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Unicellular organisms are composed of a single cell. Multicellular organisms are composed of many specialized cells. Specialized cells differ in structure (size, shape...) and function (the role they perform in the organism).
In complex multicellular organisms, cells specialize into different cell types that are adapted to particular functions. In mammals, major cell types include skin cells, muscle cells, neurons, blood cells, fibroblasts, stem cells, and others.ells are the smallest unit of life that can replicate independently, and are often called the building blocks of life. The study of cells is called cell biology. Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids.