Download 50.1 What do animals require from food?

Chapter 50 – Nutrition, Digestion, and Absorption
50.1 What do animals require from food?
Animals are heterotrophs that derive their energy and molecular building blocks, directly or
indirectly, from autotrophs.
Carbohydrates, fats, and proteins in food supply animals with metabolic energy. A measure of
the energy content of food is the kilocalorie. Excess caloric intake is stored as glycogen and fat.
Review Figure 50.2
For many animals, food provides essential carbon skeletons that they cannot synthesize
themselves. Review Figure 50.4
Humans require eight essential amino acids in the diet. Different animals need mineral elements
in different amounts. Macronutrients are needed in large quantities. Micronutrients are needed
in small amounts. Review Figure 50.5 and Table 50.1 (Part 1), Table 50.1 (Part 2) Web/CD
Activity 50.1
Vitamins are organic molecules that must be obtained in food. Review Table 50.2 (Part 1), 50.2
(Part 2), Web/CD Activity 50.2
Malnutrition results when any essential nutrient is lacking from the diet. A chronic state of
malnutrition causes a deficiency disease.
5.2 How do animals ingest and digest food?
Animals can be characterized by how they acquire nutrients: Saprobes and detritivores depend
on dead organic matter, filter feeders strain the aquatic environment for small food items,
herbivores eat plants, and carnivores eat animals. Behavioral and anatomical adaptations reflect
these feeding strategies. See Web/CD Activity 50.3
Digestion involves the breakdown of complex food molecules into monomers that can be
absorbed and utilized by cells. In most animals, digestion takes place in a tubular gut. Review
Figure 50.8
Absorptive areas of the gut are characterized by a large surface area produced by extensive
folding and numerous villi and microvilli. Review Figure 50.9
Hydrolytic enzymes break down proteins, carbohydrates, and fats into their monomeric units. To
prevent the organism itself from being digested, many of these enzymes are released as inactive
zymogens, which become activated when secreted into the gut.
50.3 How does the vertebrate gastrointestinal system
function?
The vertebrate gut can be divided into several compartments with different functions. Review
Figure 50.10, Web/CD Activity 50.4
The cells and tissues of the vertebrate gut are organized in the same way throughout its length.
The innermost tissue layer, the mucosa, is the secretory and absorptive surface. The submucosa
contains blood and lymph vessels, and a nerve plexus. External to the submucosa are two
smooth muscle layers. Between the two muscle layers is another nerve plexus that controls the
movements of the gut. Review Figure 50.11
Swallowing is a reflex that pushes the bolus of food into the esophagus. Peristalsis and other
movements of the gut move the bolus down the esophagus and through the entire length of the
gut. Sphincters block the gut at certain locations, but they relax as a wave of peristalsis
approaches. Review Figure 50.12
Digestion begins in the mouth, where amylase is secreted with the saliva. Digestion of protein
begins in the stomach, where parietal cells secrete HCl and chief cells secrete pepsinogen, a
zymogen that becomes pepsin when activated by low pH and autocatalysis. The mucosa also
secretes mucus, which protects the tissues of the gut. Review Figure 50.13
In the duodenum, pancreatic enzymes carry out most of the digestion of food. Bile from the
liver and gallbladder emulsify fats into micelles. Bicarbonate ions from the pancreas neutralize
the pH of the chyme entering from the stomach to produce an environment conducive to the
actions of pancreatic enzymes such as trypsin. Review Figure 50.15 and Table 50.3 (Part 1),
50.3 (Part 2)
Final enzymatic cleavage of polypeptides and disaccharides occurs among the microvilli of the
intestinal mucosa. Amino acids, monosaccharides, and inorganic ions are absorbed by the
microvilli. Specific transporter proteins are sometimes involved. Sodium co-transport often
powers the active transport of nutrients.
Fats broken down by lipases are absorbed mostly as monoglycerides and fatty acids and are
resynthesized into triglycerides within cells. The triglycerides are combined with cholesterol and
phospholipids and coated with protein to form chylomicrons, which pass out of the mucosal
cells and into lymphatic vessels in the submucosa. Review Figure 50.16, Web/CD Tutorial
50.1
Water and ions are absorbed in the large intestine as waste matter is consolidated into feces,
which is periodically eliminated.
Microorganisms in some compartments of the gut digest materials that their host cannot. Review
Figure 50.17
50.4 How is the flow of nutrients controlled and regulated?
Autonomic reflexes coordinate activity of the digestive tract, which has an intrinsic nervous
system that can act independently of the CNS.
The actions of the stomach and small intestine are largely controlled by the hormones gastrin,
secretin, and cholecystokinin. Review Figure 50.18
The liver plays a central role in directing the traffic of fuel molecules. During the absorptive
period, the liver takes up and stores fats and carbohydrates, converting monosaccharides to
glycogen or fats. The liver also takes up amino acids and uses them to produce blood plasma
proteins, and can engage in gluconeogenesis.
Fat and cholesterol are shipped out of the liver as low-density lipoproteins. High-density
lipoproteins act as acceptors of cholesterol and are believed to bring fat and cholesterol back to
the liver.
Insulin largely controls fuel metabolism during the absorptive period and promotes glucose
uptake as well as glycogen and fat synthesis. During the postabsorptive period, lack of insulin
blocks the uptake and utilization of glucose by most cells of the body except neurons. If blood
glucose levels fall, glucagon secretion increases, stimulating the liver to break down glycogen
and release glucose to the blood. Review Figure 50.19, Web/CD Tutorial 50.2
Food intake is governed by sensations of hunger and satiety, which are determined by brain
mechanisms. Review Figure 50.20
50.5 How do animals deal with ingested toxins?
Toxins in food may come from natural sources, but many come from human activities such as
the use of pesticides and the release of pollutants into the environment.
Toxins such as PCBs that accumulate in the bodies of prey are transferred to and further
concentrated in the bodies of their predators. This bioaccumulation produces high
concentrations of toxins in animals high up the food chain.