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The human body is made up of trillions of cells. Cells are the basic units of all living things. Each cell is surrounded by a cell membrane, which holds the cell together and controls what goes into and out of the cell. The nucleus, containing DNA, serves as the control center of the cell. The cytoplasm, a jelly-like fluid outside the nucleus, houses other organelles of the cell. Each type of cell performs a specific function. For example, red blood cells carry oxygen to all parts of the body, while white blood cells work to fight infections. Groups of cells that work together are called tissues.

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