Cell

A cell is the smallest unit considered to be alive and forms the basis of all living things. Cells can use energy, convert nutrients into other forms, and reproduce through division. Some organisms consist of a single cell, such as bacteria; others, including humans and plants, are composed of many cells that carry out different functions.

Every cell is enclosed by a membrane that separates the interior from the exterior. Inside is a fluid called cytoplasm containing small structures with specific functions. Genetic information, which determines how a cell operates and reproduces, is contained in DNA.

Major Types

Cells are divided into two major types. Prokaryotes (such as bacteria) lack a nucleus; their DNA exists freely in the cytoplasm. Eukaryotes (animals, plants, fungi, and protists) have a nucleus enclosed by a membrane and other membrane-bound compartments. Eukaryotic cells are generally larger than prokaryotic cells and have more complex internal organization.

Internal Components

The nucleus contains chromosomes. Mitochondria are the sites where energy is extracted from glucose and fat using oxygen. Ribosomes synthesize proteins. Plant cells have cell walls made of cellulose and chloroplasts that use sunlight to produce food through photosynthesis. The cell membrane controls what enters and exits the cell.

Division

New cells arise from the division of existing cells. Mitosis is the process by which one cell divides into two identical cells and is how the body grows and repairs damaged cells. Meiosis is another process that produces reproductive cells (sperm and egg), reducing the number of chromosomes by half.

History of Knowledge

The term "cell" was used centuries ago by early microscopists observing the cell walls of plant tissue, where they saw small hollow spaces arranged closely together. In the nineteenth century, the idea was established that all living things are made of cells and that every cell comes from a preceding cell. This principle forms the foundation of modern biology.