Cell structure (very basic) A cell is the fundamental unit of life. All living organisms are composed of cells, which can be broadly categorized into prokaryotic (lacking a nucleus and membrane-bound organelles) and eukaryotic (possessing a nucleus and organelles). Basic components include a cell membrane that encloses the cell, cytoplasm (the jelly-like substance filling the cell), and genetic material (DNA or RNA) that carries hereditary information. Characteristics of living things Living things exhibit several key characteristics: Movement (changing position or place), Respiration (releasing energy from food), Sensitivity (responding to stimuli), Growth (increasing in size and complexity), Reproduction (producing offspring), Excretion (removing waste products), and Nutrition (taking in and using food). These processes collectively define life.