photosynthesis

• • Chloroplasts are the organelles in plant cells where photosynthesis occurs
  • Each chloroplast is surrounded by a double-membrane envelope
  • Each of the envelope membranes is a phospholipid bilayer
  • Chloroplasts are filled with a fluid known as the stroma
  • The stroma is the site of the light-independent stage of photosynthesis
• Stroma


  A separate system of membranes found in the stroma
• Membrane system in the stroma

  • Site of the light-dependent stage of photosynthesis
  • Contains the pigments, enzymes and electron carriers required for the light-dependent reactions
• Thylakoids


  Flattened fluid-filled sacs that stack up to form structures known as grana
• Grana

  Stacks of thylakoids connected by membranous channels called stroma lamellae
• Membrane system of grana


  • Creates a large surface area to increase the number of light-dependent reactions that can occur
  • Provides a large number of pigment molecules in an arrangement that ensures as much light as necessary is absorbed
• • The stroma also contains small (70S) ribosomes, a loop of DNA and starch grains:
  • The loop of DNA codes for some of the chloroplast proteins (other chloroplast proteins are coded for by the DNA in the plant cell nucleus)
  • The proteins coded for by this loop of chloroplast DNA are produced at the 70S ribosomes
  • Sugars formed during photosynthesis are stored as starch inside starch grains
• structure of a chloroplast

  A) ribosome

  B) starch grain

  C) outer membrane

  D) inner membrane

  E) stroma

  F) thylakoid

  G) grana

  H) granum

  
• the light dependant reaction happens in the thylakoids
• • The light-dependent stage of photosynthesis occurs in the thylakoid membranes and the thylakoid spaces (the spaces inside the thylakoids)
• Thylakoid membranes


  • Contain the pigments, enzymes and electron carriers required for the light-dependent reactions
• Electron transport chain

  Electrons are carried through the membrane
• Electron energy


  Can be used to move H+ ions across the membrane
• Light-dependent reactions

  1. Electrons are donated by water molecules

  2. Electrons are accepted by NADP molecules that form NADPH
• Grana membranes


  • Create a large surface area to increase the number of light-dependent reactions that can occur
• Membrane system


  • Provides a large number of pigment molecules in an arrangement that ensures as much light as necessary is absorbed
• Photosystems

  Light-harvesting clusters of pigment molecules
• The Light-Dependent Reaction


  1. Light energy is used to breakdown water in a reaction known as photolysis

  2. This produces hydrogen ions, electrons, and oxygen in the thylakoid lumen
• Photolysis of water


  Results in a high concentration of hydrogen ions in the thylakoid lumen
• Electron transport chain

  Electrons travel through a chain of proteins within the membrane
• Reduced NADP (NADPH)


  Produced when hydrogen ions in the stroma and electrons from the electron transport chain combine with the carrier molecule NADP
• Photophosphorylation

  1. ATP is produced (ADP + Pi → ATP)

  2. Using the proton gradient between the thylakoid lumen and stroma to drive the enzyme ATP synthase
• Photoionisation


  Light is absorbed by pigments located in the thylakoid membrane, causing two electrons in the chlorophyll molecule to be excited to a higher energy level and emitted from the chlorophyll molecule
• Photophosphorylation


  The creation of a proton gradient across the thylakoid membrane drives the synthesis of ATP
• Electron transport chain

  1. Excited electron passed down

  2. Chemiosmosis occurs

  3. Proton gradient created

  4. ATP synthesis driven
• Oxygen-evolving complex


  • Water-splitting enzyme in the thylakoid
  • Catalyses the breakdown (photolysis) of water by light
• H2O → 2H+ + 2e- + ½O2
• Excited electrons pass down electron transport chain


  They are replaced by electrons from the photolysis of water
• Electron transport chain

  1. Electrons alternatively reduce and oxidise proteins

  2. Combine with H+ and NADP to give reduced NADP
• Reduced NADP (NADPH)


  Passes to the light-independent reactions to be used in the synthesis of carbohydrates
• Photophosphorylation

  The overall process of using light energy and the electron transport chain to phosphorylate ADP to ATP
• Light-dependent reaction

  Also called 'photophosphorylation'
• Photophosphorylation

  1. Energetic (excited) electrons are passed along a chain of electron carriers (electron transport chain)

  2. Electron carriers are alternately reduced (gain an electron) and then oxidised (lose the electron by passing it to the next carrier)

  3. Excited electrons gradually release their energy as they pass through the electron transport chain

  4. Released energy is used to actively transport protons (H+ ions) across the thylakoid membrane, from the stroma to the thylakoid lumen

  5. A 'proton pump' transports the protons across the thylakoid membrane

  6. Energy for proton transport comes from the excited electrons moving through the electron transport chain

  7. Creates a proton gradient, with high concentration in thylakoid lumen and low concentration in stroma

  8. Protons return to stroma (down the proton concentration gradient) through ATP synthase enzymes in a process called chemiosmosis

  9. Provides the energy needed to synthesise ATP by adding an inorganic phosphate group (Pi) to ADP (ADP + Pi → ATP)
• The whole process is known as photophosphorylation as light provides the initial energy source for ATP synthesis
• • There are two groups of pigments: primary pigments known as chlorophylls and accessory pigments known as carotenoids
  • The primary pigment in photosystem II is chlorophyll b and the primary pigment in photosystem I is chlorophyll a
  • Accessory pigments that surround the primary pigment absorb both similar and different wavelengths of light to chlorophyll, this expands the wavelength range that can be absorbed from light for use in photosynthesis
• Chromatography


  An experimental technique used to separate mixtures
• Chromatography

  1. Different components within the mixture travel through the material at different speeds

  2. This causes the different components to separate
• Retardation factor (Rf)


  Can be calculated for each component of the mixture
• Common techniques for separating photosynthetic pigments

  • Paper chromatography - mixture of pigments is passed through paper (cellulose)
  • Thin-layer chromatography - mixture of pigments is passed through a thin layer of adsorbent (eg. silica gel), through which the mixture travels faster and separates more distinctly
• Apparatus

  • Leaf sample
  • Distilled water
  • Pestle and mortar
  • Filter paper
  • Capillary tube
  • Chromatography solvent
  • Acetone
  • Pencil
  • Ruler