11

Question 1

Most of the digestive enzymes are secreted as

Answer 1

inactive zymogens.
 Zymogens contain extra amino acids prevent them from being catalytically active.

Question 2

Digestion( CHO ,Fats, Protein)

Answer 2

Breakdown of large macromolecules into smaller
components by the digestive enzymes.
CHO→ begins in mouth, continues in small intestine
and ends by monosaccharides.
Fats→ begins in the stomach & digested in small
intestine producing FAs, glycerol & cholesterol.
Protein→ begins in stomach; continues in small
intestine producing amino acids.

Question 3

Enzymes needed for degradation of most dietary
CHO are mainly:

Answer 3

1) Disaccharidases.
2) Glycosidases:→ breakdown of oligosaccharides
and polysaccharides.

Question 4

The major polysaccharides are……….&……… that consist of ……….&………

Answer 4

The major polysaccharides are glycogen & starch
that consist of amylose & amylopectin.

Question 5

DIGESTION OF CHO IN MOUTH: by

DIGESTION OF CHO IN SMALL INTESTINE: by
Final CHO Digestion by Enzymes In Intestinal Mucosal Cells:

Answer 5

DIGESTION OF CHO IN MOUTH: by salivary α-amylase randomly on starch, breaking some α(1→4) bonds.
Both branched amylopectin and glycogen containα(1→6) bonds, digestion by α -amylase

DIGESTION OF CHO IN SMALL INTESTINE: by pancreatic α-amylase

Final CHO Digestion by Enzymes In Intestinal Mucosal Cells:
 Maltase: cleaves α(1→4), 2 α-glucose molecules.
 Isomaltase: cleaves the α(1→6) , 2 α-glucose molecules.
 Sucrase: ⍺-glucose & one β-fructose molecules.
 Lactase cleaves β (1→4) one β- galactose and one β-glucose molecules.

Question 6

ABSORPTION OF MONOSACCHARIDES in …….,……………..
By 3 transport

Answer 6

The duodenum and upper jejunum
 SGLT-1 is coupled to Na+-K+ pump, allowing glucose & galactose to be transported against their concentration gradients.
 The GLUT-5 is Na+-independent facilitative transporter that allows transport of fructose, glucose & galactose with their concentration gradients.
 GLUT-2 allow exit of all the sugars
outside the cell, that is facilitative transporter.

Question 7

An adult ingests about 60-150 g of lipids/d, of which

Answer 7

> 90 % is TAG.
 The remaining 10%:
 Phospholipids,  Cholesterol,
 Cholesteryl esters,
 Unesterified free fatty acids (FFA).

Question 8

Digestion of lipids begins in ……….. by…………….,………

Answer 8

Digestion of lipids begins in stomach, by an acid-stable lingual lipase, secreted by glands at the back of tongue.for those containing fatty acids of short- or medium-chain lengths
 TAGs are also degraded by a separate gastric
lipase, secreted by the gastric mucosa.
- Both enzymes are relatively acid-stable (optimum pH 4-6).
- lipid digestion in neonates
-important in individuals with pancreatic insufficiency,

Question 9

LIPID EMULSIFICATION :
 Occurs in
Performed by two complementary mechanisms:

Answer 9

duodenum
Performed by two complementary mechanisms:
1) Detergent properties of bile salts.
2) The mechanical mixing due to peristalsis.
Emulsification ↑ surface area

Question 10

Bile salts are synthesized by …….. stored in………..
Function:

Answer 10

Bile salts are synthesized by the liver from cholesterol and are stored in the gallbladder.

They are emulsifying agents that interact with the dietary lipid particles & the aqueous duodenal contents, stabilizing the particles as they become smaller and preventing them
from coalescing.

Question 11

Dietary TAG, cholesteryl esters & phospholipids are
enzymatically degraded by

Answer 11

pancreatic enzymes

Question 12

DEGRADATION of TAG:

Answer 12

Pancreatic lipase acts on TAG & removes FAs at
carbons 1 & 3 → FFAs + 2-monoacyl glycerol.

Question 13

Cholesteryl esters are hydrolyzed by

Answer 13

pancreatic cholesteryl esterase → cholesterol + FFA.
 Its enzyme activity is greatly ↑ in the presence of
bile salts.

Question 14

PHOSPHOLIPID DEGRADATION

Answer 14

Pancreatic juice is rich in phospholipase A 2, that
is activated by trypsin & requires bile salts for optimum activity.
 Phospholipase A2 & lysophospholipase act on
phospholipids → 2 FAs + glyceryl-phosphoryl base
excreted in faeces, further degraded or absorbed.

Question 15

CONTROL OF LIPID DIGESTION:
Secretion of pancreatic enzymes is under control

Answer 15

Mucosal cells of the lower duodenum and jejunum
produce cholecystokinin (CCK) ,secretin

(CCK) hormone in response to the presence of lipids & partiallydigested proteins.
CCK acts on:
 Gallbladder → its contraction → bile release.
 Pancreas on its exocrine cells → release of its digestive enzymes.
 ↓ Gastric motility → slower release gastric
contents into the small intestine.

secretin: in response to the low pH of the chyme entering the intestine.
Secretin causes the pancreas & liver to release a watery solution rich in bicarbonate

Question 16

primary products of digestion of dietary lipids?
……..,………,………. together form………..

Answer 16

Free FA, free cholesterol & 2- monoacyl-glycerol
together with bile salts, form mixed micelles
Short-& medium-chain length FAs do not require the assistance of mixed micelles for absorption.

Question 17

RESYNTHESIS OF TAG & CHOLESTERYL ESTERS:

Answer 17

 Fatty acids are activated to → fatty acyl CoA,
 Then, fatty acyl CoA + 2-monoacyl glycerol → TAG
by triacylglycerol synthase enzyme.
 Cholesterol is esterified to → cholesteryl ester
(CE) by ACAT enzyme( acyl cholesterol acyl
transferase enzyme).

Question 18

SECRETION OF LIPIDS FROM ENTEROCYTES:

Answer 18

Short- & medium-chain length FAs are released into the portal circulation, where they are carried by serum albumin → to liver.
 Newly synthesized TAG & cholesteryl esters
(hydrophobic) are packaged & surrounded by a thin
layer of phospholipids, unesterified cholesterol &
apolipoprotein B-48 → forming chylomicrons.
Chylomicrons are released by exocytosis from
enterocytes into lymphatic system → to the blood.

Question 19

Proteolytic enzymes that degrade proteins are
produced by 3 different organs:

Answer 19

 Stomach,  Pancreas,  Small intestine.

Question 20

Digestion of proteins begins in …………which
secretes gastric juice containing …………,………

Answer 20

Digestion of proteins begins in stomach, which
secretes gastric juice containing HCL & pepsin.
1- HCl: Stomach acid is too dilute (pH 2-3).
 Its functions are to: A- Denatures proteins.
B- Hydrolyses proteins. C- Kills some bacteria.
2- Pepsin: It is an acid-stable endopeptidase.
Secreted as an inactive zymogen “pepsinogen”. It is activated to “pepsin” either by: HCl or by auto-activation by other pepsin molecules which were activated.
It releases peptides and a few free amino acids
from dietary proteins.

Question 21

In the small intestine large polypeptides are further cleaved to oligopeptides and amino acids by a group of

Answer 21

Pancreatic Proteases. The release and activation of the
pancreatic zymogens is mediated by the secretion of CCK and Secretin.

Enteropeptidase secreted by the
intestinal mucosal cells activates the
pancreatic zymogen trypsinogen to
trypsin.

Question 22

DIGESTION OF OLIGOPEPTIDES BY ENZYMES OF THE SMALL INTESTINE

Answer 22

The luminal surface of the intestine contains “aminopeptidase”.
 It is an exopeptidase that repeatedly cleaves
the amino acid at the N-terminal end

Question 23

ABSORPTION OF AMINO ACIDS , DI- & TRIPEPTIDES:

Answer 23

Free amino acids & dipeptides are taken up by a Na+-linked
Di- & tripeptides are taken up by a H+- linked

peptides are hydrolyzed in cytosol to amino acids before being released into portal system.

Question 24

The hormones that control digestion are:

Answer 24

a- Gastrin.
b- Secretin.
c- Cholecystokinin (CCK).
They are released into the blood

1) Gastrin: Causes the stomach to produce HCl for
dissolving and digesting some foods.
 It is also necessary for the normal growth of the
lining of the stomach, small intestine and colon.
2) Secretin:
It causes the pancreas to send out its digestive
juice that is rich in bicarbonate.
It stimulates the stomach to produce pepsin and
also stimulates the liver to produce bile.
3) CCK:  Causes the pancreas to grow and to produce
the enzymes of the pancreatic juice. causes the gallbladder to empty.