Lecture #15

Digestion & Nutrition

Nutrition

• an animal’s diet must satisfy three nutritional needs– 1. chemical energy for cellular processes– 2. organic building blocks for macromolecules– 3. essential nutrients

• body function depends on the chemical energy derived from food– energy is used to produce ATP

• food also provides the building blocks for biosynthesis– food provides organic carbon and organic nitrogen

• materials an animal cannot synthesize = essential nutrients

Essential Nutrients

• four classes of essential nutrients• 1. essential amino acids: 20 amino acids required by animals to

make proteins– most animals have the enzymes required to make half of these– the other half must be taken in through their food– adult humans require 8 amino acids in their diet (infants require 9 –

includes histamine)– complete proteins of meat, eggs and cheese are complete – they

provide all the essential amino acids needed in their appropriate proportions

– plant proteins are incomplete• e.g. corn is deficient in tryptophan and lysine

• 2. essential fatty acids• 3. vitamins• 4. minerals

Essential Nutrients

• four classes of essential nutrients• 2. essential fatty acids: fatty acids that contain one

or more double bonds and are unsaturated• 3. vitamins: organic molecules with diverse

functions– 13 vitamins identified for humans– classified as water soluble and fat soluble

• many water-soluble can function as co-enzymes• the fat-soluble vitamins can act as a hormones

– deficiencies result in a wide variety of diseases• e.g. vitamin C = scurvy• e.g. vitamin D = rickets

Essential Nutrients

• 4. minerals: inorganic nutrients– diverse functions from being co-factors in

reactions to functioning in osmotic balance– ingesting large amounts can disturb homeostasis– excess salt = hypertension– excess iron = liver damage and failure

Food processing

• four stages– 1. ingestion– 2. digestion– 3. absorption– 4. elimination

• digestion occurs in specialized compartments– prevents the animal from digesting itself

• compartments can be– intracellular – digestion within the cell

• within food vacuoles• occurs following phagocytosis or pinocytosis

– extracellular – digestion outside the cell• seen in most animals• digestion occurs in extracellular compartments continuous with the

outside of the body• can be followed by absorption and continued intracellular digestion

Pieces of food

Chemical digestion(enzymatic hydrolysis)

Food

Nutrientmoleculesenter bodycells

Smallmolecules

Undigestedmaterial

ELIMINATIONABSORPTIONDIGESTIONINGESTION

Mechanicaldigestion

Extracellular Digestion

• allows for the ingestion and digestion of much large pieces of food then what can be taken in via phagocytosis/pinocytosis

• simplest compartment – gastrovascular cavity– e.g. hydra – gland cells of the

gastrodermis lining the GV cavity secrete digestive enzymes into the cavity

– other cells of the gastrodermis engulf the smaller food pieces and continue digestion intracellularlly

• most animals possess a digestive tube or alimentary canal – continuous tube from mouth to anus

Gastrovascularcavity

Mouth

Food

Tentacles

Epidermis

GastrodermisMesoglea

Gland cellsFlagella

Nutritivemuscularcells

Food vacuoles

Mesoglea

Digestive Tract

• also called the alimentary canal• starts with a mouth pharynx

– esophagus– stomach– small intestine– large intestine– rectum anus

• accessory glands (shown in green) can provide additional enzymes and digestive hormones

• many specializations associated with these structures in animals– e.g. crop of birds– e.g. gastric caeca of insects

• other animals is subdivide their gut into fore-, mid and hind-gut regions rather than esophagus, SI, LI

Esophagus

Stomach

Liver

Salivaryglands

Gall-bladder

Pancreas

RectumAnus

Largeintestines

Smallintestines

Mouth

A schematic diagram of thehuman digestive system

Digestive Anatomy

• Mouth---bite, chew, swallow• Pharynx and esophagus----

transport• Stomach----mechanical

disruption; absorption of water & alcohol

• Small intestine--chemical & mechanical digestion & absorption

• Large intestine----absorb electrolytes & vitamins (B and K)

• Rectum and anus---defecation• Accessory glands – liver,

gallbladder and pancreas

Mammalian Digestion

• food enters the mouth where it is mechanically and chemically digested– digestion of carbohydrates and fats– mechanical digestion = teeth– chemical digestion = saliva containing amylase and lipase– mixing with saliva turns the ground up food into a bolus

• bolus is swallowed and travels by peristalsis down the esophagus– peristalsis – series of wavelike contractions in smooth muscle

Tongue

Pharynx

GlottisLarynx

Bolus offoodEpiglottisup

EsophagealsphinctercontractedEsophagus

To lungs To stomach Relaxedmuscles

Contractedmuscles

Sphincterrelaxed

Stomach

Trachea

Mammalian Digestion

• bolus enters the stomach – chemical and mechanical digestion– digestion of proteins and fats– mechanical digestion: three

layers of smooth muscle to churn food

– chemical digestion: production of gastric juice

– food mixes with gastric juice to become chyme

Gastric gland

Gastric pits oninterior surface

of stomach

Sphincter

Smallintestine

Epithelium

Mucous cell

Chief cell

Parietal cell

Chiefcell

Pepsinogen

Parietalcell

Pepsin

Folds ofepithelialtissue

Sphincter

Esophagus

Stomach

3

2

1

10

m

HCl

H

Cl

Mammalian Digestion

• bolus enters the stomach – chemical and mechanical digestion– gastric juice: principally water + HCl

+ pepsin + gastric lipase– stomach is lined with a gastric

mucosa that forms gastric glands– glands secrete the HCl and the

enzymes– production of H+ and Cl- ions by the

parietal cells of the gastric gland– production of pepsinogen by the

chief cells of the gastric gland• activation to pepsin accomplished

by exposure to HCl

Gastric gland

Gastric pits oninterior surface

of stomach

Sphincter

Smallintestine

Epithelium

Mucous cell

Chief cell

Parietal cell

Chiefcell

Pepsinogen

Parietalcell

Pepsin

Folds ofepithelialtissue

Sphincter

Esophagus

Stomach

3

2

1

10

m

HCl

H

Cl

Mammalian Digestion

• enters the small intestine - chemical and mechanical digestion PLUS absorption– digestion and absorption of carbs, fats & proteins plus nucleic acids– small intestine is lined with finger-like structures called villi – increases

absorptive surface area– each villus is covered with cells called absorptive cells – create a mix of

enzymes called brush-border enzymes• sucrase, maltase, lactase, aminopeptidase, dipeptidase, enterokinase

– food is digested as flows over these absorptive cells = most digestion is done in the duodenum

Vein carryingblood to liver

Muscle layers

Bloodcapillaries

Villi

Intestinal wall

Epithelialcells

Largecircularfolds

Key

Nutrientabsorption

VilliMicrovilli (brushborder) at apical(lumenal) surface

Epithelialcells

Lumen

Basalsurface

Lacteal

Lymphvessel

Mammalian Digestion

• enters the small intestine - chemical and mechanical digestion PLUS absorption– SI is also the site for the secretion of pancreatic juice – mixes with the chyme

in the duodenum• pancreatic amylase, lipase and 4 proteases• inactive proteases: trypsinogen, chymotrypsinogen, proelastase,

procarboxypeptidase• trypsin must be activated by the brush-border enzyme enterokinase before it

can work• trypsin activates the other three proteases

Mammalian Digestion

• small intestine - chemical and mechanical digestion PLUS absorption– nutrients are also absorbed by the absorptive

cells as they travel through the SI = jejunum and ileum• breakdown of carbs in the mouth and SI

monosaccharides for absorption• breakdown of proteins and peptides in the

stomach and SI amino acids for absorption

• once absorbed into the absorptive cells – digestion stops - no intracellular digestion

– monosaccharides & amino acids directly absorbed by the absorptive cells and transferred into the venous blood leaving the villus

Mammalian Digestion

• small intestine - chemical and mechanical digestion PLUS absorption

• breakdown of fats/triglycerides in mouth, stomach and SI monoglyceride and 2 fatty acids

• fatty acids & glycerol absorbed into the absorptive cells and then transferred into the lacteal of the villus– recombined to form a chylomicron

enters the lacteal– chylomicrons eventually transferred to

the blood via the subclavian veins

LUMENOF SMALLINTESTINE

Triglycerides

Epithelialcell

Fatty acidsMono-

glycerides

Triglycerides

Chylomicron

Phospho-lipids,

cholesterol,and proteins

Lacteal

Mammalian Digestion

• enters the large intestine or colon – for absorption of water and salts

• lined with absorptive cells – absorb water and salt – mainly NaCl– most water is absorbed by the SI– the last liter of water is reclaimed by the

LI– absorption of water is via osmosis and

accompanies the active pumping of Na+ and Cl- into the absorptive cells

Mammalian Digestion

• leftover, undigested food = feces– becomes more and more solid as water is

reclaimed through the LI

• digestion may take place through the action of bacterial enzymes– mostly from E.coli– by products create carbon dioxide,

methane and sulfurous compounds– some bacteria produce vitamin K, B7

and B9 in exchange

• terminal portion of the LI = rectum– storage of feces until expelled via

defecation

Accessory glands

• Liver – numerous functions – storage of iron &

copper– storage of fatty acids– production of LDL and

HDL– main digestive

function – production of bile

Fat dropletscoated withbile salts

Fat globule

Bile salts

Micelles madeup of fatty acids,monoglycerides,and bile salts

Epitheliumof smallintestineEpitheliumof lacteal

Lacteal

Accessory glands

– bile: water, cholesterol, bilirubin and salts• produced by hepatocytes &

secreted into the duodenum• emulsification of fats –

breakdown into smaller triglycerides & breakdown of TGs– A. monoglyceride

(glycerol + 1 fatty acid) – B. 2 fatty acids

• bile + monoglycerides or bile + fatty acids = micelles

• excess bile stored in the gallbladder

Fat dropletscoated withbile salts

Fat globule

Bile salts

Micelles madeup of fatty acids,monoglycerides,and bile salts

Epitheliumof smallintestineEpitheliumof lacteal

Lacteal

Accessory glands

• Pancreas – exocrine and endocrine functions in digestion– exocrine: production of

pancreatic juice– endocrine: production of

insulin, glucagon & somatostatin• glucose balance

Digestive Hormones

• production of digestive hormones– gastrin – by the G cells of the stomach lining

• stimulates production of gastric juice and encourages emptying of the stomach

– gastric inhibitory peptide – antagonist to gastrin– CCK – by the enteroendocrine cells of the SI (presence of fatty

acids)• CCK stimulates the release of pancreatic juice and bile (synthesis and

increased gallbladder contraction)• decreases gastric juice production

– secretin – by the enteroendocrine cells of the SI• secretin stimulates the release of bicarbonate from the pancreas –

neutralizes chime• decreases gastric juice production

Digestive Feedback systems

• emptying of the stomach:– gastrin – stimulates emptying– GIP/enterogastrone and CCK – inhibits emptying

• pancreatic juice production:– secretin and CCK – stimulation of production

• bile production:– CCK – stimulation of secretion

Pancreas

Stomach

Entero-gastrone

Gall-bladder

Liver

DuodenumSecretin

CCK

CCK

StimulationInhibition

Gastrin

Key

Appetite control

• satiation center = hypothalamus• ghrelin – made by the stomach wall– triggers feelings of hunger – stimulates appetite when the

stomach is empty• CCK – increases satiation (also nausea and anxiety)• insulin – secreted by the pancreas in response to

increased glucose levels– suppresses appetite when released in a slow, steady

manner• leptin – produced by adipose tissue– suppresses appetite– as body fat levels drop, so does leptin production and

appetite may increase

Digestion Adaptations

Intestine Rumen

Reticulum

Omasum

EsophagusAbomasum

• some animals have developed a complex mutalistic association with these bacteria

• allow for the digestion of plant-based materials by herbivores

• herbivores and many insects (e.g. termites) – house populations of bacteria in fermentation chambers in their alimentary canal

• location of these bacteria depends on the animal species– horses and other herbivorous mammals

– house them in the caecum– rabbits and some rodents – LI + caecum

Ruminants & Digestion

IntestineRumen

Reticulum

Omasum

EsophagusAbomasum

– ruminants – deer, sheep and cattle• stomach has four chambers: rumen, reticulum,

abomasum & omasum• rumen – chewed grass first enters where it

encounters bacteria = bolus is formed• reticulum – some of the bolus moves into the

reticulum & the bacteria continue to digest– part of the bolus (called “the cud) is

regurgitated into the mouth to be chewed again

• omasum – when the cud is re-swallowed, it ends up here

• abomasum – cud moves into the abomasum containing the ruminant’s own digestive enzymes

Digestive Adaptations• dental adaptations:• 1. carnivores – large, pointed

incisors for biting and large canines for ripping– jagged pre-molars and molars for

shredding• 2. herbivores – broad, large pre-

molars and molars for grinding– modified incisors and canines for

biting plant material– some herbivores have no canines

• 3. omnivores – mixture of diets so teeth show both kinds of adaptations

Incisors

Carnivore

CaninesPremolars

Molars

Herbivore

Omnivore

• amphibian digestion and nutrition:– most are carnivores on a wide variety of

invertebrates– larvae are herbivorous – feed on plants and algae– true tongue first appears in the amphibian– many salamanders are relatively unspecialized in

their feeding methods – using only their jaws to capture prey

– but the anurans (e.g. frogs) have more advanced specializations • use their tongue and jaws to “flip and grab” its prey• tongue attaches at the anterior margin of the jaw and

folds back into the oral cavity• the tongue is flicked out at the prey • the tongue and prey are flipped back into the mouth

• reptile digestion and nutrition:– most are carnivores– tongues of turtles and crocodilians are non-protrusible

and aid in swallowing– tongue of some lizards is sticky – for prey capture– snakes possess many unique adaptations for eating

• tongue is not for eating – sensory• quadrate bone at the back of the skull acts as a double

hinge – allows for the jaws to be thrust forward• the snake then “walks” the food into its mouth• glottis is far forward so the snake can breathe while

slowly swallowing• food movement is produced by muscular contractions of

the body wall – NOT the digestive system itself

http://www.youtube.com/watch?v=T9COATjmaHg

• fish: digestive & nutrition– most fish are predators– usually swallow their prey whole– some fish have teeth – often use a suction force that is generated by the closing of

the operculum and the opening of the mouth – generates a negative pressure that sweeps water into the mouth containing their prey

HEART

Gills

LIVER

STOMACH

SWIM BLADDER

TESTES

INTESTINE

• birds: digestion and nutrition– large appetites to support a very high metabolic

rate (flight)– in many birds – the esophagus is associated with a

pouch = crop• storage structure• allow the bird to quickly ingest food and

digest it later– stomach is modified into two regions

• 1. proventriculus – secretion of gastric juices

• 2. ventriculus – or gizzard– mechanical digestion– some birds will swallow pebbles and

sand to aid in digestion in this region– bulk of digestion and absorption occurs in the

small intestine – undigested food is eliminated through the cloaca