Carbohydrates provide your body with energy. The simplest are sugars, like glucose, fructose, sucrose and lactose. Complex carbohydrates, like starch, are made up of lots of sugar molecules joined together. The 'identity tags' (antigens) on the surface of all cells are made from carbohydrates joined to proteins. These molecules are essential for cells to recognise each other and to keep the different parts of your body working together.