When atoms become connected or bonded with other atoms, they form molecules. Molecules can be as simple as a molecule of hydrogen gas, which is made of nothing but two hydrogen atoms. Molecules can also be made up of thousands of atoms of many different elements. Many biological molecules, including proteins and DNA, are large and complex.
The bonds between the atoms in a molecule can store and release energy as the molecules are made or broken apart. This can happen because different molecular bonds can have different energy states. It takes energy to create a bond with a high energy state. This happens when green plants make carbohydrates during photosynthesis. When such bonds are broken, energy is released, as when carbohydrates are oxydized during respiration.