Food Energy and Essential Nutrients (p. 530)
25.1
Food for Energy and Growth (p. 530; Figs. 25.1, 25.2)
A. We obtain our energy from the foods we eat.
B. Different classes of nutrients, such as carbohydrates, proteins, and fats, contain the building
blocks necessary for cell growth and renewal.
C. These molecules also have energy within their bonds that cells extract to fuel their own
energy needs.
D. An optimal diet contains the right nutrients in the proper amounts.
1. Carbohydrates contain an average of 4.1 calories per gram, the same as proteins.
2. Fats, on the other hand, have 9.3 calories per gram.
3. Carbohydrates are used mostly for energy, but your body can extract energy from
proteins too, as needed.
E. Proteins and fats are used to build structural materials in cells, and fats can also be stored, for
long-term energy.
F. Obesity is a common problem in North America, primarily because of excessive intake of
fats.
G. Fiber in foods is useful to help move intestinal contents along.
H. Low-fiber diets are correlated with increased incidence of colon cancer.
I. Essential Substances for Growth
1. Essential nutrients, such as essential amino acids, are those the body cannot manufacture;
these nutrients must be included in the diet.
2. Trace Elements
a. Food must also supply the body with essential minerals and a number of trace
elements.
3. Vitamins
a. Vitamins are essential organic nutrients that often act as cofactors for cellular
enzymes.
b. Consuming sufficient quantities of essential vitamins is necessary for good health.
Digestion (p. 532)
25.2
25.3
Types of Digestive Systems (p. 532; Figs. 25.3, 25.4)
A. Heterotrophs are divided into three groups on the basis of their food sources.
1. Animals that only eat plants are herbivores.
2. Animals that rely on meat for a food source are carnivores.
3. Omnivores are those animals that eat both plants and meat.
B. Single-celled organisms and sponges digest food intracellularly.
C. Other animals digest food extracellularly, using enzymes within a digestive cavity.
D. Specialization occurs when the digestive tract has a separate mouth and anus.
E. Chemical digestion occurs as enzymes break bonds and reduce food into simple building
blocks that can be absorbed into the body.
F. Animals excrete any food molecules that cannot be absorbed.
Vertebrate Digestive Systems (p. 533; Figs. 25.5, 25.6)
A. In humans and other vertebrates, the digestive system consists of a tubular gastrointestinal
tract and accessory organs.
B. In general, carnivores have shorter intestines for their size than do herbivores, which have
long, convoluted intestines as well as chambers where bacteria can digest cellulose.
25.4
25.5
C. The tubular gastrointestinal tract of vertebrates has four layers.
1. The innermost layer, comprised of epithelium, is the mucosa.
2. The second layer, made up of connective tissue, is the submucosa.
3. Third is a double layer of muscle tissue.
4. An outer serosa layer is made up of connective tissue.
The Mouth and Teeth (p. 534; Figs. 25.7-25.10)
A. Digestion begins in the mouth.
B. Vertebrate Teeth
1. Teeth (if present) break up larger portions of food.
2. Mammals have heterodont dentition, teeth of different specialized types; the general
pattern of heterodont dentition is modified in different mammals depending on their diet.
3. In carnivorous mammals, the canines are prominent, and other teeth are more bladelike
and sharp.
4. In herbivorous mammals, incisors are well-developed for snipping, canines are reduced
or absent, and molars are large and flat, with complex ridges well suited to grinding.
5. Omnivores, such as humans, have teeth specialized for eating both plant and animal
matter.
6. Children have 20 teeth, and adults have 32 teeth.
C. Processing Food in the Mouth
1. In the mouth, the tongue helps mix the food with saliva from salivary glands.
2. Saliva contains salivary amylase, the first enzyme in digestion that goes to work on
carbohydrates.
3. Salivary secretions are controlled by the nervous system.
D. Swallowing
1. Food is moved by the tongue to the back of the mouth for swallowing.
2. The soft palate is raised, closing off the nasal cavity, and muscles push the food past the
larynx with its protective epiglottis, and into the esophagus.
The Esophagus and Stomach (p. 536; Figs. 25.11, 25.12)
A. Structure and Function of the Esophagus
1. Food is swallowed and passes through a muscular tube called the esophagus on its way
to the stomach.
2. No digestion or absorption occurs in the esophagus; it is merely a passageway.
3. Muscles in the walls of the esophagus contract in waves, called peristalsis, to propel food
along.
4. At the beginning of the stomach is a ring of muscle called a sphincter that opens so food
can move into the stomach.
B. Structure and Function of the Stomach
1. The stomach is a muscular sac that mixes foods with acid and digestive enzymes.
2. It also churns through a series of muscular contractions that function both to mix and
mechanically break up food.
3. Within the epithelial lining of the stomach are several kinds of cells that secrete digestive
juices.
4. Parietal cells secrete hydrochloric acid, and chief cells secrete an inactive form of pepsin
that, when activated by the acid in the stomach, begins the breakdown of proteins.
C. Action of Acid
1. The hydrochloric acid functions not only to keep pepsin active but also to kill bacteria
swallowed with food.
2. The mixture of partially digested food and gastric juice is called chyme.
D. Ulcers
1. The hormone gastrin regulates the amount of acid produced.
2. Susceptibility to ulcers increases when the bacterium Helicobacter pylori is present.
E. Leaving the Stomach
1. Chyme leaves the stomach through the pyloric sphincter.
2. Limited absorption occurs in the stomach; only water, aspirin, alcohol, and a few other
substances are absorbed through the stomach wall.
25.6
The Small and Large Intestines (p. 538; Fig. 25.13)
A. Digestion and Absorption: The Small Intestine
1. The small intestine is the major site of the chemical breakdown of food and absorption of
nutrients.
2. An accessory organ, the pancreas, produces digestive substances that are added to the
small intestine to facilitate digestion.
3. The liver produces bile, which contains bile salts that function to emulsify fats.
4. The first portion of the small intestine, the duodenum, receives the stomach contents and
digestive products from the pancreas and liver.
5. Most of digestion occurs in the duodenum and the next portion, called the jejunum.
6. The last portion of the small intestine is the ileum, which has the job of absorbing water
and nutrients from the digested material moving through it.
7. Fingerlike projections called villi, which in turn have cells with projections called
microvilli, increase the surface area available for absorption in the small intestine.
B. Concentration of Solids: The Large Intestine
1. The large intestine, or colon, compacts and stores feces, and reabsorbs water.
2. Feces are propelled toward the rectum, and they are eventually expelled through the anus.
25.7
Variations in Vertebrate Digestive Systems (p. 540; Figs. 25.14, 25.15)
A. Most animals lack the enzymes necessary to digest cellulose, although many harbor bacteria
that do the work for the animal host.
B. Cows, deer, and other organisms have rumens that house bacteria where cellulose breakdown
occurs.
C. In other animals, such as rodents, horses, and lagomorphs, digestion of cellulose by
microorganisms takes place in the cecum; rodents and lagomorphs also practice coprophagy
to absorb the nutrients produced by microorganisms.
D. Intestinal microorganisms also produce molecules such as vitamin K that are important to the
well-being of their vertebrate hosts.
25.8
Accessory Digestive Organs (p. 542; Figs. 25.16, 25.17)
A. The Pancreas
1. The pancreas produces digestive substances and contributes them to the small intestine.
2. Enzymes are produced to break down carbohydrates, proteins, and fats into their
component molecules.
3. Bicarbonate is also produced, which neutralizes the acidity of chyme leaving the stomach
and entering the small intestine.
4. The pancreas also produces hormones in the islets of Langerhans; the hormones insulin
and glucagon help regulate blood glucose levels.
B. The Liver and Gallbladder
1. The liver is the largest organ in the body and produces bile to aid in the digestion of fats.
2. Bile is stored in the gallbladder between meals, then released into the small intestine
during the digestive process.
3. Bile functions to emulsify fats.
C. Regulatory Functions of the Liver
1. The liver carries on a number of important metabolic functions that help regulate the
levels of nutrients in the blood.
It detoxifies alcohol, stores nutrients, and manufactures proteins, among other functions