The cell (from Latin cella, meaning "small room") is the basic structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life that can replicate independently, and cells are often called the "building blocks of life". The study of cells is called cell biology.
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- In life’s structural hierarchy, the cell is the smallest unit of organization that can perform all activities required for life. In fact, the actions of organisms are all based on the functioning of cells. For instance, the movement of your eyes as you read this sentence results from the activities of muscle and nerve cells. Even a process that occurs on a global scale, such as the recycling of carbon atoms, is the product of cellular functions, including the photosynthetic activity of chloroplasts in leaf cells. All cells share certain characteristics. For instance, every cell is enclosed by a membrane that regulates the passage of materials between the cell and its surroundings. Nevertheless, we recognize two main forms of cells: prokaryotic and eukaryotic. The cells of two groups of single-celled microorganisms— bacteria (singular, bacterium) and archaea (singular, archaean)—are prokaryotic. All other forms of life, including plants and animals, are composed of eukaryotic cells.
- Jane B. Reece, Lisa A. Urry, et al. Campbell Biology (10th ed., 2014), Ch. 1. Evolution, the Themes of Biology, and Scientific Inquiry