This web page contains notes to accompany lectures in Vertebrate Physiology, Biology 410, taught by Dr. Peter King in the Department of Biology, Francis Marion University, Florence, South Carolina, 29502, USA.

All vertebrates are heterotrophs.
i.e. they must ingest organic compounds and reorganize them internally into the compounds they need.

It is important that the energy spent finding, ingesting and digesting food is less than that provided by the food.
Some small birds and mammals need constant food supplies to maintain their high metabolic rates.
e.g. hummingbirds
Some snakes only need to feed a few times per year.
e.g. large rattlesnake

Most vertebrates swallow large chunks of food.
Prey is either swallowed whole or torn apart and swallowed.
Chewing or mastication is only found in mammals.
Mammals have specialized teeth (heterodont dentition) that have different functions.
Molars and premolars are designed to cut and grind food before it is swallowed. Canines are enlarged in carnivores and are used to catch prey. Incisors cut food.
Fish, amphibians and reptiles generally have homodont dentition.
Turtles and birds have no teeth.

For some vertebrates digestion starts before ingestion.
Snake venom is the product of modified salivary glands.
Envenomation starts the digestive process.
Some snakes strike and release their prey.

How do rattlesnakes find their meal after it runs away?
They track the chemical trail of their own venom using their vomeronasal organ.

Diets can be very specialized.
This is often for reasons other than physiology e.g. resource partitioning to reduce competition or adaptation to utilize a common resource.
Hognose snakes usually only eat toads.
Often coevolution between predator and prey takes placed as evident by specialization.
i.e. hognose snake tolerant of toad skin secretions.
young frogs have greater resistance to cottonmouth venom than adults.

Broad categories
carnivores
insectivores
piscivores
herbivores
nectivores
carion feeders
omnivores

Animal food consist mainly of
proteins
lipids (fats)
carbohydrates

Digestion
Vertebrates can not be absorbed food directly into their bodies.
Even if liquified to avoid physical breakup, the molecules of proteins, lipids and carbohydrates are too big to transport across cell membranes in the digestive tract.

These three food groups are all polymers and can be broken down by enzymes into smaller subunits by a hydrolysis reaction.
Enzymes are biological catalysts.

Enzymes facilitate a chemical reaction without being directly involved.
i.e. AB + enzyme -> A + B + enzyme
Enzymes often end in -ase and are descriptive of the molecules they act on.
i.e. protease, lipase, endopeptidase, carboxypeptidase, amylase.
Many proteases are stored in the body in inactive forms called zymogens.

Activity of enzymes are effected by
pH
temperature.

Each enzyme has an optimal range for activity.

Proteins
Proteins are made up of amino acid monomers (20 possible) joined together by peptide bonds.
Primary structue - list of AAs
Secondary structure - planes or helices
Tertiary - three dimensional shape
Quaternary - more than one chain

Protein Digestion
Proteases cleave peptide bonds.
Exopeptidases cleave the first or last peptide bond.
Endopeptidases cleave internal peptide bonds.
Proteases attract amino acids to an active site in the enzyme and cleave (break) a peptide bond associated with the amino acid.

Different proteases have unique active sites that can attract specific amino acids and only peptide bonds associated with those amino acids will be cleaved.
For example trypsin, secreted by the pancreas, cleaves bonds after lysine and arginine.

Anabolism
Only individual amino acids absorbed in intestine.
Individual amino acids are absorbed into the blood and distributed around the body.
They are recombined in cells according to RNA templates to form new proteins.

Amino acids may be broken down further in times of energy crisis.
Some amino acids can be synthesized in the body of vertebrates (non essential amino acids).
Others must be ingested (essential amino acids).
Most vertebrates must ingest about 10 essential amino acids and only slight variation exists between classes and species.

Lipid digestion
Most fats ingested are triglycerides i.e. three fatty acid chains connected to a glycerol molecule.
Lipases secreted by pancreas cleave two of the fatty acid chains from the triglyceride producing free fatty acids and monoglycerides which are absorbed.

Problem: How is a non soluble fat digested by enzyme?
Bile from the liver (via the gall bladder) emulsifies the lipids and increases surface area for action by lipases.

Monoglycerides and fatty acids combine with bile salts and lecithin to form micelles in intestine.
Micelles are small and remain suspended in water.
Micelles allow absorption of lipids by intestinal epithelium.

Triglycerides are reformed in the epithelium cells of the intestine, combined with cholesterol, phospholipids and proteins to form soluble chylomicrons.
Chylomicrons are too big to enter capillaries and are absorbed by lymph vessels (lacteals).

Carbohydrate digestion
Carbohydrates are complex sugars made up of monomers of monosaccharides.
Generally only monosaccharides are absorbed.
Animals and plants use glucose as an energy currency within the organism.
Plants store this as amylose and animals store it as glucose.

Amylases secreted in the saliva of some vertebrates (mammals) and from the pancreas break up starch and glycogen into smaller oligosaccharides.
Many specific enzymes such as sucrase, lactase, maltase are in the intestinal brush border.

Lets follow the food trail.
Mouth - the saliva of most mammals contains amylase that begins the breakdown of carbohydrates.

Mastication occurs in mammals only.
Mammals have heterodont dentition (as opposed the homodont dentition in other classes) meaning that teeth have specialized function.
Teeth are grouped as incisors, canines, premolars and molars.
Organization of teeth vary depending on diet.

Food moves from mouth to stomach via the esophagus, a muscular tube that moves food by peristalsis.
Can you stand on your head and drink water?
How does a giraffe drink from a pond?
The epiglottis closes the entrance to the larynx to prevent food entering the lungs.

Stomach - secretion of pepsinogen from chief cells in stomach lining and hydrochloric acid from parietal cells. pH is about 2 in most vertebrates. Amylase inactivated and pepsinogen activated by low pH. Only protease activity and mixing of food to form chyme.Mucus with bicarbonate buffer protects stomach lining.

Small Intestine -
Pancreas secretes bicarbonate buffer to bring pH up to about 7.
Pancreas secretes trypsinogen, chymotrypsinogen and procarboxypeptidase for protein breakdown.

Enzyme cascade
Enterokinase produced by small intestine (duodenum) brush border activates trypsinogen.
Trypsin activates trypsinogen, chymotrypsinogen and procarboxypeptidase.

Pancreas secretes amylase for carbohydrate breakdown.
Pancreas secretes nucleases for nucleic acid breakdown.
Pancreas secretes lipases for lipid breakdown.

Gall bladder secretes bile to emulsify fats.
Bile produced by liver and stored in gall bladder.
Bile salts recycled, absorbed in small intestine and returned to liver. (entero hepatic circulation)

Brush border produces enterokinase, and has other proteases and disaccharidases attached to surface.
Most of the absorption takes place in middle part of small intestine.
The surface area of the intestine is increased by villi and microvilli.
Surface area is generally proportional to body size but differs in different classes.

Large intestine
Mainly water absorption and compaction of feces.
Large bacterial community - breakdown of some cellulose.
Bacteria synthesize vitamin K that is absorbed.

Cellulose digestion by herbivores
Vertebrates lack the enzyme to cleave the bonds between successive glucose monomers of cellulose.
Herbivores have symbiotic relationships with microorganisms (bacteria and protozoans) that produce cellulase.

Species involved are more commonly mammals but birds and reptiles have also been found to utilize microbial fermentation (Willow ptarmigan, galliforme birds, green turtle, iguana).

Ruminants
Group of herbivorous mammals.
The true stomach of ruminants (abomasum) is preceded by three other chambers the rumen, reticulum and the omasum.
Rumen serves as a fermentation vat for food with large populations of bacteria and protozoans.

Glucose released either used by microbes or absorbed by vertebrate.
Fermentation products (organic acids) make up about 70% of the energy absorbed by cattle.
Other byproducts methane and carbon dioxide.
Microorganisms also synthesize protein which is later digested by vertebrate.
Urea a waste product from protein breakdown is normally excreted but is recycled in ruminants.
It is used by the microbes for protein synthesis.

Most nonruminant vertebrates herbivores have either compartmentalized stomachs where fermentation takes place or use the cecum.
Cecum not as efficient because less efficient absorption at that end of intestine

Caprophagy
Some rodents and lagermorphs send food through their digestive tract twice.
Two types of feces produced.
One is waste product the other is reingested direct from the anus.

Rabbits reingested feces come from the cecum, are surrounded by a mucus membrane and are not chewed when reingested.
They lodge in a separate part of the stomach and are not moved on directly to the small intestine.

Some birds have an expansion of the esophagus called the crop.
Fermentation has been reported in the crop of some galliforme birds but it is generally for storage and physical softening and breakdown

Control of digestive secretions
Enzyme secretion is largely controlled by endocrine secretions from cells in the gastric mucosa.
Nervous control also plays apart and the thought of food or the smell of food can induce salivary secretions in the mouth and some gastric secretion. This is called the cephalic stage.

Gastric stage.
As food enters the stomach mechanoreceptors and chemoreceptors are stimulated.
Short polypeptides and amino acids stimulate chief cells to secrete pepsinogen and parietal cells to secrete HCl.They also stimulate G cells in the stomach to secrete the hormone gastrin.

Gastrin stimulatespepsinogen and HCl secretion also. This is a positive feedback system.
Glucose has no effect on gastric secretion and lipids inhibit gastric secretion.
Negative feedback controls the system - as pH in stomach drops gastrin secretion is inhibited, stopping completely at pH of 1.

As chyme enters the duodenum its presents stimulates chemoreceptors.
Falling pH stimulate mucosal cells to secrete the hormone secretin. Secretin acts on duct cells of the pancreas to release bicarbonate rich pancreatic juice.
Secretin also stimulates the liver to increase secretiion of bile.

Lipids in the duodenum stimulate release of the hormone cholecystokinin (CCK).
CCK stimulates acinar cells in the pancreas to secrete digestive enzymes and contraction of muscle in the gall bladder to secrete bile.

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This page was created by Peter King. Please contact the author at pking@fmarion.edu with comments.
http://people.fmarion.edu/pking/vertphys/digestion.html
Last edit January 10, 2011.
Copyright Peter King