It is a reserve available to carry a plant between seasons, to sustain it during periods when photosynthesis is limited, to prepare it for times of intense energy use such as flowering, and to support its progeny in seeds before autonomous growth. Starch, for instance, represents energy stored, not for animals, but for the plant that made the starch. While digestion is aimed at breaking down molecular structure within carbohydrate molecules, there is a raft of further plant structural impediments to be overcome before most plant carbohydrates are available for digestion.
Nearly all of it is obtained from plants, and nearly all of it requires digesting before it is available for metabolism. Carbohydrate is almost universally the major dietary source of metabolic energy.
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