Photosynthesis

Photosynthesis

1. Chloroplast structure

   • Stroma - contains soluble enzymes
   • Granum - a stack of thylakoids
   • Intergranal lamellae - used to transport substances between grana
   • Thylakoid / thylakoid membrane

2. Photosystems (fixed proteins)

   • 1° pigment - chlorophyll a
   • Accessory pigments - carotenoids, xanthophylls & chlorophyll b absorb a greater number of light wavelenths

3. Light dependent reaction

   • H+ pumped from the stroma to the thylakoid space by proton pumps
   • H+ flow through ATP synthase by chemiosmosis
   • Proton motive force drives ATP synthase ADP -> ATP
   • e- energy is used to pump protons
   • Following ATP synthesis 2e- + H+ -> H2
   • H2 + 2NADP -> 2NADPH
   • Non- cyclic photophosphorylation
     • Photosystem I and II
     • Requires photolysis
     • e- recombine with protons to reduce NADP
     • Products: ATP , Oxygen, NADPH
   • Cyclic photophosphorylation
     • Photosystem I
     • e- return to PSI
     • Products: ATP

4. Photolysis

   • Energy absorbed from photosystems in the thylakoid memrane
   • H2O -> H2 + 1/2 O2
     • H2 -> 2H+ + 2e-

5. Light independent reaction

   • Fixation
     • RuBisCo enzyme binds carbon + RuBP -> 6carbon unstable intermediatE -> 2GP
       • RuBisCo (ribulosebisphosphatecarboxylate) can bind with 02 as well as CO2
         • Photorespiration
           • Stomata close to prevent water loss
           • CO2 cannot enter plant
           • O2 conc. builds up
           • Oxygen acts as a competitive inhibitor
           • Phosphogolycolate is produced instead of GP
           • Phosphogolycate is toxic and requires ATP for its breakdown
           • Approx. 25% of Calvin cycle products are lost in photorespiration
   • Reduction
     • 2GP --ATP/NADPH--> 2TP
   • Regeneration
     • 10/12 TP -> 6RuBP
     • 2/12 TP -> lipids, amino acids & glucose

6. Types of photosynthesis

   • C3 photosynthesis
     • Used by most plants
     • Most efficient in cool, wet climates with average sunshine values
   • C4 photosynthesis
     • Used in high temperatures with limited water supply
     • More efficient CO2 fixing
     • Stomata open for less time so less water loss by transpiration
     • PEP carboxylase fixes CO2 and is not inhibited by O2
     • e.g. corn
   • CAM photosynthesis
     • Plants open stomata at night so reduced water loss by transpiration
     • CO2 is converted to an acid and stored
     • Acid is broken down in the day to allow fixation of carbon using RuBisCo
     • In very dry conditions CO2 from resp. is used and O2 from PSS is used in aerobic resp. stomata remain shut
     • e.g. cacti

7. Limiting factors

   • Temperature
     • Enzyme controlled reactions in the Calvin cycle increase rate up to 25°C
     • Above 25°C the rate of PSS decreases due to denaturation of enzymes
   • Light intensity
     • Causes stomata to open, CO2 can enter
     • Excites more electrons
     • Photolysis
   • CO2 conc.
     • High conc. increases PSS, allows more carbon fixation in Calvin cycle
       • More GP
       • More TP
       • More regeneration of RuBP
       • More production of glucose, lipids & amino acids