Cells, whether single-celled organisms or the cells in our bodies, need energy to survive. The cells use their energy to perform a number of tasks, depending on what kind of cells they are and whether they're part of large bodies or tissues. Single-celled organisms use energy primarily to keep their organelles functioning, to move and to divide into new cells. Cells in an organism generally perform basic tasks, such as helping a plant to conduct photosynthesis or giving muscles the ability to move. This takes energy, which the cell needs to replenish. Cells in an advanced organism detect damaged or worn cells nearby and replace them with new versions, which requires another burst of energy. Cells also have a lifespan, and must reproduce before they die. All these different processes require continuous energy, which the cells receive from several different sources.
Cell Food Requirements
Chief among the necessary "foods" that a cell needs are carbohydrates and proteins. Carbohydrates serve as an immediate energy source for cells. These starches and simple sugars like glucose are carried to cells throughout the body of an organism. The cells absorb carbohydrates and then break them apart. As sugar molecules break apart, the atoms release the small amount of energy that was bonding them together. The escaping electrons produce chemical energy. The cell uses this energy to power its inner workings, including movements of its organelles and manufacturing necessary compounds. Nearly all organisms require simple sugars cellular energy, especially plants and animals.
Proteins, on the other hand, don't so much provide a source of chemical energy as allow cells to create more cells. Cells are made from specific kinds of proteins, and use extra proteins both to repair themselves and to build new cells. When muscles grow and cells divide, they use proteins to make the new tissue. Like carbohydrates, the cells need proteins to survive and would die without them.
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The third type of food that cells consume is actually an alternate form of carbohydrates stored in fat. Fats are compounds that hold carbohydrates, which can be extracted for later use. Certain cells in the body store these fats and convert them to carbohydrates, which can then be converted to energy as the need arises. This allows organisms to supply their cells with the necessary energy even if food sources aren't present.
Just as cells require and use food, organisms need larger types of food that can be broken down for their cells. Plants get glucose by creating sugars from nutrients drawn from the soil and energy provided by sunlight. Animals receive proteins from plants such as legumes and various types of meat, and carbohydrates from various grains and fruits. Some animals, especially those that hibernate, also build up supplies of fat, but this is an inner biological process and not related to the carbohydrates they consume. Animals (including humans) that eat fat will convert that fat directly to energy unless they need to save it or have all required energy at that time. Single-celled organisms move about seeking very small amounts of energy, often located in other organisms or cells that are devoured to gain nutrients, as can be seen when an amoeba "absorbs" its prey.