Biological Molecules

Elements and Building Blocks

Carbohydrates, fats and proteins are the main chemicals that make up living things. Each is made from only a few elements.

The elements

Small units to large molecules

Large molecules are made by joining many small molecules together. Starch, glycogen and cellulose are made from many glucose units. Proteins are made from amino acids. Fats and oils are made from fatty acids and glycerol.

Worked example (identifying a molecule): Analysis shows that a large molecule contains carbon, hydrogen, oxygen and nitrogen. Which type of molecule is it? Step 1. Carbohydrates and fats contain only carbon, hydrogen and oxygen. Step 2. Nitrogen is the extra element. Step 3. Only proteins contain nitrogen, so the molecule is a protein. Answer: A protein.

Carbohydrates

Starch, glycogen and cellulose are all long chains of glucose. They have different jobs because the chains are arranged differently.

Glucose is a small, soluble molecule. Joining many glucose units into starch or glycogen makes a large, insoluble molecule that can be stored in a cell and takes up little space.

Worked example (matching molecule to job): A molecule is found in a liver cell. It is a store of energy and is made only from glucose. Name it. Step 1. Made only from glucose, so starch, glycogen or cellulose. Step 2. It is in an animal cell and is an energy store, so it is not cellulose, which is a wall material. Step 3. Starch is stored in plants, so the molecule is glycogen. Answer: Glycogen.

Proteins and Fats

Proteins are chains of amino acids. Fats and oils are built from glycerol and fatty acids. Both are large molecules made from small units.

Proteins

Many amino acids join in a long chain to form a protein. There are many different amino acids, and the order in which they are joined differs from one protein to another.

Fats and oils

Fats and oils are made from fatty acids and glycerol. One molecule of glycerol joins to three fatty acid molecules. Fats are solid at room temperature; oils are liquid.

Worked example (naming the small units): Name the small units that make up (a) a protein and (b) a fat. Step 1. A protein is a chain of many amino acids. Step 2. A fat is made of glycerol and fatty acids. Step 3. Give both parts for the fat: one glycerol and three fatty acids. Answer: (a) amino acids; (b) glycerol and fatty acids.

Food Tests 1: Iodine and Benedict’s

Each food test uses a reagent (or reagents) that gives a visible change when one kind of molecule is present. Learn the positive and negative results.

Starch: iodine solution

Add a few drops of iodine solution to the sample. If starch is present, the colour changes from yellow-brown to blue-black. If there is no starch, it stays yellow-brown.

Reducing sugars: Benedict’s solution

If the food is solid, crush it and mix it with water first. Add Benedict’s solution, which is blue, to the sample and heat it in a hot water bath. If a reducing sugar is present, the colour changes from blue through green, yellow and orange to brick-red. The more reducing sugar there is, the further the colour goes. If there is none, the solution stays blue.

Worked example (reading a result): A food sample is heated with Benedict’s solution and turns orange. Iodine solution added to a separate portion stays yellow-brown. What does it contain? Step 1. Orange with Benedict’s means a reducing sugar is present. Step 2. Iodine stays yellow-brown, so there is no starch. Step 3. Reducing sugar is present and starch is absent. Answer: Reducing sugar, no starch.

Food Tests 2: Biuret, Ethanol and DCPIP

Three more tests identify proteins, fats and oils, and vitamin C.

Proteins: biuret test

If the food is solid, crush it and mix it with water first. Add biuret solution, which is blue, to the sample. If protein is present, the colour changes from blue to purple (mauve). If there is no protein, it stays blue.

Fats and oils: ethanol emulsion test

Crush the sample and shake it with ethanol so that any fat or oil dissolves, then pour the mixture into water. If fat or oil is present, a cloudy white emulsion forms. If not, the liquid stays clear.

Vitamin C: DCPIP

DCPIP is a blue solution. Add the sample drop by drop to a small volume of DCPIP. If vitamin C is present, the blue colour disappears and the solution becomes colourless. The fewer drops needed to decolourise it, the more vitamin C the sample contains.

Worked example (comparing two juices): 10 drops of juice A and 4 drops of juice B each decolourise the same volume of DCPIP. Which contains more vitamin C? Step 1. Fewer drops of juice are needed to decolourise the DCPIP. Step 2. Juice B needed 4 drops and juice A needed 10 drops. Step 3. Less juice does the same job, so juice B is richer in vitamin C. Answer: Juice B.

DNA

This page is Extended content: DNA is a large molecule made from small units, like the others in this chapter.

ExtendedStructure of DNA

DNA is made of two strands coiled together to form a double helix. Each strand carries a row of bases of four types: A, C, G and T. The two strands are held together by hydrogen bonds between pairs of bases.

The pairing is fixed: A pairs with T, and C pairs with G. These are called complementary base pairs.

ExtendedUsing the pairing rule

If you know the order of bases on one strand, you can write the other strand by swapping each base for its partner.

Worked example (the other strand): One strand of DNA has the bases A T T G C in order. Write the bases of the strand it pairs with. Step 1. Pair each base: A pairs with T, T pairs with A, T pairs with A, G pairs with C, C pairs with G. Step 2. Write the partners in the same order. Answer: T A A C G.