Photosynthesis

Photosynthesis

Definition and Overview

  • Photosynthesis is the process by which plants, some bacteria, and some protistans utilize energy from sunlight to produce glucose from carbon dioxide and water.

  • Products of Photosynthesis:

    • Glucose, which can be converted into pyruvate, releasing adenosine triphosphate (ATP) during cellular respiration.

    • Oxygen.

  • Word Equation for Photosynthesis:
    \text{carbon dioxide} + \text{water} \rightarrow \text{glucose} + \text{oxygen}

  • The process converts usable sunlight energy into chemical energy, primarily associated with the action of chlorophyll.

Chlorophyll and Accessory Pigments

  • Chlorophyll:

    • A complex molecule found in all photosynthetic organisms, essential for absorbing light.

    • All photosynthetic organisms contain Chlorophyll a.

  • Accessory Pigments:

    • Absorb light energy not captured by Chlorophyll a.

    • Examples include:

    • Chlorophyll b (and c, d, e in algae and protistans)

    • Xanthophylls

    • Carotenoids (e.g., beta-carotene)

  • Wavelengths of Light Absorption:

    • Chlorophyll a absorbs from the violet-blue and reddish orange-red wavelengths, with minimal absorption in the green-yellow-orange range.

  • Chlorophyll Structure:

    • Contains:

    • A lipid-soluble hydrocarbon tail (C20H39-).

    • A flat hydrophilic head containing a magnesium ion at its center.

      • Different chlorophyll types have different side-groups on the head.

    • Tail and head linked by an ester bond.

Leaf Structure and Function

  • Leaves:

    • The only photosynthetic structures among plants, serving as solar collectors filled with photosynthetic cells.

    • Function: Raw materials (water and carbon dioxide) enter cells; products (sugar and oxygen) leave.

  • Water Transport:

    • Water absorbed by roots is transported up to leaves via specialized cells (xylem vessels).

  • Gas Exchange:

    • Stomata: Specialized openings that facilitate gas exchange while minimizing water loss.

    • Singular of Stomata: Stoma, flanked by guard cells.

    • Cuticle: Waxy layer preventing carbon dioxide entry; stomata are necessary for gas exchange, which also results in significant water loss.

    • Example: Cottonwood trees can lose 100 gallons (about 450 dm³) of water per hour on hot days.

Chloroplast Structure and Function

  • Thylakoid: The key structural unit of photosynthesis, found in both prokaryotic and eukaryotic photosynthetic organisms.

    • Stacked in structures called grana.

    • Stroma: The spaces between the grana in chloroplasts.

  • Chloroplasts: Only eukaryotic cells have them, featuring three membrane systems that create three compartments.

Stages of Photosynthesis

  • Overall Process: Photosynthesis comprises two main stages:

    1. Light Reactions (Light-Dependent Reactions)

    2. Calvin Cycle (Light-Independent Reactions)

Light-Dependent Reactions

  • Mechanism:

    • Chlorophyll a absorbs light energy, leading to:

    • Photoexcitation: An electron gains energy and is excited.

    • Photoionization: The excited electron frees itself, leaving a positively charged chlorophyll ion.

  • Key Reactions:

    • Excited electrons are transferred to primary electron acceptor. Chlorophyll is oxidized and carries a positive charge.

    • Water molecules are split (photolysis):
      2H2O \rightarrow 4H^+ + O2 + 4e^-

    • Electrons reduce NADP+ to NADPH:
      NADP^+ + 2e^- + 2H^+ \rightarrow NADPH + H^+

  • Energy Carrier Production: ATP produced from ADP through photophosphorylation.

Calvin Cycle (Light-Independent Reactions)

  • Process:

    • Utilizes ATP and NADPH to convert carbon dioxide into carbohydrates, primarily glyceraldehyde 3-phosphate (GALP), a 3-carbon molecule.

  • Carbon Fixation Process:

    • Carbon dioxide combines with ribulose-1,5-bisphosphate (RuBP).

    • An unstable six-carbon sugar splits to form two glycerate 3-phosphate (GP) molecules.

  • Energy Utilization: GP is phosphorylated by ATP and reduced by NADPH to form GALP.

  • End Products:

    • Of each pair of GALP molecules produced:

    • One molecule becomes glucose and other carbohydrates, lipids, or amino acids.

    • The other is converted back to RuBP to restart the cycle.

Factors Affecting the Rate of Photosynthesis

  • Key Factors:

    • Light Intensity: Increases rate of light-dependent reactions and photosynthesis until limited by another factor.

    • Wavelength of Light: PSI absorbs most efficiently at 700 nm, and PSII at 680 nm.

    • Carbon Dioxide Concentration: Higher concentrations increase incorporation into carbohydrates during light-independent reactions.

    • Temperature: Optimal enzyme function generates maximum photosynthetic rates up to a point; too high temperatures decrease rates dramatically.

Summary of Photosynthetic Stages

  • Stages:

    • Two main stages: Light Reactions (in grana) and Calvin Cycle (in stroma).

    • Light Reactions: Generate ATP and NADPH through water photolysis and electron transport.

    • Calvin Cycle: Utilizes ATP and NADPH to fix carbon and produce glucose.

Conclusion and Quiz

  • Understanding the intricate stages and factors affecting photosynthesis is crucial for grasping how energy conversion works in living organisms.