When atoms, the basic units of chemical elements, combine into chemical compounds, they form molecules. Organisms have many different kinds of molecules, from water and simple salts to complex molecules such as carbohydrates, fats, proteins, and deoxyribonucleic acid (DNA). One protein, called hemoglobin, carries oxygen in the blood and is what makes blood red. Hemoglobin contains atoms of six different elements—carbon, hydrogen, oxygen, nitrogen, sulfur, and iron.
