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 gives molecules more kinetic energy, so the frequency of effective collisions rises. Above the optimum, denaturation changes the 3D shape of the active site and the rate falls. Cooling usually slows activity without denaturing the enzyme.

• Below the optimum: more kinetic energy, so a higher frequency of effective collisions and more enzyme-substrate complexes.
• Above the optimum: bonds holding the enzyme shape break, the 3D shape of the active site changes and the substrate is no longer complementary.
• 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.

A marked results table puts the independent variable first, the unit only in each heading, and every reading to the same number of decimal places. A boiled-enzyme control shows that the change needs active enzyme. Every source of error needs its effect: which way it pushes the reading.

• Table: independent variable in the first column; repeats and mean to the right; heading = quantity / unit; no units in the cells; same decimal places down a column.
• Boiled-enzyme control: boil the enzyme, cool it to the test temperature, then run it like the test. Starch stays, so the change needs active enzyme.
• Error + effect: name the problem, say whether it makes the reading too high or too low, and give an improvement.

Keep in mind: Keeping variables constant makes the test fair. A control set-up is a separate set-up with the tested factor removed or made inactive, such as boiled enzyme. It shows that the result needs that factor.