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Carbohydrates:
o Carbs are stored in the liver and muscle cells as glycogen
Vitamins and Minerals and essential nutrients:
o Some molecules necessary for synthesizing the carbon skeleton must be
ingested because the body cannot make them itself = essential amino acids
o Essential fatty acids need to be consumed
o Vitamins are needed in small amounts and often function as coenzymes
o Excess lipid soluble vitamins are stored as fat and excess water soluble vitamins
are excreted with urine
Proteins:
o Dietary proteins are completely digested into their constituent amino acids so
they can be easily absorbed and resynthesized into what you need
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o Kwashiorkor is a disease you get when you are protein deficient
Animal adaptations:
o Herbivores have low metabolism because vegetables are hard to digest
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o Enamel composed of calcium phosphate
o Dentine is a bony layer that the crown and root contain
o Pulp cavity is inside dentine and contains vessels, nerves, and the cells that
create dentine
o Incisors for cutting, chopping, gnawing, and canines
o Canines for stabbing, gripping, ripping
o Molars and premolars for shearing, crushing, grinding
Animal digestive systems
o Sponges have the least specialized digestive system
o The next least specialized digestive system is the gastrovascular cavity
§ One opening
§ Enzymes partially digest food
§ Cells lining cavity take in small amount of food by endocytosis
o Tubular guts have a mouth and an anus with specialized segments
§ Endosymbiotic bacteria colonize intestines and obtain their nutrition from
the food that passes by
• Provide digestive services and prevent the establishment of
harmful microorganisms
• Produce vitamin K and biotin
o Folding increases surface area for greater digestive absorption
§ Typhlosole in earthworms
§ Spiral valve in sharks
§ Villi and microvilli in humans
Enzymes:
o Hydrolytic enzymes break down macromolecules into their simplest monomeric
units
§ Proteases break down amino acids
§ Carbohydrases break down carbohydrates
§ Peptidases break down peptides
§ Lipases break down fats
§ Nucleases break down nucleic acids
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Intestinal tissue layers are arranged in concentric layers:
o The internal cavity is called the lumen
o The innermost layer is the mucosa
§ Epithelial cells with underlying connective tissue
§ Some cells secrete mucous, some cells secrete enzymes (zymogens),
some cells secrete hormones
§ Absorptive
o The next layer is the submucosa which contains blood and lymph vessels that
carry the absorbed nutrients to the body
§ Also contains a nerve network that have sensory functions (when you feel
a stomach ache) and control various secretory functions
o Next are two layers of smooth muscle
§ Longitudinal on top of circular
§ Circular muscles constrict and longitudinal muscles shorten
§ There is a nerve net between the two layers of muscle that controls the
movement of the gut
o The stomach has a third muscle layer that is oblique and helps with churning
o The nerve nets are called the enteric nervous system and only synapse with
neurons within that system
o The peritoneum is the outermost layer
§ Surrounds the gut
§ Connective and epithelial tissue
§ Secretes fluid to lubricate organs so they can slide past each other
Process of digestion
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Food enters mouth and you chew = mechanical digestion
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Tongue pushes food back
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Swallowing is automatically initiated
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Epiglottis covers glottis (opening to larynx) so food doesn’t go down trachea
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Peristalsis pushes food down esophagus
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Food enters stomach
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Gastrin is released from the stomach into the bloodstream and reenters the
stomach
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Parietal cells pump out acid to lower pH in stomach and kill harmful
microorganisms
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Higher pepsin amount activate release of more pepsinogen =
positive feedback
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Carbs are broken down into glucose
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Waves of contractions cause the pyloric sphincter to contract briefly
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The acidic chyme stimulates the small intestine to release secretin
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Digestion of carbs and proteins continues in the small intestine
High blood glucose levels target the pancreas to release insulin
Insulin circulates in the blood and promotes the cells to uptake glucose for metabolic
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activity
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Blood glucose level drops
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Glucagon circulates in blood
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Glucose is released into blood
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Mucosal epithelial cells produce small peptidases that cleave small peptides into
absorbable amino acids
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Epithelium absorbs these nutrients
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Full fat cells stimulate the release of leptin which targets the brain to make you
feel full
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Secreted through pancreatic duct
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Sphincter between duct and duodenum relaxes
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CCK targets the gallbladder to contract
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Sphincter at junction of common bile duct and small intestine relaxes in response
to waves of peristalsis
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Bile emulsifies fat → micelle
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Lipases break down micelles
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Lipid soluble = able to pass through plasma membranes into the cells of the small
intestine epithelium
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Triglyceride + cholesterol + phospholipid = chylomicron
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Chylomicrons pass into blind ended lymph vessels called lacteals that are in
each villus by exocytosis
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Most absorption of nutrients happens in ileum and jejunum
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Lipids from chylomicrons are stored as triglycerides or used to make lipoproteins
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Hepatic portal vein delivers blood to small spaces between groups of liver cells
called sinusoids
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Weak peristaltic activity continues to move gut content towards large intestine
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Segmentation movement pushes contents against large intestine walls promoting
water and ion absorption
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Water moves in the spaces between cells and carries nutrients that are in the
solution
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Anal sphincter relaxes to allow defecation