Revision: Plant Physiology >> Photosynthesis in Higher Plants Biology Science (English Medium) Class 11 CBSE

Photophosphorylation is the process of converting ADP into energy-rich ATP by adding an inorganic phosphate (Pi), using energy from light (photons).

Photolysis occurs in the grana of a chloroplast and is defined as the splitting of H₂O molecules into hydrogen ions and oxygen in the presence of light.

Photophosphorylation is the process of converting ADP into energy-rich ATP by adding an inorganic phosphate (Pi), using energy from light (photons).

Photolysis occurs in the grana of a chloroplast and is defined as the splitting of H₂O molecules into hydrogen ions and oxygen in the presence of light.

• Priestley (1770) - Plants take up CO₂ and release O₂; plants restore what animals and candles remove from the air.
• Ingenhousz (1779) - O₂ release occurs only in sunlight and only by the green parts of plants.
• Theodore de Saussure (1804) - Water is an essential requirement for photosynthesis.
• Julius von Sachs (1854) - Green parts produce glucose, stored as starch; chlorophyll is located in chloroplasts.
• T.W. Engelmann (1888) - Plotted the first action spectrum of photosynthesis; bacteria accumulated in blue and red light regions.
• C.B. van Niel (1931) - Photosynthesis is a light-dependent reaction; H from an oxidisable compound reduces CO₂ to form sugar. O₂ comes from H₂O, not CO₂.
• Hill (1937), Calvin (1954-55), Hatch & Slack (1965) - Hill: O₂ evolves in light reaction, Calvin: traced carbon fixation pathway, Hatch & Slack: discovered C4 pathway.

• Photosynthesis occurs in green parts of plants, mainly in leaves, within mesophyll cells that contain chloroplasts.
• Chloroplasts are the actual sites of photosynthesis. They contain grana, stroma lamellae, and stroma.
• Grana (thylakoids) - Responsible for trapping light energy and the synthesis of ATP and NADPH (light reactions occur here).
• Stroma - Site of dark reactions (biosynthetic phase); enzymatic reactions here synthesise sugar, which is stored as starch.
• Dark reactions do not occur in darkness — they depend on ATP and NADPH produced in light reactions, so they are indirectly light-dependent.

• Leaves contain multiple pigments such as chlorophyll a, chlorophyll b, xanthophylls, and carotenoids, which give different shades of green.
• Chlorophyll a is the chief pigment and is mainly responsible for trapping light energy and converting it into chemical energy.
• Chlorophyll b, xanthophylls, and carotenoids are accessory pigments that absorb light and transfer energy to chlorophyll a.
• Accessory pigments help in broadening the range of light absorption and also protect chlorophyll a from photo-oxidation.
• Maximum photosynthesis occurs in blue and red regions of the light spectrum.
• Absorption spectrum and action spectrum are closely related, showing that photosynthesis is highest where chlorophyll a absorbs maximum light.
• Carotenoids and xanthophylls also play a protective role and assist in capturing additional light energy.

• Light Harvesting Complexes (LHC) - Made up of hundreds of pigment molecules bound to proteins. Found in PS I and PS II. Help absorb different wavelengths of light for efficient photosynthesis.
• Antennae System - In each photosystem, all pigments except one chlorophyll a molecule form the antennae (light-harvesting system). They absorb light and pass energy to the reaction centre.
• Reaction Centre - The single chlorophyll a molecule that directly participates in the photochemical reaction. It is different in PS I and PS II.
• PS I - Reaction centre = P700 (absorbs light at 700 nm).
• PS II - Reaction centre = P680 (absorbs light at 680 nm).
• Steps of Light Reaction - Light absorption → Water splitting → Oxygen release → ATP and NADPH production.

• In Photosystem II (PS II), chlorophyll a absorbs 680 nm light (P680) and releases high-energy electrons.
• These electrons pass through an electron transport chain (cytochromes) from PS II to PS I, releasing energy used for ATP formation.
• In Photosystem I (PS I), chlorophyll a absorbs 700 nm light (P700), re-exciting the electrons to a higher energy level.
• The energised electrons are transferred to NADP⁺, reducing it to NADPH + H⁺.
• The entire flow of electrons from PS II → PS I → NADP⁺ is called the Z-scheme, due to its characteristic zig-zag shape on a redox potential graph.

• PS II continuously supplies electrons by splitting water (photolysis). Reaction: 2H₂O → 4H⁺ + O₂ + 4e⁻
• Water is split into H⁺ (protons), [O] (oxygen), and electrons; oxygen is released as a byproduct of photosynthesis.
• Electrons from water replace the electrons lost from PS II, keeping the photosystem functional.
• Water splitting occurs on the inner side of the thylakoid membrane; protons (H⁺) accumulate in the thylakoid lumen, creating a proton gradient.
• The proton gradient across the thylakoid membrane is used for ATP synthesis.

• Photophosphorylation - Synthesis of ATP from ADP and inorganic phosphate in the presence of light, occurring in chloroplasts and mitochondria.
• Non-Cyclic Photophosphorylation - Both PS II and PS I work in series (Z scheme). Produces both ATP and NADPH + H⁺. Electrons do not return to PS II.
• Cyclic Photophosphorylation - Only PS I functions; electrons circulate within the photosystem and return to PS I via the electron transport chain. Produces ATP only (no NADPH, no O₂).
• Location - Grana contain both PS I and PS II → non-cyclic occurs here. Stroma lamellae lack PS II and NADP reductase → cyclic occurs here.
• Cyclic photophosphorylation also occurs when only light beyond 680 nm is available for excitation.

• Chemiosmosis explains ATP synthesis. Requires: membrane, proton pump, proton gradient, and ATPase.
• Protons (H⁺) accumulate in the thylakoid lumen (high H⁺); the stroma has low H⁺ → creates a proton gradient.
• Proton gradient is caused by: water splitting in the lumen, electron movement through photosystems, and NADP reductase using H⁺ from the stroma.
• Protons flow back from lumen → stroma through CF₀ channel → energy released → CF₁ synthesises ATP.
• ATP synthase = CF₀ (in membrane, proton channel) + CF₁ (outside membrane, makes ATP).
• ATP and NADPH produced are used in the stroma for CO₂ fixation (dark reactions).

• CO₂ Acceptor - The primary acceptor of CO₂ is a 5-carbon ketose sugar called RuBP (Ribulose Bisphosphate), not a 2-carbon compound as initially thought.
• Carboxylation - CO₂ combines with RuBP to form 2 molecules of 3-PGA (3-carbon). Catalysed by enzyme RuBisCO (RuBP carboxylase-oxygenase). PGA is the first stable product.
• Reduction - 2 molecules each of ATP and NADPH are used per CO₂ fixed. Results in the formation of glucose.
• Regeneration - RuBP is regenerated to continue the cycle; it requires 1 ATP per molecule.
• For 1 glucose molecule, 6 CO₂ must be fixed → 6 turns of the Calvin cycle are required. Total: 18 ATP + 12 NADPH used.

• The Calvin Cycle (dark reaction) was discovered by Melvin Calvin using ¹⁴C, and the first stable product formed is a 3-carbon compound (3-PGA).
• The cycle has three stages: Carboxylation (CO₂ fixation), Reduction (formation of sugars), and Regeneration (formation of RuBP).
• RuBP is the CO₂ acceptor, and the reaction is catalysed by the enzyme RuBisCO.
• For each CO₂ molecule, the cycle requires 3 ATP and 2 NADPH.
• To produce one glucose molecule (6 CO₂), the cycle needs 18 ATP and 12 NADPH, and the process is cyclic as RuBP is regenerated.

• C₄ pathway (Hatch and Slack pathway) occurs in plants like maize and sugarcane, which are adapted to high temperature and dry conditions.
• C₄ plants show Kranz anatomy, where bundle sheath cells surround vascular bundles and contain many chloroplasts.
• The primary CO₂ acceptor is PEP (3-carbon compound), and CO₂ fixation in mesophyll cells is done by PEP carboxylase, forming oxaloacetic acid (OAA).
• OAA is converted into malic acid or aspartic acid (4-carbon compounds) and transported to bundle sheath cells.
• In bundle sheath cells, these compounds release CO₂, which enters the Calvin cycle (C₃ cycle); RuBisCO is present only here.
• The remaining 3-carbon compound returns to mesophyll cells and is converted back to PEP, completing the cycle.
• C₄ plants do not show photorespiration, have higher productivity, and perform better under high light intensity.

• Photorespiration is a process where O₂ is used, and CO₂ is released, opposite to photosynthesis, making it a wasteful process.
• It occurs when O₂ concentration is high and CO₂ is low, causing RuBisCO to act as oxygenase instead of carboxylase.
• In this process, RuBP reacts with O₂ to form one molecule of PGA and one molecule of phosphoglycolate (2C).
• Photorespiration does not produce ATP or NADPH; instead, it uses ATP and releases CO₂, reducing photosynthetic efficiency.
• It mainly occurs in C₃ plants, leading to decreased carbon fixation and lower productivity.
• C₄ plants do not show photorespiration because they increase CO₂ concentration at the RuBisCO site, ensuring proper functioning of the Calvin cycle.

• Blackman’s Law of Limiting Factors states that the rate of photosynthesis is controlled by the factor in the least supply.
• Light affects photosynthesis through intensity, quality, and duration; it shows a linear increase at low intensity and saturation at about 10% of full sunlight.
• Carbon dioxide is the major limiting factor; increasing CO₂ concentration increases photosynthesis up to a limit, after which it may become harmful.
• Temperature controls enzymatic reactions (dark reactions); C₄ plants work better at higher temperatures, while C₃ plants have a lower optimum temperature.
• Water affects photosynthesis indirectly; water stress causes stomatal closure, reducing CO₂ availability and decreasing photosynthesis.
• All factors work together, but usually one limiting factor determines the overall rate of photosynthesis.

• Light Harvesting Complexes (LHC) - Made up of hundreds of pigment molecules bound to proteins. Found in PS I and PS II. Help absorb different wavelengths of light for efficient photosynthesis.
• Antennae System - In each photosystem, all pigments except one chlorophyll a molecule form the antennae (light-harvesting system). They absorb light and pass energy to the reaction centre.
• Reaction Centre - The single chlorophyll a molecule that directly participates in the photochemical reaction. It is different in PS I and PS II.
• PS I - Reaction centre = P700 (absorbs light at 700 nm).
• PS II - Reaction centre = P680 (absorbs light at 680 nm).
• Steps of Light Reaction - Light absorption → Water splitting → Oxygen release → ATP and NADPH production.