Photosynthesis

• The process of photosynthesis converts light energy from the sun into chemical energy stored in ATP and NADPH.
• the light dependent reaction occurs in the thylakoid membranes
• the light independent reaction occurs in the stroma of the chloroplasts
• Photolysis:

  1. Light energy splits water into H+, electrons and oxygen
  2. H+ is picked up by NADP to make NADPH
  3. electrons are passed along carrier proteins in the electron transfer chain
  4. oxygen is used in respiration or diffuses out.
• Photoionisation:

  1. Light energy excites electrons in the chlorophyll increasing their energy level.
  2. Electrons then leave the chlorophyll
• Chemiosmosis:

  1. Electrons are picked up by proteins and passed along the electron transfer chain.
  2. As they move along they release energy.
  3. This energy is used to actively transport H+ across the membrane from the stroma into the thylakoid lumen.
  4. Resulting in a high concentration of H+ in the lumen causing an electrochemical gradient.
  5. H+ then move down the electrochemical gradient by facilitated diffusion via ATP synthase. Producing ATP in the process.
• what stages occur in the light dependent reaction?

  Photolysis

  Photoionisation

  Chemiosmosis
• Calvin cycle (LIR)

  1. CO2 combines with RuBP producing 2 Glycerate-3-phosphate (GP).
  2. The 2 GP are reduced to triose-phosphate (TP) using ATP and converting NADPH to NADP.
  3. TP can then be converted to organic molecules such as glucose or a hexose sugar.
  4. TP can also be used to regenerate RuBP using ATP
• Light dependent reaction:

  1. electrons are excited by light energy in chlorophyll.
  2. electrons leave the chlorophyll and move along the electron transfer chain.
  3. as they move they release energy.
  4. energy used to move H+ generating ATP through ATP synthase.
  5. photolysis of water forms H+, electrons & oxygen.
  6. electrons reduce NADP to NADPH (proton added)
• Limiting factors in photosynthesis:

  1. light intensity
  2. temperature
  3. CO2 concentration
• Increased light intensity increases the LDR during photoionisation and photolysis, so more ATP & NADPH formed which is used in the LIR.

  Light causes stomata to open, allowing more CO2 inside.
• As temperature inreases enzyme activity increases up to 25 degrees and then rate decreases due to enzymes denaturing.

  Enzymes include = ATP synthase & Rubisco

  High temperatures can cause stomata to close to prevent water loss which also reduces the CO2 intake.
• As CO2 concentration increases the LIR rate increases.

  As CO2 concentration decreases less ATP & NADPH are used up so they build up possibly slowing the electron transfer chain.