digestive system part three

Part 3

Digestive System

Digestion

Ingested materials must be broken down for absorption

Majority of absorption in small intestine

Water and alcohol in stomach mucosa

Some salts and vitamins in large intestine

Non-absorbable materials removed by defecation

Fate of Digested Materials

Carbohydrates

Glucose, fructose, and galactose are directly absorbed

Cellulose metabolized by bacteria

Vitamins K and B12 are byproducts

Figure 23.32 (1 of 4)

Carbohydrate digestion

• Glucose and galactose

are absorbed via

cotransport with

sodium ions.

• Fructose passes via

facilitated diffusion.

• All monosaccharides

leave the epithelial

cells via facilitated

diffusion, enter the

capillary blood in the

villi, and are

transported to the liver

via the hepatic portal

vein.

Starch and disaccharides

Oligosaccharides

and disaccharides

Lactose Maltose Sucrose

Glucose Fructose

Salivary

amylase

Mouth

Pancreatic

amylase

Brush border

enzymes in

small intestine

(dextrinase, gluco-

amylase, lactase,

maltase, and sucrase)

Small

intestine

Small

intestine

Foodstuff

Galactose

Path of absorption Enzyme(s)

and source

Site of

action


Proteins

Amino acids, dipeptides and some tripeptides

Absorbed by active transport

Further metabolism in cells → free amino acids → bloodstream

Figure 23.32 (2 of 4)

Protein digestion

• Amino acids are absorbed

by cotransport with

sodium ions.

• Some dipeptides and

tripeptides are absorbed

via cotransport with H+

and hydrolyzed to amino

acids within the cells.

+

• Amino acids leave the

epithelial cells by

facilitated diffusion, enter

the capillary blood in the

villi, and are transported

to the liver via the hepatic

portal vein.

Small

intestine

Small

intestine

Stomach

Foodstuff

Protein

Large polypeptides

Pepsin

(stomach glands)

in presence

of HCl

Small polypeptides,

small peptides

Pancreatic

enzymes

(trypsin, chymotrypsin,

carboxypeptidase)

Amino acids

(some dipeptides

and tripeptides)

Brush border

enzymes

(aminopeptidase,

carboxypeptidase,

and dipeptidase)


and source

Site of

action

Figure 23.33

Absorptive

epithelial

cell

Apical membrane (microvilli)

Amino

acid

carrier

Capillary

Lumen of

intestine

Pancreatic

proteases

Amino acids of protein fragments

Brush border enzymes

Na+

Na+

1 Proteins and protein fragments

are digested to amino acids by

pancreatic proteases (trypsin,

chymotrypsin, and carboxy-

peptidase), and by brush border

enzymes (carboxypeptidase,

aminopeptidase, and dipeptidase)

of mucosal cells.

2 The amino acids are then

absorbed by active transport into

the absorptive cells, and move to

their opposite side (transcytosis).

3 The amino acids leave the

villus epithelial cell by facilitated

diffusion and enter the capillary

via intercellular clefts.

Active transport

Passive transport


Lipids

Emuslification by bile salts and lipase → micelles move between

microvilli → lipids diffuse into intestinal epithelium

(bile salts later reabsorbed in ileum)

Figure 23.34

Epithelial

cells of

small

intestine

Fat droplets

coated with

bile salts

Fat globule

Lacteal

Bile salts

Micelles made up of fatty

acids, monoglycerides,

and bile salts

1 Large fat globules are emulsified

(physically broken up into smaller fat

droplets) by bile salts in the duodenum.

2 Digestion of fat by the pancreatic

enzyme lipase yields free fatty acids and

monoglycerides. These then associate

with bile salts to form micelles which

“ferry” them to the intestinal mucosa.

3 Fatty acids and monoglycerides leave

micelles and diffuse into epithelial cells.

There they are recombined and packaged

with other lipoid substances and proteins

to form chylomicrons.

4 Chylomicrons are extruded from the

epithelial cells by exocytosis. The

chylomicrons enter lacteals. They are

carried away from the intestine by lymph.


Lipids

Within intestinal cells

Triglycerides → formed proteins and cholesterol added → chylomicrons → enter lymphatics


Lipids

Plasma enzymes generate FFA’s and glycerol → pass capillary wall → protein cholesterol combo returns to liver →

additional proteins added → HDL and LDL created

LDL accumulates in plasma or on artery walls

Difficult for cells to recognize

Figure 23.32 (3 of 4)

Fat digestion

Small

intestine

Small

intestine

Foodstuff

Unemulsified

fats

Emulsification by

the detergent

action of bile

salts ducted

in from the liver

Pancreatic

lipases

Monoglycerides

and fatty acids

Glycerol

and

fatty acids


and source

Site of

action

• Fatty acids and monoglycerides enter the intestinal cells via diffusion.

• Fatty acids and monoglycerides are recombined to form triglycerides and then combined with other lipids and proteins within the cells, and the resulting chylomicrons are extruded by exocytosis.

• The chylomicrons enter the lacteals of the villi and are transported to the systemic circulation via the lymph in the thoracic duct.

• Some short-chain fatty acids are absorbed, move into the capillary blood in the villi by diffusion, and are transported to the liver via the hepatic portal vein.


Water

After digested nutrients removed, large volumes of salt and

water remain in LI

Active Na+ uptake → passive Cl- and water uptake

Undigested materials (cellulose) cause water to be retained in LI

Antibiotics may kill bacteria → digestion impaired

Disorders of the Digestive System

Colon cancer

Second most common cause of cancer death in U.S. in men

98,000 new cases annually

48,000 deaths/year

Diagnosis

Colonoscopy

Fiberoptic endoscope

Polyps often occur before tumor


Colon cancer

Contributing factors

Contact time with carcinogenic material in colon

Diets high in animal material = slowed motility

P53 (tumor suppressor gene) mutation

Hereditary component


Gallstones

A.K.A Cholelithiasis

Bile salts precipitate

Block bile ducts

Jaundice

Treatment

Lithotripsy

Medications

Cholecystectomy

Questions?

digestive system part three

Documents