Summary 5.2

Carbohydrates provide fuel and building material

A carbohydrate is a type of organic compound made from sugar molecules. Sugars are made up of elements carbon, hydrogenm and oxygen in the ratio 1 carbon: 2 hydrogen: 1 oxygen.

Simple sugars are called monosaccharides, and contain just one sugar unit. Some examples of monosaccharides are fructose, glucose, and galactose. Sugary molecules are the main fuel supply for cellular work. Cells break down the sugars and absorb the energy within them. Furthermore, cells also use the carbon skeletons of monosaccharides as raw ingredients for making other kinds of organic molecules. The glucose molecules that are not used right away are put into larger carbohydrates, or used to make fat molecules.

A disaccharide is made from the dehydration reaction. Disaccharides ("double sugar") are made up of two monosaccharides. The commonly known dissaccharide is sucrose. Sucrose is made up of a glucose monomer linked with a fructose monomer. Furthermore, sucrose is a major carbohydrate in plant sap that nourishes all parts of the plant. When sucrose is consumed, it breaks down into fructose and glucose, and these can be used right away.

Polysaccharides are long polymer chains that are made from simple sugar monomers (complex carbohydrates). Starch is a type of polysaccharide found in plants, and consists if only glucose monomers. When starch is consumed, it is kept as sugar stockpiles. Glycogen, like starch, consists only of glucose molecules. Yet, they are different in that glycogen is more highly branched. When the body requires energy, it breaks apart glycogen granules in order to release glucose. Cellulose are also made only from glucose molecules. They are found in plants cells, and protect, stiffen, and prevent the plant cell from flopping over. People, and most animals, cannot digest cellulose because the molecules necessary for breaking down cellulose are lacking. Thus, cellulose from plant foods, known as "fiber", pass through our digestive systems unchanged.

Most carbohydrates are hydrophillic due to the many hydroxyl groups within their sugar units. Yet, certain carbohydrates such as cellulose and forms of starch do not dissolve in water.

Concept Check

1.Explain the difference between a monosaccharide and a disaccharide.Give an example of each. The difference between a monosaccharide(eg.glucose) and disaccharide(eg.sucrose) is that a monosaccharides are simple sugars with just one unit of sugar, while disaccharides are two monosaccharides placed together.

2.Compare and contrast starch, glycogen, and cellulose. All starch,glycogen, and cellulose are made from glucose. Starch is found in plants and animals, and serves as a sugar storage. When a starch molecule is broken down, the glucose becomes available. Similarly, glycogen is stored as granules, and can be broken down to release glucose. Unlike starch, glycogen is more highly branched. Finally, cellulose is also made up from glucose. Cellulose serves as a building material, and unlike starch and glycogen, cannot be broken down by human bodies to extract the nutrients.

3.How do animals store excess glucose molecules.Animals store excess glucose molecules in the form of the polysaccharide glycogen.