Cell biology: structure, types, and processes

Cell as basic unit of life

The cell is the basic structural and functional unit of all forms of life. It is typically composed of a selectively permeable cell (plasma) membrane that encloses cytoplasm containing genetic material. Most cells are microscopic, can replicate, and can synthesize proteins; some specialized cells are motile. Cells are grouped into two major types: prokaryotes and eukaryotes, which differ mainly in the presence of a membrane-bound nucleus and other organelles. Prokaryotes (including bacteria and archaea) are single-celled and generally lack a nucleus, using a nucleoid region instead. They are usually smaller and simpler than eukaryotic cells and may have specialized structures such as flagella/pili for movement and communication. Eukaryotes have a nucleus enclosed by a nuclear membrane and contain multiple membrane-bound organelles (e.g., mitochondria for energy and chloroplasts in plants for photosynthesis). Multicellular organisms are made of many different cell types (somatic cells), while gametes are haploid. Cells carry out essential life processes such as cell division (binary fission in prokaryotes; mitosis/meiosis in eukaryotes), signaling, protein targeting, DNA repair, growth and metabolism, protein synthesis (transcription and translation), and motility. Programmed cell death and other death pathways help maintain organismal health. Cells likely emerged about four billion years ago, and the origin of cells is tied to the broader origin of life, potentially involving early self-replicating molecules such as RNA.

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Cell membrane and selective permeability

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