Board exam focus — Photosynthesis in Higher Plants (CBSE & HBSE)

This chapter explains how green plants synthesise food using light energy. It covers the site of photosynthesis (chloroplasts), photosynthetic pigments, the light reaction (photosystems, electron transport, cyclic and non-cyclic photophosphorylation), the chemiosmotic hypothesis of ATP synthesis, the Calvin cycle (C3 pathway), the C4 pathway, photorespiration, and the factors affecting photosynthesis. It is one of the most important chapters for NEET and board exams.

Site, Pigments and the Light Reaction

Site of Photosynthesis

Photosynthesis occurs in the chloroplast, mainly in the mesophyll cells of leaves. The chloroplast has:

• Grana (stacks of thylakoids) — site of the light reaction (photochemical phase).
• Stroma — site of the dark reaction (biosynthetic phase / Calvin cycle).

Photosynthetic Pigments

Four main pigments are present in the thylakoid membrane:

NEET Trap: Chlorophyll a is the chief pigment (reaction centre). Accessory pigments (chl b, carotenoids) widen the spectrum of light absorbed and protect chlorophyll a from photo-oxidation.

The Light Reaction

Light is absorbed by two pigment systems:

• Photosystem I (PS I) — reaction centre P700 (absorbs at 700 nm).
• Photosystem II (PS II) — reaction centre P680 (absorbs at 680 nm).

Electron Transport — Z Scheme (Non-cyclic)

1. PS II (P680) absorbs light → electrons excited and passed to an electron acceptor.
2. Electrons travel down an ETC (cytochromes) to PS I — ATP is formed.
3. Photolysis/splitting of water at PS II releases O2, protons, and electrons (which replace those lost by P680).
4. PS I (P700) re-excites electrons → passed to NADP+ → NADPH + H+.

Splitting of water (photolysis) is associated with PS II and occurs on the inner side of the thylakoid membrane. The O2 we breathe comes from water, not CO2.

Photophosphorylation

Cyclic photophosphorylation occurs when only PS I is functional (e.g. light > 680 nm) or when the cell needs more ATP than NADPH.

Chemiosmotic Hypothesis and Calvin Cycle (C3)

Chemiosmotic Hypothesis

Proposed to explain ATP synthesis in the chloroplast. ATP is synthesised due to the development of a proton gradient across the thylakoid membrane, with proton accumulation in the thylakoid lumen.

Three processes build up protons in the lumen:

1. Splitting of water releases H+ into the lumen.
2. As electrons move through the ETC, protons are transported across the membrane into the lumen.
3. NADP+ reduction on the stroma side removes H+ from the stroma.

The proton gradient is broken down as protons move through the F0-F1 ATP synthase (CF0-CF1), driving the synthesis of ATP. This is chemiosmosis (similar to mitochondria).

Key: ATP synthase has a channel (F0/CF0) that allows protons to diffuse back into the stroma; this releases energy to phosphorylate ADP → ATP.

The Calvin Cycle (C3 Pathway — Biosynthetic Phase)

Occurs in the stroma; uses ATP and NADPH from the light reaction to fix CO2 into sugar. Discovered by Melvin Calvin. Three stages:

Calvin Cycle Stoichiometry (per 1 glucose)

• 6 CO2 fixed
• 18 ATP + 12 NADPH consumed
• 6 turns of the cycle

NEET Trap: The first CO2 acceptor is RuBP (a 5-carbon), NOT a 3-carbon. The first stable PRODUCT is 3-PGA (3-carbon) — hence the name C3 pathway.

C4 Pathway, Photorespiration and Limiting Factors

C4 Pathway (Hatch and Slack Pathway)

Found in tropical plants like maize, sugarcane, sorghum. These plants have Kranz anatomy — bundle sheath cells around vascular bundles with large numbers of chloroplasts.

• First CO2 acceptor: PEP (phosphoenolpyruvate), a 3-carbon compound, in mesophyll cells.
• Enzyme: PEP carboxylase (PEPcase) → forms OAA (oxaloacetic acid, 4-carbon) — first stable product.
• OAA → malic acid, transported to bundle sheath cells, where it is decarboxylated to release CO2 for the Calvin cycle.

Advantage: C4 plants concentrate CO2 in bundle sheath cells, so RuBisCO acts as a carboxylase (not oxygenase). Hence C4 plants show NO photorespiration and are more productive.

Photorespiration

Occurs in C3 plants under high O2/low CO2. RuBisCO has affinity for both CO2 and O2; when O2 binds, RuBP forms one 3-PGA + one phosphoglycolate (2-C).

• No sugar, no ATP, no NADPH is produced.
• CO2 is released — a wasteful process.
• C4 plants avoid it by concentrating CO2.

Factors Affecting Photosynthesis — Blackman's Law of Limiting Factors (1905)

Law: If a chemical process is affected by more than one factor, its rate will be determined by the factor which is nearest to its minimal value; it is the factor which directly affects the process if its quantity is changed.

Main factors:

Overall Equation of Photosynthesis

6 CO2 + 12 H2O --(light, chlorophyll)--> C6H12O6 + 6 O2 + 6 H2O

NEET Trap: 12 water molecules are used and 6 are released — the O2 released comes from water (12 H2O), not CO2.

Frequently asked questions

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Yes — the Photosynthesis in Higher Plants notes for Biology (Class 11) on Siksha Sarovar are completely free to read, with no account required.

Do these notes follow CBSE and HBSE?

Yes. The Photosynthesis in Higher Plants notes are NCERT-aligned and include guidance for both CBSE and Haryana Board (HBSE), with important questions and MCQs for revision.

What does the Photosynthesis in Higher Plants chapter cover?

Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Photosynthesis in Higher Plants.