Photosynthesis uses light energy to build organic molecules from carbon dioxide and water, releasing oxygen. Carbon moves from atmospheric or dissolved carbon dioxide into living biomass. Cellular respiration releases usable energy from organic molecules; aerobic respiration uses oxygen and produces carbon dioxide and water. Both plants and animals respire. Photosynthesis and respiration therefore transfer carbon in different directions between organisms and their surroundings.
Decomposers break down dead organisms and wastes. They incorporate some carbon into their own biomass and release some through respiration. Not all dead material decomposes immediately: conditions affect the rate, and some carbon becomes stored in soils or buried sediments. Carbon cycling is a matter account. Energy supplied by sunlight is transferred and eventually dispersed, rather than cycling as the same reusable supply alongside carbon atoms.
Example 1
A plant incorporates carbon into a new leaf. What is the main original carbon source?
Carbon dioxide taken from air supplies carbon atoms for organic molecules through photosynthesis. Most of the carbon is not obtained from soil minerals.
Example 2
Dead leaves lose carbon while decomposer activity produces CO2. Trace the transfer.
Carbon moves from dead organic matter into decomposers and then, through their respiration, to carbon dioxide in the surroundings.
Watch out:Plants respire continuously. Photosynthesis also occurs when suitable light and other conditions are available.
Photosynthesis, respiration and decomposition practice | Year 9 Science | Ascent Arcade