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Summary BBS2041 Human Intermediary Metabolism
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Summary BBS2041: HumanIntermediary Metabolism
Case 01: Faecal Characteristics Teach Us a Lot + Lecture 02: Principles of Macronutrients
Anatomy of the GI tract
Tongue
Has intrinsic and extrinsic muscles
o Intrinsic: confined in the tongue and not attached to bone allow the tongue to
change shape: become thicker, thinner, longer, or shorter
o Extrinsic: extend to the tongue from their points of origin on bones of the skull or the
soft plate alter the tongue’s position
Saliva
Protects against microorganisms because it contains IgA antibodies, lysozyme (inhibits
bacterial growth), and defensins
Stomach
Parietal cells: secrete hydrochloric acid and intrinsic
factor
o HCl makes the stomach contents extremely
acidic (pH 1.5-3.5) optimal condition for
protein-digesting enzyme pepsin
o Intrinsic factor is needed for vitamin B12
absorption in the small intestine
Chief cells: produce pepsinogen, the inactive form of
pepsin and lipases
o Pepsinogen is activated by HCl and catalysed by
pepsin
o Lipases are fat-digesting enzymes
Enteroendocrine cells: release chemical messengers like histamine, serotonin, somatostatin
and gastrin
,Small intestine and associated structures
Highly adapted for absorbing nutrients circular
folds, villi and microvilli
Crypt epithelial cells secrete intestinal juice pH 7.4-
7.8 and isotonic with blood plasma
Enteroendocrine cells secrete secretin and
cholecystokinin
Paneth cells release antimicrobial agents such as
defensins and lysozyme
Peyer’s patches contain huge numbers of bacteria
that must be prevented from entering the
bloodstream located in lamina propria
Liver
Consists of hexagonal liver lobules consisting of plates
of liver cells or hepatocytes
At each corner is a portal triad that contains a branch of the hepatic artery, hepatic portal
vein, and a bile duct
Hepatic macrophages remove debris such as bacteria and worn-out blood cells from the
blood as it flows past
Hepatocytes
o Process bloodborne nutrients
o Store fat-soluble vitamins
o Play important roles in detoxixfication
Pancreas
Produces pancreatic juice drains from pancreas into
duodenum via the main pancreatic duct and joins with
the bile duct
Smaller accessory pancreatic duct empties directly into
the duodenum
Pancreatic islets release insulin and glucagon
carbohydrate metabolism
Pancreatic juice
o pH 8.0
o HCl of stomach is balanced by HCO3- of pancreas
o Pancreatic proteases are released inactively and
activated in the duodenum
Enteropeptidase: activates trypsinogen
and trypsin
Trypsin: activates more trypsinogen and converts procarboxypeptidase and
chymotrypsinogen to carboxypeptidase and chymotrypsin
Others: amylase, lipases, and nucleases are secreted in active form but
require presence of bile or ions for optimal activity
Large intestine
Appendix has a role in body immunity and serves as a storehouse of bacteria and recolonizes
the gut when needed; shortcoming enteric bacteria can accumulate easily and multiply
,Structure of the three macronutrients
Carbohydrates
Monosaccharides: (C-H2O)n, n>3
o Classified based on the number of carbon atoms as triose (n=3), tetrose (n=4),
pentose (n=5) etc.
o Glucose, fructose, galactose, ribose
Disaccharides: Cn(H2O)n-1, n>5
o Lactose, maltose, cellobiose, sucrose
o Two monosaccharides linked by condensation or dehydration synthesis glycosidic
bond is formed when the -OH of one sugar molecule joins with that of another sugar
molecule
Oligosaccharides are polysaccharides with a smaller number of sugar molecules
Polysaccharides: Cn(H2O)n-1, 200 < n < 2500 or (C6H10O5)n, 40 < n < 3000
o Cellulose, starch, glycogen
Proteins
, Fats
Formula: e.g. C18:2(n-3) or C18:23
o C18 = number of carbons
o 2 = number of double bonds
o N-3 or 3 = first double bond position from the omega side often 3-6 or 6-9
o Omega side = COOH side
Sn number refers to the glycerol backbone
Digestion of the three macronutrients
Carbohydrates
Monosaccharides are absorbed without further ado 3 common: glucose, fructose and
galactose
o Transport of glucose in the small intestinal epithelium: SGLT1 (classical pathway) and
GLUT2 (transmembrane carrier)
o Transport of fructose: GLUT5
Disaccharides sucrose, lactose, and maltose
Polysaccharides glycogen and starch
Intestine can absorb only monosaccharides, so all dietary carbohydrates must be digested to
monosaccharides
Most carbs are in the form of starch
Humans lack enzymes capable of breaking down most other polysaccharides such as
cellulose indigestible polysaccharides do not nourish but they do help move food along
the GI tract by providing bulk or fibre
Salivary amylase splits starch into oligosaccharides optimal pH: 6.75-7.00, which is
maintained in the mouth by the buffering effects of bicarbonate and phosphate ions in saliva
Starch digestion continues until amylase is inactivated by stomach acid and broken apart by
the stomach’s protein-digesting enzymes
Digestible carbohydrates that escape being broken down by salivary amylase are acted on by
pancreatic amylase in the small intestine
o Starch is entirely converted to various oligosaccharides, mostly maltose
Intestinal brush border enzymes further digest these products to monosaccharides
o Dextrinase and glucoamylase, which act on oligosaccharides composed of more than
three simple sugars, and maltase, sucrase, and lactase, which hydrolyse maltose,
sucrose, and lactose
Digestion officially ends in the small intestine because the colon does not secrete digestive
enzymes, but resident colon bacteria break down and metabolize the residual complex
carbohydrates and some proteins further, adding much to their own nutrition but essentially
to ours
Proteins
Proteins digested in the GI tract include not only dietary proteins, but also 15-25 g of enzyme
proteins secreted into the GI tract lumen by its various glands
Proteins are digested all the way to its amino acid monomers
Begins in the stomach when pepsinogen secreted by the chief cells is activated to pepsin
optimal in acidic pH found in the stomach: 1.5-2.5
Pepsin hydrolyses 10-15% of ingested protein and is inactivated by the high pH in the
duodenum proteolytic activity is restricted to the stomach
Trypsin and chymotrypsin secreted by the pancreas cleave the proteins into smaller peptides,
which in turn become grist for other enzymes(in the small intestine)
Pancreatic and brush border enzyme carboxypeptidase splits off one amino acid at a time
from the end of the polypeptide chain that bears the carboxyl group
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