Transcript chapter 5 Mr halex - VCE Biology Units 1 and 2

Digestive Systems in Mammals
VCE Biology Unit 1
Food Requirements
• Cows, dogs and humans have different food
requirements
– Cows (herbivores) stand around all day eating
grass
– Dogs (carnivores) gulp down food in minutes
– Their teeth are different
– Their intestines are different
Food Requirements
Food Requirements
Humans are Omnivores
• Food physically broken by teeth
• Digestion takes 12 to 72 hours
• Mucus secreted to protect lining of gut and
lubricate food (we can eat glass)
• Food moves through digestive tract and a series
of digestive enzymes released to breakdown
complex molecules for absorption.
• Useful substances absorbed; unwanted and
undigested substances eliminated as faeces
(fæces).
Humans are Omnivores
Food Reception and Transport
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Mouth and mouth cavity
Taste
Teeth
Saliva (pH 7.0 – 7.5)
Amylase
Ethanol absorbed through lining of mouth
therefore absorbed before food is digested
Food Reception and Transport
• Swallowing into oesophagus (œsophagus)
• Use back of tongue to push food against back
of pharynx (a co-ordinated reflex which
cannot be controlled)
• Swallowing causes the opening of both upper
and lower oesophagus sphincter (ring of
muscle)
• Soft palate and epiglottis move to prevent
bolus entering airway.
Food Reception and Transport
Food Reception and Transport
Digestion and Absorption
Stomach
• Food storage organ
• Changed into chyme (soft semi-fluid mixture)
• HCl, pepsinogen and gastric lipase released
• Acid breaks pepsinogen into pepsin
• pH prevents action by amylase
• Gastric lipase digests fats into fatty acids and glycerol
• Other glands produce mucus to protect stomach
• Very little absorption occurs (except alcohol and
certain drugs [aspirin and some penicillins])
Digestion and Absorption
Digestion and Absorption
Small Intestine (6 m)
• Important exchange organ
• Long, large surface area well suited for absorption
• Inner lining has million of tiny folds called villi (singular
villus)
• Thin and well supplied with blood and lymphatic
vessels
• Chyme moves through intestine by peristalsis.
• First part called duodenum (25 cm) (pH 6.0 – 7.0)
– Receives pancreatic enzymes and bile from liver to
emulsify fats and neutralises acid from stomach
Digestion and Absorption
Small Intestine (jejunum [1.4 m] and ileum [3.5] [pH 7.0 –
8.0])
• Enzymes secreted by lining of intestine
• Complete digestion of carbohydrates into simple
sugars, proteins into amino acids and fats into fatty
acids and glycerol.
• Some nutrients pass through small intestinal wall along
a concentration gradient
• Most nutrients actively transported
• Most water absorbed (90 – 95 %)
• Blood leaving the intestine pass through liver first
Digestion and Absorption
Water Absorption and Egestion
Large Intestine (Colon)
• Salts actively absorbed
• Water absorbed passively
• No digestive enzymes released
• Bacteria in colon digest fibre (cellulose) for their
own use and produce fatty acids for absorption
(insignificant for humans but important for
herbivores)
• Intestinal microorganisms produce vitamins K and
B12 which are absorbed by colon.
Water Absorption and Egestion
Large Intestine (Colon and Rectum)
• Involved in storage of faeces
• Faeces is made up of 75% water and 25% solid
matter (largely dead bacteria and indigestible
fibre)
• Bacteria in colon main source of farts
Water Absorption and Egestion
Water Absorption and Egestion
Herbivores Utilise Cellulose
• Cellulose too large to be absorbed
• Cellulose broken down by cellulase (produced
by fungi, protozoans and bacteria)
• These organisms live in gut of herbivores
(including termites and cockroaches)
• Symbiotic partnership (mutualism)
• Environment in gut warm and moist but low in
oxygen, therefore breakdown of cellulose
occurs anaerobically by fermentation.
Hindgut Fermenters
• Fermentation occurs in caecum (enlarged pouch
at junction of small and large intestines)
• Position of fermentation chamber limits
absorption of nutrients
• Examples: Horses, koalas, wombats, possums and
rabbits
– Possums and rabbits produce two kinds of faeces: one
directly from caecum at night which is re-ingested to
go through intestine again.
Hindgut Fermenters
Foregut Fermenters
• Fermentation chamber located before stomach
• Chamber called a rumen in ruminants (cattle,
sheep, kangaroos and wallabies)
• Food regurgitated (cud) into mouth for further
physical breakdown (rumination)
• Better absorption of nutrients from breakdown
(before small intestine
• Negative aspect: can take hours to days for
breakdown of cellulose
Foregut Fermenters