5.1 Photosynthesis

• What are the stages

  of photosynthesis?

  1. Light dependent reaction

  ○ Thylakoid membrane of chloroplast

  2. Light independent reaction

  ○ Stroma of chloroplast
• Chloroplast structure:

  A) double membrane

  B) stroma

  C) thylakoid membrane

  D) small - 70S ribosomes

  E) circular DNA

  F) starch granules/lipid droplets

  G) lamella

  H) grana

  
• Describe photoionisation in the light-dependent reaction (LDR)

  ● Chlorophyll absorbs light energy which excites its electrons (higher energy level)

  ● So electrons are released from chlorophyll (chlorophyll becomes positively charged)
• Describe what happens after photoionisation in the LDR

  Some energy from electrons released in photoionisation is conserved in the production of ATP / reduced NADP

  (chemiosmotic theory):

  1. Electrons move along electron transfer chain (electron carriers), releasing energy

  2. This energy is used to actively pump protons from stroma into thylakoid

  3. Protons move by facilitated diffusion down electrochemical gradient into stroma via ATP synthase

  4. Energy used to join ADP and Pi to form ATP (photophosphorylation)

  5. NADP accepts a proton and an electron to become reduced NADP
• Describe photolysis of water in the LDR

  ● Water splits to produce protons, electrons and oxygen (H2O → 1⁄2 O2 + 2e- + 2H+)

  ○ Electrons replace those lost from chlorophyll
• Describe the light-independent reaction of photosynthesis (Calvin cycle)

  1. CO2 reacts with ribulose bisphosphate (RuBP)

  ○ Catalysed by the enzyme rubisco

  2. Forming 2 glycerate 3-phosphate (GP) molecules

  3. GP reduced to triose phosphate (TP)

  ○ Using products from light-dependent reaction - reduced NADP and energy from ATP

  4. Some TP converted to useful organic substances (eg. glucose)

  5. Some TP used to regenerate RuBP in the Calvin cycle (using energy from ATP)
• Light independent reaction:

  A) ATP

  B) ADP + Pi

  C) NADPH

  D) NADP

  E) ATP

  F) ADP + Pi

  
• Describe and explain how temperature affects rate of photosynthesis

  1. As temperature increases, rate increases

  ○ Enzymes eg. rubisco gain kinetic energy

  ○ So more enzyme-substrate complexes form

  2. Above an optimum temperature, rate decreases

  ○ Enzymes denature as H bonds in tertiary structure break

  ○ So fewer enzyme-substrate complexes form
• Describe and explain how light intensity affects rate of photosynthesis

  1. As light intensity increases, rate increases

  ○ Light-dependent reaction increases (eg. more photoionisation of

  chlorophyll) so more ATP and reduced NADP produced

  ○ So light-independent reaction increases as more GP reduced to

  TP and more TP regenerates RuBP

  2. Above a certain light intensity, rate stops increasing

  ○ Another factor is limiting eg. temperature / CO2 concentration
• Describe and explain how CO2 concentration affects rate of photosynthesis

  1. As CO2 concentration increases, rate increases

  ○ Light-independent reaction increases

  ○ As more CO2 combines with RuBP to form GP

  ○ So more GP reduced to TP

  ○ So more TP converted to organic substances and more

  RuBP regenerated

  2. Above a certain CO2 concentration, rate stops increasing

  ○ Another factor is limiting eg. temperature / light intensity
• why does reducing the transfer of electrons down the electron transfer chain, reduce the rate of photosynthesis?

  • less protons actively pumped into thylakoid
  • less ATP produced
  • less NADPH produced
  • light independent reaction slows