Gas required for photosynthesis:
Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy in the form of glucose. This process occurs in the presence of several factors, including sunlight, chlorophyll, water, and gases. Out of these gases, carbon dioxide is the primary gas required for photosynthesis.

Explanation:
Photosynthesis is a complex biochemical process that takes place in the chloroplasts of plant cells. It involves several stages and requires specific conditions to occur. One of the most important requirements for photosynthesis is carbon dioxide (CO2).

Role of carbon dioxide:
1. Carbon dioxide is the source of carbon atoms used in the synthesis of glucose molecules during photosynthesis. The carbon atoms in carbon dioxide are fixed and converted into glucose through a series of chemical reactions.
2. The carbon dioxide molecules are taken up by the plant through tiny openings called stomata, which are present on the surface of leaves and stems.
3. Inside the chloroplasts, carbon dioxide combines with water and sunlight to produce glucose and oxygen. This process is known as the Calvin cycle or the dark reaction of photosynthesis.
4. The glucose produced during photosynthesis serves as an energy source for the plant and is used for various metabolic processes such as growth, respiration, and reproduction.
5. Additionally, the oxygen produced as a byproduct of photosynthesis is released into the atmosphere, contributing to the oxygen content of the air.

Other gases:
While carbon dioxide is the primary gas required for photosynthesis, other gases also play important roles in the process:
1. Oxygen: Although oxygen is not directly involved in photosynthesis, it is a byproduct of the process and is released into the atmosphere. Oxygen is essential for the respiration of plants and other organisms.
2. Nitrogen: Nitrogen is an important element for plant growth and is required for the synthesis of proteins and nucleic acids. However, it is not directly involved in photosynthesis.
3. Hydrogen: Hydrogen is a component of water, which is essential for photosynthesis. However, it is not considered a gas required for the process.

Conclusion:
In conclusion, the gas required for photosynthesis is carbon dioxide (CO2). It is essential for the synthesis of glucose and provides the carbon atoms necessary for plant growth. While other gases such as oxygen, nitrogen, and hydrogen play important roles in plant metabolism, they are not directly involved in the photosynthetic process.