The Digestive System Heterotrophs take in organic molecules synthesized by other animals....

The Digestive SystemThe Digestive System

Heterotrophs take in organic molecules Heterotrophs take in organic molecules synthesized by other animals.synthesized by other animals.

1. 1. DigestionDigestion provides energy needed to carry provides energy needed to carry out routine metabolic activities and maintain out routine metabolic activities and maintain homeostasis. homeostasis. 2. 2. The digestive tractThe digestive tract: : a. ingests food, a. ingests food, b. breaks down food into small molecules b. breaks down food into small molecules that can cross plasma membranes. that can cross plasma membranes. c. absorbs these nutrient molecules, and c. absorbs these nutrient molecules, and d. eliminates nondigestible remains. d. eliminates nondigestible remains.

Incomplete versus Complete Incomplete versus Complete Digestive SystemDigestive System

Incomplete gutIncomplete gut – Digestive system contains only a mouth, a Digestive system contains only a mouth, a

pharynx, and an intestine. pharynx, and an intestine. – Digestive enzymes in gastrovascular cavity Digestive enzymes in gastrovascular cavity

allow some extracellular digestion. allow some extracellular digestion. – Digestion is finished intra-cellularly by cells Digestion is finished intra-cellularly by cells

that line the cavity; food diffuses to nearby that line the cavity; food diffuses to nearby cells. cells.

– Digestive system lacks regions of specialized Digestive system lacks regions of specialized function. function.

Incomplete versus Complete Incomplete versus Complete Digestive SystemDigestive System

Complete digestive tractComplete digestive tract is composed of a tube with a is composed of a tube with a mouth at one end and an anus at the other end.mouth at one end and an anus at the other end.– Different regions have specialized functions (e.g., Different regions have specialized functions (e.g.,

ingestion, mechanical digestion, etc.). ingestion, mechanical digestion, etc.). – Muscular Muscular pharynxpharynx draws in food with sucking action. draws in food with sucking action.– CropCrop is storage area with expansive walls. is storage area with expansive walls.– GizzardGizzard has thick muscular walls to grind food. has thick muscular walls to grind food.– Digestion occurs in intestine, outside of cellsDigestion occurs in intestine, outside of cells– Surface area for absorption is increased by intestinal Surface area for absorption is increased by intestinal

folding. folding. – Undigested remains exit the body at anusUndigested remains exit the body at anus

Continuous versus Continuous versus Discontinuous FeedersDiscontinuous Feeders

Continuous feedersContinuous feeders, often called a , often called a filter feederfilter feeder..– Water moves into a mantle cavity through an incurrent Water moves into a mantle cavity through an incurrent

siphon and deposits particles on gills.siphon and deposits particles on gills.– Cilia move particles to labial palps which direct them Cilia move particles to labial palps which direct them

into mouth and to stomach.into mouth and to stomach.– Digestive enzymes from a digestive glands help Digestive enzymes from a digestive glands help

complete digestion.complete digestion.– Marine fanworms are sessile filter feeders; only small Marine fanworms are sessile filter feeders; only small

particles are consumed.particles are consumed.– Baleen whales are active filter feeders; baleen (fringe) Baleen whales are active filter feeders; baleen (fringe)

filters small krill from water. filters small krill from water.

Continuous versus Continuous versus Discontinuous FeedersDiscontinuous Feeders

Discontinuous feedersDiscontinuous feeders– Squids are an example Squids are an example

a. Head of a squid has ten arms; two arms seize a. Head of a squid has ten arms; two arms seize prey and bring it to the squid's mouth. prey and bring it to the squid's mouth. b. Beak-like jaws and a radula (toothy tongue) b. Beak-like jaws and a radula (toothy tongue) reduce food to pieces. reduce food to pieces. c. Esophagus leads to a stomach that holds food c. Esophagus leads to a stomach that holds food until digestion is complete. until digestion is complete. d. d. Discontinuous feedersDiscontinuous feeders require a storage require a storage region in their gut. region in their gut.

Adaptation to DietAdaptation to Diet

Animals are:Animals are:– herbivoresherbivores (eat plants) or; (eat plants) or;– carnivorescarnivores (eat animals) or; (eat animals) or; – omnivoresomnivores (eat both). (eat both).

Mammal dentition differs according to mode of Mammal dentition differs according to mode of nutrition. nutrition. – Omnivores, including humans, have dentition that Omnivores, including humans, have dentition that

accommodate a vegetable diet and a meat diet.accommodate a vegetable diet and a meat diet.– Omnivore teeth include Omnivore teeth include incisorsincisors (shearing), (shearing), caninescanines

(tearing), (tearing), premolarspremolars (grinding), and (grinding), and molarsmolars (crushing). (crushing).– Herbivores have large, flat premolars and molars for Herbivores have large, flat premolars and molars for

grinding plant matter. grinding plant matter.

Adaptation to DietAdaptation to Diet

– Grazers (e.g., horses) also have sharp incisors for clipping off grass and Grazers (e.g., horses) also have sharp incisors for clipping off grass and leaves. leaves.

– Hard to digest plant material requires extensive grinding to disrupt plant Hard to digest plant material requires extensive grinding to disrupt plant cell walls. cell walls.

– Animals that feed on plants may have long and complex digestive tracts Animals that feed on plants may have long and complex digestive tracts and bacteria in their digestive and bacteria in their digestive tracts that can digest cellulose, producing nutrients that an animal can tracts that can digest cellulose, producing nutrients that an animal can use. use.

– Some grazers have a Some grazers have a rumenrumen to digest chewed grasses; partially digested to digest chewed grasses; partially digested cud is rechewed. cud is rechewed.

– Carnivores' pointed incisors and canines tear off pieces small enough to Carnivores' pointed incisors and canines tear off pieces small enough to swallow. swallow.

» Meat is rich in protein and fatty acids and is easier to digest than Meat is rich in protein and fatty acids and is easier to digest than plant material.plant material.

» Carnivores have fewer molars for grinding and a shorter digestive Carnivores have fewer molars for grinding and a shorter digestive tract with less specialization. tract with less specialization.

Human Digestive Tract Human Digestive Tract

1. 1. Human digestive tract is complete and complexHuman digestive tract is complete and complex2. Each part of the digestive system has a specific 2. Each part of the digestive system has a specific

functiofunctio3. Digestion of food in humans is an extracellular 3. Digestion of food in humans is an extracellular

process.process.4. Enzymes are secreted into the digestive tract by 4. Enzymes are secreted into the digestive tract by

nearby glands which never contain food nearby glands which never contain food themselves.themselves.

5. Digestion requires a cooperative effort by 5. Digestion requires a cooperative effort by production of hormones and actions of nervous production of hormones and actions of nervous system. system.

Human Digestive TractHuman Digestive TractMouthMouth

Human dentition has many specializations because humans are Human dentition has many specializations because humans are omnivores.omnivores.

Food is chewed in the mouth and mixed with saliva.Food is chewed in the mouth and mixed with saliva.– Three pairs of Three pairs of salivary glandssalivary glands secrete saliva by way of secrete saliva by way of

ducts into the mouth.ducts into the mouth.– Salivary amylaseSalivary amylase is enzyme that begins starch digestion; is enzyme that begins starch digestion;

maltose is common end product. maltose is common end product.

Salivary amylaseSalivary amylase starch + H2O starch + H2O maltose maltose

Food is manipulated by a muscular tongue with touch and Food is manipulated by a muscular tongue with touch and pressure receptors. pressure receptors.

Food is chewed and mixed with saliva to form a bolus in Food is chewed and mixed with saliva to form a bolus in preparation for swallowing. preparation for swallowing.

Human Digestive TractHuman Digestive TractPharynx and the Esophagus Pharynx and the Esophagus

Digestive and respiratory passages come Digestive and respiratory passages come together in together in pharynxpharynx, then separate. , then separate. – During swallowing, path of air to lungs could During swallowing, path of air to lungs could

be blocked if food entered trachea.be blocked if food entered trachea.– EpiglottisEpiglottis covers opening into covers opening into tracheatrachea as as

muscles move bolus through pharynx into muscles move bolus through pharynx into esophagus. esophagus.

– EsophagusEsophagus is a muscular tube that moves is a muscular tube that moves swallowed food to swallowed food to stomachstomach by by peristalsisperistalsis. .

Human Digestive TractHuman Digestive TractStomach Stomach

StomachStomach stores a liters of partially digested stores a liters of partially digested food freeing humans from continual eating.food freeing humans from continual eating.– Gastric juiceGastric juice produced by cells of produced by cells of gastric gastric

glandsglands..– Walls of the stomach contract vigorously and Walls of the stomach contract vigorously and

mix food with juices secreted when food enters. mix food with juices secreted when food enters. – Gastric juice contains Gastric juice contains hydrochloric acidhydrochloric acid and and

another digestive substance, another digestive substance, pepsinpepsin..– Gastric juices are produced independently of Gastric juices are produced independently of

protective mucous secretions.protective mucous secretions.

Human Digestive TractHuman Digestive TractStomachStomach

– Hydrochloric acid (HCl)Hydrochloric acid (HCl) lowers pH of the gastric lowers pH of the gastric contents to about 2.contents to about 2.

» Epithelial lining of the stomach has millions of gastric pits Epithelial lining of the stomach has millions of gastric pits leading to gastric glands. leading to gastric glands.

» This acid kills most bacteria and other microorganisms.This acid kills most bacteria and other microorganisms.

» Low pH also stops activity of salivary amylase and promotes Low pH also stops activity of salivary amylase and promotes activity of pepsin. activity of pepsin.

– PepsinPepsin is a hydrolytic enzyme that acts on proteins to is a hydrolytic enzyme that acts on proteins to produce peptides. produce peptides.

PepsinPepsin protein + H2O protein + H2O peptides peptides

Human Digestive TractHuman Digestive TractStomachStomach

A thick layer of A thick layer of mucusmucus protectsprotects wall of the stomach and first part of wall of the stomach and first part of duodenum from duodenum from HCl and pepsinHCl and pepsin..– UlcersUlcers develop when lining is exposed to digestive action; recent develop when lining is exposed to digestive action; recent

research indicates this is usually due to research indicates this is usually due to infection by infection by Helicobacter pyloriHelicobacter pylori bacteria. bacteria.

Stomach contents, a thick, soupy mixture, are called Stomach contents, a thick, soupy mixture, are called chymechyme.. At base of the stomach is a narrow opening controlled by a At base of the stomach is a narrow opening controlled by a sphinctersphincter

(a circular muscle valve).(a circular muscle valve).– When the sphincter relaxes, chyme enters When the sphincter relaxes, chyme enters duodenumduodenum; a neural ; a neural

reflex causes the sphincter to contract closing off the opening.reflex causes the sphincter to contract closing off the opening.– DuodenumDuodenum is first part of the is first part of the small intestinesmall intestine. .

c. The sphincter relaxes and allows more chyme to enter c. The sphincter relaxes and allows more chyme to enter the duodenum. the duodenum. d. The slow, rhythmic pace with which chyme exits the d. The slow, rhythmic pace with which chyme exits the stomach allows thorough digestion. stomach allows thorough digestion.

Human Digestive Tract Human Digestive Tract Small Intestine Small Intestine

Human Human small intestinesmall intestine is a coiled muscular tube about three is a coiled muscular tube about three meters long. meters long.

Mucous membrane lining has ridges and furrows; surfaces Mucous membrane lining has ridges and furrows; surfaces are covered by villi. are covered by villi. – VilliVilli are finger-like projections whose surface cells are covered by are finger-like projections whose surface cells are covered by

microvilli.microvilli.» MicrovilliMicrovilli are minute projections, a brush border, of surface cells of are minute projections, a brush border, of surface cells of

intestinal villi. intestinal villi.

– Ridges, furrows, villi, and microvilli greatly increase effective Ridges, furrows, villi, and microvilli greatly increase effective surface area of small intestine. surface area of small intestine.

As chyme enters duodenum, proteins and carbohydrates are As chyme enters duodenum, proteins and carbohydrates are partly digested; no fat digestion occurs. partly digested; no fat digestion occurs.

Human Digestive Tract Human Digestive Tract Small IntestineSmall Intestine

Additional digestion is aided by secretions from Additional digestion is aided by secretions from liverliver and and pancreaspancreas. . – BileBile is a secretion of is a secretion of liverliver temporarily stored in temporarily stored in gallbladdergallbladder

before sent to duodenum.before sent to duodenum.– Bile emulsifies fat; bile is a green byproduct of the breakdown Bile emulsifies fat; bile is a green byproduct of the breakdown

of hemoglobin. of hemoglobin. – Bile contains bile salts that help in emulsification of fat.Bile contains bile salts that help in emulsification of fat.

» Emulsification breaks fat globules into microscopic droplets. Emulsification breaks fat globules into microscopic droplets.

Bile saltsBile salts fat fat fat droplets fat droplets

» This increases fat digestion by increasing surface area of fat globules This increases fat digestion by increasing surface area of fat globules exposed to enzymes. exposed to enzymes.


Pancreatic juicePancreatic juice secreted by secreted by pancreaspancreas contains the following: contains the following: 1) 1) Sodium bicarbonateSodium bicarbonate [NaCO3] that [NaCO3] that neutralizes acidity of chymeneutralizes acidity of chyme; ; pH of small intestine is slightly basic; pH of small intestine is slightly basic; 2) 2) Pancreatic amylasePancreatic amylase that digests starch to maltose; that digests starch to maltose;

– pancreatic amylasepancreatic amylase starch + H2O starch + H2O maltose maltose

3) 3) trypsintrypsin and other enzymes that digest protein to peptides; and other enzymes that digest protein to peptides;

– trypsintrypsin protein + H2O protein + H2O peptides peptides

4) 4) LipaseLipase that digests fat droplets to glycerol and fatty acids. that digests fat droplets to glycerol and fatty acids.

– lipaselipase

fat droplets + H2O fat droplets + H2O glycerol + fatty acids glycerol + fatty acids


Epithelial cells of villi produce intestinal enzymes Epithelial cells of villi produce intestinal enzymes attached to plasma membrane of microvilli.attached to plasma membrane of microvilli.

Intestinal secretions complete digestion of Intestinal secretions complete digestion of peptides and sugars; peptides and sugars; – peptides are digested by peptides are digested by peptidasespeptidases to amino acids; to amino acids;

peptidasespeptidases peptides + H2O peptides + H2O amino acids amino acids

– maltose from the first step in starch digestion is maltose from the first step in starch digestion is converted by maltase to glucose;converted by maltase to glucose;

maltasemaltase maltose + H2O maltose + H2O glucose glucose


Absorption by VilliAbsorption by Villi 1. 1. Small intestineSmall intestine is specialized for absorption by the huge is specialized for absorption by the huge

number of villi that line the intestinal wall. number of villi that line the intestinal wall. 2. If a smooth tube, intestine would have to be 500-600 m long to 2. If a smooth tube, intestine would have to be 500-600 m long to have a comparable surface area. have a comparable surface area. 3. Each 3. Each villusvillus contains blood vessels and a lymphatic contains blood vessels and a lymphatic lacteallacteal..

4. 4. LactealLacteal is lymphatic vessel in an intestinal villus that aids in is lymphatic vessel in an intestinal villus that aids in absorption of fats. absorption of fats. 5. Sugars and amino acids enter villi cells and are absorbed into 5. Sugars and amino acids enter villi cells and are absorbed into bloodstream. bloodstream. 6. Glycerol and fatty acids enter villi cells; reassembled into fat 6. Glycerol and fatty acids enter villi cells; reassembled into fat molecules, they move into lacteals. molecules, they move into lacteals. 7. Absorption involves diffusion and active transport requiring 7. Absorption involves diffusion and active transport requiring expenditure of cellular energy. expenditure of cellular energy.

Control of Digestive JuicesControl of Digestive Juices

GastrinGastrin is produced by cells in gastric glands of stomach wall; is produced by cells in gastric glands of stomach wall; stimulates gastric glands and increases gastric motility; its stimulates gastric glands and increases gastric motility; its secretion is stimulated by a meal rich in protein. secretion is stimulated by a meal rich in protein.

SecretinSecretin is produced by cells in duodenal wall; stimulates is produced by cells in duodenal wall; stimulates pancreas to secrete fluids rich in NaCO3 into duodenum; pancreas to secrete fluids rich in NaCO3 into duodenum; secretion is stimulated by acid chyme. secretion is stimulated by acid chyme.

Cholecystokinin (CCK)Cholecystokinin (CCK) produced by duodenal wall stimulates produced by duodenal wall stimulates pancreas to increase pancreatic juice and liver to increase pancreas to increase pancreatic juice and liver to increase output of bile; causes gallbladder to release bile; secretion is output of bile; causes gallbladder to release bile; secretion is stimulated by fats. stimulated by fats.

Gastric inhibitory peptide (GIP)Gastric inhibitory peptide (GIP) from duodenal wall inhibits from duodenal wall inhibits gastric gland secretion and stomach motility. gastric gland secretion and stomach motility.

Accessory OrgansAccessory Organs Pancreas Pancreas

PancreasPancreas lies deep within abdominal cavity, just lies deep within abdominal cavity, just below stomach, and rests on posterior abdominal below stomach, and rests on posterior abdominal wall. wall.

It is an elongated and somewhat flattened organ.It is an elongated and somewhat flattened organ. As an endocrine gland, it secretes glucogon and As an endocrine gland, it secretes glucogon and

insulin hormone into bloodstream. insulin hormone into bloodstream. As an exocrine gland, it secretes pancreatic juice. As an exocrine gland, it secretes pancreatic juice.

1) Pancreatic juice contains sodium bicarbonate 1) Pancreatic juice contains sodium bicarbonate that neutralizes acidic chyme. that neutralizes acidic chyme. 2) Digestive enzymes digest carbohydrates, fats 2) Digestive enzymes digest carbohydrates, fats and proteins. and proteins.

Accessory OrgansAccessory OrgansLiverLiver

LiverLiver is a large glandular organ that fills the top of is a large glandular organ that fills the top of abdominal cavity, just below diaphragm. abdominal cavity, just below diaphragm.

Liver has numerous functions: Liver has numerous functions: 1) It detoxifies blood by removing and 1) It detoxifies blood by removing and metabolizing poisonous substances. metabolizing poisonous substances. 2) It makes plasma proteins including albumin and 2) It makes plasma proteins including albumin and fibrinogen. fibrinogen. 3) Liver destroys old red blood cells; converts 3) Liver destroys old red blood cells; converts hemoglobin to bilirubin and biliverdin in bile. hemoglobin to bilirubin and biliverdin in bile. 4) It produces bile stored in 4) It produces bile stored in gallbladdergallbladder before before entering duodenum to emulsify fats. entering duodenum to emulsify fats.

Accessory OrgansAccessory OrgansLiverLiver

5) It stores glucose as glycogen; breaks down glycogen to maintain constant 5) It stores glucose as glycogen; breaks down glycogen to maintain constant blood glucose concentration. blood glucose concentration. – Blood vessels from large and small intestines lead to liver as hepatic Blood vessels from large and small intestines lead to liver as hepatic

portal vein. portal vein. – Liver maintains blood glucose level at 0.1% by removing glucose from Liver maintains blood glucose level at 0.1% by removing glucose from

hepatic portal vein to store as glycogen; when needed, glycogen is hepatic portal vein to store as glycogen; when needed, glycogen is broken down and glucose enters hepatic vein. broken down and glucose enters hepatic vein.

6)Liver produces urea from amino groups and ammonia. 6)Liver produces urea from amino groups and ammonia. – Amino acids can be converted to glucose but deamination (removal of Amino acids can be converted to glucose but deamination (removal of

amino groups) must occur.amino groups) must occur.– Using complex metabolic pathway, liver converts amino groups to Using complex metabolic pathway, liver converts amino groups to

urea. urea. – Urea is most common human nitrogenous waste it is transported by Urea is most common human nitrogenous waste it is transported by

blood to kidneys. blood to kidneys.

Large IntestineLarge Intestine

Large intestineLarge intestine is region following the small intestine. is region following the small intestine. It has four parts: cecum, colon, rectum, and anal canal.It has four parts: cecum, colon, rectum, and anal canal. AppendixAppendix

– This is finger-like projection extending from This is finger-like projection extending from cecumcecum, a blind sac at junction , a blind sac at junction of small and large intestine. of small and large intestine.

– It may play a role in fighting infections.It may play a role in fighting infections.– If infected appendix bursts, it results in general abdominal infection If infected appendix bursts, it results in general abdominal infection

((peritonitisperitonitis). ). 4. 1.5 liters of water enter digestive tract daily from drinking; another 8.5 liters 4. 1.5 liters of water enter digestive tract daily from drinking; another 8.5 liters

enter from various secretions. enter from various secretions. About 95% of this total liquid is reabsorbed by small intestine; remainder by About 95% of this total liquid is reabsorbed by small intestine; remainder by

cells of colon. cells of colon. If water is not reabsorbed, it causes If water is not reabsorbed, it causes diarrheadiarrhea which can cause serious which can cause serious

dehydration and ion loss. dehydration and ion loss.

Large IntestineLarge Intestine

Large intestine functions in ion regulation, Large intestine functions in ion regulation, absorbing salts plus vitamin K produced by absorbing salts plus vitamin K produced by intestinal bacteria. intestinal bacteria.

Large intestine that terminates at the Large intestine that terminates at the anusanus, an , an external opening.external opening.

FecesFeces– Feces consists of 75% water and 25% solid matter.Feces consists of 75% water and 25% solid matter.– One-third of the solid matter is intestinal bacteria.One-third of the solid matter is intestinal bacteria.– Remainder is undigested wastes, fats, organic material, Remainder is undigested wastes, fats, organic material,

mucus, and dead cells from intestinal lining. mucus, and dead cells from intestinal lining.

The Digestive System Heterotrophs take in organic molecules synthesized by other animals....

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