Chapter 46: Nutrition and Digestion (Part 2, Week 12)

Question 1

[Start 46.2 General Principles of Digestion and Absorption of Nutrients]

What are the two ways food is digested when it comes to cells?

Answer 1

Intracellularly or, more commonly, extracellularly.

Intracellular digestion occurs only in some very simple invertebrates such as sponges and single-celled organisms and to a limited extent in cnidarians.

It involves using phagocytosis to bring food particles directly into a cell, where the food is segregated from the rest of the cytoplasm in food vacuoles. Once inside these vacuoles, the macromolecules in the food are digested by hydrolytic enzymes into monomers (the building blocks of polymers), which then are moved out of the vacuole to be used directly by that cell.

Intracellular digestion cannot support the metabolic demands of an active animal, because only extremely tiny bits of food can be phagocytosed at one time.

This form of digestion also does not provide a mechanism for storing large quantities
of food so that ananimal can digest it slowly while engaging in its other activities.

Question 2

What is the simplest form of extracellular digestion that is seen in invertebrates such as flatworms and cnidarians? Mainly, the digestive structure.

Answer 2

The digestive cavity has one opening that serves as both an entry and an exit port.

This is called a GASTROVASCULAR cavity. In certain invertebrates such as cnidarians, a body cavity with a single opening to the outside; it functions as both a digestive system and circulatory system.

Because not only does digestion occur within it, but fluid movements in the cavity also serve as a circulatory—or vascular—system to distribute digested nutrients throughout the animal’s body.

Question 3

In contrast to the gastrovascular cavities of certain invertebrates, the digestive systems of all other animals consist of?

Answer 3

A single elongated tube, or alimentary canal, with an opening at both ends through which food passes from one end to the other.

Along its length, the tube usually contains smooth muscle. The contractions of this muscle help churn up the ingested food so that it is mechanically broken into smaller fragments.

Question 4

What do the epithelial cells within the alimentary canal perform?

Answer 4

These cells synthesize and secrete digestive enzymes and other factors into the hollow cavity (or lumen, as such cavities are called) of the alimentary canal, and they secrete certain hormones into the blood that help regulate digestive processes.

The cells are also involved with transporting digested material out of the canal and into an animal’s body fluids. In addition, a diverse collection of microorganisms, called amicrobiome, inhabit the alimentary canal. The microbiome plays a key role in digestion and even affects the growth of young animals.

Question 5

By what processes do nutrients get absorbed by epithelial cells that line specialized portions of the alimentary canal?

Answer 5

This occursin different ways, either by simple or facilitated diffusion or by active transport.

Question 6

Facts about Aborption as Passive and Active

Small, hydrophobic molecules such as fatty acids diffuse down concentration gradients across the epithelium. Ions and other molecules are transported by facilitated diffusion or active transport.

Minerals are ions and therefore, like all polar substances, do not readily cross plasma membranes. Instead, they are usually actively transported across the membranes of epithelial cells by ATP-dependention pumps. In other cases, small, hydrophilic organic nutrients are transported by secondary active transport, usually with Na+.

After nutrients enter the epithelial cells of the alimentary canal, the cells use some of the nutrients for their own requirements.

In animals with a closed circulatory system, most of the nutrients are transported across the basolateral surface of the epithelial cells (the side opposite the lumen), where they can enter into nearby blood or lymph vessels and circulate to the other cells of the body.

Thus, nutrients enter the alimentary canal in food, are digested within the canal into monomers or other small fragments that can be transported into epithelial cells, and from there are released into the blood, where they can reach all ofthe body’s cells. In the special case of water, osmotic gradients established by the transport of ions and other nutrients out of the epithelial cells draw water by osmosis from the canal, across the epithelial cells, and from there into the blood.

Answer 6

N/A

Question 7

[Start 46.3 Overview of Vertebrate Digestive Systems]

What, in animals, is the long tube through which food is processed? In a vertebrate, this system consists of the alimentary canal plus several associated structures.

Answer 7

Digestive system

Also known as the gastrointestinal (or GI) tract—plus several accessory organs.

Question 8

What does the human alimentary canal consist of?

Answer 8

Oral cavity (mouth), pharynx, esophagus, stomach, small and large intestines, and anus.

The accessory organs, not all of which are found in all vertebrates, are the salivary glands, liver, gallbladder, and pancreas.

Question 9

What begins digestion and breaks up food for swalling and what secretes saliva?

Answer 9

Mouth and salivary glands.

Question 10

What is the pathway to the __________ which transports food to the stomach?

Answer 10

The pharynx is the pathway to the esophagus.

Question 11

What stores and mechanically disrupts food and digests some proteins?

Answer 11

The stomach

Question 12

What produces bile to assist in fat digestion?

Answer 12

The liver

Question 13

What stores bile until needed and secretes bile into the small intestine?

Answer 13

Gallbladder

Question 14

What secretes digestive enzymes and HCO3- into the small intestine?

Answer 14

Pancreas. HCO3- is bicarbonate.

Question 15

What absorbs water and minerals and prepares wastes for defecation?

Answer 15

Large intestine

Question 16

What is the site for most absorption and digestion?

Answer 16

Small intestine

Question 17

What is part of the large intestine and stores wastes like feces?

Answer 17

Rectum

Question 18

What eliminates waste in defecation?

Answer 18

Anus

Question 19

What are the three general sections of the alimentary canal?

Answer 19

Anterior end which functions primarily in the ingestion of food which contains the mouth (with accesory organs, the salivary glands), pharynx (throat), and esophagus.

The middle section, which functions in the storage and initial digestion of food, contains one or more food storage or digestive organs, including the crop, gizzard, and stomach, depending on the species. This section also contains the upper part of the small intestine—where most of the digestion and absorption of food takes place—and accessory organs that connect with the intestine, including the pancreas, liver, and gallbladder.

The third section, the posterior part of the canal, functions in final digestion and absorption and the elimination of nondigestible wastes. It consists of the remainder of the small intestine and, in most vertebrates other than fishes, a large intestine.

Undigested material is defecated through an opening called an anus or, in many amphibians, reptiles, and birds, a cloaca (a common opening for the digestive and urogenital tracts).

Question 20

What is the lumen lined with within the GI tract?

Answer 20

Epithelial and glandular cells.

Included in the epithelial layer are secretory cells that release a protective coating of mucus into the lumen of the tract, and for this reason this layer of cells is also referred to as the mucosa.

Other cells in the mucosal layer release hormones into the blood in response to the presence of food.

Question 21

What passes through the mucosal layer?

Answer 21

Ducts from secretory glands that release acid, enzymes, water, and ions into the lumen.

Question 22

What are some places, such as the small intestine, have its apical surface of the mucosa highly convoluted?

Answer 22

A feature that increases the surface area available for digestion and absorption.

Question 23

What is the epithelial cell layer surrounded by layers of within the alimentary canal?

Answer 23

Smooth muscles, neurons, connective tissue, and blood vessels.

The neurons are activated by signals originating from regions of the central nervous system that respond to the sight and smell of food.

The neurons of the alimentary canal are also activated directly by the presence of food in the tract. Contraction of the smooth muscles is controlled by these neurons and results in mechanical mixing of the contents within the stomach and intestine. This helps speed up digestion and also brings digested foods into contact with the epithelium to facilitate absorption.

Question 24

When does ingestion and the start of digestion begin?

Answer 24

Once food enters the mouth.

Question 25

In terrestrial vertebrates, what does the presence of food stimulate?

Answer 25

Salivary glands in and around the mouth, cheeks, tongue, and throat to produce a flow of saliva—a watery fluid containing proteins, mucus, and antibacterial substances that keeps the mouth moist and clean.

Unlike terrestrial vertebrates, fishes, which lack true salivary glands, secrete mucus from specialized cells in their mouth and pharynx.

Question 26

What are several functions of saliva?

Not of all which pertain to vertebrates.

Answer 26

to moisten and lubricate food to facilitate swallowing

to dissolve food particles to facilitate the ability of specialized chemical-sensing structures called taste buds to taste food

to kill ingested bacteria with a variety of antibacterial compounds, including antibodies

to initiate digestion of carbohydrates through the action of a secreted enzyme called amylase

Question 27

What is a digestive enzyme in saliva and the pancreas involved in the digestion of carbohydrates?

Answer 27

Amylase

In humans and other primates, salivary amylase is present and accounts for only a very small percent of total carbohydrate digestion.

The other functions of saliva, however, are very important. For example, imagine trying to swallow food with a perfectly dry mouth.

Also, the antibiotic properties of saliva help keep the mouth free of infection. In people who have had cancerous salivary glands removed, the teeth and gums often become so diseased that tooth loss occurs.

Question 28

What is the purpose of the pharynx and esophagus?

Answer 28

These structures, although not contributing to digestion or absorption, serve as a pathway to a storage organ.

Question 29

What, in animals, is the rhythmic, spontaneous waves of muscle contractions that propel food through the digestive system?

Answer 29

Peristalsis; begin near the mouth and end at the stomach

The wavelike action of peristalsis ensures that swallowed food is moved toward the stomach and does not remain in the esophagus or even move backward into the pharynx if the head is lowered.

Question 30

What is a storage organ that is a dilation of the lower esophagus; found in most birds and many invertebrates, including insects and some worms?

Answer 30

Crop

Food is stored and softened by watery secretions in the crop, but little or no digestion occurs there.

Because they process large amounts of tough food, birds that eat primarily grains and seeds have larger crops than do birds that eat only insects and worms.

The material that birds regurgitate to their young comes from the crop. In some species, such as pigeons and doves, the epithelial cells that line the crop lumen secrete a lipid-rich watery solution called crop milk or pigeon milk into the material to be regurgitated.

Question 31

What is the storage organ?

Answer 31

The stomach

Book Definition

A saclike organ in some animals that most likely evolved as a means of storing food; it partially digests some of the macromolecules in food and regulates the rate at which its contents empty into the small intestine.

Question 32

What is a true stomach?

Answer 32

Defined as one that produces hydrochloric acid, HCI.

The stomach partially digests some of the macromolecules in food and regulates the rate at which its contents empty into the small intestine.

Question 33

What are the two secretions by the glands within the stomach mucosa?

Answer 33

HCI and an inactive protein called pepsinogen into the stomach lumen.

Question 34

What are the roles of the HCI secreted by glands within the stomach mucosa?

Answer 34

It converts inactive pepsinogen into the active enzyme pepsin, which is a protease and begins the digestion of protein.

HCl kills many of the microorganisms that may have been ingested with food.

Helps dissolve the particulate matter in food.

Question 35

Why do stomach gland cells secrete pepsinogen instead of pepsin?

Answer 35

The answer is that if the cells produced active pepsin, they would digest their own cellular proteins.

The epithelium of the stomach is coated with a layer of alkaline mucus that protects the stomach lining from the effects of HCl.

Question 36

What does the acid environment in the stomach or gastric lumen alter and why?

Answer 36

Alters the ionization of polar molecules, especially proteins.

This disrupts the structural framework of the tissues in food and makes the proteins more accessible to pepsin, which partially digests some proteins into smaller polypeptide fragments.

Question 37

T/F No significant digestion of carbohydrates or lipids occurs in the stomach.

Answer 37

True

Question 38

Specializations of the Stomach in Vertebrates Facts

• In birds, the stomach is divided into two parts: the proventriculus and the gizzard
• The proventriculus is the glandular portion of the stomach that secretes acid and pepsinogen. Partially digested and acidified food then moves to the
  gizzard, a muscularstructure with a rough inner lining that grinds food into smaller fragments. This increases the surface area available to digestive enzymes that act in the small intestine.
• The gizzard contains sand or tiny stones swallowed by the bird. The gritty sand and stones take the place of teeth and help mash and grind ingested food. Eventually, the pebbles in the gizzard become smaller as they are worn away, and they are eliminated.
• Thus, birds must occasionally restock the gizzard with new grinding stones.
• Grain-eating birds, particularly chickens and other fowl, many passerines (perching birds), and pigeons and doves, generally have more muscular gizzards than do insectivorous birds, because of the difficulty in breaking down plant matter.

Gizzards, incidentally, are not unique to birds. Certain reptiles that are closely related to birds, such as crocodiles, also contain muscular gizzards. In addition, some species of herbivorous fishes (forexample, members of the family Acanthuridae) ingest quantities of inorganic grit with their meals, which helps to grind up food in a portion of the stomach that is modified into a strong, muscular grinding organ like a gizzard.

Answer 38

N/A

Question 39

What is a solution of water, ions, molecular fragments of proteins, nucleic acids, and carbohydrates, droplets of fat, and various other small molecules produced in the vertebrate stomach?

Answer 39

Chyme

Therefore, little or no absorption of nutrients occurs in the stomach.

Question 40

What, in the human diet, is also called fiber or roughage, and is useful in helping eliminate solid wastes?

Answer 40

Cellulose, important in the diet of herbivores and omnivores.

The human digestive system is not equipped with the enzymes to digest cellulose, and it therefore passes through the system intact.

Question 41

What are animals such as sheep, goats, llamas, and cows that have complex stomachs consisting of several chambers?

Answer 41

Ruminants

Also lack the enzymes, but they are able to digest cellulose with the help of their microbiome.

Microorganisms living within their digestive tracts break down the cellulose into monosaccharides that can be absorbed along with other by-products of microbial digestion, such as fatty acids and some vitamins.

In this way, bacteria and protists predigest the food, and the animal absorbs the broken-down cellulose and uses its sugar as a food source.

Question 42

What, in vertebrates, is the part of the alimentary canal that leads from the stomach to the large intestine (or to the anus or cloaca in animals that lack a large intestine) and that carries out nearly all digestion of food and absorption of food nutrients and water?

Answer 42

The small intestine

Hydrolytic enzymes break down macromolecules of organic nutrients into smaller monomers. Some of these hydrolytic enzymes are on the apical membrane of the intestinal epithelial cells; others are secreted by the pancreas and enter the intestinal lumen through a connecting duct.

The products of digestion are absorbed across the epithelial cells and enter the blood. Vitamins and minerals, which do not require enzymatic digestion, are also absorbed in the small intestine. Water is absorbed by osmosis from the small intestine in response to the movement of nutrients across the intestinal epithelium.

Question 43

What aids the small intestine to carry out the bulk of digestion?

Answer 43

Aided by mucosal infoldings and specializations along its length.

Question 44

What are the finger-like projections extending from the inner surface into the lumen of the vertebrate small intestine; these are specializations that aid in digestion and absorption?

Answer 44

Villi (singular, villus)

Question 45

The surface of the villus is covered with a layer of epithelial cells, the plasma membranes of which form small projections called what?

Answer 45

Microvilli

Question 46

What is the collective name for the microvilli in the small intestine and the proximal tubules of the kidneys in vertebrates?

Answer 46

The brush border

Question 47

How much of the small intestine’s surface area is increased because of the combination of folded mucosa, villi, and microvilli?

Answer 47

600-fold above that of a flat-surfaced tube having the same length and diameter

The small intestine is small in diameter compared with the large intestine, but it is very long—3 m
in an adult human (if removed from the abdomen, the small intestine can expand to almost twice as long as its normal length, because the muscles relax).

This brings the total surface area of the human small intestine to about 300 m^2—roughly the size of a tennis court!

This enormous surface area means that the likelihood of an ingested food particle encountering a digestive enzyme and being absorbed across the epithelium is very high, so digestion and absorption proceed rapidly.

Question 48

What is a lymphatic vessel in the center of each intestinal villus; lipids are absorbed by the lacteals, which eventually empty into the circulatory system?

Answer 48

Lacteal; this is part of the lymphatic system

Most of the fat absorbed in the small intestine exists as bulky protein-coated particles that are too large to enter capillaries.

Consequently, absorbed fat enters the larger, wider lacteals. Materialabsorbed by the lacteals eventually empties into the circulatory system. Other nutrients are absorbed directly into the capillaries and from there into veins.

Question 49

What are the three organs that secrete substances that flow via ducts into the first portion of the small intestine?

Also, what is this part of the intestine called?

Answer 49

Pancreas, Liver, and Gallbladder

Duodenum

Question 50

What advantage does an animal gain by having a gallbladder?

Answer 50

A gallbladder stores bile and releases it precisely when it is needed, in response to a meal, which is particularly useful for animals that consume large or infrequent meals.

In the absence of a gallbladder, bile flows into the intestine continuously and cannot be increased to match the amount or timing of food intake.

Question 51

What, in vertebrates, is an elongated gland located behind and below the stomach that secretes digestive enzymes and a fluid rich in bicarbonate ions?

Answer 51

Pancreas

The glandsecretes digestive enzymes and a fl uid rich in bicarbonate ions (HCO3–).

The HCO3– neutralizes the acidity of chyme, which would otherwise inactivate the pancreatic enzymes in the small intestine and could also damage the intestinal epithelium.

Question 52

What is an organ in vertebrates that performs diverse metabolic functions and is the site of bile production?

Answer 52

The liver.

Question 53

What is a substance produced by the liver that contains bicarbonate ions, cholesterol, phospholipids, a number of organic wastes, and a group of substances derived from cholesterol and collectively termed bile salts?

Answer 53

Bile

Bile emulsifies fats so that they can be absorbed by the small intestine.

bile salts
A group of substances produced in the liver that solubilize dietary fat and increase its accessibility to digestive enzymes.

Question 54

What, in many vertebrates, is a small sac underneath the liver that is a storage site for bile; allows the release of large amounts of bile to be precisely timed to the consumption of fats?

Answer 54

The gallbladder

During a meal, the smooth muscles inthe gallbladder contract, secreting the bile solution into a connecting duct called the common bile duct.

The opening ofa sphincter allows the bile to flow into the lumen of the small intestine. The gallbladder, therefore, is a storage organ that allows the release of large amounts of bile to be precisely timed to the consumption of fats.

However, many animals such as horses and doves that secrete bile donot have a gallbladder. In humans, the gallbladder can be surgically removed without impairing bile secretion by the liver or its flow into the intestinal tract.

People without a gallbladder can still digest fat but may need to limit the amount of fat they eat at one time because bile secretion can no longer be well timed to a meal.

The digested nutrients, along with water, are absorbed across the plasma membranes of the brush border cells. Peristalsis slowly propels the remaining contents through the posterior two portions of
the small intestine, called the jejunum and the ileum, where further absorption occurs. Finally, depending on the animal, the remaining material enters the large intestine or the anus or cloaca.

Question 55

What are other two sections of the small intestine called?

Answer 55

The jejunum and the ileum.

Question 56

What is the dimensions of the large intestine?

Answer 56

6 cm in diameter and 1-1.5 m long.

Question 57

What is the first portion of a vertebrate’s large intestine?

Answer 57

The cecum.

Forms a small pouch from which extends the appendix, a finger-like projection having no certain essential function but may contribute to the body’s immune defense mechanisms.

Question 58

What is a part of a vertebrate’s large intestine consisting of three relatively straight segments—the ascending, transverse, and descending portions?

Answer 58

The colon.

The terminal portion of the descending colon is S-shaped, forming the sigmoid colon, which empties into the rectum.

Question 59

What is the last segment of the large intestine of vertebrates that ends at the anus, the posterior opening of the alimentary canal to the external environment?

Answer 59

Rectum

Question 60

What is the opening at the posterior end of the alimentary canal through which solid wastes are expelled?

Answer 60

Anus

Question 61

What is the primary functions of the large intestine?

Answer 61

To store and concentrate fecal material before defecation and to absorb some of the remaining ions and water that were not absorbed in the small intestine.

Question 62

What is the expulsion of feces that occurs through the anus of an animal’s digestive canal?

Answer 62

Defecation

Occurs when contractions of the rectum andrelaxation of associated sphincter muscles expel the feces through theanus

Located within the microbiome of the large intestine are large populations of various bacteria, some of which provide benefits to animals.

For example, some bacteria release by-products such as certain vitamins into the lumen of the large intestine, where these substances can be absorbed across the intestinal epithelium. Although this source of vitamins generally provides only a small part of the normal daily requirement, it may make a significant contribution when dietary vitamin intake is low.

Sometimes people develop a vitamin deficiency if treated with antibiotics that kill these species of bacteria. Other bacterial products include gas (flatus), which is a mixture of nitrogen, carbon dioxide, hydrogen, methane, and hydrogen sulfide. Bacterial processing of undigested polysaccharides produces these gases, except for nitrogen, which is derived from swallowed air.

Certain foods (for example, beans) contain large amounts of carbohydrates that cannot be digested by intestinal enzymes but are readily metabolized by bacteria in the large intestine, producing gas. The bacteria are continually eliminated in the feces along with undigested material, so their populations normally stay in check.