Plants are autotrophic organisms, meaning they generate their own food, as opposed to animals, heterotrophs, which need to actively acquire their food from autotrophs or other heterotrophs. Photosynthesis is the process by which plants take in atmospheric carbon dioxide and generate organic molecules. Respiration is the utilization of energy within the plant and results in the release of oxygen back into the atmosphere.
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Photosynthesis occurs in two phases: the light reaction and the dark reaction. During the light reaction, sunlight causes electrons to become excited within the chlorophyll, the green pigment that we see when we look at plants. As the electrons become excited, they have more potential energy available. This energy is used to split molecules of water, thereby releasing oxygen, and to form adenosine triphosphate (ATP), which is a compound used to store energy for later use. During the dark reaction, ATP formed during the light reaction is used to attach a hydrogen ion to carbon dioxide and eventually converts it to a sugar. This process is also known as carbon fixation. However, during photosynthesis, the carbon fixed is not directly used for plant growth and cellular maintenance. Rather, respiration is the metabolic process by which carbon compounds, such as glucose, are utilized for energy.
Cellular respiration is carried out by all plants, animals and soil microbes, and can be thought of as the reverse process of photosynthesis. Respiration is the metabolic process by which energy is harvested from glucose through the consumption of oxygen. Whereas carbon dioxide and water are consumed during photosynthesis, both of these compounds are produced during respiration. Respiration requires the use of available energy in the form of ATP, but the process results in the generation of a net gain in ATP, meaning more energy is available for continued photosynthesis and respiration. Plants use respiration to obtain energy from the carbon fixed during photosynthesis. Conversely, heterotrophs use respiration to yield energy from the food that they consume.
Carbon dioxide is a well-known and extensively studied greenhouse gas. In the atmosphere, carbon dioxide acts as a blanket preventing heat from escaping into space. The cycle of carbon begins in the atmosphere and enters the biosphere, life on earth, through photosynthesis which incorporates the gaseous form of carbon into solid organic matter. This organic matter can move throughout the food web as other animals consume plants and are then in turn consumed by other animals. Finally, as plants and animals die, their tissue is decomposed by soil microbes and the organic matter is consumed as a food source, and the microbes return the carbon to the atmosphere through respiration.