Weather & Climate | VOL. 11, ISSUE 69, November-December 2011

The Carbon Cycle

Carbon, an essential element, occurs in organic substances such as carbohydrates, proteins, fats and nucleic acids such as deoxyribonucleic acid (DNA). Most carbon on Earth was originally released from the interior of the Earth as gaseous carbon dioxide, which now constitutes about 0.03-0.04 per cent by volume of the air. This gas helps insulate the Earth, retaining enough of the Sun’s heat to keep the planet warm and sustain life. Carbon dioxide is used in photosynthesis by plants to make carbohydrates. This locked carbon is transferred to animals that eat the plants. During respiration, all organisms break down food releasing energy and carbon dioxide, which is released back into the air every time an animal exhales. Some of the carbon dioxide from respiration is used by plants in photosynthesis, while some of the oxygen made in photosynthesis can be used in respiration. For part of the day, the rate of photosynthesis exceeds the rate of respiration leading to an excess of oxygen. During night, photosynthesis stops and respiration continues so that a balance between carbon dioxide and oxygen production is achieved. Many plants in middle latitudes remain dormant through the cold winter, photosynthesis slows, but animal and plant respiration continues – thus increasing atmospheric concentration of carbon dioxide. It decreases again in spring as plant growth resumes. When organisms die, their bodies are broken down...

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