Carbohydrates are a major class of biomolecules made mainly from carbon, hydrogen, and oxygen. They are important because they provide quick energy, store energy, and build structural materials such as plant cell walls. Glucose, fructose, sucrose, starch, glycogen, and cellulose are all carbohydrates with different sizes and shapes.
Their chemistry explains how food energy is stored, released, and used by living cells.
Many simple sugars can exist as both open-chain molecules and ring-shaped molecules in water. A glucose molecule forms a ring when one of its hydroxyl groups reacts with its carbonyl group inside the same molecule. Sugars join together through glycosidic bonds, usually by a dehydration reaction that removes water.
The type of bond and the way monomers repeat determine whether a carbohydrate becomes a digestible energy store like starch or a strong structural polymer like cellulose.
Key Facts
- Many carbohydrates have an approximate formula of Cn(H2O)n, but not all carbohydrates fit this exactly.
- Glucose has the molecular formula C6H12O6.
- Monosaccharides are single sugar units, such as glucose, fructose, and galactose.
- A disaccharide forms when two monosaccharides join: glucose + fructose -> sucrose + H2O.
- A glycosidic bond is a covalent bond that links sugar units through an oxygen atom.
- Polysaccharides are long carbohydrate polymers, such as starch, glycogen, and cellulose.
Vocabulary
- Monosaccharide
- A monosaccharide is a single sugar molecule that cannot be broken into smaller carbohydrates by hydrolysis.
- Disaccharide
- A disaccharide is a carbohydrate made of two monosaccharides joined by a glycosidic bond.
- Polysaccharide
- A polysaccharide is a large polymer made of many monosaccharide units linked together.
- Glycosidic bond
- A glycosidic bond is the covalent linkage that connects one sugar molecule to another.
- Dehydration reaction
- A dehydration reaction joins molecules by removing the atoms needed to form water.
Common Mistakes to Avoid
- Calling all carbohydrates sugars is wrong because sugars are small carbohydrates, while starch, glycogen, and cellulose are large polysaccharides.
- Forgetting the water molecule in disaccharide formation is wrong because joining two monosaccharides by dehydration releases H2O.
- Assuming glucose is always a straight chain is wrong because glucose exists mostly in ring form when dissolved in water.
- Thinking starch and cellulose are the same because both contain glucose is wrong because their different glycosidic bonds create very different shapes and functions.
Practice Questions
- 1 A glucose molecule has the formula C6H12O6. What is the total number of atoms in one glucose molecule?
- 2 A polysaccharide chain is made from 25 glucose monomers. If each glycosidic bond forms by releasing one water molecule, how many water molecules are released when the chain forms?
- 3 Starch and cellulose are both polymers of glucose, but humans digest starch much more easily than cellulose. Explain how bond type and molecular shape can affect digestibility.