The animals on earth are very different. From fleas to elephants, from mosquitoes to goldfish, from earthworms to peacocks, from spiders to pythons, it seems that there are no similarities between them. In fact, they have many things in common, one of which is that all animals have to eat.
In order to live, many animals have to look around for food every day. It can be said that this is the most important instinct of animals. Chicks will eat when hatched from eggs, and piglets will eat milk when born. Once the food is cut off, all animals will face death. Why?
We know that all life activities need to consume energy. Breathing, heartbeat and exercise are realized through muscle contraction. Energy is also needed for nerve cells to send out electric pulses, digestive organs to secrete digestive juice, and cells to synthesize macromolecules in the body. Just as power plants need to burn coal or natural gas to supply electricity, the energy needed by animals can only be obtained by "burning fuel".
For animals, this "fuel" is food, and an important role of food is to provide energy. In fact, the principle of animals using food is the same as that of power plants burning fuel to generate electricity. Power plants burn coal and turn it into carbon dioxide and water. The energy released during combustion is converted into electric energy. Animals also "burn" food, which is converted into carbon dioxide and water. The released energy is not used to generate electricity, but to synthesize ATP (adenosine triphosphate), a "high-energy compound", which provides energy for various life activities.
In addition to providing energy, the second important role of food is to provide the "parts" needed by the substitute body. Amino acids that make up protein molecules, fatty acids and glycerol that make up fats, vitamins necessary for maintaining normal life activities (as proteins that catalyze chemical reactions - the "additional component" of enzymes) need to be "extracted" from food.
Metabolism is accompanied by the life of organisms, and their bodies need to be constantly updated. Inanimate objects do not renew themselves. A rock can change very little in billions of years. Its age can be calculated from the proportion of isotopes it contains; The composition of deep Arctic ice is almost unchanged for a long time. The composition of air hundreds of thousands of years ago can be known from the bubbles in it. However, organisms are highly complex and fragile. The "components" in organisms need to be continuously repaired and updated in order to continue life.
Many animals can live for more than ten to dozens of years, but most cells in the body can't live that long. Human intestinal wall cells should be replaced every two or three days, epithelial cells should be replaced every 28 days or so, and liver cells should be replaced about once a year. It is the bone that people think will not grow and change in adulthood. The bone cells should be renewed every 10 years or so.
In this way, the "life span" of the molecules in the cell is shorter. Some protein molecules have a "life span" of only a few minutes. Some "long-lived" cells, such as nerve cells, are also constantly replaced by molecules. After 10 years, especially for adults, although it seems that there is no earth shaking change from 10 years ago, almost all the cells and molecules in the body have been replaced in these 10 years.
The continuous renewal of animal body, the repair and renewal of "components" in animal body and the replacement of molecules in cells all need food to provide energy.