Photosynthesis

Question 1

LDR

Answer 1

1. Light absorption by photosystems:
   • Photosystem II (PSII) absorbs light, exciting electrons in chlorophyll.
   • Excited electrons jump to a higher energy level and are passed to the electron transport chain (ETC).
   • The loss of electrons from PSII is replaced by photolysis of water:
   2H_2O \rightarrow 4H^+ + 4e^- + O_2
   • This releases electrons, protons (H⁺), and oxygen gas (O₂).

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2. Electron Transport Chain (ETC):
   • High-energy electrons move down the ETC from PSII to Photosystem I (PSI).
   • As electrons move down, energy is released and used to pump protons (H⁺ ions) from the stroma into the thylakoid lumen.
   • This creates a proton gradient across the thylakoid membrane.

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3. ATP production (photophosphorylation):
   • Protons flow back into the stroma through ATP synthase channels due to the proton gradient.
   • The flow drives the synthesis of ATP from ADP + Pi (photophosphorylation).

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4. Photosystem I and NADPH production:
   • Electrons arrive at PSI, are re-excited by light.
   • High-energy electrons are transferred to NADP⁺ along with a proton (H⁺) to form NADPH:
   NADP^+ + 2e^- + H^+ \rightarrow NADPH

Question 2

LIR

Answer 2

1. Carbon fixation:
   • CO₂ combines with ribulose bisphosphate (RuBP) (a 5-carbon molecule).
   • The enzyme rubisco catalyses this reaction.
   • This forms an unstable 6-carbon intermediate that immediately breaks down into two molecules of glycerate-3-phosphate (GP) (3-carbons each).

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2. Reduction:
   • GP is reduced to triose phosphate (TP) using energy from ATP and reducing power from NADPH (both produced in the light-dependent reaction).
   • ATP provides energy, NADPH donates electrons (hydrogen atoms).

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3. Regeneration of RuBP:
   • Some TP molecules go on to regenerate RuBP using ATP.
   • This keeps the cycle going.
   • The rest of TP is used to make glucose and other carbohydrates.

Question 3

Limiting Factors of Photosynthesis and Their Effects

1. Light Intensity

Answer 3

• Light provides the energy for the light-dependent reactions.
• Low light intensity → slower rate because less energy to excite electrons in chlorophyll.
• As light intensity increases, the rate increases up to a point.
• Beyond a certain intensity, rate plateaus because other factors become limiting (e.g., CO₂ or temperature).

Question 4

Limiting Factors of Photosynthesis and Their Effects CO2

Answer 4

• CO₂ is fixed in the Calvin cycle.
• Low CO₂ → slower rate because less substrate for carbon fixation.
• Increasing CO₂ concentration increases rate until the enzyme rubisco is saturated.
• After that, rate plateaus as light or temperature limit.

Question 5

Limiting Factors of Photosynthesis and Their Effects temp

Answer 5

• Affects enzyme activity (rubisco and others).
• Low temperature → slower rate because enzymes work slower.
• Rate increases as temperature rises up to the optimum (~25–35°C).
• Above optimum, enzymes denature → rate drops sharply.

Question 6

Limiting Factors of Photosynthesis and Their Effects other factors

Answer 6

• Water availability: affects stomatal opening and turgor, indirectly influencing CO₂ uptake.
• Chlorophyll concentration: affects light absorption.