Photosynthesis

Photosynthesis

1. 2. Light independent reactions

   • The Calvin Cycle
     • 1. Carbon dioxide diffuses into the plant, into the cell and into the stroma of the chloroplasts
       • 2. The carbon dioxide fixes with the 5-carbon molecule Ribulose bisphosphate (RuBP). This is carbon fixation
         • 3. The unstable 6-carbon molecule formed immediately breaks into 2 glycerate 3-phosphate molecules (2 x GP)
           • 4. Each GP is converted to triose phosphate (TP) using a H atom fron NADP and energy from an ATP
             • 5. 1/6 of the carbons from the TP are used to make organic molecules (lipids, amino acids, glucose) and the other 5/6 carbons are used to regenerate the RuBP --> regeneration requires ATP
         • *catalysed by ribulose bisphosphate carboxylase (RuBisCO)*
     • Fixation, Reduction, Regeneration
   • Occurs in the stroma
     • "Stroma, independent woman"

2. 1. Light dependent reactions

   • 1. Non-cyclic photophosphorylation
     • 1. Electrons in photosystem II are excited by the sunlight
       • 2. The electrons are released from the reaction centre where the wavelengths of light are absorbed
         • 3. The electrons pass through the ETC to create ATP by chemiosmosis & using ATPsythase
           • 4. Excited electrons are released from the reaction centre at PSI and are pased to another ETC and ATP is again produced by chemiosmosis & ATPsythase
             • 5. Electrons leaving the ETC after PSI are accepted along with a H ion by NADP (coenzyme) which provides the reducing power in the production of organic molecules
         • These electrons are replaced by the electrons released by the photolysis of water
           • Water molecules are split into H ions, electrons and oxygen
             • Electrons used to replace those lost in PSII
             • H ions / protons used are released into the lumen of the thylakoid to increase the proton concentration gradient across the membrane
               • When they move back through the membrane, they drive the formation of ATP and once they're back in the stroma, they combine with NADP and an electron from PSI to form NADPH for the light-independent reactions
             • Oxygen released as a waste by-product
   • 2. Cyclic photophosphorylation
     • 1. Electrons leaving the ETC after PSI can return instead of forming NADPH
       • 2. This allows PSI to make ATP without electrons being supplied by PSII
     • This happens because the amount of ATP produced in non-cyclic often isn't enough for the Calvin Cycle
   • Occurs in and across the thylakoid membranes

3. Limiting factors

   • Light intensity
     • The higher intensity the more energy available
     • Blue and Red wavelengths are absorbed the most, green is reflected
     • Photosystems have more than pigment to absorb as many wavelengths as possible
     • Low light intensity means the products of light dependent are in short supply (NADPH and ATP)
       • Slows the conversion of GP to TP so the levels of GP will rise
   • Temperature
     • Optimum is 25 degrees
     • Below 10 degrees the enzymes are inactive but over 45 degrees they denature
       • Inactivity means levels of RuBP, GP and TP will fall
   • Water availability
     • *not classed as a limiting factor because the plant has some stored in cells if need be and controls how much it loses with the stomata*
     • Less water = less photolysis so the electrons at PSII won't be replenished
   • Carbon dioxide availability
     • More carbon dioxide increases the rate of photosynthesis but if its too high, the stomata close --> optimum is 0.4%
     • Low carbon dioxide means that carbon fixation to RuBP is slow so production of GP is also slow
       • So levels of RuBP rise while GP and TP fall

Images and files in the map