Cell Biology Learning Path

Cell as basic unit of life

The cell is the basic structural and functional unit of all living organisms. It consists of a selectively permeable cell (plasma) membrane that encloses cytoplasm containing genetic material. Most cells are microscopic, and—except for highly specialized types such as red blood cells and gametes—most can replicate and synthesize proteins. Cells can also be motile, and they arose on Earth about four billion years ago. All organisms are grouped into prokaryotes and eukaryotes. Prokaryotes (including bacteria and archaea) are single-celled and lack a membrane-bound nucleus, instead having a nucleoid region for DNA. Eukaryotes have a membrane-bound nucleus and can be single-celled or multicellular; multicellular organisms are made of many different cell types (somatic cells), while gametes are haploid. Eukaryotic cells contain additional membrane-bound organelles such as mitochondria (energy), chloroplasts in plants (photosynthesis), and other organelles involved in specialized functions. Cellular life also depends on key processes such as cell division (binary fission in prokaryotes; mitosis and cytokinesis in eukaryotes), DNA replication, signaling, protein targeting, DNA repair, growth and metabolism, protein synthesis (transcription and translation), and motility. Cells can undergo programmed cell death (e.g., apoptosis) or other death pathways, and their origin is linked to the broader origin of life, with cells likely emerging around 4 billion years ago.

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Cell membrane and selective permeability

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