4. VMED 5109 - Digestion and Absorption

Foreign food protein, which would be very antigenic, does not normally enter the blood. An interesting exception is the protein toxin (Botox) that causes botulism, produced by the bacterium __________. This protein is resistant to digestion and is thus intact when it is absorbed into the blood.

Clostridium botulinum

Peptidase enzymes and functions?

1. Aminopeptidase - produces free amino acids, dipeptides, and tripeptides 2. Enterokinase - cleaves trypsinogen to active trypsin; deficiency results in protein malnutrition

Chitin?

Chitin is similar to cellulose. β1-4 linkage requires cellulase. Exoskeleton, lobster shell.

Absorption of di- and tri-peptides in the small intestine by co-transport with H+ ions via a transporter called _______.

PepT1.

In the luminal membrane, ______ transports/ absorbs Glucose and Galactose.

SGLT1

Digestion and absorption mainly occur in the _______.

Small Intestine

Newborn babies appear to be capable of absorbing a substantial amount of undigested proteins, hence they can absorb _______ ( γ-globulins: IgG, IgA and IgM) from colostrum which is their mother's first milk. In adults, however, only the _______ enter the portal vein.

antibodies free amino acids

Triglycerides, phospholipids, and cholesterol are then combined with protein (________) inside the enterocytes to form small particles called ________.

apoprotein chylomicrons

Cellulose is a glucose polymer in which repeated glucose units are bonded by______ linkages.

beta (β1-4)

Brush Border Enzymes are not secreted into the lumen, but instead attached to the ___________.

cell membrane of microvilli in the small intestine with their active sites exposed to the chyme

β1-4 linkage breakdown requires _______.

cellulase

Plant fibers are made from _______.

cellulose

These chylomicrons are secreted into the _________ (lymphatic capillaries) of the intestinal villi.

central lacteals

Pancreatic enzyme ________ digest Cholesteryl esters into additional FAAs and free cholesterol.

cholesterol ester hydrolase

Fat digestion occurs at the surface of the droplets through the enzymatic action of pancreatic lipase which is aided in its action by a protein called _______ (also secreted by the pancreas) that coats the emulsification droplets and "anchors" the lipase enzyme to them.

colipase

The small Intestine has a large surface area for both _______ and ________.

digestion absorption

The arrival of lipids (primarily triglyceride, or fat) in the _______ serves as a stimulus for the secretion of _______ from the liver.

duodenum bile

In a process called ________, bile salt are secreted into the duodenum and act to break up the fat droplets into tiny emulsification droplets of triglycerides called micelles.

emulsification

Inside the _______, free fatty acid, monoglyceride and lysophospholipids resynthesize triglycerides and phospholipids. This process is different from the absorption of amino acids and monosaccharides, which pass through the enterocytes without being altered.

enterocytes

Brush border enzymes: In addition to providing a large surface area for absorption' the cell membranes of the microvilli contain digestive enzymes that hydrolyze disaccharides, polypeptides, and other substrates. These brush border enzymes are not secreted into the lumen, but instead remain attached to the cell membrane with their active sites exposed to the chyme. One brush border enzyme, ____________ is required for activation of the protein digesting enzyme trypsin, which enters the small intestine in pancreatic juice.

enterokinase (enteropeptidase)

Absorption occurs at a rapid rate as a result of _________ of the intestinal mucosa, which greatly increases its absorptive surface area.

extensive foldings

Once inside the cell, specific _______ in cytoplasm completely disassemble the remaining peptides into individual amino acids which move across the cell and pass into the blood.

peptidases

The mucosa and submucosa form large folds, called __________, which can be observed with the unaided eye.

plicae circulares

In adults, fat digestion mainly occurs in the _______ by pancreatic ______.

small intestine lipase

Digestion of proteins is the first enzymatic action in the _______, but occurs mainly in the _______.

stomach small intestine

The emulsification of lipid aids digestion because the smaller and more numerous emulsification droplets present a greater ________ than the unemulsified fat droplets that originally entered the duodenum.

surface area

Free fatty acid, ______ and _______ can leave the micelles and enter enterocytes (the epithelial cells of small intestine) by directly passing through the membrane of microvilli.

monoglyceride lysolecithin

Carbohydrate digestion starts in the ______, but primarily takes place in the ________.

mouth small intestine

Gastric pepsinogen is activated by ______ to pepsin starting hydrolysis of peptide linkages. Pepsin also attacks ________ (major component of connective tissue and is not digested by other proteases ).

HCl collagen

Absorption of amino acids via _______ and ________ that is specific for acidic, basic, neutral amino acids

Na+ amino acid co-transport

Exceptions of intact proteins that can be absorbed without digestion?

1. Immunoglobulin (antibodies) 2.Botulinum toxin (BoNT, Botox)

Disaccharides?

1. Maltose: glucose + glucose 2. Sucrose: fructose + glucose 3. Lactose: galactose + glucose

Polysaccharides?

1. Starch: from plants -Amylose (unbranched) and Amylopectin (branched) 2. Cellulose: plant fibers (unbranched) 3. Glycogens: "animal starch" (branched)

Disaccharidase enzymes and functions?

1. Sucrase - digests sucrose to fructose; deficiency produces GI disturbances 2. Maltase - digests maltose to glucose 3. Lactase - digests lactase to glucose and galactose; deficiency produces GI disturbances (lactose intolerance)

Brush border enzymes?

Aminopolypeptidase and several dipeptidases split the remaining polypeptides into di- and tri-peptides and a few amino acids. These two peptidases also facilitate peptide movement into the enterocyte.

How are carbohydrates, proteins and fats digested? How are the end products absorbed?

Food → digestion → monomers → absorption in small intestine → blood or lymph

In the basal membrane, ______ transports/absorbs glucose and galactose.

GLUT2

In the basal membrane, _______ transports/absorbs fructose.

GLUT5

In the luminal membrane, ______ transports/absorbs fructose.

GLUT5

________ have bacterial cellulase. Most _______ animals do not have cellulase.

Herbivores monogastric

Absorbed lipids thus pass through the _______ system eventually entering the venous blood by way of the thoracic duct. By contrast, amino acids and monosaccharides enter the _________.

Iymphatic hepatic portal vein

Pancreas-derived enzymes and their functions?

Trypsin and chymotrypsin cleave peptide bonds. Carboxypolypeptidase cleaves individual amino acids off the carboxyl end of the polypeptide. Elastase attacks elastin fibers.

Starch is a glucose polymer in which repeated glucose units are bonded by ________ linkages.

alpha (α1-4)

Fat is hydrolyzed by the action of _____.

lipase

Once the chylomicrons are in the blood, their triglyceride content is removed by the enzyme _______, which is attached to the endothelium of blood vessels. This enzyme hydrolyzes triglycerides and thus provides free fatty acids and glycerol for use by the cells.

lipoprotein lipase

Pancreatic enzymes ______, ________, _________ and _________ continue hydrolysis in the small intestines.

trypsin chymotrypsin carboxypolypeptidase elastase

Through hydrolysis, pancreatic lipase removes ______ of the three fatty acids from each triglyceride molecule and thus liberates two free fatty acids (FAAs) and one 2-monoglyceride.

two

Pancreatic lipase digest most fats into:

two FFAs + one 2-monoglyceride !!!

The surface area is further increased by microscopic folds of mucosa, called _______, and by foldings of the apical cell membrane of epithelial cells (which can be seen only with an electron microscope), called ________.

villi microvilli