Biological Molecules
Elements in Biological Molecules
| Molecule | Elements |
|---|---|
| Carbohydrates | Carbon (C), Hydrogen (H), Oxygen (O) |
| Proteins | Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N) [+ sometimes Sulfur (S)] |
| Lipids | Carbon (C), Hydrogen (H), Oxygen (O) |
Nitrogen is the Key
Proteins are the only major biological molecule to always contain nitrogen. This is useful for identifying molecules from element lists.
Structure of Biological Molecules
Carbohydrates
- Large molecules built from simple sugars (monosaccharides) such as glucose.
- Starch = many glucose units joined; storage in plants.
- Glycogen = many glucose units joined; storage in animals and fungi.
- Glycosidic bonds join monomers together.
Proteins
- Large molecules built from amino acids joined by peptide bonds.
- The sequence and type of amino acids determines the protein’s shape and function.
- Functions: enzymes, structural proteins (e.g. collagen), hormones (e.g. insulin), antibodies.
Lipids (Fats and Oils)
- Built from one glycerol molecule + three fatty acid chains (triglyceride).
- Ester bonds join fatty acids to glycerol.
- Fats = solid at room temperature; oils = liquid at room temperature.
- Functions: energy storage, insulation, cell membranes (phospholipids).
Food Tests (Spec 2.9)
| Food substance | Reagent | Positive result |
|---|---|---|
| Glucose (reducing sugar) | Benedict’s solution + heat | Blue → brick red/orange precipitate |
| Starch | Iodine solution | Yellow-brown → blue-black |
| Protein | Biuret reagent (NaOH + CuSO₄) | Blue → purple/lilac |
| Fat | Ethanol emulsion test: dissolve in ethanol, add to water | Clear water → cloudy white emulsion |
Method for Each Test
Benedict’s (glucose):
- Add Benedict’s solution to the food sample in a test tube.
- Place in a water bath at ~80 °C for 2–3 minutes.
- Observe colour change.
Iodine (starch):
- Add iodine solution (yellow-brown) directly to the food sample.
- Observe immediately.
Biuret (protein):
- Add NaOH solution, then CuSO₄ solution (or use pre-made Biuret reagent).
- Mix and observe.
Ethanol emulsion (fat):
- Add the food sample to ethanol and shake to dissolve any fat.
- Pour the solution into a test tube of water.
- Observe for cloudiness.
Exam Questions
Q: Identify the chemical elements found in proteins but not in carbohydrates.
Nitrogen (and sometimes sulfur). Both carbohydrates and proteins contain carbon, hydrogen and oxygen, but only proteins contain nitrogen.
Q: Describe the structure of a starch molecule.
Starch is a large polysaccharide molecule made up of many glucose units (monosaccharides) joined together by glycosidic bonds.
Q: Describe the Biuret test for protein and state the positive result.
Add sodium hydroxide solution to the sample, then add copper sulfate solution (or Biuret reagent). Mix and observe. A positive result is a colour change from blue to purple/lilac.
Q: A student adds iodine solution to a food sample and it turns blue-black. What can they conclude?
The food contains starch.
Exam Tip
For food tests, you must describe what you do (the reagent and method), what colour you start with, and what colour indicates a positive result. A colour change alone without stating the starting/ending colours will not score full marks.