Sonoma State University
Department of Biology - Hanes
Animal Physiology
Chapter 15: Feeding, Digestion & Absorption
Feeding MethodsEndocytosis
Filter Feeding
Fluid Feeding
Seizing Prey
GrazingAlimentary Systems
Digestive Enzymes & Hormones
Nutritional Requirements
Schemes for cellulose & vitamins
Application
Discussion
Reflection
Endocytosis occurs when individual cells take up food directly. Protozoa take in nutrients by diffusion across the cell membrane and by encapsulating food in cell membranes called food vacuoles that merge with lysosomes. Porifera have ameboid cells that feed similarly and then seem to wander - passing out nutrients? Arthropods have specialized cells in the green gland that are ameboid and aid in digestion. Although vertebrates digest mainly in their hollow gut, absorbing by diffusion or active transport, we do have a few blood related cells that are still very ameboid in their feeding.
Animals from protozoa to whales can be filter feeders. By definition, these animals select food based on size. A good example is found in bivalves which use their gills to filter passing water. Solid material sticks to a mucous track and is carried to the mouth by cilia where it is ingested.
An external type of digestion is sucking or fluid feeding. A wide variety of animals and insects are adapted to take in liquid food, either from plants or animal juices. All spiders use sucking as their digestive means. They inject digestive enzymes into the prey and allow the exoskeleton to contain the juice as internal body parts are liquified.
Seizing food and Trituration refers to the technique of breaking up food mechanically. The term is derived from grinding corn to a powder. Animals may make their prey smaller by chewing, picking it apart, etc... Teeth, claws, gizzards, etc. may be used to subdivide food.
Grazing as done with a radula as in molluscs and grinding tough plant food with molars.
Most alimentary canals contain at least some of the following divisions:
The digestive tract is imbued with an evolutionary ancient nervous system called Auerbach's plexus. It is a network of individual nerve cells that have synapses in passing much like hollow bodied animals like the sea anemone. Neurotransmitters from these cell are more related to brain and spinal cord neurotransmitters than those we are more familiar with in the periphery. The following, while being listed as hormones, are most likely synthesized by sensory nerve cells serving the enterogastric mucosa. They most likely influence other, motor nerve cells that also serve the enterogastric mucosa. It is common in this type of nervous system to release neurotransmitters in the general area of other cells and not onto receptors directly. In this way, the chemicals can be considered neuro-endocrine in nature.
Gastrin - Polypeptides or alcohol in the pyloric stomach stimulate its release. Gastrin, in turn, stimulates hydrochloric acid secretion from oxyntic cells in the fundic stomach. A pH below 2 in the pyloric stomach will halt the process. This is a beautiful example of negative feedback.
Cholecystokinin (CCK) - Fats or succus entericus in the duodenum stimulate its release from the intestinal mucosa. It causes the release of pancreatic enzymes and contraction of the gall bladder.
Secretin - Acid in the duodenum stimulates its release from the intestinal mucosa. Secretin causes HCO3- secretion from Brunner's glands, pancreas, and synthesis of bile in the liver. Secretin also inhibits gastric motility slowing emptying of the stomach. An "Oh my" - secretin was the first hormone ever discovered. As I recall it was discovered by Baylis and Starling, two English physiologists.
Gastric Inhibitory Polypeptide (GIP) - Monosaccharides and fats in the duodenum stimulate its release from the intestinal mucosa. GIP inhibits gastric secretions and motility and stimulates Brunner's glands. Slowing the emptying of the stomach via control of motility and slowing the opening of the pyloric sphincter allows more time for fat digestion which is slower than digestion of other food groups.
Salivary secretions are under the control of the autonomic nervous system. Sympathetic stimulation causes the release of mucous with little fluid giving one a "dry" mouth. Thought or taste of food stimulates the parasympathetic system to cause the release of copious fluid, HCO3-, mucous, and the enzyme alpha-amylase which breaks off maltose and malto-triose units from starches.
The gastric secretions of mucous (Goblet cells), HCl, and the enzyme pepsin are under the control of both the autonomic nervous system and hormones listed above. Pepsinogen is synthesized in chief cells found in the lower reaches of gastric glands. Contact with HCl chops off a few amino acids and converts pepsinogen to pepsin. Pepsin is an endoprotease that breaks peptide bonds that are next to aromatic amino acids. HCl is under nervous and hormonal control (histamine also stimulates its release). It is synthesized and released by oxyntic or parietal cells and made from NaCl, CO2, and H2O. HCl goes to the stomach and NaHCO3 is released into the blood to be later released by the pancreas, liver, and Brunner's glands. Infants of mammals also release the enzyme rennin which especially helps to break down milk protein. This ability is lost as the infant matures.
Pancreatic juice is of two types. Secretin causes a HCO3- rich secretion that is poor in enzymes. CCK on the other hand causes a rich mixture of proenzymes to be released. Trypsinogen is converted to trypsin by trypsin and enterokinase (an enzyme synthesized and released by intestinal mucosal cells). In this way, conversion normally does not take place until the proenzymes or zymogen reach the gut lumen. Trypsin breaks peptide bonds on the carboxyl side of dibasic amino acids (arginine & lysine). Chymotrypsinogen is converted to chymotrypsin by trypsin. It breaks peptide bonds on the carboxyl side of the aromatic amino acids (phenylalanine, tyrosine, and tryptophan) and leucine and methionine. In a similar manner, other enzymes are activated in the intestine including: carboxypeptidases (break off the carboxyl terminal amino acid), aminopeptidases (break off the amino terminal amino acid), lipase (breaks off the outside fatty acids from glycerol and eventually the inside one), alpha-amylase is also secreted by the pancreas, and there are others as elastase and nucleases (deoxy- and ribo-).
Bile, synthesized by the liver, contains HCO3-, bile salts and bile pigments. The pigments are waste products from heme destruction and are used to color eggs and feces. Bile salts are detergents that are necessary to keep fats in small globules so that the surface area is great and they can be attacked by water soluble lipase. The bile salts are derived from cholesterol.
Succus entericus is the secretion of the intestine. It amounts to about 6 liters/day. It comes mostly from the mucosa in Crypts of Lieberkuhns and is alkaline. Enzymes are added to it from the death of cells at the tops of villi as they are scrapped off and release their lysosomes. New cells are constantly being made in the Crypts which migrate up the villi. The enzymes include peptidases, nucleotidases, nucleosidases, and disaccharidases such as maltase, lactase, and sucrase.
Some alcohol can be absorbed from the stomach, but most absorption takes place in the small intestine. It is here that we have absorption of amino acids, minerals, glucose and other monosaccharides, monoglycerides, glycerol, fatty acids, etc... Many of these use mucosal cell membrane transport molecules to speed their uptake. Water soluble molecules are absorbed into the blood stream which drains to the liver via the hepatic portal vein. The liver has a chance to modify chemicals before they enter the general circulation. Fats and fatty acids are resynthesized into triglycerides and phospholipids and accumulate in interstitial spaces where they are picked up by the lacteals and eventually find their way to the blood via the subclavian lymphatic drainage. The fats are mostly in the form of chylomicrons which are very low density, meaning that they have little protein in the particle. Chylomicrons are picked up by liver cells which have receptors for them and they are processed into low density lipoproteins which are picked up by fat cells among others. Fat cells release high density lipoproteins when called upon to do so. The large intestine absorbs minerals and water primarily, compacting the feces and recovering much of the succus entericus.
The greatest of these is water.
Minerals are often divided into major and minor categories depending on the amount required. The major minerals follow with an example of their uses:
Minor minerals include the following:
Vitamins have become well known because of the money that can be made in promoting their use. An average American diet contains lots of them and the only time a person should supplement these is during pregnancy, illness, and childhood. Vitamins are defined as organic compounds required in the diet in minute amounts or a specific deficiency syndrome will develop. Thus, they have little in common with each other chemically. They are all coenzymes, often complex chemicals. Synthesizing them would require a number of enzymes that humans do not have. Still, the vitamin requirements of different animals are very different. For instance, rats do not require vitamin C; they can synthesize it. It is my contention that these chemicals are so universal in plant and animal life; it is an advantage for an animal to save its DNA to make other things and get vitamins through diet. Some of the fat soluble vitamins can be harmful in large amounts. High concentrations of vitamin A make polar bear liver poisonous. A pregnant woman taking vitamin supplements and drinking lots of milk and sunbathing could over dose on vitamin D.
The following are lipid soluble vitamins with their major functions:
The following are water soluble vitamins with their related functions:
For the most part, fats and lipids are already well represented in our diets. We do have a dietary requirement for some special fatty acids. These are known as the omega 6 and omega 3 acids. The numbers refer to the position of the first double bond counting from the non-reactive or omega end of the molecule. It seems that we do not have enzymes that can put double bonds in these positions, but once we have one acid of these types, we can lengthen or shorten the chain from the alpha end. These are so ubiquitous in our diets that it is difficult to detect a deficiency. New born babies who have obstructed digestive tracts are kept alive with I.V. feeding. If the I.V. nutrients do not contain these essential fatty acids a deficiency will develop. These fatty acids are common in structural phospholipids in cell membranes.
You have obviously heard of saturated fats (those that are saturated with hydrogen) and unsaturated fats (those with double bonds between carbons). We can make omega 9 unsaturated fats like oleic acid. Polyunsaturated fats have more than one double bond. The double bonds are always 3 carbons apart from each other. At each double bond the carbon chain has a permanent bend in it, so that if four double bonds are present, the tail nearly reaches back to the head. We will find later that the unsaturated fats in cells membranes are often broken down and used for messenger molecules either in cells or between cells. Lipids are organic molecules that dissolve in organic solvents. Fats are molecules that have a fatty acid moiety. Most fatty acids are found in triglycerides or phospholipids. Phospholipids have glycerol, a phosphate group or an organic phosphate group, and two fatty acid moieties. All fats produce about 9 Kcal/g when burned, nearly twice as much per gram as other fuels. Unfortunately most flavors are found in fats.
Starches and sugars, carbohydrates, are all treated in the same manner by the body. They are essential for normal metabolism and should make up the bulk of the diet. Some are absorbed more quickly than others, but it is not as simple as sugars versus starches. The most common monosaccharides are glucose, fructose, and galactose. Some common disaccharides are sucrose (glucose + fructose), lactose (glucose + galactose), and maltose (glucose + glucose). Some common polysaccharides are amylose (many plant starches) and glycogen (a common animal starch). Starches and sugars produce about 5.0 Kcal/g. Cellulose is a structural polysaccharide composed of glucose moieties linked by ß linkages. You should know what this means. Chitin is a polysaccharide composed of aminoglucose moieties.
Proteins are composed of amino acids. There are many types of amino acids, but essentially all living beings only use 20 types to make protein. In humans, 10 of these are considered essential and are necessary in the diet. Others can be made from sugars or from other amino acids. When using amino acids for fuel, ammonia (quite toxic) is produced and must be excreted or metabolically changed. High protein diet supplements may upset mineral balances as they are most often salts of proteins. Amino acids are joined into proteins by peptide bonds (amine to carboxyl). Protein gives off about 4.5 Kcal/g.
Ruminants, such as cows, goats, deer, etc., have digastric stomachs. These may have varying numbers of compartments, but two that are physiologically different. They survive on nutritionally poor quality food. It has been shown that a cow can survive on vitamin A, urea and newsprint. These animals may eat fast and swallow relatively unchewed material that enters a stomach compartment. The compartment does not secrete digestive enzymes, but rather secretes sodium bicarbonate and urea. Bacterial and protozoan commensuls live in the compartment and produce digestive enzymes including cellulase. The bacterial digestion produces acids, methane, and CO2 as well as more micro-organisms. The bicarbonate keeps the compartment from becoming too acid and the urea not only helps clear the blood of a waste product, but also acts as a source of nitrogen for the micro-organisms to make amino acids. This mixture may be brought up and chewed thoroughly. It then passes to other compartments and eventually to the abomasum or true stomach that produces pepsin and HCl. By this time much of the food has been converted to short-chain fatty acids and micro-organisms. The cow garners its sustenance from the micro-organisms.
Rabbits also live on poor quality food. They have large caecae in which bacteria are allowed to proliferate. At night, they will produce a different type of fecal pellet from the caecum that is about 95% bacteria. They will eat this pellet directly from the anus and thus acquire vitamins and protein. This is called coprophagia. Rats also practice coprophagia and will develop vitamin deficiencies if not allowed access to their feces.
: (Don't forget those at the end of the chapter.)
What are some specific uses of specific minerals and vitamins?
Know the stimulus, originating tissue, hormone, effect of the
hormone, site of activity of the hormones mentioned.
Know the same for enzymes.
Describe the anatomy and physiological adaptions necessary for
ruminants.
Be able to trace the important events that are triggered by and
happen to fats, proteins, and lipids during digestion.
Explain the advantages of coprophagia, rumination, trituration.
Once the most often prescribed drug was TAGAMET a blocker of the H2 histamine receptor. Why would you guess?
If we cover this, what were some of the definitive experiments that proved the existence of secretin & gastrin. How do we know they are hormones? How do we know that they act via the nervous system for both release and effect?