In photosynthesis which gas is absorbed by plants from the atmosphere ?

Question

Accepted Answer

The correct answer is **3. Carbon dioxide**.

### Explanation:

Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy in the form of glucose, using carbon dioxide and water. This process primarily occurs in the chloroplasts of plant cells, where chlorophyll captures sunlight.

1. **Carbon Dioxide (CO₂)**: During photosynthesis, plants absorb carbon dioxide from the atmosphere through small openings in their leaves called stomata. This gas is essential for the photosynthetic process, as it combines with water (absorbed by the roots) and sunlight to produce glucose and oxygen. The overall chemical equation for photosynthesis can be summarized as:
   $$
   6CO₂ + 6H₂O + light \ energy \rightarrow C₆H₁₂O₆ + 6O₂
   $$
   Here, carbon dioxide is a key reactant.

2. **Oxygen (O₂)**: While oxygen is produced as a byproduct of photosynthesis, it is not absorbed by plants. Instead, it is released into the atmosphere after the conversion of carbon dioxide and water into glucose.

3. **Nitrogen (N₂)**: Although nitrogen is a vital nutrient for plants, it is not directly involved in the photosynthesis process. Plants absorb nitrogen primarily in the form of nitrates from the soil, which is essential for the synthesis of amino acids and proteins, but not for photosynthesis itself.

4. **Water Vapour (H₂O)**: Water is indeed a crucial component of photosynthesis, but it is absorbed by the roots from the soil rather than from the atmosphere in the form of water vapor. While some water can enter the plant through transpiration, the primary source for photosynthesis is liquid water.

### Conclusion:
In summary, the correct answer is carbon dioxide because it is the gas that plants absorb from the atmosphere to carry out photosynthesis. The other options, while important in different contexts, do not play a direct role in this specific process.