Chemistry is used in biology
Knowledge of chemistry can help aid our understanding and practice of biological studies. For example, a great number of chemical reactions take place in the body, thanks to the different cells, hormones and proteins that are floating around inside us. An understanding of chemistry can therefore help us to investigate how these chemical reactions take place, and why, for instance, they make us sick. If we understand how the reactions occur, we can develop ways of controlling or treating the undesirable reactions, in the study of a field we call medicine. Even something as simple as the biological process of breathing (or respiration) is represented through a chemical equation - glucose + oxygen = carbon dioxide + water, or C_6 H_12 O_6 + O_2 =C_O2 + H2O. All biological process, in some way or another, can be represented through chemical equations, which is why knowledge of chemistry is vital for a good grasp on biology.
Chemistry is used in physics
Physics often examines substances on a molecular level - it takes a solid, a liquid or a gas and the breaks it down into its smallest parts in order to study them. This requires a knowledge of chemistry, as chemistry is used to help break the substance down in order for us to access its smallest parts. In addition, physics examines almost everything in the universe in order for us to understand how and why the universe works - and chemistry is involved in this. One scientific theory states that the rapid heating and cooling of our universe allowed it to form into the state it is in today - this can be explained through a chemical reaction process.