The cell is the smallest unit of life that exists and that constitutes all living organisms and body tissues including unicellular and multicellular organisms. Due to this, cells are considered as the fundamental unit of life as even the smallest and simplest living organisms are made of cells or at least made of one cell.
Cells are commonly made up of three main parts: the cell membrane which surrounds the cell and controls the substances that enter and leave it; the nucleus which is the structure within the cell that contains the nucleolus and most of the cell's DNA; and the cytoplasm that is the fluid inside the cell that contains other tiny cell particles with specific functions, such as the Golgi apparatus, mitochondria, and the endoplasmic reticulum. This applies to all cells except to prokaryotic cells because these do not contain a nucleus but a chromosome in the middle of the cell.
Red blood cells contain enormous amounts of a protein called hemoglobin. This iron-containing molecule binds oxygen as oxygen molecules enter blood vessels in the lungs. Hemoglobin is also responsible for the characteristic red color of blood.
