Carbohydrates are one of the major classes of biological molecules and are a main source of energy for many cells. They are built from carbon, hydrogen, and oxygen atoms, often in a ratio close to 1:2:1. Simple sugars can be used quickly for cellular respiration, while larger carbohydrates store energy or provide support.
Understanding carbohydrates helps explain nutrition, metabolism, plant structure, and how cells manage energy.
Key Facts
- General carbohydrate formula for many simple sugars: (CH2O)n
- Glucose molecular formula: C6H12O6
- Monosaccharides are single sugar units, such as glucose, fructose, and galactose.
- Disaccharides form when two monosaccharides join by dehydration synthesis, releasing H2O.
- Polysaccharides are long chains of sugars, including starch, glycogen, and cellulose.
- Cellular respiration uses glucose to release energy: C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy
Vocabulary
- Carbohydrate
- A biological molecule made mainly of carbon, hydrogen, and oxygen that provides energy, stores energy, or forms structural materials.
- Monosaccharide
- A single sugar molecule that is the basic building block of larger carbohydrates.
- Disaccharide
- A carbohydrate made of two monosaccharides joined by a glycosidic bond.
- Polysaccharide
- A large carbohydrate made of many monosaccharides linked together in chains or branches.
- Glycosidic bond
- A covalent bond that links sugar molecules together in disaccharides and polysaccharides.
Common Mistakes to Avoid
- Calling all carbohydrates sugars is incorrect because sugars are only the smaller carbohydrates, while starch, glycogen, and cellulose are large polysaccharides.
- Thinking cellulose is used by humans for energy is wrong because humans lack the enzymes needed to break its beta glycosidic bonds efficiently.
- Confusing starch and glycogen leads to errors because starch is the main storage carbohydrate in plants, while glycogen is the main storage carbohydrate in animals.
- Forgetting water in dehydration synthesis is a mistake because joining two monosaccharides releases one H2O molecule as a glycosidic bond forms.
Practice Questions
- 1 A starch fragment contains 25 glucose units. If each glucose has the formula C6H12O6 before bonding, how many carbon atoms are in the fragment?
- 2 A cell breaks down 3 glucose molecules completely during cellular respiration. Using C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy, how many O2 molecules are required and how many CO2 molecules are produced?
- 3 Starch, glycogen, and cellulose are all made from glucose monomers. Explain why they can have different functions in organisms even though they are built from the same basic sugar.