2.06: Carbohydrates

Monosaccharides

Three common sugars share the same molecular formula: C₆H₁₂O₆. Because of their six carbon atoms, each is a hexose. Three atom-labeled ring structures appear left to right. Glucose and galactose are six-membered rings with a ring oxygen at the top, several ring carbons, H and OH substituents, and an upward CH2OH group; small carbon numbers, including 6, 4, 3, and 2, are visible. Their main visible difference is at the left-side ring carbon: glucose has H above and OH below, while galactose has HO above-left and H below-left. Fructose is a five-membered ring with an oxygen at the top, two CH2OH groups extending upward on the left and right, and multiple H and OH substituents around the ring. The labels beneath the structures read GLUCOSE, GALACTOSE, and FRUCTOSE. Some tiny carbon numerals are difficult to resolve.

They are:

• glucose, "blood sugar", the immediate source of energy for cellular respiration
• galactose, a sugar in milk (and yogurt)
• fructose, a sugar found in honey

Although all three share the same molecular formula (C₆H₁₂O₆), the arrangement of atoms differs in each case. Substances such as these three, which have identical molecular formulas but different structural formulas, are known as structural isomers. Glucose, galactose, and fructose are "single" sugars or monosaccharides. Two monosaccharides can be linked together to form a "double" sugar or disaccharide.

Disaccharides

Three common disaccharides:

• sucrose — common table sugar = glucose + fructose
• lactose — major sugar in milk = glucose + galactose
• maltose — product of starch digestion = glucose + glucose

Although the process of linking the two monomers is rather complex, the end result in each case is the loss of a hydrogen atom (H) from one of the monosaccharides and a hydroxyl group (OH) from the other. The resulting linkage between the sugars is called a glycosidic bond. The molecular formula of each of these disaccharides is

$$\begin{array}{}\text{(}\text{2.6.1}\text{)}& C_{12}{H}_{22}{O}_{11}=2C_6{H}_{12}{O}_6-H_{2}O\end{array}$$

All sugars are very soluble in water because of their many hydroxyl groups. Although not as concentrated a fuel as fats, sugars are the most important source of energy for many cells. Carbohydrates provide the bulk of the calories (4 kcal/gram) in most diets, and starches provide the bulk of that. Starches are polysaccharides.

Polysaccharides

There are three primary polysaccharide polymer systems of interest: Starches, Glycogen and Cellulose.

Starches

Four glucose rings form a horizontal lower chain, connected through oxygen bridges. Two additional rings form an upper branch. The upper branch connects through oxygen to the CH2 group at carbon 6 of a lower-chain glucose. Numbers 1 through 6 identify positions on that lower ring. The drawing contrasts links along the main chain with the branch linkage; bonds at the edges indicate that the polymer continues.Starches are polymers of glucose. Two types are found:

• amylose consists of linear, unbranched chains of several hundred glucose residues (units). The glucose residues are linked by a glycosidic bond between their #1 and #4 carbon atoms.
• amylopectin differs from amylose in being highly branched. At approximately every thirtieth residue along the chain, a short side chain is attached by a glycosidic bond to the #6 carbon atom (the carbon above the ring). The total number of glucose residues in a molecule of amylopectin is several thousand.

Potato starch Amylase