1. Essentials of the Living World
Second Edition
George B. Johnson
Jonathan B. Losos
Chapter 27
The Path of Food Through the Animal Body
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. 27.1 Food for Energy and Growth
The food animals eat provides both a source of energy and essential molecules that the animal body is not able to manufacture for itself
an optimal diet contains more carbohydrates than fats and also a significant of protein

3. Figure 27.1 The pyramid of nutrition

4. 27.1 Food for Energy and Growth
Carbohydrates are obtained primarily from cereals, grains, and breads
on the average, carbohydrates contain 4.1 calories per gram
the body uses carbohydrates for energy
Dietary fats are obtained from oils, margarine, and butter and are abundant in fried foods, meats, and processed snack foods
fats contain 9.3 calories per gram
the body uses fats to construct cell membranes, to insulate nervous tissue, and to provide energy

5. 27.1 Food for Energy and Growth
Proteins can be obtained from many foods, including poultry, fish, meat, and grains
proteins have 4.1 calories per gram
proteins are used for energy and as building materials for cell structures, enzymes, hemoglobin, hormones, and muscle and bone tissue

6. Figure 27.2 The protein content of a variety of common foods

7. 27.1 Food for Energy and Growth
In wealthy countries, being significantly overweight is common
this is due to habitual eating and high-fat diets, in which fats constitute over 35% of the total caloric intake
the standard measure of appropriate body weight is the body mass index (BMI), estimated as your body weight in kg, divided by your height in meters squared

8. Figure 27.3 Are you overweight?
64.5% of Americans are overweight with a BMI of 25 or more

9. 27.1 Food for Energy and Growth
One essential characteristic of food is its fiber content
fiber is the part of plant food that cannot be digested by humans
diets that are low in fiber result in a slower passage of food through the colon
low fiber is thought to be associated with incidences of colon cancer

10. 27.1 Food for Energy and Growth
Over the course of evolution, many animals have lost their ability to manufacture certain substances they need
many vertebrates are unable to manufacture one or more of the 20 amino acids used to make proteins
humans are unable to synthesize eight amino acids, which must be obtained from proteins in food
these are called essential amino acids

11. 27.1 Food for Energy and Growth
In addition to supplying energy, food must also supply essential minerals, such as calcium and phosphorous
some minerals are required in very small amounts and are called trace minerals
Essential organic substances that are used in trace amounts are called vitamins
many vitamins are required cofactors for enzymes

12. 27.2 Types of Digestive Systems
Heterotrophs are divided into three groups on the basis of their food sources
herbivores eat plants exclusively
carnivores are meat eaters
omnivores eat both plants and animals
Single-celled animals, as well as sponges digest their food intracellularly
All other animals digest their food extracellularly, within a digestive cavity

13. 27.2 Types of Digestive Systems
A gastrovascular cavity is found in cnidarians and flatworms
this cavity has only a single opening that serves as both a mouth and an anus
there is no specialization within this type of digestive system because every cell is exposed to all stages of digestion

14. Figure 27.4 The gastrovascular cavity

15. 27.2 Types of Digestive Systems
The alimentary canal is a digestive tract with a separate mouth and anus
this permits specialization and the transport of food is one way
physical forces, such as chewing and grinding, first breaks the ingested food into smaller fragments
chemical digestion involves hydrolysis reactions that liberate the subunits of food
the products of digestion are absorbed into the blood
any molecules in the food that are not absorbed by the animal are excreted through the anus

16. Figure 27.5 One-way digestive tracts

17. 27.3 Vertebrate Digestive Systems
In humans and other vertebrates, the digestive system consists of a tubular gastrointestinal tract and accessory organs
in general, carnivores have shorter intestines for their size than herbivores
herbivores ingest a large amount of plant cellulose, which resists digestion
the tubular gastrointestinal tract has a layered structure

18. Figure 27.6 The human digestive system

19. Figure 27.7 The layers of the gastrointestinal tract

20. 27.4 The Mouth and Teeth
Many vertebrates have teeth and chewing (mastication) breaks up food into small particles and mixes it with fluid secretions
carnivorous mammals have pointed teeth that lack flat grinding surfaces
these teeth are adapted for cutting and shearing
herbivores have large, flat teeth with complex ridges well suited to grinding

21. Figure 27.8 Diagram of generalized vertebrate dentition

22. 27.4 The Mouth and Teeth
Humans are omnivores and human teeth are specialized for eating both plant and animal material
humans are carnivores in the front of the mouth and herbivores in the back
children have only 20 teeth but these are lost during childhood and replaced by 32 adult teeth

23. Figure Human teeth
The tooth is a living organ.

24. 27.4 The Mouth and Teeth
Inside the mouth, the tongue mixes food with a mucous solution called saliva
saliva moistens and lubricates food so that it is easier to swallow
saliva also contains a hydrolytic enzyme called amylase
this enzyme initiates the breakdown of starch into the disaccharide maltose

25. 27.4 The Mouth and Teeth
When food is ready to be swallowed, a sequence of events occur that cause food to go into the esophagus
food is prevented from going into the respiratory tract by the epiglottis
Figure The human pharynx, palate, and larynx.

26. 27.5 The Esophagus and Stomach
The esophagus is a muscular tube that connects the pharynx to the stomach
the upper third is enveloped in skeletal muscle for voluntary control of swallowing
the lower two-thirds is surrounded by involuntary smooth muscle
rhythmic waves of contractions, called peristalsis, propel food towards the stomach

27. Figure 27.12 The esophagus and peristalsis

28. 27.5 The Esophagus and Stomach
The movement of food from the esophagus into the stomach is controlled by a ring of circular smooth muscle, called a sphincter
contraction of the sphincter prevents food in the stomach from moving back into the esophagus
in humans, stomach contents can be brought back out during vomiting
the relaxing of the sphincter may lead to acid reflux, which is when stomach acid moves into the esophagus
this produces a burning sensation known as heartburn

29. 27.5 The Esophagus and Stomach
The stomach is a saclike portion of the digestive tract
the stomach contains an extra layer of smooth muscle for churning food
gastric juice is released by gastric glands in the lining of the stomach
parietal cells secrete hydrochloric acid (HCl)
chief cells secrete pepsinogen
pepsinogen requires a low pH to be activated into pepsin, a protease that begins the digestion of proteins

30. Figure 27.13 The stomach and gastric glands

31. 27.5 The Esophagus and Stomach
Gastric juice has a pH of 2, much more acidic than the 7.4 pH of blood
the low pH helps to denature protein, keep pepsin active, and kill most bacteria
active pepsin hydrolyzes food proteins into short chains of polypeptides that are not fully digested until the mixture enters the small intestine
chyme is the name for the mixture of partially digested food and gastric juice

32. 27.5 The Esophagus and Stomach
Overproduction of gastric acid can occasionally eat a hole through the wall of the stomach, called a gastric ulcer
normally the stomach epithelial cells are protected by alkaline mucus
susceptibility to ulcers is increased by an infection of the bacterium Helicobacter pylori

33. 27.5 The Esophagus and Stomach
The parietal cells of the gastric glands also produce intrinsic factor, which is necessary for the intestinal absorption of vitamin B12
this vitamin is necessary for the production of red blood cells
persons who lack sufficient intrinsic factor develop pernicious anemia

34. 27.6 The Small and Large Intestines
The intestine is the true digestive vat of the body
only relatively small portions of chyme are introduced into the small intestine at one time
this allows time for acid to be neutralized and enzymes to act
in the small intestine, carbohydrates, protein, and lipids are broken down and absorbed into the bloodstream

35. 27.6 The Small and Large Intestines
While some enzymes necessary for digestion are secreted by the cells of the intestinal wall, most are made in the pancreas
the pancreas is an exocrine gland, meaning it secretes through ducts
the pancreas sends it products via a duct that empties into the first part of the small intestine, the duodenum

36. 27.6 The Small and Large Intestines
much of the food energy the vertebrate body harvests is obtained from fats
fat digestion involves bile salts that are secreted into the duodenum by the liver
the bile salts act like detergents and make drops of fat into microscopic droplets
this process is known as emulsification
this increases the surface area for the enzyme lipase to work on in order to breakdown the fat

37. 27.6 The Small and Large Intestines
The small intestine also includes
jejunem where digestion continues
ileum where water and digested products are absorbed
The lining of the small intestine is folded into ridges, which are covered with fine projections called villi (singular, villus)
each of the cells covering the villus is covered by a field of microprojections called microvilli

38. Figure 27.14 The small intestine

39. 27.6 The Small and Large Intestines
The large intestine has a wider diameter than the small intestine
no digestion takes place here
only about 6% to 7% of fluid absorption occurs here
some water, sodium, and vitamin K
the main function of the large intestine is to compact and store undigested material as feces

40. 27.7 Variations in Vertebrate Digestive Systems
Most animals lack the enzymes necessary to digest cellulose
but the digestive tract of some animals contain prokaryotes and protists that convert cellulose into substances the host can digest
ruminants have large divided stomachs
one section, the rumen, harbors symbiotic prokaryotes and protists
cows and deer are examples of ruminants

41. Figure 27.15 Four-chambered stomach of a ruminant

42. 27.7 Variations in Vertebrate Digestive Systems
Other herbivores, such as rodents, horses, and rabbits harbor microorganisms that can digest cellulose in their cecums
because the cecum is below the stomach, these organisms cannot regurgitate like ruminants
in rodents and rabbits, they ingest feces in order to process further the cellulose
this is known as coprophagy

43. Figure 27.16 The digestive systems of different mammals reflect their diets

44. 27.8 Accessory Digestive Organs
The pancreas secretes fluid through the pancreatic duct into the duodenum
the fluid contains a host of enzymes
trypsin and chymotrypsin digest proteins
pancreatic amylase digests starch
lipase digests fats
the pancreas secretes bicarbonate, which neutralizes the HCl from the stomach

45. 27.8 Accessory Digestive Organs
In addition to being an exocrine gland, the pancreas is also an endocrine gland
it produces hormones in the Islets of Langerhans
the two most important pancreatic enzymes are insulin and glucagon

46. 27.8 Accessory Digestive Organs
The liver is the largest internal organ of the body
the liver produces bile and stores it in the gallbladder where it is concentrated
if the bile duct becomes blocked, a gallstone forms
the arrival of fatty food in the duodenum triggers a neural and endocrine reflex that stimulates the gallbladder to contract

47. Figure 27.17 The pancreatic and bile ducts empty into the duodenum

48. 27.8 Accessory Digestive
The liver removes toxins, pesticides, carcinogens and other poisons by converting them into less toxic forms
excess amino acids that may be present in the blood are converted to glucose
an amino group (-NH2) is removed from the amino acid to become ammonia (NH3)
NH3 then combines with CO2 to form urea, which then goes to the liver

49. Figure 27.18 The organs of the digestive system and their functions

50. Inquiry & Analysis
How long does it take for the diabetic person’s blood glucose levels to return to the level before the test dose?
Why do you suppose the diabetic individual took so much longer to recover from the test dose?
Graph of Effects of Eating on Blood Glucose Levels