VH

Comprehensive Notes on Photosynthesis

Photosynthesis & Cellular Respiration

C1: Photosynthesis

  • Explored questions:
    • How is solar energy captured, transferred to electrons, and used to split water?
    • How are ATP and NADPH produced?

Photosynthesis Review

  • Why Plants, algae, and some bacteria undergo photosynthesis is to combine carbon dioxide, water, and energy from the sun to synthesize glucose.

  • Photosynthesis Location: Takes place in the chloroplasts of plants.

  • Two types of reactions that allow photosynthesis to occur

Chloroplasts Review

  • Chloroplasts: The site of photosynthesis within plant cells.
  • Bound by inner and outer membranes.
  • Stroma: The inner fluid of the chloroplast, containing a concentrated mixture of proteins and chemicals used for glucose synthesis.

Chloroplasts Review

  • Thylakoids: Flattened, interconnected sacs within the chloroplast that contain chlorophyll.
  • Chlorophyll: A green pigment responsible for trapping solar energy.
  • Grana: Stacks of thylakoids connected by lamellae.

Photosynthesis Overview

  • Inputs: Light, H2O, CO2, NADP^+, ADP + P_i
  • Outputs: Starch, Sugar ([CH2O], Sucrose), O2, ATP, NADPH
  • Light Reactions: Occur in the thylakoids and produce ATP and NADPH.
  • Calvin Cycle: Occurs in the stroma and uses ATP and NADPH to fix carbon and produce sugar.

Photosynthesis Reactions

  • Light-Dependent Reactions (Photo)
  • Light-Independent Reactions (Synthesis)
  • Oxygen is a by-product of photosynthesis.

Light-Dependent Reactions

  • Location: Thylakoid membrane of the chloroplast.
  • Inputs: Light, H2O, NADP^+, ADP + Pi
  • Outputs: O_2, ATP, NADPH
  • Calvin Cycle Inputs: CO_2, ATP, NADPH
  • Calvin Cycle Outputs: Phosphoglycerate, G3P, Sugar, Starch, Amino Acids, Fatty Acids, Sucrose (export).

Light Dependent Reactions Stages

  • Stage 1: Capturing solar energy and transferring it to electrons through splitting of water.
  • Stage 2: Using energy to make ATP and transferring electrons to make NADPH.

Light Dependent Reaction Components

  • Photosystems: Capture light energy.
  • Oxidation and Reduction Reactions: Transfer electrons.
  • Electron Transport Chain: Transports electrons and generates a proton gradient.

Photosystems

  • Photosystem I (PSI)
  • Photosystem II (PSII)

Light Dependent Reactions Products

  • Reducing power for light-independent reactions (NADPH).
  • Energy for light-independent reactions (ATP).
  • Water splits: H2O othe 2H^+ + 1/2 O2

Leaf Color

  • Chlorophyll is the main photosynthetic pigment.
  • 'White' light consists of all colors (wavelengths).
  • Green light is reflected, while blue and red wavelengths are absorbed.

Photosystem Components

  • Photosystem: Clusters of chlorophyll and other pigments embedded in the thylakoid membrane.
  • Components: Antenna pigment molecules, reaction center chlorophyll, primary electron acceptor.
  • Photosystems: Photosystem I (PSI) and Photosystem II (PSII).
  • Photosystem II (PSII) comes first in the light reaction.

Stage 1: Capture Solar Energy: Photosystem II

  • Light strikes chloroplast resulting in photons excite chlorophyll molecules in thylakoid membranes.
  • Chlorophyll captures light energy by absorbing photons and passing the energy to electrons.
  • Energy is transferred to the reaction center.

Stage 1: Capture Solar Energy: Photosystem II

  • Photolysis: Light energy splits water.
  • Occurs in the thylakoid lumen, dividing water into hydrogen ions, oxygen, and electrons.
  • Oxygen is released into the atmosphere through the stoma.
  • Hydrogen ions (H^+) are formed and remain in the lumen.

Where photolysis occurs?

  • Photolysis occurs in the thylakoid lumen; water is broken down into hydrogen ions, oxygen, and electrons.

Stage 1: Capture Solar Energy: Photosystem II Electron Excitation

  • Electrons are excited and move from the lumen to chlorophyll molecules in Photosystem II.
  • Electrons are then transferred to Photosystem I via the electron transport chain.

Analogy

  • Photons excite electrons in Photosystem II.
  • Electron transport chain leads to ATP production (mill makes ATP).
  • Photons excite electrons in Photosystem I.
  • NADPH is produced.

Products of Photolysis

  • Hydrolysis (splitting of H2O) creates O2 and H^+ ions.
  • Oxygen is released to the environment.
  • H^+ accumulates inside the thylakoid lumen, creating a concentration gradient.

Photosystem II

  • Photon strikes Photosystem II (P680).
  • Water is split, producing O_2 and H^+ ions.
  • Electrons move through the electron transport chain, providing energy for ATP synthesis by chemiosmosis.

Electron Transport Chain

  • Location: Along the thylakoid membrane.
  • Electrons move along this membrane, releasing energy as they move in a step-by-step manner.
  • Electrons pass from one carrier to another, moving down the ETC as redox reactions occur.

Stage 2: Making NADPH | Photosystem I

  • Electrons move from PSII to PSI using the ETC.
  • Every step down the excited electron takes, it gives up some stored (potential) energy.
  • This energy is used to make ATP.
  • Once the electron reaches PSI, it is hit with light again and excited.
  • The energy released by Photosystem I is used to rejoin the high energy electrons with the hydrogen ions and NADP^+ to produce NADPH, the final electron acceptor.

Photosystem I

  • Photons strikes Photosystem I (P700).
  • Electrons are re-energized and used to produce NADPH.

Reduction and Oxidation Reactions

  • Photosynthesis uses electrons (e^-) to transfer energy.
  • Electrons are passed around in the process.
  • When electrons transfer in a chemical equation, this is called a redox reaction.
  • General form: Xe^- + Y othe X + Ye^-

Mnemonic for Redox Reactions

  • LEO the lion says GER
  • LEO: Loss of Electrons = Oxidation.
    • The reducing agent.
  • GER: Gain of Electrons = Reduction.
    • The oxidizing agent.

Making ATP | ATP Synthase

  • When electrons are passed down the ETC from PSII to PSI:
    • The energy released is used to pull (against the concentration gradient) H^+ in the thylakoid lumen.
    • This creates a positive charge in the thylakoid lumen and a steep concentration gradient.

ETC & ATP Synthase

  • Even though H^+ ions would likely diffuse back across, they cannot because the membrane is impermeable to them, so, they are stuck there!
  • How do they get out? = Chemiosmosis!

ETC & ATP Synthase

  • Because the membrane is impermeable to H^+, a special structure called ATP synthase, embedded in the thylakoid membrane, provides the only pathway for the H^+ to move out.
  • The process for synthesizing ATP using the energy from a H^+ gradient and the ATP synthase enzyme is called CHEMIOSMOSIS.

Light Dependent Reaction Summary

  • Requirements: Light energy, water.
  • Products: Oxygen, NADPH, ATP.