HKDSE Biology - Photochemical reactions and Carbon fixation 

• Outline the major steps of photochemical reactions and carbon fixation.
• Understand the dependence of carbon fixation to the photochemical reaction.

👆🏻This photo showing the steps in photochemical reaction including light absorption, photolysis of water in the generation of NADPH as well as generation of ATP.

🔺Photochemical reaction takes place in the thylakoid membranes of grana.

1. Light absorption

• The light is absorbed by chlorophyll and electron is boosted to a high energy level.
• The high energy electron replacing the electron in water which split the water into Oxygen (O2) and H⁺ ions.

2. Generation of NADPH

• Travelling down an electron transport chain, high energy electrons passed to NADP⁺ and leads to the formation of NADPH.

3. Generation of ATP

• Travelling down an electron transport chain, high energy electrons will release energy which drives the pumping of H⁺ ions into the interior of the thylakoid from the stroma.
• H⁺ ions moving down the concentration gradient from the interior of the thylakoid to the stroma.
• ATP synthase is carried out and generate ATP from ADP.

👆🏻This photo showing the steps in carbon fixation - Calvin cycle.

🔺Carbon fixation takes place in the stroma

1. Carbon fixation 

• A five-carbon molecule (RuBP) is combined with a Carbon dioxide (CO₂) molecule.
• A six-carbon molecule is formed and splitting into two three-carbon molecules (3-PGA).
• The reaction is catalysed by an enzyme ➡️rubisco.

2. Reduction

• Three-carbon molecule (3-PGA) is converted into three-carbon sugar (G3P) using ATP and NADPH.
• ATP ➡️ ADP
• NADPH ➡️NADP⁺

3. Regeneration 

• Three-carbon sugar (G3P) turns into product glucose using ATP.
• ATP ➡️ ADP

👆🏻This photo showing the linkage between photochemical reaction and Calvin cycle.

• ATP and NADPH are generated in photochemical reaction.
• ATP and NADPH are essential in Clavin cycle.
• ATP 👇🏻
• Three-carbon molecule (3-PGA) is converted into three-carbon sugar (G3P)
• Three-carbon sugar (G3P) turns into product glucose
• NADPH 👇🏻
• Three-carbon molecule (3-PGA) is converted into three-carbon sugar (G3P)

Written by Bella | HKDSE Biology Specialist @ TUTTEE

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References 

1. Khan Academy. Light-dependent reactions (photosynthesis reaction) (article). Khan Academy. https://www.khanacademy.org/science/biology/photosynthesis-in-plants/the-light-dependent-reactions-of-photosynthesis/a/light-dependent-reactions.
2. Khan Academy. The Calvin cycle (article) | Photosynthesis. Khan Academy. https://www.khanacademy.org/science/biology/photosynthesis-in-plants/the-calvin-cycle-reactions/a/calvin-cycle.