Photosynthesis and Plant Biochemistry

Photosynthesis: Overview

Topics Covered

  • Structure of leaves and chloroplasts

  • Phases of photosynthesis

  • Light-dependent phase

  • Calvin cycle

  • Photorespiration

  • Photosynthetic adaptations

Organic Compounds

Types of Organisms Based on Energy and Carbon Sources

  • Energy Source: The source of energy for organisms can be classified based on their metabolic processes.

    • Light

    • Phototrophs: Organisms that use light as an energy source.

    • Chemoautotrophs: Organisms that derive energy from chemical reactions involving inorganic compounds (e.g. sulfur- , iron- , nitrogen- oxides, and carbon monoxide-oxidizing bacteria).

    • Chemoheterotrophs: Organisms that obtain energy from organic compounds (all animals, most fungi, protozoa, and bacteria).

    • Photoheterotrophs: Organisms like green nonsulfur bacteria and purple nonsulfur bacteria that utilize organic compounds but also have a photosynthetic capability.

  • Carbon Source: Organisms can also be classified based on their carbon sources.

    • Carbon Dioxide (CO2): Used by plants and photosynthetic organisms.

    • Organic Compounds: Utilized by heterotrophs (animals, fungi).

  • Electron Acceptors:

    • Chemoautotrophs use substances other than O2 for metabolism depending on their environment.

    • Primary Electron Acceptors: For phototrophs, oxygen is produced as a byproduct in oxygenic photosynthesis from water.

    • Non-oxygenic Photosynthesis: Anoxygenic photosynthetic bacteria use other compounds instead of water to generate energy without releasing O2.

    • Example: Fermentative organisms (e.g., Streptococcus).

Photosynthesis Overall Process

  • Main Inputs and Outputs:

    • Inputs: CO2 + H2O + Light Energy

    • Outputs: Sugars (glucose) + O2

  • Overall Chemical Reaction:

    • 6CO2 + 6H2O
      ightarrow C6H{12}O6 + 6O2

    • Summary: Light energy captured by chlorophyll drives the synthesis of glucose from carbon dioxide and water, releasing oxygen in the process.

Structure of Leaves

  • Key Components:

    • Cuticle: Protective layer preventing water loss.

    • Epidermis: Upper and lower protective layers.

    • Mesophyll:

    • Palisade Layer: Contains tightly packed cells with chloroplasts for maximum light absorption.

    • Spongy Layer: Looser cells with air spaces for gas exchange.

    • Stomata: Pores that allow CO2 uptake and O2/water vapor release.

  • Chloroplast Structure:

    • Components:

    • Outer Membrane

    • Inner Membrane

    • Thylakoid Membrane: Contains thylakoids stacked in structures called grana.

      • Thylakoid Lumen: Fluid-filled interior of thylakoids where light-dependent reactions occur.

    • Stroma: Fluid surrounding thylakoids where the Calvin cycle occurs.

Light Absorption in Chloroplasts

  • Chlorophyll Molecule:

    • Absorbs light, raising electrons to an excited state.

    • Excited State: Higher energy condition post photon absorption.

    • Chlorophyll a (main pigment) absorbs maximally at 670 nm; Chlorophyll b absorbs at 640 nm.

  • Energy Transfer: Upon the electron reaching an excited state, energy is used in photosystems to synthesize ATP and NADPH.

Light-Dependent Reactions

Overview

  • Primary Goal: Generate ATP and NADPH from light energy.

  • Pathway:

    • Photosystem II (PSII):

    • Absorbs light at 680 nm, oxidizing water to produce O2 and providing electrons to the electron transport chain (ETC).

    • Photosystem I (PSI):

    • Absorbs light at 700 nm, uses electrons to reduce NADP+ to NADPH.

  • Electron Transport Chain Cycle:

    • Electrons flow through various proteins, pumping protons into the thylakoid lumen (creating a proton gradient).

    • Protons flow through ATP synthase, leading to the production of ATP (chemiosmosis).

Noncyclic vs Cyclic Electron Transport

  • Noncyclic Electron Transport:

    • Produces both ATP and NADPH.

    • Water split, providing electrons to PSII.

  • Cyclic Electron Transport:

    • Happens when ATP is low, NADPH is high. Electrons cycle through PSI only to generate more ATP without producing NADPH.

  • Key Application: Understand the differences between oxidative (mitochondria) and photophosphorylation (chloroplasts).

Differences Compared to Cellular Respiration

  • Oxidative Phosphorylation:

    • Occurs in mitochondria, uses organic molecules, oxygen is the final electron acceptor.

  • Photophosphorylation:

    • Occurs in chloroplasts using sunlight, with NADP+ as final electron acceptor.

Light-Independent Reactions: Calvin Cycle

Overview

  • Main Goal: Fix carbon dioxide into glucose.

  • Three Phases:

    1. Carbon Fixation:

    • Enzyme RuBisCO converts CO2 and RuBP into 3-phosphoglycerate (3PG).

    1. Reduction Phase:

    • ATP and NADPH are used to convert 3PG to glyceraldehyde-3-phosphate (G3P)

    1. Regeneration of RuBP:

    • G3P is used to regenerate RuBP, allowing the cycle to continue.

  • Phase-Specific Reactions:

    • Global regulation by light; light influences enzymes that mediate the Calvin cycle.

  • Energy Forms in Calvin Cycle:

    • ATP provides energy, while NADPH provides reducing power for the synthesis of glucose.

Final Summary

  • Photosynthesis encompasses a series of complex reactions that convert solar energy into chemical energy. Each part of the process, from light absorption to carbon fixation, plays a crucial role in ensuring plant growth and energy production. Profound understanding of both light-dependent and light-independent reactions is essential for comprehending plant biology and biochemistry.