Biological Molecules

Explained

Three families of molecule, and the element that separates them

Carbohydrates, proteins and lipids are all built from small units joined into larger ones. What distinguishes them is which elements they contain and which small unit they are built from, and almost every question here is testing one of those two things.

The elements

Carbohydrates contain carbon, hydrogen and oxygen.

Lipids contain carbon, hydrogen and oxygen.

Proteins contain carbon, hydrogen, oxygen and nitrogen, and many also contain sulfur.

So nitrogen is the whole distinction. Carbohydrates and lipids have exactly the same three elements as each other, and the only element that tells you a molecule is a protein is nitrogen. If you remember one fact from this page, remember that.

It also explains something from another topic. Plants need nitrate from the soil because photosynthesis gives them carbon, hydrogen and oxygen only, and none of those can be turned into protein without a source of nitrogen.

The building blocks

Starch, glycogen and cellulose are all made from glucose. Same unit, joined differently, which is why starch stores energy in plants, glycogen stores it in animals, and cellulose builds plant cell walls.

Proteins are made from amino acids joined in a chain. There are about twenty different amino acids, and the order they are joined in decides which protein you get and what shape it folds into.

Lipids, meaning fats and oils, are made from three fatty acids joined to one molecule of glycerol. That three to one ratio is worth stating, because it is the detail that distinguishes a full answer.

Notice the pattern. Carbohydrates and proteins are each built from many copies of one type of unit; a lipid is built from two different components. That is why lipids do not form long chains in the way the other two do.

Testing for each

Starch gives a blue black colour with iodine solution, which is orange brown to begin with.

Reducing sugars such as glucose give a brick red precipitate when heated with Benedict's solution, which starts blue.

Protein turns Biuret solution from blue to purple or lilac.

Lipids give a milky white emulsion in the ethanol emulsion test, where the sample is shaken with ethanol and the mixture then poured into water.

For each one, state the reagent, any conditions such as heating, the starting colour and the final colour. A colour change described without its starting point is only half a change.

Why the shape matters

Proteins are the family where structure decides function, and it is worth seeing why.

A chain of amino acids folds into a specific three dimensional shape, and that shape is determined by the order of the amino acids in the chain. An enzyme's active site is part of that shape, and it fits one particular substrate because of it.

This is the link between three topics that are usually taught separately. A mutation changes the order of bases in a gene, which changes the order of amino acids in the protein, which changes the shape it folds into, which changes whether it works. Denaturing does the same damage by a different route, unfolding a shape that was already correct.