There are four major types of macromolecules; carbohydrate, protein, lipid and nucleic acid. Carbohydrate functions as structure maintenance, transportable and storage forms of energy. Glucose is a common source of energy for many animals and cellulose, a type of polysaccharide, works in maintaining structure in plants. Protein functions in various areas like the immune system, nutrition and signalling. Enzymes, which are crucial for chemical reactions, are proteins. All proteins have a nitrogen atom bound to it. Lipid can work as a storage of energy or as hormones. Since one gram of lipid can contain more than double the energy of carbohydrate or protein, it is an efficient source of energy. Nucleic acid consists of nucleotides and functions in storing genetic information.