Photosynthesis__1_

Energy Acquisition in Organisms

• Autotrophs: Organisms that produce their own food by harnessing energy from the environment.

  • Photoautotrophs: Include plants, algae, and some photosynthetic bacteria that utilize light energy. Known as producers.

• Heterotrophs: Organisms that cannot produce their own food and must consume plants, animals, or decomposed organic material.

Photosynthesis Overview

• Definition: The process of converting light energy into chemical energy.

• Location: Takes place in chloroplasts, primarily located in the mesophyll of leaves.

  • Gas Exchange: Carbon dioxide (CO₂) enters and oxygen (O₂) exits through small pores known as stomata.

Structure of Chloroplasts

• Membranes: Consist of two outer membranes with a thick fluid-filled inner compartment called the stroma.

• Thylakoids: Form another inner compartment; stacks of thylakoids are called grana.

• Chlorophyll: The light-absorbing pigment located in thylakoids, essential for converting solar energy into chemical energy.

Redox Reactions in Photosynthesis

• Comparison: Photosynthesis and cellular respiration are opposite reactions; the reactants of one are the products of the other.

• Processes: Both processes involve redox (reduction-oxidation) reactions where molecules gain electrons (reduced) or lose electrons (oxidized).

• Overall Equation: Focus on balancing energy transformations and molecular conversions.

Two Stages of Photosynthesis

• Light Reactions (light-dependent): Occur in thylakoids; convert light energy to chemical energy, releasing O₂.

• Dark Reactions (light-independent): Also known as the Calvin cycle, occur in the stroma and use ATP and NADPH to synthesize sugars.

Sunlight as Energy Source

• Nature of Light: Electromagnetic radiation that travels in rhythmic waves.

• Wavelength: Distance between crests; visible light ranges from 380-750 nm and is crucial for photosynthesis.

  • Photon Concept: Light behaves as packets of energy; shorter wavelengths correspond to higher energy.

Photosynthetic Pigments

• Role: Molecules that absorb specific wavelengths of light.

• Chlorophyll a: Most common pigment, absorbs blue-violet and red light, reflects green.

• Chlorophyll b: Absorbs blue and orange light, serving as an accessory pigment.

• Function of Accessory Pigments: They protect plant cells from UV damage and broaden the spectrum of light absorption.

Photosystems in Photosynthesis

• Photosystems: Clusters of chlorophyll organized in thylakoid membranes, consisting of both reaction-center and light-harvesting complexes.

  • Light-Harvesting Complex: Various pigments that pass energy to the reaction center.

  • Reaction Center: Contains chlorophyll a and the primary electron acceptor, essential for initiating electron transport.

Mechanism of Light Reactions

1. Energy Transfer: Light energy is captured by Photosystem II.

2. Electron Capture: Excited electrons are captured by a primary electron acceptor.

3. Water Splitting: Water molecules are split to replace lost electrons in Photosystem II, releasing O₂.

4. Electron Transport Chain: Electrons move through this chain, contributing to ATP and NADPH production.

5. Photon Excitation: Another photon energizes electrons in Photosystem I, which are also passed to a primary electron acceptor.

ATP Production through Chemiosmosis

• H+ Gradient: The concentration of H+ ions drives ATP production via ATP synthase through a process called photophosphorylation.

Calvin Cycle: The Second Stage of Photosynthesis

• Purpose: Produces sugars in the stroma by fixing carbon from CO₂.

• Role of Rubisco: An enzyme that facilitates the attachment of CO₂ to ribulose biphosphate (RuBP).

Steps of the Calvin Cycle

1. Carbon Fixation: Enzyme rubisco attaches CO₂ to RuBP, forming an intermediate molecule PGA.

2. Reduction Phase: ATP and electrons from NADPH convert PGA into G3P.

3. Release Phase: 1 G3P is released from the cycle for glucose production for every 3 CO₂ fixed.

4. Regeneration: Uses ATP to convert G3P back into RuBP.

Types of Plants and Their Photosynthetic Adaptations

• C3 Plants: Utilize the Calvin cycle for carbon fixation and manage water conservation by closing stomata, which can lead to photorespiration.

• Photorespiration: Occurs when Rubisco reacts with O₂ instead of CO₂, inefficiently releasing CO₂ and consuming energy.

• C4 Plants: Use an alternate fixation pathway to minimize photorespiration and enhance CO₂ capture with adaptations in mesophyll and bundle-sheath cells.

• CAM Plants: Stomata open at night to capture CO₂ and use it during the day in the Calvin cycle, effectively managing water usage in arid climates.

Significance of Photosynthesis

• Essential Process: The ultimate source of organic compounds for almost all organisms and provides the O₂ necessary for cellular respiration.

• Impact of Climate Change: The greenhouse effect and resulting climate changes can affect photosynthetic efficiency and overall plant health, influencing global ecosystems and agriculture.