Class 11 Biology - Photosynthesis in Higher Plants

Key Concepts

Introduction

Definition: A physico-chemical process by which green plants use light energy to drive the synthesis of organic compounds (food).
Importance: It is the primary source of all food on earth and is responsible for the release of oxygen into the atmosphere.

Early Experiments

Joseph Priestley (1770): Revealed the essential role of air in the growth of green plants. Plants restore to the air whatever breathing animals and burning candles remove.
Jan Ingenhousz (1779): Showed that sunlight is essential for the plant process that purifies air. Green parts of plants release oxygen in the presence of sunlight.
Julius von Sachs (1854): Provided evidence for production of glucose in green parts, which is stored as starch.
T.W. Engelmann (1843-1909): Described the first action spectrum of photosynthesis using a prism, green alga (Cladophora), and aerobic bacteria. Red and blue light were most effective.
Cornelius van Niel (1897-1985): Demonstrated that photosynthesis is a light-dependent reaction where hydrogen from an oxidisable compound reduces CO₂ to carbohydrates. In plants, H₂O is the hydrogen donor and is oxidised to O₂.

Where does Photosynthesis take place?

Occurs in the Chloroplasts of mesophyll cells in leaves.
Grana (Membrane system): Responsible for trapping light energy and synthesis of ATP and NADPH (Light Reaction).
Stroma (Fluid matrix): Enzymatic reactions incorporate CO₂ into the plant leading to sugar synthesis (Dark Reaction).

Pigments involved in Photosynthesis

Chlorophyll a: Bright or blue-green; the chief pigment associated with photosynthesis.
Chlorophyll b: Yellow-green.
Xanthophylls: Yellow.
Carotenoids: Yellow to yellow-orange.
Accessory Pigments: Chl b, xanthophylls, and carotenoids absorb light and transfer energy to Chl a. They protect Chl a from photo-oxidation and broaden the range of light wavelengths utilised.

Light Reaction (Photochemical Phase)

Includes light absorption, water splitting, oxygen release, and formation of ATP and NADPH.
Photosystems: PS I (Reaction centre P700) and PS II (Reaction centre P680).
Z-Scheme (Non-cyclic Photophosphorylation):
1. Electrons excited in PS II are picked up by an acceptor and passed through the ETS (Cytochromes) to PS I.
2. Splitting of water ( $2H_2O \rightarrow 4H^+ + O_2 + 4e^-$ ) provides electrons to PS II.
3. PS I electrons are excited and passed to NADP⁺, reducing it to NADPH.
Cyclic Photophosphorylation: Occurs when only PS I is functional (in stroma lamellae). Results only in ATP synthesis.

Chemiosmotic Hypothesis

Explains ATP synthesis linked to a proton gradient across the thylakoid membrane.
Protons accumulate in the lumen due to water splitting and the H-carrier in ETS.

Proton Gradient Direction In chloroplasts, protons accumulate inside the thylakoid lumen, whereas in mitochondria, they accumulate in the intermembrane space. The breaking of this gradient provides the energy for ATP synthesis.

The gradient is broken down as protons move through the CF₀-CF₁ ATPase channel to the stroma, providing energy to synthesize ATP.

The Calvin Cycle (C3 Pathway)

Occurs in all photosynthetic plants in three stages:

Carboxylation: Fixation of CO₂ into 3-phosphoglyceric acid (PGA) by the enzyme RuBisCO using RuBP (5C).
Reduction: Series of reactions using 2 ATP and 2 NADPH per CO₂ to form glucose.
Regeneration: RuBP is regenerated using 1 ATP.

Total for 1 Glucose (6 turns): 18 ATP and 12 NADPH.

The C4 Pathway (Hatch and Slack Pathway)

Found in tropical plants (e.g., Maize, Sorghum).
Kranz Anatomy: Large bundle sheath cells around vascular bundles with many chloroplasts.
Mechanism: CO₂ fixed as oxaloacetic acid (OAA, 4C) in mesophyll by PEPcase. OAA is converted to malic acid, transported to bundle sheath, and broken down to release CO₂ for the Calvin Cycle.
Advantage: Lack of photorespiration; high productivity and tolerance to high temperatures.

Photorespiration

A wasteful process in C3 plants where RuBisCO binds with O₂ instead of CO₂.
Results in the release of CO₂ with the use of ATP; no synthesis of sugar or ATP/NADPH.

Factors affecting Photosynthesis

Blackman’s Law of Limiting Factors: The rate is determined by the factor nearest its minimal value.
Light: Saturation point is 10% of full sunlight. High intensity can cause chlorophyll breakdown.
CO₂ Concentration: Major limiting factor. C4 plants saturate at 360 µL/L; C3 plants at 450 µL/L.
Temperature: Enzymatic dark reactions are more sensitive. C4 plants have higher optimum.
Water: Stress causes stomata closure and leaf wilting, indirectly reducing photosynthesis. nthesis.

Key Concepts

Definition: A physico-chemical process by which green plants use light energy to drive the synthesis of organic compounds (food).

Importance: It is the primary source of all food on earth and is responsible for the release of oxygen into the atmosphere.

Joseph Priestley (1770): Revealed the essential role of air in the growth of green plants. Plants restore to the air whatever breathing animals and burning candles remove.

Jan Ingenhousz (1779): Showed that sunlight is essential for the plant process that purifies air. Green parts of plants release oxygen in the presence of sunlight.

Julius von Sachs (1854): Provided evidence for production of glucose in green parts, which is stored as starch.

T.W. Engelmann (1843-1909): Described the first action spectrum of photosynthesis using a prism, green alga (Cladophora), and aerobic bacteria. Red and blue light were most effective.

Cornelius van Niel (1897-1985): Demonstrated that photosynthesis is a light-dependent reaction where hydrogen from an oxidisable compound reduces CO₂ to carbohydrates. In plants, H₂O is the hydrogen donor and is oxidised to O₂.

Occurs in the Chloroplasts of mesophyll cells in leaves.

Grana (Membrane system): Responsible for trapping light energy and synthesis of ATP and NADPH (Light Reaction).

Stroma (Fluid matrix): Enzymatic reactions incorporate CO₂ into the plant leading to sugar synthesis (Dark Reaction).

Chlorophyll a: Bright or blue-green; the chief pigment associated with photosynthesis.

Chlorophyll b: Yellow-green.

Xanthophylls: Yellow.

Carotenoids: Yellow to yellow-orange.

Accessory Pigments: Chl b, xanthophylls, and carotenoids absorb light and transfer energy to Chl a. They protect Chl a from photo-oxidation and broaden the range of light wavelengths utilised.

Includes light absorption, water splitting, oxygen release, and formation of ATP and NADPH.

Photosystems: PS I (Reaction centre P700) and PS II (Reaction centre P680).

Z-Scheme (Non-cyclic Photophosphorylation):

Electrons excited in PS II are picked up by an acceptor and passed through the ETS (Cytochromes) to PS I.
Splitting of water ( $2H_2O \rightarrow 4H^+ + O_2 + 4e^-$ ) provides electrons to PS II.
PS I electrons are excited and passed to NADP⁺, reducing it to NADPH.

Cyclic Photophosphorylation: Occurs when only PS I is functional (in stroma lamellae). Results only in ATP synthesis.

Explains ATP synthesis linked to a proton gradient across the thylakoid membrane.

Protons accumulate in the lumen due to water splitting and the H-carrier in ETS.

The gradient is broken down as protons move through the CF₀-CF₁ ATPase channel to the stroma, providing energy to synthesize ATP.

Occurs in all photosynthetic plants in three stages:

Carboxylation: Fixation of CO₂ into 3-phosphoglyceric acid (PGA) by the enzyme RuBisCO using RuBP (5C).

Reduction: Series of reactions using 2 ATP and 2 NADPH per CO₂ to form glucose.

Regeneration: RuBP is regenerated using 1 ATP.

Total for 1 Glucose (6 turns): 18 ATP and 12 NADPH.

Found in tropical plants (e.g., Maize, Sorghum).

Kranz Anatomy: Large bundle sheath cells around vascular bundles with many chloroplasts.

Mechanism: CO₂ fixed as oxaloacetic acid (OAA, 4C) in mesophyll by PEPcase. OAA is converted to malic acid, transported to bundle sheath, and broken down to release CO₂ for the Calvin Cycle.

Advantage: Lack of photorespiration; high productivity and tolerance to high temperatures.

A wasteful process in C3 plants where RuBisCO binds with O₂ instead of CO₂.

Results in the release of CO₂ with the use of ATP; no synthesis of sugar or ATP/NADPH.

Blackman’s Law of Limiting Factors: The rate is determined by the factor nearest its minimal value.

Light: Saturation point is 10% of full sunlight. High intensity can cause chlorophyll breakdown.

CO₂ Concentration: Major limiting factor. C4 plants saturate at 360 µL/L; C3 plants at 450 µL/L.

Temperature: Enzymatic dark reactions are more sensitive. C4 plants have higher optimum.

Water: Stress causes stomata closure and leaf wilting, indirectly reducing photosynthesis. nthesis.

Class 11 Biology - Photosynthesis in Higher Plants

Class 11 Biology - Photosynthesis in Higher Plants

Notes

Key Concepts

Introduction

Early Experiments

Where does Photosynthesis take place?

Pigments involved in Photosynthesis

Light Reaction (Photochemical Phase)

Chemiosmotic Hypothesis

The Calvin Cycle (C3 Pathway)

The C4 Pathway (Hatch and Slack Pathway)

Photorespiration

Factors affecting Photosynthesis

On this page

Class 11 Biology - Photosynthesis in Higher Plants

Class 11 Biology - Photosynthesis in Higher Plants

Notes

Key Concepts

Introduction

Early Experiments

Where does Photosynthesis take place?

Pigments involved in Photosynthesis

Light Reaction (Photochemical Phase)

Chemiosmotic Hypothesis

The Calvin Cycle (C3 Pathway)

The C4 Pathway (Hatch and Slack Pathway)

Photorespiration

Factors affecting Photosynthesis

On this page