Digestion in Animals PDF

Digestion in Animals At the end of the lecture, students should be able to: Define digestion Distinguish among intracellular, extracellular and external digestions and give examples in animals. Take note of structural modifications of the digestive tracts, of selected animals,...

Digestion in Animals At the end of the lecture, students should be able to: Define digestion Distinguish among intracellular, extracellular and external digestions and give examples in animals. Take note of structural modifications of the digestive tracts, of selected animals, that enhance digestion. State the functions of various enzymes in carbohydrate, protein and fat digestions.  Digestion in Animals Animal food consists of organic material, most of which belong to proteins, fats and carbohydrates. Organic compounds dominate the makeup of virtually all plants and animals. Digestion: A process by which large and complex molecules in food are broken up with the aid of enzymes so that they become absorbable and available for use in the body. Can take place within cells (intracellular), outside cells, but within the body (extracellular) and outside the body (external).  Digestion in Animals External digestion (digestion outside the body) E.g. Spiders pierce prey with their hollow jaws and pump digestive juices into prey’s body (often large and usually covered with hard chitin). Tissues are liquefied and the spider sucks the prey empty. Intracellular digestion Examples include Digestion in digestive vacuoles in protozoans Sponges and to some extent turbellarians (intra and extracellular digestions). Intracellular and extracellular digestions take place in a number of more complex organisms. - E.g. Clams  Digestion in Animals Extracellular digestion Advantage: permits ingestion of large pieces of food. Usually associated with a well-developed digestive tract or gut that allows secreted enzymes to act on food. Digestive tract may have one opening as in flatworms. Digestive tract is tubular In more complex animals it has two openings, extending from the mouth to the anus or cloacal opening.  Digestion in Animals Extracellular digestion Digestive tract Made up of buccal cavity (mouth), pharynx and alimentary canal. Alimentary canal is divided into the esophagus, stomach, small and large intestines. In most vertebrates, alimentary canal ends in a cloaca, a terminal chamber that receives: - faecal materials from the intestines and - products of urogenital tract.  Digestion in Animals – Extracellular digestion Digestive tract – Structural modifications E.g. Planarian flatworms (Class Turbellaria) Gastrovascular cavity is widely branched, increasing its total surface area for: - transport of food to all parts of the body. - Extracellular digestion. Smaller food particles are digested intracellularly. biologyjunction.com - Extensive branching also aids in absorption of digested food.  Digestion in Animals – Extracellular digestion Digestive tract of amphibians – structural modifications Most amphibians swallow prey whole without much chewing so they possess voluminous stomachs. Short oesophagus lined with cilia help move food to stomach. Mucus produced by glands in mouth and pharynx eases food passage. Chitinase produced in stomach helps digest chitinous cuticle of arthropod prey (Dorit et al., 1991) tutorvista.com Coiled small intestine and short straight large intestine that empties into a cloaca.  Digestion in Animals – Extracellular digestion Digestive tract of birds – structural modifications Birds have a crop, special storage organ, at the base of oesophagus. Proventriculus secretes acid (HCl) and enzymes e.g. pepsin to start digestion. Gizzard (ventriculus) has very strong muscular wall. Contains grit swallowed by bird. - Grit along with muscle breakinggroundagscience.wordpress.com movement grinds hard foods such as cereal grains.  Digestion in Animals – Extracellular digestion Monogastric organism: Has a simple single-chambered stomach. Examples include: - omnivores: e.g. humans, rats and pigs. - carnivores: e.g. dogs & cats, and - herbivores: e.g. horses and rabbits. Hindgut fermenters - monogastric herbivores that can digest cellulose nearly as well as ruminants. - Colonic fermenters: tend to be larger species such as horses and rhinos. - Caecal fermenters: are smaller animals e.g. rabbits & rodents.  Digestion in Animals – Extracellular digestion Monogastric organism: Hindgut fermenters - Enlarged large intestine and caecum (functional caecum), occupied by cellulose digesting micro- organisms. Non-ruminant herbivores. E.g. Horse, rabbit, rodents.  Digestion in Animals – Extracellular digestion Digestion in Herbivores Plants not easily digested. Thus, herbivores eat large quantities to obtain all they require. They have teeth adapted to crush and grind. Guts tend to be lengthy. Herbivores digest cellulose via microbial fermentation by way of symbiotic gut bacteria. - Energy extraction from cellulose digestion is less efficient than in ruminants.  Digestion in Animals – Extracellular digestion Ruminant Has four-chambered complex stomach. Has cud-chewing behavior. Cud is a regurgitated food bolus.  Digestion in Animals – Extracellular digestion Digestive tract – structural modifications Ruminant About 150 different species including cows, sheep, goats, deer, antelope, buffalo and giraffe. http://farmingwithaflare.blogspot.com/2011/06/moose -on-loose.html Rumen: - large storage space for food quickly consumed. Also a large fermentation vat. - contains bacteria & protozoa (in billions) that carry out initial digestion of cellulose by anaerobic fermentation - Cellulose: glucose polymer, extremely insoluble, main energy source for many thekebun.wordpress.com vertebrates, most important structural material of plants. Fibrous feeds e.g. grass, hay,  Digestion in Animals – Extracellular digestion Ruminant microbial-flora-and-feed-fermentation-7-638.jpg (638×479) (slidesharecdn.com)  Digestion in Animals – Extracellular digestion Digestive tract – structural modifications Ruminant Rumen - Fermentation products include acetic acid, propionic acid and butyric acid are absorbed. CO2 and methane escape by belching. Reticulum: close to rumen and their contents mix. Fluid helps separate particles, contraction sends larger particles to rumen & finer ones to omasum. Omasum – relatively little, if any, digestive activity. Water absorbed. Abomasum – "true stomach". - Similar in function to the stomach of non-ruminants. - secretion of enzymes and acids to breakdown nutrients.  Digestion in Animals – Extracellular digestion Carnivorous animals Examples: animals in the cat and dog families, polar bears, seals, crocodiles and birds of prey. Usually have specialized teeth. Have sleek bodies, strong, sharp claws and keen senses of smell, hearing and sight. Often cunning, alert and have aggressive nature. Meat provides a very concentrated source of nutrients. Guts are usually shorter and less complex than those of herbivores, as meat is easier to digest than plant material.  Digestion in Animals – Extracellular digestion Omnivorous animals Feed on both animal and vegetable material. Food is diverse. Examples: humans, rats, pigs, few primates e.g. chimpanzees, belong to this category. Lack the specialized teeth and guts of carnivores and herbivores.  Digestion in Animals – Extracellular digestion Digestive tract – structural modifications Simple-stomached animals E.g. humans, dogs or pigs Human digestive system ww2.valdosta.edu  Digestion in Animals – Enzymatic digestion Protein Digestion Protein is a polypeptide, a linear polymer of many amino acids, linked by peptide bonds. rpi.edu Protein-digesting enzymes are grouped into two. - Exopeptidases: hydrolyse terminal peptide bonds in a long peptide chain. E.g. Carboxypeptidase (secreted by pancreas), aminopeptidase (secreted by intestine) and dipeptidases (from enterocytes) in intestine, colon & appendix.  Digestion in Animals – Enzymatic digestion Protein Digestion - Endopeptidases: act in the interior of the peptide chain. E.g. Pepsin and trypsin. - Vertebrate stomach secretes pepsin as pepsinogen (inactive precursor), which is activated in acidic medium (HCl secreted by stomach). - Trypsin is secreted by pancreas as trypsinogen, which is activated in the intestine by enterokinase and active trypsin (autocatalytic activation).  Digestion in Animals – Enzymatic digestion Fat Digestion Similar in vertebrates and invertebrates. Vertebrate pancreas secretes fat-hydrolysing enzyme lipase. With the aid of bile acids (secreted by liver) and mechanical intestinal movements, fats are emulsified. Fatty acids are then kept in solution with the aid of bile salts and can be absorbed. Carbohydrate Digestion Large number of plants store starch (relatively insoluble & polymer of glucose units) as a major energy reserve. Amylase is secreted in saliva of humans and some other animals. Larger amounts are secreted by pancrease.  Digestion in Animals – Enzymatic digestion Carbohydrate Digestion Amylase easily attacks heated (cooked) starchy foods e.g. potatoes, bread etc. Cellulose: a complex carbohydrate, or polysaccharide, extremely insoluble glucose polymer and the most important structural material of plants. Many vertebrates - depend on cellulose as their main energy source, but rely on symbiotic microorganisms in digestive tract to digest it in a process called fermentation. - Cellulases are absent from their digestive secretions. - Examples: ruminants and many nonruminant mammals.  Digestion in Animals – Enzymatic digestion Carbohydrate Digestion Products of fermentation mostly organic acids such as acetic, propionic & butyric acids are absorbed and utilized in oxidative metabolism. By-products of fermentation: carbon dioxide (CO2) and methane (CH4) escape by eructation (belching). Microbial fermentation may occur in - specialized stomachs (gastric fermentation) e.g. Rumen in ruminants or - in the intestine (caeca) as in non-ruminants (intestinal fermentation).  Digestion in Animals – Enzymatic digestion Carbohydrate Digestion Some invertebrates also rely on symbiotic microorganisms for cellulose digestion. - E.g. Intestinal tract of wood-eating termites is virtually crammed with several kinds of flagellates and bacteria. - Symbiotic flagellates (obligate anaerobic organisms) from termites are selectively killed on exposure to oxygen at 3.5 atmospheric pressure, termites survive unharmed. - Defaunated termites cannot survive when fed on wood indicating that flagellates are responsible for cellulose digestion.