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Photosynthesis Overview

# Overview of Photosynthesis

Photosynthesis consists of light reactions and dark reactions (Calvin Cycle). The light reactions convert light energy into chemical energy stored in ATP and NADPH, while the dark reactions use that energy to synthesize glucose from carbon dioxide.

# Light Reactions

  • Electron Flow and ATP Generation

    • No NADPH is generated during the initial electron flow, only ATP (as stated, 'No NADPH is generated. Right? It's not gonna generate any NADP, any reduced electron carrier in this case.')

    • Involves only one of the photosystems: Photosystem II (P680) and Photosystem I (P700).

    • The process is complex and requires a diagram for better understanding and visualization.

Thylakoid Structure

  • The site of light reactions.

    • Composed of thylakoid membranes organized in stacks (grana) with the thylakoid lumen within them.

Photosystem II

  • Also known as P680 because of the wavelength of light (680 nm) that excites the chlorophyll.

  • Absorption of light energy ejects an electron from chlorophyll.

  • The electron travels through an electron transport chain (ETC) after being excited.

Electron Transport Chain

  • The electron moves through the molecules of the ETC, causing a proton pump action.

  • Protons are pumped into the thylakoid lumen, creating a proton gradient analogous to mitochondria where protons are pumped into the intermembrane space.

  • The purpose of the proton buildup is to use ATP synthase for ATP production.

ATP Synthase

  • Functions like a channel for protons, creating ATP through chemiosmosis.

    • Reaction: ADP + P_i
      ightarrow ATP

  • Harvests energy from proton gradient generated by the ETC.

Photosystem I

  • Known as P700, it's the first photosystem discovered.

  • Absorbs light energy to energize the electron again and eject it.

  • Process is less energetic compared to P680 due to longer wavelength (700 nm).

NADPH Generation

  • The electron from P700 is passed on to NADP+, reducing it to NADPH.

    • This involves two electrons and a proton. - NADPH formation is similar to the way NAD+ is converted to NADH.

Source of Electrons

  • Water Hydrolysis: To replace lost electrons in the photosystems, water molecules are split:

    • Hydrolysis of water produces electrons, protons, and oxygen.

    • Oxygen is released as a waste product of photosynthesis.

Summary of Light Reactions

  • Electrons transferred from P680 create an electron transport chain and generate ATP.

  • Electrons from P700 lead to NADPH production.

  • Water provides replacement electrons and produces oxygen as a byproduct.

# Dark Reactions (Calvin Cycle)

  • The cycle consists of three phases: carbon fixation, reduction, and regeneration.

Phase 1: Carbon Fixation

  • Involves the incorporation of carbon dioxide into the organic molecule ribulose bisphosphate (RuBP).

  • This step is catalyzed by an enzyme called Rubisco (ribulose bisphosphate carboxylase/oxygenase).

  • Reaction:

    • CO2 + RuBP → an unstable six-carbon intermediate → immediately splits into two three-carbon molecules (3-PGA).

Phase 2: Reduction

  • Utilizing ATP and NADPH produced in light reactions, 3-PGA is reduced to form glyceraldehyde-3-phosphate (G3P). - G3P is a key intermediate that can enter metabolic pathways.

  • Two G3P molecules can be combined to form glucose and/or other carbohydrates.

Phase 3: Regeneration

  • Remaining G3P molecules are used to regenerate RuBP, allowing the cycle to continue.

  • ATP is used in this step as well to facilitate the conversion.

Overview of Calvin Cycle

  • The process overall is cyclical, regenerating starting materials while synthesizing sugar.

  • Relationship between the light reactions and Calvin Cycle shows the transfer of energy through ATP and NADPH.

# Key Points

  • Photophosphorylation vs Oxidative Phosphorylation:

    • Both mechanisms generate ATP through chemiosmosis, but differ in electron sources (light energy vs. organic molecules).

  • Plants perform photosynthesis AND cellular respiration, utilizing their produced glucose for energy.

  • Importance of water hydrolysis in replenishing electron deficits in photosystems is crucial for continuous ATP and NADPH production capability.

  • Both light and dark reactions are integrated parts of the broader photosynthetic process, making energy and organic compounds available to the plant and thus supporting life on Earth.