Cambridge O Level5090

Biological molecules

Biology 5090 Chapter Notes

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Biological molecules
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In this chapter

  1. 1The Building Blocks of Life
  2. 2Carbohydrates: Energy and Structure
  3. 3Lipids: Long-Term Energy Storage
  4. 4Proteins: The Molecules of Action
  5. 5Food Tests: Identifying Molecules
  6. 6DNA and the Importance of Water

1. The Building Blocks of Life

All life is built from large molecules called biological molecules or macromolecules. These are based on carbon atoms, which can form long chains and rings. The main elements are Carbon (C), Hydrogen (H), and Oxygen (O). Proteins also contain Nitrogen (N). Most large biological molecules are polymers, which are long chains made of repeating smaller units called monomers. Think of a polymer as a long train and each monomer as a single carriage.

Key term

Polymer: A large molecule made from many smaller, repeating sub-units called monomers joined together.

Examiner insight

Examiners expect you to clearly define 'monomer' and 'polymer' and give a named example of each, such as glucose being the monomer for starch.

Common pitfall

Students often mix up monomers and polymers. Remember: 'mono' means one, 'poly' means many. Glucose is the monomer; starch is the polymer.

Fun fact

The carbon atoms in your body were once part of the carbon dioxide in the air, fixed by plants during photosynthesis!

Worked example 13 marks

Explain what is meant by the terms 'monomer' and 'polymer', using a named biological example.

  1. 1

    A monomer is a small, simple molecule that acts as a repeating sub-unit. [1 mark]

  2. 2

    A polymer is a large, complex molecule formed when many monomers are joined together in a chain. [1 mark]

  3. 3

    For example, glucose is a monomer, and many glucose molecules join together to form the polymer starch. [1 mark]

Recap

  • Biological molecules are large molecules essential for life, based on carbon.
  • They are primarily made of carbon, hydrogen, and oxygen.
  • Polymers are long chains of repeating units called monomers.
  • Carbohydrates, proteins, and nucleic acids are all examples of polymers.

Quick check

  1. Name the three main chemical elements found in all carbohydrates and lipids.1 mark
  2. What is the general name for the sub-unit that makes up a polymer?1 mark

2. Carbohydrates: Energy and Structure

Carbohydrates are molecules made of carbon, hydrogen, and oxygen, and their primary role is to provide energy. The simplest carbohydrates are single sugar units called monosaccharides, like glucose (C₆H₁₂O₆). Two monosaccharides joined together form a disaccharide (e.g., sucrose). Many monosaccharides joined together form a polysaccharide. Important polysaccharides include:

  • Starch: The energy storage molecule in plants. It's a large, insoluble polymer of glucose.
  • Glycogen: The energy storage molecule in animals, found in the liver and muscles. It's also a polymer of glucose but is more branched than starch.
  • Cellulose: A structural component of plant cell walls. It's a polymer of glucose arranged in straight, strong fibres, making it insoluble and indigestible by humans.

Glucose: C₆H₁₂O₆

Key term

Monosaccharide: The simplest form of carbohydrate, a single sugar unit, such as glucose.

Examiner insight

Marks are often awarded for linking the structure of a polysaccharide to its function, for example, 'starch is insoluble so it does not affect the water potential of the cell'.

Common pitfall

Confusing the roles of starch, glycogen, and cellulose. Remember: Starch is plant storage, Glycogen is animal storage, and Cellulose is plant structure.

Fun fact

A single potato can contain over 100 trillion starch granules, each packed with glucose molecules for energy.

Worked example 14 marks

Glycogen and cellulose are both carbohydrates made from glucose. Describe how their structures and functions differ.

  1. 1

    Structure: Glycogen is a highly branched polymer of glucose, while cellulose is made of long, straight, unbranched chains of glucose. [1 mark]

  2. 2

    Structure: The straight chains in cellulose are cross-linked by hydrogen bonds to form strong fibres, which is not the case for glycogen. [1 mark]

  3. 3

    Function: Glycogen is an energy storage molecule found in animals (in the liver and muscles). [1 mark]

  4. 4

    Function: Cellulose is a structural molecule that provides strength and support to plant cell walls. [1 mark]

Recap

  • Carbohydrates are made of C, H, and O, and are a primary source of energy.
  • Glucose is a monosaccharide, the basic monomer for complex carbohydrates.
  • Starch is the energy storage polysaccharide in plants.
  • Glycogen is the energy storage polysaccharide in animals.
  • Cellulose is a structural polysaccharide that forms plant cell walls.

Quick check

  1. Name the storage polysaccharide found in animal muscle cells.1 mark
  2. What is the chemical formula for glucose?1 mark

3. Lipids: Long-Term Energy Storage

Lipids are a group of substances that include fats (solid at room temperature) and oils (liquid at room temperature). Like carbohydrates, they are made of carbon, hydrogen, and oxygen, but they contain much less oxygen. A fat molecule is made from two types of smaller molecules: one molecule of glycerol and three molecules of fatty acids. Lipids are not polymers. They are vital for:

  • Energy Storage: They provide more than twice as much energy per gram as carbohydrates.
  • Insulation: Fat stored under the skin reduces heat loss.
  • Protection: Fat surrounds organs like the heart and kidneys, cushioning them from injury.
  • Cell Membranes: They are an essential component of all cell membranes.

Fat molecule = 1 Glycerol + 3 Fatty Acids

Key term

Lipid: An organic molecule including fats and oils, made of glycerol and fatty acids, used for energy storage, insulation, and in cell membranes.

Examiner insight

Candidates who state that lipids provide more energy 'per gram' than carbohydrates often gain credit over those who just say they provide 'more energy'.

Common pitfall

Mistakenly calling lipids 'polymers'. They are large molecules (macromolecules) but are not made of repeating monomer units.

Fun fact

Polar bears have a layer of blubber (fat) up to 10 cm thick, which is crucial for surviving the freezing Arctic temperatures.

Worked example 14 marks

Describe the chemical composition of a fat molecule and state two of its functions in the human body.

  1. 1

    A fat molecule is made from one molecule of glycerol. [1 mark]

  2. 2

    And it is joined to three molecules of fatty acids. [1 mark]

  3. 3

    Function 1: It serves as a long-term energy store. [1 mark]

  4. 4

    Function 2: It provides thermal insulation OR it protects organs from physical shock. [1 mark]

Recap

  • Lipids are fats and oils, made from one glycerol and three fatty acids.
  • They are composed of C, H, and O, with a low proportion of oxygen.
  • Lipids are not polymers.
  • Their main functions are energy storage, insulation, protection, and forming cell membranes.

Quick check

  1. Name the two types of molecule that combine to form a fat.2 marks
  2. Which biological molecule provides the most energy per gram?1 mark

4. Proteins: The Molecules of Action

Proteins are polymers made from monomers called amino acids. There are about 20 different types of amino acids. The specific sequence of these amino acids causes the protein chain to fold into a unique three-dimensional shape. This shape is crucial for its function. If the shape is changed (denatured) by high temperatures or extreme pH, the protein stops working. Proteins have a vast range of functions:

  • Enzymes: Biological catalysts that speed up chemical reactions.
  • Structural: Provide support and strength (e.g., collagen in skin).
  • Hormones: Chemical messengers (e.g., insulin).
  • Antibodies: Part of the immune system, fighting pathogens.
  • Transport: Carry substances (e.g., haemoglobin transports oxygen).

Protein = A chain of amino acids

Key term

Amino Acid: The monomer unit from which proteins are made; there are about 20 different types.

Examiner insight

Examiners reward answers that explicitly link a protein's specific 3D shape to its specific function, especially for enzymes and antibodies.

Common pitfall

Forgetting that proteins are made of amino acids. Students sometimes incorrectly say they are made of 'peptides' (peptides are short chains of amino acids, but the monomer is the amino acid itself).

Fun fact

Your body contains tens of thousands of different types of proteins, each with a specific job. Haemoglobin, the protein that carries oxygen, gives your blood its red colour.

Worked example 13 marks

Explain why a high fever can be dangerous, with reference to the structure of proteins like enzymes.

  1. 1

    Proteins, such as enzymes, have a specific three-dimensional shape which is essential for their function. [1 mark]

  2. 2

    High temperatures (like a high fever) can break the bonds holding this shape, causing the protein to change shape permanently. This is called denaturation. [1 mark]

  3. 3

    If enzymes in the body become denatured, they can no longer catalyse vital metabolic reactions, which can be life-threatening. [1 mark]

Recap

  • Proteins are polymers made from amino acid monomers.
  • The sequence of amino acids determines the protein's specific 3D shape.
  • A protein's shape is critical to its function.
  • High temperatures or extreme pH can denature a protein, changing its shape and stopping it from working.
  • Proteins have many functions, including as enzymes, hormones, and antibodies.

Quick check

  1. Name the monomer that makes up proteins.1 mark
  2. State two different functions of proteins in the body.2 marks

5. Food Tests: Identifying Molecules

In the lab, we use specific chemical tests to find out which biological molecules are present in a food sample. For a solid food, you must first grind it up with some water and filter it to get a liquid extract.

  • Test for Starch: Add a few drops of iodine solution. A positive result is a colour change from yellow-brown to blue-black.
  • Test for Reducing Sugars (e.g., Glucose): Add an equal volume of Benedict's solution and heat the mixture in a water bath for a few minutes. A positive result is a colour change from blue to green, yellow, orange, or brick-red.
  • Test for Protein: Add Biuret solution. A positive result is a colour change from blue to purple/lilac.
  • Test for Fats (Emulsion Test): Add ethanol to the sample and shake to dissolve the fat. Then, pour this mixture into a test tube of water. A positive result is the formation of a cloudy white emulsion.

Key term

Benedict's solution: The chemical reagent used to test for the presence of reducing sugars, which turns from blue to brick-red on heating.

Examiner insight

For the Benedict's test, you must mention the need for heating to gain full marks. For the emulsion test, you must mention both ethanol and water.

Common pitfall

Forgetting the initial colour of the reagent. For example, stating 'it turns blue-black' for starch without mentioning the initial yellow-brown colour of iodine.

Fun fact

The Biuret test doesn't actually contain biuret. It's named after the compound biuret, which gives a similar positive result with the same reagents.

Worked example 12 marks

A student tests a sample of milk. They add Benedict's solution and heat it, and the solution turns orange. They then test a separate sample with Biuret solution, and it turns purple. What conclusions can be drawn about the biological molecules present in milk?

  1. 1

    The orange result with Benedict's solution after heating shows that a reducing sugar is present. [1 mark]

  2. 2

    The purple result with Biuret solution shows that protein is present. [1 mark]

Worked example 23 marks

Describe the method to test a potato for the presence of starch.

  1. 1

    Cut a piece of the potato to expose a fresh surface. [1 mark]

  2. 2

    Add a few drops of iodine solution to the surface. [1 mark]

  3. 3

    Observe the colour change. If starch is present, the yellow-brown iodine will turn blue-black. [1 mark]

Recap

  • Use iodine solution to test for starch (positive = blue-black).
  • Use Benedict's solution and heat to test for reducing sugars (positive = green/yellow/orange/brick-red).
  • Use Biuret solution to test for protein (positive = purple/lilac).
  • Use ethanol and water (emulsion test) to test for fats (positive = cloudy white emulsion).
  • Safety precautions, like wearing eye protection, are important during food tests.

Quick check

  1. What reagent is used to test for protein?1 mark
  2. What colour change indicates a positive test for starch?1 mark

6. DNA and the Importance of Water

DNA (Deoxyribonucleic acid) is the molecule of inheritance, carrying the genetic instructions for all known organisms. Like proteins and carbohydrates, DNA is a polymer. Its monomers are called nucleotides. The structure of DNA is a double helix, like a twisted ladder. This structure is key to its ability to store vast amounts of information and to be copied accurately. Water (H₂O) is not an organic biological molecule, but it is essential for life, making up about 70% of the human body. Its importance lies in its properties as a solvent. Many substances, like glucose, amino acids, and mineral ions, dissolve in water. This allows them to be transported around an organism (e.g., in blood plasma) and to take part in chemical reactions inside cells (in the cytoplasm).

Water: H₂O

Key term

Solvent: A substance, typically a liquid, in which other materials dissolve to form a solution.

Examiner insight

When asked about water, always link its role as a solvent to transport of substances in blood or plants, or as the medium for metabolic reactions in cytoplasm.

Common pitfall

Confusing DNA with proteins. DNA carries the *code* for making proteins, but they are completely different molecules.

Fun fact

If you could unravel all the DNA in all the cells in your body, it would stretch from the Earth to the Sun and back over 600 times!

Worked example 13 marks

Explain why water is important for transport in mammals.

  1. 1

    Water is the main component of blood plasma. [1 mark]

  2. 2

    It is an excellent solvent, meaning many substances can dissolve in it. [1 mark]

  3. 3

    This allows soluble substances like glucose, amino acids, and urea to be transported around the body in the blood. [1 mark]

Recap

  • DNA is a polymer of nucleotides and carries genetic information.
  • DNA has a double helix structure.
  • Water is essential for life, making up a large proportion of cells and body fluids.
  • Water acts as a solvent, allowing transport of substances and metabolic reactions to occur.

Quick check

  1. What is the name of the monomer that makes up DNA?1 mark
  2. State one reason why water is important for living organisms.1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. List the chemical elements present in (i) lipids and (ii) proteins.2 marks
  2. Describe the test you would carry out to show that a food contains protein, including the positive result.3 marks
  3. Starch and glycogen are both polysaccharides. State where each is found and describe their common function.4 marks
  4. Explain the difference between a monomer and a polymer, using carbohydrates as an example.3 marks
  5. A fat molecule is composed of two different types of smaller molecules. Name these two types of molecule.2 marks
  6. A student tested an unknown food solution. The results were: - Iodine test: yellow-brown - Benedict's test (with heating): brick-red - Biuret test: blue What conclusions can you draw about the nutrients present in this food solution? Explain your reasoning.4 marks
  7. Explain why the specific sequence of amino acids in a protein is important for its function as an enzyme.4 marks
  8. Compare the structure and function of cellulose with that of starch.5 marks
  9. Water is described as an important solvent in organisms. Explain what this means and give one example of its importance as a solvent in the human body.3 marks
  10. A person on a low-carbohydrate diet gets most of their energy from lipids. State two advantages and one disadvantage of using lipids rather than carbohydrates as a primary energy source.3 marks

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