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Photosynthesis: Light Reaction Notes

Light Reaction and Dark Reaction

  • Photosynthesis is split into two processes: light reaction and dark reaction.

  • Multiple names for light and dark reactions:

    • Light-dependent vs. light-independent reaction.

Learning Objectives

  • Explain how energy is absorbed by pigments and transferred via NADP+ reduction to NADPH. The energy is then transferred to ATP via chemiosmosis.

  • Explain how light-dependent reaction products reduce carbon in light-independent reactions to produce glucose.

  • Describe where these processes occur in the chloroplast.

Photosynthesis Equation

  • Carbon dioxide + water + sun's energy → glucose (sugars) + oxygen.

  • General formula is important.

Overview

  • Uses energy to "fix" atmospheric carbon into sugar.

    • "Fix" means incorporating inorganic carbon (like CO_2) into organic molecules (like glucose).

  • Light reaction: harnesses sunlight to generate ATP and NADPH.

  • Dark reaction: uses ATP and NADPH to fix carbon into sugar.

  • Products of the light reaction are used in the dark reaction.

Chloroplast Review

  • Thylakoids: Small discs.

  • Grana: Stack of thylakoids.

  • Lumen: Space inside the thylakoid; also called thylakoid space.

  • Millamella: Connects multiple grana.

  • Stroma: Space outside thylakoids and grana.

Key Players

  • Electron Carriers:

    • NADP^+ / NADPH: Photosynthesis.

    • NAD^+ / NADH: Cellular respiration.

    • Mnemonic: NADP+ has a "P" for photosynthesis.

Light Reaction Overview

  • Light-dependent reaction:

    • Takes place in the thylakoid membrane.

  • Light-independent reaction (Calvin Cycle):

    • Happens in the stroma.

  • Reactants:

    • Water and sunlight.

  • Products:

    • Oxygen, ATP, and NADPH.

Light Independent Reaction

  • Also known as the Calvin Cycle.

  • Products of the light reaction are used, then remade in a cycle.

Light Reaction Details

  • Occurs in the thylakoid membrane.

  • Proteins are scattered throughout.

    • Including cholesterol and sugars.

  • Proteins are called photosystems.

    • Photosystems are complex and embedded in the membrane.

    • Found in plants, algae, and cyanobacteria.

  • Two important photosystems:

    • Photosystem I

    • Photosystem II

  • Named in order of discovery, not process order.

Light Reaction Steps

  1. Photon of light is absorbed by chlorophyll in photosystem II (PSII).

    • Photosystems contain chlorophyll molecules (pigments).

    • Chlorophyll makes plants green, but its primary role is light absorption.

  2. Light energy (photon) excites an electron in the photosystem.

    • Photons are passed between chlorophyll molecules until they reach an electron.

    • The electron gains energy.

  3. The excited electron leaves photosystem II.

    • To keep photosynthesis occurring, this electron must be replaced.

  4. Water molecules are split (photolysis) inside the lumen.

    • H2O umber{\longrightarrow} H^+ + O2 + e^-

    • Water splits into hydrogen ions (H^+), oxygen (O_2), and electrons (e^-).

    • Electrons replace those lost from photosystem II.

    • Two water molecules are consumed for every four electrons transferred.

  5. The excited electron leaves photosystem II and goes down the electron transport chain.

    • A series of proteins.

    • As the electron moves through the chain, energy is released into the thylakoid space.

    • This energy causes hydrogen ions (H^+) to be pumped from the stroma into the thylakoid space, creating a concentration gradient.

  6. Photosystem I repeats the process.

    • The electron from the electron transport chain enters photosystem I and is re-excited by another photon.

    • It then goes down a shorter electron transport chain to the final electron acceptor.

    • The final electron acceptor is NADP^+.

    • NADP^+ + e^-
      umber{\longrightarrow} NADPH

    • The NADPH then goes to the dark reaction.

  7. Chemiosmosis generates ATP.

    • The purpose of the light reaction is to produce a small amount of ATP for the dark reaction.

    • High concentration of $H^+ builds up in the lumen.

    • H^+ moves from high concentration (lumen) to low concentration (stroma) through a protein called ATP synthase.

    • ATP synthase: synthesizes ATP.

    • As $H^+ moves through ATP synthase, adenosine diphosphate (ADP) is converted into adenosine triphosphate (ATP).

Analogy

  • Photon bumps up the electron (catapult).

  • Slides it down (takes energy from the electron).

  • Falls into another system (buoy).

  • Gets hit again by a photon (re-excited).

  • Meets up with NADP^+ to turn into NADPH.