CHAPTER 5 - Photosynthesis

• Coenzymes


  Organic, non-protein molecules that assist enzymes in catalysing reactions
• Coenzymes


  • The enzyme remains unchanged, but the structure of the coenzyme is changed
  • The coenzyme binds to the active site, donates energy or molecules, and then cannot be immediately reused
  • After the reaction, the coenzyme leaves the enzyme and is recycled by accepting more energy, so it can then go on to assist in more reactions
• Coenzymes in Photosynthesis


  • ADP/ATP
  • NADP+/NADPH
• ATP


  • The main energy coenzyme of the cell, it donates energy to catalyse reactions
  • Upon releasing energy, ATP loses a phosphate group and becomes ADP
  • ADP can have a phosphate group re-added to become ATP again
• NADP+ and NADPH


  • Work like ATP as a coenzyme
  • NADP+ picks up a hydrogen to become NADPH
  • NADPH breaks off the hydrogen and releases a small amount of energy, turning back into NADP+
• Photosynthesis


  • It is carried out by green plants, some bacteria and some protists
  • In plants and photosynthetic protists, it takes place in the chloroplasts (in bacteria it takes place in the cell but not in a particular organelle)
• Types of autotrophs


  • Photoautotrophs: use sunlight energy for photosynthesis (e.g. plants)
  • Chemoautotrophs: use chemical energy for photosynthesis (e.g. bacteria that use sulfide or methane)
• Leaves for photosynthesis


  • Large surface area to maximise amount of sunlight hitting the surface
  • Mesophyll cells contain chloroplasts (the site of photosynthesis), split into 2 layers (palisade & spongy)
  • Chloroplasts contain pigment chlorophyll (captures and is energised by light energy)
  • Stomata open to allow carbon dioxide in, can close to prevent water loss
  • Root hair cells absorb water from the soil, transported through xylem to photosynthesising cells
• C3 plants


  The majority of plant species on Earth use C3 photosynthesis, the first carbon compound produced contains three carbon atoms
• Photosynthesis


  1. Light-dependent stage involving trapping of light energy

  2. Light-independent stage, known as the carbon fixation, in which energy is trapped in the first stage is used to make/synthesise glucose from carbon dioxide and water
• Light-dependent stage


  1. Only occurs when light is present

  2. Occurs in the chlorophyll-filled thylakoid membranes

  3. Purpose is to generate high energy coenzymes NADPH and ATP to power the second stage of photosynthesis

  4. Inputs: H2O, NADP+, ADP + Pi

  5. Outputs: O2, NADPH, ATP
• Light-dependent stage


  1. Water enters the thylakoids

  2. Light energy splits the water into hydrogen and oxygen

  3. Oxygen leaves the chloroplast

  4. Hydrogen is picked up by NADP+ to form NADPH

  5. ADP + Pi form some ATP
• Light-independent stage


  1. Does not require light to occur

  2. Reactions are energised by ATP and NADPH coenzymes produced by the light dependent reactions

  3. Occurs in the stroma

  4. Involves multiple reactions in the Calvin cycle

  5. Inputs: 6CO2, 12 NADPH, 18 ATP

  6. Outputs: glucose, 6H2O, 12 NADP+, 18 ADP + 18 Pi
• Light-independent stage


  1. CO2 enters the Calvin cycle

  2. Carbon fixation - Carbon from CO2 combines with a 5 carbon molecule (RuBP), then splits into two x 3 carbon molecules

  3. Reduction - NADPH molecules donate their H+ ions and electrons, and ATP molecules break into ADP + Pi to release energy to facilitate further changes to carbon molecules

  4. One specific three carbon molecule (PGAL) is created and leaves the cycle, going on to contribute to the formation of glucose

  5. Regeneration - The start molecule 5 carbon RuBP re-formed to begin cycle again

  6. Some of the oxygen molecules, leftover from breaking the CO2 at the beginning of the cycle, combine with H+ ions from NADPH to create the output of water
• How to tell if photosynthesis has occurred
• Rubisco


  An enzyme involved in the light independent stage of photosynthesis that is responsible for the initial changes to carbon dioxide at the beginning of the Calvin cycle
• Rubisco in light-independent stage

  1. Binds to CO2 and facilitates further reactions to make glucose

  2. Sometimes binds to oxygen instead and initiates photorespiration
• Photorespiration


  A wasteful process that occurs when Rubisco binds to oxygen instead of CO2, disrupting photosynthesis and reducing glucose production
• Factors influencing Rubisco binding

  • Temperature
  • Substrate (CO2 or O2) concentration
• Increased O2 concentration


  Leads to photorespiration
• Hot and arid/dry weather

  Increases likelihood of Rubisco binding to O2 and photorespiration occurring
• C3 plants


  Plants with no evolved adaptation to minimise photorespiration
• C4 plants


  Plants that minimise photorespiration by separating initial carbon fixation and the remainder of the Calvin cycle
• C4 plant adaptations


  • Use PEP carboxylase instead of Rubisco for initial carbon fixation
  • Separate carbon fixation and Calvin cycle in different cells to reduce exposure to O2
• CAM plants


  Plants that minimise photorespiration by separating initial carbon fixation and the remainder of the Calvin cycle over time
• CAM plant adaptations


  • Perform carbon fixation at night when stomata are open
  • Store fixed carbon as malate or other organic compounds to use during the day when stomata are closed
• Closed stomata in CAM plants

  Prevents CO2 entry and leads to O2 buildup, increasing photorespiration
• C4 and CAM photosynthesis require more ATP than C3 photosynthesis
• C4 and CAM plants have a competitive advantage over C3 plants in hot, dry, or low CO2 conditions due to reduced photorespiration
• PBS Nova: 'Great Rubisco video'
• Key terms


  • Rubisco
  • Carbon fixation
  • Photorespiration