HUMAN BIOLOGY

204 Chapter 11

+ 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
duodenum First sec- small Intestine
tion of the small intestine,
where chyme and digestive
enzymes enter.

ileum Last section of
the small intestine, where
absorption is completed
and residues move toward
the large intestine.

jejunum Middle section of
the small intestine, where
most nutrients are digested
and absorbed.

lacteals Lymph vessels that
take up triglycerides from
digested fat and deliver
them to the bloodstream.

segmentation Mechani-
cal mixing of digested food
moving through the small
intestine.

hoW are different kinds of nutrients

absorbed in the small intestine?

• In the small intestine, chemical and mechanical processes
  break down large organic molecules to smaller molecules that
  can be absorbed.


• Enzymes from the pancreas act on carbohydrates, fats,
  proteins, and nucleic acids in chyme. Bile salts emulsify
  large fat globules, allowing fats to be more easily digested.


• Simple sugars and amino acids pass through brush border
  cells that line the surface of intestinal villi, then move into
  the blood.


• Digested lipids pass through brush border cells, then into
  lacteals, then into the bloodstream.

taKe-Home message

Digestion and absorption in the small intestine

n Absorption moves nutrients into the internal environment—

tissue fluid and the bloodstream.

n    Links to Buffers 2.7, Osmosis 3.10

Each day about 9 liters (10 quarts) of fluid enters the first

section of the small intestine, the duodenum (doo - oh-dee-

num). This fluid includes chyme along with enzymes and

other substances from the pancreas, liver, and gallblad-

der. Most digestion and nutrient absorption occurs in the

next section, the 3-foot-long jejunum. Some nutrients are

absorbed while the remaining material is moving through

the ileum, the last section of the small intestine, on its way

to the large intestine.

Chyme entering the duodenum triggers hormone sig-

nals that stimulate a brief flood of digestive enzymes from

the pancreas. As part of pancreatic juice, these enzymes

act on carbohydrates, fats, proteins, and nucleic acids

(Table 11.1 and Figure 11.11). For example, like pepsin in

the stomach, the pancreatic enzymes trypsin and chymo-

trypsin digest the polypeptide chains of proteins into pep-

tide fragments. The fragments are then broken down to

amino acids by different peptidases (which are on the sur-

face of the intestinal mucosa). Recall from Section 11.5 that

the pancreas also secretes bicar-

bonate that buffers stomach acid,

maintaining a chemical environ-

ment in which pancreatic enzymes

can function.

Fat digestion requires enzymes

called lipases. Bile salts in bile

secreted by the liver (and deliv-

ered via the gallbladder) make fat

digestion more efficient. Bile salts

are like a detergent—they emul-

sify, or break up, large units of fat

into smaller ones. How does this

process work? Most fats in the aver-

age diet are triglycerides, which

tend to clump into big fat globules

in chyme. When peristalsis mixes

chyme, the globules break up into

droplets that become coated with

bile salts (see step 5 in Figure 11.11).

These droplets, called micelles (my-

cells), give fat-digesting enzymes a much greater surface

area to act on. So, because triglycerides are emulsified, they

can be broken down much faster to monoglycerides and

fatty acids, molecules that are small enough to be absorbed.

Micelles also may contain fat-soluble vitamins.

When a substance is absorbed, it crosses the intestine

lining into the bloodstream. Due partly to the vast absorp-

tive surface area of the small intestine, this process is very

efficient. Segmentation helps, too. In this process, rings

of smooth muscle in the wall repeatedly contract and

relax. The result is a back-and-forth movement that mixes

digested material and forces it against the wall:

11.6

By the time food is halfway through the small intes-

tine, most of it has been broken apart and digested. Water

crosses the intestine lining by osmosis, and cells in the lin-

ing also selectively absorb minerals. Transport proteins in

the plasma membrane of brush border cells actively move

some nutrients, such as the monosaccharide glucose and

amino acids, across the lining. After glucose and amino

acids are absorbed, they move into tissue fluid and then

directly into blood vessels (steps 1–4 in Figure 11.11).

Additional steps occur before digested lipids move into

the bloodstream. After lipases digest micelles, the fatty

acids and monoglycerides enter brush border cells, just as

glucose and amino acids do. (The bile salts that formed the

droplets are recycled.) There, fatty acids and monoglycer-

ides quickly reunite into triglycerides. Then triglycerides

combine with proteins into particles that leave the cells

by exocytosis and enter tissue fluid. They don’t directly

enter blood vessels, however. Instead they cross into lymph

vessels known as lacteals, which drain into the general

circulation.

© Cengage Learning

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).