Revision: Plant Physiology >> Photosynthesis in Higher Plants Biology (Theory) ISC (Science) ISC Class 11 CISCE

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.

• 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.

• 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.

• 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.

• 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.

• 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.