The cell is the smallest unit capable of carrying out the basic processes of life in the human body.
Human cells maintain internal conditions, obtain and use energy, respond to signals, grow, and reproduce through division. They contain a plasma membrane and specialized internal structures, and most have a nucleus that stores nuclear genetic material. Together, these features let cells function as organized living systems.
Cells become specialized through differences in gene activity. Muscle cells contract, neurons transmit signals, and epithelial cells form protective or absorptive surfaces. Groups of similar cells form tissues, tissues combine into organs, and organs cooperate in organ systems. Organelles such as mitochondria and ribosomes perform essential tasks inside cells but are not themselves independent living units.