Carbon, Nitrogen, Oxygen and Hydrogen. All of them are in fact quite important, Carbon is for respiration, without it there would be very little amounts of energy. Oxygen would be needed for respiration so that there would be an aerobic respiration, creating more energy. Hydrogen is used for generating the ATP (energy). Nitrogen is used in amino acids, without it, it would only be carbohydrates, and it prevents the whole world from exploding when a match lights.