Photosynthesis Structures and Process

Photosynthesis Structures

• Mesophyll: Middle layer of leaf where photosynthesis occurs.
• Stoma: Small openings on the surface of leaves that allow for gas exchange, including the release of oxygen produced during photosynthesis.
• Chloroplast: Organelle responsible for photosynthesis in plants.
  • Thylakoids: Membranous structures within chloroplasts that contain chlorophyll.
  • Chlorophyll: Pigment responsible for absorbing light energy.
    • Pigment: A molecule that absorbs light.
• Granum: Stack of thylakoids.
• Thylakoid Lumen: Internal space enclosed by the thylakoid membrane.

Photosynthesis Process

• Photosynthesis is the process by which plants convert light energy into chemical energy stored in glucose.
• Light-dependent reactions: Occur in the thylakoid membrane and require light energy.
  • Energy from sunlight is used to split water molecules, releasing oxygen as a byproduct.
  • Electrons are transferred along an electron transport chain, generating ATP and NADPH.
• Light-independent reactions (Calvin Cycle): Occur in the stroma and do not directly require light.
  • ATP and NADPH produced in the light-dependent reactions are used to fix carbon dioxide and synthesize glucose.
  • The Calvin Cycle has three main stages:
    • Fixation: Carbon dioxide is incorporated into an organic molecule.
    • Reduction: ATP and NADPH are used to reduce the organic molecule, forming glucose.
    • Regeneration: The starting molecule for the Calvin Cycle is regenerated.

Photosystem II

• Energy is passed from pigment to pigment until it reaches the reaction center.
• Energy is transferred to P680, boosting an electron to a high energy level.
• The high-energy electron is passed to an acceptor molecule and replaced with an electron from water (this splitting of water releases the oxygen we breathe).
• The high-energy electron travels down an electron transport chain, losing energy as it goes, which is used to generate ATP.

Photosystem I

• The electron arrives at photosystem I and joins the P700 special pair of chlorophylls in the reaction center.
• When light energy is absorbed by pigments and passed inward to the reaction center, the electron in P700 is boosted to a very high energy level and transferred to an acceptor molecule.
• The special pair’s missing electron is replaced by a new electron from PSII (arriving via the electron transport chain).
• The high-energy electron travels down a short second leg of the electron transport chain.
• At the end of the chain, the electron is passed to NADP+ to make NADPH (Nicotinamide Adenine Dinucleotide phosphate).

Calvin Cycle

• The Calvin Cycle, also known as the light-independent reactions, is the second stage of photosynthesis.
• It occurs in the stroma of chloroplasts.
• The Calvin Cycle uses ATP and NADPH produced in the light-dependent reactions to convert carbon dioxide into glucose.
• The Calvin Cycle has three main stages:
  • Fixation: Carbon dioxide is incorporated into an organic molecule, RuBP (ribulose 1,5-biphosphate), by the enzyme Rubisco.
  • Reduction: ATP and NADPH are used to reduce the organic molecule, resulting in the formation of G3P (glyceraldehyde 3-phosphate).
  • Regeneration: The starting molecule, RuBP, is regenerated.

Energy Cycle

• Photosynthesis is the primary source of energy for most living things on Earth.
• Organisms convert light energy into chemical energy stored in glucose.
• When organisms break down food, the chemical energy in glucose is released and used to perform work.
• Thus, photosynthesis drives the energy cycle in ecosystems.