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Learning objectives
By the end of this chapter, students should be ableto:
Describe the mechanism of feeding
State and explain four stages of food processingincluding hormonal control.
Differentiate thevariation in vertebratedigestive system.
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Main feeding mechanismsSuspension feeders
Substrate feeders
Fluid feeders
Bulk feeders
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Main feeding mechanisms Suspension feeder/ filter feeders
Sieve small food particles from water
E.g :whales, clams and flamingos
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Main feeding mechanisms Substrate feeders
Animals that live in/on their food sourceEat their way through the food
E.g : earthworms and termites
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Main feeding mechanisms Fluid feeders
Suck nutrient-rich fluid from a living host
E.g : mosquito, aphids
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Main feeding mechanisms Bulk feeders
Eat relatively large pieces of food (swallow altogether)
Spend a long time to digest their food
E.g : snake
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Mouth
Esophagus
Stomach
Large intestine
Rectum
Anus
Tongue
Glands in mouth that make saliva
Pancreas
Liver
Gallbladder
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The mammalian digestive system consists of analimentary canal and accessory glands thatsecrete digestive juices through ducts
Mammalian accessory glands are the salivaryglands, the pancreas, the liver, and thegallbladder
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Stages of Food Processing
Ingestion is the act of eating
Digestionis the process of breaking food downinto molecules small enough to absorb In chemical digestion, the process ofenzymatic hydrolysis
splits bonds in molecules with the addition of water
Absorption is uptake of nutrients by body cells
Eliminationis the passage of undigested materialout of the digestive compartment
Fi
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Fig. 41-7
Ingestion Digestion Absorption Elimination
Undigestedmaterial
Chemical digestion
(enzymatic hydrolysis) Nutrientmoleculesenter bodycells
Smallmolecules
Mechanicaldigestion
Food
Piecesof food
1 2 3 4
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Digestion:
Mechanical
&
Chemical
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Animals with simple body plans have agastrovascular cavitythat functions in both
digestion and distribution of nutrients.
More complex animals have a digestive tube
with two openings, a mouth and an anus -called a complete digestive tract or analimentary canal
Digestive Compartments
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Gastrovascular Cavity
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Fig 41
-10a
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Fig. 41-10a
Cecum
Anus
Ascendingportion oflarge intestine
Gall-bladder
Small
intestineLargeintestine
Smallintestine
Rectum
Pancreas
Liver
Salivary glands
Tongue
Oral cavity
PharynxEsophagus
Sphincter
Stomach
Sphincter
Duodenum ofsmall intestine
Appendix
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Variation of alimentary canal
related organ Crop is a pouch like organ in which food is usually
softened, moistened and stored temporarily
Gizzards Actively churn and grind the food (physicalfragmentation)
Fig 41-9a
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Fig. 41 9a
Esophagus
Mouth
Pharynx
Crop Gizzard
Typhlosole
Intestine
Lumen of intestine
Anus
(a) Earthworm
Fig. 41-9b
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Fig. 41 9b
(b) Grasshopper
Foregut
Mouth
Crop
Gastric cecae
Esophagus Rectum
Anus
Midgut Hindgut
Fig. 41-9c
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Fig. 41 9c
(c) Bird
Stomach
GizzardIntestine
Esophagus
Anus
Crop
Mouth
Fig. 41-10b
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g 4
Anus
Liver
Pancreas
Smallintestine
Large
intestine
Rectum
Stomach
Gall-bladder
A schematic diagram of thehuman digestive system
Esophagus
Salivaryglands
Mouth
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The first stage of digestion is mechanical and
takes place in the oral cavity- Human teeth suit their omnivorous lifestyle.
- They are simple carnivorous at their mouth front by having
cuspids and incisors.
- Behind the cuspids are two premolars and three molars usedfor grinding and crushing food.
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Fig. 41-18
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g
Incisors
(c) Omnivore
Molars
(b) Herbivore
(a) Carnivore
CaninesPremolars
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The palate, the bone-reinforced section of the mouthprovides a hard surface for the tongue to press the food
in order to mix with the saliva.
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As for the tongue, besides equipped with taste buds to
help us taste the food, the tongue helps shaped it intoa bolus.
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Salivary gland produce and secrete saliva that
* Cleanses the mouth* Moistens and dissolves food chemicals
* Aids in bolus formation
* Contains salivary amilase which initiates thechemical digestion (breaking down) of starch into
sugar.
Three types salivary glands
(parotid, sublingual and submandibular)
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The esophagus is a muscular tube that conveysboluses from the pharynx to the stomach byperistalsis.
The region we call our throat is the pharynx, ajunction that opens to both the esophagus andthe trachea (windpipe)
The trachea leads to the lungs
Esophagus
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Esophagus From esophagus to the anal canal of the walls of the GI
(gastrointestinal tract) have the same four tunics:
- mucosa, submucosa, mascularis externa and serosa.
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During swallowing, the esophageal sphincter muscle is relaxed
allowing bolus to enter the esophagus
epiglottis is down
glottis is closed
- Coughing occurs when the swallowing reflexfails and food or liquids reach the windpipe
Fig. 41-11-2
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Larynx
Trachea
Epiglottis
upPharynxTongue
Glottis
Esophagus
Esophagealsphinctercontracted
Food
Tostomach
Tolungs
Epiglottisdown
Esophageal
sphincterrelaxedGlottis upand closed
Fig. 41-11-3
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Larynx
Trachea
Epiglottis
upPharynxTongue
Glottis
Esophagus
Esophagealsphinctercontracted
Food
Tostomach
Tolungs
Epiglottisdown
Esophageal
sphincterrelaxedGlottis upand closed
Epiglottisup
Esophagealsphinctercontracted
Sphincterrelaxed
Relaxedmuscles
Contractedmuscles
Relaxedmuscles
Stomach
Glottisdownand open
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In the esophagus, peristalsis happens
A wavelike contraction that squeezes a bolusdownwards to the stomach
The movement of food is controlled by a sphincter.
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Both layers of muscularis externa
contract involuntarily, meaning
that whenever one muscle layer
contract the other one relax.
This causes the peristalsis
process to happen and brings the
bolus to the stomach.
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Stomach The stomach is
convulated, enablingto fold up when empty
and open out like anexpanding balloonwhenever it is full offood
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STOMACH
A bolus is moved down through the
esophagus through peristaltic contractions
When the sphincter at the entrance ofthe
stomach opens, food enters the stomach
Pyloric sphincter causes the chyme
to enter the small intestine one
squirt at a time (2 6 hours)
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Small Intestine
The small intestine is about 4.5 m long where thefirst 25 cm is the duodenum; the remainder isdivided into jejunum and ileum.
The small intestine is the longest section of thealimentary canal
It is the major organ of digestion and absorption
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Small Intestine
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Small intestines
The epithelial wall of the small intestine is coveredwith tiny, fingerlike projection calledvilli.
The epithelial cells lining the villi have manycytoplasmic extensions called the microvilli.
This greatly increases the surface area of the smallintestine that helps in the absorption process.
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Small intestines
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Large Intestine The large intestine (colon) is much shorter than the small
intestine but it possesses a larger diameter.
No digestion takes place within the large intestine and onlyabout 4% of fluid absorption happens here.
Undigested material is compacted and stored.
Bacterial fermentation happens at the colon and produces
gases.
Compacted feces will be driven by peristalsis from the largeintestine into a short tube called rectum.
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Large intestine
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Large intestineTwo sphincters control passage to the anus;i) composed of smooth muscles that open involuntarily
in response to pressure inside the rectum.
ii) composed of striated muscle that can be voluntarilycontrolled by the brain.
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Accessory organ
Consists of :
pancreas
gallbladderliver
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Pancreas
The pancreas is an exocrine organ pancreatic fluid issecreted through the pancreatic duct
The pancreatic fluid contains hydrolytic enzymes:
1. Trypsin & chymotrypsin (protein digestion)
2. Pancreatic amylase (carbohydrate digestion)
3. Lipase (fat digestion)
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Pancreas The enzymes are released as inactive enzymes called
zymogens which will then be activated by the brushborder enzymes of the small intestine.
Pancreatic fluid also contains bicarbonate thatfunction in neutralizing the HCl from the stomach.
The pancreas also plays a role as an endocrine gland.
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Liver The main exocrine secretion of the liver is bile which is
a mixture ofbile pigments and bile salts.
The bile pigments (by-products of red blood celldestruction) did not participate in the digestionprocess. It is eliminated with feces.
Bile saltswill play an important role in fat digestion(emulsification process).
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Gallbladder Gallbladder functions in storage and
concentration of bile salts.
The arrival of fatty food to the
duodenum triggers a reflex, causingcontraction and injection of
gallbladder to the duodenum.
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Human Digestive System
Chemical Digestion
Accomplished through the use of chemicals known asdigestive enzymes
These complex molecules, such as fats, proteins, andcarbohydrates, are digested (broken down) intosmaller molecules.
These smaller molecules can then be absorbed for use
by cells.
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Chemical digestion MOUTH
The salivary amylase enzyme begins hydrolyzingstarch in the food
Salivary amylase will turn starch to oligosaccharide
and dissaccharides (maltose).salivary amilase
polysaccharides ------------------ > maltose
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Chemical digestion STOMACH
The stomach secretes gastric juice from gastric glands
Gastric juice contain mucus, enzymes (pepsin &renin) and strong acid (HCL pH 1.5 2).
Mucus functions in lubricating and protecting the cell
lining in the stomach from the acidity of the gastricjuice
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Chemical digestion - STOMACH
The enzyme secreted is pepsinogen, an inactive formof the digestive enzyme pepsin.
Acids converts pepsinogen to active pepsin byremoving a small portion of the molecule andexposing the active sites
HCL
Pepsinogen------- > Pepsin
Fig. 41-12
Esophagus
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Interior surfaceof stomach
p g
Chief cells
Smallintestine
Epithelium
Stomach
Sphincter
Parietal cell
Pepsinogen and HClare secreted.
HCl convertspepsinogen to pepsin.
Pepsin activatesmore pepsinogen.
Chief cell
Folds ofepithelialtissue
Pepsin
Sphincter
Pepsinogen
HCl
H+Cl
Parietal cells
Mucus cells
Gastric gland
1
2
2
3
3
1
5m
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Chemical digestion - STOMACH
Pepsin will digest protein into short polypeptides tomake easier for the protein to undergo furtherdigestion in the small intestine.
pepsin
Protein ------------ > short polypeptides
Contraction of the muscles in stomach will aids inchemical digestion.
The stomach will mix the food boluses with the gastricjuice , forming a mixture called chyme.
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Chemical digestion SMALL INTESTINE
The first portion of the small intestine is the duodenum,
where acid chyme from the stomach mixes with digestivejuices from the pancreas, liver, gallbladder, and the smallintestine itself.
The epithelial lining of the small intestine is called thebrush border that released enzymes.
Chyme entering the duodenum will trigger the release of
pancreatic juice.
The enzymes in the pancreatic juice break downcarbohydrates, proteins, fats and nucleic acids
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Chemical digestion SMALL INTESTINE
1. Protein digestion
Trypsin
Chymotrypsin
Protein ----------------------- > smaller polypeptides
Carboxypeptidase
Aminopeptidase Brush border enzyme
Dipeptidase
Small polypetides ------------------------- > Amino acids
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Chemical digestion SMALL INTESTINE
2. Carbohydrate digestion
Pancreatic amylase
Polysaccharides -------------------- > di/mono saccharides
Disaccharides brush border
Disaccharides ---------------------- > monosaccharides
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Chemical digestion SMALL INTESTINE
3. Nucleic acid digestion
Pancreatic nucleases
DNA & RNA-------------------- > nucleotides
IntestinalnucleasesNucleotides ---------------------- > bases / sugar
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Chemical digestion SMALL INTESTINE
4. Fat digestion
Bile saltsFat globules ------------------- > Fat droplets (Emulsified)
PancreaticlipaseFat droplets ------------------- >fatty acids + glycerol
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Chemical digestion SMALL INTESTINE
For fat digestion, a process called emulsificationhappens in the smallintestine. This process functionsin accelerating fat digestion.
Fats are triglycerides (not water-soluble). In thechyme, they will clump to form fat globules
As the fat globules move in the intestinal wall, themovement of the muscle layers breaks apart the fatglobules into small droplets that get coated with bilesalts.
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Chemical digestion SMALL INTESTINE
Bile salts are negatively charged, making the dropletsrepel each other (separated with each other) and forman emulsion.
Emulsion droplets, which are separated, give fatdigesting enzyme lipase a greater surface area to acton.
By the time peristalsis has moved the chyme mixturethrough the duodenum, chemical digestion of ourmeal is just about to complete.
Fig
. 41-13
Oral cavity,
Carbohydrate digestion
P l h id Di h id
Protein digestion Nucleic acid digestion Fat digestion
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Oral cavity,pharynx,esophagus
Stomach
Lumen ofsmall intes-tine
Epitheliumof smallintestine(brushborder)
Polysaccharides
Smaller polysaccharides,maltose
Polysaccharides
Maltose and other
disaccharides
Disaccharides
Proteins
Small polypeptides
Pepsin
Pancreatic amylases
Salivary amylase
Disaccharidases
Monosaccharides
Small peptides
Amino acids
Amino acids
Polypeptides
Smallerpolypeptides
Pancreatic trypsin andchymotrypsin
Pancreatic carboxypeptidase
Dipeptidases, carboxypeptidase,and aminopeptidase
DNA, RNA
Pancreaticnucleases
Fat globules
NucleotidesFat droplets
Nucleosides
Nitrogenous bases,sugars, phosphates
Nucleotidases
Nucleosidasesandphosphatases
Glycerol, fattyacids, monoglycerides
Bile salts
Pancreatic lipase
(starch, glycogen) (sucrose, lactose)
Fig. 41-13a
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Oral cavity,pharynx,
esophagus
Stomach
Lumen ofsmallintestine
Epitheliumof smallintestine(brushborder)
Carbohydrate digestion
Polysaccharides
Smaller polysaccharides,maltose
Polysaccharides
Maltose and otherdisaccharides
Disaccharides
Pancreatic amylases
Salivary amylase
Disaccharidases
Monosaccharides
(starch, glycogen) (sucrose, lactose)
Protein digestion
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Stomach
Lumen ofsmallintestine
Epitheliumof smallintestine(brushborder)
Proteins
Polypeptides
Smallerpolypeptides
Pancreatic trypsin andchymotrypsin
Pepsin
Dipeptidases, carboxypeptidase,and aminopeptidase
Monosaccharides
Small polypeptides
Amino acids
Pancreatic carboxypeptidase
Amino acids
Small peptides
Fig. 41-13c
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Lumen ofsmallintestine
Epitheliumof smallintestine(brushborder)
Nucleic acid digestion
DNA, RNA
Nucleotides
Pancreaticnucleases
Nucleosidasesandphosphatases
Nucleosides
Nucleotidases
Nitrogenous bases,sugars, phosphates
Fig. 41-13d
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Lumen ofsmallintestine
Fat digestion
Fat globules
Fat droplets
Pancreatic lipase
Bile salts
Glycerol, fatty
acids, monoglycerides
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Absorption in the Small Intestine
The small intestine has a huge surface area, due to
villiand microvillithat are exposed to theintestinal lumen
The enormous microvillar surface greatlyincreasesthe rate of nutrient absorption
Fig. 41-15
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Muscle layers
Microvilli (brushborder) at apical(lumenal) surface
Vein carrying blood
to hepatic portal vein
Villi
Intestinal wall
Key
Nutrientabsorption
Largecircularfolds
Bloodcapillaries
Epithelialcells
Villi
Lymph
vessel
Basalsurface
Lacteal
Epithelial cells
Lumen
Fig. 41-15b Microvilli (brushborder) at apical
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) p(lumenal) surface
Key
Nutrientabsorption
Bloodcapillaries
Epithelialcells
Villi
Lymphvessel
Basal
surface
Lacteal
Epithelial cells
Lumen
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Each villus contains a network of blood vessels and a
small lymphatic vessel called a lacteal
After glycerol and fatty acids are absorbed by
epithelial cells, they are recombined into fats withinthese cells
These fats are mixed with cholesterol and coatedwith protein, forming molecules calledchylomicrons, which are transported into lacteals
Fig. 41-16Lumenof smalli i
Triglycerides
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intestine
Lacteal
Chylomicron
Phospholipids,cholesterol,and proteins
Triglycerides
MonoglyceridesFatty acids
Epithelialcell
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Amino acids and sugars pass through the epithelium
of the small intestine and enter the bloodstream
Capillaries and veins from the lacteals converge in
the hepatic portal vein and deliver blood to theliver and then on to the heart
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Hormonal ControlFour hormones altogetheri. Gastrin
ii. Cholecytoskinin (CCK)
iii. Secretin
iv. Enterogastrone
Fig. 41-14
Gallbladder
Liver
Bile
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Secretinand CCK
Stomach
+
Duodenum of
small intestine
Gastrin
Secretin
Pancreas
CCK
CCK
Key
Stimulation
Inhibition
+
+
++
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Hormonal control
Gastrin (from stomach) stimulates the production ofgastric juice.
CCK (from duodenum) stimulates the release ofdigestive enzymes from the pancreas and bile saltsfrom the bladder
Secretin (from duodenum) stimulates the release ofbicarbonate from the pancreas
Enterogastrone (from duodenum) inhibits peristalsisand acid secretion from the stomach slowingdigestion of fat
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Vertebrate digestive system Most herbivores lack enzyme that digest cellulose in
the vegetation cell wall, so they depend onmicroorganisms role to aid in their digestion.
Ruminants such as cows and other herbivores, theyhave multiple stomach chambers in which cellulose isslowly broken down.
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Ruminant digestive system-cow The first chamber contains of a rumen and a smaller
chamber called reticulum and the second portionconsists of two additional chambers called the
omasum and abomasum.
The breakdown of cellulose in tough plant materialshappens in the first and second stomach chamberwhere bacterial symbionts release digestive enzymes todigest the nutrients in cellulose.
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Ruminant digestive system-cow The cow will then regurgitates and rechews the
contents of the first sac before swallowing again.
This process is called rumination. This action exposes
more surface area for the enzymes to react, resulting inmore nutrients to be released for the hosts benefit.
In the omasum water is absorbed
In the abomasum, digestion is carried out by the cow'sown enzyme. Absorption completed here.
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Cow digestive system
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Vertebrate digestive system In animals like rodents, horses, deer and rabbits, thedigestion of cellulose is carried out by microorganismsin the enlarged cecum.
Because it is located beyond the stomach,regurgitation is impossible.
Therefore, they swallowed their feces in order for theabsorption process to happen.
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Rabit digestive system The ingested litter is called cecotropes and rabbitsneed to ingest it to avoid malnutrition.
The fecal pellets are dry, consist of undigested fibercompared to the mucus-coated cecotropes.
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Rabit digestive system
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Carnivore & Herbivore digestive system Carnivores usually have large expandable stomach
because it is harder for them to catch prey.
Shorter alimentary canal because it is easier todigest meat as compared to vegetation.
Herbivores generally have longer alimentary canalsthan carnivores, reflecting the longer time neededto digest vegetation
Fig. 41-19
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Cecum
Small intestine
HerbivoreCarnivore
Colon(largeintestine)
StomachSmallintestine
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Tutorial Divide yourself into four groups. Each group will have to answer one
question only and topic will be given as listed below. Discuss andpresent.
Q1: Trace a bite of food through the human digestive tract, listing eachstructure to which it passes.
Q2 : Summarize step-by-step digestion of a
a) carbohydrates b) fat c) proteins
Q3 : How does absorption of fat differ from the absorption of glucose?
Q4 : Give the functions of three types of accessory glands that secretedigestive juices in vertebrate Identify their secretions