1. Define food substances
B. List class of food substances, chemical composition and their importance.
We do not do your homework for you. Although it might take more effort to do the work on your own, you will profit more from your effort. We will be happy to evaluate your work though.
However, I will start you out from the Internet:
Food is mainly composed of water, lipids, proteins and carbohydrates. Minerals (e.g salts) and organic substances (e.g vitamins) can also be found in food. Plants, algae and some microorganisms use photosynthesis to make their own food molecules.