Biological molecules

1. Overview

Biological molecules are the fundamental chemical building blocks that make up all living organisms. Understanding their structure, composition, and how to identify them is essential for studying how organisms grow, repair tissues, and obtain energy.

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Key Definitions

• Monomer: A small, single molecule that can be bonded to other identical molecules to form a polymer (e.g., glucose).
• Polymer: A large molecule made up of many repeating smaller units (monomers) joined together (e.g., starch).
• Reducing Sugar: A type of sugar (like glucose or maltose) that can donate electrons to another chemical, resulting in a color change during a chemical test.
• Double Helix: The structural shape of DNA, consisting of two strands coiled around each other.

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Core Content

A. Chemical Elements in Biological Molecules

All living things are composed primarily of four elements: Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N).

• Carbohydrates: Contain Carbon, Hydrogen, and Oxygen.
• Fats (Lipids): Contain Carbon, Hydrogen, and Oxygen.
• Proteins: Contain Carbon, Hydrogen, Oxygen, and Nitrogen (some also contain Sulfur).

B. Large Molecules from Smaller Units

Large biological molecules (macromolecules) are synthesized from smaller subunits:

1. Starch, Glycogen, and Cellulose: All are large polysaccharides made from glucose units.
   • Starch: Energy storage in plants.
   • Glycogen: Energy storage in animals/fungi.
   • Cellulose: Structural component of plant cell walls.
2. Proteins: Formed from long chains of amino acids.
3. Fats and Oils (Lipids): Each lipid molecule is made of one glycerol molecule attached to three fatty acids.

C. Food Tests (Step-by-Step)

Test	Substance	Reagent	Method	Positive Result
Starch	Starch	Iodine Solution	Add a few drops of iodine to the sample.	Blue-black
Reducing Sugars	Glucose/ Maltose	Benedict’s Solution	Add reagent and heat in a water bath (approx. 80°C).	Green $\rightarrow$ Yellow $\rightarrow$ Orange $\rightarrow$ Brick red
Proteins	Amino acids/ Protein	Biuret Reagent	Add Biuret solution to the sample.	Purple / Violet
Fats & Oils	Lipids	Ethanol Emulsion	Dissolve sample in ethanol, then pour into water.	Cloudy-white emulsion
Vitamin C	Ascorbic acid	DCPIP	Add sample drop-by-drop to a fixed volume of DCPIP.	Blue $\rightarrow$ Colorless

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Extended Content (Extended Only)

DNA Structure

Deoxyribonucleic acid (DNA) is the molecule that contains genetic information. Its structure is linked closely to its function of storing and copying data.

• Shape: It consists of two long strands coiled together to form a double helix.
• Bases: Each strand contains chemical groups called bases. There are four types: A, T, C, and G.
• Pairing: The two strands are held together by bonds between these bases.
• Complementary Base Pairing: Bases always pair specifically:
  • A always pairs with T
  • C always pairs with G

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Key Equations

While there are no complex mathematical formulas for this topic, you should understand the molecular ratio of a lipid:

• 1 Lipid Molecule = 1 Glycerol + 3 Fatty Acids
• Numerical Data: In experiments (like DCPIP titration), you may need to calculate the volume of juice required to decolorize the reagent.
  • Example: If 5.0ml of Juice A decolorizes DCPIP, and 1.1ml of Juice B decolorizes it, Juice B has a higher concentration of Vitamin C.

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Common Mistakes to Avoid

• ❌ Wrong: Forgetting to heat the Benedict's test.
  • ✓ Right: Benedict’s reagent only reacts with reducing sugars if it is heated in a water bath.
• ❌ Wrong: Stating that proteins contain only C, H, and O.
  • ✓ Right: You must mention Nitrogen; it is the distinguishing element for proteins.
• ❌ Wrong: Describing the Biuret result as "pink."
  • ✓ Right: Use the terms purple or violet to ensure full marks.
• ❌ Wrong: Confusing the bases in DNA (e.g., pairing A with C).
  • ✓ Right: Remember the rule: A-T and C-G.

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Exam Tips

• Command Word "State": This is the most common command word for this topic (found 10x in recent papers). It requires a brief, one-word or one-sentence answer (e.g., "State the elements in protein" $\rightarrow$ "C, H, O, and N").
• Food Test Tables: You are frequently asked to identify an "unknown" solution based on a table of test results. Look for the color changes: Blue-black always means starch; Brick-red always means reducing sugar.
• Real-World Context: Questions often involve testing fruit juices for Vitamin C or identifying the contents of a specific food item like milk or bread.
• Typical Values: Be prepared to handle small decimal numbers like 1.1 or 5.0 in titration/Vitamin C tables; always include units (ml or $cm^3$) if they are not provided in the table header.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0610 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

(a) State the four chemical elements that are always present in a protein molecule. [4]

(b) State one other chemical element that may also be present in some protein molecules. [1]

Worked Solution:

(a)

1. Carbon is present. Proteins are organic molecules, containing carbon.
2. Hydrogen is present. Proteins are organic molecules, containing hydrogen.
3. Oxygen is present. Proteins are organic molecules, containing oxygen.
4. Nitrogen is present. Nitrogen is a key element in the amino group of amino acids.

How to earn full marks:

• Four elements stated correctly.
• Spelling must be correct.

(b)

1. Sulfur is present. Some amino acids contain sulfur atoms.

How to earn full marks:

• Sulfur stated correctly.
• Do not accept any other element.

Common Pitfall: Many students forget that all four elements (C, H, O, N) are required for part (a). For part (b), be careful not to suggest elements that are always present, like phosphorus.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

(a) State the name of the smaller molecule that is used to make a protein. [1]

(b) Describe the Biuret test used to test a food sample for the presence of protein. [5]

Worked Solution:

(a)

1. An amino acid. Amino acids are the monomers of proteins.

How to earn full marks:

• Correct spelling is required.

(b)

1. Add sodium hydroxide solution to the food sample. This makes the solution alkaline.
2. Add copper(II) sulfate solution. This is the Biuret reagent.
3. Observe any colour change. The colour change indicates the presence or absence of protein.
4. A positive result is a change to purple. This indicates protein is present.
5. A negative result is that the solution remains blue. This indicates protein is absent.

How to earn full marks:

• Correct order of steps.
• Correct reagents named.
• Correct observation for positive and negative result.
• Must mention both positive and negative results for full marks.

Common Pitfall: Students often mix up the order of adding the reagents or forget to mention both the positive and negative results. Remember that the Biuret test requires both sodium hydroxide and copper(II) sulfate.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

(a) Explain how a molecule of starch is formed from smaller molecules. [3]

(b) Describe the iodine test for the presence of starch in a potato. [5]

Worked Solution:

(a)

1. Starch is a polysaccharide. Starch is a large molecule.
2. Starch is made up of many glucose molecules. Glucose is the monomer.
3. Glucose molecules join together by condensation reactions. Water is eliminated in this process.

How to earn full marks:

• Must mention glucose as the monomer.
• Must mention condensation reaction (or removal of water).

(b)

1. Cut the potato open to expose the inner surface. This allows the reagent to react with the potato cells.
2. Add a few drops of iodine solution to the cut surface of the potato. Iodine is the reagent.
3. Observe the colour change. The colour change indicates the presence or absence of starch.
4. A positive result is a change to blue-black. This indicates starch is present.
5. A negative result is that the iodine solution remains brown/yellow. This indicates starch is absent.

How to earn full marks:

• Correct method described.
• Correct observation for positive and negative result.
• Must mention both positive and negative results for full marks.
• Do not penalise if cutting the potato is not mentioned.

Common Pitfall: Many students only describe the positive result and forget to mention what a negative result would look like. Also, be sure to state the initial colour of the iodine solution.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

(a) State the full name of the molecule abbreviated to DNA. [1]

(b) Describe the structure of a DNA molecule. [8]

Worked Solution:

(a)

1. Deoxyribonucleic acid. This is the full name of DNA.

How to earn full marks:

• Correct spelling is required.

(b)

1. DNA is a double helix. This is the overall shape.
2. It consists of two strands. These strands are intertwined.
3. Each strand contains chemicals called bases. These bases are the genetic code.
4. The four bases are adenine (A), thymine (T), cytosine (C) and guanine (G). These are the four building blocks of DNA.
5. Bases on opposite strands are linked by bonds. These bonds hold the two strands together.
6. A always pairs with T, and C always pairs with G. This is complementary base pairing.
7. The sequence of bases determines the genetic information. This sequence codes for proteins.
8. The sugar-phosphate backbone is the support structure of each strand. This backbone provides structural integrity.

How to earn full marks:

• Must mention double helix.
• Must mention two strands.
• Must name all four bases.
• Must describe correct base pairing (A with T, C with G).
• Must mention sugar-phosphate backbone.

Common Pitfall: Students often forget to mention the sugar-phosphate backbone or confuse the base pairings. Remember that adenine (A) always pairs with thymine (T), and cytosine (C) always pairs with guanine (G).