We associate energy with movement, but any living organism needs energy for fueling life processes, even long ones, like growth and reproduction. On Earth, the whole life energy comes from the Sun. Plants capture the Sun's energy, animals eat plants as food.
Plants can store the sun energy in chemicals. The other organisms take the energy indirectly from the plants, through parasitism (fungi) or eating them (plants). Other can eat the plants' consumers, but they use in fact the same energy.
Plants turn using sun energy simple inorganic chemicals into sugars, proteins and fats, energy and functional chemicals. What the plants need for these syntheses are water, carbon dioxide, nitrates, phosphates and salts (especially of potassium). The last three categories form the "fertilizers". In fact, the carnivorous plants just "hunt" the nitrogen from their prey's body. The main light absorbing pigment is chlorophyll.
The main chemical synthesized by plants is cellulose (a sugar), which wraps their cells, but starches and other simple sugars are more easily used by animals.
Animals are lucky if they find what they need in food. The milk of the mammals is the perfect food for the offspring, selected from a much poorer diet. Imagine a koala, and the difference between the Eucalyptus leaves consumed by the mother, and the milk for her baby.
This is one of the key of mammal's
success: the young do not lose energy digesting complex food. The eucalyptus leaves, behind being toxic for most animals, are rich in cellulose and poor in proteins and fats. The adult must detoxify leaves (with energy expense), while cellulose is hard to break down.
All animals, no matter if carnivorous or plant eating, must break down the complex proteins, fats and sugars from the food into smaller easy to assimilate molecules. The process is called digestion and is made using catalyst proteins called digestive enzymes. Humans, for example, have enzymes for breaking down starch or simple sugars, but not for cellulose. That's why cellulose is not digested in the human gut and cellulose (fiber) rich food is indicated for people willing to lose weight.
Fats are the easiest to break down; proteins too are easier than sugars to be broken down.
Through the gut wall, simple chemicals enter the blood and through the blood they reach cells. Sugars are digested as glucose which is burned by the cells for getting energy (the way gas is burned into a car's engine for getting energy).
The energy is stored in ATP molecules, as glucose and oxygen may vary. ATP acts like a battery, storing the energy and releasing it at request. When the burning is made using oxygen (aerobic burning), the energy released is high. This is the effort of a marathon runner, which must have oxygenated muscles.
A sprinter has sudden burst of energy, more rapid than that delivered by supplying the muscle cells with oxygen. He must make an anaerobic effort. When finishing the race, he/she must stop for panting: the energy balance is remade, and the lactic acid formed by the glucose break down in the absence of oxygen must be removed. This acid causes the tiresome sensation. The anaerobic metabolism cannot be sustained more than for a few minutes. That's why cheetah's race finishes in 20 seconds, leaving the animal exhausted, while migrating birds rely on aerobic metabolism. The rapid contraction fibers explains how the archer fish can "shoot" insects found close to the water or jump out of the water for those it cannot shoot.
The aerobic metabolism fuels fibers of "slow contraction", for long distances, while anaerobic metabolism fuel fibers of "rapid contraction", for high speed. Both fibers may be found in a muscle, but their proportions vary depending on the utilization of the muscle.
For example, tunas need "slow contraction" fibers as they constantly swim for bringing oxygenated water to the gills. But they also need "rapid contraction" fibers when attacking prey or escaping predators. That's why tunas have two sets of muscles on the flanks: white muscle for rapid contraction (based on ATP) and red muscles (due to numerous blood vessels supplying constantly with glucose and oxygen), which never get tired (otherwise the fish would die suffocated).
In the case of the humans, such a muscle is the heart.
In the case of grasshoppers and locusts, the insects block their rear legs and the large muscles slowly contract, depositing energy like in an elastic band. The muscular contraction lasts 0.5 seconds, but the final movement of the legs, which makes the jump, less than 0.03 seconds. The "elastic" energy is released thus 15 times faster.
When burning the glucose, the animal organism loses 70 % of the energy as heat. Body's energy is measured in calories, one calorie being the heat amount required for increasing the temperature of a gram of water by 1o C. One gram of glucose contains 4.1 calories; so does a gram of protein, while one gram of fats contains 9.3 calories.
A balanced human diet must be made 55-60 % of sugars (carbohydrates), 25-30 % of fats, and 10-15 % of proteins.
Dead animals are decomposed by fungi, bacteria and protozoa. The process function just like a digestion. But the final products are simpler, and easy to use by the plants. This way forms a cycle of nutritive chemicals (nitrates, phosphates and potassium).