Tishk International University

Faculty of Science

Department of Medical Analysis

Advanced Clnical Biochemistry

Gastrointestinal tract

Grade 4-Fall 2021-2022

Dr. Rundk A. Hwaiz

Introduction:

The digestive system, also called the alimentary canal or

gastrointestinal tract, begins with the mouth, where food enters

the body, and ends with the anus, where solid waste material

leaves the body, the major functions are;

A- digestion

B- absorption

C- elimination

The major organs of digestive system are;

1. Mouth

2. Pharynx

3. Esophagus

4. Stomach

5. Small intestine

6. Large intestine

A. Digestion of carbohydrates

Dietary carbohydrates (mainly starch, sucrose, and lactose)

constitute about 60% of the calories in the average diet.

1. Starch, the storage form of carbohydrates, is similar

in structure to glycogen.

2. Sucrose contains glucose and fructose residues

linked via their anomeric carbons.

3. Lactose (milk sugar) contains galactose-linked

to glucose

Digestion of dietary carbohydrates in the mouth

In the mouth, salivary α-amylase cleaves starch by

breaking α-1,4 linkages between glucose residues

within the chains. Dextrins (linear and branched

oligosaccharides) are the major products that

enter the stomach.

Digestion of carbohydrates in the intestine

1. The stomach contents pass into the intestine, where

bicarbonate (HCO3) secreted by the pancreas neutralizes

the stomach acid, raising the pH into the optimal range for

the action of the intestinal enzymes.

2. Digestion by pancreatic enzymes:

a. The pancreas secretes an α-amylase that acts in the

lumen of the small intestine and, like salivary amylase,

cleaves α -1,4 linkages between glucose residues.

b. The products of pancreatic α-amylase are the

disaccharides maltose and isomaltase, trisaccharides, and

small oligosaccharides containing α-1,4 and α-1,6

linkages.

CLINICAL CORRELATES:

Serum amylase is elevated in cases of pancreatitis, and the test

to measure amylase is often ordered in patients to evaluate

such a condition. However, serum lipase is another marker of

pancreatitis that demonstrates higher sensitivity and specificity

compared with amylase.

Digestion by enzymes of intestinal cells

Complexes of enzymes, produced by intestinal epithelial cells

and located in their brush borders, continue the digestion of

carbohydrates.

• Maltases

• Sucrase

• Lactase

CLINICAL CORRELATES:

Lactase deficiency (lactose intolerance) occurs in more than

80% of Native, African, and Asian Americans. Lactose is not

digested at a normal rate and accumulates in the gut, where it

is metabolized by bacteria. Bloating, abdominal cramps, and

watery diarrhea result.

Acarbose, an α-glucosidase inhibitor, works in the intestine,

slowing down digestion of carbohydrates and lengthening the

time it takes for carbohydrates to be converted to glucose,

which facilitates better postdigestive blood glucose control.

CASE HISTORY

A 12-year male had complained of abdominal discomfort,

a feeling of being bloated, increased passage of urine and

development of diarrhea after taking milk.

Questions

A. Name the probable disorder

B. Cause of disorder

C. What will you suggest the patient to relieve the

symptoms?

Carbohydrates that cannot be digested

Indigestible polysaccharides are part of the dietary fiber

that passes through the intestine into the feces. For

example, because enzymes produced by human cells

cannot cleave the β-1,4 bonds of cellulose, this

polysaccharide is indigestible.

B. Absorption: means the transfere of nutrients from lumen

into the blood.

Fructose absorbs passively into the cells via GLUT5 and then absorbs

into the blood via GLUT2

Galactose absorbs actively into the cells via SGLT1 and then absorbs

into the blood via GLUT2

Glucose absorbs passively into the cells via GLUT5 and then absorbs

into the blood via GLUT2 in case of low blood glucose.

Glucose absorbs actively into the cells via SGLT1 and then absorbs

into the blood via GLUT2 in case of normal or high blood glucose.

Glucose, galactose and fructose are alcohols and they are polar

thererefore need carrier which made of proteins to cross them into the

lipid bilayer of cell membranes.

Glucose carriers

Na dependent Na independent

Insuline dependent Insuline independent

GLUT4 GLUT1

GLUT2

GLUT3

GLUT5

GLUT7

SGLT1

SGLT2

SGLT1: found in the brush boarder (lumen) of intestine

responsible for glucose uptake in the intestine.

SGLT2: found in the proximal convuluted tubule of kidney,

responsible for glucose reabsorption in the kidney.

GLUT4: found in the muscle and adipose tissue, responsible

of glucose uptake and it is insulin dependent carrier.

GLUT1: found in the brain and RBC

GLUT2: found in the basal membrane of intestine

GLUT3: found in the brain

GLUT5: Found in the brush boarder of intestine.

GLUT7: found in the hepatocytes (liver cells)