A cell is the basic structural and functional unit of the human body. All living organisms are made up of cells, and the human body contains trillions of cells that perform different functions such as nutrition, respiration, growth, reproduction, movement and response to stimuli. Cells vary in size, shape and function, but most have a common basic organization. The basic structure of a cell consists of the cell membrane, cytoplasm and nucleus. The cell membrane is the outer covering of the cell. It is selectively permeable and controls the movement of substances into and out of the cell. The cytoplasm is a jelly-like substance present between the cell membrane and nucleus. It contains various cell organelles, such as mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus and lysosomes. Mitochondria produce energy in the form of ATP and are therefore called the powerhouses of the cell. Ribosomes help in protein synthesis. The endoplasmic reticulum participates in the synthesis and transport of proteins and lipids, while the Golgi apparatus modifies, packages and distributes cellular products. Lysosomes contain enzymes that help digest waste materials and damaged cell components. The nucleus is the control centre of the cell. It contains genetic material in the form of DNA and regulates cell growth, metabolism and reproduction. Tissues are groups of similar cells that work together to perform a particular function. The human body has four major types of tissues: epithelial, connective, muscular and nervous tissue. Epithelial tissue covers the external surface of