Photosynthesis – The Life-Giving Process of Plants
Photosynthesis – NCERT Notes, Light Reaction, Calvin Cycle & NEET Biology
Photosynthesis is one of the most important processes studied in Class 11 Biology and is a high-value topic for NEET Biology. NCERT questions can be based on the site of photosynthesis, pigments, light reactions, photolysis of water, electron transport, ATP and NADPH, the Calvin cycle and factors affecting photosynthesis.
Looking for more NCERT-focused Biology notes, diagrams, MCQs and NEET preparation resources?
Explore NEET Biology →🌿 What is Photosynthesis?
Photosynthesis is the process by which green plants and other photosynthetic organisms use light energy to synthesize organic compounds from carbon dioxide and water.
In green plants, photosynthesis mainly occurs in the chloroplasts, where chlorophyll and other pigments absorb light energy.
Overall Photosynthesis Equation
The simplified equation above represents the overall process. In detailed biochemical representation, water participates in both formation and release during the reactions.
☀️ Essential Factors for Photosynthesis
- Light: Provides the energy required to drive the photosynthetic reactions.
- Chlorophyll and other pigments: Absorb light energy and transfer it to the photosynthetic reaction centres.
- Water: Provides electrons and hydrogen; water is split during the light reaction.
- Carbon dioxide: Provides the carbon that is incorporated into carbohydrates during carbon fixation.
🍃 Where Does Photosynthesis Occur?
Photosynthesis occurs in the chloroplast. The chloroplast has two major regions that are important for photosynthesis:
| Structure | Major Event |
|---|---|
| Grana / Thylakoid membranes | Light-dependent reactions |
| Stroma | Calvin cycle / carbon fixation reactions |
🌱 Two Major Stages of Photosynthesis
1. Light-Dependent Reaction
The light-dependent reactions occur mainly on the thylakoid membranes of the chloroplast.
- Light energy is absorbed by photosynthetic pigments.
- Photosystems participate in light absorption and electron transfer.
- Water undergoes photolysis.
- Oxygen is released as a by-product.
- ATP is synthesized.
- NADPH is produced.
Light Reaction → ATP + NADPH + O₂
2. Calvin Cycle / Light-Independent Reactions
The Calvin cycle occurs in the stroma of the chloroplast. It uses ATP and NADPH generated during the light-dependent reactions to drive carbon fixation and carbohydrate synthesis.
The Calvin cycle is traditionally described in three major stages:
- Carboxylation
- Reduction
- Regeneration of RuBP
🔬 Photosystems in Photosynthesis
| Photosystem | Reaction Centre | Key Point |
|---|---|---|
| Photosystem I | P700 | Important in NADPH formation |
| Photosystem II | P680 | Involved in water splitting and oxygen evolution |
P680 → Photosystem II
P700 → Photosystem I
💧 Photolysis of Water
During the light-dependent reactions, water is split in a process called photolysis.
This process provides electrons and protons and results in the release of molecular oxygen.
In oxygenic photosynthesis, the oxygen released into the atmosphere originates from water.
🌿 Calvin Cycle – Quick Revision
The Calvin cycle fixes atmospheric CO₂ into organic molecules.
Three Main Steps
- Carboxylation: CO₂ combines with RuBP with the help of the enzyme RuBisCO.
- Reduction: ATP and NADPH are used in the reduction reactions.
- Regeneration: RuBP is regenerated so that the cycle can continue.
🧪 What is RuBisCO?
RuBisCO is the enzyme involved in the carboxylation of RuBP during the Calvin cycle.
It is one of the most important enzymes associated with carbon fixation in photosynthesis.
📈 Factors Affecting Photosynthesis
The rate of photosynthesis is affected by several external and internal factors.
- Light intensity
- Carbon dioxide concentration
- Temperature
- Water availability
- Chlorophyll content
- Leaf age and physiological condition
🌍 Importance of Photosynthesis
- Converts light energy into chemical energy.
- Produces organic food molecules.
- Provides the primary source of energy for most ecosystems.
- Releases oxygen during oxygenic photosynthesis.
- Helps regulate atmospheric carbon dioxide.
🧠 NEET Quick Revision Table
| Topic | Remember |
|---|---|
| Light reaction | Thylakoid membrane |
| Calvin cycle | Stroma |
| PS II | P680 |
| PS I | P700 |
| Photolysis | Splitting of water |
| Oxygen source | Water |
| CO₂ fixation enzyme | RuBisCO |
| Calvin cycle inputs | CO₂, ATP and NADPH |
📝 NEET Practice MCQs
1. The light-dependent reactions of photosynthesis occur mainly in:
A. Cytoplasm
B. Thylakoid membranes
C. Nucleus
D. Mitochondrial matrix
Answer: B. Thylakoid membranes
2. The reaction centre of Photosystem II is:
A. P700
B. P680
C. P600
D. P760
Answer: B. P680
3. Oxygen released during photosynthesis comes primarily from:
A. Carbon dioxide
B. Glucose
C. Water
D. Chlorophyll
Answer: C. Water
4. The Calvin cycle occurs in the:
A. Thylakoid lumen
B. Stroma
C. Cytoplasm
D. Mitochondrial matrix
Answer: B. Stroma
🎯 NEET Preparation Strategy
For NEET, do not study photosynthesis only as a definition-based topic. Connect the entire process:
After reading this article, revise the corresponding NCERT Biology chapter and practise MCQs based on diagrams, pathways, pigments, photosystems and experimental concepts.
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