Chapter 43: Animal nutrition (pg. 1010-1024)
43.3 How are nutrients digested and absorbed?
Digestion is the breakdown of food into small enough pieces for absorption—the uptake of nutrients.
Digestion takes place in the digestive tract (Figure 43.5)-also known as the alimentary canal or
gastrointestinal tract (GI), beginning in the mouth and ending in the anus.
Figure 43.6 summarizes the chemical changes that occur as food moves through the digestive tract.
Tubelike digestive tract has 3 advnatges:
Digestion of material inside tract allows animals to feed on large pieces of food.
Different chemical and physical processes can be separated within digestive tract, so that
they occur independently of each other and in a sequence.
Tubelike digestive tract provides one way flow and aloows wastes to exit body through
Several vital organs and glands connected to digestive tract. These structures contribute digestive
enzymes and other products to specific portions of tract. Include salivary glands, liver, gallbladder and
Digestion begins with tearing and crushing of teeth. Chewing reduces size of food and softens them.
Chemical breakdown of carbohydrates begins in the mouth, continues in the stomach and is completed
in the small intestine.
Carbs lipids and proteins must be broken down during digestion.
Nutrients are absorbed in the small intestine and more water is absorbed in the large intestine. In large
intestine more water is absorbed and feces are generated and excreted. The mouth and the esophagus
Amylase contained in saliva cleaves the bonds that link
glucose monomers in starch, glycogen, and other glucose
amylase cleaves bonds that release maltose dimers from
starch, glycogen and other glucose polymers. Amylase not the
only ingredient in saliva.
Cells in the tongue also synthesize and secrete lipase,
which begins the breakdown of lipids (triglycerides) into fatty
acids and monoglycerides.
Salivary glands in the mouth also release water and
glycoproteins called mucins.
When mucins contact water, they form the slimy
substance called mucus. The combination of water and mucus
makes food soft and slippery enough to be swallowed.
Once food is swallowed it enters the esophagus—a
muscular tube connecting the mouth and stomach—and is
propelled to the stomach by peristalsis—waves of muscle contractions (Figure 43.7).
Because material is actively moved along esophagus, you can swallow even when your mouth is lower
than stomach such as when you bend over to drink from a stream. Upper third of the esopgahus consists of SM, lower third is composed of smooth muscle and middle
third contains a mix of both.
In experiment, established that peristalsis in upper third of esophagus occurs when series of nerve cells
that originate at base of brain sends electrical signals in a precise sequence. Each of these nerves
terminates at SM at different location along esophagus.
Some of the muscles wrap around esophagus; others run along its length. In response to nerve signals,
muscle contracts or relaxes in a coordinated fashion.
In this way, wave of muscle contractions propagates down the tube, propelling food mass ahead of it.
Action of nerves not a conscious choice. System is reflex and automatic reaction to a stimulus.
The stomach is a tough, muscular pouch bracketed on both ends by valves called sphincters.
When a meal fills the stomach, muscular contractions result in churning that mixes contents and reduces
food in a uniform consistency and solute , or osmolarity.
A certain amount of chemical breakdown also occurs in response to this churning. Other main fcn of
stomach is the partial digestion of proteins.
Enzyme responsible for digesting proteins in stomach is called
pepsin. Because it destroys proteins, biologists hypothesized that pepsin
must be synthesized and stored in cells while it is inactive.
Precursos compound found in chief cells, which came to be
called pepsinogen, is converted to active pepsom by contact with acidic
environment of stomach
After eating, muscular contractions in the stomach result in
churning that breaks the food down mechanically and mixes it with
hydrochloric acid (HCl) and pepsin in the stomach to digest protein
Secretion of protein digesting enzymes in inactive form is
important, because it prevents destruction of proteins in cells where the
enzyme is synthesized.
Which cells produce stomach acid?
Parietal cells are the source of HCl in gastric juice, whose pH can be as low as 1.5.
Goblet cells secrete mucus, which lines the gastric epithelium and protects the stomach from damage by
HCl. How do parietal cells secrete HCL?
The enzyme carbonic anhydrase exists in high concentration in parietal cells.
It catalyzes the formation of carbonic acid (H CO ) from carbon dioxide and water.
In solution, the carbonic acid formed immediately dissociates into the bicarbonate ion (HCO –) and3a
CO +2H O 2 H CO → 2 + H3O – 3
The current model for HCl production is shown in Figure 43.9b.
Ruminants have a complex, four–chambered stomach specialized for digesting cellulose rather than
protein. It consists of the rumen, reticulum, omasum, and abomasum (Figure 43.10).
The rumen is the largest chamber and serves as a fermentation vat. Rumen and adjacent chamber called
The reticulum is adjacent to the rumen. Packed with symbiotic bacteria and protists. These organism
have enzymes capable of breaking apart chemical bonds in cellulose, yielding glucose.
Rumen as oxygen free environment, and symbiotic organisms produce ATP from this glucose via
After plant material partially digested in rumen and in the reticulum, the ruminant regurgitates portions
of material into its mouth forming a cud. Chew on Cud to enhance mechanical breakdown and
As it passes the stom