Question 1

What is photosynthesis, and why is it essential for life on Earth?
Answer:
Photosynthesis is a biochemical process by which green plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. The overall equation is:
6CO2+6H2O+light energy→C6H12O6+6O26CO_2 + 6H_2O + light \, energy \rightarrow C_6H_{12}O_6 + 6O_26CO2​+6H2​O+lightenergy→C6​H12​O6​+6O2​
It is essential because it:

  1. Produces oxygen necessary for respiration.
  2. Forms the base of most food chains.
  3. Sequesters carbon dioxide, reducing atmospheric CO₂ levels.

Question 2

Describe the structure of chloroplasts and their role in photosynthesis.
Answer:
Chloroplasts are double-membraned organelles found in plant cells and algae. Key parts include:

  1. Thylakoids: Contain chlorophyll and are the site of light-dependent reactions.
  2. Grana: Stacks of thylakoids.
  3. Stroma: Fluid-filled space where the Calvin cycle occurs.
  4. Inner and Outer Membranes: Protect the chloroplast.

The chloroplast absorbs sunlight and converts it into ATP and NADPH during the light reactions. These products power the Calvin cycle to synthesize glucose.

Question 3

Explain the light-dependent reactions of photosynthesis.
Answer:
Light-dependent reactions occur in the thylakoid membranes and involve:

  1. Photolysis of Water: Splits water into oxygen, electrons, and protons.
  2. Photosystem II: Captures photons and uses energy to transfer electrons to the electron transport chain (ETC).
  3. Electron Transport Chain (ETC): Produces ATP via chemiosmosis.
  4. Photosystem I: Further excites electrons, which reduce NADP⁺ to NADPH.

The products—ATP, NADPH, and O₂—are vital for the subsequent dark reactions.

Question 4

What is the role of chlorophyll in photosynthesis?
Answer:
Chlorophyll is the primary pigment in plants, located in the thylakoid membranes. It absorbs light most effectively in the blue and red wavelengths and reflects green light, giving plants their green color. Chlorophyll’s role includes:

  1. Capturing light energy.
  2. Exciting electrons to initiate the light-dependent reactions.
  3. Transferring energy to the ETC for ATP and NADPH synthesis.

Question 5

What happens during the Calvin cycle (dark reactions)?
Answer:
The Calvin cycle occurs in the stroma and involves three main stages:

  1. Carbon Fixation: CO₂ is fixed to ribulose-1,5-bisphosphate (RuBP) by rubisco, forming 3-phosphoglycerate (PGA).
  2. Reduction: PGA is converted into glyceraldehyde-3-phosphate (G3P) using ATP and NADPH.
  3. Regeneration: RuBP is regenerated from G3P to continue the cycle.

For every 6 molecules of CO₂, one glucose molecule is synthesized.

Question 6

Differentiate between cyclic and non-cyclic photophosphorylation.
Answer:

  1. Non-cyclic Photophosphorylation:
    • Involves both Photosystem I and II.
    • Produces ATP, NADPH, and O₂.
    • Electrons are not recycled.
  2. Cyclic Photophosphorylation:
    • Involves only Photosystem I.
    • Produces ATP only.
    • Electrons return to Photosystem I after passing through the ETC.

Question 7

What is photolysis, and where does it occur?
Answer:
Photolysis is the splitting of water molecules using light energy during the light reactions. It occurs in the thylakoid lumen of chloroplasts. The reaction produces:

  1. Oxygen (released as a by-product).
  2. Protons (used in ATP synthesis).
  3. Electrons (transferred to Photosystem II).

Question 8

How is ATP synthesized during photosynthesis?
Answer:
ATP is synthesized via chemiosmosis during the light reactions:

  1. As electrons move through the ETC, energy pumps protons into the thylakoid lumen, creating a proton gradient.
  2. Protons flow back into the stroma through ATP synthase, driving ATP formation from ADP and inorganic phosphate (Pi).

Question 9

Describe the role of NADPH in photosynthesis.
Answer:
NADPH is a key electron carrier produced in the light reactions. It provides high-energy electrons required to reduce 3-phosphoglycerate into glyceraldehyde-3-phosphate during the Calvin cycle.

Question 10

Explain the significance of stomata in photosynthesis.
Answer:
Stomata are small pores on leaf surfaces that:

  1. Allow CO₂ to enter for carbon fixation.
  2. Facilitate the release of O₂.
  3. Regulate water loss through transpiration, balancing photosynthetic efficiency.

Question 11

What are C3, C4, and CAM pathways?
Answer:

  • C3 Pathway: The Calvin cycle; common in temperate plants.
  • C4 Pathway: Carbon fixation occurs in mesophyll cells and Calvin cycle in bundle-sheath cells, reducing photorespiration.
  • CAM Pathway: Plants fix CO₂ at night to conserve water, common in arid regions.

Question 12

What factors affect photosynthesis?
Answer:

  1. Light intensity.
  2. CO₂ concentration.
  3. Temperature.
  4. Water availability.
  5. Wavelength of light.

Question 13

Discuss photorespiration and its disadvantages.
Answer:
Photorespiration occurs when rubisco binds O₂ instead of CO₂, producing glycolate. It:

  1. Wastes energy and reduces photosynthetic efficiency.
  2. Occurs at high temperatures and low CO₂ levels.

Question 14

How do accessory pigments contribute to photosynthesis?
Answer:
Accessory pigments like carotenoids and xanthophylls absorb light in wavelengths chlorophyll cannot, extending the range of usable light and protecting the plant from photodamage.

Question 15

What is the significance of G3P in the Calvin cycle?
Answer:
G3P is a 3-carbon sugar that serves as:

  1. A precursor to glucose and other carbohydrates.
  2. An energy source for cellular functions.

Question 16

Compare light-dependent and light-independent reactions.
Answer:

  • Light-Dependent: Require light, occur in thylakoids, produce ATP, NADPH, and O₂.
  • Light-Independent: Do not require light, occur in stroma, use ATP and NADPH to fix CO₂ into glucose.

Question 17

What is the role of water in photosynthesis?
Answer:
Water provides electrons for the ETC, protons for ATP synthesis, and oxygen as a by-product through photolysis.

Question 18

Why is rubisco considered inefficient?
Answer:
Rubisco has a dual affinity for CO₂ and O₂, leading to photorespiration, which reduces photosynthetic efficiency.

Question 19

How do environmental conditions influence photosynthesis?
Answer:

  • High light intensities and moderate temperatures enhance photosynthesis.
  • Water scarcity and high oxygen levels increase photorespiration.

Question 20

Why are C4 and CAM plants better adapted to hot and dry conditions?
Answer:

  • C4 Plants: Minimize photorespiration by spatially separating carbon fixation and the Calvin cycle.
  • CAM Plants: Fix CO₂ at night to reduce water loss during the day.

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