Carbohydrates are the main source of energy for the cell, provide some structural support for cells, and function in cell recognition.
A carbohydrate is an organic molecule that contains carbon, hydrogen, and oxygen. Carbohydrates are vital to all cells, providing energy, structural support, and identity markers for cellular signaling and recognition. A monosaccharide, known also as a simple sugar, is the most basic unit of carbohydrates, as it is a single sugar molecule. A monosaccharide cannot be broken down into smaller carbohydrates. In contrast, a polysaccharide is a carbohydrate formed from many monosaccharides bonded together by glycosidic linkages, named covalent bonds. Polysaccharides, like all polymers (chains) of biological molecules, are formed through dehydration reactions, chemical reactions that combine two molecules together with the elimination of a water molecule. An oligosaccharide is a polysaccharide made up of just a few monosaccharides. Sugar is a common term used to refer to various water-soluble monosaccharides and disaccharides (two-unit carbohydrates). All carbohydrates contain carbon, hydrogen, and oxygen atoms at a ratio of 1:2:1.
A major function of carbohydrates is to provide fuel, or energy, for the cell. Photosynthesis builds the monosaccharide glucose using carbon dioxide from the air and water, and energy from sunlight, which provides usable energy to other organisms within an ecosystem. Glucose is an important energy source because it is the substrate of glycolysis, a series of reactions nearly universal among organisms that breaks glucose down into two pyruvate molecules and releases a small amount of adenosine triphosphate (ATP), the cell's energy carrier. In oxygen-using organisms that can perform cellular respiration, glucose is further processed to drive the formation of a significant amount of ATP.
Because of the utility of glucose in providing energy for the cell, many polysaccharides are made up of repeating units of glucose. This allows organisms to store energy for later use. In plants, the glucose polysaccharide that stores energy is starch. In animals, the glucose polysaccharide that stores energy is glycogen. Humans are only able to store a small amount of glucose as glycogen. Most excess energy in humans is stored as fat. Carbohydrates also provide structural support to cells and organisms. Organisms that rely heavily on carbohydrates for structural support are plants, arthropods (animals with a hard exoskeleton, such as insects), and some fungi. In plants, the carbohydrate that provides structural support is cellulose. Cellulose is the most abundant biological molecule on the planet and is responsible for the tough, woody stems of plants such as trees. In arthropods and some fungi, a modified carbohydrate that provides structural support is chitin. Chitin gives rigidity to the exoskeletons of insects and marine arthropods. Another function of carbohydrates is cell recognition. An example of this in complex organisms is the immune response that is predicated on the immune system's ability to recognize familiar cells in contrast with foreign bodies. To do this, the immune cells determine which cells are foreign based on the carbohydrates on the surface of the other cell. These carbohydrates have particular molecular makeups, allowing the body to recognize which cells are its own and which are foreign. For example, the array of human blood types is based on carbohydrates on the surface of the red blood cells. People with type A blood have red blood cells with the type A carbohydrate, while people with type B blood have red blood cells with the type B carbohydrate. Those with type AB blood have red blood cells with both types of carbohydrate, and those with type O blood have red blood cells lacking any surface carbohydrates. If a person with blood type A receives a transfusion of type B blood, their immune system creates antibodies to B antigens, which attack the invading blood cells. The allergic reaction can lead to flu-like symptoms and possibly death, as blood clots can occur.