Cambridge IGCSE0610

Biological molecules

Biology 0610 Chapter Notes

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Biological molecules
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In this chapter

  1. 1Monomers, Polymers, and Key Reactions
  2. 2Carbohydrates: Energy and Structure
  3. 3Lipids: Energy, Insulation, and Membranes
  4. 4Proteins: The Body's Workforce
  5. 5Water: The Essential Inorganic Molecule

1. Monomers, Polymers, and Key Reactions

Biological molecules are the essential organic compounds that build living things. Most of the large molecules, called polymers, are constructed from smaller, repeating units known as monomers. Think of it like a train (the polymer) made of many individual carriages (the monomers). Organisms build the polymers they need through condensation reactions, where monomers join together and a molecule of water is released. Conversely, to break down polymers, for example during digestion, organisms use hydrolysis reactions, where a water molecule is added to break the bond between monomers.

Monomer + Monomer → Polymer (or Dimer) + Water (Condensation)

Polymer + Water → Monomer + Monomer (Hydrolysis)

Key term

Polymer: A large molecule made up of many smaller, repeating subunits called monomers linked by chemical bonds.

Examiner insight

Examiners reward students who can clearly define and differentiate between condensation and hydrolysis, often using simple word equations or diagrams to support their answer.

Common pitfall

Confusing condensation with hydrolysis. Remember: 'condensation' creates water (like on a cold window), while 'hydrolysis' uses water to split a molecule ('hydro' for water, 'lysis' for splitting).

Worked example 12 marks

The diagram shows two glucose molecules joining to form maltose and water.(i) Name this type of reaction.(ii) Name the reverse reaction that would split maltose back into glucose.

  1. 1

    (i) The reaction joins two smaller molecules (monomers) into a larger one and produces water. This is a condensation reaction.

  2. 2

    (ii) The reaction that uses water to break down a larger molecule into its smaller subunits is called hydrolysis.

Worked example 23 marks

Describe how a large molecule like starch is formed from its monomers.

  1. 1

    Starch is a polymer made from many glucose monomers.

  2. 2

    Many glucose molecules are joined together in a long chain.

  3. 3

    Each time two glucose molecules join, a condensation reaction occurs, and one molecule of water is removed.

  4. 4

    This process is repeated many times to form a large polysaccharide molecule.

Recap

  • Large organic molecules are called polymers; their subunits are called monomers.
  • The four main types of organic molecules are carbohydrates, lipids, proteins, and nucleic acids.
  • Condensation reactions join monomers together and release a water molecule.
  • Hydrolysis reactions use a water molecule to break down polymers into monomers.
  • These reactions allow organisms to build and break down molecules for growth, repair, and energy.

Quick check

  1. What is the name of the reaction that joins two amino acids together?1 mark
  2. Name the small, repeating units that make up a protein.1 mark

2. Carbohydrates: Energy and Structure

Carbohydrates are molecules made of carbon, hydrogen, and oxygen, and they are a primary source of energy for living organisms. The simplest carbohydrates are single sugar units called monosaccharides, like glucose. Glucose is small, soluble in water, and easily transported in the blood, making it perfect for delivering quick energy to cells. When two monosaccharides join, they form a disaccharide. When many join, they form a large, complex carbohydrate called a polysaccharide. Key polysaccharides include starch (energy storage in plants), glycogen (energy storage in animals), and cellulose (forms strong cell walls in plants). Because polysaccharides are large and insoluble, they are excellent for storage as they don't affect the water balance of the cell.

General formula for a monosaccharide: C6H12O6 (e.g., Glucose)

(Glucose)n → Starch / Glycogen (Polysaccharide)

Key term

Monosaccharide: The simplest form of carbohydrate, a single sugar unit (e.g., glucose), which is the building block for more complex carbohydrates.

Common pitfall

Mixing up the roles of starch, glycogen, and cellulose. A simple way to remember is: Starch is storage for Plants, Glycogen is storage for Animals ('-gen' as in 'genesis' of energy for us), and Cellulose is for Cell walls.

Fun fact

Although humans cannot digest cellulose, it is vital for our diet as 'dietary fibre', helping to keep the digestive system healthy.

Worked example 14 marks

Compare the properties and functions of glucose and starch.

  1. 1

    Glucose is a small monosaccharide, while starch is a large polysaccharide.

  2. 2

    Glucose is soluble in water, making it suitable for transport in the blood. Starch is insoluble, so it's a good storage molecule that doesn't affect osmosis.

  3. 3

    Glucose is used for immediate energy release in respiration. Starch acts as a long-term energy store in plants, which can be hydrolysed to glucose when needed.

Worked example 23 marks

Explain why glycogen is a suitable molecule for energy storage in animals.

  1. 1

    Glycogen is a large molecule, so it cannot pass out of the cell.

  2. 2

    It is insoluble in water, so it does not affect the water potential (osmotic balance) of the cell.

  3. 3

    It is a compact molecule, so a large amount of energy can be stored in a small space.

  4. 4

    It is highly branched, meaning it can be broken down quickly by enzymes to release glucose for respiration when energy is needed.

Recap

  • Carbohydrates contain carbon, hydrogen, and oxygen.
  • Monosaccharides like glucose are small, soluble sugars for immediate energy.
  • Polysaccharides are large, insoluble polymers of monosaccharides, used for storage or structure.
  • Starch is the energy storage carbohydrate in plants.
  • Glycogen is the energy storage carbohydrate in animals.
  • Cellulose provides structural support to plant cell walls.

Quick check

  1. Name the polysaccharide that forms the cell wall of plants.1 mark
  2. State one reason why glucose is well-suited for transport in the blood.1 mark

3. Lipids: Energy, Insulation, and Membranes

Lipids, commonly known as fats and oils, are a diverse group of organic molecules. They are made of carbon, hydrogen, and oxygen, but contain a much lower proportion of oxygen than carbohydrates. The most common type of lipid is a triglyceride, formed by a condensation reaction between one molecule of glycerol and three fatty acid molecules. Lipids are insoluble in water. They serve several vital functions: as a long-term energy store (providing more than double the energy per gram compared to carbohydrates), for thermal insulation, to protect vital organs, and as a crucial structural component of cell membranes.

1 Glycerol + 3 Fatty Acids → 1 Triglyceride + 3 Water

Key term

Triglyceride: A lipid molecule made from one molecule of glycerol and three molecules of fatty acids, serving as the main energy storage compound in animals.

Examiner insight

Students who can explain *why* lipids are good energy stores (due to the high number of energy-rich carbon-hydrogen bonds) often achieve higher marks.

Fun fact

The hump of a camel is not filled with water, but with fat! When this fat is metabolised, it releases a large amount of metabolic water, a crucial adaptation for desert survival.

Worked example 13 marks

Describe the chemical structure of a triglyceride.

  1. 1

    A triglyceride is a type of lipid.

  2. 2

    It is formed from two types of smaller molecules: glycerol and fatty acids.

  3. 3

    One molecule of glycerol is joined to three molecules of fatty acids.

  4. 4

    The molecules are joined by condensation reactions, forming ester bonds.

Worked example 24 marks

Explain two functions of lipids in the human body.

  1. 1

    Function 1: Energy Storage. Lipids are a long-term energy store, releasing more energy per gram than carbohydrates, which is useful during fasting or prolonged exercise.

  2. 2

    Function 2: Insulation. The layer of fat under the skin (adipose tissue) is a poor conductor of heat, which reduces heat loss from the body and helps maintain a constant body temperature.

Recap

  • Lipids are fats and oils, composed of carbon, hydrogen, and oxygen.
  • A triglyceride is formed from one glycerol molecule and three fatty acid molecules.
  • Lipids are insoluble in water.
  • Lipids function as a major energy store, for insulation, for protection, and in cell membranes.
  • Lipids release more than twice the energy per gram as carbohydrates.

Quick check

  1. Name the two types of molecule that combine to form a lipid like a triglyceride.2 marks
  2. State one function of lipids in the body, other than energy storage.1 mark

4. Proteins: The Body's Workforce

Proteins are highly complex polymers that perform a vast range of tasks in the body. They are made from monomers called amino acids. All proteins contain the elements carbon, hydrogen, oxygen, and nitrogen; some also contain sulfur. There are 20 different types of amino acids, and they can be linked together in any sequence to form a long chain called a polypeptide. This chain then folds into a unique and specific three-dimensional (3D) shape. This 3D shape is critical to the protein's function. For example, enzymes have an active site with a specific shape to bind their substrate. If a protein is exposed to high temperatures or extreme pH levels, the bonds holding its shape are broken, and it becomes denatured. This change in shape is permanent and stops the protein from working.

(Amino Acid)n → Polypeptide + (n-1)Water

Key term

Denaturation: The irreversible change in the specific three-dimensional shape of a protein (e.g., an enzyme) due to factors like extreme heat or pH.

Common pitfall

Stating that enzymes are 'killed' by heat. The correct biological term is 'denatured'. Enzymes are molecules, not living organisms, so they cannot be killed.

Fun fact

Keratin, the protein that makes up your hair and nails, is also the primary component of reptile scales, bird feathers, and the horns of a rhinoceros.

Worked example 14 marks

Explain why the specific 3D shape of an enzyme is essential for its function.

  1. 1

    Enzymes are biological catalysts that speed up reactions.

  2. 2

    Each enzyme has a region called the active site, which has a specific 3D shape.

  3. 3

    This shape is complementary to the shape of its specific substrate molecule.

  4. 4

    This allows the substrate to bind to the active site, forming an enzyme-substrate complex, which enables the reaction to occur.

  5. 5

    If the shape changes (denaturation), the substrate can no longer bind, and the enzyme will not function.

Worked example 23 marks

List three different types of protein and state one function for each.

  1. 1
    1. Enzymes (e.g., amylase): Act as biological catalysts to speed up metabolic reactions like digestion.
  2. 2
    1. Antibodies: Part of the immune system, they have specific shapes to bind to pathogens and help destroy them.
  3. 3
    1. Haemoglobin: Found in red blood cells, it transports oxygen around the body.

Recap

  • Proteins are polymers made from amino acid monomers.
  • Proteins contain carbon, hydrogen, oxygen, and nitrogen.
  • The sequence of amino acids determines a protein's unique 3D shape.
  • A protein's specific shape is essential for its function.
  • High temperatures or extreme pH can cause a protein to denature, losing its shape and function.
  • Proteins have diverse roles, including as enzymes, antibodies, and structural components.

Quick check

  1. What are the monomers of proteins called?1 mark
  2. What is the term for the irreversible change in a protein's shape caused by high temperature?1 mark

5. Water: The Essential Inorganic Molecule

Although not an organic molecule, water (H₂O) is absolutely essential for life. Its unique properties make it the medium in which most of life's chemistry happens. Water is an excellent solvent, meaning many substances like glucose, amino acids, and mineral ions can dissolve in it. This allows them to be transported around an organism (e.g., in blood plasma) and to react with each other inside cells (in the cytoplasm). Water is also a key reactant in all hydrolysis reactions, such as the breakdown of food during digestion. Its presence is crucial for nearly every metabolic process.

Key term

Solvent: A substance, usually a liquid, in which other substances (solutes) dissolve to form a solution.

Examiner insight

Clear, specific examples are key. Instead of just saying 'water transports things', a better answer is 'water in blood plasma transports dissolved glucose and amino acids'.

Worked example 13 marks

Explain the importance of water as a solvent in the human body.

  1. 1

    Many essential substances, like glucose, amino acids, and mineral salts, are soluble in water.

  2. 2

    This allows them to be transported around the body in the blood, which is mainly water (plasma).

  3. 3

    Metabolic reactions, like those in respiration, occur in the cytoplasm, which is mostly water. The dissolved reactants can move and collide, allowing reactions to happen.

  4. 4

    Waste products like urea are dissolved in water to be excreted in urine.

Worked example 23 marks

Describe the role of water in the chemical digestion of a large food molecule like starch.

  1. 1

    The chemical digestion of starch is its breakdown into smaller glucose molecules.

  2. 2

    This breakdown is an example of a hydrolysis reaction.

  3. 3

    In hydrolysis, water molecules are added to the starch chain to break the chemical bonds holding the glucose monomers together.

  4. 4

    This process is catalysed by enzymes like amylase.

Recap

  • Water is an essential inorganic molecule for all living organisms.
  • Water is an excellent solvent, allowing transport of substances and metabolic reactions to occur.
  • Blood plasma, tissue fluid, and cytoplasm are all mainly water.
  • Water is a reactant in hydrolysis, the process that breaks down polymers into monomers.

Quick check

  1. State one property of water that makes it vital for living organisms.1 mark
  2. Name the type of reaction where water is used to break down a large molecule.1 mark

End-of-chapter exercise

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

  1. List the four main groups of organic molecules found in living organisms and state one function for each group.4 marks
  2. Define the terms (i) monomer, (ii) polymer, and (iii) hydrolysis.3 marks
  3. Compare the structure and function of starch and glycogen.4 marks
  4. Describe how a triglyceride molecule is formed from its component parts. You should name the components and the type of reaction.4 marks
  5. Proteins are polymers of amino acids. Explain how the structure of a protein is related to its function. Use the term 'denature' in your answer.5 marks
  6. Explain why lipids are a more efficient long-term energy store than carbohydrates.3 marks
  7. A meal consists of bread (mainly starch) and chicken (mainly protein). Describe the chemical digestion of these two food types into molecules that can be absorbed by the body. For each, name the monomer produced and the type of reaction involved.6 marks
  8. Cellulose and glycogen are both polymers of glucose, but they have very different properties and functions. Explain these differences, relating structure to function.5 marks
  9. State two functions of water in a living organism, and for each function, explain why it is important.4 marks
  10. Explain why a person with a very high fever may find that their ability to digest food is reduced. Your answer should refer to enzymes.5 marks

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