|Human Brain Cells
All cells share four characteristics. First, every cell is enclosed in a thin cell membrane, which provides shape and acts as a barrier between the cell and its environment. The membrane is semipermeable, composed largely of lipids (fats, oils, and fatty substances such as cholesterol), with embedded proteins that regulate the transport of molecules into and out of the cell.
Second, all cells are filled with cytoplasm. Cytosol, the fluid portion of cytoplasm, contains nutrients, enzymes, and other dissolved materials vital for cell metabolism. Also in the cytoplasm are specialized structures called organelles, held in place by a network of protein filaments.
Third, within the cytoplasm of all cells are organelles called ribosomes, which manufacture proteins for the cell. Some of the proteins are structural components; others are enzymes that control chemical reactions in the cell.
Fourth, all cells contain chromosomes composed of genetic material, which encodes instructions for making the hundreds or even thousands of different proteins found in a cell.
Homeostasis and Enzymes
If the internal environment of a cell (or an organism as a whole) changes significantly, the cell dies. Thus, it is important that a cell maintains homeostasis: a relatively stable internal environment regardless of changes in the external environment.
Cells perform many metabolic reactions to maintain homeostasis. Proteins called enzymes act as catalysts of these reactions. Enzymes are very specific; many catalyze only a single reaction whereas others are able to catalyze a few closely related reactions. Some enzymes break compounds into smaller molecules. Others act to combine small molecules into a larger molecule. In either process, the enzymes are neither changed nor destroyed; they can be reused again and again.
If temperature, acidity, or other environmental conditions vary outside a narrow range, the rate of a catalytic reaction is affected. For instance, in humans, the optimal temperature for catalytic reactions is normal body temperature, approximately 98.6 degrees F. If body temperature gets too cold, enzymes are unable to bind to molecules quickly enough and the rate of the reaction will decrease, possibly to levels that cannot sustain life. If body temperature rises too high, the heat causes enzymes to denature and lose their catalytic ability.
Cells obtain nutrients and other needed materials from their environment, and excrete wastes and other substances into the environment. These materials must move through the cell membrane, which is selectively permeable; that is, it controls the passage of materials between the cell and its environment, allowing some substances in while keeping others out. Two basic methods transport materials through the membrane.
Passive transport, including diffusion and osmosis, requires no energy. Small molecules such as oxygen, carbon dioxide, and water plus ions such as those of sodium and calcium pass freely across the membrane, from an area of higher concentration to an area of lower concentration.
Active transport moves substances from an area of low concentration to an area of high concentration. The cell must expend energy for active transport. For example, in a process called endocytosis, human white blood cells engulf harmful invaders, which it then destroys. In a reverse process, exocytosis, a cell can expel large waste molecules.
Another type of active transport relies on membrane proteins called pumps. They capture molecules from one side of the membrane and release them on the other side. (New York Times ‘Guide to Essential Knowledge’)