With the advent of technology the fuel to power the work of the animals moved from carbohydrates to hydrocarbons because machines prefer the hydrocarbons to carbohydrates. In both cases (machine and animal) the organism strips off the hydrogen to combust it with oxygen to make energy + water. The oxygen is in the air and the hydrogen is in the fuel.
The plant kingdom uses the energy of the sun to go in the reverse direction and strip off the hydrogen from the carbon to make the high growth-rate building materials that constitute vegetable matter. Plants are rooted, they are planted in the earth, as opposed to animals that move around. So animals use energy to maintain their bodies at a sufficiently high temperature so they can move around while plants use energy to build up structural matter so they can keep growing. Plants keep on growing more structural matter even after they've been cut because the growth of structural matter is what they're about. They don't use energy to find food because the food is made directly available from the sky: water (rain) to provide the hydrogen, and sunlight. The rest they need merely to get from the atmosphere in their covenantal exchange of carbon with the animal kingdom.