TOPIC 6 PHOTOSYNTHESIS_JANNAH

Chapter 6: Photosynthesis Overview

• Hours: 1 Lecture + 5 Tutorials

Table of Contents

1. Overview of Photosynthesis

2. Absorption Spectrum of Photosynthetic Pigments

3. Light-Dependent Reactions

4. Light-Independent Reactions / Calvin Cycle

5. Alternative Mechanisms of Carbon Fixation: C4 and Crassulacean Acid Metabolism (CAM) pathways

6.1 Overview of Photosynthesis

• Definition: Photosynthesis converts light energy into chemical energy stored in glucose and organic molecules.

• Chemical Equation:

  • 6CO₂ + 6H₂O --> C₆H₁₂O₆ + 6O₂

  • CO₂ (carbon dioxide) + H₂O (water) --> C₆H₁₂O₆ (glucose) + O₂ (oxygen)

• Photosynthesis occurs in chloroplasts, specifically in the green parts of plants.

Complete Process of Photosynthesis

• Stages:

  • Light-Dependent Reactions

    • Occur in the thylakoid membrane of chloroplasts.

    • Require light energy, produce ATP, NADPH, and O₂.

  • Light-Independent Reactions (Calvin Cycle)

    • Occur in the stroma of chloroplasts.

    • Do not directly require light energy; use ATP and NADPH from the light reactions to fix CO₂ into glucose.

6.2 Absorption Spectrum of Photosynthetic Pigments

• Photosynthetic Pigments: Located in thylakoid membranes.

  • Chlorophyll a (P680 and P700)

  • Chlorophyll b

  • Carotenoids (e.g., carotene and xanthophyll)

• Absorption of Light: Visible light (380-740 nm) absorbed by pigments.

  • Highest absorption for chlorophylls in blue and red light; least in green light.

Types of Photosynthetic Pigments

• Primary Pigment:

  • Chlorophyll a: Main pigment, absorbs blue-green and red light.

• Accessory Pigments:

  • Chlorophyll b: Absorbs light for photosynthesis.

  • Carotenoids: Protect plants from excessive light and absorb additional wavelengths.

6.3 Light-Dependent Reactions

• Photosynthesis Stages:

  1. Non-Cyclic Photophosphorylation/Linear Electron Flow

  2. Cyclic Photophosphorylation/Cyclic Electron Flow

• Occur in thylakoid membranes, require light energy and water. Produce ATP, NADPH, and release O₂.

• Photosystem Organization:

  • Composed of reaction center and light-harvesting complex.

  • Photosystem I & II: Work together to convert light energy into chemical energy (ATP and NADPH).

Non-Cyclic Photophosphorylation

• Involves both Photosystem I (P700) and Photosystem II (P680).

• Sequence:

  1. Light excitation of electrons in PS II

  2. Splitting of water (photolysis) to replace lost electrons, producing O₂.

  3. Electron transport through enzyme complexes generates ATP.

  4. Electrons excited in PS I lead to NADPH formation.

Cyclic Photophosphorylation

• Involves only Photosystem I (PS I).

• Electrons are cycled back to PS I to create additional ATP but do not reduce NADP+.

6.4 Light-Independent Reactions / Calvin Cycle

• Calvin Cycle Phases:

  1. Carbon Fixation: CO₂ attached to RuBP, catalyzed by rubisco.

  2. Reduction Phase: ATP and NADPH reduce 3-phosphoglycerate to glyceraldehyde 3-phosphate (G3P).

  3. Regeneration: Spent G3P is recycled to regenerate RuBP.

• Role of ATP & NADPH: Essential for energy transfer and reducing power during the cycle.

6.5 Alternative Mechanisms of Carbon Fixation

1. C4 Pathway:

   • Utilizes PEP carboxylase, which has higher CO₂ affinity.

   • Example plants: Corn, sugarcane.

2. Crassulacean Acid Metabolism (CAM):

   • Operates at night to fix CO₂ into organic acids and utilizes it during the day.

   • Example plants: Cactus, pineapple.

Photorespiration

• Occurs in C3 plants when CO₂ concentration is low and O₂ concentration is high, leading to inefficient photosynthesis.

• Impacts: Consumes energy and reduces sugar production.

Key Differences C3, C4, and CAM Pathways

• C3 Plants: Directly fix CO₂; susceptible to photorespiration.

• C4 Plants: Efficient in hot climates; minimize photorespiration through spatial separation of initial carbon fixation and Calvin Cycle.

• CAM Plants: Fix CO₂ at night, store it for daytime use, reducing water loss.

Summary Points

• Photosynthesis has two major stages: light-dependent and light-independent reactions.

• Different plant types utilize various mechanisms to optimize photosynthesis according to their environments.

• Overall, plants play a crucial role in converting solar energy into chemical energy, sustaining life on Earth.

During hot and dry days, C3 plants face increased risks of photorespiration due to lower CO₂ concentrations and higher O₂ concentrations inside their leaves. This happens because the stomata close to conserve water, limiting the influx of CO₂ needed for carbon fixation. As a result, rubisco preferentially uses O₂ over CO₂, leading to inefficient photosynthesis and reduced sugar production. This can negatively impact the overall growth and productivity of the plant under stressful environmental conditions.

During hot and dry days, C3 plants experience increased photorespiration due to the closure of stomata to conserve water. This closure limits the uptake of CO₂, leading to lower internal CO₂ concentrations. As a result, the enzyme rubisco preferentially binds to O₂ instead of CO₂, causing a decrease in the efficiency of carbon fixation. This inefficiency leads to reduced sugar production and can negatively impact plant growth and productivity.

Photorespiration and Photosynthetic Efficiency in Soybean Plants

Definition of Photorespiration: Photorespiration is a process that occurs in plants, including soybeans, when the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) catalyzes a reaction with oxygen (O₂) instead of carbon dioxide (CO₂).

Impact on Photosynthesis in Soybeans:

• Reduced CO₂ Fixation: During photorespiration, CO₂ fixation is diminished as rubisco preferentially binds to O₂. This reduces the availability of CO₂ for the Calvin cycle, crucial for converting light energy into stored chemical energy in the form of glucose.

• Energy Waste: The photorespiration process consumes ATP and NADPH, which are produced during the light-dependent reactions of photosynthesis but do not contribute to glucose production. This results in an overall energy loss for the plant.

• Increased Oxygen Levels: High O₂ concentrations, which can occur in hot and dry conditions due to stomatal closure for water conservation, exacerbate photorespiration in soybeans, leading to lower photosynthetic efficiency.

• Signs of Stress: The inefficiency caused by photorespiration can manifest as reduced growth, lower yields, and decreased productivity in soybean plants, especially under suboptimal environmental conditions.

Conclusion: In summary, photorespiration reduces photosynthetic efficiency in soybean plants by limiting CO₂ availability, wasting energy, and being aggravated by environmental stresses, all of which can negatively impact plant growth and overall soybean yields.