Biological Molecules and Enzymes Notes

1. Biological Molecules

Carbohydrates, fats, and proteins are all important biological molecules made from specific chemical elements. Carbohydrates are made of carbon, hydrogen, and oxygen (CHO). Fats and oils are also made of carbon, hydrogen, and oxygen, but they contain proportionally much more hydrogen than oxygen. Proteins are made of carbon, hydrogen, oxygen, and nitrogen (CHON), and some proteins also contain small amounts of sulfur. Knowing the elements present helps explain the different properties and roles of these molecules in living organisms.

Large biological molecules are built from smaller subunits. Starch, glycogen, and cellulose are all large carbohydrate molecules made from many glucose molecules joined together. Although they are all made from glucose, they have different structures and functions: starch is used for energy storage in plants, glycogen is used for energy storage in animals, and cellulose forms plant cell walls. Proteins are large molecules made from smaller units called amino acids, which join together in long chains. Fats and oils are formed from fatty acids and glycerol, with three fatty acid molecules attached to one glycerol molecule. These smaller building blocks combine to form complex molecules needed for growth, repair, and energy storage.

Different food tests are used to identify these biological molecules. The iodine solution test is used to detect starch; iodine turns from brown to blue-black if starch is present. Benedict’s solution is used to test for reducing sugars such as glucose; when heated, the solution changes from blue to green, yellow, orange, or brick-red depending on sugar concentration. The biuret test detects proteins and produces a purple or lilac color if proteins are present. The ethanol emulsion test is used to identify fats and oils; ethanol is added and shaken, then water is added—if fats are present, a cloudy white emulsion forms.

Task: In groups, research one of the four tests to present to class.

Questions:

1. What are the basic units of:
   • a) fats
   • b) carbohydrates
   • c) proteins?
2. Give two differences between the structures of a protein and a carbohydrate.
3. Describe what you would see if you tested samples of the followig separately with i) Benedict's solution ii) iodine solution:
   • a) glucose syrup
   • b) a cake made with flour, table sugar (sucrose), fat, and eggs. 
4. Expain how you would test seeds to see if they contained stores of:
   • a) fat
   • b) protein
5. Which of the following elements is found in proteins but not in carbohydrates or lipids?
   • A Carbon
   • B Hydrogen
   • C Nitrogen
   • D Oxygen
6. A sample of bread was ground up. Some of the breadcrumbs were tested with Benedict's solution and some with iodine solution. The rest of the crumbs were mixed with Substance A. After 20 minutes, some of the mixture was tested with Benedict's reagent and some with iodine solution. The results of the tests are shown in the table.

   • 	Test with Benedict's solution	Test with iodine solution
     Before adding Substance A	no precipitate	change to blue-black colour
     After 20 minutes with Substance A	orange-red precipitate	no colour change

   • a) Describe what the results show.
   • b) Suggest what Substance A was. Explain.
7. Before solid foods are tested for the presence of different food substances it is usually best to cut or grind them into small pieces. Suggest an explanation for this.