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Unformatted text preview: Diet & Digestion • Macronutrients
nutrition – Tiny amounts of vital elements & compounds our body
cannot adequately synthesize
5. Minerals • Hydration
6. Water Hydrolytic Enzymes 1. Ingestion
Consuming food 2. Motility
– Swallowing, peristalsis, segmentation, defecation 3. Physical disruption
– Mastication, dissolution, denaturation , emulsification 4. True digestion
– Hydrolysis of polymers into monomers 5. Absorption
– Moving nutrients from G.I. Tract into circulatory system 6. Assimilation
– Incorporate nutrient molecules into tissues Vertebrate Digestive Tract Energy for all our endergonic processes
Monomers to build our polymers
Protein • Micronutrients Food Processing
– Why eat? Hydrolysis reactions catalyzed by enzymes
– Remember, enzyme names end with “-ase” •
• Amylase: hydrolyze carbohydrates
Protease: hydrolyze proteins
Peptidase; hydrolyze peptides
Lipase: hydrolyze lipids
Nuclease: hydrolyze nucleic acids Digestive System • Alimentary Canal (26-foot long tube)
– Gastrointestinal [G.I.] tract (“gut”)
• Small Intestine
• Large Intestine
• Accessory Organs
– Salivary Glands
– Liver & Gall Bladder
– Pancreas Heyer 1 Diet & Digestion Digestion Begins in Mouth Swallowing
• Salivary glands – Water & mucus: moisten/lubricate food for swallowing • Pharynx – Initiate swallowing reflex
– Divert food bolus from respiratory tract • Esophagus – uses peristaltic action • Salivary glands produce Amylase
– Enzyme hydrolyzes starch
– Cleans teeth • Saliva also contains mucous, buffers,
• Slug of mashed food and saliva is a Bolus Peristalsis Gut Motility
• Peristalsis — waves of
– Esophagus, stomach,
small intestines • Segmentation —
– Stomach, small intestines • Sphincters — muscular
rings that control
– Esophageal sphincter
– Pyloric sphincter
– Anal sphincter Stomach
• Distensible: stores swallowed food pending further
processing. Sealed by sphincters at either end.
• Mucosa wrinkled to form rugae that increase surface area
• Highly motile. Regulates rate of passage to intestines.
• Many gastric pits leading
to gastric glands secreting
• Masticated food
+ digestive juices
• Acid in gastric juice
disinfects the chyme Heyer Muscular action in the human gut Gastric Glands Gastric Juice:
A. Goblet cells ‡ alkaline mucus
• Helps protect mucosa from auto-digestion.
B. Parietal cells ‡ hydrochloric acid [ HCl ] ‡ pH < 2
• Disinfects chyme ; denatures protein
C. Chief cells ‡ pepsinogen ( a weak protease)
• Auto-hydrolyzes to form pepsin (stronger protease)
D. Other cells produce hormones 2 Diet & Digestion Protein Digestion
Begins in the Stomach Regulation of Gastric Activity 1. Acid denatures protein
in the chyme
– Unfolded protein structure
more vulnerable to
hydrolysis 2. Pepsin hydrolyzes
polypeptides into short
3. Digestion and
absorption completed in
small intestines Digestion Completed in Small Intestine • Gastric secretion and motility stimulated
by parasympathetic neurons, intrinsic
stretch reflex, gastrin hormone, and
amino acids in chyme (positive
• Inhibited by stretch reflex and hormones
from intestines, or chyme pH <1.5. Visceral Digestive Accessory Glands • Chyme enters via the pyloric
• First segment = duodenum:
secretions from accessory
glands enter via common bile
• First 40% = duodenum +
jejunum: most digestion and
absorption of macro- &
• Final 60% = ileum: most
water and salts. Liver: the biochemistry lab Liver to Gall Bladder:
Bile Salts: Fat Emulsifier
• Liver produces bile salts &
• Bile is stored & released by
Gall Bladder into Small
• Emulsify: Bile salts break
large fat globs into small
droplets (detergent action). Heyer 3 Diet & Digestion Small
Intestine • Exocrine component — Acini: Pancreas – Secrete pancreatic juice.
• Bicarbonate to neutralize acid chyme
• Suite of hydrolytic enzymes • Endocrine component — Islets:
– Secrete insulin and glucagon. Carbohydrate digestion
salivary & pancreatic amylase Absorption enhanced by large surface area & high vascularization:
• Lining wrinkled into ridges ( plicae ) and finger-like protrusions ( villi).
• Apical surface of mucosal epithelium wrinkled into microvilii (brush border).
• Core of each villus bears dense capillary beds surrounding a central lacteal
• Capillaries carry absorbed substances to the hepatic portal vein and then to
the liver. Lacteal transports to central veins and general circulation. Carbohydrate Digestion — Summary
1. Hydrolysis of polysaccharides (poly-glucose) starts
with salivary amylase. • Minor significance — amylase denatured by gastric acid. brush border enzymes 2. Pancreatic amylase in duodenum & jejunum
hydrolyzes polysaccharides into maltose & maltriose.
3. Brush border enzymes hydrolyze tri-/disaccharides
4. Brush border transporters absorb monosaccharides
by active cotransport into hepatic portal vein.
5. Insulin dependent: monosaccharides taken up by liver
for glycogenesis, or circulate as energy substrate for
glycolysis or lipogenesis. Protein Digestion — Summary
1. Proteins are denatured by gastric acid.
2. Hydrolysis of exposed polypeptides starts with pepsin
in gastric juice under acidic pH.
3. Pancreatic proteases & peptidases, plus brush
border peptidases in duodenum & jejunum hydrolyze
poly-/oligopeptides into tri-/dipeptides & amino acids.
4. Brush border transporters absorb tri-/dipeptides &
amino acids into mucosal epithelia. Intracellular
peptidases hydrolyze tri-/dipeptides into amino acids.
Amino acids absorbed into hepatic portal vein.
5. Insulin dependent: amino acids taken up by liver for
gluconeogenesis, or circulate as substrate for protein
synthesis Heyer Lipid Digestion — Summary
1. Fats & oils are emulsified by bile salts (detergent) in
duodenum & jejunum .
2. Pancreatic lipases in duodenum & jejunum hydrolyze
triglycerides into monoglycerides & fatty acids.
3. Monoglycerides & fatty acids, plus cholesterol, coalesce
with bile salts to form micelles.
4. Brush border transporters absorb micelles into
mucosal epithelia. Intracellular liposynthases
condense monoglycerides & fatty acids into new
triglycerides. Triglycerides & cholesterol are bound to
carrier proteins to form chylomicron lipoprotein.
5. Chylomicrons are absorbed into the lacteal and systemic
circulation. Lipoprotein-lipase of vessel endothelia
releases fatty acids & glycerol for tissue lipogenesis. 4 Diet & Digestion Absorption/Assimilation of Lipids Large Intestine
• Final water absorption
• Compaction, storage, & excretion of feces
– 1/3 bacteria by weight (some make K & B vitamins).
– Brown color from bile pigments of RBC breakdown. Insert fig. 18.36 Carnivores Have Short Tracts
Herbivores Have Long Tracts
stomachs. WHAT HAPPENS TO IT ALL? • Aided by
or bacteria. WHAT HAPPENS TO IT ALL? Fate of Chemical Energy
• Stored as
– circulating energy substrates
– liver & muscle glycogen
– body fat • Stored energy used for
– 1) metabolism
– 2) growth
– 3) reproduction Heyer 5 ...
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