Typically, in anabolism, small precursor molecules are assembled into larger organic molecules. Most of them use the energy of sunlight to assemble inorganic precursors, chiefly carbon dioxide and water, into the array of organic macromolecules of which they are made. The process is photosynthesis . Photosynthesis makes the ATP needed for the anabolic reactions in the cell. We may eat beef but the steer ate grass.
Humans are heterotrophs.
We are totally dependent on ingested preformed organic molecules to meet all our energy needs. We are also dependent on preformed organic molecules as the building blocks to meet our anabolic needs. See the discussion of human nutrition. See the discussion on the human gastrointestinal tract for the details. Absorption into cells Within cells, these molecules are further degraded into still simpler molecules containing two to four carbon atoms.