Photosynthesis and Cellular Respiration

Photosynthesis Study Notes

Learning Objectives

  • Understand the properties of light.
  • Analyze how pigment structure and organization allow cells to acquire energy from light.
  • Explain how photosystems use light energy to produce chemical energy.
  • Understand the importance of water.
  • Identify the reactants, products, and energy transformations in the light reactions and the Calvin cycle.
  • Relate the structure of chloroplasts to their function.
  • Compare and contrast cellular respiration and photosynthesis.

Photosynthesis Overview

  • Definition: Photosynthesis is the process where organisms use sunlight to manufacture carbohydrates.
  • Autotrophs: Refer to photosynthetic organisms that create their own food from inorganic substances such as ions and molecules.
    • Example: Plants, algae.
  • Heterotrophs: Refers to organisms that must acquire sugars and organic compounds from other organisms.
    • Example: Animals, fungi.

Energy Transformation in Photosynthesis

  • Photosynthesis converts electromagnetic energy into chemical energy.
  • Key Inputs: Requires sunlight, carbon dioxide (CO₂), and water (H₂O).
  • Key Outputs: Produces glucose (C₆H₁₂O₆) and oxygen (O₂) as a by-product.
    • The process oxidizes 3 carbon compounds.

The Two Linked Sets of Photosynthetic Reactions

  1. Light-Capturing Reactions:

    • Produce oxygen (O₂) from water (H₂O).
    • Utilize electrons and ATP to reduce carbon dioxide (CO₂).

  2. Calvin Cycle Reactions:

    • Produce sugar (glucose) from carbon dioxide (CO₂).
    • Water is split to release O₂ gas.
    • Energy from light excites electrons, which leads to ATP formation and the creation of NADPH.

Chloroplasts: The Site of Photosynthesis

  • Location of Photosynthesis: Occurs in chloroplasts, which possess a double membrane.

    • Structure:
    • Interior contains flattened vesicle-like structures called thylakoids.
    • Thylakoids are often organized into stacks known as grana.
    • Fluid-filled space between thylakoids is known as the stroma.

  • Function: Chloroplast structure is intricately related to its function in photosynthesis.

Properties of Light

  • Electromagnetic Radiation: A form of energy that travels in waves.
    • Light: A type of electromagnetic radiation that exhibits both wave-like and particle-like properties.
    • Wave Characterization: Defined by wavelength, which is the distance between two crests of a wave.
    • Particle Characterization: Exists in discrete packets known as photons.

The Electromagnetic Spectrum

  • The electromagnetic spectrum encompasses various types of radiation categorized by their wavelengths:

    • Ranges from gamma rays (shorter wavelengths) to radio waves (longer wavelengths).
    • Visible light ranges from approximately 400 nm (blue) to 710 nm (red).

  • Energy Correlation: Higher energy corresponds to shorter wavelengths, while lower energy corresponds to longer wavelengths.

Pigments in Chloroplasts

  • Thylakoid Membranes: Contain pigments that absorb specific wavelengths of light while reflecting others.
    • Color Perception: Pigments appear colored because they reflect the wavelengths of light not absorbed.
  • Chlorophyll: The most common pigment found in thylakoids.
    • Absorption: Reflects green light; responsible for the green color of plants.

Structure of Chlorophyll

  • Similar Structures: Chlorophyll a and b both have:
    • Long isoprenoid tails: Interact with proteins in thylakoid membranes.
    • Head: Comprising a large ring structure with a magnesium atom at the center, which is crucial for light absorption.

Accessory Pigments

  • Carotenoids: Pigments that extend the range of wavelengths that can drive photosynthesis by absorbing UV and blue-green light, appearing yellow, orange, or red.
  • Role: Work alongside chlorophyll, aiding in light absorption and transfer of energy.
  • Seasonal Changes: As chlorophyll degrades in trees, carotenoids become visible in autumn foliage.

Excitation of Electrons

  • Photon Absorption: When chlorophyll absorbs photons:
    • The energy from the photon excites electrons to a higher energy state.
  • Energy Release: If the excited electron returns to the ground state, energy can be lost as heat or emitted as light, a phenomenon known as fluorescence.

Photosystems: Structures for Light Capture

  • Definition: Photosystems are complexes of chlorophyll and accessory pigments.
    • Function: Serve as antenna pigments by gathering light energy and directing it toward the reaction center.

Electron Transport Chain (ETC)

  • Comparison with Mitochondria: Thylakoid ETC shares similarities in structure and function with mitochondrial ETC.
    • Both utilize quinones and cytochromes to transport electrons.
    • Redox reactions in both systems result in proton movement across internal membranes, generating a proton-motive force that drives ATP production via ATP synthase.

Photosynthetic Electron Transport Chain Flow

  • Photosystem II (PS II): Produces ATP via light energy absorbed.

  • Electrons from PS II: Are sent to the cytochrome complex, generating a proton gradient across the thylakoid membrane:

    • High Concentration of H+: Acts on one side of the membrane, driving active transport and creating a concentration gradient to store potential energy.

  • Photosystem I (PS I):

    • Produces NADPH when NADP+ acts as the final electron acceptor.

Water Splitting and O2 Production

  • Water Splitting Reaction:
    • Splitting of water (2 H2O ightarrow 4 H^+ + 4 e^- + O2): Replaces the electrons lost from chlorophyll in PS II, producing O₂ as a waste product.

The Z-Scheme Model of Photosynthesis

  • Link between Photosystems: Demonstrates the transfer of electrons from water to NADP+:
    • Input: H_2O + light
    • Output: O_2 + ATP + NADPH
    • Sequence of Events: PS II generates ATP and PS I produces NADPH after water is split.

The Calvin Cycle

  • Carbon Fixation: Addition of carbon atoms from inorganic CO₂ to form useful organic compounds.
    • Enzyme: RuBisCO serves in this crucial reaction, making it the most abundant enzyme in leaf tissue.
  • Biological Importance: Carbon fixation is the most critical chemical reaction on Earth due to its role in sustaining life.

The Calvin Cycle Phases

1. Fixation Phase:

  • CO₂ reacts with RuBP (ribulose bisphosphate).
  • Produces two molecules of 3PGA (3-phosphoglycerate).

2. Reduction Phase:

  • 3PGA is phosphorylated by ATP and reduced by electrons from NADPH to produce G3P (glyceraldehyde-3-phosphate).

3. Regeneration Phase:

  • Utilizes ATP to regenerate RuBP, ensuring the cycle continues.

Summary of Products from the Calvin Cycle

  • The Calvin cycle results in the production of:
    • G3P: Can be utilized to form glucose, fructose, and sucrose.
    • Storage: Glucose can be polymerized into starch for storage; cellulose for cell walls.
    • Utility: All organic carbon compounds can trace back to the process of photosynthesis.

Interconnection of Photosynthesis and Cellular Respiration

  • Respiration formula: 6CO2 + 6H2O + energy
    ightarrow C6H{12}O6 + 6O2
  • Photosynthesis vs. Cellular Respiration:
    • Different Organelle: Photosynthesis occurs in chloroplasts; respiration occurs in mitochondria.
    • Different Enzymes: Diverse sets of enzymes facilitate these processes.
    • Diverse Reactions: Photosynthesis captures energy, while cellular respiration releases it.

The Carbon Cycle and Climate Change

  • Photosynthesis Role: Critical in sequestering CO₂ from the atmosphere, helping mitigate climate change.
    • Balance: Photosynthetic activity by producers and respiration by consumers and decomposers maintain ecological balance.