Inside a Cell Quiz
The building blocks of life
Every living thing is made of cells. Some, like bacteria, are a single cell. A human body is made of about 37 trillion of them, of hundreds of different kinds.
Most cells are far too small to see without a microscope. The English scientist Robert Hooke gave them their name in 1665, after looking at thin slices of cork and seeing rows of tiny boxes that reminded him of monks' small rooms, or cells.
Bacteria are simpler than our cells: their DNA sits in a loose region of the cell instead of being wrapped up in a nucleus. Cells like ours, with a nucleus, are found in animals, plants and fungi.
Parts of an animal cell
The cell membrane is a thin, flexible skin around the cell. It controls what goes in and out. Inside is cytoplasm, a jelly-like fluid where much of the cell's chemistry happens.
The nucleus holds most of the cell's DNA, the instructions for building and running it. Ribosomes, tiny structures by the thousands, read copies of those instructions and make proteins.
Mitochondria release energy from food, using oxygen. Busy cells, like muscle cells, are packed with them.
Plant cells and other special cells
Plant cells have three things animal cells don't. A stiff cell wall made of cellulose surrounds the membrane. Chloroplasts capture light to make food. And a large vacuole filled with watery sap presses outward and keeps the plant firm; when it loses water, the plant wilts.
Some cells are built for one job. Red blood cells lose their nucleus as they mature, leaving more room to carry oxygen. Nerve cells can be extremely long: some reach from the base of the spine to the toes, about a meter.