Photosynthesis

• the light-dependent reaction of photosynthesis involves the capture of light whose energy is used for…

  • to add an inorganic phosphate (pi) molecule to ADP to make ATP.
  • to split water into H+ ions (protons) and OH- ions. This process is called photolysis
• What are the 4 stages in the light-dependent reaction?

  photolysis

  photoionisation of chlorophyll

  chemiosmosis

  production of ATP and reduced NADP
• explain photolysis 


  photo(light) lysis (splitting)

  light energy is absorbed by chlorophyll and splits water into oxygen H+ and e-.

  H2O —> 1/2 O2 + 2e- + 2H+

  the e- are passed along a chain of electron carrier proteins (electron transport chain)

  the H+ is picked up by NADP to form NADPH and is used in the LIR.

  Oxygen is either used for respiration or diffuses out of the leaf through the stomata.
• Photoionisation and photophosphorylation

  1. Light is absorbed by chlorophyll in the thylakoid membrane

  2. Two electrons in the chlorophyll molecule are exited to a higher energy level and are emitted from the chlorophyll molecule

  3. Each exited electron is passed down a chain of electron carrier proteins (the electron transport chain)

  4. During this process chemiosmosis occurs: as electrons release energy as they move along the ETC H+ ions are actively transported from the stroma to the thylakoid lumen (against concentration gradient)

  5. The protons then move by facilitated diffusion through ATP synthase by chemiosmosis

  6. The process provides the energy needed to synthesis ATP (ATP + Pi —>ATP)

  7. The proton combines with the co-enzyme NADP to become reduced NADP. Because the protons move from a high to low concentration gradient
• Give examples of complex organic molecules produced in the light independent stage of photosynthesis

  Starch (for storage)

  sucrose (For translocation)

  cellulose (for making cell walls)
• what are the three main steps within the Calvin cycle

  1. rubisco catalyses fixation of carbon dioxide by combination with a molecule of riblose (RuBP) a 5C compound which produces 2 Glycerate 3-phosphate (GP) molecules a 3C compound.
  2. reduced NADPH is used to reduce glycerate 3-phosphate to triose phosphate.
  3. NADP is re-formed and can now be reduced again int he light-dependent reaction
• carbon fixation

  1. carbon dioxide combines with ribulose bisphosphate (RuBP)
  2. an enzyme called rubisco (ribulose bisphosphate carboxylase) catalyses this reaction
  3. RuBP is unstable and splits into two molecules known as glycerate 3-phosphate (GP)
• reduction of glycerate 3-phosphate

  1. energy from ATP and HYDROGEN FROM REDUCED NADP are used to reduced glycerate 3 -phosphate into triose phosphate (TP)
• what is triose phosphate used to produce ?

  hexose phosphate - can be used to produced starch, sucrose or cellulose

  lipids for cell membrane

  amino acids for protein synthesis
• What are factors that limit the rate of photosynthesis ?

  1. presence of photosynthetic pigments
  2. a supply of water
  3. light energy
  4. temperature
  5. carbon dioxide
• how does an increase in light intensity increase the rate of photosynthesis?

  more energy supplied to the plant the faster the light dependent stage of photosynthesis can occur

  this produces more ATP and reduced NADP for the Calvin cycle
• how does an increase in carbon dioxide increase the rate of photosynthesis?

  more carbon dioxide the faster the rate of carbon fixation. the quicker the rate of the Calvin cycle the faster the overall rate of photosynthesis
• what stage of photosynthesis is affected by temperature ?

  the Calvin cycle as the light-independent reactions are enzyme controlled reactions for example rubisco catalysing the reaction between CO2 and ribulose bisphoasphate
• what other factor excluding enzymes is affected by temperature?

  increasing the temperature causes the stomata on the leaf to close in order to reduced water loss. when the stomata are closed CO2 cannot enter the leaves therefore reducing the rate of the Calvin cycle.
• Describe how reduced NADP is produced in the light-independent reactions of photosynthesis 


  Light is absorbed by chlorophyll molecules (in thylakoid membrane)

  electrons are emitted to a higher energy level

  electrons are passed along ETC and then accepted by co-enzyme (NADP)

  water is split (photolysis)into its ions H+ and OH-

  electrons from water molecule replace electrons in chlorophyll

  protons from water reduce NADP