport, proton pumping, and energy conversion is clear.)
