Biological Molecules

Building biological molecules

Living organisms need carbohydrates, lipids and proteins to build cells and carry out life processes. Many of these molecules can be assembled from smaller units and broken down again. Think of the smaller units as pieces that can be joined into larger structures: which pieces are used determines which biological molecule is made.

Simple sugars are the building units of many carbohydrates. Glucose is one example. Joining many sugar units produces large carbohydrates such as starch in plants and glycogen in animals. When these carbohydrates are broken down, smaller sugars become available again. Cells can use glucose in respiration, while plants can also use sugars to make other substances.

Proteins are long chains of amino acids. Cells join amino acids in particular orders to make different proteins, including enzymes and structural proteins. Breaking a protein down releases amino acids that can be absorbed from food and used to make new proteins. Lipids, including fats and oils, are made by joining fatty acids to glycerol. Their breakdown releases fatty acids and glycerol. Lipids are not long chains of repeating units in the way that starch and proteins are.

Joining small units involves forming chemical bonds; a condensation reaction produces water as the units join. Breaking these bonds by hydrolysis uses water. During digestion, enzymes help break large food molecules into smaller molecules that can be absorbed. The smaller molecules can then be used to build the organism’s own carbohydrates, lipids and proteins. Breaking a molecule into its units is not the same thing as respiration, although some of those units can later be used as fuels.

Finding molecules in food

A qualitative food test tells you whether a type of molecule is present from a visible change. It does not, by itself, give an exact concentration. A solid food sample can be crushed and mixed with water before testing a little of the mixture; use a separate portion for each test so the reagents do not interfere with one another.

To test for starch, put a little sample on a spotting tile and add a few drops of iodine solution (iodine dissolved in potassium iodide). A blue-black colour shows starch; if starch is absent, the solution remains yellow-brown. This test detects the larger carbohydrate starch, not all sugars.

To test for reducing sugars, including glucose, add Benedict’s solution to a sample in a test tube and warm it in a hot water bath. A negative result stays blue. A positive result may turn green, yellow, orange or brick red; the progression generally indicates increasing reducing-sugar concentration. Benedict’s solution therefore suggests the presence of glucose but cannot prove that glucose, rather than another reducing sugar, is present.

To test for protein, carry out the Biuret test: add potassium hydroxide to the food solution, followed by copper sulfate solution. Purple indicates protein; without protein, it remains blue. To test for lipid, shake a separate food sample with ethanol to dissolve any fat or oil, then add water. A cloudy white emulsion indicates lipid; a clear mixture is a negative result. Keep ethanol away from flames because it is flammable.

These tests connect a food’s visible test results to the molecules it contains. They can identify starch, reducing sugar, protein or lipid in an unknown sample, but a positive result does not tell you the exact amount present.

Building and breaking down

• Simple sugars join to make carbohydrates such as starch and glycogen; breakdown releases smaller sugars.
• Amino acids join in chains to make proteins; breakdown releases amino acids.
• Fatty acids join to glycerol to make lipids; breakdown releases fatty acids and glycerol. Lipids are not polymers of repeating units.
• Condensation joins units and produces water. Hydrolysis uses water to break bonds. Digestion supplies small molecules for absorption and reuse.

Qualitative food tests

A qualitative result shows presence, not an exact amount. Benedict’s test is not specific to glucose.