Digestion

Different trophic levels 1. Autotrophs use simple inorganic materials to manufacture complex organic compounds whereas heterotrophs consume complex organic food material. Heterotrophic Nutrition.  An important group are the saprophytes or saprobionts, which include all bacteria and some fungi.

They feed by secreting enzymes onto the food material outside the body and then absorb the soluble products across the cell membrane by diffusion. This is known as extracellular digestion. Heterotrophs with Guts In heterotrophs food is processed as it passes along the gut. These processes are ingestion, digestion, absorption and egestion In simple organisms, feeding on only one type of food, the gut is undifferentiated. In more advanced organisms, with a varied diet, the gut is divided into various parts along its length and each part is specialised to carry out particular functions. The method of digestion which an animal uses depends on its diet i.e. carnivore, herbivore or omnivore.  Carnivores Generally, carnivores consume animal tissue which is similar to their own; therefore all the body needs to do is break down the tissue and absorb the different components which can then be used in the carnivores own body.   Because meat is easily digested, the gastric system of carnivores is typically short and simple. They are monogastric meaning they have only one stomach (unlike a ruminants’ stomach which has four chambers). Due to the ease at which components required for growth are obtained from food, some carnivores have lost the ability to synthesis them (e.g. cats are unable to synthesis taurine). Due to the lack of salivary enzymes, food spends little time in the mouth of a carnivore, it is shortly swallowed and travels down the oesophagus. The oesophagus is a tube which runs from the pharynx (back of the oral cavity) to the stomach. The next stop is the stomach, the stomach has multiple roles in digestion, including: A reservoir for food A sterilising chamber, due to the low pH (high acid content – HCl) A churning chamber to mix food with digestive gastric juices a The initial site of protein digestion, primarily by pepsin – secreted by the epithelial lining of the stomach Food is moved to the next site of digestion, the small intestine, by peristalsis. The small intestine is a long and narrow ‘tube’ with a structure and epithelium that maximises surface area. This is important because the small intestine is the primary site of digestion by enzymes. Food continues to travel along the small intestine by peristalsis. The small intestine can be divided into the duodenum, jejunum and the ileum. The pancreatic duct connects the pancreas to the duodenum – the majority of the digestive enzymes enter the small intestine by this duct. To aid in lipid digestion, bile is secreted by the liver (stored in the gallbladder). Bile emulsifies lipids which gives them a larger surface area, increasing enzyme efficiency. The small intestine joins to the large intestine, which consists of the caecum, colon and rectum. In carnivores the caecum has no function (as it is used in herbivores/omnivores as a site of bacterial fermentation of plant matter). The colon absorbs minimal nutrients from the ingested food; instead its primary role is the reabsorption of water, vitamins and electrolytes from the mixture of food, saliva and gastric & pancreatic juices passing through. This prevents excessive water loss and therefore dehydration. The remnants are excreted via the rectum and anal sphincters. Herbivores Herbivores consume plant matter which is more difficult to break down than tissue. Therefore herbivores have evolved fermentation systems which contain specific microflora, the microflora breakdown the plant material releasing useful nutrients which the herbivore utilises.  Herbivores only consume plant material which is very difficult to digest. No vertebrates make an enzyme capable of breaking down cellulose, the tough sugar that makes up plant cell walls which is unfortunate as its digestion yields glucose. As the diet includes large amounts of fibre the digestive tract of herbivores is comparatively much longer than carnivores, due to fibre being much more difficult to digest. To overcome this herbivores have developed a symbiotic relationship with a population of microflora that inhabit a specialised region of the gut for fermentation e.g. the caecum or rumen of ruminants. The microflora population of the gut is able to breakdown cellulose and use the glucose for its own metabolic needs. As a waste product of this process, the microflora population releases volatile fatty acids (e.g. acetate, butyrate & propionate) which the herbivore utilises for energy. The production of these fatty acids is known as fermentation (fermentation also produces heat which keeps the animal warm) Omnivores have very similar digestive systems to carnivores with the addition of a caecum.

Ruminants such as cow and sheep eat mainly grass, a large proportion of which consists of cellulose cell walls. Ruminants have a specialised stomach or rumen in which mutualistic bacteria live. The presence of these bacteria together with their modified gut enables ruminants to achieve a more complete breakdown of cellulose. Click here to see a video of all the different ruminants.

Adapatations in Heterotrophs for different diets.

Dentation

Teeth are used in mechanical digestion in order to increase surface area for enzyme action. Mammals have evolved different types of teeth with each type being specialised for a different function, incisors, canines, premolars and molars. There are differences between the teeth of carnivores and herbivores reflecting their differing diets. The teeth of carnivores are sharp and strong, this makes it easy to rip and tear meat from bones of prey. When possible, the meat is broken down further by the teeth to ensure maximum surface area for digestion by enzymes in the stomach and small intestines. True carnivores do not have digestive enzymes in their saliva. In herbivores the jaw moves in a horizontal plane whereas in carnivores the jaw moves vertically.