All living things have something basic in common: cells. Most organisms consist of only one cell. The rest of us, from humans to fleas to flowers, are multi-celled. Whatever the organism, its cells are usually very small. A typical cell is about 1/1,000 of an inch (0.0025 centimeters) in diameter. It takes a microscope to see them. Added together, the cells in a human total more than …
Every cell has work to do. But different cells may have different jobs. In multicellular organisms, cells with the same type of job often work together. These groups of specialized cells form tissue. In turn, tissues often group together to form larger units, called organs. The heart is an organ; so is the stomach. Humans have over 200 kinds of cells. The cells of each organ or tissue have …
Cells differ in how long they live. Human skin cells, for instance, live about 28 days. Our white blood cells live about 13 days. Red blood cells, though, live up to four months. In humans, billions of body cells die everyday. If new cells didn't replace them, we would soon die, too. So most cells have instructions to make copies of themselves. That way, their work can continue—and…
Usually cells function properly. They grow and repair themselves to do their work. They also divide in an orderly way. But things go wrong from time to time. Cancer cells can break away from where they started and spread the cancer to other parts of the body. Sometimes certain cells in multicellular organisms cannot stop dividing. These wild cells, called cancer cells, cause problems. They…
In the 1830's, German scientists Mathias Schleiden and Theodor Schwann called cells the basic unit of life. Since that time, scientists have learned a great deal about the cell. Gregor Mendel's experiments with garden peas in the mid-1800's showed that genes were the basis of heredity. A hundred years later, other scientists were studying DNA's structure and chemical m…
antibodies (AN-ti-bod-ees): weapons produced by white blood cells to fight disease bacteria (bak-TIHR-ee-uh): common single-celled organisms that sometimes harm humans botulism (BACH-eh-lizm): a type of poisoning caused by a bacteria's toxin cells (selz): basic units of life chlorophyll (KLOR-uh-fil): green matter inside chloroplasts that absorb the Sun's rays chromosomes (KROH-muh-…