Biological Molecules: Questions and Answers

O Level Biology Questions and Answers

Subject: Biology

Level: O Level

Topic: Biological Molecules

This resource provides a complete set of O Level Biology Questions and Answers on biological molecules – the chemicals that make up living organisms. You will explore the structure and functions of carbohydrates, proteins, and lipids, plus the essential role of enzymes. Perfect for mastering these core biochemical concepts.

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Topic Overview

Biological molecules are large organic compounds essential for life. Carbohydrates provide energy and are made of simple sugars. Proteins are made of amino acids and are used for growth, repair, and enzymes. Lipids (fats and oils) store energy and form cell membranes. Enzymes are protein catalysts that speed up reactions. Food tests (Benedict’s, iodine, Biuret, ethanol) identify these molecules. Understanding their structure and function is fundamental to O Level Biology.

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Exam Questions and Answers

Question 1

Question:
Name the three main types of biological molecules and state one function of each.

Answer:
1. Carbohydrates – provide energy.
2. Proteins – growth and repair of tissues; act as enzymes.
3. Lipids – energy storage; form cell membranes.

Explanation:
Carbohydrates are broken down into glucose for respiration. Proteins are structural and functional. Lipids store more energy per gram than carbohydrates and are a major component of membranes.

Exam Tip:
Learn at least one specific function for each. For proteins, mention “enzymes” or “antibodies” for higher marks.

Question 2

Question:
Describe the test for starch and the result you would expect for a positive test.

Answer:
Add iodine solution to the sample. A positive result is a blue‑black colour change.

Explanation:
Iodine reacts with starch to form a characteristic blue‑black complex. No colour change (remains brown/orange) indicates starch is absent.

Exam Tip:
Always state “iodine solution” and “blue‑black”. Do not confuse with Benedict’s test for reducing sugars.

Question 3

Question:
Explain how you would test a food sample for reducing sugars.

Answer:
Add Benedict’s solution to the sample and heat in a water bath. A positive result is a colour change from blue to green, yellow, or brick‑red precipitate.

Explanation:
Reducing sugars reduce copper(II) ions in Benedict’s solution to copper(I) oxide, which forms a coloured precipitate. The colour intensity indicates sugar concentration.

Exam Tip:
Mention “heat” and “colour change”. For non‑reducing sugars, you need to hydrolyse with acid first.

Question 4

Question:
What is the chemical test for protein? State the positive result.

Answer:
The Biuret test. Add dilute copper(II) sulfate and sodium hydroxide. A positive result is a lilac/purple colour change.

Explanation:
The Biuret reagent reacts with peptide bonds in proteins to form a violet complex. The test works for proteins and short polypeptides.

Exam Tip:
Remember “Biuret” and “purple”. Some exam papers accept “violet” as correct.

Question 5

Question:
Describe how to test for the presence of lipids (fats and oils).

Answer:
The ethanol emulsion test. Add ethanol to the sample, shake, then pour into water. A positive result is a milky white emulsion.

Explanation:
Lipids dissolve in ethanol but not in water. When water is added, they form a fine suspension (emulsion) that appears cloudy or milky.

Exam Tip:
Always use the phrase “emulsion test”. If you are asked to test a solid sample, crush it first with ethanol.

Question 6

Question:
Explain why enzymes are important in digestion.

Answer:
Enzymes are biological catalysts that speed up the breakdown of large, insoluble food molecules into smaller, soluble molecules that can be absorbed into the bloodstream.

Explanation:
Without enzymes, digestion would be too slow to sustain life. Each enzyme is specific to one substrate (e.g., amylase breaks starch into maltose).

Exam Tip:
Use terms like “catalyst”, “specific”, and “substrate”. Link enzyme action to absorption of nutrients.

Question 7

Question:
What is the “lock and key” model of enzyme action?

Answer:
The lock and key model states that the active site of an enzyme has a specific shape that fits only its complementary substrate, like a key fits a lock.

Explanation:
When the substrate binds to the active site, an enzyme‑substrate complex forms, and the reaction occurs. Any change in shape (e.g., by denaturation) prevents binding.

Exam Tip:
Draw a simple diagram in your answer. Mention “active site”, “enzyme‑substrate complex”, and “specificity”.

Question 8

Question:
Describe how temperature affects the rate of an enzyme‑controlled reaction.

Answer:
As temperature increases, rate increases up to an optimum (around 40°C) because particles have more kinetic energy. Above the optimum, the enzyme denatures and the rate drops sharply.

Explanation:
Denaturation changes the shape of the active site so substrate can no longer bind. Low temperatures reduce molecular movement, slowing the reaction.

Exam Tip:
Use the term “denature” for high temperatures. Draw a graph showing a peak at the optimum.

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Key Concepts Summary

• Carbohydrates: energy source; tested with iodine (starch) or Benedict’s (reducing sugars).
• Proteins: growth and repair; tested with Biuret reagent (purple).
• Lipids: energy storage, membranes; tested with ethanol emulsion (milky).
• Enzymes: biological catalysts; lock and key model; affected by temperature and pH.
• Denaturation: permanent loss of enzyme function due to heat or extreme pH.

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Frequently Asked Questions

What is the difference between a reducing sugar and a non‑reducing sugar?

Reducing sugars (e.g., glucose) give a positive Benedict’s test directly. Non‑reducing sugars (e.g., sucrose) must be hydrolysed into reducing sugars before they give a positive test.

Why do enzymes have an optimum pH?

Enzymes have a specific shape. Changes in pH alter the charge of amino acids in the active site, affecting substrate binding. Extreme pH denatures the enzyme.

Are all lipids fats?

No. Lipids include fats (solid at room temperature), oils (liquid), and phospholipids (found in cell membranes).

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Related Topics

• O Level Biology Cell Structure Questions and Answers Live
• O Level Biology Enzymes Questions and Answers Live 
• O Level Biology Human Nutrition Questions and Answers Live 

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Conclusion

Mastering O Level Biology Questions and Answers on biological molecules gives you a strong foundation in biochemistry. Know your food tests, understand enzyme function, and practise explaining the lock and key model. These concepts appear frequently in both theory papers and practical exams.