4.1.8 Overall Mechanism of Photosynthesis

Introduction

The process of photosynthesis is a complex series of reactions that convert light energy into chemical energy. It is divided into two main phases: the light-dependent phase (light reaction) and the light-independent phase (dark reaction). Each phase occurs in a specific part of the chloroplast and has specific inputs and outputs.

Definition

Photosynthesis: Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. It is divided into two phases: light-dependent and light-independent.

In simple words: Photosynthesis is the process by which plants make food using sunlight, carbon dioxide, and water.

Core Concept

• The process of photosynthesis has been divided into two phases:
• The light-dependent phase occurs in the thylakoid membranes.
• The light-independent phase occurs in the stroma.
• The light-dependent phase produces ATP and NADPH.
• The light-independent phase uses ATP and NADPH to fix CO₂ into G3P.

Detailed Explanation

Two Phases of Photosynthesis

1. Light-Dependent Phase (Light Reaction)

Definition: The first phase is called the light-dependent phase (light reaction) because it can take place only in the presence of light.

Location: The light-dependent phase occurs in the thylakoid membranes.

• Absorption of light by the photosystems
• Excitation and flow of electrons through an electron transport chain
• Chemiosmotic synthesis of ATP
• Reduction of NADP to NADPH

• Light (energy source)
• H₂O (electron donor)
• ADP + Pi (for ATP synthesis)
• NADP⁺ (for NADPH synthesis)

• ATP (assimilating power)
• NADPH (reducing power)
• O₂ (byproduct)

Summary Equation: 2H₂O + 2NADP⁺ + 3ADP + 3Pi Light→ O₂ + 2NADPH + 3ATP

2. Light-Independent Phase (Dark Reaction)

Definition: The second phase of photosynthesis is called the light-independent phase (dark reaction) because it can take place whether light is present or not.

Location: The light-independent phase occurs in the stroma.

• This phase actually requires the products of light reaction, i.e., ATP and NADPH.
• Since these products are available in the day, therefore, the dark reaction also occurs in the daytime.
• In this phase, CO₂ acts as input which is converted into glyceraldehyde-3-phosphate (G3P), the output of this phase.
• The ATP is hydrolyzed to ADP and Pi (H₃PO₄) and its energy is incorporated in this phase.
• NADPH provides energized electron and hydrogen for the formation of G3P, which is an energy-rich molecule.

• CO₂ (carbon source)
• ATP (energy source)
• NADPH (reducing power)

• G3P (glyceraldehyde-3-phosphate)
• ADP + Pi
• NADP⁺

Summary Equation: 3CO₂ + 6NADPH + 9ATP → G3P + 3H₂O + 9ADP + 9Pi + 6NADP⁺

Overall Mechanism of Photosynthesis

Summary of the Overall Mechanism

• Location: Thylakoid membranes
• Inputs: Light, H₂O, ADP, NADP⁺
• Outputs: ATP, NADPH, O₂
• Function: Convert light energy into chemical energy

• Location: Stroma
• Inputs: CO₂, ATP, NADPH
• Outputs: G3P, ADP, NADP⁺
• Function: Fix CO₂ into organic molecules

Overall Reaction: 6CO₂ + 12H₂O [→Chlorophyll]Light C₆H₁₂O₆ + 6O₂ + 6H₂O

Intermediate Level

Light-Dependent Phase in Detail

• Light absorbed by PS-II (P680).
• Water split (photolysis) → O₂ + H⁺ + e⁻.
• Electrons flow through ETC → ATP produced.
• Electrons reach PS-I (P700).
• Light absorbed by PS-I.
• Electrons passed to NADP⁺ → NADPH.
• ATP and NADPH move to stroma.

Light-Independent Phase in Detail

• Carbon fixation: CO₂ + RuBP → 3-PGA (by RuBisCO).
• Reduction: 3-PGA + ATP + NADPH → G3P.
• Regeneration: G3P → RuBP (using ATP).
• G3P is used to make glucose and other organic compounds.

Comparison of Light and Dark Reactions

Importance of G3P

• G3P is the output of the dark reaction.
• It is an energy-rich molecule.
• It is used to make:
• Glucose
• Fructose
• Sucrose
• Maltose
• Starch
• Cellulose
• Other organic compounds

Important Points

• 1. The process of photosynthesis has been divided into two phases.
• 2. The first phase is called light-dependent phase (light reaction) because it can take place only in the presence of light.
• 3. The light-dependent phase occurs in the thylakoid membranes.
• 4. In this phase, light energy is used to make ATP (assimilating power) and NADPH (reducing power).
• 5. Water and oxygen are supposed to be input and output respectively.
• 6. The second phase of photosynthesis is called the light-independent phase (dark reaction) because it can take place whether light is present or not.
• 7. This phase actually requires the products of light reaction, i.e., ATP and NADPH.
• 8. Since these products are available in day therefore, dark reaction also occurs in day time.
• 9. In this phase, CO₂ acts as input which is converted into glyceraldehyde-3-phosphate (G3P), the output of this phase.
• 10. The ATP is hydrolyzed to ADP and Pi (H₃PO₄) and its energy is incorporated in this phase.
• 11. NADPH provides energized electron and hydrogen for the formation of G3P, which is an energy-rich molecule.

Remember

• - Photosynthesis = two phases: light-dependent and light-independent
• - Light-dependent phase = thylakoid membranes; makes ATP and NADPH
• - Light-independent phase = stroma; makes G3P
• - Light reaction = needs light
• - Dark reaction = does not need light directly
• - Inputs of light reaction: Light, H₂O, ADP, NADP⁺
• - Outputs of light reaction: ATP, NADPH, O₂
• - Inputs of dark reaction: CO₂, ATP, NADPH
• - Outputs of dark reaction: G3P, ADP, NADP⁺
• - G3P = glyceraldehyde-3-phosphate; used to make glucose
• - **ATP** = assimilating power
• - NADPH = reducing power

Common Mistakes

• 1. Thinking the dark reaction occurs only in the dark – The dark reaction occurs during the day as well; it does not require light directly.
• 2. Confusing the locations of the two phases – Light reaction = thylakoid membranes; Dark reaction = stroma.
• 3. Confusing the inputs and outputs – Light reaction uses light, H₂O, ADP, NADP⁺; Dark reaction uses CO₂, ATP, NADPH.
• 4. Thinking ATP and NADPH are produced in the dark reaction – They are produced in the light reaction.
• 5. Thinking CO₂ is used in the light reaction – CO₂ is used in the dark reaction.
• 6. Thinking O₂ is produced in the dark reaction – O₂ is produced in the light reaction.
• 7. Confusing G3P with glucose – G3P is the output of the dark reaction; glucose is made from G3P.
• 8. Forgetting the role of ATP and NADPH – They provide energy and reducing power for the dark reaction.
• 9. Thinking the light reaction only produces ATP – It also produces NADPH and O₂.
• 10. Thinking the dark reaction only produces G3P – It also regenerates ADP and NADP⁺.
• 11. Confusing the overall reaction – 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O.
• 12. Forgetting the role of G3P – G3P is used to make glucose and other organic compounds.

Quick Revision

• The process of photosynthesis has been divided into two phases.
• The first phase is called light dependent phase (light reaction) because it can take place only in the presence of light.
• The light-dependent phase occurs in the thylakoid membranes.
• In this phase light energy is used to make ATP (assimilating power) and NADPH (reducing power); whereas, water and oxygen are supposed to be input and output respectively.
• The second phase of photosynthesis is called the light independent phase (dark reaction) because it can take place whether light is present or not.
• This phase actually requires the products of light reaction i.e., ATP and NADPH.
• Since these products are available in day therefore, dark reaction also occurs in day time.
• In this phase CO₂ acts as input which is converted into glyceraldehyde-3-phosphate (G3P), the output of this phase.
• The ATP is hydrolyzed to ADP and Pi (H₃PO₄) and its energy is incorporated in this phase; whereas, NADPH provides energized electron and hydrogen for the formation of G3P, which is an energy-rich molecule.

60-Second Revision

• Photosynthesis = two phases: light-dependent and light-independent
• Light-dependent phase = thylakoid membranes; makes ATP and NADPH
• Light-independent phase = stroma; makes G3P
• Light reaction = needs light
• Dark reaction = does not need light directly
• Inputs of light reaction: Light, H₂O, ADP, NADP⁺
• Outputs of light reaction: ATP, NADPH, O₂
• Inputs of dark reaction: CO₂, ATP, NADPH
• Outputs of dark reaction: G3P, ADP, NADP⁺
• G3P = glyceraldehyde-3-phosphate; used to make glucose
• **ATP** = assimilating power
• NADPH = reducing power
• Warning: The dark reaction does not occur only in the dark; it occurs during the day as well. Light reaction = thylakoid membranes; Dark reaction = stroma. ATP and NADPH are produced in the light reaction, not the dark reaction. CO₂ is used in the dark reaction, not the light reaction. O₂ is produced in the light reaction. G3P is the output of the dark reaction.