The document defines the cell as the structural and functional unit of living organisms. It describes cells as the smallest units that make up tissues and organs, which work together in biological systems. The structures of cells include a cell membrane, cytoplasm, nucleus, and organelles. The cell membrane supports and protects the cell, controls movement of substances in and out, and separates the cell from the external environment. The nucleus contains DNA and sends signals for cell growth, maturation, division, and death. Organelles like the mitochondria and endoplasmic reticulum perform specific functions to carry out life's processes. The cell theory states that all living things are made of cells, cells arise from pre-existing cells, and the cell is the basic