Photosynthesis: Process, Pigments, and Significance

Photosynthesis is the process by which green plants, algae and some bacteria convert light energy into chemical energy. Using carbon dioxide and water in the presence of chlorophyll, it produces carbohydrates and releases oxygen.

For prelims revision, the topic is important for understanding chloroplast structure, pigments, light reaction, Calvin cycle, alternative carbon fixation pathways and photorespiration.

Basic Concept and Site of Photosynthesis

Photosynthesis is a physicochemical process in which light energy is trapped and converted into chemical energy stored in sugars. The process is represented by the overall balanced equation:

6CO2 + 12H2O → C6H12O6 + 6H2O + 6O2

• Site: Photosynthesis occurs inside chloroplasts, double-membrane-bound organelles found mainly in the mesophyll cells of leaves.
• Stroma: The inner fluid matrix contains enzymes for carbohydrate synthesis, double-stranded circular DNA and 70S ribosomes.
• Thylakoids: Membranous sacs embedded in the stroma; their stacks form grana.
• Stroma lamellae: Flat membranous connections between grana.
• Reaction sites: Light-dependent reactions occur on thylakoid membranes, while light-independent reactions occur in the stroma.

Photosynthetic Pigments and Absorption

Photosynthetic pigments absorb specific wavelengths of visible light, broadly in the 380 nm to 760 nm range, which is called Photosynthetically Active Radiation (PAR).

• Chlorophyll a: Primary reaction-centre pigment in all oxygenic photosynthetic organisms; absorbs blue-violet and red light.
• Chlorophyll b: Accessory pigment that absorbs blue and red-orange light and transfers energy to chlorophyll a.
• Carotenoids: Include carotenes such as beta-carotene and xanthophylls such as lutein; they absorb violet-blue light and protect chlorophyll from photo-oxidation.
• Phycobilins: Water-soluble accessory pigments found in cyanobacteria and red algae; they include phycocyanin and phycoerythrin.

Light Reaction or Photochemical Phase

The light reaction takes place in the thylakoid membranes and involves light absorption, water splitting, oxygen release, and the synthesis of ATP and NADPH.

• Photosystems: The pigments are organized into two photochemical complexes, Photosystem I (PS I) with reaction centre P700 and Photosystem II (PS II) with reaction centre P680.
• Non-cyclic photophosphorylation: Electrons ejected from P680 move through plastoquinone, cytochrome b6f and plastocyanin to PS I, leading to the formation of ATP and NADPH.
• Photolysis of water: Occurs on the inner surface of the thylakoid membrane associated with PS II and requires manganese and chloride ions.

2H2O → 4H+ + 4e- + O2

• Cyclic photophosphorylation: Operates when light of wavelengths beyond 680 nm is available or when the ATP/NADPH ratio needs adjustment; it involves only PS I and produces ATP without NADPH or oxygen.
• Chemiosmotic hypothesis: Proposed by Peter Mitchell, it explains ATP synthesis through ATP synthase driven by a proton gradient across the thylakoid membrane into the lumen.

Dark Reaction or Biosynthetic Phase

The dark reaction occurs in the chloroplast stroma and does not directly require light. It uses ATP and NADPH from the light reaction to reduce carbon dioxide into sugars.

• C3 pathway or Calvin cycle: The primary CO2 acceptor is ribulose-1,5-bisphosphate (RuBP), and the process is catalysed by RuBisCO.
• Stages of Calvin cycle: Carboxylation, reduction and regeneration.
• Energy requirement: Fixation of one CO2 molecule requires 3 ATP and 2 NADPH.
• Glucose formation: One glucose molecule requires six turns of the cycle and consumes 18 ATP and 12 NADPH.

Alternative Carbon Fixation Pathways

Some plants use modified pathways to reduce water loss and limit the effect of RuBisCO oxygenation under high temperature and drought conditions.

• C3 pathway: Primary CO2 acceptor is RuBP; the first stable product is 3-PGA.
• C4 pathway (Hatch-Slack): Primary CO2 acceptor is PEP; the first stable product is oxaloacetic acid.
• CAM pathway: Uses PEP at night; the first stable product is oxaloacetic acid.
• Leaf anatomy: C4 plants show Kranz anatomy, while CAM plants have succulent leaves without Kranz anatomy.
• Stomatal behaviour: C3 and C4 plants open stomata during the day, while CAM plants open them at night.
• Photorespiration: High in C3 plants, absent or negligible in C4 plants, and negligible in CAM plants.
• Optimum temperature: C3 plants: 20°C–25°C; C4 plants: 30°C–45°C; CAM plants: above 35°C.
• Examples: C3 plants include rice, wheat, soybean and potato; C4 plants include maize, sugarcane, sorghum and amaranth; CAM plants include cactus, pineapple, agave and sedum.

Photorespiration and Limiting Factors

Photorespiration occurs when oxygen concentration is high and carbon dioxide is low, causing RuBisCO to act as an oxygenase instead of a carboxylase. This forms 3-PGA and 2-phosphoglycolate and begins a wasteful pathway involving the chloroplast, peroxisome and mitochondrion.

• Effect: It produces neither ATP nor NADPH and releases previously fixed CO2.
• Efficiency loss: In C3 plants, photosynthetic efficiency may fall by up to 25%.
• Limiting factors: Photosynthesis is governed by internal and external factors under Blackman’s Law of Limiting Factors (1905).
• Light: Intensity, quality and duration affect the rate; light saturation occurs at about 10% of full sunlight.
• Carbon dioxide: A major limiting factor in nature; atmospheric concentration is about 0.03% to 0.04% (300–400 ppm).
• Temperature: Dark reactions are enzymatic and temperature-sensitive; C4 plants have a higher thermal optimum than C3 plants.
• Water: Water stress causes stomatal closure, reduces CO2 entry and lowers the leaf surface available for light interception.

Key Prelims Takeaways

• Chloroplast parts: Stroma, thylakoids, grana and stroma lamellae are core terms to remember.
• Reaction centres: PS I = P700 and PS II = P680.
• Non-cyclic flow: Produces both ATP and NADPH, while cyclic photophosphorylation produces only ATP.
• Water splitting: Photolysis releases oxygen and occurs with PS II.
• Calvin cycle energy cost: 3 ATP and 2 NADPH per CO2; 18 ATP and 12 NADPH per glucose.
• C4 advantage: Kranz anatomy and low photorespiration are important distinguishing features.
• CAM feature: Stomata open at night, helping reduce water loss.