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Gastrointestinal System > Digestion and Absorption > Flashcards

Digestion and Absorption Flashcards

1
Q

Digestion of Carbohydrates

A
  1. The action of salivary amylase in the mouth begins to digest the starch to shorter glucose chains
  2. As soon as food reaches the stomach,the stomach acid inactivates the amylase and protease destoys the enzyme.
  3. In the small intestin,pancreatic amylase completes the digestion of starch to maltose.Specific enzymes digest disaccharides to monosaccharides which are absorbed in the bloodstream
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2
Q

Types of Carbohydrates

A

Vegetable starches- Polysaccharides:amylopectin and amylose

Sucrose-Disaccharides: Glucose and fructose

Lactose-Disaccharide: Glucose and galactose which is found in MMP

Dietary fibre-Indigestible polysaccharides

Inositol-Essential vitamin like components

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3
Q

What does the salivary gland secrete

A

The salivary gland secretes about 1 litre of fluid per day containing
salivary mucin and salivary a-amylase - both glycoproteins.

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4
Q

What is the importance of the Salivary

mucin

A

is important for lubrication and for dispersion of the

polysaccharides.

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5
Q

Salivary alpha-amylase

A

alpha-Amylase randomly hydrolyses internal -1,4
glycosidic bonds between glucose units within amylopectin, amylose, and
glycogen, converting the large polysaccharides into smaller intermediate
oligosaccharides, both unbranched oligosaccharides and branched
oligosaccharides (called alpha-limit dextrins).

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6
Q

Pancreatic Alpha-amylase

A

The digestive process continues as chyme moves from the stomach into the upper part of the
small intestine. After neutralisation of the acidic chyme by HCO3 -to obtain an optimal pH,

Pancreatic alpha-amylase continues to hydrolyse the alpha-1,4 glycosidic bonds in the remaining
fragments of starch.

Pancreatic amylase is produced in larger amounts than salivary amylase.

The products of starch and glycogen degradation at this stage are :
1. Maltoses (disaccharides
containing glucose units connected by alpha-1,4 bonds)

  1. Isomaltoses (connected by alpha-1,6
    bonds)
  2. Maltotrioses and
  3. Some intermediate oligosaccharides (containing from 3 to 9 glucose residues, some including the -1,6 branching bond) (known as alpha-limit dextrins).

All these molecules now enter the jejunum.

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7
Q

Role of brush border glycosidases

A

Glycosidases, enzymes that hydrolyse glycosidic bonds, include the
brush border disaccharidases.

They are present as a number of large
glycoprotein enzyme complexes attached to the brush border membranes
of absorptive cells in the intestinal villi.

Their active sites protrude from
the microvilli membrane toward the intestinal lumen.

They are, therefore refered to as ecto-enzymes.

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8
Q

List the ecto-enzymes:

A
  1. Sucrase-isomaltase complex
  2. Gluco-amylase complex
  3. Lactase
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9
Q

Sucrase-isomaltase complex

A

Operates as two separate subunits
extracellularly, although still attached to each other.

The complex hydrolyses sucrose, maltose, and isomaltose. Its concentration is highest
in the jejunum.

Together, the sucrase subunit and the isomaltase subunit of the complex
account for about 80% of the maltase activity in the small intestine

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10
Q

Sucrase subunit:.

A
  1. Hydrolyses sucrose to glucose and fructose. But it also has maltase activity:
    cleaves the a-1,4 bond of maltose
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11
Q

Isomaltase subunit:

A

1.Performs most of the hydrolysis of a-1,6
bonds between glucose residues in the isomaltoses and a-dextrins formed from starch and glycogen. It also has maltase activity, hydrolysing the a-1,4 bond of maltose.

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12
Q

Gluco-amylase complex

A

The remainder of the maltase activity is found in this luminal membrane glycoprotein.

Both subunits hydrolyse a-1,4 bonds between glucose units in the small unbranched oligosaccharides present along the length of the small intestine.

Its highest activity is in the ileum. They start at the nonreducing end of the chains -the last glucose units - and are therefore exoglucosidases.

They also cleave
the a-1,4 bond of maltose and, thus, act as maltases.

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13
Q

Lactase

A

A b-glycosidase complex, is another large glycoprotein found in the brush bordermembranes, especially in the jejunum.

It hydrolyses the b-1,4 bond between galactose and
glucose in lactose.

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14
Q

Lactase Activity

A

lactase activity increases during the late gestational
period (27-32 weeks), and remains elevated until about 5-7 years of age.

After this time, it falls to adult levels, which are less than 10% of that present in infants.

“Late-onset lactase
deficiency” is, however, a common brush border disorder in up to 90% of adult
Mediterranean’s, Africans, Asians, and South Americans.

They may present with lactose
intolerance (pain, nausea, flatulence, and diarrhoea after ingestion of milk and dairy
products).

The condition called “secondary lactase deficiency” is due to injury of the absorptive cells of the intestinal villi, diminishing lactase activity:

  1. Kwashiorkor (protein
    malnutrition) ,
  2. Colitis,
  3. Gastroenteritis, and
  4. Excessive alcohol consumption fall into this
    category.
    Smith,
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15
Q

Dietary Fibre

A

Dietary fibre is the portion of the diet that is not hydrolysed by human digestive enzymes and thus does not directly serve as a source of food.

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16
Q

The types of dietary fibres.

A

Insoluble and Soluble fibre

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17
Q

Insoluble fibre:

A

Cellulose, hemicellulose, and lignins (which are not

carbohydrates, but polymers of phenylpropane)

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18
Q

Soluble fibre:

A

Pectins, mucilages and gums

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19
Q

Why can’t human
enzymes of the
intestinal tract not
digest dietary fibre

A

Cellulose, a polysaccharide where the glucose molecules are joined
by b-1,4 glycosidic bonds, cannot be digested by pancreas and
salivary a-amylase, which only cleave a-1,4 bonds between glucose
units.

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20
Q

Benefits of Dietary Fibre:

A

Bacterial flora in the normal human gut may degrade the more soluble dietary fibres - producing
gases and short-chain fatty acids. Remember that we may obtain as much as 10% of our total
kilojoules from compounds produced by bacterial digestion of substances in the GIT.

Fibre “softens” the stool, thereby reducing pressure on the colonic wall and enhancing expulsion
of faeces.

Certain types of fibre (e.g., pectins) may be able to lower blood cholesterol levels by binding
bile acids.

Pectins may also have a beneficial effect in the diet of individuals with diabetes mellitus by
slowing the rate of absorption of simple sugars and preventing high blood glucose levels
following meals.

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21
Q

What are the end-products of carbohydrate digestion:

A

Glucose (80%),

Galactose (from milk) and

Fructose (from table sugar).

These monosaccharides are transported across the enterocytes and into the portal circulation

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22
Q

What is the meaning of the term Glycaemic Index(GI)f a food

A

Describes how rapidly blood
glucose levels rise after ingestion of the particular food

The GI of a carbohydrate-rich food thus compares the blood glucose response to the body’s reaction to pure glucose, which is given the value of 100.

Carbohydrate-rich foods can have a high GI (>70),
moderate GI (56-70), or a low GI (<55)
23
Q

How is glucose transported across absorptive cells of the intestine(mature enterocytes)

A

Glucose is transported by using two types of glucose transport proteins which occupy positions in the brush border membrane which are adjacent to disaccharide complexes.

These complexes release glucose and other hexoses on the surface of the membrane ready for transfer to the carrier proteins

24
Q

The glucose molecule is
very polar and cannot diffuse through the hydrophobic phospholipid
bilayer of the cell membrane

A

Each -OH of the glucose molecule forms at least two hydrogen bonds with
water molecules, and random movement would require energy to dislodge the
polar OH groups from their hydrogen bonds and to disrupt the Van der Waals’
forces between the hydrocarbon tails of the fatty acids in the membrane
phospholipid.

25
Q

The two mechanisms involved in the absorption of D-glucose are:

A
  1. ## Secondary active transport: glucose enters the enterocyte via a membrane spanning Na+glucose cotransporter in the brush border (also called SGLT-1 or Na+
    -dependent glucose
    transporter), enabling the enterocytes to concentrate glucose from the intestinal lumen. The
    uptake of glucose is stimulated by an increased luminal Na+
    concentration - a molecule of
    glucose only binds to the carrier if 2 Na+ bind simultaneously.
  2. Facilitated diffusion: the glucose molecule also binds to another family of membrane-spanning
    facilitative transport proteins. Some are located on the luminal side (GLUT5), but the isoform
    GLUT2, which is in high concentration on the serosal side of the enterocytes (basolateral
    membranes), is responsible for moving glucose from the high concentration inside the cell to
    the lower concentration in the portal blood.
26
Q
  1. Secondary active transport:
A

glucose cotransporter in the brush border (also called SGLT-1 or Na+
-dependent glucose
transporter), enabling the enterocytes to concentrate glucose from the intestinal lumen. The
uptake of glucose is stimulated by an increased luminal Na+
concentration - a molecule of
glucose only binds to the carrier if 2 Na+ bind simultaneously.

27
Q

Facilitated diffusion:

A

the glucose molecule also binds to another family of membrane-spanning
facilitative transport proteins. Some are located on the luminal side (GLUT5), but the isoform
GLUT2, which is in high concentration on the serosal side of the enterocytes (basolateral
membranes), is responsible for moving glucose from the high concentration inside the cell to
the lower concentration in the portal blood.

28
Q

Absorption of D-Galactose

A

absorbed via the same mechanisms as glucose. It enters the enterocytes on the
luminal side via the Na+
-dependent glucose transporters and is transported through the serosal side
on the facilitative glucose transporters (GLUT2).

29
Q

Absorption of D-Fructose

A

both enters and leaves enterocytes via Na+
-independent facilitated diffusion. The
transporter on the luminal side has been identified as GLUT 5, with a high affinity for fructose. (For
reasons as yet unknown, perhaps because other fructose transport proteins may also be present,
fructose is absorbed at a much more rapid rate when it is ingested as sucrose than when it is ingested
alone.)

30
Q

Oral Rehydration Therapy

A

During severe diarrhoea large amounts of Na+
, K+
and water are lost.
Glucose and its polymers can be used to retain Na+
in diarrhoea disease

Na+
-dependent transporters for glucose and
amino acids not affected

• Oral rehydration therapy:

Isotonic or hypotonic solution
of glucose and NaCl-uptake of glucose and Na+, followed by Cl- and water

31
Q

Pathogenesis of V.Cholera

A

Cholera is an acute watery diarrhoeal disorder caused by the waterborne, Gram-negative bacterium Vibrio cholerae.

During epidemics,
the infection is spread by large numbers of V. cholerae that contaminate water sources (like rivers) from the voluminous liquid
stools.

It is a problem particularly in areas of extreme poverty where plumbing and modern waste-disposal systems are primitive or nonexistent.

After being ingested, the V. cholerae organisms attach
to the brush border of the enterocytes and secrete an exotoxin.

This exotoxin increases cAMP levels in the enterocyte, resulting in a markedly decreased absorption of Na+
and water from the gut lumen into the intestinal cell.

The exotoxin also stimulates the crypt cells to
secrete chloride, accompanied by cations and water, from the bloodstream into the lumen of the gut.

The excessive loss of electrolyterich diarrhoeal fluid may, in severe cases, exceed 1 litre/hour, leading
to rapid dehydration and even death

32
Q

Proteins

A

In normal humans essentially all ingested protein is digested and
absorbed.

The RDA for protein intake is about 60 gram/day for men and
50 gram/day for women, containing all of the essential amino acids.

In
addition to dietary or exogenous protein which is ingested, about 25 to
35 gram of endogenous protein (enzymes, mucins, shedded epithelial
cells from the tips of villi) is digested each day.

33
Q

Kwashikor

A
  1. Deficiency of protein in a diet that is otherwise adequate in kilojoules. Due to the
    lack of essential amino acids, existing proteins are broken down to
    replenish the amino acid pool
  2. Endogenous protein:-existing proteins broken down
    They may suffer from muscle atrophy
    Edema-as a result of increase inn the interstitial fluid with a distended abdomen
    These problems may be by a decreased synthesis of
    digestive enzymes and
    intestinal epithelium
34
Q

Digestion of proteins

A

Stomach-Pre-intestinal digestion

Intestinal lumen-Intestinal digestion

Intestinal Cell-Brush border digestion and intracellular digestion

35
Q

Activation of proteolytic enzymes (proteases):

A

Enzymes that digest proteins are produced as inactive precursors (proenzymes, or zymogens),
which are larger than the active enzymes.

When entering the lumen of the stomach and upper small intestine, they are cleaved to smaller peptides that have proteolytic activity.

Active enzymes have
different specificities; no single enzyme can completely digest a protein

36
Q

Protein digestion in the stomach:

A

The chief cells of the stomach secrete pepsinogen.

The low pH in the gastric lumen alters the
conformation of pepsinogen so that it can cleave itself, producing the active protease pepsin
(autocatalytic activation).

 The HCl in the stomach denatures dietary proteins - the partially unfolding increases enzymesubstrate binding.

 Pepsin acts optimal at a pH of 2.0 to 3.0. It is an endopeptidase, cleaving peptide bonds at
various points within the protein chain where the carboxyl group is provided by an aromatic or
acidic amino acid.

Smaller peptides and some free amino acids are produced. About 15% of dietary protein is digested by pepsin

37
Q

Digestion of short peptides at the brush border of enterocytes:

A
  1. Aminopeptidases.

2. Dipeptidase

38
Q

Aminopeptidase:

A

Produced by jejunal epithelial cells, act within the brush border membranes
to cleave one amino acid at a time from the N-terminal side upon contact with short peptides in
the chyme. They are, therefore, cellular exopeptidases

39
Q

Dipeptidases:

A

A small group of brush border which split peptide bonds of remaining di-, tri- and oligopeptides.

Together these membrane-bound peptidases are responsible for the degradation of about 60% of ingested protein.

(Luminal digestion, producing either free amino acids or di- and tripeptides, represents
degradation of about 40% of ingested protein).

Unlike carbohydrate digestion, where only monosaccharides are absorbed, many di- and
tripeptides are absorbed intact by the enterocytes from the chyme.

Actually, about 25% of
ingested protein leaves the enterocytes still intact as di- and tripeptides. However, some
cytosolic peptidases also act on these di- and tripeptides to produce free amino acids.

40
Q

Absorption of Amino Acids

A

75% of the end-products of protein digestion are free amino acids.

From the intestinal lumen, only L-amino acids are absorbed.

This occurs
mainly by:

  1. Secondary active Na+
    - dependent cotransport systems.
  2. Small amount is absorbed from the lumen by Carrier-mediated facilitated
    diffusion.
41
Q

Semi-specific Na+-dependent cotransport proteins:

A

Tansport both
Na+
and an amino acid into the epithelial cell from the intestinal
lumen. (At least 7 different amino acid carriers had been located in
the apical brush border membrane of enterocytes). The mechanism
is secondary active cotransport.

42
Q

Na+,K+-ATPases:

A

Creating a low intracellular Na+
concentration,
which drives the Na+
-dependent cotransport proteins and thereby
concentrating amino acids from the intestinal lumen.

43
Q

Facilitated transporters in the basolateral membrane:

A

About 3 carriers
transfer amino acids out of the cell down their concentration gradient
into the interstitial fluid and portal vein. (Some amino acids are also
transported across the luminal membrane by facilitated transport
carriers).

44
Q

Dietary lipid:

A
  • 60-100 g / day
  • mainly triacylglycerols

• Small quantities:

  • Phospholipids
  • Cholesterol esters

They are all water soluble

45
Q

Pre-intestinal digestion:

A

Digestive enzymes:

Lingual lipases-secreted by cells at the back of the tongue

Gastric lipases-secreted by cells in stomach

Function:

Hydrolyse short- and medium chain fatty acids -
≤ 12 carbons

Most active in infants, young children

In the stomach, the high acidity together with agitation contributes to breaking of fat globules into smaller sizes in an aqueous environment,
called emulsification.

This is a coarse emulsification, producing 100nm
lipid globules (large droplets
46
Q

Digestion of lipid in the small intestine:

A

In the small intestine, two substances (bile salts and pancreatic lipase) play a major role in fat digestion.

47
Q

Action of bile salts

A

Bile salts are amphipathic compounds, acting as detergents in the lumen of the upper small
intestine.

They bind to the globules of dietary fat to decrease the surface tension, and the
globules are then broken up by peristaltic agitation. A 5nm emulsion of small lipid droplets is
formed.

This process is called emulsification of lipid globules.

The total surface layer of emulsified fat globules is greatly increased-Enhancing the action of pancreatic lipase.

48
Q

Action of pancreatic lipase

A

The major enzyme that digests the dietary triacylglycerols in the emulsified fat globules is
pancreatic lipase (a glycerol ester hydrolase), secreted in enormous quantities along with
another protein known as colipase.

The contents of the intestinal lumen have to be at a pH of
7.0 - 8.0, which is optimal for the action of all of the digestive enzymes of the small intestine.

Pancreatic lipase hydrolyses fatty acids of all chain lengths from positions 1 and 3 of the
glycerol portion of the triacylglycerol, producing free fatty acids and 2-monoacylglycerol.

The pancreas also produces a non-specific cholesterol ester hydrolase
(also known as carboxylester hydrolase) that removes fatty acids from
compounds such as cholesterol esters, and phospholipases (mostly A2)
that digest phospholipids to their component parts.

49
Q

Colipase

A

The colipase binds to the fat-water interface, displacing a bile salt molecule, allowing
pancreatic lipase to digest triacylglycerols.

Colipase also binds to the lipase, anchoring the
lipase, and thus increases lipolytic activity.

50
Q

Bile salts have two functions:

A

The bile salts have two important actions in the GIT:

  1. They play a role in emulsification of dietary lipid droplets, thereby providing a larger surface area for lipolysis through
    the action of pancreatic lipase.
  2. Their role in mediating fat absorption is even more
    important. They form mixed micelles to keep the products of fat digestion in solution and to transport them to the brush border of enterocytes
51
Q

Micelle formation:

A

• Small spherical cylindrical globules 3-6nm in diameter
• Composed of 20-40 bile salt molecules
– fat-soluble sterol nucleus projects inward
– hydrophilic polar groups project outward
 simple micelles

• Lipids collect in micelles  mixed micelles
– fatty acids, 2-monoacylglycerol (2-MG) ‘dissolve’
into micelles
– cholesterol, fat-soluble vitamins in hydrophobic
centre
 micelles keep lipids in solution
• Mixed micelles are
soluble in chyme

 free fatty acids
 2-MG
 phospholipids
 cholesterol
 fat soluble vitamins
 (A, D, E and K)

Ferrying function
of micelles
micelles NOT
absorbed

Bile salts:
• Mixed micelles are constantly broken down and
formed again
• Ionised, conjugated bile salts
– poorly absorbed  form more mixed micelles
– absorbed in ileum → secondary active transport
• Deconjugated bile salts
not in micellar structures
may be passively absorbed
 enterohepatic recirculation
of bile salts

52
Q

Short and medium chain

fatty acids:

A 
• contain 4 to 12 carbons
• do not require bile salts for
absorption  passive diffusion
• not packaged in chylomicrons
 transported bound to albumin
53
Q

Chylomicron

A 
• TG in blood would fuse together  obstruct
blood flow
• Packaged in lipoprotein particles
 chylomicrons
• Also contain:
– cholesterol
– fat soluble vitamins
54
Q

Absorption of water

in the GIT:

A 
• 7-8 liters per day (capacity > 20L)
• through osmosis
• hyperosmotic solutions in the lumen
 water will move in the opposite direction
(Worksheet: Questions 4 &amp; 5)
Absorption of Sodium:
• 20 – 30g Na+
secreted daily in intestinal
secretions
• Average daily intake: 5 – 8g Na+
  intestinal absorption of 25 – 35g Na+ daily
During
diarrhoea?
secondary
active transport
active
transport
maintains
flow
gradient
for Na
\+
to
enter cell
(Guyton: Fig. 66.8)
Na
\+
transport
HCO3
- + H+
H2CO3
CO2 + H2O
Ion
transport
into the blood

Gastrointestinal System (14 decks)

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