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

• 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