Study Notes on Phototrophy and Photosynthesis

Chapter 11: Catabolism/Phototropy

Energy Release and Conservation

  • Introduction to phototrophy.

Phototropic Organisms

Diversity of Phototrophic Microorganisms

  • Eukaryotes:

    • Multicellular green, brown, and red algae.

    • Unicellular protists such as euglenoids, dinoflagellates, diatoms.

  • Bacteria:

    • Cyanobacteria

    • Green sulfur bacteria

    • Green nonsulfur bacteria

    • Purple sulfur bacteria

    • Purple nonsulfur bacteria

    • Heliobacteria

    • Acidobacteria

  • Archaea:

    • Halophiles

Phototrophic Fueling Reactions

Energy, Electrons, and Carbon

  • Different types of fueling reactions based on the source of carbon and energy:

    • Photoautotrophs (Photo - troph):

    • Energy: Light

    • Carbon: Inorganic CO2

  • Litoautotrophs:

    • Energy: Inorganic compounds

    • Carbon: Inorganic CO2

  • Organoheterotrophs:

    • Energy: Organic compounds

    • Carbon: Organic compounds

  • Photogens:

    • Light energy is donated to chlorophyll or bacteriochlorophyll which then fuels electron transport chain (ETC).

    • Generates NADPH and ATP through photo-phosphorylation.

Overview of Phototrophy and Photosynthesis

Phototrophy Explained

  • Defined as the use of light energy to fuel a variety of cellular reactions (excluding CO2 fixation)

  • Photosynthesis:

    • Defined as the use of light energy to fuel carbon fixation to produce organic compounds such as glucose.

    • Two-part process:

    • Light Reactions:

      • Light energy is captured and converted into chemical energy.

    • Dark Reactions:

      • Energy from light reactions is used to reduce CO2 and synthesize cellular constituents.

Types of Photosynthesis

Two Main Types:

  1. Oxygenic Photosynthesis:

    • Involves the production of oxygen.

  2. Anoxygenic Photosynthesis:

    • Does not produce oxygen.

Chlorophyll-Based Phototrophy

Process Overview

  • Light energy absorbed by chlorophyll or bacteriochlorophyll initiates photon capture, leading to:

    • The electron transport chain.

    • Production of NAD(P)+ and its reduction to NAD(P)H.

    • Generation of ATP through proton motive force (PMF).

Chlorophyll-Based System Properties

Properties Overview

  • Eukaryotes:

    • Photosynthetic pigment: Chlorophyll a

    • Number of photosystems: 2

    • Photosynthetic electron donors: H₂O

    • Oxygen production: Oxygenic

    • Primary products: ATP + NADPH

    • Carbon source: CO2

  • Cyanobacteria:

    • Photosynthetic pigment: Chlorophyll a¹

    • Number of photosystems: 2 (has divinyl chlorophyll a and b)

    • Photosynthetic electron donors: H₂O

    • Oxygen production: Oxygenic³

    • Primary products: ATP + NADPH

    • Carbon source: CO2

    • Additional notes:

      • Some cyanobacteria can function anoxygenically; for example, Oscillatoria can use H₂S as an electron donor.

  • Other Bacteria:

    • Contains distinct types of bacteriochlorophyll.

    • Electron donors include H₂, H₂S, S, and organic matter.

    • Process is anoxygenic.

    • Key products: ATP.

    • Carbon sources can be organic or CO2.

Chloroplast Anatomy

Structure

  • Chloroplast Components:

    • Thylakoid

    • Stroma

Organization of Photosynthetic Pigments

Components of Photosynthesis

  • Antennas

  • Photosystems

  • Flow of electrons within the system

Photosystems and Electron Transport Chain

Mechanism

  • Photosystem II and I Non-cyclic Electron Flow:

    • Leads to ATP and NADPH production with inputs from H₂O and sunlight

    • Outputs involve the Calvin cycle and reduction of CO2 to CH₂O

Light Reactions Explained

Photons and Energy Transfer

  • Brief outline of the flow of electrons through the thylakoid membranes.

  • Linking of photosystems with electron acceptors and chemiosmosis to ensure energy transfer and ATP production.

  • Balance of H⁺ ions across membranes establishing a proton gradient (PMF).

  • NADP+ is reduced to NADPH facilitating other synthetic processes.

Cyclic vs Non-Cyclic Electron Flow

Differences in Pathways

  • Non-cyclic Electron Flow:

    • ATP + NADPH produced (also known as noncyclic photophosphorylation).

  • Cyclic Electron Flow:

    • ATP produced (change in energy levels).

Overview of Oxygenic Photosynthesis

Reaction Summary

  • Overall equation:
    6CO<em>2+6H</em>2O<br>ightarrow6O<em>2+C</em>6H<em>12O</em>66 CO<em>2 + 6 H</em>2O <br>ightarrow 6 O<em>2 + C</em>6H<em>{12}O</em>6

  • This encompasses light reactions occurring in the thylakoid membranes and the Calvin cycle occurring in the stroma.

Anoxygenic Photosynthesis

Characteristics

  • Variability among bacteria regarding electron donors and metabolic pathways.

  • Examples of identified processes in photosynthetic bacteria.

Example of Bacterial Anoxygenic Photosynthesis

Mechanism

  • Specific processes detailed regarding electron transport, ATP synthesis in the presence of light energy.

Post-Light Reaction Processes

Energy Utilization

  • After light reactions:

    • Energy harvested is converted to glucose in dark reactions (Calvin-Benson cycle).

    • Utilized in other biosynthetic reactions.