Carbohydrates
Before a big cross-country meet, lots of high school runners eat a plate of pasta the night before, taking in a store of glucose that the plant packaged as starch and that their body will later store as glycogen. In this section you'll go from the simplest sugar molecule to the polymer and see how the same chemical unit —a monosaccharide— can serve as an immediate fuel or as a structural material, depending on how it is bonded and folded.
Monosaccharides: the basic unit
If you dissolve a spoonful of glucose in water and add digestive enzymes or mild acid, the molecule does not break down into smaller carbohydrates: you have reached the limit of the sugars. What chemical features define this basic unit, and how does its structure determine the function it performs?
Monosaccharides are the cell's quick fuel: cells break down glucose to make ATP. They are also the building blocks (monomers) of larger carbohydrates. Glucose and fructose are two common examples. Fructose is the sugar in fruit and honey.
What is a monosaccharide? Give two examples and state the main function of monosaccharides in the cell.
What is a monosaccharide? Give two examples and state the main function of monosaccharides in the cell.
Disaccharides: the glycosidic bond
Classify the following carbohydrates as monosaccharides, disaccharides, or polysaccharides: glucose, sucrose, starch, fructose, lactose, and cellulose.
Classify the following carbohydrates as monosaccharides, disaccharides, or polysaccharides: glucose, sucrose, starch, fructose, lactose, and cellulose.
Sucrose is formed by the joining of a glucose molecule and a fructose molecule. What type of bond is formed between the two monosaccharides, what is the reaction that…
Sucrose is formed by the joining of a glucose molecule and a fructose molecule. What type of bond is formed between the two monosaccharides, what is the reaction that forms it called, and what molecule is released during it?
Polysaccharides: energy storage and structure
A cotton T-shirt (Texas grows a lot of cotton) and a spoonful of flour are made of exactly the same monomer —D-glucose—, yet the T-shirt neither dissolves nor can we digest it, while flour thickens when cooked and nourishes us. How do cells achieve such opposite properties from the same building block?
In animals, extra glucose is stored as glycogen, mainly in the liver (which releases glucose to keep blood sugar steady) and in skeletal muscle (fuel for the muscle itself).
State the biological function of glycogen in animals and in which organs it is mainly stored.
State the biological function of glycogen in animals and in which organs it is mainly stored.
The diagram shows how the chains of two polysaccharides made exclusively of glucose are arranged. State which corresponds to starch and which to cellulose, explain what…
The diagram shows how the chains of two polysaccharides made exclusively of glucose are arranged. State which corresponds to starch and which to cellulose, explain what difference exists in the bonding of their monomers (α or β bond), and reason how the shape of their chains justifies why one serves as an energy reserve and the other has a structural function.
Carbohydrates in the diet and metabolism
Back to the start: The runner eats pasta because its starch has $\alpha(1\to 4)$ bonds that our enzymes readily hydrolyze into free glucose; this passes into the blood and is stored in the muscle as glycogen, a compact reserve ready to release fuel quickly during the race.
Pentoses, isomerism and recognizing structures
Ribose is a pentose found in RNA; 2-deoxyribose, found in DNA, has H instead of OH on carbon 2. Glucose, galactose and fructose are hexoses. Glucose and fructose differ in their carbonyl group (aldose/ketose); glucose and galactose differ in the configuration of one asymmetric carbon (epimers).