Biology

• Photosynthesis


  The biochemical pathway that plants and other photosynthetic organisms use to convert solar energy (sunlight) into chemical potential energy (glucose)
• Chloroplast


  • All plants contain chlorophyll, a green pigment that is able to absorb different light wavelength (colours), except for green
  • There are other pigments that may mask the green chlorophyll and the plant may not appear green
  • These pigments broaden the range of light that can be absorbed by the plant
• Grana


  • Made of thylakoid membrane
  • Coated in chlorophyll (green pigment)
  • Where photons of light are absorbed
  • First phase of photosynthesis occurs (light – dependent reaction)
• Stroma


  • The space inside a chloroplast that has fluid along with chloroplast's own DNA and ribosomes
  • Where the second phase of photosynthesis happens (light independent reaction)
  • Not to be confused with the stoma (stomata)
• Light - dependent stage

  1. Occurs in the grana (thylakoid membrane) where molecules of chlorophyll are embedded that absorb the light energy

  2. During this process water is split into hydrogen ions and oxygen gas

  3. The coenzyme NADP+ transfers the H+ ions to become NADPH

  4. The coenzyme ADP + Pi transfers energy to become ATP for an input of energy into the second stage
• Light - independent stage

  1. Occurs in the stroma (fluid matrix)

  2. It requires NADPH & ATP produced in the light-dependent stage

  3. Energy (ATP) is required to combine CO2 with H+ the ions (carried by NADPH) to form glucose (C6H12O6)
• Photosynthesis Inputs


  • H2O
  • CO2
  • Light
• Photosynthesis Outputs


  • O2
  • C6H12O6
• Photosynthetic pathways


  • C3 photosynthesis
  • C4 photosynthesis
  • CAM photosynthesis
• C3 Plants


  • Use the original Calvin cycle
  • Always use the Calvin cycle to fix carbon directly from carbon dioxide
  • Rubisco is a critical enzyme that brings carbon dioxide from the air into the Calvin cycle where glucose is made
• Rubisco in C3 plants


  • Relatively slow compared to other enzymes
  • Inefficient in creating sugar molecules because its lack of specificity for carbon dioxide
  • Oxygen competes with carbon dioxide for the active site, leading to the pathway photorespiration
• Higher temperatures in C3 plants


  • Loss of fixed carbon is even greater
  • Plants close their stoma (pores in leaf surface) to reduce water loss by evaporation
  • Oxygen builds up inside the leaf, leading to a low CO2 : O2 ratio and increasing the chances of oxygen binding to Rubisco
• C4 Plants


  • The light-dependent reactions and the light-independent reactions are physically separated, with the light-dependent stage occurring in the mesophyll cells and the light-independent stage occurring bundle-sheath cells
  • Mesophyll cells are closer to the stomata where oxygen molecules are present
  • Bundle-sheath cells are deeper in the plant away from the stomata, therefore less oxygen molecules are present to bind to rubisco
• PEP Carboxylase


  Can only bind to carbon dioxide molecules, so photorespiration cannot occur
• C4 Photosynthesis


  1. CO2 is fixed in the mesophyll cells to form oxaloacetate, then converted into malate

  2. Malate is transported into the bundle-sheath cells where it is broken down into CO2

  3. A steady supply of carbon dioxide raises the carbon dioxide concentration and as a result the Rubisco preferentially binds carbon dioxide, not oxygen, and brings it into the Calvin cycle to produce glucose
• CAM Plants


  • Separate the light-dependent reactions and the use of CO2 in the light in-dependent stage in time
  • At night, CAM plants open their stomata, allowing CO2 to diffuse into the leaves
  • CO2 is fixed into oxaloacetate by PEP carboxylase then converted to malate
  • Malate is stored inside vacuoles until the next day
• CAM Photosynthesis


  1. In the daylight, the CAM plants close their stomata, but are still able to photosynthesise

  2. Malate is transported out of the vacuole and broken down to release CO2, which enters the Calvin cycle

  3. This controlled release maintains a high concentration of CO2 around rubisco without losing water
• Photosynthetic pathway characteristics


  • Separation of initial CO2 and Calvin cycle
  • Stomata open
  • Best adapted to
• C3 plants have no separation, open stomata during the day, and are best adapted to cool, wet environments
• C4 plants have separation between mesophyll and bundle-sheath cells, open stomata during the day, and are best adapted to hot, humid environments
• CAM plants have separation between night and day, open stomata at night, and are best adapted to very hot, dry environments