2c Biological Molecules¶

Part of 2 Structure and Functions in Living Organisms.

Biological molecules supply energy, build structures and let reactions run at useful rates. Three ideas connect this topic: the units that make up carbohydrates, proteins and lipids, the tests that detect them, and how an enzyme's shape decides what it can do.

What You Need to Learn¶

Further detail: Pearson Edexcel International GCSE Biology specification.

The structure and uses of carbohydrates, proteins and lipids, the practical tests for glucose, starch, protein and fat, and how enzymes work, including the effects of temperature and pH.

Carbohydrates, Proteins and Lipids¶

All three groups contain carbon, hydrogen and oxygen. Proteins also contain nitrogen, and sometimes sulfur and phosphorus. Because they are large biological molecules, the three groups are often called macromolecules.

A polymer is a large molecule made of many repeating smaller units joined together. Starch, glycogen and proteins are polymers. Lipids are large molecules too, but they are built differently.

Group	Made of	Examples and uses
Carbohydrates	Sugars. Starch and glycogen are polymers of glucose	Glucose is the sugar used in respiration. Starch is the storage carbohydrate in plants, and glycogen is the storage carbohydrate in animals and fungi
Proteins	Amino acids joined in a sequence, which decides the protein's shape and function	Growth and repair, and making enzymes
Lipids (fats and oils)	One glycerol molecule joined to three fatty acid molecules	Energy storage and insulation

Food Tests¶

Each test links a chemical present in a sample to a colour you can see.

Test for	Method	Positive result	Negative result
Glucose (Benedict's)	Add Benedict's solution and heat in a hot water bath at 60–70 °C for five minutes	An orange or brick-red precipitate, so the mixture turns cloudy and is no longer clear	Stays blue
Starch (iodine)	Add drops of iodine solution	Blue-black	Stays brown or orange
Protein (Biuret)	Add Biuret solution and leave for one minute	Purple	Stays blue
Fat (ethanol emulsion)	Add 2 cm³ of ethanol and shake, then add 2 cm³ of distilled water	A milky white emulsion	Stays colourless

Benedict's solution has to be heated, because the glucose reacts only in the heat. A positive result is a precipitate, not just a slight tint. With small amounts of glucose the colour can stop at green or yellow, and a larger amount gives orange and then brick-red.

Worked example: reading food test results

A food sample gives these results: Benedict's solution stays blue, iodine turns blue-black, Biuret turns purple, and the ethanol test stays colourless.

Blue Benedict's means no glucose. Blue-black iodine means starch is present. Purple Biuret means protein is present. A colourless ethanol test means no fat. The sample contains starch and protein only.

Exam technique

For a food test, give the reagent, any condition needed (heat for Benedict's), and the colour change from start to finish. "It goes blue-black" earns less than "iodine solution turns from brown to blue-black".

Enzymes and the Active Site¶

Enzymes are proteins that act as biological catalysts. They speed up reactions in cells without being used up. An enzyme is a protein, so its three-dimensional shape decides its function.

Each enzyme has an active site of a particular shape. The enzyme is specific, which means it normally works on one substrate or a very small group of similar ones. The active site is complementary to that substrate, which means the shapes fit together. The two words are not interchangeable: specific describes which substrate the enzyme works on, and complementary describes how the substrate matches the active site. In the lock-and-key model, the substrate binds to the active site and an enzyme-substrate complex forms. The reaction is catalysed, the products leave, and the enzyme is free to catalyse the reaction again.

Use the interactive below to watch the lock-and-key model step by step, from substrate binding to product release. Open full interactive.

Effect of temperature. Raising the temperature increases the rate up to the optimum, which is around 37 °C for most human enzymes. The particles have more kinetic energy, so enzyme and substrate molecules collide more often and more enzyme-substrate complexes form each second. Above the optimum, the bonds holding the enzyme in shape begin to break. The active site changes shape, so the substrate no longer fits, and the enzyme is denatured.

Effect of pH. Most enzymes have an optimum pH of around 7, though an enzyme that works in acid conditions, such as pepsin in the stomach, has a lower optimum. Moving away from the optimum in either direction reduces activity, because the active site changes shape and fewer substrate molecules fit. At more extreme pH values the forces holding the protein in shape are disrupted further, and the enzyme is denatured.

The interactive below compares the two rate-factor graphs. Notice that temperature first raises the collision rate and then denaturation takes over, whereas pH changes the shape of the active site on either side of the optimum. Open full interactive.

Investigating enzyme activity. Amylase breaks down starch, so iodine solution can show how fast it works. The mixture changes from blue-black (starch present) to orange-brown (starch absent). The time taken for the starch to disappear measures the activity: a shorter time means higher activity.

Exam technique

When explaining the effect of temperature, say what changes in the enzyme. Above the optimum it is the active site that changes shape, so the substrate no longer fits. "The enzyme dies" is wrong, because enzymes are molecules and are not alive. Say that it is denatured.

Common Confusions¶

Catalyst vs reactant: A catalyst speeds a reaction up without being used up by it.
Specific vs complementary: Specific describes which substrate an enzyme works on. Complementary describes how the substrate's shape matches the active site.
Substrate binding: The substrate binds to the active site, not vaguely to the enzyme as a whole.
Benedict's test: A positive result needs heating and gives a precipitate, not just a slight colour tint.
Denaturation: Denaturation changes the shape of the active site. It does not mean the enzyme slows down for no structural reason.
Starch vs glucose: Starch is a polymer made of many glucose units. Glucose gives a positive Benedict's test, and starch does not.

Check Yourself¶

Name the three main groups of biological molecules and state one use of each.
Describe how to test a sample for glucose, including the result if glucose is present.
A sample turns purple with Biuret solution and stays blue with Benedict's solution. State what it contains and what it lacks.
Explain the difference between the words specific and complementary when describing an enzyme.
Explain why the rate of an enzyme-catalysed reaction rises with temperature and then falls sharply above the optimum.
A student adds iodine solution to a mixture of starch and amylase every minute. The colour stays blue-black for 6 minutes and then turns orange-brown. Explain what has happened, and what the student could change to find out how pH affects amylase.

Answers

Carbohydrates are used for energy (glucose in respiration) or storage, proteins for growth and repair or making enzymes, and lipids for energy storage or insulation.
Add Benedict's solution and heat in a hot water bath at 60–70 °C for about five minutes. If glucose is present, an orange or brick-red precipitate forms. If it is absent, the solution stays blue.
It contains protein (purple with Biuret). It lacks glucose (Benedict's stays blue).
Specific means the enzyme normally works on only one substrate, or a very small group of similar substrates. Complementary means the shape of the active site matches the shape of the substrate.
Up to the optimum, particles have more kinetic energy, so they collide more often and more enzyme-substrate complexes form. Above the optimum, the bonds holding the enzyme in shape break, so the active site changes shape and the substrate no longer fits. The enzyme is denatured and the rate falls.
The amylase has broken down all the starch after about 6 minutes, so the iodine no longer turns blue-black. To test pH, repeat with buffer solutions of different pH and compare the time taken for the starch to disappear, keeping temperature and concentrations the same.

Key Terms¶

Carbohydrate: a biological molecule made from carbon, hydrogen and oxygen.
Protein: a biological molecule made from amino acids.
Lipid: a biological molecule made from fatty acids and glycerol.
Amino acid: the monomer building block from which proteins are assembled.
Enzyme: a protein that acts as a biological catalyst, speeding up reactions without being used up.
Active site: the region of an enzyme that is complementary in shape to a specific substrate.
Substrate: the molecule that binds to an enzyme's active site and is converted into products.
Enzyme-substrate complex: the temporary combination formed when a substrate binds to the active site.
Denature: to irreversibly change an enzyme's shape so it can no longer catalyse reactions.
Optimum: the temperature or pH at which an enzyme works at its fastest rate.
Glycogen: a storage carbohydrate in animals and fungi made from many glucose units.
Starch: a storage carbohydrate in plants made from many glucose units.