Unit 2

Plant Physiology

Chapter 2.2

Photosynthesis

Class 12 Biology – Photosynthesis Notes PDF

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Introduction

Definition Photosynthesis is the synthesis of carbohydrates by the green parts of plants in the presence of light with the help of carbon dioxide and water. It is an anabolic, endothermic and reductive process.

6CO₂ + 12H₂O — light + chlorophyll → C₆H₁₂O₆ + 6H₂O + 6O₂

Photosynthesis converts light energy into chemical energy and stores that energy in organic food. It is the basic process by which green plants manufacture carbohydrates and release oxygen.

1. Site of Photosynthesis

The chloroplast is the principal site of photosynthesis in green plant cells. The two major phases of photosynthesis occur in different regions of the chloroplast:

Light reaction takes place mainly on the thylakoid membranes of the grana.
Dark reaction / carbon fixation takes place in the stroma of the chloroplast.

Chlorophyll pigments present in the thylakoid membranes absorb light energy. The energized chlorophyll initiates reactions that convert solar energy into chemical energy.

Remember Grana / thylakoids → light reaction
Stroma → carbon-fixation reactions

2. Structure of Chloroplast

Chloroplasts are green plastids. They are commonly discoidal or lens-shaped and are generally about a few micrometres in size. Each chloroplast is enclosed by a double membrane called the chloroplast envelope.

The internal fluid matrix is called the stroma. The stroma contains enzymes, DNA, RNA, ribosomes and other substances required for chloroplast function. Inside the stroma lies a system of flattened membranous sacs called thylakoids. Stacks of thylakoids form grana, while intergranal or stromal lamellae connect different grana.

Fig. 1 – Labelled Structure of a Chloroplast

Redrawn to follow the same labelled chloroplast structure shown in the handwritten PDF.

Part	Main Role
Chloroplast envelope	Double membrane surrounding the chloroplast.
Stroma	Fluid matrix containing enzymes, DNA, RNA and ribosomes; site of carbon-fixation reactions.
Thylakoid	Flattened membrane sac containing chlorophyll and electron-transfer components.
Granum	Stack of thylakoids; major site of the light reaction.
Intergranal lamella	Membranous connection between grana.
Starch grain	Temporary storage of photosynthetic carbohydrate.
Chloroplast DNA and ribosomes	Support limited independent synthesis of some chloroplast proteins.

3. Photosynthetic Pigments

Photosynthetic pigments absorb light energy. The principal pigment of green plants is chlorophyll a. Other pigments act as accessory pigments and broaden the range of light that can be absorbed.

Chlorophyll a

Primary photosynthetic pigment directly involved in the reaction centres.

Chlorophyll b

Accessory pigment that transfers absorbed energy to chlorophyll a.

Carotenoids

Accessory pigments including carotenes and xanthophylls; they also help protect the photosynthetic system from excess light.

Location

These pigments are associated with the thylakoid membranes of chloroplasts.

Fig. 2 – Light Absorption by Photosynthetic Pigments

4. Mechanism of Photosynthesis

Photosynthesis can be understood in two broad stages:

Light Reaction

Requires light directly. Light energy is absorbed and converted into chemical energy in the form of ATP and reducing power. Oxygen is released from water.

Carbon-Fixation Reaction

Uses ATP and reducing power generated by the light reaction to reduce carbon dioxide and form carbohydrate.

Fig. 3 – Overall Mechanism of Photosynthesis

5. Light Reaction

The light reaction takes place on the thylakoid membranes. Chlorophyll absorbs photons and becomes energized. The energy drives electron transfer, formation of ATP and reduction of electron carriers.

Major Events

Absorption of light by photosynthetic pigments.
Excitation of reaction-centre chlorophyll.
Transfer of high-energy electrons through electron carriers.
Photolysis of water and release of oxygen.
Formation of ATP by photophosphorylation.
Formation of reducing power for use in carbon fixation.

Photosystems

Two major photosystems participate in the light reaction: Photosystem II (PS II) and Photosystem I (PS I). Each contains pigment molecules organized around a reaction centre.

Fig. 4 – Simplified Light Reaction / Z-Scheme

Photolysis of Water

In association with PS II, water is split under the influence of light. This process supplies electrons, contributes protons and releases oxygen.

2H₂O → 4H⁺ + 4e⁻ + O₂

6. Photophosphorylation

Definition Formation of ATP from ADP and inorganic phosphate using light energy is called photophosphorylation.

Non-Cyclic Photophosphorylation

Both PS II and PS I participate. Electrons move from water through the photosystems to a final electron acceptor. ATP, reducing power and oxygen are produced.

Cyclic Photophosphorylation

Mainly PS I participates. Excited electrons return to the same photosystem through electron carriers. ATP is produced, but oxygen and reducing power are not generated by this cycle.

Fig. 5 – Cyclic vs Non-Cyclic Photophosphorylation

Feature	Cyclic	Non-Cyclic
Photosystem involved	Mainly PS I	PS II and PS I
Electron path	Returns to reaction centre	Does not return to original chlorophyll
ATP	Produced	Produced
Oxygen release	No	Yes
Reducing power	Not the main product	Produced

7. Dark Reaction / Calvin Cycle

The carbon-fixation reactions occur in the stroma. They do not use light directly, but they depend on ATP and reducing power produced by the light reaction.

Main Steps of the Calvin Cycle

Carboxylation: carbon dioxide combines with RuBP in a reaction catalysed by Rubisco, forming an unstable compound that yields 3-carbon products.
Reduction: ATP and reducing power are used to reduce the carbon compounds and form triose phosphates.
Regeneration: part of the triose phosphate is used to regenerate RuBP so the cycle can continue.
Carbohydrate formation: some triose phosphate leaves the cycle and contributes to glucose and other carbohydrates.

Fig. 6 – Simplified Calvin Cycle

8. Factors Affecting Photosynthesis

External Factors

1. Light

Light intensity, quality and duration affect photosynthetic rate. At very low light, photosynthesis is limited; beyond an optimum, further increase may produce little benefit or cause inhibition.

2. Carbon Dioxide

Carbon dioxide is a raw material for carbon fixation. Increasing its concentration can increase the rate until another factor becomes limiting.

3. Temperature

Photosynthesis depends on enzyme-controlled reactions, so it increases with temperature up to an optimum and then declines when enzymes and membranes are adversely affected.

4. Water

Water is a raw material and also influences stomatal opening. Water shortage causes stomatal closure and can lower photosynthetic rate.

Internal Factors

Chlorophyll content.
Leaf age and condition.
Number and opening of stomata.
Internal carbon dioxide concentration.
Enzyme activity and protoplasmic condition.
Accumulation and transport of photosynthetic products.

Fig. 7 – Effect of Light Intensity on Photosynthetic Rate

Limiting Factor When several factors affect photosynthesis, the rate is controlled by the factor that is least favourable or in shortest supply.

9. Importance of Photosynthesis

Produces organic food that supports nearly all food chains.
Stores solar energy as chemical energy.
Releases oxygen required for aerobic respiration.
Removes carbon dioxide from the atmosphere.
Produces raw materials used to form starch, cellulose, fats, proteins and many other compounds.
Maintains the broad balance between atmospheric oxygen and carbon dioxide.

10. Quick Revision & Exam Points

Important Questions

Define photosynthesis and write its overall equation.
Why is photosynthesis described as anabolic, endothermic and reductive?
Describe the structure of a chloroplast with a labelled diagram.
State the sites of light reaction and carbon-fixation reaction.
Write the functions of grana, thylakoids and stroma.
What are photosynthetic pigments?
Explain the major events of the light reaction.
What is photolysis of water?
Differentiate cyclic and non-cyclic photophosphorylation.
Explain the major steps of the Calvin cycle.
Describe external and internal factors affecting photosynthesis.

One-Minute Revision

Photosynthesis makes carbohydrate from CO₂ and water.
It requires light and photosynthetic pigments.
Chloroplast is the main organelle of photosynthesis.
Grana contain stacks of thylakoids.
Light reaction occurs on thylakoid membranes.
Carbon fixation occurs in the stroma.
Chlorophyll a is the primary pigment.
Photolysis releases oxygen from water.
Photophosphorylation forms ATP using light energy.
Non-cyclic flow involves PS II and PS I.
Cyclic flow mainly involves PS I.
Calvin cycle fixes CO₂.
Light, CO₂, temperature and water affect the rate.

Diagram Practice Practice the chloroplast diagram first. It is the major labelled diagram shown directly in the handwritten PDF. Also practice the light-reaction flow, cyclic/non-cyclic photophosphorylation and Calvin cycle.

Source handling: The definition, equation, photosynthesis site discussion and chloroplast labels were reconstructed closely from the visible handwritten PDF page. Later sections expand the Grade 12 photosynthesis topic into a complete study article while keeping the same note-style structure.