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Photosynthesis in Plants Class 11: NCERT Notes, Light Reaction & NEET MCQs

🌿 Photosynthesis in Plants — Class 11 Biology

Understand photosynthesis with NCERT-focused concepts covering chloroplasts, light reaction, photophosphorylation, Calvin cycle, C3 and C4 pathways, photorespiration and NEET Biology MCQs.

Photosynthesis is the process by which green plants and other photosynthetic organisms use light energy to synthesise organic compounds from carbon dioxide and water.

For NEET Biology, this chapter becomes easier when you understand the sequence: light absorption → electron transport → ATP and NADPH formation → carbon fixation → carbohydrate synthesis.

1. What Is Photosynthesis?

Photosynthesis takes place mainly in the chloroplasts of plant cells. Chlorophyll and other photosynthetic pigments absorb light energy required for the process.

The overall process can be represented in a simplified form as:

Carbon dioxide + Water + Light Energy → Organic Compounds + Oxygen

Chloroplast: Site of Photosynthesis

The chloroplast contains a double membrane and internal membrane structures. The flattened membrane sacs are called thylakoids. Stacks of thylakoids are called grana.

The fluid-filled region surrounding the grana is the stroma.

Structure Important Role
Thylakoid membrane Contains photosystems and components of the electron transport chain.
Grana Stacks of thylakoids.
Stroma Site of reactions associated with carbon fixation.
🧠 NEET Memory Trick:

Grana → Light reactions
Stroma → Calvin cycle

2. Light Reaction and Photophosphorylation

The light-dependent reactions occur in the thylakoid membranes. Light energy is captured by photosynthetic pigments associated with photosystems.

Photosystems

Two major photosystems participate in the light reactions: Photosystem I (PS I) and Photosystem II (PS II).

The reaction centres are commonly identified as:

  • PS II → P680
  • PS I → P700

Photolysis of Water

In the light reaction, water is split to provide electrons and protons, with oxygen released as a by-product.

🎯 High-Yield Point:

The oxygen released during photosynthesis comes from water, not directly from carbon dioxide.

Photophosphorylation

The formation of ATP using light energy during photosynthesis is called photophosphorylation.

It can occur through:

  • Non-cyclic photophosphorylation
  • Cyclic photophosphorylation

Non-Cyclic Photophosphorylation

Non-cyclic electron flow involves both photosystems and results in the formation of ATP and NADPH, while oxygen is released through water splitting.

Cyclic Photophosphorylation

Cyclic electron flow involves Photosystem I and produces ATP but does not result in the production of NADPH or oxygen through water splitting.

Feature Cyclic Non-Cyclic
Photosystems PS I PS I + PS II
ATP Produced Produced
NADPH Not produced Produced
Oxygen evolution No Yes

3. Calvin Cycle, C3 and C4 Pathways

The Calvin cycle occurs in the chloroplast stroma. It uses ATP and NADPH generated during the light reactions to drive carbon fixation and the subsequent reactions leading toward carbohydrate formation.

Three Broad Stages

  • Carboxylation
  • Reduction
  • Regeneration

Role of RuBisCO

RuBisCO is an important enzyme involved in carbon fixation in the Calvin cycle.

In C3 plants, the first stable product of carbon fixation is a three-carbon compound.

C4 Pathway

C4 plants possess specialised leaf anatomy and initially fix carbon dioxide into a four-carbon compound.

A common example is maize.

C4 plants are adapted to conditions in which photorespiration can otherwise be significant. Their carbon-concentrating mechanism helps reduce the oxygenase activity of RuBisCO in the bundle sheath cells.

🌱 NEET Memory Point:

C3 → first stable product has 3 carbons
C4 → initial fixation produces a 4-carbon compound

C3 vs C4 Plants

Feature C3 C4
Initial carbon fixation 3-carbon product 4-carbon product
Specialised anatomy Generally absent Kranz anatomy
Photorespiration More significant Strongly reduced
Example Many common plants Maize

4. Photorespiration and NEET Biology MCQs

What Is Photorespiration?

Photorespiration is associated with the oxygenase activity of RuBisCO. It becomes particularly relevant when the oxygen concentration relative to carbon dioxide is high.

Unlike photosynthesis, photorespiration does not result in the efficient production of sugars and can lead to loss of previously fixed carbon.

Factors Affecting Photosynthesis

The rate of photosynthesis is influenced by environmental and internal factors including:

  • Light intensity
  • Carbon dioxide concentration
  • Temperature
  • Water availability
  • Chlorophyll and leaf condition

📝 NEET Biology Practice MCQs

Q1. The light-dependent reactions of photosynthesis occur mainly in the:

A. Stroma
B. Thylakoid membranes
C. Cytoplasm
D. Nucleus

Answer: B. Thylakoid membranes

Q2. The reaction centre of Photosystem II is:

A. P700
B. P680
C. P600
D. P760

Answer: B. P680

Q3. The reaction centre of Photosystem I is:

A. P680
B. P700
C. P500
D. P760

Answer: B. P700

Q4. The Calvin cycle occurs in the:

A. Thylakoid lumen
B. Stroma
C. Cytoplasm
D. Mitochondrial matrix

Answer: B. Stroma

Q5. Which enzyme is central to carbon fixation in the Calvin cycle?

A. ATP synthase
B. RuBisCO
C. Amylase
D. DNA polymerase

Answer: B. RuBisCO

Q6. Which of the following is a C4 plant?

A. Maize
B. Pea
C. Wheatgrass

Answer: A. Maize

Q7. The oxygen released during photosynthesis originates primarily from:

A. Carbon dioxide
B. Water
C. Glucose
D. Chlorophyll

Answer: B. Water

🎯 60-Second Photosynthesis Revision

Chloroplast → Site of photosynthesis
Grana → Stacks of thylakoids
Thylakoid membrane → Light reactions
PS II → P680
PS I → P700
Water splitting → Oxygen release
Calvin cycle → Stroma
RuBisCO → Carbon fixation
C4 example → Maize

🔗 Continue Your NEET Biology Preparation

This article is part of the NEET Biology for Odisha Students topic cluster. Continue with NCERT-focused Biology notes, Botany revision and MCQs.

Explore NEET Biology Resources →

Frequently Asked Questions

Where does photosynthesis occur?

Photosynthesis occurs mainly in chloroplasts. The light-dependent reactions occur in the thylakoid membranes, while the Calvin cycle occurs in the stroma.

What is photophosphorylation?

Photophosphorylation is the formation of ATP using light energy during the light-dependent reactions of photosynthesis.

What is the difference between C3 and C4 plants?

C3 plants initially produce a three-carbon compound during carbon fixation, whereas C4 plants initially fix carbon into a four-carbon compound and possess specialised carbon-concentrating anatomy.

What is RuBisCO?

RuBisCO is the enzyme that catalyses the initial carbon-fixation step of the Calvin cycle in C3 plants and is also involved in photorespiration.

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