3.1 identify the chemical elements present in carbohydrates, proteins and lipids

Carbohydrates contain 3 elements: carbon, hydrogen and oxygen (CHO). The ratio of hydrogen atoms to oxygen atoms is always 2:1. e.g. sucrose: C₁₂ H₂₂ O₁₁

Lipids contain the same 3 elements as carbohydrates: carbon hydrogen and oxygen (CHO). However, the proportion of oxygen in a lipid is much lower.

Proteins contain 4 elements: carbon, hydrogen, oxygen and nitrogen (CHON). Proteins are made of chains of amino acids. There are about 20 different amino acids, two of these also contain sulphur.

External links:
http://alevelnotes.com/Biological-Molecules/38
http://lesiuk-biology.wikispaces.com/Unit+C+-+Biological+Molecules
http://ibguides.com/biology/notes/digestion