Carbohydrates & Lipids

You’ll be able to

Identify monomers and polymers of carbohydrates and describe their roles
Explain how dehydration synthesis and hydrolysis build and break polymers
Contrast the structure of saturated and unsaturated lipids and predict their properties

Building big from small

Most large biological molecules are polymers: long chains built from repeating subunits called monomers, like beads on a string. Cells assemble them with one universal trick — dehydration synthesis: joining two monomers by removing a water molecule (one loses an –OH, the other an –H, which together make H₂O). To break the chain, cells run it in reverse: hydrolysis adds a water molecule to split a bond ("hydro" = water, "lysis" = split). These two reactions build and recycle nearly every macromolecule.

Dehydration synthesis / hydrolysis

monomer–OH + H–monomer ⇌ monomer–monomer + H₂O

Left-to-right removes water to bond (dehydration synthesis); right-to-left adds water to break (hydrolysis).

Carbohydrates: sugars and their chains

Carbohydrates follow the general formula (CH₂O)ₙ — carbon "hydrated" with water. The monomer is a monosaccharide such as glucose (C₆H₁₂O₆). Two joined together form a disaccharide (glucose + fructose → sucrose). Many joined together form a polysaccharide: plants store energy as starch and build structure with cellulose, while animals store energy as glycogen. Same glucose monomer, but different bonding geometry makes cellulose rigid and indigestible while starch is easily broken down.

Lipids: the nonpolar family

Lipids are not polymers, but they group together because they are all largely nonpolar and hydrophobic. A fat (triglyceride) is a glycerol backbone linked to three fatty acid tails. If every carbon–carbon bond in the tails is a single bond, the tails are saturated (packed with hydrogen), pack tightly, and the fat is solid at room temperature — think butter. If the tails contain one or more C=C double bonds, they are unsaturated; each double bond puts a kink in the tail that prevents tight packing, so the fat is liquid — think oil. Phospholipids, with a polar phosphate head and two nonpolar tails, spontaneously form the cell membrane bilayer.

Worked example

Two glucose molecules join to form the disaccharide maltose. What kind of reaction is this, what is produced besides maltose, and how would maltose later be broken apart?

1.Joining two monomers into a larger molecule is dehydration synthesis: an –OH is removed from one glucose and an –H from the other.
2.The removed –OH and –H combine, so one molecule of water (H₂O) is released as a byproduct.
3.To break maltose back into two glucose molecules, a cell runs hydrolysis: it adds a water molecule across the bond, restoring the –OH and –H.

Answer: Dehydration synthesis forms maltose and releases one H₂O; hydrolysis (adding water back) later splits it into two glucose molecules.

Checkpoint

A cell links 100 glucose monomers into a single starch molecule. How many water molecules are released?

Watch out

Lipids are the odd family out: they are not polymers and their fatty acids are not true monomers strung into long chains. Group them by their shared property — hydrophobic and nonpolar — not by a monomer–polymer relationship.

Checkpoint

Olive oil is liquid at room temperature, while beef fat is solid. What best explains the difference?

Tip

Remember the vocabulary by the chemistry: saturated means the carbons are "saturated" with the maximum hydrogen (all single bonds, straight, solid); unsaturated means room for more hydrogen (double bonds, kinked, liquid).

Answer the 2 checkpoints as you read.