The study of the chemistry of living things is called biochemistry. A very important part of biochemistry deals with how food becomes fuel for the cells of our body. The fuel part of food consists of sugars, starches, fats, and protein. Sugars and starches are broken down into a simple sugar called glucose. Glucose and oxygen combine to give cells energy through a process called cell respiration.
However, cells cannot use glucose directly to make energy. Instead, oxygen and glucose react to form a molecule called ATP. ATP is the energy molecule. It is a compound which reacts with water inside of cells and releases energy. One molecule of glucose results in the production of 38 molecules of ATP. However, two molecules of ATP are used as energy to start the process of cell respiration. Therefore, one molecule of glucose results in a net production of 36 ATPs.
Cell respiration is called aerobic respiration when glucose breaks down in the presence of oxygen, When this occurs, a net of 36 ATP molecules are formed and carbon dioxide and water are given off as by-products. However, if your cells are using energy too fast, and your breathing cannot supply the oxygen needed for aerobic respiration, anaerobic respiration occurs. Anaerobic respiration involves fermentation.
In fermentation, glucose is broken down without the use of oxygen. Fermentation also results in the production of ATP, but it is not as efficient as aerobic respiration. While aerobic respiration results in a net of 36 ATP molecules for every one glucose molecule, fermentation results in a net of only two ATP molecules for every glucose molecule. In fermentation, carbon dioxide and water are not produced as by-products. Instead, lactic acid is produced. It is lactic acid that makes muscle cells hurt during anaerobic respiration.
All organisms get energy from the break down of food. However, only green plants and certain green one-celled organisms can make food. These food making organisms trap energy from sunlight in a green substance called chlorophyll. The energy trapped in chlorophyll is used to make food in a process called photosynthesis. The energy in chlorophyll splits water into hydrogen and oxygen. The hydrogen is joined with carbon dioxide to form glucose. Plants then use this glucose for cell respiration which occurs in the presence of oxygen. However, all of the oxygen is not used in cell respiration, so oxygen is given off as a by-product. Animals, like us, then breath this oxygen to use in our own cell respiration.
The glucose made by green plants is then eaten by animals and
becomes fuel for energy. Other animals then eat these animals.
Other animals, such as people, eat both plants and animals. But
ultimately, all food comes from plants, and all energy comes from
the sun, since this is the energy used by plants to make food.