Activities of Cells
Cells carry out complex processes. Among these are the synthesis (production) of compounds used by the cell, the selection of the materials that enter the cell, and the excretion from the cell of toxic or useless waste substances. Cells can grow; they can repair and replace their own parts; and they can duplicate themselves.
The different cells that compose complex multicellular organisms are highly specialized, carrying out extremely varied activities. Some cells are specialized for sensitivity to light, heat, sound, or pressure; others for the synthesis of materials such as hair, bone, shell, or bark; and still others for the synthesis of such substances as milk, hormones, and poisons.
In animals, examples of specialized cells are nerve cells, which transmit electrical impulses, and cells that produce antibodies (special proteins that attack invading organisms). In plants, examples are tracheid cells, which conduct water and help support the plant; palisade chlorenchyma cells, which produce food materials; and sieve tube cells, which transport food materials.
All cellular processes require energy, and among the basic properties of cells is the ability to generate energy. The energy is obtained through controlled oxidation (rearrangement of molecules so as to release energy) of foods. The oxidation of foods in cells is known as cellular respiration. For cells of certain living things, including some bacteria, certain protists, and animals, the main food substances oxidized are carbohydrates taken in from the organism's environment. Other living things, including certain bacteria, algae, and plants, make their own carbohydrates by a process known as photosynthesis and in turn oxidize them to obtain energy.
Most cells use free, or molecular, oxygen in the oxidation process. This type of respiration is known as aerobic respiration. Some cells carry on oxidation of food substances without using free oxygen. This type of respiration is known as anaerobic respiration.
After carbohydrates and other foods have been oxidized, the chemical energy thus obtained is locked into the chemical structure of certain specific compounds within the cell, mainly adenosine triphosphate (ATP). When a process requiring energy is to be carried out, the cell utilizes the ATP, releasing chemical energy by breaking down the chemical structure of ATP. The cell must then replenish its supply of ATP through the oxidation of food substances.