Photosynthesis in Higher Plants Revision Notes, Key Concepts & Important Questions

Photosynthesis in Higher Plants, explains how green plants convert light energy into chemical energy to produce food. The chapter covers the structure and role of chloroplasts, the pigments involved in photosynthesis, the light reactions, photophosphorylation, and the Calvin cycle. It also discusses the C3 and C4 pathways, photorespiration, and factors affecting the rate of photosynthesis.

These revision notes on Photosynthesis in Higher Plants, prepared by ALLEN’s expert faculty, offer a clear summary of the chapter’s essential concepts, processes, and mechanisms. Designed for quick and efficient revision, they help students reinforce key ideas, improve conceptual clarity, and prepare effectively for board and competitive exams.

1.0Concept-Wise Important Revision Notes

1. What is photosynthesis in higher plants?

Photosynthesis is the process by which light energy is converted into chemical energy. In this process, plants synthesise organic compounds from carbon dioxide and water using light energy.

It is a light-driven oxidation–reduction (redox) reaction in which:

• Water is oxidised, releasing oxygen and hydrogen ions
• Carbon dioxide is reduced to form organic molecules or carbohydrates

Photosynthesis is important because:

• It is the primary source of food on earth
• It releases oxygen into the atmosphere

Organisms that perform photosynthesis are called autotrophs since they synthesise glucose from carbon dioxide and water using sunlight.

2. Where does photosynthesis take place in plants?

Photosynthesis mainly occurs in the green leaves, particularly in the mesophyll cells which contain numerous chloroplasts.

Structure and functions inside chloroplast:

• Grana and thylakoid membranes – trap light energy and produce ATP and NADPH (light reactions).
• Stroma – contains enzymes that synthesise sugars which may later form starch (dark reactions).

Chloroplasts usually align along the cell walls of mesophyll cells to receive optimum light.

3. What are the pigments involved in photosynthesis?

Pigments are substances that absorb light at specific wavelengths.

Major leaf pigments include:

• Chlorophyll a – most abundant pigment (blue-green)
• Chlorophyll b – yellow-green pigment
• Xanthophylls – yellow pigment
• Carotenoids – yellow to orange pigments

These pigments are organised into light-harvesting complexes (LHC) in Photosystem I (PS I) and Photosystem II (PS II).

Key points:

• Antenna pigments capture light energy.
• Reaction centre contains one chlorophyll a molecule.
• Reaction centre wavelengths:
• P700 in PS I
• P680 in PS II

Accessory pigments broaden the range of absorbed wavelengths and protect chlorophyll from photo-oxidation.

4. What is the light reaction of photosynthesis?

The light reaction is the first stage of photosynthesis where solar energy is converted into chemical energy.

Main events of light reaction:

• Light absorption by pigments
• Splitting of water (photolysis)
• Release of oxygen
• Formation of ATP and NADPH

Since these reactions are driven directly by light, they are called photochemical reactions.

5. What is photophosphorylation and the Electron Transport System (ETS)?

Photophosphorylation is the synthesis of ATP from ADP and inorganic phosphate in the presence of light during the light reaction of photosynthesis.

The Electron Transport System (ETS) is a chain of electron carriers that transfer electrons released from photosystems and help generate energy for ATP formation.

Two types of photophosphorylation occur:

1. Non-cyclic photophosphorylation

• Both PS II and PS I are involved.
• Electrons move from PS II → electron transport chain → PS I.
• Electrons ultimately reduce NADP⁺ to NADPH + H⁺.
• Both ATP and NADPH are produced.
• Photolysis of water replaces electrons lost from PS II.

2. Cyclic photophosphorylation

• Only PS I participates.
• Excited electrons from P700 return to the same photosystem through the electron transport chain.
• Only ATP is produced.
• No NADPH formation and no photolysis of water occur.
• Often occurs when light of wavelength beyond 680 nm is available.

Thus, the electron transport system transfers electrons between photosystems and generates energy that drives ATP synthesis.

6. What is the chemiosmotic hypothesis in photosynthesis?

The chemiosmotic hypothesis explains ATP synthesis through a proton gradient across the thylakoid membrane.

Requirements for chemiosmosis:

• Membrane
• Proton pump
• Proton gradient
• ATP synthase enzyme

Formation of proton gradient occurs due to:

• Photolysis of water releasing protons into the lumen
• Electron transport chain transporting protons across the membrane
• Reduction of NADP⁺ removing protons from the stroma

ATP synthesis mechanism:

• Protons move from lumen to stroma through ATP synthase.
• Energy released drives ATP formation.

ATP synthase has two components:

• CF₀ – forms proton channel in the membrane
• CF₁ – catalyses ATP formation in the stroma.

7. Where are ATP and NADPH used in photosynthesis?

ATP and NADPH produced in light reactions are used in carbon dioxide fixation to synthesise carbohydrates.

General reaction:

NADPH + ATP + CO₂ → (CH₂O)ₙ + NADP⁺ + ADP + iP

These reactions regenerate:

• NADP⁺
• ADP
• Inorganic phosphate

This recycling is essential because the supply of these metabolites is limited.

8. Explain the stages of the Calvin cycle.

The Calvin cycle occurs in the stroma and consists of three stages:

1. Carboxylation

• CO₂ combines with RuBP (5-carbon compound)
• Catalysed by RuBisCO
• Produces two molecules of PGA

2. Reduction

• PGA is reduced using ATP and NADPH
• Produces G3P (glyceraldehyde-3-phosphate)

3. Regeneration of RuBP

• Some G3P molecules regenerate RuBP
• Enables the cycle to continue.

G3P can also form:

• glucose
• fructose
• sucrose
• starch
• cellulose.

9. What is the C4 pathway (Hatch and Slack pathway)?

The C4 pathway occurs in plants adapted to dry tropical regions.

Characteristics of C4 plants:

• Tolerate high temperatures
• Adapted to high light intensity
• Show higher biomass productivity
• Photorespiration is absent

The first stable compound formed is oxaloacetic acid (4-carbon).

C4 plants show Kranz anatomy where:

• Bundle sheath cells surround vascular bundles
• Chloroplasts are dimorphic
• Mesophyll chloroplasts contain grana
• Bundle sheath chloroplasts lack grana and contain starch.

10. What are the steps involved in the C4 (Hatch and Slack) pathway?

The C4 pathway involves four main steps:

1. Carboxylation

• Occurs in mesophyll chloroplasts
• PEP + CO₂ → Oxaloacetate
• Enzyme: PEP carboxylase

2. Breakdown

• Oxaloacetate forms malate or aspartate
• These move to bundle sheath cells.

3. Splitting

• Malate/aspartate release CO₂
• CO₂ enters Calvin cycle
• Pyruvate is produced.

4. Phosphorylation

• Pyruvate returns to mesophyll cells
• Converted back to PEP using ATP
• Enzyme: pyruvate phosphodikinase.

Also Read: C3 and C4 pathways

11. What is photorespiration?

Photorespiration occurs when RuBisCO binds oxygen instead of carbon dioxide.

Process:

• RuBP reacts with O₂
• Produces phosphoglycerate and phosphoglycolate

Characteristics:

• No synthesis of sugars
• No ATP production
• CO₂ is released
• ATP is consumed.

Photorespiration occurs in C3 plants but does not occur in C4 plants because they maintain high CO₂ concentration around RuBisCO.

12. What are the factors affecting photosynthesis?

There are various internal and external factors affecting photosynthesis. Lets discuss in detail.

Internal factors

• Number and size of leaves
• Orientation and age of leaves
• Mesophyll cells and chloroplasts
• Internal CO₂ concentration
• Amount of chlorophyll

External factors

• Light intensity and duration
• Carbon dioxide concentration
• Water availability
• Temperature

The rate of photosynthesis is controlled by Blackman’s Law of Limiting Factors, which states that when several factors affect a process, the rate is determined by the factor that is nearest to its minimum value.

2.0Memory Tricks

Products of Light Reaction: “OAN”

• O – Oxygen
• A – ATP
• N – NADPH

Chemiosmosis Requirements: “MPGA”

• M – Membrane
• P – Proton pump
• G – Proton gradient
• A – ATP synthase

Calvin Cycle Stages: “CRR”

• C – Carboxylation
• R – Reduction
• R – Regeneration

3.0Previous Year Questions on Photosynthesis in Higher Plants

Question: Which micronutrient is required for splitting of the water molecule during photosynthesis?

1. Molybdenum
2. Magnesium
3. Copper
4. Manganese

Answer: Manganese

Question: How many ATP and NADPH₂ molecules are required for the synthesis of one molecule of glucose during the Calvin cycle?

1. 18 ATP and 12 NADPH₂
2. 12 ATP and 16 NADPH₂
3. 18 ATP and 16 NADPH₂
4. 12 ATP and 12 NADPH₂

Answer: 18 ATP and 12 NADPH₂

Question: The first stable product of CO₂ fixation in sorghum (a C₄ plant) is:

1. Pyruvic acid
2. Oxaloacetic acid
3. Succinic acid
4. Phosphoglyceric acid

Answer: Oxaloacetic acid