Future Scientists

Revision note 1.8

Biological Molecules and Enzymes

Carbohydrates, lipids and proteins build cells and supply energy. Enzymes control the reactions that make, break down and transform these molecules.

CoreRequired practicalsRate calculations

What you need to know

  • State the roles and smaller units of carbohydrates, lipids and proteins.
  • Describe the chemical tests for starch, reducing sugars, protein and lipids.
  • Explain enzyme specificity using an active-site model.
  • Explain effects of temperature, pH and substrate concentration.
  • Calculate and interpret enzyme reaction rates.

Major biological molecules

MoleculeBuilt fromRoles
CarbohydratesSimple sugars such as glucoseRespiration, energy storage and structures such as cellulose cell walls.
ProteinsAmino acidsEnzymes, antibodies, hormones and structural or contractile materials.
LipidsFatty acids and glycerolEnergy storage, insulation and cell membranes.

Large molecules can be broken down into smaller soluble molecules, while smaller units can be joined to build larger molecules. These reactions are controlled by enzymes.

Required practical: food tests

Use a separate clean sample for each test. Wear eye protection, especially when heating Benedict's solution.

SubstanceMethodPositive result
StarchAdd iodine solution.Orange-brown changes to blue-black.
Reducing sugarAdd Benedict's solution and heat in a hot water bath.Blue changes through green, yellow and orange to brick-red, depending on concentration.
ProteinAdd Biuret reagent, or sodium hydroxide followed by copper sulfate.Blue changes to lilac or purple.
LipidAdd ethanol, shake, then add water.A cloudy white emulsion forms.
Valid comparison

When comparing concentrations using colour, keep sample volume, reagent volume, heating time and temperature the same. A colorimeter can make the measurement more objective.

Misconception AlertBenedict's test and the iodine test both require heating.Select to reveal the correctionSelect to hide the correction
Correct understanding

Benedict's test for reducing sugars requires heating in a water bath. The iodine test for starch does not require heating.

How enzymes work

An enzyme is a biological catalyst: it increases the rate of a reaction without being used up. Enzymes are proteins folded into specific three-dimensional shapes.

The active site is the region where a substrate binds. Its shape is complementary to a particular substrate. The substrate binds, the reaction takes place, products leave, and the enzyme can be reused.

Substrate approachesRandom movement brings it to the enzyme.
BindingThe substrate fits the active site.
ReactionAn enzyme–substrate complex forms.
Products leaveThe unchanged enzyme can work again.

The lock-and-key model is useful but simplified. It represents the idea that only a substrate with a complementary shape fits the active site.

Factors affecting enzyme activity

Temperature

At low temperatures, particles move slowly and successful collisions are infrequent. As temperature rises, collision frequency and reaction rate increase. Above the optimum, bonds maintaining the enzyme's shape break. The active site changes shape and the enzyme becomes denatured, so the substrate no longer fits.

pH

Each enzyme has an optimum pH. Moving away from this pH can alter charges and bonds in the enzyme, changing the active site's shape. Extreme pH can denature the enzyme.

Substrate concentration

Increasing substrate concentration initially increases collision frequency and reaction rate. Eventually every active site is occupied most of the time. Enzyme concentration becomes the limiting factor and the rate levels off.

Misconception AlertHigh temperatures kill enzymes.Select to reveal the correctionSelect to hide the correction
Correct understanding

Enzymes are not alive. High temperatures can denature an enzyme by changing the shape of its active site.

reaction rate = amount of product formed ÷ timeor amount of substrate used ÷ time

Required practical: pH and amylase

Amylase breaks starch into sugars. Iodine solution can show when starch has disappeared.

  1. Place drops of iodine solution into wells of a spotting tile.
  2. Add amylase and a pH buffer to starch, keeping total volumes constant.
  3. Start the timer and keep the reaction mixture in a water bath at constant temperature.
  4. Every fixed interval, transfer one drop of reaction mixture to a fresh iodine drop.
  5. Record the time when iodine remains orange-brown, showing that no starch remains.
  6. Repeat at several pH values and calculate a mean time.
rate = 1 ÷ timeUseful when the same amount of starch is broken down in every test

Independent variable: pH. Dependent variable: time for starch to disappear or 1 ÷ time. Control temperature, enzyme concentration, starch concentration, volumes and sampling interval.

Evaluation

The exact colour-change endpoint is subjective. Use consistent iodine volumes, frequent sampling, repeats and a colorimeter if available.

Quick retrieval check

1. What are proteins built from?
Amino acids.
2. What is the positive result for the Biuret test?
A lilac or purple colour.
3. Why does an enzyme usually work with only one substrate?
Its active site has a specific shape complementary to that substrate.
4. Why does reaction rate fall rapidly above the optimum temperature?
The enzyme denatures, changing the active-site shape so the substrate no longer fits.

Exam connection

Question

At pH 6, starch disappears after 80 seconds. At pH 7, it disappears after 50 seconds. Calculate both rates and identify the faster condition.

Show the answer

pH 6 rate = 1 ÷ 80 = 0.0125 s−1. pH 7 rate = 1 ÷ 50 = 0.020 s−1. The reaction is faster at pH 7.