Feeding and Digestion: Intestinal Morphology and FunctionMany heterotrophic organisms can digest and assimilate food:

bacteria fungi many protists parasitic and carnivorous plants

Only animals have a true digestive system:a collection of organs cooperating to break down food for digestion and absorption.The gastrointestinal tract (GIT)

is derived from the embryonic blastopore and archenteron largely of endodermal origin has some ectodermal and mesodermal components can be considered (along with integumentary system)

the most ancient organ system gastric - of or pertaining specifically to the stomach enteric - of or pertaining to the intestine (including stomach)Digestion is the physiological function of processing ingested food into smaller molecules that can be absorbed by the body.Absorption is the physiological process of bringing molecules into the living tissues (in this case, from the lumen of the intestine).Incomplete Digestive SystemThe incomplete digestive system is a blind-end sac.One opening serves the function of both mouth and anus.This was the first type of gastrointestinal tract to evolve. Animals with a simple gut

ingest food through the mouth process it expel unused food back out the mouth

Complete Digestive SystemEvolution of the complete digestive system was a major evolutionary innovation.The complete digestive tract

has an opening at each end may have contributed to the evolutionary success and diversification of animals

The animal digestive tract (particularly the vertebrate tract) can be divided into the headgut consisting of the lips (in mammals) mouth, tongue, and pharynx foregut consisting

o esophaguso stomacho crop (some species)o gizzard (some species)

midgut consisting of the small intestine. hindgut consisting of the large intestine.

Not All Guts Are Created EqualIn general...

carnivores have shorter, simpler GI tracts. herbivores have longer, more complex GI tracts.

o Herbivores have more acutely critical intestinal microbiome symbioses.

o Herbivore intestinal microbiome disruption (as by certain antibiotics) can be a serious, sometimes lethal, insult.

Herbivorous fermenters house anaerobic, fermenting microbes in highly specialized intestinal regions.

Anatomy and Function of the Gastrointestinal Tract

Each portion of the Gastrointestinal Tract (GIT) has a specific anatomy, physiology, and role in digestion.

Headwaters of the HeadgutThe mouth and associated structures are known as the headgut.The headgut performs...

minimal processing in animals that ingest food whole initial processing in animals that chew their food (saliva contains digestive enzymes)

There are about as many headgut specializations as there are animals.The simplest mouth merely engulfs food and sends it to the GIT.But many lineages have headgut specializations specific to their feeding modes. Arthropod HeadgutArthropods have evolved a diverse array of mouthparts, each specialized for its particular natural history.

o mandibleso cheliceraeo modified versions of the above

Vertebrate HeadgutPrimitively, vertebrates are homodont and polyphyodont.

Homodont teeth are relatively uniform in shape. Vertebrate teeth are primitively conical. Polyphyodont teeth are constantly lost and replaced. Fish, toothed amphibians, and reptiles are polyphyodonts.

Many vertebrates have evolved headgut specializations: conical teeth of different sizes reduction or loss of teeth differentiated teeth loss of constant tooth replacement

Primitive Vertebrate DentitionFish exhibit the primitive homodont dental condition

similar, conical teeth these regrow and are replaced when lost But many fish show dental specializations

Tuataras, Lizards, Snakes, and crocodilians exhibit the primitive conical tooth pattern can replace lost teeth

Viperid and Elapid snakes employ elongated fangs to deliver venom.

<-- Some Colubrid snakes are rear-fanged and venomous.

Derived Vertebrate DentitionBirds have evolutionarily lost their teeth.They feed with a keratinous bill.Bill morphologies have diversified via natural selection to meet the needs of each species' diet.

Mammals are diphyodonts Unlike polyphyodonts, teeth are replaced only once. deciduous ("baby") teeth fall out permanent teeth

replace themMammals are also heterodonts

incisors canines premolars molars

The dental formula is a mammalian species diagnostic character (i.e., one that can be used to identify the species).

Mammalian dentition has diversified via natural selection, reflecting the species' diet and natural history.Cetaceans (whales, dolphins, porpoises) have secondarily reverted to the homodont condition.

Some mammals are hypsodonts: Teeth grow continually throughout the animal's life. Teeth wear against each other during chewing, maintaining proper length. Correct dental occlusion is critical for maintenance of healthy tooth length.

Some hypsodonts: rodents lagomorphs horses elephants many herbivorous mammals

Foregut

The embryonic foregut develops into esophagus - pushes food from the headgut into the stomach crop - a short-term storage compartment (saurischians) gizzard - is a grinding apparatus (saurischians) stomach breaks up food with muscular action, acids, and digestive enzymes.

o stomach environment may be highly acidic (pH ~ 0.8 in humans) o stomach secretes pepsins to digest proteins

proximal duodenum portion of small intestine just distal to the stomach gallbladder - stores bile, a lipid emulsifier pancreas - secretes digestive enzymes (exocrine function) and insulin (endocrine function) liver

bile production and excretion bilirubin excretion cholesterol, hormone, and toxin excretion protein, carbohydrate, and lipid metabolism

MidgutThe embryonic midgut develops into

distal duodenum jejunum ileum caecum appendix ascending colon first ~65% of transverse colon

In mammals, the smaller diameter of the segments anterior to the colon give them the collective name "small intestine" Not all animals have a difference in diameter between midgut and hindgut.

HindgutThe embryonic hindgut develops into

last ~35% of transverse colon descending colon sigmoid colon rectum proximal anus or cloaca

The hindgut reabsorbs water and electrolytes stores fecal matter between defecations

Of Arthropods and CloacasThe foregut and hindgut of arthropods is of ectodermal origin.It is lined with a very thin layer of chitin.The midgut is of endodermal origin. It lacks the chitinous cuticle.In insects, the excretory system and digestive system are associated with each other.The Malpighian tubules collect nitrogenous waste.They then empty waste into the intestine at the midgut/hindgut junction.In many species, the reproductive tract also joins the GIT in the hindgut, so the posterior opening of the gut serves as the exit for

nitrogenous/excretory waste reproductive products (eggs or sperm) fecal waste

A multi-purpose opening like this is known as a cloaca.(Cloaca is Latin for "sewer".) Cloacae are found in

many invertebrates most fish most amphibians all reptiles and birds

monotreme mammals (platypus, echidna)

Moving Food Through the GutGut wall muscles are arranged in

an outer layer of longitudinal muscles an inner layer of circular muscles

Gut Motility is generated by peristalsis - wavelike constrictions of the intestine that push food

caudally towards the anus/cloaca segmentation - organized circular muscle constrictions that push

food back and forth in the intestine.Ileus is a pathological condition in which peristalsis ceases.If not treated, this can result in intestinal obstruction and sometimes life-threatening complications.A static intestine is fertile ground for overgrowth of harmful bacteria, as chemical conditions in the lumen change.This can lead to intestinal dysbiosis: an imbalance of the normal microbial flora.

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Commercial Reactors are Analogous to Food Processing SystemsA commercial reactor is a vessel used to perform industrial chemical or microbial reactions to produce various products.Three different types of reactors are analogous to animal food processing systems.

Batch ReactorA batch reactor processes one batch at a time, emptying out its products between batches.Animals with a simple gut are somewhat analogous to this type of reactor.Large snakes that ingest a single meal and process it before feeding again have a digestive system that can be likened to a continuous flow reactor.Each meal is processed before the next meal enters. There is no mixing of food ingested at different times.

Continuous Flow Reactor (No Mixing)A continuous flow reactor is a tube in which batches line up in the order in which they were placed in the reactor.Batches do not mix together, but can be processed for a longer time than in a batch reactor.A typical mammalian carnivore or omnivore intestine functions like this type of reactor.Multiple meals per day travel through the intestine, but do not mix.

Continuous Flow Reactor (With Mixing)A continuous flow reactor (with mixing) is a tube with an intervening, enlarged vessel that allows mixing of batches that have been added to the reactor at different times.The intestine of an herbivorous fermenter functions like a this type of reactor.Different species of fermenters have evolved different mechanisms to facilitate microbial fermentation, but all have specialized, enlarged chambers (e.g., rumen or cecum) in the GI tract that serve as mixing vessels like those of continuous flow reactors with mixing.

Biochemistry Fits the ReactorEnzymatic digestion (like ours) works best on food not already subjected to enzymatic breakdown.

Monogastric animals (those with a single, undivided stomach) employ this process.o primates (including us) o canids (dogs) o felids (cats) o suids (pigs) o etc.

Microbial fermentation works best on a mixture of previously ingested food (already colonized by microbes) and recently ingested food.

Ruminant animals (those with a multi-chambered stomach) employ this process.

FermentationHerbivores that digest food via enteric fermentation house a complex ecosystem of microorganisms in a specialized fermentation location in the intestine. These microbes break down complex carbohydrates into their simpler components. Herbivorous animals can be categorized by the location of the fermentation vat.

foregut fermenters midgut fermenters hindgut fermenters

Foregut FermentersForegut fermenters begin the digestion process by sending chewed food to a stomach populated with a complex ecosystem of fermenting microbes.The stomach of a foregut fermenter is specialized into either

four chambers (ruminants, which chew cud)o Antilocapridae (Pronghorn Antelope) o Bovidae (cattle, goats, sheep, and true antelope) o Cervidae (deer and moose) o Giraffidae (giraffes and okapi) o Moschidae (musk deer) o Tragulidae (mouse deer)

two or three chambers (pseudoruminants)o Tylopoda (camels, llamas, alpacas, vicunas, which chew cud) o Kangaroos (which sometimes regurgitate stomach contents to re-chew) o Hippopotamidae (hippopotami, which do not chew cud)

Fermentation in RuminantsThe stomach of a true ruminant is divided into four chambers. rumen

large, non-acidic fermenting vat housing microbes diverse array of bacteria and yeasts producing enzymes that are

o proteolytic o lipolytic o amylolytic o cellulolytic o pectinolytic o hemicellulolytic o deaminative o ureolytic o methanogenic

One milliliter of rumen fluid contains 1010 bacteria! Fermentation processing of food begins here. Quality of forage can strongly affect rumen pH.

Much research done on proper nutrient balance in cattle because of economic importance. reticulum

has a honeycomb-like appearance involved in separation of particles of different sizes Cooked, it is known as tripe Eaten, it is totally gross.

omasum involved in water re-absorption highly folded ("like the pages of a book")

abomasum acidic essentially similar to the monogastric animal stomach secretes a cocktail of digestive enzymes

o chymosin (a protease) produced mostly in young animals, this curdles milk proteins

o pepsin (a protease) o lipases

...collectively known as rennet.Food travel path:

esophagus --> rumen --> reticulum --> esophagus --> mouth (for rumination)--> omasum --> abomasum -->small intestine --> large intestine --> cornhole

The process of rumination is commonly called "chewing cud". Fun (or disturbing) Facts

o Rennet is employed in traditional cheese making to curdle milk.o Most commercial cheese fermentation is now done with chymosin extracted from GMO microbes (bacteria, yeasts)

expressing bovine chymosin genes. o Chymosin is produced mainly by newborn and young ruminants to aid in milk digestion. o Some other animals (pigs, seals, cats) also produce chymosin when young. o Humans have a chymosin pseudogene, but no longer produce chymosin.

Cellulose and Hemicellulose FermentationMicrobial fermentation of cellulose and hemicellulose produces

short-chain fatty acids (SCFAs) (2 - 6 carbons per molecule) These are also known as volatile fatty acids (VFAs) They are easily absorbed and metabolized as an energy source by the animal. More than 70% of a typical ruminant's energy supply comes from VFAs. VFAs are metabolized into compounds (e.g., acetyl coenzyme A) entering the Krebs

Cycle. VFAs produced are primarilyo acetic acido propionic acido butyric acid

VFAs constantly move down their concentration gradient through the rumen epithelium into the bloodstream and on to the liver.

This constant flow prevents buildup of VFAs which would dangerously lower rumen pH, killing symbiotic bacteria.

Essential amino acids and B vitamins are also produced by enteric fermentation.Enteric Fermentation and Climate ChangeOther products of enteric fermentation include

CO2

CH4

Both are important Greenhouse Gases.Because many human populations rely on fermenting herbivores for sustenance, this is contributing significantly to climate change.Quality of ruminant feed can have a significant effect on Greenhouse Gas emission.

corn and soy increase methane production grass reduces methane production (but is low in energy, so animal growth suffers)

cows fed on pastures mimicking wild prairies, with a mix of grasses and high energy flowering plants (e.g., legumes) grow better than on plain grass, and produce less methane than corn- or soy-fed cattle.

Ongoing research in this area is directed at optimizing livestock growth while reducing carbon emissions.

Urea Nitrogen SalvagingThe ruminant gut is is able to recycle nitrogen.In ruminants, as in other animals, toxic ammonia is a waste product of protein catabolism.In the liver, ammonia is converted to urea via the ornithine/urea cycle (left).Urea enters the bloodstream and is excreted in urine. Then...

Urea can enter the rumen from the bloodstream. Ureolytic rumen bacteria use urease to break down urea into ammonia and CO2. Bacteria convert the ammonia into amino acids and peptides. These amino acids and peptides can be re-absorbed into the bloodstream and used by the

animal.This process is known as Urea Nitrogen Salvaging (UNS).Various species of vertebrates and some invertebrates recycle nitrogen via UNS.All use partnerships with mutualistic bacterial gut flora.UNS serves many important functions

maintenance of nitrogen balance in ruminants regulation of blood pH water conservation in desert herbivores maintenance of lean mass in hibernating mammals

Midgut and Hindgut FermentersIn many herbivorous vertebrates (and some invertebrates), a different portion of the intestine is enlarged to house a cellulose- and hemicellulose-fermenting microbial ecosystem.

Midgut FermentersMidgut fermenters have expanded part of the small intestine to house mutualistic microbes that break down plant fiber.Many commercially important herbivorous freshwater fish are midgut fermenters

Tilapia catfish Bluegill

Many herbivorous insects also appear to be midgut fermenters.Studies of their midgut microbiomes have revealed the presence of fermenting microbes.

Arthropod Midgut FermentersHerbivorous insects may have highly specialized microbiomes, depending on their diet.Termites rely on ancient flagellate (protist) symbionts

Trichonympha (basal flagellate protist) Personympha (basal flagellate protist) anaerobic bacteria and archaeans

... to ferment cellulose and hemicellulose into usable acetic acid.Scarab beetles also employ bacterial midgut symbionts to break cellulose into usable VFAs and other essential nutrients.Some researchers are exploring the potential for using scarabs to produce biofuels via their fermentation process.

Unlike vertebrates, insects cannot synthesize sterols.Their microbial partners manufacture sterol precursors for them.

Microbe Symbionts Help More Than HerbivoresBlood-feeding arthropods and annelids generally cannot digest blood without the help of symbiotic bacteria.The symbionts also may produce antibiotics to guard against bacterial overgrowth in the relatively slow-moving ectothermic poikilotherm digestive tract.Hindgut FermentersHindgut fermenters expand either the caecum or the colon (or both) to house fermenting microbes.Lagomorphs and some rodents are cecal fermenters.

The system is like a continuous flow reactor without mixing.

Hence, they must reingest their cecotropes to get essential, microbe-manufactured nutrients.

The animal senses the cecotrope passing from the rectum and reaches down to snatch it before it touches the ground.

Cecotrophy (ingestion of cecotropes) should not be confused with coprophagy (ingestion of feces).

Cecotropes are functionally more like cud than like true fecal matter.

Other hindgut fermenters include herbivorous ground-dwelling birds

o galliforms o some anatidso ostriches (but not emus, rheas, or cassowaries!)

some marine herbivorous fish equines (horses, donkeys, zebras) rhinos elephants herbivorous primates and apes koalas caviid rodents (guinea pigs, capybaras, maras)

The Chemistry of DigestionAnimals use a variety of digestive enzymes

at different physical locations in the digestive tract at different times during the digestion process

Because digestive enzymes are a product of natural selection, their identities and properties differ among species.The descriptions below describe a generalized vertebrate intestine, unless otherwise stated.

Notice the great variety of immune system components.The GIT performs many immune system functions.(Why might this be adaptive in the intestine?)

Digestive EnzymesEnzymes at any point in the gut may be

intraluminal - mediating extracellular digestion taking place in the lumen of the gut

membrane-associated - mediating extracellular digestion via binding of food molecules at the intestinal epithelium's apical membrane surface

intracellular - mediating digestion inside the cellIn the section of intestinal epithelium illustrated:

enterocytes - secrete and contain digestive enzymes goblet cells - produce components of mucus Paneth cell - secrete anti-microbial compounds into the lumen, maintaining the

gastrointestinal barrier M cells - transport antigens from lumen to immune cells, initiating immune

response dendritic cells - monitor intestinal contents and blood in vessels vascularizing

the intestine by collecting antigens. They cano suppress inflammatory reactions against foodo protect against invading pathogens

Peyer's Patch - a small pocket of lymphoid follicles associated with immune system functions

Hydrolytic EnzymesEnzymes are specialized to break down carbohydrates, lipids, and proteins, and are named for what they hydrolyze.

Carbohydrate-digesting Enzymes (Amylases, etc.) amylases break starches into their component monosaccharides and disaccharides sucrase breaks down sucrose lactases break down milk sugars chitinase breaks down chitin

Lipid-digesting Enzymes (Lipases)Lipases operate at lipid/water interfaces.

Bile salts emulsify lipids. This creates tiny, bile-salt encrusted lipid globules suspended in the aqueous gut

contents. Bile salts block the binding of lipases. A peptide coenzyme, colipase, secreted by the pancreas as inactive procolipase. Procolipase is activated into colipase by trypsin. Colipase binds to lipase, changing it to active form. Lipase binds to the surface of the fat globule and begins hydrolyzing the

triglycerides at the surface into usable fatty acids.

Protein-digesting Enzymes (Proteases)Proteases (a.k.a. peptidases) are diverse within and among species.

endopeptidases break amino acids apart within a peptide chain exopeptidases remove amino acids from the amino (N-) or acid (C-) terminals

Proteases often have a specific recognition site where the peptide bond is cleaved usually function only under very specific environmental conditions

The three main proteolytic enzymes vertebrates use to digest protein are pepsin

o secreted primarily by gastric cellso secreted in small amounts by esophageal cellso functions in the stomacho secreted as inactive zymogen pepsinogen

o converted to active pepsin by acid stomach conditionso cleaves polypeptides at aromatic amino acid residues (e.g., phe, trp, tyr) towards the C-

terminal trypsin

o is secreted as inactive zymogen trypsinogen by the pancreaso is converted to active trypsin in the small intestineo cleaves polypeptides at basic amino acid residues (e.g., lys, arg) towards the C-terminalo very similar to chymotrypsin in structure and function

chymotrypsino is secreted as inactive chymotrypsinogen by the pancreaso is converted to active chymotrypsin in the small intestineo cleaves polypeptides at aromatic amino acid residues (e.g., phe, trp, tyr) towards the C-

terminalo very similar to trypsin in structure and function

Intracellular proteases further hydrolyze small peptides transported into the cell.Finally, free amino acids are transported into the bloodstream for distribution to tissues where they are needed.

AbsorptionExtracellular absorption is the process of digestive products being transferred across epithelia into the living tissues of the animal.

Intestinal Villi: Site of AbsorptionIntestinal villi provide vast surface area for absorption.Polar nutrients are transported across the apical surface of intestinal epithelia via

cotransporters countertransporters

Nonpolar nutrients (especially VFAs) are transported across the apical surface of intestinal epithelia via simple diffusion.

Inside cells, VFAs and other small lipids are converted into phospholipids triacylglycerols cholesterol esters

These aggregate with proteins to form chylomicrons, which can be transported throughout the body via the bloodstream.

Hormonal and Neurological Responses to FeedingFeeding is controlled by a complex interaction of hormones and nervous system feedback.

The Stomach is the MessengerIn mammals, chewing and secretion of saliva initiates carbohydrate breakdown via salivary amylase.Swallowing initiates involuntary peristalsis of the esophagus.

upper esophageal sphincter transiently opens food is pushed down the esophagus lower esophageal sphincter (a.k.a. cardiac sphincter) transiently opens this allows food into the stomach.

The presence of food in the stomach stimulates endocrine G cells to secrete a peptide hormone, gastrin.Gastrin stimulates

stomach epithelial cells to secrete acid and pepsinogens peristaltic muscle contractions

As food is processed, most of it sloshes around repeatedly in the stomach, with only a measured portion passing through the pyloric sphincter to the midgut.In response to entry of acidified food from the stomach, endocrine cells in the upper midgut secrete

secretin and cholecystokinin (CCK) which stimulateo secretion of bile by the hepatic (liver) biliary sytemo secretion of pancreatic enzymes

gastric inhibitory polypeptide (GIP) which induces insulin secretionPeristalsis and segmentation are mediated by multiple hormones and neurological interactions.

Hunger and SatietyFasting causes endocrine cells in the stomach and midgut to secrete the peptide hormone ghrelin.Ghrelin

stimulates hunger increases food intake promotes fat storage acts on hypothalamic receptors

Sufficient food ingestion causes stomach stretch receptors to send feedback that triggers different mechanisms that stop feeding, including the release of leptin a polypeptide hormone, by white adipose tissue.Leptin secretion is proportional to body white adipose content mediates long-term energy balance reduces appetite suppresses food intake acts on hypothalamic receptorsOther tissues have leptin receptors, but their function is not yet fully understood.

Feeding Responses to Environmental ChangeAnimals can acclimatize their intestines to take best advantage of a new diet.This is mediated by upregulation and downregulation of various genes.Full acclimatizatoin can take days or weeks.The end result is an animal better physiologically adapted to its new diet.(Think: Changing to a vegetarian diet from an omnnivorous diet)Diet can alter gene expression, changing epigenetic markers.Thus, diet can have long-term health effects, though their nature and magnitude are not yet fully understood.Many animals also change their eating habits in response to seasonal changes, gaining fat when preparing for hibernation, migration, or reproduction.

Research on the digestive system has been ongoing for longer than on any other organ system.But many questions remain to be answered.