Plants manufacture their own food by photosynthesis, but all animals including humans have to take in readymade food. Most part of such food consists of complex organic molecules (carbohydrates, proteins and fats) which have to be broken down into simpler forms before they can be absorbed into the body. Such breaking down of the food and subsequent absorption of food constituents occur inside the digestive tract (alimentary canal). The digestive tract together with the associated glands constitute the digestive system.