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WHAT IS CELL? Cells are the structural and functional units of all living organisms. Some organisms, such as bacteria, are unicellular , consisting of a single cell. Other organisms, such as humans, are multicellular , or have many cells—an estimated 100,000,000,000,000 cells! Each cell is an amazing world unto itself: it can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary. Even more amazing is that each cell stores its own set of instructions for carrying out each of these activities.

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MULTICELLULAR ORGANISMS Multicellular organisms are organisms with more than one cell . This is the case for most animals and plants that can be seen (without the use of a microscope ). In such organisms, cells are usually specialised . All the cells with the same function group together. Such a group of cells is then called a tissue . Multicellular organisms have a set of cells that specialize in reproduction. Reproduction in such organisms is usually sexual . The sex cells are either sperm or ovum (also named "egg") cells. If they are sperm cells, the organism is male , if they are egg cells it is female . If both are present, the organism is a hermaphrodite .

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UNICELLULAR ORGANISMS Unicellular organisms are organisms that have one cell . They are divided into two quite different types, from different classification kingdoms . The prokaryotes , bacteria and archaea , have cells with no nucleus and a simple cell structure. Eukaryotes have a nucleus , and a more complex cell structure. The differences between the prokaryota and eukaryota are significant. Eukaryotes possess a nucleus, while prokaryotes lack it, and eukaryotes possess a range of subcellular organs called organelles , which prokaryotes do not.

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