Biological molecules, at a glance

Carbohydrates provide an immediate energy source, fats provide insulation and long-term energy storage, and proteins support cell growth and repair.

• Carbohydrates and fats contain carbon, hydrogen and oxygen.
• Proteins contain carbon, hydrogen, oxygen and nitrogen; some also contain sulfur.
• Food types can contain several molecule groups. A food name is not a chemical test result.

Keep in mind: The types of atoms are the same, but their proportions and arrangement differ. This produces different molecules with different properties and roles.

Glucose units build starch, glycogen and cellulose. Amino acids build polypeptides and proteins. Glycerol and fatty acids build fats.

• Starch, glycogen and cellulose all use glucose units, joined in different arrangements.
• Amino acid sequence affects the shape and function of a protein.
• A fat molecule is assembled from glycerol and fatty acids; it is not a long chain of glucose.

Keep in mind: Proteins are made from amino acids. Fats are made from glycerol and fatty acids. Glucose builds starch, glycogen and cellulose.

Iodine solution tests for starch. Heated Benedict's solution tests for reducing sugars. A result identifies what the test detects, not every carbohydrate present.

• Starch: iodine in potassium iodide changes from yellow-brown to blue-black.
• Reducing sugar: Benedict's solution is heated in a water bath; a coloured precipitate is a positive result.
• A negative Benedict's test does not prove that a food contains no carbohydrate.

Keep in mind: Benedict's test detects reducing sugars, not every carbohydrate. Starch may still be present, so choose a separate appropriate test.

Biuret solution gives a violet or purple colour with protein. The ethanol emulsion test gives a cloudy white emulsion when fat is present.

• Protein: add biuret reagent; blue changes to violet or purple.
• Fat: mix with ethanol, then add the ethanol extract to water; a cloudy white emulsion indicates fat.
• Keep ethanol away from flames and heat sources. Food-test samples must not be tasted.

Keep in mind: The fat test produces a cloudy white emulsion after ethanol extraction and addition to water. Benedict's solution is used with heating to test for reducing sugars.

A suitable substrate binds to an enzyme's active site. The enzyme lowers the activation energy for the reaction and is available to act again after the products leave.

• Enzymes are biological catalysts: they increase reaction rate without being used up.
• The lock-and-key model links a complementary active-site shape to enzyme specificity.
• The enzyme-substrate complex is temporary; products leave and the enzyme can be reused.

Keep in mind: Enzymes lower the activation energy of the reaction. They do not supply extra energy to the substrate or get used up as a fuel.

Warming initially increases productive collisions. Above the optimum, denaturation changes the active site and reduces the rate. Cooling usually slows activity without denaturing the enzyme.

• Below the optimum: higher temperature increases particle movement and productive collisions.
• Excessive heat changes enzyme shape; the substrate fits less well or not at all.
• For the same endpoint, relative rate can be compared using 1/time. Keep other variables constant.

Keep in mind: Low temperature can slow collisions while leaving the active site intact. A low rate alone does not establish denaturation; the cause and conditions matter.

Each enzyme has a suitable pH range. Moving away from its optimum can change the active site and reduce activity; extreme pH can denature it.

• Different enzymes can have different optimum pH values.
• A buffer keeps the reaction mixture near a chosen pH.
• When changing pH, hold temperature, enzyme concentration and substrate concentration constant.

Keep in mind: Enzymes have different pH ranges. Some stomach enzymes are adapted to acidic conditions. Explain the response of the particular enzyme using its optimum and active site.