Photosynthesis

Question 1

What are the main components of a chloroplast, and where do light and dark reactions occur?

Answer 1

1. Thylakoid membrane: Site of light-dependent reactions.
2. Stroma: Site of the Calvin Cycle (dark reactions).
3. Lumen: Space inside the thylakoids where proton gradient builds.

Question 2

What are the key steps in light reactions of photosynthesis?

Answer 2

1. Light energy excites electrons in PSII → passed down the ETC.
2. H2O undergoes photolysis, producing O2, protons (H+), and electrons.
3. Proton gradient drives ATP synthesis via ATP synthase.
4. Electrons re-energized in PSI → reduce NADP+ to NADPH.

Question 3

What is photolysis, and why is it critical in photosynthesis?

Answer 3

Photolysis is the splitting of H2O by light in PSII, producing:
- Electrons: Replace those lost in PSII.
- Protons (H+): Contribute to the proton gradient.
- Oxygen: Released as a byproduct.

Question 4

Describe the Z-scheme electron flow during light-dependent reactions.

Answer 4

1. PSII: Excited electrons passed to PQ, b6f complex, and PC, releasing energy for proton pumping.
2. ATP synthesis: Proton gradient drives ATP production.
3. PSI: Electrons re-energized and reduce NADP+ to NADPH via Fd and NADP+ reductase.

Question 5

What experiment proved that chloroplasts split water using light energy?

Answer 5

The Hill Reaction: Demonstrated that isolated chloroplasts can split water, release O2, and transfer electrons to an artificial acceptor using light.

Question 6

What are the differences between cyclic and non-cyclic photophosphorylation?

Answer 6

• Non-cyclic: Produces ATP, NADPH, and O2 (involves both PSII and PSI).
• Cyclic: Produces only ATP (electrons cycle through PSI and Cyt b6f complex).

Question 7

Compare Photosystem I (PSI) and Photosystem II (PSII).

Answer 7

• PSII: Located first, absorbs light (680 nm), drives photolysis of H2O.
• PSI: Absorbs light (700 nm), produces NADPH via electron transfer.

Question 8

How is the proton gradient formed in light reactions?

Answer 8

1. H+ pumped into thylakoid lumen via ETC (PQ → b6f complex).
2. H+ from photolysis contributes to lumenal acidity.
3. Protons flow back to stroma via ATP synthase → ATP production.

Question 9

What are the three main phases of the Calvin Cycle?

Answer 9

1. Fixation: CO2 + RuBP → 2x 3-PGA (via Rubisco).
2. Reduction: 3-PGA → GA-3-P using ATP and NADPH.
3. Regeneration: GA-3-P → RuBP (requires ATP).

Question 10

How much energy is required to produce one glucose molecule in the Calvin Cycle?

Answer 10

18 ATP and 12 NADPH are required for six turns of the cycle, producing one glucose molecule.

Question 11

What is Rubisco’s limitation, and how does it cause photorespiration?

Answer 11

Rubisco can bind O2 instead of CO2, producing 2-phosphoglycolate instead of 3-PGA, which:
- Consumes energy and releases CO2, reducing photosynthetic efficiency.

Question 12

What did the Emerson Enhancement Effect demonstrate?

Answer 12

Photosynthesis rate increases when plants are exposed to both red and far-red light, proving the existence of two photosystems.

Question 13

What are the differences between C3 and C4 photosynthesis?

Answer 13

• C3: Direct Calvin Cycle (common in cool climates).
• C4: Spatial separation; CO2 is fixed in mesophyll cells and transported to bundle sheath cells, reducing photorespiration.

Question 14

How does CAM photosynthesis minimize water loss?

Answer 14

CO2 is fixed at night (stomata open) and stored as malic acid. During the day, CO2 is released for the Calvin Cycle while stomata remain closed.

Question 15

Where does photorespiration occur, and what are its consequences?

Answer 15

Occurs in chloroplasts, peroxisomes, and mitochondria.
- It wastes energy, releases CO2, and reduces photosynthetic efficiency.

Question 16

Why are C4 plants more efficient than C3 plants?

Answer 16

C4 plants concentrate CO2 in bundle sheath cells, minimizing photorespiration and enabling efficient photosynthesis in high temperatures.

Question 17

Name two significant CAM plants and their agricultural importance.

Answer 17

Pineapple and Agave – they are adapted to arid environments and produce economically valuable products.

Question 18

What experiment validated the Z-scheme model?

Answer 18

Researchers measured the stepwise increase in electron energy in PSII and PSI, supporting the sequential electron flow model.

Question 19

How can light reactions be optimized in plants?

Answer 19

Introducing bacterial rhodopsins (absorb additional wavelengths) or altering PSI/PSII to absorb more red light.

Question 20

How do cyanobacteria improve CO2 concentration for photosynthesis?

Answer 20

Cyanobacteria use carboxysomes, protein shells that concentrate CO2 and house Rubisco, increasing photosynthetic efficiency.

Question 21

Summary of C4

Answer 21

• C4 Photosynthesis: A biochemical CO₂-concentrating mechanism (CCM) used by some plants (e.g., maize, sugarcane) to efficiently fix CO₂.
• Initial CO₂ Fixation: CO₂ is first fixed in mesophyll cells by the enzyme PEP (phosphoenolpyruvate carboxylase) carboxylase, forming a 4-carbon compound (oxaloacetate).
• Transport to Bundle Sheath Cells: The 4-carbon compound is transported to the bundle sheath cells, where CO₂ is released in a concentrated form near Rubisco.
• Reduced Photorespiration: By concentrating CO₂ around Rubisco, the likelihood of Rubisco fixing O₂ (and causing photorespiration) is minimized, enhancing efficiency.
• Additional ATP Requirement: The C4 pathway requires additional ATP to transport the 4-carbon compound into the bundle sheath cells, but this is compensated by increased photosynthetic efficiency in hot, dry conditions.
• Key Enzyme: PEP carboxylase is highly efficient at fixing CO₂ and has a low affinity for O₂, making it more effective than Rubisco in CO₂ fixation.

Question 22

CAM summary

Answer 22

• Definition: CAM is a photosynthetic pathway found in some plants, primarily in arid environments, where stomata open at night to minimize water loss.
• CO2 Fixation: At night, CAM plants fix CO2 into malate (a 4-carbon acid) using PEP carboxylase.
• Storage: Malate is stored in vacuoles overnight.
• Daytime: During the day, malate is decarboxylated to release CO2, which is then used by RuBisCO in the Calvin cycle to produce sugars.
• Advantage: CAM reduces water loss by opening stomata at night instead of during the day.
• Examples: Succulents, cacti, and some orchids.