CO₂ and O₂

Which organisms require CO₂?

Life on this planet is characterized by the need to acquire carbon to make the organic molecules that compose and are used by an organism’s cells. Some organisms, like plants, some protists, and many bacteria, are able to extract carbon dioxide gas (CO₂) from the environment and convert it into organic carbon. In photosynthesis, for example, the carbon in CO₂ becomes part of a sugar molecule, which becomes a source of energy as well as building materials. The gas CO₂ is thus a key molecule in organisms that make their own food.

Oxygen burning in a flame

Which organisms require O₂?

Another critical gas is oxygen gas (O₂). Much of life on this planet is aerobic, meaning oxygen is required for survival. Oxygen serves as a key constituent in the process that releases the energy stored in food. The oxygen is used in cell respiration, which is a process that is much like burning a candle. When a candle is burned, O₂ combines with the chemicals that store energy in wax, producing light and heat energy. When food is burned, O₂ combines with sugar, making its energy available to fuel cell processes.
Something that surprises many people is that plants and other photosynthesizes require O₂ as well as CO₂. Photosynthesis is indeed the process by which plants make food, but once this food is made, cellular respiration is required to release its energy. Plants thus require both gases for survival.

Is O₂ always required to “burn” food for energy?

Many organisms, even humans, are capable of anaerobic energy production in a process known as fermentation. Fermentation processes do not produce as much energy as aerobic reactions and often generate harmful byproducts. An example in humans is the production of lactic acid by lactic acid fermentation in muscle cells during exercise. This occurs when O₂ supplies in muscles are insufficient and the body adjusts by using fermentation as an energy reaction. This leads to the familiar burning sensation in overworked muscles. Fermentation is the process that creates many food products: yogurt, wine, and cheese, for example. In addition, there are certain bacteria, archaea, and protists that are strictly anaerobic, meaning oxygen is poisonous. Their only energy reaction is fermentation. These organisms live in habitats such as sediments or lakes that totally lack oxygen.

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