Asked by: Marina Paya, Barcelona
Both fats and oils are molecules shaped like a capital E with a glycerol spine and arms made of fatty acid chains.
In animals, the carbon atoms in the fatty acids are saturated with hydrogen atoms. This allows them to lie straight and the molecules will pack neatly into a solid arrangement. In plants, missing hydrogen atoms cause kinks in the fatty acids. This reduces the amount of intermolecular bonding that can occur and keeps the molecules liquid at room temperature.
The reason they have evolved this way is probably due to differences in plant and animal metabolisms.