The basic membrane-bound unit that contains the fundamental molecules of life and of which all living things are composed
Cell
A single cell is often a complete organism in itself, such as a bacterium or yeast
Cells acquire specialized functions as they mature
Cells cooperate with other specialized cells and become the building blocks of large multicellular organisms, such as humans and other animals
Cells are much larger than atoms, but still very small
The smallest known cells are a group of tiny bacteria called mycoplasmas; some of these single-celled organisms are spheres as small as 0.2 μm in diameter (1μm = about 0.000039 inch), with a total mass of 10−14 gram—equal to that of 8,000,000,000 hydrogen atoms
Cells of humans typically have a mass 400,000 times larger than the mass of a single mycoplasma bacterium, but even human cells are only about 20 μm across
It would require a sheet of about 10,000 human cells to cover the head of a pin, and each human organism is composed of more than 30,000,000,000,000 cells
Cytoplasm
Surrounds the cell's specialized structures, or organelles
Ribosomes
Sites of protein synthesis, found free in the cytoplasm or attached to the endoplasmic reticulum, through which materials are transported throughout the cell
Mitochondria
Release energy needed by the cell
Golgi complex
Stacks of flattened sacs that process and package materials to be released from the cell in secretory vesicles
Lysosomes
Contain digestive enzymes
Peroxisomes
Contain enzymes that detoxify dangerous substances
Centrosome
Contains the centrioles, which play a role in cell division
Microvilli
Fingerlike extensions found on certain cells
Cilia
Hairlike structures that extend from the surface of many cells, can create movement of surrounding fluid
Nuclear envelope
A double membrane surrounding the nucleus, contains pores that control the movement of substances into and out of the nucleoplasm
Chromatin
A combination of DNA and proteins that coil into chromosomes, makes up much of the nucleoplasm