Chapter 5: Photosynthesis

Question 1

Photosynthesis

Equation

Answer 1

light energy
6CO₂ + 6H₂O ——> C₆H₁₂O₆ + 6O₂

This means that carbon dioxide (CO₂) and water (H₂O) are used, along with light energy, to produce glucose (C₆H₁₂O₆) and oxygen (O₂).

Question 2

Key events:

Light-dependent

Answer 2

1. Light energy strikes P680, exciting its electrons.
2. P680 loses an electron, which is passed to the primary electron acceptor.
3. To replace the lost electron, water molecules are split (photolysis), releasing oxygen, protons (H⁺), and electrons. The protons are released into the thylakoid lumen.
4. The excited electron from P680 is passed to plastoquinone (PQ), a molecule that carries the electron through the electron transport chain (ETC).
5. As the electron moves through the protein complexs, energy is used to pump protons (H⁺) from the stroma into the thylakoid lumen.
6. The electron continues along the ETC and reaches Photosystem I (P700) through plastocyanin, where it is re-excited by another photon of light.
7. Ferredoxin then picks up the re-exited electron.
8. The re-excited electron is used to reduce NADP⁺ to NADPH in the stroma.
9. The proton gradient created by the pumping of protons into the thylakoid lumen drives protons back into the stroma through ATP synthase, providing energy for ATP synthesis.

This process results in the production of ATP and NADPH, which are used in the Calvin cycle for glucose production.

Question 3

Key events:

Light-independent

Answer 3

• Carbon Fixation:
  
  • CO₂ attaches to RuBP (5-carbon molecule) with the enzyme RuBisCO.
  • Forms an unstable 6-carbon compound that splits into two 3-carbon molecules (3-PGA).
• Reduction Reactions:
  
  • ATP and NADPH convert 3-PGA into G3P (3-carbon sugar).
  • ATP adds a phosphate group to 3-PGA.
  • NADPH donates electrons to reduce 3-PGA to G3P.
• Regeneration of RuBP:
  
  • Some G3P molecules are used to regenerate RuBP.
  • 15 carbons from G3P are rearranged to form 3 RuBP molecules using ATP.
  • This allows the cycle to continue and produces G3P for glucose and other carbohydrates.

Question 4

Define photorespiration

Answer 4

is an unfavourable process which decreases the production of sugars by photosynthesis.

Question 5

Comparing photosynthesis and cellular respiration

Primary Energy Source

Answer 5

Comparison of Photosynthesis and Cellular Respiration

1. Primary Energy Source:
   
   • Photosynthesis: Light energy (from the sun) is the primary energy source.
   • Cellular Respiration: Glucose (from food) is the primary energy source.

Question 6

Comparing photosynthesis and cellular respiration

Final electron acceptor

Answer 6

2. Final Electron Acceptor:
   
   • Photosynthesis: NADP⁺ is the final electron acceptor, forming NADPH.
   • Cellular Respiration: Oxygen (O₂) is the final electron acceptor, forming water (H₂O).

Question 7

Comparing photosynthesis and cellular respiration

Location of ATP Accumulation

Answer 7

3. Location of ATP Accumulation:
   
   • Photosynthesis: ATP is accumulated in the thylakoid membrane of the chloroplast during the light-dependent reactions.
   • Cellular Respiration: ATP is accumulated in the mitochondria, mainly in the inner mitochondrial membrane during the electron transport chain (ETC).