Carbohydrate Structure, Bonding, and Biological Roles

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What are carbohydrates, and what are three major roles they perform?

Carbohydrates are biomolecules containing carbon, hydrogen, and oxygen, although some also contain other elements. They can provide or store energy, support structures, and contribute sugars to the nucleotides of DNA and RNA.

How are monosaccharides classified by size?

They are classified by the number of carbon atoms. The prefix indicates that number and the suffix -ose denotes a saccharide: for example, triose has three carbons, pentose five, and hexose six.

Why do many monosaccharides form ring structures?

In sugars with four or more carbon atoms, a carbonyl group can react with a hydroxyl group elsewhere on the same flexible chain, closing it into a ring. The cyclic form is generally more stable.

What is a glycosidic bond?

It is the covalent bond joining monosaccharide units. It forms between hydroxyl groups in a dehydration synthesis reaction, which releases water.

How do starch, glycogen, and cellulose differ in function?

All are glucose-based polysaccharides. Starch is the main energy-storage carbohydrate in plants, glycogen stores energy in animals and bacteria, and cellulose provides structural support in plant cell walls.

Quiz questions

1. Which empirical formula describes many carbohydrates?Answer: $(CH_2O)_n$
2. How can a monosaccharide with four or more carbon atoms form a ring?Answer: A carbonyl group reacts with a hydroxyl group elsewhere on the same sugar chain.
3. How does a glycosidic bond form between two monosaccharides?Answer: Hydroxyl groups react in a dehydration synthesis reaction that releases water.
4. Which monosaccharides combine to form lactose?Answer: One galactose molecule and one glucose molecule.
5. Which glucose-based polysaccharide stores energy in animals and bacteria?Answer: Glycogen