The macromolecular constituents of cells are important biochemical elements so that their organelles can perform their biological functions to keep them alive. Thus, aside from containing water and ions, cells are composed of four major organic constituents: 1) proteins; 2) carbohydrates; 3) lipids (fats); and 4) nucleic acids.
Proteins- They consist of one or more chains of amino acids, with each one having an amino (-NH2) and a carboxyl (COOH) group. Plants produce their own amino acids and proteins from carbon dioxide, nitrogen, and water. However, animals cannot synthesize them as they have to depend on dietary proteins obtained from animal and plant sources when they eat them.
Carbohydrates- Hepatocytes (liver cells) convert the simplest form of carbs, glucose, into glycogen whenever there is an excess. This is called stored carbohydrate. Glycogen is a polysaccharide which can quickly be turned back into glucose during fasting, whenever the body cells need it.
Lipids- The body cells store fat in its simpler form; fatty acid, which is released when lipase breaks down a molecule of fat in the duodenum: two molecules of fatty acid and one glycerol. When there is not enough glucose, many body cells mitochondria employ fatty acid as fuel to produce ATP (energy). The liver cells also use this molecule to synthesize ketone bodies, which are also used as fuel by muscle cells and neurons to produce ATP.
Nucleic acids- They are complex molecules, which are the basic components of DNA and ARN. They are the carrier of hereditary information and control the synthesis of proteins.