Photosynthesis and Respiration in Plants

Introduction to Photosynthesis

  • Definition: The process by which plants convert solar energy into chemical energy for use in the plants.

  • Key Components:

    • Light energy

    • Oxygen (O₂)

    • Sugar (produced from CO₂ and H₂O)

    • Carbon dioxide (CO₂)

    • Minerals

    • Water (H₂O)

Learning Objectives

  • Understand the following:

    • The equation showing the production (photosynthesis) and breakdown (metabolism) of glucose.

    • Identification of which wavelengths of light are absorbed versus reflected.

    • The flow of water from plant roots to oxygen released by the plant.

    • The path of atmospheric CO₂ to fixed carbons in glucose.

    • The role and location of various plant pigments.

    • Substrates and products of the light reaction and Calvin cycle.

    • The location of the reactions in photosynthesis.

    • Comparison of the reactants and products of photosynthesis with those of metabolism.

Relationship between Photosynthesis and Aerobic Respiration

  • Photosynthesis occurs in chloroplasts and involves:

    • Input:

    • Carbon Dioxide (CO₂)

    • Water (H₂O)

    • Output:

    • Oxygen (O₂)

    • Sugars (e.g., glucose)

  • Aerobic Respiration occurs in mitochondria and involves:

    • Input:

    • Oxygen (O₂)

    • Sugars

    • Output:

    • Carbon Dioxide (CO₂)

    • Water (H₂O)

    • ATP (chemical energy)

  • The relationship:

    • Photosynthesis reduces CO₂ and H₂O to build sugar.

    • Respiration oxidizes sugars to generate ATP.

Overview of Photosynthesis Processes

  • Two main processes:

    • Light Reactions

    • Calvin Cycle

  • Light Reactions occur in the chloroplasts and involve:

    • Input: Water (H₂O), Light Energy

    • Outputs: NADPH, ATP, Oxygen (O₂)

  • Calvin Cycle occurs in the stroma and involves:

    • Input: Carbon Dioxide (CO₂)

    • Output: Sugars (e.g., [CH₂O])

Autotrophs and Photoautotrophs

  • Autotrophs produce complex organic molecules from simple inorganic molecules:

    • Example: glucose and starch from CO₂ and H₂O.

  • Photoautotrophs transform radiant energy into chemical energy (ATP).

  • Importance: They store potential energy in the bonds of sugars and starches, which humans (heterotrophs) utilize for energy.

Structure of Leaves and Photosynthesis

  • Leaf Anatomy:

    • Upper surface of the leaf is adapted to maximize light capture.

    • Stomata facilitate the exchange of CO₂ and O₂.

    • Photosynthetic cells are called mesophyll.

  • Stomatal Function:

    • Can open and close to regulate water loss.

    • Controlled by ion transporters and gradients.

The Electromagnetic Spectrum and Light Absorption

  • Definition of Light:

    • A form of energy that travels in wavelengths.

    • Different wavelengths correspond to different energy levels.

  • Visible Light Range:

    • Approximately 380 nm (violet) to 750 nm (red).

  • Plant Pigments:

    • Main pigment: Chlorophyll (gives leaves green color).

    • Absorbs all wavelengths except for those in the green range (500-600 nm).

    • Reflects green light, perceived by the brain as green.

Plant Pigments and Their Functions

  • Types of Pigments:

    • Chlorophyll a and b (variations in absorption spectra).

    • Carotenoids (accessory pigments that capture additional wavelengths).

  • Pigment Structure:

    • Chlorophyll contains a porphyrin ring for light absorption and a hydrocarbon tail to interact with thylakoid membranes.

Light Reactions of Photosynthesis

  • Location: Thylakoids within chloroplasts.

  • Description:

    • Light energy is absorbed by chlorophyll in Photosystem II (PS II) and excitatory electrons are donated to the electron transport chain (ETC).

    • Water is split to replace lost electrons in PS II.

    • Production of NADPH and ATP through electron transport processes.

  • Chemiosmosis:

    • Involves the creation of a proton gradient across the thylakoid membrane leading to ATP synthesis.

Calvin Cycle

  • Location: Stroma of chloroplasts.

  • Phases of the Calvin Cycle:

    1. Fixation: 3 RuBP + 3 CO₂ -> 6 3-phosphoglycerate.

    2. Reduction: 6 3-phosphoglycerate + 6 ATP + 6 NADPH -> Glyceraldehyde-3-phosphate (G3P).

    3. Regeneration: 5 G3P + 3 ATP -> 3 RuBP.

  • Outputs:

    • Production of sugars (organic molecules).

Summary of Photosynthesis Products

  • Light Reactions Produce:

    • ATP

    • NADPH

    • O₂

  • Calvin Cycle Produces:

    • Sugars (e.g., glucose).

    • Uses ATP and NADPH from light reactions for sugar production.

Transport and Storage of Sugars

  • Sugars are transported as sucrose through phloem to different parts of the plant.

  • Glucose produced in leaves can be converted into:

    • Sucrose for transport.

    • Starch for energy storage in chloroplasts.

    • Cellulose and other organic compounds for structural use.

Starch Storage and Energy Usage

  • Many plants, such as potatoes, store potential energy as starch, which is utilized later for ATP production in mitochondria.

  • Starch is predominantly stored in potato tubers underground, aiding growth and asexual reproduction.

Conclusion

  • Overall, photosynthesis is critical for the conversion of solar energy into food sources (sugars) for plants and, subsequently, for the wider ecosystem, including humans and animals that rely on plants for energy and oxygen.