Photosynthesis

Chapter 10: Photosynthesis Overview

10.1 Introduction to Photosynthesis

  • Photosynthesis is a process that converts solar energy into chemical energy.

  • Occurs within chloroplasts in plants.

  • Nourishes nearly all living organisms directly or indirectly.

10.2 Role of Autotrophs and Heterotrophs

Autotrophs

  • "Self-feeders" that produce organic molecules from CO2 and inorganic sources.

  • Photoautotrophs (e.g., plants, algae) use sunlight for energy.

  • Various forms of autotrophs include multicellular plants, unicellular protists, and cyanobacteria.

Heterotrophs

  • Obtain organic material from consuming other organisms or organic waste (decomposers).

  • All heterotrophs depend on autotrophs, directly or indirectly, for sustenance.

  • Fossil fuels are ancient organic materials formed from past organisms.

10.3 The Chloroplast Structure

  • Chloroplasts evolved from photosynthetic bacteria.

  • Composed of an envelope with two membranes containing a dense fluid called stroma.

  • The thylakoids form a third membrane system, organized into stacks called grana.

  • Chlorophyll, a key pigment, resides in thylakoid membranes, giving leaves their green color.

10.4 Photosynthesis Equation

  • Overall reaction: 6 CO2 + 12 H2O + Light energy → C6H12O6 + 6 O2 + 6 H2O.

  • Photosynthesis involves splitting water, incorporating hydrogen into sugars, releasing oxygen.

10.5 Redox Process of Photosynthesis

  • Photosynthesis involves a redox process where H2O is oxidized and CO2 is reduced.

  • Utilizes light energy, making it an endergonic process.

10.6 Two Stages of Photosynthesis

Light Reactions

  • Occur in thylakoids: split H2O, release O2, generate ATP, and reduce NADP+ to NADPH.

  • They convert solar energy to chemical energy (ATP and NADPH).

Calvin Cycle (Light Independent Reactions)

  • Occurs in the stroma: uses ATP and NADPH to convert CO2 into sugars.

  • Involves carbon fixation, reduction phase, and regeneration phase of ribulose bisphosphate (RuBP).

10.7 Photosynthetic Pigments

  • Different pigments absorb different wavelengths of light, notably:

    • Chlorophyll a: primary light-capturing pigment.

    • Chlorophyll b: accessory pigment that assists chlorophyll a.

    • Carotenoids: pigments that provide broader light absorption and protection against damage.

10.8 Mechanism of Light Reactions

  • Light energy excites electrons in chlorophyll, leading to their transfer to a primary electron acceptor.

  • Two photosystems (PS I and PS II) facilitate two routes for electron flow: cyclic and linear.

10.9 Linear Electron Flow Steps

  1. Photon Absorption: Light energy excites electrons in PS II.

  2. Water Splitting: Enzyme splits H2O, replenishing electrons to PS II, releasing O2.

  3. Electron Transport Chain: Electrons move through a series of proteins, releasing energy to pump H+ ions, creating a gradient.

  4. ATP Production: Proton gradient drives ATP synthesis via ATP synthase.

  5. NADPH Formation: Electrons are transferred to NADP+ forming NADPH.

10.10 Light Independent Cycle (Calvin Cycle)

  • Fixes CO2 and synthesizes G3P from ATP and NADPH.

  • For one G3P molecule, consumes 9 ATP and 6 NADPH.

  • G3P serves as a precursor for glucose and starch synthesis.

10.11 Importance of Photosynthesis

  • Photosynthesis is essential for life on Earth, providing energy and organic materials for ecosystems.

  • Sugars formed in chloroplasts supply energy and carbon skeletons needed for cellular functions.

  • Plants store excess energy in the form of starch for later use.


Light Dependent Reaction

  • Inputs: Light energy, water (H2O), NADP+, ADP + Pi

  • Outputs: Oxygen (O2), NADPH, ATP

  • Location: Thylakoids of chloroplasts

  • Main Purpose: Convert solar energy into chemical energy in the form of ATP and NADPH, while releasing oxygen as a byproduct.

Light Independent Reaction (Calvin Cycle)

  • Inputs: Carbon dioxide (CO2), ATP, NADPH

  • Outputs: G3P (glyceraldehyde 3-phosphate), ADP + Pi, NADP+

  • Location: Stroma of chloroplasts

  • Main Purpose: Use ATP and NADPH from the light-dependent reactions to convert CO2 into organic molecules, primarily sugars.

Photosystems

  • Definition: Photosystems are complexes of proteins and chlorophyll pigments in the thylakoid membranes. There are two types: Photosystem I (PS I) and Photosystem II (PS II).

  • Importance: They are critical for the absorption of light energy and the initiation of the electron transport chain, which leads to the production of ATP and NADPH.

Importance of the Proton Gradient

  • Creation: The proton gradient is created during the light-dependent reactions through the splitting of water molecules and the movement of electrons through the electron transport chain, which pumps protons into the thylakoid lumen.

  • Use: The gradient is utilized by ATP synthase to produce ATP. As protons flow back into the stroma through ATP synthase, ADP and inorganic phosphate (Pi) are combined to form ATP.

Consequences of a Non-Functional Proton Gradient

If the proton gradient is not functioning, ATP synthase would not be able to produce ATP effectively. This would impede the energy supply for the Calvin Cycle and other cellular functions, leading to a decreased rate of photosynthesis and compromising plant health and