Notes

Living things are made up of four main groups of carbon-based molecules called organic compounds.
One of these groups is carbohydrates.
The other three groups are lipids, proteins, and nucleic acids.
All carbohydrates are made up of the same three chemical elements: carbon, hydrogen, and oxygen. But carbohydrate molecules can come in different forms. The smallest carbohydrates are called simple sugars. Simple sugars can be linked together in long chains to make larger carbohydrates, called complex carbohydrates.
Carbohydrates can also have different functions. An important function of carbohydrates is to provide the primary source of energy for cells. Cells can quickly break down many types of carbohydrates to release energy, so these carbohydrates provide energy for cells' immediate use. Carbohydrates can also form cell structures. One complex carbohydrate, called cellulose, forms the cell walls in plant cells. Cellulose helps to give plants structure and make them strong.