Every living thing is made up of structures called cells. The cell is the smallest unit with the basic properties of life. Some organisms, or living things, consist of a single cell. Bacteria and protozoans are such single-celled organisms. Large plants and animals are composed of many billions of cells. Human beings are made up of more than 75 trillion cells. The study of cells is a branch of biology. In 1655 Robert Hooke was the first to describe cells. With a microscope, he was able to see the very small, boxlike units that made up a piece of cork. Hooke called these structures cells because he thought the “little boxes” looked like monastery cells, or rooms. Improvements in microscopes since then have helped scientists learn much more about these tiny structures. General featuresCells exist in a variety of shapes and sizes. They may, for example, be cube-shaped or flat. A single cell may be as large as a tennis ball or so small that thousands would fit on the period at the end of this sentence. About 10,000 human cells could fit on the head of a pin. Bacteria cells can be much smaller. Most many-celled organisms have cells of various kinds. The cells form different structures and perform different functions. This is called specialization. Certain types of animal cells, for example, form muscles, eyes, or teeth. Some plant cells form flowers, fruits, or seeds. Regardless of its shape or size, a cell can digest its own nutrients, provide its own energy, and make copies of itself. In a many-celled organism, each cell cooperates with other cells. To do this, the cell must keep in constant communication with its neighbors. Cooperative groups of similar cells form tissues. Cooperation between tissues in turn forms organs, the units that keep organisms functioning. Parts of a cellAll cells consist of three main parts—the cell membrane, cytoplasm, and nucleus. The cell membrane is like the skin of the cell. The cytoplasm performs many functions, including digestion. The nucleus can be thought of as the cell's brain. Cell membraneThe cell membrane is a thin but tough wall that encloses the entire cell. The membrane allows useful substances to enter the cell and blocks the entry of harmful substances. The membrane also forces out, or excretes, waste products. CytoplasmThe cytoplasm is composed mainly of water, with some solids. Most of the cell's constant work of keeping alive is performed in the cytoplasm. Special organs within the cytoplasm make and store proteins. Other organs digest food molecules and change them into the material needed for energy and growth. A network of tubes transports material within the cell. In many-celled organisms, the cytoplasm in the specialized cells may produce substances needed by the rest of the organism. For example, certain cells in the digestive canal of an animal produce digestive juices. NucleusThe nucleus of a cell is located near its center. The nucleus acts as the control center of the cell. It regulates the cell's growth and division. Bacteria and blue-green algae do not have nuclei. The nucleus is surrounded by its own membrane. The nucleus contains a liquid called nucleoplasm. It also contains strands of genetic material called deoxyribonucleic acid (DNA). A strand of DNA consists of a long series of genes. Genes determine the characteristics of the cell. When the cell divides, the genes pass on the cell's traits to the next generation of cells. (See also genetics.) Differences between plant and animal cellsA great difference between plant and animal cells is in the cell membrane. In a typical plant cell, the cell membrane has a protective wall of a woody substance called cellulose. A plant's combined cell walls give it stiffness and help it grow tall. The membranes of animal cells contain a flexible protein material. The material provides protection yet permits the cells to change in size and shape. This flexibility makes it possible for animals to move about. Most many-celled animals need structures such as bones to support their bodies and shells or tough skins to protect them. Special cells produce the substances needed to form bone and skin tissues. In addition, most plants contain a chemical called chlorophyll, which makes the plants green. The chlorophyll is found within the cytoplasm of the plant's cells. Chlorophyll helps the plant make its own food from the energy in sunlight. This process is called photosynthesis. Animal cells do not contain chlorophyll. |