Revision: Class 11 >> Photosynthesis in Higher Plants NEET (UG) Photosynthesis in Higher Plants

Releasing electrons and dividing the water molecule (H2O) into its two components (Hydrogen and Oxygen). Photolysis is the term used to describe this reaction, which is characterised by the fracturing of molecules by light (photo = light, lysis = breaking).

Photosynthesis is the process by which living plant cells, containing chlorophyll, produce food substances (glucose and starch) from carbon dioxide and water by using light energy. Plants release oxygen as a byproduct during photosynthesis.

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.

• Photosynthesis converts sunlight into chemical energy using CO₂ and water, producing glucose and releasing oxygen as a byproduct.
• Green plants are autotrophs (make their own food); all other organisms are heterotrophs. All life depends on sunlight for energy.
• Three essentials for photosynthesis: chlorophyll, light, and CO₂.
• Experiment 1 - Variegated leaf tested for starch showed that photosynthesis occurs only in green parts in the presence of light.
• Experiment 2 - Leaf part enclosed with KOH-soaked cotton (absorbs CO₂) tested negative for starch → proved CO₂ is necessary for photosynthesis.

• 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).

• The biosynthetic phase uses ATP and NADPH (from light reactions) to synthesise sugars. Takes place in the stroma of chloroplasts.
• Called "dark phase", but this is misleading — it does not require darkness but is independent of direct light. It depends on ATP, NADPH, CO₂, and H₂O.
• After light is removed, biosynthesis continues briefly, then stops. When light returns, it resumes — showing indirect light dependence.
• Melvin Calvin used radioactive ¹⁴C to discover that the first stable product of CO₂ fixation is 3-phosphoglyceric acid (PGA) — a 3-carbon compound → called the Calvin cycle.
• C3 pathway - First product of CO₂ fixation = PGA (3-carbon). Found in C3 plants.
• C4 pathway - First product of CO₂ fixation = Oxaloacetic acid/OAA (4-carbon). Found in C4 plants.

• 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.