The biogeochemical cycle in which carbon is exchanged between Earth's terrestrial biosphere, hydrosphere, geosphere, and atmosphere is called the carbon cycle. The global carbon budget is the balance of the fluxes of carbon between these four reservoirs.
Membrane proteins can function as enzymes to speed up chemical reactions, act as receptors for specific molecules, or transport materials across the cell membrane. Carbohydrates, or sugars, are sometimes found attached to proteins or lipids on the outside of a cell membrane.
