Plants absorb energy from the sun and use photosynthesis to make sugars. Animals have mitochondria that use the sugars provided by plants to produce their own cellular energy. Plants that produce their own food, and food for other plants and animals using photosynthesis, are called autotrophs.
The sun provides energy for plants that absorb it into their chloroplasts. Chloroplasts use this energy to create sugar molecules that help the plants grow and reproduce. Then, animals come along and eat the plants and absorb their energy. They use the energy obtained from the plants to produce their own energy and convert it into water and carbon dioxide. Plants use the carbon dioxide and water, and the cycle begins again. In order to obtain energy, animals do not always have to eat plants. They can also get energy from eating other animals that eat plants.
There are many different processes that go on in plants and animals that require energy. Synthetic work involves things like the production of DNA, and it requires energy to occur. The mechanical work involved in moving muscles requires energy, as do the electrical impulses that travel from the brain to the rest of the body. Without enough energy, these processes become difficult or impossible.