Major food groups Carbohydrates = sugars = saccharides Lipids = fats Proteins nucleic acids Carbohydrates, proteins and nucleic acids (but not fats)

Major food groups• Carbohydrates = sugars = saccharides• Lipids = fats• Proteins• nucleic acids

Carbohydrates, proteins and nucleic acids (but not fats) are long chains of smaller subunits, which are monomers. The combinations are polymers.

Polymers in which the order of monomers provides information: proteins and nucleic acids.

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Nucleic acid digestion- pancreatic ribonuclease & deoxyribonuclease; nucleosidases & phosphatases; act in small intestines


Composition and Function of Pancreatic Juice

• Many enzymes are secreted in inactive form:

Examples include:– Trypsinogen is activated to trypsin– Pepsinogen activated to pepsin by HCl– Procarboxypeptidase is activated to

carboxypeptidase


• Digestive enzymes that are produced by the small intestine are all brush border enzymes (membrane-bound)

• Advantages and disadvantages of having membrane-bound enzymes


Figure 23.27 Activation of pancreatic proteases in the small intestine.

Stomach

Pancreas

Epithelialcells

Trypsinogen(inactive)Chymotrypsinogen(inactive)Procarboxypeptidase(inactive)

Trypsin

Chymotrypsin

Carboxypeptidase

Membrane-boundenteropeptidase


Protein digestion

+

• Amino acids enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.

Smallintestine

Smallintestine

Stomach

Foodstuff

Protein

Large polypeptides

Pepsin(stomach glands)in presence of HCl

Small polypeptides,small peptides

Pancreaticenzymes (trypsin, chymotrypsin,carboxypeptidase)

Amino acids

Brush border enzymes (small intestine)

Path of absorptionEnzymes

and sourceSite ofaction


Figure 23.33 Protein digestion and absorption in the small intestine.

Absorptiveepithelialcell

Apical membrane (microvilli)

Aminoacid carrier

Capillary

Lumen of intestine

Pancreaticproteases

Amino acids of protein fragmentsBrush border enzymes

1Protein fragments (peptides) are digested to amino acids by brush border enzymes of mucosal cells.

2 The amino acids are then absorbed by active transport into the absorptive cells

3 The amino acids leave the villus epithelial cell by facilitated diffusion and enter the capillary via intercellular clefts.

Active transport

Passive transport


Carbohydrate digestion

• All monosaccharides enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.

Starch and disaccharides

Oligosaccharidesand disaccharides

Lactose Maltose Sucrose

Glucose Fructose

Salivaryamylase

Mouth

Pancreaticamylase

Brush borderenzymes in small intestine

Smallintestine

Smallintestine

Foodstuff

Galactose

Path of absorptionEnzyme(s)and source

Site ofaction


Fat digestion

Small intestine

Small intestine

Foodstuff

Unemulsifiedfats

Emulsification by the detergent action of bile salts from theliver

Pancreatic lipases

fatty acids glycerol


Site ofaction

• Fatty acids and glycerol 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 to make chylomicrons, which 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.

Note: fats are NOT made of a long chain of monomers (unlike proteins and polysaccharides)


Figure 23.34 Emulsification, digestion, and absorption of fats.

Epithelialcells ofsmallintestine

Fat dropletscoated withbile salts

Fat globule

Lacteal

Bile salts

Fatty acidsand 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 glycerol, still associated with bile salts

3 Fatty acids and glycerol diffuse into epithelial cells. There they are recombined and packaged with other lipids 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.


Nucleic acid digestion

• Subunits are absorbed into capillary blood in the villi and transported to the liver via the hepatic portal vein.

Smallintestine

Smallintestine

Foodstuff

Nucleic acids

Pancreatic ribo-nuclease and deoxyribonuclease

Brush borderenzymes(nucleosidasesand phosphatases)

Pentose sugars,N-containing bases,

phosphate ions


Site ofaction


In the large intestine:

• Other than digestion by enteric bacteria, no further digestion takes place– Bacteria synthesize B complex vitamins and vitamin K

• water, and electrolytes are absorbed

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Major food groups Carbohydrates = sugars = saccharides Lipids = fats Proteins nucleic acids Carbohydrates, proteins and nucleic acids (but not fats)