Download lecture 15

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
•
•
•
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four classes of essential nutrients
1. essential amino acids:
2. essential fatty acids
3. vitamins
4. minerals
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 –
including 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
Vegetarian? Don’t worry you’re
covered
Essential Nutrients
• 2. essential fatty acids: fatty acids that contain one or more
double bonds and are unsaturated
– omega-3 and omega-6 families of fatty acids
– deficiencies in fatty acids are rare
Essential Nutrients
• 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 – e.g. vitamin B1
• the fat-soluble vitamins can act as hormones – e.g. vitamin D
– deficiencies result in a wide variety of diseases
distinct chemicals despite sharing a similar
name
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
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–
–
–
1. ingestion
2. digestion
3. absorption
4. elimination
Small
molecules
Pieces
of food
Mechanical
digestion
Chemical digestion
Nutrient
(enzymatic hydrolysis) molecules
enter body
cells
Undigested
material
Food
INGESTION
DIGESTION
• digestion occurs in specialized compartments
ABSORPTION
ELIMINATION
– prevents the animal from digesting itself
• compartments can be
– intracellular – digestion within the cell
• within food vacuoles
• occurs following phagocytosis or pinocytosis
• sponges use intracellular digestion entirely
– 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
– e.g. hydra
• 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
– also plays a role in the distribution of the
digested and absorbed nutrients
– other cells of the gastrodermis engulf the
smaller food pieces and continue
digestion intracellularlly
– waste is expelled out the same way the
food enter – via the mouth
• most animals possess a digestive tube
or alimentary canal – continuous tube
from mouth to anus
Extracellular
Digestion
Mouth
Tentacles
Gastrovascular
Food cavity
Epidermis
Mesoglea
Gastrodermis
Nutritive
muscular
cells
Flagella
Gland cells
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
• the pharynx can often be used like
the stomach in many animals
• other animals is subdivide their gut
into fore-, mid and hind-gut regions
Salivary
glands
Mouth
Esophagus
Gallbladder
Liver
Pancreas
Stomach
Small
intestines
Large
intestines
Rectum
Anus
A schematic diagram of the
human 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
• food is swallowed and travels by
peristalsis down the esophagus
• enters the stomach – chemical
and mechanical digestion
• enters the small intestine chemical and mechanical
digestion and absorption
• enters the large intestine absorption
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
– reflex that it controlled at the level of the medulla oblongata
– peristalsis = series of wavelike contractions in smooth muscle
– swallowing means you can’t breathe!
Bolus of
food
Tongue
Epiglottis
up
Pharynx
Esophageal
sphincter
contracted
Glottis
Larynx
Trachea
Esophagus
To lungs
Relaxed
muscles
Contracted
muscles
Sphincter
relaxed
To stomach
Stomach
The Stomach
• enters the stomach 
chemical and mechanical
digestion
– digestion of proteins and fats
– absorption of water and
some drugs
– mechanical digestion: three
layers of smooth muscle to
churn food
– chemical digestion:
production of gastric juice
– food mixes with gastric juice
to become chyme
The Stomach
Esophagus
Sphincter
Stomach
Sphincter
10 m
– chemical digestion is through
gastric juice: principally water and
mucus
– stomach is lined with a gastric
mucosa that folds in to form gastric
glands
– glands secrete the HCl and the
enzymes pepsin & lipase
– production of H+ and Cl- ions by the
parietal cells of the gastric gland
– production of gastric lipase &
pepsinogen by the chief cells of the
gastric gland
Small
intestine
Folds of
epithelial
tissue
Epithelium
Gastric pits on
interior surface
of stomach
3
Pepsinogen
Pepsin
2
Gastric gland
HCl
Chief
cell
Mucous cell
Chief cell
Parietal cell
1
Cl
H
Parietal
cell
The Small Intestine
• 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
Vein carrying
blood to liver
Villi
Microvilli (brush
border) at apical
(lumenal) surface
Lumen
Epithelial
cells
Blood
capillaries
Epithelial
cells
Muscle layers
Villi
Intestinal wall
Large
circular
folds
Basal
surface
Lacteal
Key
Nutrient
absorption
Lymph
vessel
The Small Intestine
• enters the small intestine  chemical and mechanical digestion
PLUS absorption
– 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 the chime flows over these absorptive cells = most
digestion is done in the duodenum
The Small Intestine and the Pancreas
– SI is also the site for the secretion of pancreatic juice – mixes
with the chyme in the duodenum
• pancreatic amylase, lipase and 4 proteases
• made as 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
The Small Intestine
– 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
• breakdown of proteins and peptides in the
stomach and SI  amino acids
• 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
Fat Absorption
• but the absorption of fats is different
• 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 into triglycerides in the
absorptive cell and combined with proteins,
cholesterol and phospholipids to form a
chylomicron  enters the lacteal
– chylomicrons eventually transferred to the
blood via the subclavian veins
LUMEN
OF SMALL
INTESTINE
Triglycerides
Epithelial
cell
Fatty acids
Triglycerides
Phospholipids,
cholesterol,
and proteins
Chylomicron
Lacteal
Monoglycerides
The Large Intestine
• enters the large intestine or colon  for absorption of water
and salts
• NO HUMAN ENZYME BASED DIGESTION!!!
The Large Intestine
• 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 Clinto the absorptive cells
The Large Intestine
• leftover, undigested food = feces
• digestion may take place in the colon
through the action of bacterial
enzymes
– mostly from E. coli
– by products are carbon dioxide, methane,
hydrogen sulfide, nitrogenous 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 in digestion
– storage of iron & copper
– storage of fatty acids
– production of LDL and
HDL
– main digestive function –
production of bile
Bile
– bile: water, cholesterol, bilirubin
and salts
• produced by hepatocytes
• secreted into the duodenum
• emulsification of fats – breakdown of
larger fats into smaller triglycerides
• PLUS – the breakdown of TGs into
monoglyceride (glycerol + 1 fatty
acid) and 2 fatty acids
• bile + monoglycerides or bile + fatty
acids form into micelles
• excess bile stored in the gallbladder
Fat globule
Bile salts
Fat droplets
coated with
bile salts
Micelles made
up of fatty acids,
monoglycerides,
and bile salts
Epithelium
of small
intestine
Epithelium
of 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 by the stomach and
small intestine
• stomach:
– gastrin – by the G cells of the stomach lining
• stimulates production of gastric juice and encourages emptying of the
stomach
• also relaxes the sphincter between the ileum and cecum (ileocecal)
Digestive Hormones
• small intestine:
– gastric inhibitory peptide (enterogastrone) – 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)
• also slows the emptying of the stomach
• 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
• decreases gastrin production and release
• increases pepsinogen
Digestive Feedback systems
• emptying of the stomach:
– production of gastrin – stimulates emptying
– production of GIP/enterogastrone and CCK – inhibits emptying
• pancreatic juice production:
– secretin and CCK – stimulation of production
• bile production:
– CCK – stimulation of secretion
Key
Liver
Enterogastrone
Gallbladder
Stimulation
Inhibition
Gastrin
CCK
Stomach
Pancreas
Secretin
Duodenum
CCK
Appetite control
• satiation center = hypothalamus
• ghrelin – made by the stomach wall
– triggers feelings of hunger – stimulates appetite when the stomach is empty
– levels increase in dieters!
• 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
– surges of insulin can result in feelings of hunger quicker!
• leptin – produced by adipose tissue
– suppresses appetite
– as body fat levels drop, so does leptin production and appetite may increase
• some animals have developed a complex
mutalistic association with these bacteria
• allow for the digestion of plant-based
materials by herbivores
– e.g. digestion of cellulose 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
Digestion
Adaptations
– ruminants – deer, sheep and cattle
• specialized adaptations to their gut
• stomach has four chambers: rumen, reticulum, abomasum & omasum
• rumen – chewed grass first enters rumen where it encounters bacteria = bolus is
formed (20 to 80 quarts of saliva produced a day!)
– largest of the four chambers – 25 gallons
– storage area for food
– digestion of cellulose starts here
Intestine
Rumen
Reticulum
Abomasum
Esophagus
Omasum
• reticulum – honeycombed in structure
– bolus moves into the reticulum & the bacteria continue to digest
– ruminant periodically regurgitates and re-chews parts of the bolus (called “the
cud) in its mouth to continue physically breaking down the grass
» because cattle don’t spend much time actually chewing the first time
– chemical digestion of cellulose continues in reticulum
Intestine
Rumen
Reticulum
Abomasum
Esophagus
Omasum
• omasum – when the cud is re-swallowed, the bolus will eventually end up
here
– water is removed
• abomasum – “true stomach”
– cud moves into the abomasum containing the ruminant’s own digestive enzymes =
protein digestion
• food then moves into the intestine for absorption – 20X length of animal
Intestine
Rumen
Reticulum
Esophagus
Abomasum
Omasum
Windows into the Cow Gut
• farmers install “windows” (i.e. fistula) in some cows
• allows them to see and sample the food being digested in the cow’s
stomach
• cow is not bothered by this fistula
• also a port for the administration of medicines or beneficial microbes
Digestive Adaptations to the Jaw
• dental adaptations:
• 1. carnivores – large, pointed
incisors for biting and large canines
for ripping
– jagged pre-molars and molars for
shredding
Incisors
Premolars
Carnivore
• 2. herbivores – broad, large premolars 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
Molars
Canines
Herbivore
Omnivore
• amphibian digestion and nutrition:
–
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–
–
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
• some can “flick” tongues out at incredible speeds https://www.youtube.com/watch?v=zcWxAfl0okE
• amphibian digestion and nutrition:
– 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
• may take approx. 0.05 to 0.15 seconds for the complete
flip and grab
• the prey is pressed up against the teeth and the muscles
of the mouth push it into the esophagus
https://www.youtube.com/watch?v=MFlHHG57V2U
https://www.youtube.com/watch?v=CmoVURYBc4Q
https://www.youtube.com/watch?v=wXqK5QulbJ8
liver
stomach
heart
intestine
liver
gallbladder
intestine
stomach
Bullfrog diet
• 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
• https://www.youtube.com/watch?v=cwAHjl9PZH4
• some demonstrate incredible speeds and accuracy https://www.youtube.com/watch?v=eN8L2ESXMPM
• reptile digestion and nutrition:
– snakes possess many unique adaptations for eating
• tongue is not for eating – sensory
• modifications to the skull - lower jaw can spread apart to allow for swallowing of
whole prey
• quadrate bone at the back of the skull acts as a double hinge – allows for the
lower jaw to be unhinged and 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
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most fish are predators
usually swallow their prey whole
some fish have teeth – capture and hold prey
some teeth are modified to crush the shells of molluscs and arthopods
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
SWIM BLADDER
LIVER
INTESTINE
Gills
HEART
STOMACH
TESTES
• birds: digestion and nutrition
– large appetites to support a very high metabolic
rate (flight)
– bills and tongues are modified for a variety of
feeding habitats and food sources
– in many birds – the esophagus is associated with
a pouch = crop
• storage structure
• allow the bird to quickly ingest food and
digest it later
• also used to store food for regurgitation to
young
– stomach is modified into two regions
• 1. proventriculus
– secretes gastric juices to initiate
digestion
• 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
•
•
•
•
aided by secretions from the pancreas and liver
contain bacteria for cellulose digestion
undigested food is eliminated through the cloaca
however owls will expel their “pellets” through the
mouth