All living organisms are made up of cells with internal structures that have specific functions. Each cell is at least five times smaller than substances visible to the unaided eye. Cells contain structures and systems in the same way the human body has structures (e.g., bones, muscles, blood vessels) and systems (e.g., circulatory, digestive). Cells can be categorized as prokaryotic or eukaryotic cells. Prokaryotic cells have minimal complexity, lacking a true nucleus and membrane-bound organelles. Members of the kingdoms Eubacteria and Archaebacteria are prokaryotes. Eukaryotic cells have a nucleus and membrane-bound organelles. Eukaryotes make up the kingdoms Fungi, Plantae, and Animalia, and a group of organisms called "Protists."
At A Glance
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Cells are the smallest unit of life. All organisms are composed of cells.
- Prokaryotic cells and eukaryotic cells differ in size and complexity, but they contain some common structures, including chromosomes, a cell membrane, cytoplasm, and ribosomes.
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Prokaryotes are a type of cell lacking a nucleus and other membrane-bound organelles.
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Eukaryotic cells contain a nucleus that directs the functions and activities of the cell's organelles.
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Plant cells are eukaryotic cells with a cell wall and organelles (called chloroplasts) involved in photosynthesis.
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Animal cells are eukaryotic cells that generally lack a cell wall.
