Photosynthesis in Higher Plants: Class 11 Biology NCERT Guide

The Engine of Life: What is Photosynthesis and Where Does it Occur?

Photosynthesis is a physico-chemical process used by autotrophs to synthesize organic compounds, primarily sugars, from inorganic substances like carbon dioxide and water, using light energy. It is an anabolic (building-up) and endergonic (energy-requiring) process. The overall balanced equation is:

6CO₂ + 12H₂O + Light Energy → C₆H₁₂O₆ + 6O₂ + 6H₂O

In higher plants, the primary site of photosynthesis is the leaf, and within the leaf cells, it occurs in specialized organelles called chloroplasts. A chloroplast has a double membrane. Inside, there is a fluid-filled space called the stroma. Embedded within the stroma is a system of interconnected membranous sacs called thylakoids. These thylakoids are often stacked in piles called grana (singular: granum). This specific structure is crucial because the two main stages of photosynthesis are spatially separated:

1. Light-Dependent Reactions: Occur in the thylakoid membranes, where pigments capture light energy.
2. Light-Independent Reactions (Calvin Cycle): Occur in the stroma, where the captured energy is used to fix CO₂ into sugar.

The Light Catchers: Pigments and Photosystems

Photosynthetic Pigments

Substances that absorb light at specific wavelengths. The main ones are Chlorophyll a (the primary pigment), and accessory pigments like Chlorophyll b, Xanthophylls, and Carotenoids. Accessory pigments absorb light and transfer the energy to Chlorophyll a, and also protect it from photo-oxidation.

Photosystem I (PSI)

A cluster of pigments and proteins in the thylakoid membrane. Its reaction centre has an absorption peak at 700 nm, hence it is called P700. It is involved in both cyclic and non-cyclic photophosphorylation.

Photosystem II (PSII)

Another pigment-protein complex in the thylakoid membrane. Its reaction centre has an absorption peak at 680 nm, so it's called P680. It is primarily involved in non-cyclic photophosphorylation and is responsible for the splitting of water (photolysis).

Photophosphorylation

The synthesis of ATP from ADP and inorganic phosphate (Pi) using light energy. It occurs during the light reactions of photosynthesis.

The Mechanism: Light and Dark Reactions

1. Stage 1: Light-Dependent Reactions (Photochemical Phase) — This phase occurs in the thylakoid membranes and directly uses light energy. It involves a series of events: (a) Light Absorption: Pigments in PSII and PSI absorb photons of light. (b) Water Splitting (Photolysis): In PSII, light energy splits water molecules (H₂O) into protons (H⁺), electrons (e⁻), and oxygen (O₂). This is the source of oxygen released during photosynthesis. (c) Electron Transport: The excited electrons travel through an electron transport system from PSII to PSI. This transport pumps protons into the thylakoid lumen, creating a proton gradient. (d) ATP & NADPH Synthesis: As protons flow back into the stroma through ATP synthase, ATP is produced (photophosphorylation). At the end of the chain, electrons, along with protons from the stroma, reduce NADP⁺ to NADPH. The products, ATP and NADPH, are the 'assimilatory power' needed for the next stage.
2. Stage 2: Light-Independent Reactions (Biosynthetic Phase / Calvin Cycle) — This phase occurs in the stroma and does not directly require light, but it depends on the products of the light reaction (ATP and NADPH). (a) Carboxylation: Carbon dioxide (CO₂) from the atmosphere is 'fixed' by combining it with a 5-carbon molecule, Ribulose-1,5-bisphosphate (RuBP). This reaction is catalyzed by the enzyme RuBisCO and forms an unstable 6-carbon compound that immediately splits into two molecules of a 3-carbon compound (3-PGA). (b) Reduction: The 3-PGA molecules are converted into triose phosphates (sugars) in a two-step process that uses the energy from ATP and the reducing power of NADPH. (c) Regeneration: For every 6 molecules of CO₂ that enter the cycle, 12 molecules of triose phosphate are formed. 2 of these are used to make one glucose molecule, while the other 10 are used to regenerate the initial 6 molecules of RuBP, a process that requires more ATP. The cycle is now ready to accept more CO₂.

Exam Focus: C3 vs. C4 Pathways - A Common Trap

A very common topic for exam questions is the difference between C3 and C4 plants. The key distinction lies in the first product of carbon fixation.

• C3 Pathway (Calvin Cycle): The first stable product is a 3-carbon compound, 3-phosphoglyceric acid (3-PGA). This is the 'standard' pathway described above, found in most plants like rice, wheat, and potatoes.

• C4 Pathway (Hatch-Slack Pathway): The first product is a 4-carbon compound, oxaloacetic acid (OAA). These plants (e.g., maize, sugarcane) have a special leaf anatomy called Kranz anatomy. They perform initial CO₂ fixation in mesophyll cells and then shuttle the 4-C acid to bundle sheath cells, where the Calvin cycle occurs.

Why is this important? The C4 pathway is an adaptation to hot, dry climates. It minimizes a wasteful process called photorespiration by concentrating CO₂ around the RuBisCO enzyme in the bundle sheath cells. This makes C4 plants more photosynthetically efficient than C3 plants under high light intensity and high temperatures.

Practice Questions with Solutions

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