One of the interesting Biology Topics is the study of animal behavior and how it is influenced by genetics and the environment.
Nutrition and Digestion In Grass Eating Animals – Ruminants
The herbivorous animals such as cattle (cows, buffaloes), goat, sheep, deer, antelope and giraffe, etc., eat mainly grass and other plant leaves as food. Grass is rich in a carbohydrate called cellulose (which acts as a source of energy). Cellulose carbohydrate is a quite tough material so ‘more times’ it is chewed by the animal, the better.
The cellulose carbohydrate present in grass can be digested by the action of certain bacteria which are present only in the stomach of animals called ruminants like cattle (cows, buffaloes), goat, sheep, deer, antelope, giraffe, etc. Thus, in herbivorous animals (grass eating animals), the stomach is adapted to digest the food like grass and leaves. We will now describe the digestion in grass-eating animals like cow in detail.
Digestion In Grass-Eating Animals
The animals (such as cow) which eat grass have a special stomach to digest the tough cellulose carbohydrate present in grass. The stomach of a cow is large and consists of four compartments (or four chambers). The first compartment of a cow’s stomach is the biggest and it is called ‘rumen’ (see Figure). When cow eats grass as food while grazing, it does not chew it completely (because it has no time for it). The cow swallows the grass quickly with little chewing and stores it in the rumen (which is the first compartment of stomach). The rumen contains cellulose digesting bacteria. The bacteria present in the rumen of a cow start to digest cellulose carbohydrate present in grass food. Thus, the grass is partially digested in the rumen. The partially digested food (or partially digested grass) in the rumen of a cow is called cud.
After some time, when the cow is resting, the cud from rumen is brought back to the mouth of cow in small amounts at a time. The cow now chews the cud thoroughly. This is why many times we see a cow (or buffalo) moving its jaws from side to side and chewing continuously even when it is not eating grass. The process by which the cud (partially digested food) is brought back from the stomach to the mouth of the animal and chewed again is called rumination (or chewing the cud). All the animals which chew the cud are called ruminants.
When the cud is thoroughly chewed in the mouth of the cow, it is swallowed again. But this chewed cud does not go back to rumen. The thoroughly chewed cud now goes into the other compartments of the cow’s stomach and then into the small intestine for complete digestion and absorption.
The ruminants (like cow) cannot chew their food (grass) completely at the time when they eat it because they have very short time available for grazing grass. So, they want to store as much grass as possible in their stomach in the short time available to them.
The animals called ruminants can survive on grass as food because they have certain bacteria in the,.rumen part of their stomach which can digest the cellulose carbohydrate present in grass (and other plant leaves). The cellulose digesting bacteria are not present in the body of human beings due to which human beings cannot digest cellulose carbohydrate present in plant foods.
We (human beings) cannot survive by eating only grass (or raw leafy vegetables) because these foods contain mainly cellulose carbohydrate and our body does not have bacteria which can digest cellulose carbohydrate. So far we have discussed the digestion in human beings and other animals which possess digestive system (made up of a number of organs working together). There are, however, many small animals which do not have a mouth and a digestive system. One example of an organism having no fixed mouth and no digestive system is a tiny animal called Amoeba. We will now discuss how such organisms take in food (ingest food) and digest it.
Amoeba is a microscopic organism (very small organism) which consists of a single cell (see Figure). Actually, Amoeba is the simplest animal. Amoeba lives in pond water. Amoeba is so small that it can hardly be seen with naked eyes. The structure of Amoeba given in Figure is as seen through a microscope. Amoeba has a cell membrane, a round, dense nucleus, and many small bubble-like vacuoles in its cytoplasm.
The vacuoles in Amoeba are of two types : food vacuoles and contractile vacuoles. A food vacuole consists of a bubble of water containing the food particle captured by Amoeba. The contractile vacuole is a bubble of liquid. It controls the amount of water present in the body of Amoeba. The most important feature of Amoeba is that it has no fixed shape. The shape of Amoeba keeps on changing constantly.
The shape of Amoeba changes because it can make its cytoplasm flow iri any direction it wants to. The body of Amoeba has finger-like projections which are called pseudopodia (or false feet). Amoeba can produce pseudopodia on any side by pushing the cytoplasm in that side. Amoeba moves very slowly with the help of pseudopodia (which keep on appearing and disappearing when it moves). Amoeba also uses its pseudopodia to catch food particles from the surroundings.
Feeding and Digestion in Amoeba
Amoeba eats (or feeds on) tiny plants and animals present in pond water where it lives. Amoeba takes in food (or ingests food) by using pseudopodia. When a food particle comes near Amoeba, then Amoeba produces two pseudopodia around the food particle and surrounds it [see Figure (a)]. The two pseudopodia then join around the food particle and trap the food particle with a little water forming a food vacuole inside the Amoeba [see Figure (b)], The food vacuole is like a temporary stomach of Amoeba. The food is digested in food vacuole. This happens as follows : The surrounding cytoplasm secretes digestive enzymes into the food vacuole [see Figure (b)].
The digestive enzymes break the food particle into simpler substances. In this way, the food gets digested. The digested food present in food vacuole is absorbed directly into the cytoplasm by the process of diffusion [see Figure (c)]. After the absorption of food, the food vacuole disappears. The absorbed food is used for the production of energy, maintenance and growth of Amoeba [see Figure (d)], Amoeba has no fixed place (like anus) for removing the undigested food. When a considerable amount of undigested food collects inside Amoeba, then its cell membrane suddenly ruptures at any place and the undigested food is thrown out (or egested) from the body of Amoeba [see Figure (e)].
A similarity between the nutrition in Amoeba and human beings is that in both the cases digestive juices break down the complex food particles into simpler substances (which can be absorbed). A difference is that Amoeba has no mouth and no digestive system whereas a human being has a mouth and a digestive system made up of many organs.