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A cell is the smallest unit of life. Cells are often called the “building blocks of life”. Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids. The cell was discovered by Robert Hooke in 1665, who named the biological units for their resemblance to cells inhabited by Christian monks in a monastery. Cells are of two types, eukaryotic, which contain a nucleus, and prokaryotic, which do not.
Prokaryotic cells were the first form of life on Earth, characterised by having vital biological processes including cell signaling and being self-sustaining. The genetic material is freely found in the cytoplasm. On the outside, flagella and pili project from the cell’s surface. Plants, animals, fungi, slime moulds, protozoa, and algae are all eukaryotic.
These cells are about fifteen times wider than a typical prokaryote and can be as much as a thousand times greater in volume. The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present. The eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are associated with histone proteins. All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane. Many eukaryotic cells are ciliated with primary cilia.