U4L2: Sugar is Food

• Calvin cycle


  The second stage of photosynthesis in which carbon atoms from carbon dioxide are combined, using the energy in ATP and NADPH, to make glucose
• Chemosynthesis


  The process of using the energy in chemical compounds to make food
• Chlorophyll


  Green pigment in a chloroplast that absorbs sunlight in the light reactions of photosynthesis
• Electron transport chain


  Series of electron-transport molecules that pass high-energy electrons from molecule to molecule and capture their energy
• Grana


  Within the chloroplast, consists of sac-like membranes known as thylakoid membranes
• Light reactions


  First stage of photosynthesis in which light energy from the sun is captured and changed into chemical energy that is stored in ATP and NADPH
• Photosystem


  A group of molecules, including chlorophyll, in the thylakoid membrane of a chloroplast that captures light energy
• Stroma


  Space outside the thylakoid membranes of a chloroplast where the Calvin cycle of photosynthesis takes place
• Thylakoid membrane


  Membrane in a chloroplast where the light reactions of photosynthesis occur
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• Photosynthesis


  The process by which plants and other organisms use sunlight, water, and carbon dioxide to create oxygen and energy in the form of sugar
• Light-dependent reactions


  • Use light energy to make ATP and NADPH
  • Take place in the thylakoid membranes of chloroplasts
  • Involve photosystems I and II
• Photosystem


  A large complex of proteins and pigments (light-absorbing molecules) that is optimized to harvest light
• Reaction center chlorophylls


  P700 in photosystem I

  P680 in photosystem II
• Non-cyclic photophosphorylation


  1. Light absorption in PSII

  2. ATP synthesis

  3. Light absorption in PSI

  4. NADPH formation
• Cyclic photophosphorylation is another form of the light-dependent reactions where electrons cycle through PSI and the electron transport chain but do not pass through PSII
• Resonance energy transfer


  When a pigment absorbs a photon and transfers the energy to a neighboring pigment through direct electromagnetic interactions
• The pigment molecules in a photosystem collect energy and transfer it to the reaction center chlorophyll
• Photosystems


  Complexes made up of proteins and pigments that harvest light and convert it to chemical energy
• Light-harvesting complexes


  • Contain proteins, 300-400 chlorophylls, and other pigments
• Resonance energy transfer


  When a pigment is excited by light, it transfers energy to a neighboring pigment through direct electromagnetic interactions
• Reaction center


  Central part of the photosystem where energy is transferred
• Photosystems are structures within the thylakoid membrane that harvest light and convert it to chemical energy
• Photosystem II (PSII)


  • Absorbs light at 680nm, primary electron acceptor is pheophytin
• Photosystem I (PSI)


  • Absorbs light at 700nm, primary electron acceptor is chlorophyll A0
• Electron transport chain


  Electrons are passed from PSII to PSI, losing energy along the way
• When the P680 special pair of PSII absorbs energy


  It enters an excited state and can transfer its electron to the primary electron acceptor, pheophytin
• Water splitting in PSII

  Manganese center binds 2 water molecules, extracts 4 electrons, releases 4 H+ ions, and produces 1 O2 molecule
• About 10% of the oxygen produced is used by mitochondria in the leaf, the rest escapes to the atmosphere
• Electron transport chain in PSII

  Electrons are passed from PSII to plastoquinone, cytochrome complex, and plastocyanin, releasing energy to pump protons into the thylakoid lumen
• Electron transport in PSI

  Electrons are re-excited by light and passed from PSI to ferredoxin and NADP+ reductase, reducing NADP+ to NADPH
• ATP synthesis


  Protons flow down their concentration gradient through ATP synthase, driving the production of ATP
• Cyclic photophosphorylation


  Electrons flow cyclically through PSI instead of linearly to NADPH, producing ATP but not NADPH
• Cyclic electron flow may occur when the ratio of NADPH to NADP+ is too high, or in cell types with high ATP needs
• Calvin cycle


  1. Carbon fixation

  2. Reduction

  3. Regeneration
• Carbon fixation


  CO2 molecule combines with a five-carbon acceptor molecule (RuBP), forming an unstable six-carbon compound that splits into two molecules of a three-carbon compound (3-PGA)
• Rubisco


  • Enzyme that catalyzes the carbon fixation reaction
• Reduction stage


  1. 3-PGA receives a phosphate group from ATP, forming 1,3-bisphosphoglycerate

  2. 1,3-bisphosphoglycerate is reduced by NADPH, forming glyceraldehyde 3-phosphate (G3P)