10. Which element forms the central metal atom in chlorophyll?
[WBCS Mains, Science and Technology 2018]
- (A) Zinc
- (B) Iron
- (C) Magnesium
- (D) Cobalt
Answer: (C) Magnesium
Explanation: The porphyrin-like ring of chlorophyll coordinates a magnesium ion at its centre. This pigment is essential for photon absorption in photosynthesis. Iron and other micronutrients are needed for various plant functions, but magnesium is the characteristic metal of chlorophyll.
9. In photosynthesis, solar energy is ultimately converted into which form of energy?
[CGPSC (Chhattisgarh PCS), Prelims 2017]
- (A) Heat energy only
- (B) Kinetic energy
- (C) Potential energy stored in chemicals
- (D) Mechanical energy
Answer: (C) Potential energy stored in chemicals
Explanation: The correct idea is chemical potential energy: plants use absorbed sunlight to build energy-rich molecules from carbon dioxide and water. That energy remains stored in molecular bonds and can later be used through respiration. Conversion solely to heat or motion is not photosynthesis.
8. Consider two statements about cactus:
1. Leaves are modified into spines.
2. The green stem carries out photosynthesis.
Which is correct?
[UPSC NDA-II, General Ability 2016]
- (A) Statement 1 only
- (B) Statement 2 only
- (C) Both statements 1 and 2
- (D) Neither statement
Answer: (C) Both statements 1 and 2
Explanation: In cacti, the leaves are usually transformed into spines, reducing exposed surface and deterring herbivores. Green succulent stems contain chloroplasts and undertake photosynthesis. Their CAM adaptation further helps conserve water by moving much carbon dioxide uptake to nighttime.
7. Which gas is released by green plants during photosynthesis?
[UPSC NDA-II, General Ability 2015]
- (A) Carbon dioxide
- (B) Oxygen
- (C) Hydrogen
- (D) Methane
Answer: (B) Oxygen
Explanation: Photosynthetic water splitting releases molecular oxygen as a by-product. Green plants use light energy to fix carbon dioxide into carbohydrates. The oxygen evolved comes from water rather than directly from carbon dioxide, a fact established by isotopic tracer experiments.
6. Which energy transformation correctly describes photosynthesis?
[UPSC Civil Services Preliminary Examination 2014]
- (A) Potential energy becomes free energy and is released
- (B) Free energy is converted into stored potential energy
- (C) Food is oxidised to release carbon dioxide and water
- (D) Oxygen is consumed as the major raw material
Answer: (B) Free energy is converted into stored potential energy
Explanation: Photosynthesis captures incoming solar energy and stores some of it as chemical potential energy in synthesised organic compounds. Food oxidation with oxygen describes aerobic respiration, not photosynthesis. Energy storage in carbohydrates supports the transfer of energy through ecosystems.
5. From which substance does the oxygen released in photosynthesis originate?
[UP Review Officer/Assistant Review Officer, Mains 2014]
- (A) Water
- (B) Carbon dioxide
- (C) Chlorophyll
- (D) 3-phosphoglycerate
Answer: (A) Water
Explanation: Photosystem II extracts electrons from water through the oxygen-evolving complex, releasing protons and molecular oxygen. Carbon dioxide supplies carbon for sugar production, not the evolved oxygen. Oxygen-isotope labelling experiments confirmed that photosynthetic oxygen originates from water.
4. Some photosynthetic organisms can use an invisible, infrared region of light. These include certain:
[UPPSC Combined State/Upper Subordinate Services, Prelims 2013]
- (A) Trees
- (B) Algae only
- (C) Bacteria
- (D) Fungi
Answer: (C) Bacteria
Explanation: Certain anoxygenic photosynthetic bacteria possess bacteriochlorophyll pigments capable of absorbing longer wavelengths, including near-infrared light. Higher plants predominantly use visible wavelengths through chlorophylls a and b. Fungi lack the photosynthetic machinery needed to capture light for food production.
3. Which mineral element is present at the centre of the chlorophyll molecule?
[UPPSC Combined State/Upper Subordinate Services, Prelims 2012]
- (A) Iron
- (B) Copper
- (C) Magnesium
- (D) Manganese
Answer: (C) Magnesium
Explanation: A magnesium ion sits in the central ring structure of chlorophyll, the light-absorbing pigment of green plants. Magnesium availability is important for chlorophyll synthesis and normal leaf colour. Iron, copper and manganese take part in other plant processes but are not the central metal.
2. Which mechanism helps remove part of the excess atmospheric carbon dioxide through the oceans?
[UPSC Civil Services Preliminary Examination 2011]
- (A) Escape of carbon dioxide into the upper stratosphere
- (B) Photosynthesis by oceanic phytoplankton
- (C) Permanent trapping of most carbon dioxide in polar ice
- (D) Conversion of carbon dioxide directly into nitrogen gas
Answer: (B) Photosynthesis by oceanic phytoplankton
Explanation: Marine phytoplankton absorb dissolved carbon dioxide during photosynthesis and convert it into organic carbon. Some carbon enters deep water through the biological pump. This contributes to carbon sequestration; the other proposed mechanisms do not explain substantial oceanic photosynthetic carbon uptake.
1. Which gas is absorbed by green plants during photosynthesis?
[Uttarakhand PCS Preliminary Examination 2002]
- (A) Ammonia
- (B) Carbon dioxide
- (C) Chlorine
- (D) Sulphur dioxide
Answer: (B) Carbon dioxide
Explanation: Carbon dioxide enters leaves, usually through stomata, and is fixed into carbohydrates in the Calvin cycle. Water supplies electrons, and light supplies energy for ATP and NADPH synthesis. Oxygen is typically released as a by-product of photosynthetic water splitting.
