Biological Molecules

Every food molecule is built from a small set of chemical elements. You must recall which elements are in each food group.

Food group	Elements present
Carbohydrate	Carbon (C), Hydrogen (H), Oxygen (O)
Lipid (fat / oil)	Carbon (C), Hydrogen (H), Oxygen (O)
Protein	Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N) — and often Sulfur (S)

• Carbohydrates and lipids contain the same three elements (C, H, O).
• The thing that makes a protein different is the extra nitrogen — and proteins also sometimes contain sulfur.

Memory hook. "ProteiN" has an N for Nitrogen — the giveaway element for protein.

Exam tip. A very common one-mark question is "State the element present in proteins that is not present in carbohydrates." The answer is nitrogen (sulfur is also accepted, but nitrogen is the safe answer).

Large biological molecules are too big to be absorbed — they are built by joining many small basic units together, like beads on a string.

Large molecule	Built from (basic units)
Starch	many simple sugars (glucose)
Glycogen	many simple sugars (glucose)
Protein	many amino acids
Lipid	fatty acids + glycerol

• Starch is the plant carbohydrate store; glycogen is the animal/fungal carbohydrate store. Both are made of glucose units.
• Proteins are long chains of amino acids (about 20 different amino acids exist — the order gives each protein its job).
• A lipid is different: it is not a long chain. One lipid molecule = 1 glycerol joined to 3 fatty acids.

Exam tip. Learn each pairing as a flashcard — examiners love the direct recall: "Name the basic units that make up a protein." → amino acids.

Carbohydrates (C, H, O) range from small simple sugars to large storage molecules.

• A simple sugar such as glucose is small and soluble — easily transported and used in respiration.
• When a cell has spare glucose, it joins the molecules together into a large, insoluble store:
  • Starch — the storage carbohydrate in plants.
  • Glycogen — the storage carbohydrate in animals (and fungi).
• Both starch and glycogen are made of many glucose units joined together.

Why store as starch/glycogen, not as glucose?

• They are insoluble, so they do not dissolve and affect the cell's water balance (osmosis).
• They are compact — a lot of glucose is packed into one molecule.

Exam tip. "Plants store starch, animals store glycogen — both are made of glucose." Memorising this single sentence reliably scores multiple marks.

Proteins (C, H, O, N — and sometimes S) are built from amino acids.

• There are about 20 different amino acids.
• They join together in a long chain to make a protein.
• The order of the amino acids decides the shape — and the shape decides the protein's job (e.g. enzymes, antibodies, muscle, haemoglobin).
• The element that makes proteins special is nitrogen (N) — carbohydrates and lipids have no nitrogen. Some amino acids also contain sulfur (S).

Exam tip. If asked "What is a protein made from?" the answer is amino acids — not "nitrogen" (nitrogen is an element, an amino acid is the basic unit). Don't mix up the two ideas.

Lipids are fats and oils (C, H, O). A lipid is built from two kinds of basic unit:

• Fatty acids — long molecules; three of them per lipid.
• Glycerol — one per lipid; it links the three fatty acids together.

So one lipid molecule = 1 glycerol + 3 fatty acids.

• Fats are solid at room temperature (e.g. in animals); oils are liquid at room temperature (e.g. in plants) — but both are lipids made from the same units.
• Lipids are a concentrated energy store and provide insulation.

Exam tip. The marker is looking for both units: "fatty acids and glycerol". Writing only "fatty acids" usually loses a mark. Remember the ratio 3 fatty acids : 1 glycerol.