Intro to Biology, BIO Ch. 3 | Source: Textbook Ch. 3 / Lecture notes
Tags: macromolecules, polymers, monomers, carbohydrates, monosaccharides, polysaccharides, dehydration synthesis, hydrolysis, glucose, starch, glycogen, cellulose, chitin, organic molecules
Difficulty: Introductory Prerequisites: Basic chemistry (atoms, bonds, functional groups from Ch. 1-2).
This chapter introduces the four classes of large biological molecules that make up all living things: carbohydrates, lipids, proteins and nucleic acids. Before you can understand how cells work (Ch. 4 onwards), you need to know what they are built from. The unifying thread is that most macromolecules are polymers, assembled from smaller monomers by the same basic reaction (dehydration synthesis) and broken apart by its reverse (hydrolysis). If you are comfortable with covalent bonds and functional groups, you are ready for this material.
Living organisms are built from four families of large molecules: carbohydrates, lipids, proteins and nucleic acids. Most of these are polymers, long chains of repeating monomer units snapped together by dehydration reactions and broken apart by hydrolysis. Carbohydrates range from simple sugars (glucose, fructose) to massive storage and structural polymers (starch, glycogen, cellulose, chitin), and understanding how monomers link up is the key to the whole chapter.
Polymer
A large molecule made of many small, repeating subunits bonded together in a chain. Think of it as a train: each carriage is a monomer, and the whole train is the polymer.
Monomer
A small molecule that can bond with others of its kind to build a polymer. In simple terms, monomers are the individual building blocks (like Lego bricks) that snap together.
Dehydration reaction (condensation reaction)
A chemical reaction that joins two monomers by removing a water molecule (one monomer loses an -OH, the other loses an -H). Think of it as: two pieces click together and a drop of water pops out as a byproduct.
Hydrolysis
The reverse of dehydration synthesis. A water molecule is added across the bond between two monomers, splitting the polymer apart. In simple terms, "hydro" (water) + "lysis" (breaking) = water breaks the bond.
Macromolecule
A very large organic molecule. The four classes in biology are carbohydrates, lipids, proteins and nucleic acids.
Monosaccharide
The simplest carbohydrate, a single sugar unit with the general formula CH₂O (repeated). Examples: glucose (C₆H₁₂O₆), fructose. Think of it as the single sugar molecule that all larger carbs are built from.
Disaccharide
A carbohydrate made of two monosaccharides joined by a dehydration reaction. Example: sucrose (glucose + fructose).
Polysaccharide
A carbohydrate polymer made of many monosaccharide units. Examples: starch, glycogen, cellulose, chitin. Think of it as a long chain (or branched chain) of sugars strung together for storage or structure.
Glycosidic linkage
The covalent bond that joins two monosaccharides together, formed during a dehydration reaction.
Carbonyl group (C=O)
A functional group consisting of a carbon double-bonded to an oxygen. Its position on the sugar ring distinguishes one monosaccharide from another (e.g. glucose vs. fructose).
Alpha (α) glucose vs. Beta (β) glucose
Two structural forms of glucose that differ in the orientation of the hydroxyl group (-OH) on carbon 1. Alpha glucose links into starch and glycogen; beta glucose links into cellulose. Think of it as: same ingredients, different arrangement, completely different properties.
Living organisms are composed primarily of carbon (C), hydrogen (H), oxygen (O), nitrogen (N) and phosphorus (P). These elements combine to form the four classes of biological macromolecules:
Carbohydrates
Lipids
Proteins
Nucleic acids
Most macromolecules are polymers, built by linking monomer subunits together.
Dehydration synthesis (condensation reaction): joins monomers together by removing a water molecule. This is how polymers are built.
Hydrolysis: breaks polymers apart by adding a water molecule across the bond. This is how polymers are digested.
These two reactions are mirror images of each other. Every time you eat starch and your body breaks it into glucose, that is hydrolysis. Every time a plant cell strings glucose units into starch for storage, that is dehydration synthesis.
Monosaccharides are the simplest sugars, with the general formula (CH₂O)ₙ.
Glucose (C₆H₁₂O₆): the most common monosaccharide. In solution it forms a six-membered ring. When the ring closes, the carbonyl group (C=O) is replaced by a hydroxyl group.
Fructose: same molecular formula as glucose, but the carbonyl group sits in a different position on the carbon skeleton. This makes fructose a structural isomer of glucose.
Monosaccharides differ from one another by the placement of the carbonyl group and by the number of carbons in the chain.
The prefix "glyco-" means sugar.
Two monosaccharides joined by a dehydration reaction form a disaccharide.
Sucrose (table sugar) = glucose + fructose.
The covalent bond between them is a glycosidic linkage.
Polysaccharides are long chains (or branched chains) of monosaccharides. They serve two broad purposes: energy storage and structural support.
Storage polysaccharides:
Starch: the storage polysaccharide of plants. Found in potatoes, grains and other plant tissues. Made of alpha (α) glucose units.
Glycogen: the storage polysaccharide of animals. Stored in liver and muscle cells. More highly branched than starch. Also made of alpha glucose. Contains no double bonds in the carbon backbone.
Structural polysaccharides:
Cellulose: the main structural component of plant cell walls. Made of beta (β) glucose units. The different linkage geometry means most animals cannot digest cellulose (they lack the enzyme).
Chitin: a structural polysaccharide found in fungal cell walls and the exoskeletons of arthropods (insects, crustaceans). Extremely strong.
The difference between alpha and beta glucose is the orientation of the hydroxyl group (-OH) on carbon 1.
Alpha glucose: -OH points downward. Produces starch, glycogen.
Beta glucose: -OH points upward. Produces cellulose.
This single structural difference leads to completely different biological functions (digestible energy storage vs. rigid structural fibre).
General formula for monosaccharides: (CH₂O)ₙ
Glucose molecular formula: C₆H₁₂O₆
Dehydration synthesis: Monomer-OH + H-Monomer → Monomer-Monomer + H₂O
Hydrolysis: Monomer-Monomer + H₂O → Monomer-OH + H-Monomer
Key structural distinction: the position of -OH on carbon 1 determines alpha vs. beta glucose, which in turn determines whether the resulting polysaccharide is starch/glycogen or cellulose.
When you eat a potato, your digestive enzymes perform hydrolysis on starch, breaking it into individual glucose molecules that your cells can use for energy.
Cellulose is the reason wood is rigid and cotton is fibrous. Despite being made of the same monomer as starch (glucose), the beta linkage makes it indigestible to humans and structurally tough.
Chitin is used in biodegradable packaging and surgical sutures because of its strength and biocompatibility.
Students often think "dehydration reaction" and "hydrolysis" are the same thing. They are opposites. Dehydration builds polymers (removes water); hydrolysis breaks them (adds water).
Students sometimes assume all polysaccharides are digestible. Cellulose is a polysaccharide of glucose, but humans cannot break its beta linkages. Starch (alpha linkages) is the one we digest.
"Saturated" and "unsaturated" are lipid terms, not carbohydrate terms. Students occasionally confuse the two when discussing bonds in polysaccharides.
Sucrose is a disaccharide, not a monosaccharide. It is made of glucose + fructose joined together. Glucose and fructose individually are the monosaccharides.
⚠️ Be able to distinguish dehydration synthesis from hydrolysis, and explain which direction builds a polymer vs. breaks one down.
⚠️ Know the storage vs. structural polysaccharides: starch and glycogen (storage, alpha glucose) vs. cellulose and chitin (structural, beta glucose).
⚠️ Understand that the position of the carbonyl group distinguishes different monosaccharides (glucose vs. fructose).
⚠️ Be prepared to explain alpha vs. beta glucose and why the distinction matters for digestibility and function.
⚠️ "Glyco-" = sugar. This prefix appears often in exam questions about carbohydrates.
True or false: Hydrolysis requires the removal of a water molecule. Answer: False. Hydrolysis adds water to break a bond. Dehydration synthesis removes water.
Fill in the blank: The storage polysaccharide found in animal liver and muscle cells is ______. Answer: Glycogen.
True or false: Cellulose and starch are both made from glucose monomers. Answer: True. They differ in the type of glucose linkage (beta vs. alpha).
Fill in the blank: The general formula for a monosaccharide is ______. Answer: (CH₂O)ₙ.
True or false: Fructose and glucose have different molecular formulas. Answer: False. They share the formula C₆H₁₂O₆ but differ in the placement of the carbonyl group (they are structural isomers).
Q: What is the difference between a dehydration reaction and hydrolysis?
A: A dehydration reaction (condensation reaction) joins two monomers by removing a water molecule, building a polymer. Hydrolysis does the opposite: it adds a water molecule to break the bond between monomers, splitting the polymer apart.
Q: Name two storage polysaccharides and state where each is found.
A: Starch is found in plant cells (e.g. potatoes). Glycogen is found in animal cells, specifically in the liver and skeletal muscles.
Q: Why can humans digest starch but not cellulose, even though both are made of glucose?
A: Starch is made of alpha glucose linked by alpha glycosidic bonds. Cellulose is made of beta glucose linked by beta glycosidic bonds. Human digestive enzymes can break alpha linkages but lack the enzyme needed to break beta linkages.
Q: What determines whether a monosaccharide is classified as glucose, fructose, or another sugar?
A: The placement of the carbonyl group (C=O) on the carbon skeleton, along with the number of carbon atoms in the chain.
Q: Describe the relationship between monomers and polymers using carbohydrates as an example.
A: Monosaccharides (e.g. glucose) are the monomers. They are joined by dehydration reactions through glycosidic linkages to form polysaccharides (polymers) such as starch, glycogen or cellulose. Hydrolysis reverses the process, breaking the polysaccharide back into monosaccharides.
This material connects directly to lipids, proteins and nucleic acids (Part 2 of these notes), which are also built through dehydration synthesis and broken by hydrolysis. The enzyme concepts here ("humans lack the enzyme to digest cellulose") will reappear in the enzymes and metabolism chapters. Cell membrane structure (Ch. 4+) depends on understanding both carbohydrates and lipids.
Macromolecules, biological molecules, organic molecules, polymers, monomers, carbohydrates, carbs, sugars, monosaccharides, disaccharides, polysaccharides, glucose, fructose, sucrose, starch, glycogen, cellulose, chitin, dehydration synthesis, condensation reaction, hydrolysis, glycosidic linkage, alpha glucose, beta glucose, carbonyl group, C₆H₁₂O₆, CH₂O, energy storage molecules, structural polysaccharides, plant cell wall, exoskeleton, intro to biology, BIO Ch. 3