Explanation carbon cycle

Carbon enters life forms through the process of photosynthesis. Photosynthesis is the process by which plants prepare food in the presence of sunlight and chlorophyll. During photosynthesis, carbon dioxide and water combine to produce glucose and oxygen. This changes the atmospheric carbon into glucose molecules. 
Glucose, which is a source of food, is utilized by organisms to produce energy during respiration. During this process, glucose is broken down in the presence of oxygen to produce carbon dioxide. 
Thus, through the processes of photosynthesis and respiration, carbon is utilized and then returned to the environment. 
All organisms do not need oxygen to break down glucose and produce energy. The organisms which survive in the absence of oxygen are called anaerobes. 
Another process that releases carbon dioxide is called combustion. It is the process of burning. Many substances release carbon dioxide on burning. Vehicular emissions, industrial fumes, and the gases released during the process of cooking are some instances during which carbon dioxide is released.