Overall Word Equation:
Water+ Carbon Dioxide( Carbohydrates+ Oxygen ((: light + chlorophyll)
1. Light Dependent Stage/ photochemical reaction/ light reactions:
Word Equation:
Water ( Hydrogen ions + Oxygen ((: light + chlorophyll)
Site: On the Thylakoid membrane of the chloroplast
Major events:
• Light absorption
o Chlorophyll in the thylakoids absorbs light. The energy absorbed is used for photolysis (splitting of water molecules).
• Photolysis of water (Photodissociation)
o The light energy absorbed split H2O molecules into H+ ions and O2
▪ 2H2O(4H+ +4e- + O2
▪ O2 is released as a gas to the atmosphere (as a by-product)
▪ H+ ions is accepted by NADP to form NADPH2
• NADPH2 acts as a reducing agent by donating H+ ions in Calvin Cycle.
• Generation of ATP (Adenosine triphosphate)
o The light energy absorbed is used to form ATP (photophosphorylation)
o In this way, light energy is converted to chemical energy stored in ATP
▪ Used to drive the reactions in Calvin Cycle
Products formed in this stage:
• Photolysis of water molecules to give H+ ions and O2
• ATP molecules: as energy source for subsequent synthesis of carbohydrates in light dependent reactions.
The flow of electrons in Photosynthesis
[pic]
Photosystem: Consists of chlorophyll molecules & accessory pigments e.g. carotenoid accessory pigments trap light energy and pass to the chlorophyll.
Results of chromatography:
2. Light independent stage/ carbon fixation/ dark reactions
Word Equation:
Carbon dioxide + Hydrogen ions ( Carbohydrates + Water ((enzymes)
• Site: In the stroma of chloroplast with the presence of enzymes for dark reactions
• Requires the products of light dependent stage (ATP, NADPH2), takes place whether or not light is present.
• Details of this stage was analysed by Melvin Calvin and his co-workers and the process is called the Calvin Cycle.
[pic]
Major events occurring in Calvin Cycle:
1. Carbon dioxide fixation and formation of 3-C compound
2. Reduction of 3-C compound and formation of TP (Triose Phosphate)
3. Regeneration of carbon acceptor
[pic]
Pi: Inorganic phosphate (PO3)
Role/ functions of the different substances in photosynthesis
1. Photolysis:
• H2O molecules (2H2O(4H+ +4e- + O2(g)
• H+ : Taken up by NADP to form NADPH2
• e- : Captured by the excited chlorophyll molecules to fill up their positively charged holes
• O2: Released as a by-product
2. 5C-compound (ribulose bisphosphate) at the beginning of the Calvin cycle
• Acts as a carbon dioxide acceptor
3. Triose phosphate (TP) in carbon fixation
• Some of the triose phosphate are used to regenerate the 5-C compound (RuBP)
• Some act as the raw material for the production of glucose and other carbohydrates.
4. ATP ( Supplies energy for
o Reduction of a 3-C compound into 6-C sugar (glucose)
o Conversion of triose phosphate into ADP
5. NADP
• Act as the hydrogen acceptor. It takes up H+ ions from photolysis of water and e- from electron transport system to form NADPH2 which is an importance reducing agent used in carbon fixation
6. CO2
• Essential raw material for the synthesis of carbohydrates in carbon fixation
7. 1st stable product of photosynthesis: PGA (a 3-C compound)
8. 1st carbohydrates formed: Triose phosphate
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