Ch7 Part1

Overview of Photosynthesis

Photosynthesis is a crucial biological process that transforms light energy into chemical energy. It serves as the counterpart to cellular respiration, where cells convert glucose into energy.

Photosynthesis Stages

Photosynthesis takes place in two stages linked by ATP and NADPH, an electron carrier similar to NADH in cellular respiration.

1. Light Reactions

  • Occur in the thylakoids of chloroplasts.

  • Reactants:

    • Water (H2O)

    • Light energy

  • Products:

    • Oxygen (O2)

    • ATP

    • NADPH (carries excited electrons)

  • Light is absorbed by chlorophyll pigments in thylakoid membranes, exciting electrons and facilitating photoexcitation.

    • The light reaction involves two photosystems connected by an electron transport chain, similar to processes in cellular respiration, which ultimately produce ATP and reduce NADP+ to NADPH. The first photosystem to receive the excited electron is Photosystem II.

  • Electrons are derived from water, which is split to release oxygen as a byproduct. The splitting of water occurs at the start of photosynthesis and is critical for providing electrons.

  • Exergonic reaction as ATP is produced and energy is released in this stage.

2. Calvin Cycle

  • Utilizes ATP and NADPH produced in the light reactions, along with CO2.

  • The Calvin cycle is a cyclical process that recycles NADP+, which goes back for reuse in the light reactions. It also uses all the ATP produced.

  • Overall photosynthesis is an endergonic process, while the light reaction is exergonic.

  • Ultimately, the Calvin cycle produces sugars via carbon fixation, specifically Glyceraldehyde 3-phosphate (G3P), which is a precursor to glucose, but glucose itself is not a direct product of the cycle.

  • Key stages:

    1. Carbon fixation (CO2 is fixed)

    2. Reduction stage (production of G3P)

    3. Regeneration of RuBP (to continue the cycle)

  • Most plants are C3 plants, where the first product of carbon fixation is a three-carbon compound called 3-PGA.

  • C4 plants have evolved in hot, dry climates. They differ by fixing CO2 initially into a four-carbon compound instead of a three-carbon compound to conserve water. Their stomata are largely closed in hot and dry weather, which prevents water loss.

Photosynthesis Fundamentals

Photosynthesis utilizes the products of cellular respiration as reactants.

  • Reactants: Carbon Dioxide (CO2), Water (H2O), Energy from Sunlight

  • Main Product: Glyceraldehyde 3-phosphate (G3P), used to form glucose.

  • Byproduct: Oxygen (O2), essential for aerobic life.

Energy Flow in Ecosystems

  • Unidirectional flow of energy originates from sunlight, is converted to chemical energy through photosynthesis, and ultimately leaves the system as heat. Chemical cycling exists but differs from the unidirectional flow of energy.

Structure and Function in Plant Cells

  • Chloroplasts: Organelles in plant cells where photosynthesis occurs, key distinguishing feature of plant cells.

  • Photosynthesis Location: Most occurs in the leaves of plants, leaf cross-section shows numerous chloroplast-filled cells.

Gas Exchange in Leaves

  • Stomata: Tiny pores in leaves that allow for the exchange of gases; CO2 enters and O2 exits.

Chloroplast Structure

  • Thylakoids: Membranous sacs within chloroplasts functioning in light absorption, organized in stacks called granum.

  • Stroma: The fluid surrounding thylakoids inside the chloroplast, analogous to cytoplasm in the cell.

  • Chlorophyll: Green pigment located in thylakoid membranes that captures light energy essential for photosynthesis.

Photosynthesis as a Redox Process

  • Involves reduction-oxidation (redox) reactions where electron movement is critical. Chlorophyll captures light energy, leading to excitation of electrons which move down electron transport chains to produce energy and form glucose from CO2 and H2O. Mnemonic: "LEO goes GER" (Lose Electrons Oxidized, Gain Electrons Reduced).

Photosynthesis Equation Exercise

  • Reactants: CO2 + H2O

  • Products: Glucose + O2

  • Consider whether CO2 is oxidized to form glucose or reduced, and similarly for oxygen in water.