1.3 Introduction to Macromolecules · Educerie AP Biology

Educerie · AP · Biology

AP Biology 1.3 Introduction to Macromolecules

In one breath

Macromolecules are mostly polymers, chains of monomers held by covalent bonds: monosaccharides make polysaccharides, amino acids make proteins, nucleotides make nucleic acids; fats are built from glycerol and fatty acids but are not polymers.
Dehydration synthesis joins two molecules by removing an –OH from one and an –H from the other, releasing water; repeated, it is polymerization.
Hydrolysis adds water across the bond, the H going to one monomer and the OH to the other, and splits it, as in digestion.
Building takes energy; breaking releases a little and frees the monomers.
A chain of n monomers has n − 1 bonds, so n − 1 water molecules, and rates are change over time with units.

What you must be able to do

Describe hydrolysis: water added to break the covalent bond between monomersIdentify the reaction in a diagram; say which monomer gains the H and which the OH
Describe dehydration synthesis: two molecules joined by a covalent bond as the equivalent of water is removedIdentify the reaction; count water molecules released when a chain forms
Describe polymerization as many monomers joined in a chainName the monomer of each class of polymer
Predict and calculate what happens to water and mass when polymers are built or broken"How many water molecules are needed to hydrolyse a chain of 50 amino acids?"

Once you have read the note, tick off each one you can do in Check your understanding.

In the exam. Section I multiple choice shows a drawing of two monomers joining or a polymer splitting and asks you to name the reaction, say where the water goes, or count the water molecules used or released; data items give the products of digestion over time and ask for a rate. In Section II it supplies points in Analyze model or visual representation (describe the reaction a diagram shows), Conceptual analysis (explain how a polymer is broken down in digestion) and Interpreting and evaluating experimental results (an enzyme breaking down a polymer, with a rate to calculate).

1Monomers and polymers

Three of the four classes of biological molecule are true polymers:

Polymer class	Monomer	Example polymers
Carbohydrates (polysaccharides)	monosaccharides, such as glucose	starch, glycogen, cellulose
Proteins	amino acids	enzymes, haemoglobin, keratin
Nucleic acids	nucleotides	DNA, RNA

Lipids are the exception. A fat is built from smaller subunits, one glycerol and three fatty acids, joined by the same dehydration synthesis reaction, but those subunits are different from each other and do not repeat in a long chain, so a fat is not a polymer (topic 1.5).

The order and kind of monomers decide what a polymer does. A protein's sequence of amino acids decides its shape (topic 1.7); a DNA strand's sequence of nucleotides is its genetic information (topic 1.6). Structure determines function: that phrase runs through the whole course.

2Dehydration synthesis: building with water removed

Figure 1 walks through one dehydration synthesis and its reverse, using two glucose molecules joining to make the sugar maltose. You do not need to draw glucose; the ring is there so you can see exactly where the water comes from.

Figure 1 · Dehydration synthesis and hydrolysis: two glucose and maltose

The atoms that will leave. One glucose gives up a hydroxyl group (–OH, in amber on the left). The other gives up a hydrogen atom (–H, from the –OH on the right-hand glucose). Together they make one molecule of water, H₂O.

Dehydration synthesis. With those atoms gone, the oxygen left on one glucose forms a new covalent bond to a carbon of the other. The two monomers are now one molecule, maltose, and one water molecule has been released. "Dehydration" means water removed; "synthesis" means something built.

Hydrolysis, the way back. Add a water molecule across the covalent bond between the two units. The bond breaks; the H from the water goes to one glucose and the OH to the other, so each is whole again. "Hydro" is water, "lysis" is splitting.

Repeat step 2 at the end of a growing chain again and again and you get polymerization: a third glucose joins maltose and releases a second water molecule, a fourth releases a third, and so on. The same pattern builds every polymer class: amino acids join by a covalent peptide bond (topic 1.7), nucleotides by a covalent bond between the sugar of one and the phosphate of the next (topic 1.6).

Dehydration synthesis removes water to join monomers; hydrolysis adds water to split them.

3Hydrolysis: breaking with water added

Hydrolysis cleaves covalent bonds, breaking big molecules into smaller ones. It happens whenever cells or organisms need the monomers back:

Digestion. In your gut, starch is hydrolysed to sugars, proteins to amino acids and nucleic acids to nucleotides, small enough to be absorbed.
Releasing stores. Liver cells hydrolyse glycogen to glucose when blood sugar falls (topic 1.4).
Recycling. Cells hydrolyse old or damaged proteins and reuse the amino acids.

Both reactions in cells are sped up by enzymes, which you meet in Unit 3. Without them, hydrolysis in water would be far too slow to matter.

4Energy in building and breaking

The CED's energetics question for this unit asks about the role of energy in making and breaking polymers. The short answer:

Building a polymer by dehydration synthesis takes an input of energy. Cells supply it, ultimately from ATP made in respiration (Unit 3). This is why a growing organism needs both raw materials (topic 1.2) and energy.
Breaking a polymer by hydrolysis releases a little energy, and it releases the monomers, which can then be broken down further in respiration to release much more.

So growth (building) and digestion (breaking) are the two halves of one system: an organism hydrolyses the polymers in its food and uses the monomers and the energy to build its own polymers by dehydration synthesis.

5Counting water and mass: worked examples

Each new bond in a chain releases one water molecule. A chain of n monomers has n − 1 bonds.

Example 1. How many water molecules are released when 120 glucose units join into one unbranched chain?

bonds = n − 1 = 120 − 1 = 119, so 119 water molecules are released

Example 2. Glucose has a relative molecular mass of 180, water 18. What is the mass of a chain of three glucose units?

3 × 180 = 540

2 bonds, so 2 water molecules lost: 2 × 18 = 36

540 − 36 = 504

Example 3. How many water molecules does it take to hydrolyse a fat completely into glycerol and fatty acids? A fat is one glycerol joined to three fatty acids by three bonds, so three water molecules.

The commonest slip is to answer n instead of n − 1. Picture three beads on a string: two links, not three.

6Reading a hydrolysis rate

An invented experiment. A polysaccharide solution was mixed with an enzyme that hydrolyses it, and the glucose concentration was measured every 2 minutes.

Time / min	0	2	4	6	8	10
Glucose / mmol L⁻¹	0.0	1.6	3.2	4.6	5.4	5.8

The formula sheet's rate is dY ÷ dt.

0 to 4 min: 3.2 − 0.0 ÷ 4 = 0.8 mmol L⁻¹ min⁻¹

6 to 10 min: 5.8 − 4.6 ÷ 4 = 0.3 mmol L⁻¹ min⁻¹

Claim: the rate of hydrolysis fell over time. Evidence: 0.8 mmol L⁻¹ min⁻¹ in the first 4 minutes against 0.3 in the last 4. Reasoning: each hydrolysis uses up one of the bonds between glucose units, so as the polymer is broken down there are fewer bonds left to break and glucose is released more slowly. A good control would be the same solution with no enzyme (or with boiled enzyme), to show the glucose came from enzyme-driven hydrolysis.

Practice questions

5 questions on this note, from easy to hard. Each one you get right lights a star of this topic's constellation.