Summary BBS2041 Human Intermediary Metabolism

Summary BBS2041: Human
Intermediary 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 C18:23
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