HUMAN BIOLOGY

DiGestioN aND NUtritioN 205

+ fat globules

(triglycerides)

emulsified fat droplets

free fatty acids + monoglycerides + bile salts

+ monoglycerides

triglycerides

carbohydrates

bile salts

monosaccharides

proteins

amino acids

triglycerides lipoproteins

+ proteins

free fatty acids

Interstitial fluid inside

a villus

Brush

border

cell

Lumen (interior) of

small Intestine

6 Pancreatic enzymes

digest the droplets to fatty

acids and monoglycerides.

7 Monoglycerides and

fatty acids diffuse across

the plasma membrane’s

lipid bilayer, into brush

border cells.

8 In a brush border cell, the

products of fat digestion form

triglycerides.

9 Triglycerides associate

with proteins to form lipo-

proteins, which are expelled

by exocytosis into tissue fluid.

1 Enzymes break

polysaccharides down

to monosaccharides

(simple sugars).

2 Monosaccharides are

actively transported into

brush border cells, then

out into tissue fluid.

3 Proteins are broken

into polypeptides, then

amino acids.

4 Amino acids are

actively transported into

brush border cells, then

out into tissue fluid.

5 Movements of the

intestinal wall break up

fat globules into small

droplets. Bile salts coat

the droplets, so they

remain separated.

enzyme Released by: active in: Breaks down: Resulting Products

Digesting carbohydrates

Salivary amylase Salivary glands Mouth, stomach Polysaccharides Disaccharides, oligosaccharides

Pancreatic amylase Pancreas Small intestine Polysaccharides Disaccharides, monosaccharides

Disaccharidases Intestinal lining Small intestine Disaccharides Monosaccharides* (e.g., glucose)

Digesting Proteins

Pepsins Stomach lining Stomach Proteins Protein fragments

Trypsin and chymotrypsin Pancreas Small intestine Proteins Protein fragments

Carboxypeptidase Pancreas Small intestine Peptides amino acids*

Aminopeptidase Intestinal lining Small intestine Peptides amino acids*

Digesting Fats

Lipases Pancreas Small intestine Triglycerides Fatty acids, monoglycerides*

Digesting Nucleic acids

Pancreatic nucleases Pancreas Small intestine DNA, RNA Nucleotides*

Intestinal nucleases Intestinal lining Small intestine Nucleotides Nucleotide bases, monosaccharides*

Table 11.1 Major Enzymes of Digestion and What They Do

Fig u r e 11.11 Animated! Different types of nutrients are absorbed by different mechanisms. (© Cengage Learning)

*Products small enough to be absorbed into the bloodstream.

The four basic kinds of biological molecules are shown in blue type.

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