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) are located in the thylakoid membranes.

Z-Scheme (Non-cyclic Photophosphorylation)

This represents the non-cyclic, continuous flow of electrons from water to $\text{NADP}^+$ , resulting in the synthesis of both ATP and NADPH.

PS II Excitation & Photolysis: PS II absorbs light ( $680\text{ nm}$ ). Excited electrons are replaced by splitting water: $2\text{H}_2\text{O} \rightarrow 4\text{H}^+ + 4\text{e}^- + \text{O}_2 \uparrow$
Electron Transport Chain (ETC): Electrons are accepted by a primary acceptor (Pheophytin) and passed through Plastoquinone (PQ), Cytochrome $b_6f$ , and Plastocyanin (PC) to PS I.
PS I Excitation & NADPH Production: PS I ( $700\text{ nm}$ ) excites its electrons, which are passed via Ferredoxin (Fd) to $\text{NADP}^+$ Reductase to generate $\text{NADPH} + \text{H}^+$ .

Cyclic Photophosphorylation

Occurs in the stroma lamellae (which lack PS II and $\text{NADP}^+$ reductase).
Electrons excited from PS I (P700) flow through Ferredoxin $\rightarrow$ Plastoquinone $\rightarrow$ Cytochrome $b_6f$ $\rightarrow$ Plastocyanin $\rightarrow$ PS I.
Results solely in ATP synthesis (no NADPH or $O_2$ production).

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 the stroma of all photosynthetic plants and consists of three stages:

Carboxylation: Carbon dioxide is fixed into 3-phosphoglyceric acid (3-PGA) by RuBisCO reacting with RuBP (5C). $\text{RuBP (5C)} + \text{CO}_2 + \text{H}_2\text{O} \overset{\text{RuBisCO}}{\longrightarrow} 2 \times \text{3-Phosphoglycerate (3C)}$
Reduction: Series of reactions using 2 ATP (for phosphorylation) and 2 NADPH (for reduction) per fixed $\text{CO}_2$ to form Glyceraldehyde-3-phosphate (G3P).
Regeneration: RuBP acceptance is regenerated using 1 ATP.

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

The C4 Pathway (Hatch and Slack Pathway)

Found in tropical plants (e.g., maize, sugarcane, sorghum). It bypasses photorespiration by physically separating initial carbon capture from the Calvin Cycle.

Kranz Anatomy: Mesophyll cells capture $\text{CO}_2$ as oxaloacetic acid (OAA, 4C) using PEP carboxylase. Malate is then transported to the Bundle Sheath cells.
Decarboxylation: Malate is broken down in bundle sheath cells to release $\text{CO}_2$ locally, which then saturates RuBisCO and feeds into the Calvin Cycle.
PEP Regeneration: Pyruvate is sent back to mesophyll cells, where it requires the equivalent of 2 ATP to regenerate PEP.
Total Energy for C4 Cycle: Requires 30 ATP and 12 NADPH per glucose molecule (additional 12 ATP are consumed to regenerate PEP compared to C3).

Photorespiration (C2 Cycle)

A wasteful process in $C_3$ plants that occurs when $\text{O}_2$ binds to RuBisCO instead of $\text{CO}_2$ , requiring the cooperation of three organelles:

Chloroplast: $\text{RuBP} + \text{O}_2 \rightarrow \text{PGA} + \text{2-Phosphoglycolate}$ . Glycolate is exported.
Peroxisome: Glycolate is oxidized to Glyoxylate, then transaminated to Glycine.
Mitochondria: Two molecules of Glycine condense to form one Serine, releasing $\text{CO}_2 \uparrow$ and $\text{NH}_3 \uparrow$ .

No ATP or NADPH is synthesized. In fact, ATP is consumed during Serine/Glycerate recovery, making it highly wasteful.

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.

Class 11 Biology - Photosynthesis in Higher Plants

Notes

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) are located in the thylakoid membranes.

Z-Scheme (Non-cyclic Photophosphorylation)

This represents the non-cyclic, continuous flow of electrons from water to $\text{NADP}^+$ , resulting in the synthesis of both ATP and NADPH.

PS II Excitation & Photolysis: PS II absorbs light ( $680\text{ nm}$ ). Excited electrons are replaced by splitting water: $2\text{H}_2\text{O} \rightarrow 4\text{H}^+ + 4\text{e}^- + \text{O}_2 \uparrow$
Electron Transport Chain (ETC): Electrons are accepted by a primary acceptor (Pheophytin) and passed through Plastoquinone (PQ), Cytochrome $b_6f$ , and Plastocyanin (PC) to PS I.
PS I Excitation & NADPH Production: PS I ( $700\text{ nm}$ ) excites its electrons, which are passed via Ferredoxin (Fd) to $\text{NADP}^+$ Reductase to generate $\text{NADPH} + \text{H}^+$ .

Cyclic Photophosphorylation

Occurs in the stroma lamellae (which lack PS II and $\text{NADP}^+$ reductase).
Electrons excited from PS I (P700) flow through Ferredoxin $\rightarrow$ Plastoquinone $\rightarrow$ Cytochrome $b_6f$ $\rightarrow$ Plastocyanin $\rightarrow$ PS I.
Results solely in ATP synthesis (no NADPH or $O_2$ production).

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.

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 the stroma of all photosynthetic plants and consists of three stages:

Carboxylation: Carbon dioxide is fixed into 3-phosphoglyceric acid (3-PGA) by RuBisCO reacting with RuBP (5C). $\text{RuBP (5C)} + \text{CO}_2 + \text{H}_2\text{O} \overset{\text{RuBisCO}}{\longrightarrow} 2 \times \text{3-Phosphoglycerate (3C)}$
Reduction: Series of reactions using 2 ATP (for phosphorylation) and 2 NADPH (for reduction) per fixed $\text{CO}_2$ to form Glyceraldehyde-3-phosphate (G3P).
Regeneration: RuBP acceptance is regenerated using 1 ATP.

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

The C4 Pathway (Hatch and Slack Pathway)

Found in tropical plants (e.g., maize, sugarcane, sorghum). It bypasses photorespiration by physically separating initial carbon capture from the Calvin Cycle.

Kranz Anatomy: Mesophyll cells capture $\text{CO}_2$ as oxaloacetic acid (OAA, 4C) using PEP carboxylase. Malate is then transported to the Bundle Sheath cells.
Decarboxylation: Malate is broken down in bundle sheath cells to release $\text{CO}_2$ locally, which then saturates RuBisCO and feeds into the Calvin Cycle.
PEP Regeneration: Pyruvate is sent back to mesophyll cells, where it requires the equivalent of 2 ATP to regenerate PEP.
Total Energy for C4 Cycle: Requires 30 ATP and 12 NADPH per glucose molecule (additional 12 ATP are consumed to regenerate PEP compared to C3).

Photorespiration (C2 Cycle)

A wasteful process in $C_3$ plants that occurs when $\text{O}_2$ binds to RuBisCO instead of $\text{CO}_2$ , requiring the cooperation of three organelles:

Chloroplast: $\text{RuBP} + \text{O}_2 \rightarrow \text{PGA} + \text{2-Phosphoglycolate}$ . Glycolate is exported.
Peroxisome: Glycolate is oxidized to Glyoxylate, then transaminated to Glycine.
Mitochondria: Two molecules of Glycine condense to form one Serine, releasing $\text{CO}_2 \uparrow$ and $\text{NH}_3 \uparrow$ .

No ATP or NADPH is synthesized. In fact, ATP is consumed during Serine/Glycerate recovery, making it highly wasteful.

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.

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)

Z-Scheme (Non-cyclic Photophosphorylation)

Cyclic Photophosphorylation

Chemiosmotic Hypothesis

The Calvin Cycle (C3 Pathway)

The C4 Pathway (Hatch and Slack Pathway)

Photorespiration (C2 Cycle)

Factors affecting Photosynthesis

On this page

BioBot

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)

Z-Scheme (Non-cyclic Photophosphorylation)

Cyclic Photophosphorylation

Chemiosmotic Hypothesis

The Calvin Cycle (C3 Pathway)

The C4 Pathway (Hatch and Slack Pathway)

Photorespiration (C2 Cycle)

Factors affecting Photosynthesis

On this page