Year 11 Biology - Digestion in Herbivores

Why do you think bacteria are needed to break the cellulose down? There are no enzymes present in herbivores (or humans). While humans can’t digest cellulose it is still a really important part of our diet and is called dietary fibre and bulks up our faeces. It is really good for our digestive system. About 1/3 of the carbohydrates in plants is cellulose. It is the main component of cell walls. What is cellulose? It is a polysaccharide; a very long chain of over 10,000 glucose units. This probably sounds similar to starch. They are both polysaccharides made only of glucose. However their differences arise in the orientation of the glucose molecules. In starch they are all in a straight line but in cellulose they are rotated. How do the bacteria break down the cellulose? They produce enzymes called cellulases which are able to break down the cellulose. Some termites and fungi can also produce cellulases. How is this a symbiotic relationship? The host provides the food and “accommodation” for the bacteria and then they get the byproducts of fermenation (including fatty acids and sugars). Fermentation – organic compounds (such as carbs) are oxidised. (lactic acid produced, anaerobic)Carbohydrates – our main source of energy. Broken down into glucose and what is not used is stored as glycogen (fat).Fats – another source of energy. Higher energy content than carbs. Longer to break down?Proteins – used for growth and repair of muscle and tissues. Plant matter is fairly low in protein and fat and the majority of nutrients are located within tough cell walls.
Let’s review the path that food travels when it enters the body. While the food doesn’t actually pass through the liver, gall bladder or pancreas these organs still play an important role. Liver – absorbs nutrients from the small intestine. Gall bladder – makes and stores bile then releases it into the duodenum to help digest fats. Pancreas – secretes digestive enzymes into the duodenum.
Adaptation – structure, function or behaviour that improves their chances of survival. These adaptations take place in the digestive system.
Where does absorption take place? Small intestineSo, by the time that the food is broken down into small enough units to be absorbed it has already passed through the small intestine. These animals then need to eat huge amounts of food to get enough energy. They spend about ¾ of their day eating. They are called grazers. This also means that they are at more risk to predators as they have to keep their heads down while they are eating. They excrete lots of undigested food and will sometimes eat their own wastes. Koala’s have an really long caecum – about 2m in length. This is because their diet is almost all eucalyptus leaves which are basically indigestible to other animals and have very little nutritional value. They need the long caecum in order to digest these leaves. The caecum alters the state of the leaves as to produce digestible nutrients.
When chewing the cud occurs lots of saliva is added to the cud. In sheep and goats this is about 10 or 15L per day. In cows its 200L per day!The fourth component is called the true stomach and is the equivalent of the human stomach. This is where gastric juices are secreted and some of the cows own enzymes break down the food. The bacteria break down the cellulose into sugars and fatty acids. But they also synthesize vitamins, detoxify plant poisons, and recycle nitrogenous compounds into proteins - all actions which help the microbes survive. Unfortunately for the microbes, as fluids are selectively filtered out of the fermentation chambers, the microorganisms are also carried to the stomach to be digested: ruminants actually derive the majority of their nutrients from microbes, not directly from plant material!
When the cellulose is broken down not all of the nutrients go to the host. A lot go to the bacteria. Such as the sugar which is produced from the fermentation. The host absorbs fatty acids produced by the bacteria, which are then transformed into glucose by the liver. Protein in the food is digested by enzymes of the bacteria. Some of the amino acids are absorbed through the wall of the reticulum (second compartment) but most as absorbed by the bacteria for growth and reproduction. The bacteria eventually die and move into the fourth compartment where they are digested and become an important source of amino acids for the host. Bacteria also digest some food in the colon of ruminants.
Cellulose is the most common organic compound on Earth. The bacteria make many vitamins (particularly group B) which are able to be absorbed by the host animal. The ruminants need to get vitamins A and D in their diets.
Muscle = protein. Specific enzymes. The Tasmanian Devil primarily eats dead animals. It has really strong jaws and sharp teeth and is able to eat every part of a dead animal, including the skull. Protein is much easier to digest than cellulose. They therefore have a much shorter digestive system and a reduced or small caecum. Humans are omnivores. We eat both meat and plants.
The nutrients that are obtained from our food absorbed into the blood stream and then taken to all of the cells. They are then used in the process called cellular respiration. This is occurring continuously. This process uses the nutrients such as glucose and combines it with oxygen to form energy. This is why we expire CO2. All of this takes place in the mitochondria. The type of energy that we can use in the body is called ATP.

Year 11 Biology - Digestion in Herbivores

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Year 11 Biology - Digestion in Herbivores

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