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Biological carbon fixation, or сarbon assimilation, is the process by which living organisms convert inorganic carbon (particularly carbon dioxide, CO 2) to organic compounds. These organic compounds are then used to store energy and as structures for other biomolecules .
Through photosynthesis, plants use CO 2 from the atmosphere, water from the ground, and energy from the sun to create sugars used for growth and fuel. [22] While using these sugars as fuel releases carbon back into the atmosphere (photorespiration), growth stores carbon in the physical structures of the plant (i.e. leaves, wood, or non-woody stems). [23]
CAM plants, such as cacti and succulent plants, also use the enzyme PEP carboxylase to capture carbon dioxide, but only at night. Crassulacean acid metabolism allows plants to conduct most of their gas exchange in the cooler night-time air, sequestering carbon in 4-carbon sugars which can be released to the photosynthesizing cells during the day.
There are also many varieties of anoxygenic photosynthesis, used mostly by bacteria, which consume carbon dioxide but do not release oxygen. [13] [14] Carbon dioxide is converted into sugars in a process called carbon fixation; photosynthesis captures energy from sunlight to convert carbon dioxide into carbohydrates.
Most carbon leaves the terrestrial biosphere through respiration. When oxygen is present, aerobic respiration occurs, producing carbon dioxide. If oxygen is not present, e.g. as is the case in marshes or in animals' digestive tracts, anaerobic respiration can occur, which produces methane. About half of the gross primary production is respired ...
Carbon dioxide equilibrates between the atmosphere and the ocean's surface layers. As autotrophs add or subtract carbon dioxide from the water through photosynthesis or respiration, they modify this balance, allowing the water to absorb more carbon dioxide or causing it to emit carbon dioxide into the atmosphere. [2]
Carbon dioxide observations from 2008 to 2017 showing the seasonal variations and the difference between northern and southern hemispheres. The concentrations of carbon dioxide in the atmosphere are expressed as parts per million by volume (abbreviated as ppmv, or ppm(v), or just ppm).
Others may breathe atmospheric air while remaining submerged, via breathing tubes or trapped air bubbles, though some aquatic insects may remain submerged indefinitely and respire using a plastron. A number of insects have an aquatic juvenile phase and an adult phase on land. In these case adaptions for life in water are lost at the final ecdysis