Plants provide all energy for maintenance, growth, reproduction, and locomotion of every living organism on our planet. That energy, originating from the Sun, flows from plants through a web of herbivores, carnivores, and decomposers. This trophic chain – “who eats whom” – gradually returns carrier CO2 molecules to the atmosphere. Fires, occurring naturally from lightening strikes, or provoked by human activities, are a more sudden, but chemically similar, release of solar energy accumulated by plants.

Humans and some other animals also use plant material (biomass) for construction but humans alone have combusted them under controlled conditions to provide heat for warmth, cooking, and both stationary power and traction. Once, animals were the only source of traction and, in the eighteenth century, consumed as much as one third of agricultural production. Biomass accumulated by plants during previous geological periods formed coal and oil (fossil fuels) that have driven the development of transportation, agriculture, and industry during recent centuries.

Agricultural systems have developed predominantly to provide food for humans in plant and animal products, but they also provide fiber and fuel. This chapter describes the chemical and energetic content of plant products and explains their relationship to nutritive value and carrying capacity of land for animals used in agriculture and for humans. Questions of energy use in agriculture and its potential to supply a greater proportion of society's demand for non-dietary energy, including the current focus on biofuel, are discussed further in Chapter 15.