Source: Biology, University of Michigan
Tags: carbohydrates, sugars, monosaccharides, disaccharides, polysaccharides, glycosidic linkage, starch, glycogen, cellulose, chitin, glucose, fructose, sucrose, macromolecules
Difficulty: Introductory | Prerequisites: Basic chemistry (atoms, bonds, functional groups)
Carbohydrates are one of the four major classes of macromolecules found in all living cells, alongside proteins, lipids and nucleic acids. This chapter sits at the foundation of the course: understanding what carbohydrates are, how they are built and what they do will come back repeatedly when you study cellular respiration (Chapter 7), photosynthesis and metabolism. You should already be comfortable with covalent bonds, functional groups (hydroxyl, carbonyl) and the idea that water is released or consumed when bonds form or break. If those terms feel unfamiliar, review Chapter 2 first.
Carbohydrates are sugars and chains of sugars built from carbon, hydrogen and oxygen. Simple sugars (monosaccharides) are the cell's quick fuel; link two together and you get a disaccharide such as sucrose; link many and you get polysaccharides such as starch, glycogen and cellulose, which store energy or provide structural support. All of these bonds form by dehydration reactions and break by hydrolysis.
Carbohydrate
An organic compound composed of carbon, hydrogen and oxygen, typically in a ratio approximating CH₂O. In simple terms, carbohydrates are sugars and the molecules built from sugars.
Monosaccharide
The simplest carbohydrate unit, a single sugar molecule such as glucose or fructose. Think of it as the single Lego brick from which all larger carbohydrates are assembled.
Disaccharide
A sugar formed by linking two monosaccharides via a glycosidic linkage. Sucrose (table sugar) is the most familiar example: one glucose joined to one fructose.
Polysaccharide
A long chain (polymer) of monosaccharide units linked by glycosidic bonds. In simple terms, this is a complex carbohydrate, such as starch, glycogen or cellulose.
Glycosidic linkage
The covalent bond that joins two monosaccharides, formed by a dehydration (condensation) reaction. Think of it as the clip that snaps two sugar bricks together while releasing a water molecule.
Dehydration reaction (condensation reaction)
A chemical reaction that bonds two molecules together by removing a water molecule (one monomer donates –OH, the other donates –H). This is how all carbohydrate polymers are built.
Hydrolysis
The reverse of dehydration: a water molecule is added to break a covalent bond, splitting a polymer back into its monomers.
Starch
A polysaccharide used for energy storage in plants, made of alpha-glucose units. In simple terms, starch is a plant's way of keeping glucose on hand for later.
Glycogen
A highly branched polysaccharide used for energy storage in animals, found mainly in the liver and muscles. Think of it as the animal equivalent of starch, but larger and more branched.
Cellulose
A structural polysaccharide that forms plant cell walls, made of beta-glucose units linked in parallel strands joined by hydrogen bonds. In simple terms, cellulose is the fibre that makes plants rigid.
Chitin
A structural polysaccharide found in the exoskeletons of arthropods (insects, crustaceans) and in fungal cell walls. Similar concept to cellulose, but in animals and fungi rather than plants.
All carbohydrates contain carbon (C), hydrogen (H) and oxygen (O)
The empirical formula for monosaccharides is CH₂O (or multiples of it, e.g. C₆H₁₂O₆ for glucose)
They are one of the four macromolecule families; the others are proteins, lipids and nucleic acids
The monomer unit of all carbohydrates
Contain a carbonyl group (C=O) and multiple hydroxyl groups (–OH)
Classified by the number of carbon atoms (triose = 3C, pentose = 5C, hexose = 6C) and by the position of the carbonyl group (aldose vs ketose)
Most common monosaccharide: glucose (C₆H₁₂O₆), the primary fuel for cellular work
Fructose and galactose are other common hexoses
Simple sugars enter the bloodstream quickly after digestion
Formed when two monosaccharides join via a glycosidic linkage (a dehydration reaction)
Most common disaccharide: sucrose (glucose + fructose), ordinary table sugar
Other examples: lactose (glucose + galactose), maltose (glucose + glucose)
Long chains (polymers) of monosaccharide units
Digested more slowly than simple sugars because the body must break many glycosidic bonds
Storage polysaccharides
Starch: energy storage in plants
Made from alpha-glucose molecules
A major dietary energy source for humans
Takes longer to break down in the body than simple sugars
Glycogen: energy storage in animals
Stored primarily in the liver and skeletal muscles
More highly branched and larger than starch
Broken down between meals to maintain blood glucose
Structural polysaccharides
Cellulose: structural component of plant cell walls
Made from beta-glucose units
Parallel strands held together by hydrogen bonds, forming rigid microfibrils
Humans cannot digest cellulose (we lack the enzyme cellulase), so it passes through as dietary fibre
Chitin: structural component of arthropod exoskeletons and fungal cell walls
Also composed of parallel strands joined by hydrogen bonds
Similar architecture to cellulose, different monomer (N-acetylglucosamine)
Dehydration reaction (condensation): removes a water molecule to form a glycosidic bond between two sugars. One monomer contributes –OH, the other contributes –H.
Hydrolysis: adds a water molecule to break a glycosidic bond, releasing individual sugars. This is how digestion works at the molecular level.
Starch and glycogen are the reason athletes "carb-load" before endurance events: eating starch-rich foods tops up glycogen stores in liver and muscle, providing a slow-release fuel reservoir. Cellulose is the raw material behind paper, cotton fabric and wood construction. Chitin is being studied as a biodegradable packaging material and as a wound-healing scaffold in medicine.
Students often assume all polysaccharides are digestible. They are not. Cellulose is a polysaccharide, but humans cannot break it down because we lack cellulase.
"Complex carb" does not automatically mean "healthy" and "simple sugar" does not automatically mean "unhealthy." The distinction is about chain length and digestion speed, not nutritional value in isolation.
Students sometimes list lipids as polymers because they are built by dehydration reactions. Lipids are not true polymers; they do not consist of repeating monomer units the way polysaccharides, proteins and nucleic acids do.
Glycogen and starch are often confused. Both store energy, but starch is the plant version (alpha-glucose, less branched) and glycogen is the animal version (alpha-glucose, highly branched, stored in liver and muscle).
⚠️ Know the three classes (mono-, di-, polysaccharide) and be able to give an example of each.
⚠️ Be able to explain the difference between a dehydration reaction and hydrolysis, and state which builds polymers and which breaks them.
⚠️ Distinguish storage polysaccharides (starch, glycogen) from structural polysaccharides (cellulose, chitin) by function, organism and glucose type (alpha vs beta).
⚠️ The glycosidic linkage is the bond name you need. Expect a question asking you to name the bond that joins monosaccharides.
True or false: Glycogen is the storage polysaccharide found in plants. (False, glycogen is the animal storage polysaccharide; starch is the plant version.)
Fill in the blank: A dehydration reaction joins two monosaccharides by removing a molecule of ______. (water)
True or false: Cellulose and starch are both made of glucose, but humans can only digest starch. (True)
Fill in the blank: The covalent bond between two monosaccharides is called a ______ linkage. (glycosidic)
True or false: Lipids are true polymers made of repeating fatty-acid monomers. (False, lipids are not true polymers.)
Q: What type of bond joins two monosaccharides, and what kind of reaction forms it?
A: A glycosidic linkage, formed by a dehydration (condensation) reaction that releases one water molecule.
Q: Compare starch and glycogen. Where is each found, and which is more branched?
A: Starch is the energy-storage polysaccharide in plants; glycogen is the energy-storage polysaccharide in animals (liver and muscles). Glycogen is more highly branched than starch.
Q: Why can humans digest starch but not cellulose, even though both are made of glucose?
A: Starch is made of alpha-glucose with alpha-glycosidic linkages, which human enzymes (amylases) can break. Cellulose is made of beta-glucose with beta-glycosidic linkages, and humans lack the enzyme cellulase needed to break those bonds.
Q: Name two structural polysaccharides and state where each is found.
A: Cellulose, found in plant cell walls, and chitin, found in arthropod exoskeletons and fungal cell walls.
Q: Explain the relationship between dehydration reactions and hydrolysis.
A: They are reverse processes. Dehydration removes water to form a covalent bond between monomers (building a polymer). Hydrolysis adds water to break that bond (splitting a polymer into monomers).
Carbohydrates connect directly to cellular respiration (Chapter 7), where glucose is broken down to produce ATP. Understanding glycosidic bonds here also sets up the logic of peptide bonds (proteins) and phosphodiester bonds (nucleic acids), since all three form by dehydration reactions and break by hydrolysis. The structural role of cellulose will come back when you study plant cell structure and cell walls.
carbohydrates, sugars, monosaccharide, disaccharide, polysaccharide, glucose, fructose, sucrose, lactose, maltose, starch, glycogen, cellulose, chitin, glycosidic linkage, dehydration reaction, condensation reaction, hydrolysis, alpha glucose, beta glucose, CH₂O, carbonyl group, hydroxyl group, energy storage, structural carbohydrate, dietary fibre, complex carbs, simple sugars, macromolecules, BIO 101, Chapter 5