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Photosynthesis Processes
Overview• Absorption of light
energy– Sun hits chloroplasts
• Conversion to chemical energy– Light dependant
reaction• Storage of energy in
sugars– Light independent
reaction
2 Stages of Photosynthesis
1. Light Dependant Reaction • Takes place in Grana
(thylakoid membrane)• Products – ATP, O₂,
NADPH
2. Light Independent Reaction – Calvin Cycle• Takes place in the stroma• Products – Glucose,
NADP⁺, ADP
Light Dependant Reaction
• Takes place in thylakloid membrane of the chloroplast
Light Dependant Reaction
Light Dependant Reaction
1. Light strikes PSII– 2 electrons are excited– Simultaneously - H₂O split into to H⁺ and ½ O₂ (photolysis)
2. Electrons flow down ETC (electron transport chain) – Providing energy for production of ATP– Electrons enter photosystem I
3. Light strikes PSI– Exciting electrons
4. Electrons flow down to NADP⁺ electron acceptor– Provide energy for production of NADPH (used in light
independent reactions)
Light Dependant Reaction
Light Independent Reactions
Calvin Cycle• No light required• Carbohydrates are
synthesized from CO₂ using the ATP and NPDH formed in light reactions
Calvin Cycle• 4 Steps
1. Carbon Fixation2. Energy conversion3. Release one
molecule of sugar G3P
4. Regeneration of RuBP
Calvin Cycle• Step 1 – Carbon Fixation– Three molecules of CO₂
combine with 5-C sugars RuBP (Ribulose biphosphate)
• Enzyme is Rubisco
– Each CO₂ forms a 6-C molecule which splits into two 3-C acids (PGA)
Calvin Cycle• Step 2 – Energy Conversion– 2 chemical reactions
Use 6 molecules of ATPATP ADP + P
AndUse 6 molecules of NAPDH
NADPH NADP⁺
– Producing 6 molecules of 3-C sugar G3P (phosphoglyeralehyde)
Calvin Cycle• Step 3 – Release one
molecule of G3P– Five of the G3P
molecules remain in the cycle
– One G3P molecule in released as a product
Calvin Cycle• Step 4 – Regeneration
of RuBP– Series of chemical
reactions• Use 3 ATP to convert
5 G3P molecules into 3 RuBP molecules
Calvin Cycle• Takes two molecules of G3P to produce one
molecule of glucose– So• 6 molecules of CO₂• 18 molecules of ATP• 12 molecules of NADPH
Photosynthesis Recap