The three main food molecules
Carbohydrates, proteins and fats (lipids) are large biological molecules built from smaller subunits joined together. Living organisms make these large molecules from their subunits and break them down again during digestion. Knowing their elements and building blocks is essential for IGCSE Biology, because it explains both how they are built and how they are digested.
Key idea
Carbohydrates and fats contain carbon, hydrogen and oxygen only. Proteins contain carbon, hydrogen, oxygen and nitrogen (and often sulfur).
Building blocks (subunits)
| Molecule | Elements | Building blocks |
| Carbohydrate | C, H, O | Simple sugars (e.g. glucose) |
| Protein | C, H, O, N | Amino acids |
| Fat (lipid) | C, H, O | 3 fatty acids + 1 glycerol |
Many glucose molecules join to form large carbohydrates such as starch (in plants), glycogen (stored in animals) and cellulose (plant cell walls). Amino acids join in long chains that fold into a specific shape to make a protein.
Functions
- Carbohydrates — the main source of energy for respiration.
- Fats — long-term energy store, insulation and protection of organs.
- Proteins — growth and repair of tissues; also used to make enzymes, antibodies and haemoglobin.
Because fats and carbohydrates both store energy, gram for gram fats store about twice as much energy as carbohydrates. This is why fat is used for long-term storage while carbohydrate (glucose) is used for quick, everyday energy release in respiration.
Remember
- Only proteins contain nitrogen.
- A fat = 1 glycerol + 3 fatty acids.
- Starch, glycogen and cellulose are all made of glucose units.