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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