NSEJS Biology: Plant Physiology Photosynthesis

Photosynthesis is one of the most important topics in the Plant Physiology section of NSEJS Biology. Questions from this chapter test your understanding of concepts, experiments, plant adaptations, and biological processes. If your basics are clear, you can solve both direct and application-based questions with confidence.

Here, you will learn how photosynthesis takes place, where it occurs, the role of chloroplasts, the light and dark reactions, important experiments, different photosynthetic pathways, and the factors that affect the rate of photosynthesis.

Also Read: 

• NSEJS Syllabus
• NSEJS Previous Question Papers

What Is Photosynthesis?

Photosynthesis is the process through which green plants prepare food by using sunlight. During this process, plants convert light energy into chemical energy and store it in the form of carbohydrates.

Plants use carbon dioxide from the atmosphere and water absorbed from the soil. Chlorophyll captures sunlight and helps drive this process.

The overall reaction is:

Carbon Dioxide + Water + Sunlight + Chlorophyll → Carbohydrate + Oxygen

Photosynthesis is important because it:

• Produces food for plants.

• Forms the base of every food chain.

• Releases oxygen into the atmosphere.

• Stores solar energy in the form of chemical energy.

Although leaves perform most of the photosynthesis, other green parts of a plant, such as young stems, can also carry out this process.

Where Does Photosynthesis Take Place?

Photosynthesis takes place inside the chloroplast, an organelle found in plant cells. A chloroplast contains the following parts:

• Outer membrane

• Inner membrane

• Stroma

• Thylakoids

• Grana

• Stroma lamellae

Each part has a specific role.

Grana

Grana are stacks of thylakoids. They contain chlorophyll and other pigments that absorb sunlight. The light reaction takes place here.

Stroma

The stroma is the fluid-filled region around the grana. It contains enzymes required for sugar formation. The dark reaction or Calvin cycle, takes place in the stroma.

Light Reaction and Dark Reaction

Photosynthesis occurs in two stages.

Light Reaction

The light reaction requires sunlight. It takes place on the thylakoid membrane inside the grana.

During this stage:

• Chlorophyll absorbs light.

• Water molecules split.

• Oxygen is released.

• ATP is produced.

• NADPH is formed.

ATP and NADPH provide energy for the next stage.

Dark Reaction

The dark reaction takes place in the stroma. It does not directly require sunlight. Instead, it uses ATP and NADPH produced during the light reaction. In this stage, carbon dioxide is converted into carbohydrates.

Important Experiments in Photosynthesis

Several classic experiments explain how photosynthesis works. These experiments are frequently asked in NSEJS Biology.

1. Variegated Leaf Experiment

A part of the leaf is covered so that it does not receive sunlight. After the starch test:

• The covered region does not contain starch.

• The exposed region contains starch.

Conclusion: Light is necessary for photosynthesis.

2. Moll's Half Leaf Experiment

Part of a leaf is placed inside a tube containing potassium hydroxide. Potassium hydroxide absorbs carbon dioxide.

After the starch test:

• The portion without carbon dioxide does not form starch.

• The exposed portion forms starch.

Conclusion: Carbon dioxide is essential for photosynthesis.

3. Priestley's Bell Jar Experiment

Joseph Priestley placed a mouse and a burning candle inside a sealed jar. Both survived longer when a green plant was present.

Conclusion: Green plants release oxygen during photosynthesis.

4. Engelmann's Experiment

Theodor Engelmann passed different colours of light through a prism onto green algae. Aerobic bacteria gathered mainly near the blue and red regions.

Conclusion: Photosynthesis is highest in blue and red light.

5. Van Niel's Hypothesis

Van Niel studied photosynthetic bacteria. He proposed that water supplies hydrogen during photosynthesis in green plants. He also showed that the oxygen released during photosynthesis comes from water.

Photosynthetic Pigments

Plants contain different pigments that absorb light.

Chlorophyll a

• Primary photosynthetic pigment

• Reaction centre pigment

• Bright green in colour

Chlorophyll b

• Accessory pigment

• Transfers absorbed energy to chlorophyll a

• Yellowish-green in colour

Carotenoids and Xanthophylls

These pigments:

• Absorb additional wavelengths of light.

• Transfer energy to chlorophyll a.

• Protect chlorophyll from excessive light damage.

Together, these pigments form a photosystem.

Photosystem I and Photosystem II

Two photosystems participate in the light reaction.

Photosystem II (P680)

• Absorbs light best at 680 nm.

• Functions first during the light reaction.

• Participates only in non-cyclic photophosphorylation.

Photosystem I (P700)

• Absorbs light best at 700 nm.

• Participates in both cyclic and non-cyclic photophosphorylation.

Cyclic and Non-Cyclic Photophosphorylation

Photophosphorylation is the formation of ATP using light energy.

Cyclic Photophosphorylation

Characteristics include:

• Only Photosystem I participates.

• Electrons return to the same photosystem.

• Only ATP is produced.

• NADPH and oxygen are not produced.

Non-Cyclic Photophosphorylation

Characteristics include:

• Both Photosystem II and Photosystem I participate.

• Water undergoes photolysis.

• ATP is produced.

• NADPH is formed.

• Oxygen is released.

The movement of electrons follows the Z-scheme.

Chemiosmosis and ATP Formation

ATP is produced through chemiosmosis. During the light reaction:

• Protons accumulate inside the thylakoid lumen.

• This creates a proton gradient.

• Protons move through ATP synthase.

• ATP synthase converts ADP into ATP.

This process explains how energy from electron transport is converted into chemical energy.

Calvin Cycle (C3 Pathway)

The Calvin cycle is also called the C3 pathway. It takes place in the stroma. The first stable product is 3-phosphoglyceric acid (3-PGA).

The cycle has three stages.

• Carboxylation: Carbon dioxide combines with RuBP. The reaction is catalysed by RuBisCO.

• Reduction: ATP and NADPH convert 3-PGA into G3P.

• Regeneration: Most G3P regenerates RuBP. The remaining G3P is used to form glucose. All green plants perform the Calvin cycle.

C4 Pathway (Hatch and Slack Pathway)

Some plants grow in hot and dry environments. Examples include:

• Maize

• Sugarcane

These plants use the C4 pathway.

The first stable product is oxaloacetic acid, a four-carbon compound.

Kranz Anatomy

C4 plants possess Kranz anatomy. Their leaves contain:

• Mesophyll cells

• Bundle sheath cells

Carbon dioxide is first fixed in mesophyll cells. It is then transported to bundle sheath cells, where the Calvin cycle occurs.

Advantages of C4 Plants

C4 plants:

• Reduce water loss.

• Minimise photorespiration.

• Perform better at high temperatures.

• Show higher photosynthetic efficiency under intense sunlight.

Photorespiration (C2 Cycle)

Photorespiration occurs mainly in C3 plants. It begins when RuBisCO reacts with oxygen instead of carbon dioxide.

This process:

• Produces phosphoglycolate.

• Consumes ATP.

• Does not produce sugar.

• Reduces photosynthetic efficiency.

Photorespiration involves:

• Chloroplast

• Peroxisome

• Mitochondrion

C4 plants largely avoid this process because they maintain a high carbon dioxide concentration around RuBisCO.

CAM Pathway

CAM stands for Crassulacean Acid Metabolism. It is found in many desert plants and succulents.

In CAM plants:

• Stomata open at night.

• Carbon dioxide is stored as organic acids.

• During the day, stomata remain closed.

• Stored carbon dioxide is used for the Calvin cycle.

This adaptation helps plants conserve water.

Factors Affecting the Rate of Photosynthesis

Several internal and external factors influence the rate of photosynthesis.

Internal Factors

These include:

• Leaf age

• Leaf size

• Number of chloroplasts

• Number of mesophyll cells

• Leaf orientation

• Internal carbon dioxide concentration

External Factors

The major external factors are:

• Light intensity

• Light quality

• Duration of light

• Carbon dioxide concentration

• Temperature

• Water availability

Blackman's Law of Limiting Factors

Blackman's Law states that when several factors affect a process, the factor available in the least amount limits the overall rate.

For example:

If light intensity is low, increasing carbon dioxide alone will not increase the rate of photosynthesis.

Similarly, if carbon dioxide is limited, increasing light intensity will have little effect.

Understanding this principle helps explain why photosynthesis does not always increase under favourable conditions.

Quick Revision Points for NSEJS

• Photosynthesis converts light energy into chemical energy, producing carbohydrates and releasing oxygen.

• The light reaction occurs in the grana, and the dark reaction occurs in the stroma.

• Chlorophyll a is the primary pigment; chlorophyll b, carotenoids, and xanthophylls are accessory pigments.

• Photosystem II participates only in non-cyclic photophosphorylation, while Photosystem I participates in both cyclic and non-cyclic pathways.

• Cyclic photophosphorylation produces only ATP; non-cyclic photophosphorylation produces ATP, NADPH, and oxygen.

• Chemiosmosis explains ATP formation through a proton gradient across the thylakoid membrane.

• C3 pathway is universal to all plants; C4 pathway is found in plants adapted to hot, dry conditions.

• Photorespiration is a loss process seen mainly in C3 plants, involving the chloroplast, peroxisome, and mitochondrion.

• CAM plants open stomata at night to conserve water in arid conditions.

• Blackman's Law of Limiting Factors decides which factor controls the overall rate of photosynthesis.

Photosynthesis is one of the foundation topics in NSEJS Biology Plant Physiology. If you understand the structure of the chloroplast, the light and dark reactions, important experiments, photosynthetic pathways, and limiting factors, you will be able to answer a wide range of questions with confidence.

While revising, focus on comparing C3, C4, and CAM pathways. Also practise questions based on experiments and chloroplast structure. Regular revision of these concepts will strengthen your preparation for the NSEJS examination.