Biological Molecules

During synthesis, glucose molecules join in long chains via condensation reactions to form polymers like starch, glycogen, or cellulose. Each link between the sugars is a strong covalent bond called a glycosidic bond.

$$\text{Glucose} + \text{Glucose} \xrightarrow{\text{Condensation}} \text{Maltose} + \text{Water}$$

The structure of these molecules determines their biological importance. Large polymers like starch are insoluble, meaning they do not affect the osmotic potential of a cell (water will not rush in via osmosis), making them perfect for energy storage. When energy is needed, digestive enzymes like amylase and maltase use hydrolysis to break these polymers back into small, soluble glucose monomers, which easily transport in the blood for cellular respiration.

Synthesis and Breakdown of Proteins

Proteins contain carbon, hydrogen, oxygen, and importantly, nitrogen. There are 20 different types of amino acid monomers that join together to form protein polymers (also called polypeptides).

When amino acids join, a condensation reaction removes a water molecule to form a peptide bond. Proteins are broken back down into amino acids via hydrolysis, a reaction catalysed by protease enzymes.

$$\text{Amino Acid A} + \text{Amino Acid B} \xrightarrow{\text{Condensation}} \text{Dipeptide} + \text{Water}$$

The importance of proteins lies in their sequence. The specific order of amino acids determines the protein's unique 3D shape. This shape is essential for their function, whether they are acting as enzyme active sites, antibodies, or hormones.

Synthesis and Breakdown of Lipids

Lipids are composed of carbon, hydrogen, and oxygen, but they have a much lower proportion of oxygen than carbohydrates. A single lipid molecule (a triglyceride) is composed of one glycerol backbone attached to three fatty acid chains.

During synthesis, three water molecules are removed to form three ester bonds. During breakdown, lipase enzymes use three water molecules to hydrolyse these bonds. Bile from the liver emulsifies large lipid droplets into smaller ones, increasing the surface area for lipase to work faster.

$$\text{Glycerol} + 3\,\text{Fatty Acids} \xrightarrow{\text{Condensation}} \text{Triglyceride} + 3H_2O$$

Crucially, lipids are classified as large macromolecules, but they are NOT polymers because they are not made of repeating, identical monomer units. They are vital for providing compact energy storage (yielding $37\,\text{kJ/g}$ compared to carbohydrates at $17\,\text{kJ/g}$) and structural insulation, such as the myelin sheath around nerves.

Testing for Biological Molecules (PAG 2)

To identify the presence of these biological molecules in food, specific qualitative laboratory tests are used. For solid foods, you must first crush the sample using a pestle and mortar, mix it with distilled water, and filter the solution (except for the lipid test, as lipids will stick to the filter paper).

1. Testing for Reducing Sugars Add an equal volume of blue Benedict's solution to your sample. You must heat the mixture in a water bath at $80\text{\u2013}100^{\circ}\text{C}$ for 3\u20135 minutes. If reducing sugars are present, the colour changes on a scale based on concentration: Blue $\rightarrow$ Green $\rightarrow$ Yellow Orange Brick-red precipitate. Note that sucrose is a non-reducing sugar and requires acid hydrolysis first.

2. Testing for Starch Add a few drops of orange-brown iodine solution to the sample. If starch is present, it will immediately turn blue-black.

3. Testing for Proteins Perform the Biuret test by adding potassium hydroxide and copper(II) sulfate to the sample. The initial blue solution will turn purple/lilac if peptide bonds are present. Holding the tube against a white tile helps you see faint colour changes. This test does NOT work on single amino acids because they lack peptide bonds.

4. Testing for Lipids Perform the emulsion test by shaking the sample with ethanol for one minute, allowing the lipids to dissolve. Next, pour this liquid into a test tube of distilled water. A positive result forms a milky-white emulsion. Be aware that ethanol is highly flammable, so it must be kept away from Bunsen burners.