The four macromolecules of life and how cells build them
TEAS 7 outline: Structure and function of basic biological macromolecules
Also covers HESI A2 Biology (Biological molecules) · HESI A2 Chemistry (Biochemistry)
- Four classes: carbohydrates, lipids, proteins, and nucleic acids.
- Dehydration synthesis joins monomers and releases water; hydrolysis adds water to split.
- Plants store glucose as starch; animals store it as glycogen.
- Proteins are amino acid chains, and their shape decides their function.
- Enzymes speed reactions by lowering activation energy and are not used up.
Key facts
- The four classes of biological macromolecules are carbohydrates, lipids, proteins and nucleic acids.
- Dehydration synthesis joins monomers and releases a water molecule; hydrolysis adds water to split a polymer into monomers.
- Monosaccharides such as glucose are the monomers of carbohydrates; plants store glucose as starch and animals store it as glycogen, mainly in the liver and muscles.
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- A fat (triglyceride) is one glycerol attached to three fatty acids; saturated fats have only single bonds and are usually solid at room temperature.
- Proteins are chains of amino acids joined by peptide bonds; 20 common amino acids make up all proteins, and a protein's shape determines its function.
- Almost all enzymes are proteins; they speed reactions by lowering activation energy and are not used up by the reaction.
- Nucleotides (sugar + phosphate + nitrogenous base) build DNA and RNA; DNA has deoxyribose and thymine, RNA has ribose and uracil.
Quick check1 question · 2 more at the end
Which pairing correctly matches a monomer with a polymer built from it?
Correct answer: C.
Each class of macromolecule has its own building block: monosaccharides build carbohydrates such as starch and glycogen, amino acids build polypeptides (proteins), and nucleotides build nucleic acids (DNA and RNA). Starch is a long chain of glucose, a monosaccharide, which plants use to store energy. When you see a polymer, first name its class, then recall that class's monomer.
A Amino acids are the monomers of proteins; DNA is a nucleic acid built from nucleotides.
B This swaps the building blocks: nucleotides build nucleic acids, while polypeptides are built from amino acids.
C Correct: starch is a polysaccharide made of many glucose units, and glucose is a monosaccharide.
D Glycogen is a polysaccharide made of glucose; fatty acids are parts of lipids such as triglycerides and phospholipids.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: Living things are built from four classes of macromolecules: carbohydrates (sugars, made of monosaccharides), lipids (fats, phospholipids and steroids), proteins (chains of amino acids) and nucleic acids (DNA and RNA, made of nucleotides). Cells join small units by dehydration synthesis, which releases water, and break them apart by hydrolysis, which adds water.
1 What are macromolecules made of?Macromolecules are the large molecules that make up most of a cell's dry mass. 1 min
Macromolecules are the large molecules that make up most of a cell's dry mass. There are four classes: carbohydrates, lipids, proteins and nucleic acids. All of them are organic, which means they are built around carbon bonded to hydrogen, often with oxygen, nitrogen and a few other elements.
Most macromolecules are polymers: long chains made by linking many small building blocks called monomers. Picture a beaded necklace. Each bead is a monomer, and the finished necklace is the polymer.
| Macromolecule | Building blocks | Main jobs | Examples |
|---|---|---|---|
| Carbohydrates | Monosaccharides (simple sugars) | Quick energy, energy storage, structure | Glucose, sucrose, starch, glycogen, cellulose |
| Lipids | Glycerol + fatty acids (in fats) | Long-term energy storage, insulation, membranes, hormones | Triglycerides, phospholipids, cholesterol |
| Proteins | Amino acids | Enzymes, transport, structure, defense, some hormones | Hemoglobin, collagen, keratin, antibodies, insulin |
| Nucleic acids | Nucleotides | Store and use genetic information | DNA, RNA |
2 How do cells build polymers and break them down?Cells join monomers by dehydration synthesis. 1 min
Cells join monomers by dehydration synthesis. One monomer gives up a hydrogen and the other gives up a hydroxyl group (OH). Those pieces leave together as a water molecule, and the two monomers become linked by a covalent bond. The name means "putting together while losing water."
The reverse reaction is hydrolysis. Water is added across the bond, which splits the polymer back into monomers. Digestion is hydrolysis: specific enzymes break each kind of food molecule apart. Amylase and lactase work on carbohydrates, proteases such as pepsin work on proteins, and lipases work on fats.
3 What do carbohydrates do?Carbohydrates contain carbon, hydrogen and oxygen in a 1:2:1 ratio, written (CH₂O)n. 1 min
Carbohydrates contain carbon, hydrogen and oxygen in a 1:2:1 ratio, written (CH₂O)n. The smallest are monosaccharides such as glucose, fructose and galactose. Glucose is a major fuel: cells release its energy during cellular respiration to make ATP.
Two monosaccharides joined by a glycosidic bond form a disaccharide. Sucrose (table sugar) is glucose plus fructose, lactose (milk sugar) is glucose plus galactose, and maltose is two glucose units.
Long chains are polysaccharides. Plants store glucose as starch, while humans and other vertebrates store it as glycogen, mainly in the liver and muscle cells. Cellulose gives plant cell walls their strength, and chitin forms the exoskeletons of arthropods such as insects and crabs and fungal cell walls.
4 Why don't lipids mix with water?Lipids are mostly nonpolar, so they are hydrophobic (water-fearing). 1 min
Lipids are mostly nonpolar, so they are hydrophobic (water-fearing). They store long-term energy and provide insulation. They also don't follow the simple long-chain pattern of the other classes: a fat, for example, is built from just four parts.
- A fat, or triglyceride, is one glycerol attached to three fatty acids. Saturated fatty acids have only single bonds, pack tightly and are usually solid at room temperature. Unsaturated fatty acids have double bonds that kink the chain, so they are usually liquid (oils).
- A phospholipid has a water-loving phosphate head and two water-fearing tails. Phospholipids line up in a double layer to form the cell membrane.
- Steroids have four fused carbon rings. Cholesterol is the most common one and is used to make hormones such as testosterone and estradiol, plus vitamin D and bile salts.
5 Why does a protein's shape matter?Proteins are chains of amino acids. 1 min
Proteins are chains of amino acids. Each amino acid has a central carbon bonded to an amino group, a carboxyl group, a hydrogen and a variable R group. Twenty common amino acids, arranged in different orders, make up all proteins. Amino acids link by peptide bonds formed through dehydration synthesis, creating a polypeptide.
The amino acid order is the primary structure. The chain then coils into alpha helices or folds into beta-pleated sheets (secondary structure), folds into an overall 3D shape (tertiary), and sometimes joins other chains (quaternary). Shape decides function. Heat, extreme pH or certain chemicals can denature a protein: it loses its shape, keeps its amino acid sequence, and may stop working. Frying an egg is a familiar example.
Enzymes deserve special attention. Almost all enzymes are proteins that act as catalysts: they speed up a reaction by lowering its activation energy.
The reactant, called the substrate, binds to the enzyme's active site, which fits only certain substrates. The enzyme comes out of the reaction unchanged, so it can be used again. Because the active site depends on shape, an enzyme works best within a certain temperature and pH range.
6 How do DNA and RNA differ?Nucleic acids are chains of nucleotides. 1 min
Nucleic acids are chains of nucleotides. Each nucleotide has three parts: a five-carbon sugar, a phosphate group and a nitrogenous base. Sugars and phosphates form the backbone, and the bases stick out.
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, G, C | A, U, G, C |
| Shape | Double helix | Usually single strand |
| Main job | Stores genetic information | Helps make proteins (mRNA, tRNA, rRNA) |
In DNA, bases pair inside the helix: A with T and G with C, held by hydrogen bonds. Genetic information flows from DNA to RNA (transcription) to protein (translation).
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: Dehydration synthesis releases water and hydrolysis uses water, not the other way around ('hydro-lysis' = water splits).
Watch out: Glycogen is the animal storage form of glucose and starch is the plant form; cellulose is a plant structural carbohydrate, not a storage one.
Watch out: Cholesterol is a lipid (a steroid), not a carbohydrate or a protein.
Watch out: Denaturing a protein changes its shape, not its amino acid sequence, and the shape change is what stops it from working.
Watch out: Uracil replaces thymine in RNA; DNA does not contain uracil, and RNA does not contain thymine.
How to remember it
Memory hook: "Cute Little Puppies Nap": Carbohydrates, Lipids, Proteins, Nucleic acids. Then pair each with its block: sugar, glycerol and fatty acids, amino acid, nucleotide. For the reactions, dehydration "dries out" the pair by removing water; hydrolysis uses water to "lyse" (split) the bond.
Self-check:
- Name the monomer and one example for each of the four macromolecules.
- Explain which reaction releases water and which one uses it.
- Predict what would happen to an enzyme heated far above its ideal temperature, and why.