Cell (biology), basic unit of life. Cells are the smallest structures capable of basic life processes, such as taking in nutrients, expelling waste, and reproducing. All living things are composed of cells. Some microscopic organisms, such as bacteria and protozoa, are unicellular, meaning they consist of a single cell. Plants, animals, and fungi are multicellular; that is, they are composed of a great many cells working in concert. But whether it makes up an entire bacterium or is just one of trillions in a human being, the cell is a marvel of design and efficiency. Cells carry out thousands of biochemical reactions each minute and reproduce new cells that perpetuate life.
Eukaryotic Animal Cells
To stay alive, cells must be able to carry out a variety of functions. Some cells must be able to move, and most cells must be able to divide. All cells must maintain the right concentration of chemicals in their cytoplasm, ingest food and use it for energy, recycle molecules, expel wastes, and construct proteins. Cells must also be able to respond to changes in their environment.
Movement of Prokaryotes
Movement of Eukaryotes
Movement in eukaryotes is also accomplished with cilia, short, hair like proteins built by centrioles, which are barrel-shaped structures located in the cytoplasm that assemble and break down protein filaments. Typically, thousands of cilia extend through the plasma membrane and cover the surface of the cell, giving it a dense, hairy appearance. By beating its cilia as if they were oars, an organism such as the paramecium propels itself through its watery environment. In cells that do not move, cilia are used for other purposes. In the respiratory tract of humans, for example, millions of ciliated cells prevent inhaled dust, smog, and micro organisms from entering the lungs by sweeping them up on a current of mucus into the throat, where they are swallowed. Eukaryotic flagella and cilia are formed from basal bodies, small protein structures located just inside the plasma membrane. Basal bodies also help to anchor flagella and cilia.
All cells require nutrients for energy, and they display a variety of methods for ingesting them. Simple nutrients dissolved in pond water, for example, can be carried through the plasma membrane of pond-dwelling organisms via a series of molecular pumps. In humans, the cavity of the small intestine contains the nutrients from digested food, and cells that form the walls of the intestine use similar pumps to pull amino acids and other nutrients from the cavity into the bloodstream. Certain unicellular organisms, such as amoebas, are also capable of reaching out and grabbing food. They use a process known as endocytosis, in which the plasma membrane surrounds and engulfs the food particle, enclosing it in a sac, called a vesicle, that is within the amoeba’s interior.
A typical cell must have on hand about 30,000 proteins at any one time. Many of these proteins are enzymes needed to construct the major molecules used by cells—carbohydrates, lipids, proteins, and nucleic acids—or to aid in the breakdown of such molecules after they have worn out. Other proteins are part of the cell’s structure—the plasma membrane and ribosomes, for example. In animals, proteins also function as hormones and antibodies, and they function like delivery trucks to transport other molecules around the body. Hemoglobin, for example, is a protein that transports oxygen in red blood cells. The cell’s demand for proteins never ceases.
Before a protein can be made, however, the molecular directions to build it must be extracted from one or more genes. In humans, for example, one gene holds the information for the protein insulin, the hormone that cells need to import glucose from the bloodstream, while at least two genes hold the information for collagen, the protein that imparts strength to skin, tendons, and ligaments. The process of building proteins begins when enzymes, in response to a signal from the cell, bind to the gene that carries the code for the required protein, or part of the protein. The enzymes transfer the code to a new molecule called messenger RNA, which carries the code from the nucleus to the cytoplasm. This enables the original genetic code to remain safe in the nucleus, with messenger RNA delivering small bits and pieces of information from the DNA to the cytoplasm as needed. Depending on the cell type, hundreds or even thousands of molecules of messenger RNA are produced each minute.