 In this lecture, we are going to talk about the digestive system and the nutrition, goals of this lecture are to describe and discuss nutrition and the related processes, various structures in the human digestive system and functions of various parts of the human digestive system, few common diseases associated with the human digestive system. We start with the lecture, nutrition, what is nutrition? Nutrition is the acquiring energy from the environment, converting this energy into the usable form by the organism itself and then assimilating this energy into its body for performing its various functions or making its own structures and then eliminating the remains. So, the process of nutrition and digestion starts from acquiring energy. Acquiring energy from the environment is a property of living organisms and this is one of the two major properties of living organisms that they acquire energy from the environment and then they utilize that energy for their own processes by converting it into a chemical form of energy or we can say the biochemical form of energy which is usable by the organisms. There are various types of strategies that organisms assume or adopt to acquire energy from the environment. There are two major categorizations of organisms as we know, the plants and the animals. These are the organisms that acquire energy from sunlight and make their own carbohydrates or we can say their own food by themselves and also produces energy from sunlight. We call them producers that is they can make their own food by themselves. They comes at the first level of the food chain. The animals on the other hand have to acquire energy by either eating plants or eating other animals. Animals sometimes also acquire energy by eating up the dead organic matter that is the dead organisms, dead living dead plants for example, the branches of plants, the leaves of plants present on the soil or maybe mixed up with the soil the old dead organic matter. According to their energy acquiring habit, if they are eating other plants or other animals we categorize them into different categories. We can call them consumers if they eat up the plants or animals. We call them decomposers if they decompose or break up the dead organic matter. So plants they are producers according to their nutrition, according to their mode of nutrition their eating habit and the animals may be consumers or the decomposers. Many of the microorganisms like bacteria are also decomposers. They also decompose or break down the dead organic matter present in the soil, in water or other places. Now we talk about the various modes of nutrition in animals. There are four major categories of animals according to their modes of nutrition. Perhaps maybe are herbivores, carnivores, omnivores and detritivores, herbivores. Herbivores are those organisms which eat up other the plants. For example, goats they eat up the leaves of different plants, the buffaloes which eat up the grass and we know that grass is also a plant, the cows which also eat up the grass and the plants. The herbivores are those animals which eat up or which acquire their energy or their food from plants. Then comes the carnivores. Carnivores are the organisms which acquire their energy or their food in the form of meat or proteins. We can call them sometimes flesh eaters. They eat up proteins, they eat up other animals. For example, the lion, they eat the deers, they eat the monkeys, they eat other animals like cows or buffaloes. The carnivores are those organisms hence which eat the meat or the flesh. Then comes the omnivores. These are the third category of animals which can eat plants and meat both. Like human being, ourselves, we, we are omnivores. We eat meat, we eat various plants like vegetables, we are omnivores. Cows, we observe that cows eat meat and the plant material also eat it. So, crows they are also omnivores like us. Then there is a category called the deteriorative ores or the decomposers. These are the animals or the organisms which eat up the dead organic matter. For example, the earthworm. Earthworm lives inside the soil in the upper layer of the soil and it eat up the dead organic matter. Break it down, break down the dead organic matter and produces its, takes out its food or we can say acquire its energy from the dead organic matter. So, there are four modes of nutrition in animals. The herbivores, the carnivores, the omnivores and the deteriorative ores. Interestingly, the animals have particular adaptations according to their mode of nutrition. As we know that flesh eaters or the meat eaters have to tear large or big pieces of meat and meat is harder. For example, lion sometimes have to break up the bones of the prey. Sometimes they have to tear bigger, larger parts of the flesh. The herbivores however have to take the grass. For example, the cows have to eat up the grass and grass is harder to chew. They do not have to tear it, but they have to grind it hardly. So, according to their mode of nutrition, animals have different types of teeth. Their dentition we can say is different from each other. If we observe the cats around us or if we have a chance to go to a zoo or if we observe the National Geographic, we can observe that the cats and the lions or the tigers, they have very large conical teeth. If you look into the diagram, in the second diagram, say canine. These are the specific teeth which are modified in the carnivores. These are very hard and long. These are the teeth which we can see on the sides of the cat or in the zoo. We call them canines. Their function is to make the pieces of the meat cheaper. This is the reason that these lions and the cats do have long canines. You can see at the first the incisors. The incisors are the cutting teeth. Those organisms which have to cut down things like for example, a rabbit. If you look at a rabbit, it have long incisors. In our cartoon films, we generally observe that the rabbit have two long teeth in front. These are the incisors because they have to cut the carrots and they have to cut other vegetables. These are the cutting teeth. Because their habit is to eat the carrots or vegetables, they have cutting teeth more advanced. And the others are, we can say, less developed. In the predators, the canines are more highly developed. In those organisms who have to chew or grind the hard plants like grass in buffaloes and in horses and in cows, they have their premolars and molars highly developed. You can see in the diagram, the other two categories other than incisors and canine, the premolars and molars. The horses, the cows, the buffaloes, they have complicated premolars and molars. These premolars and molars have wavy or curved surfaces which help these organisms to grind the hard grasses. So it means that according to the mode of nutrition, organisms have different types of teeth. There are other different adaptations in organisms. Mouth parts are very important according to the mode of nutrition. We know that frog, for example, have a very long tongue and this tongue is inverted. That is, it is attached on the base and then it goes inside out because and it is sticky because the frogs, they capture the insects and to capture the insects, they need a long tongue and they need a sticky one. When they throw their tongue on an insect, it is due to because the tongue is sticky, it is attached to the tongue and they take it back. Just like that, there are various mammals which have specific types of snouts. You may have seen in natural geographics, a pangolin. Pangolin is also called sometimes a scaly anteater. It have a long snout. It as the name says anteater, it eats up the ants, it have a long snout. It can exert its snout in the ant hole and open it up and then the ants have to come out and it eats upon them. You can look at a beautiful picture of Mekau, a big parrot. You can see its beak is very hard and long and curved because these parrots eat upon different types of fruits and hard nuts. They have to break up the nuts. So they have very hard beaks. There are different types of beaks according to the food of the organisms. We see that the beak of sparrow is different. Beak of a fish-eater bird is very, very long like that of a falcon. And beak of the parrots is different. Beak of some other bird which is for example a kingfisher which have to eat fish is very long because they have to capture fish from the water. So the mouth parts are modified according to the mode of nutrition of the organism. These mouth parts help the organism to acquire the food from the environment.