3.37 Carbohydrate Classification
A:You may eat a wide variety of carbohydrates—from sugars in fruits to starches in potatoes. However, body cells
use only sugars for energy.
Sugars
Sugars are simple carbohydrates. Molecules of sugars have relatively few carbon atoms. Glucose (C 6 H 12 O 6 ) is one
of the smallest sugar molecules. Plants and some other organisms make glucose in the process of photosynthesis.
Living things that cannot make glucose obtain it by consuming plants or these other organisms.
In theFigure3.69, you can see structural formulas for glucose and two other sugars, named fructose and sucrose.
Fructose is a sugar that is found in fruits. It is an isomer of glucose. Isomers are compounds that have the same
atoms but different arrangements of atoms. Do you see how the atoms are arranged differently in fructose than in
glucose? You’re probably most familiar with the sugar sucrose, because sucrose is table sugar. It’s the sugar that
you spoon onto your cereal or into your iced tea.
FIGURE 3.69
Note: Each unlettered point where lines
intersect represents a carbon atom.Q:Compare the structure of sucrose with the structures of glucose and fructose. How is sucrose related to the other
two sugars?
A:Sucrose consists of one molecule of glucose and one molecule of fructose bonded together.
Starches
Starches are complex carbohydrates. They are polymers of glucose. A polymer is a large molecule that consists
of many smaller, repeating molecules, called monomers. The monomers are joined together by covalent bonds.
Starches contain hundreds of glucose monomers. Plants make starches to store extra glucose. Consumers get
starches by eating plants. Common sources of starches in the human diet are pictured in theFigure3.70. Our
digestive system breaks down starches to sugar, which our cells use for energy.