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Herbivores – such as cattle, gorillas, sea urchins, and snails, eat mainly autotrophs (plants
and algae)
Carnivores - such as lions, hawks, spiders, and whales, mostly eat other animals
Omnivores – ingest both plants and animals
Suspension Feeders – Extract food particles suspended in the surrounding water
Substrate feeders – live in or on their food source eat their way out of it
Fluid feeders - obtain food by sucking nutrient-rich fluids from a living host, either a plant
or an animal
Bulk feeders - ingest large pieces of food
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Digestion in an animal break the polymers in food into monomers as shown in Figure 5
•Protiens amino acids
•Carbohydrates monosaccharides
•Nucleic acids nucleotides
•Fats glycerol and fatty acids
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Diagram 1: The four main stages of food processing
1. Ingestion – The act of eating
2. Digestion – the breaking down of food into molecules small enough for the
body to absorb. Digestion typically occurs in two phases:
i.
Food may be mechanically broken into smaller pieces with chewing or
tearing, breaking large chunks of food into smaller ones
ii. Digestion is the chemical breakdown process called hydrolysis.
Catalyzed by specific enzymes, hydrolysis breaks chemical bonds in
food molecules by adding water to them
3. Absorption – the cells lining the digestive tract take up (absorb) the products
of digestion – small molecules such as amino acids and simple sugars. From
the digestive tract, these nutrients travel in the blood to body cells, where
they are joined together to make the macromolecules of the cells or broken
down further to provide energy
4. Elimination - undigested material passes out of the digestive tract
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The simplest of all digestive compartments are food vacuoles within a cell
Most animals, however, uses other specialized compartments to break down larger foods
•Gastrovascular cavity – a digestive tract compartment with a single opening, the
mouth
•Diagram 2:
1. Gland cells lining the gastrovascular cavity secrete digestive
enzymes that
2. Break down the soft tissues of the prey
3. Other cells engulf small food particles which
4. Are broken down in food vacuoles
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Most animals have an alimentary canal, a digestive tract with two openings, a mouth and
an anus.
Food entering the mouth usually passes into a pharynx, or throat. Depending on the
species, the esophagus may channel food to a crop, gizzard, or a stomach. A crop is a
pouch-like organ in which food is softened and stored. Stomachs and gizzards may also
store food temporarily, but they are more muscular and they churn and grind the food.
Chemical digestion and nutrient absorption occur mainly in the intestine. Undigested
material are expelled through the anus.
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Diagram 3:
•Food enters the mouth, is chewed in the oral cavity, and then pushed by the
tongue into the pharynx. Once food is swallowed, muscles propel it through the
alimentary canal by peristalsis, alternating waves of contraction and relaxation of
the smooth muscles lining the canal. Sphincters regulate the passage of food into
and out of the stomach. The final steps of digestion and nutrient absorption occur
in the small intestine over a period of 5-6 hours. Undigested material moves slowly
through the large intestine (taking 12-24 hours), and feces are stored in the rectum
and then expelled through the anus.
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Figure 7:
Digestion begins in the oral cavity in the form of chemical and mechanical digestion.
The saliva excreted by the salivary glands contains the digestive enzyme, amylase,
which begins to break down the starch in your food. This is a chemical digestion.
Mechanical digestion occurs by the chewing and smashing of food performed by
the teeth.
As you anticipate your apple, cheese, and crackers, your salivary glands may start delivering
saliva through ducts to the oral cavity even before you take a bite. This is a response to the
sight or smell (or even thought) of food. The presence of food in the oral cavity continues
to stimulate salivation. In a typical day, your salivary glands excrete more than a liter of
saliva. You can see the duct opening and salivary glands in this figure.
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Diagram 4: Most of the time the esophageal opening is closed off by a sphincter (blue
arrows). Air enters the larynx, the voice box containing the vocal cords, and flows the
through trachea to the lungs (black arrows). This situation changes when you start to
swallow. The tongue pushes the bolus of food into the pharynx, triggering the swallowing
reflex; the esophageal sphincter relaxes and allows the bolus to enter the esophagus (green
arrow). At the same time, the larynx moves upward and tips the epiglottis (a flap of
cartilage and fibrous connective tissue) down over the opening to the larynx. In this
position, the epiglottis prevents food from passing into the trachea. You can see this
motion in the bobbing of your larynx (also called your Adam’s apple) during your
swallowing. After the bolus enters the esophagus, the larynx moves back downwards, the
epiglottis tips up again, and the breath passage reopens (right side of the figure). The
esophageal sphincter contracts above the bolus
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The esophagus is a muscular tube that conveys food boluses from the pharynx to the
stomach. The muscles at the very top of the esophagus are under voluntary control; thus,
the act of swallowing begins voluntarily. But then involuntary waves of contraction by
smooth muscles in the rest of the esophagus take over. Figure 8 shows how waves of
muscles contraction – peristalsis – squeeze a bolus toward the stomach. As food is
swallowed, muscles above the bolus contract (blue arrows), pushing the bolus downwards.
At the same time, muscles around the bolus relax, allowing the passageway to open.
Muscles around the bolus relax, allowing the passageway to open. Muscles contractions
continue in waves until the bolus enters the stomach.
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The Heimlich maneuver was invented by Dr. Henry Heimlich in the 1970’s. It allows people
with little medical training to step in and aid a choking victim. The maneuver is often
performed on someone who is seated or standing up. Stand behind the victim and place
your arms around the victim’s waist. Make a fist with one hand, and place it against the
victim’s upper abdomen, well below the rib cage. Then place the other hand over the first
and press into the victim’s upper abdomen with a quick upward thrust. When done
correctly, the diaphragm is forcibly elevated, pushing air into the trachea. Repeat this
procedure until the object is forced out of the victim’s airway.
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Some chemical digestion occurs in the stomach. The stomach secretes gastric juice, which
is made up of mucus, enzymes and strong acid. The pH of gastric juice is about 2. One
function of the acid is to break apart the cells in food. The acid also kills most bacteria and
other microbes that are swallowed with food. The interior surface of the stomach wall is
highly folded, and as the diagram shows, it is dotted with pits leading into tubular gastric
glands. The gastric glands have three types of cells that secrete different components of
the gastric juice. Mucous cells (shown here in dark pink) secrete mucus, which lubricates
and protects the cells lining the stomach. Parietal cells (yellow) secrete hydrogen ions and
chloride ions, which combine in the lumen (cavity) of the stomach to form hydrochloric
acid (HCl). Chief cells (light pink) secrete pepsinogen, an inactive form of the enzyme
pepsin. The diagram on the far right of the figure indicates how pepsinogen, HCl, and
pepsin interact:
1. Pepsinogen and HCl are secreted into the lumen of the
stomach
2. Next the HCl converts pepsinogen to pepsin
3. Pepsin then activates more pepsinogen, starting a chain
reaction
When you see, smell, or taste food, a signal from your brain to your stomach stimulates
your gastric glands to secrete gastric juice. Once
you have food in your stomach, substances in the food stimulate cells in the stomach wall
to release the hormone gastrin into the circulatory
system. Gastrin circulates in the bloodstream, returning to the stomach wall. When it
arrives there, it stimulates additional secretion of
gastric juice.
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A stomachful of digestive juice laced with strong acid breaks apart the cells in our food, kills
bacteria, and begins the digestion of proteins. At the same time, these chemicals, acidic
enough to dissolve iron nails, can be harmful. The opening between the esophagus and the
stomach is usually closed until a bolus arrives. Occasionally, however, there is a acid reflux.
This backflow of chyme (the mixture of partially digested food and digestive juices formed
in the stomach) into the lower end of the esophagus causes the feeling we call heartburn.
The low pH of the stomach kills most microbes, but not the acid tolerant Helicobacter
pylori (Figure 9). This bacterium burrows beneath the mucus and releases harmful
chemicals. Growth of H. pylori seems to result in a localized loss of protective mucus and
damage to the cells lining the stomach. Numerous white blood cells move into the stomach
wall to fight the infection, and their presence is associated with mild inflammation of the
stomach, called gastritis. Gastric ulcers develop when pepsin and HCl destroy cells faster
than the cells can regenerate. Eventually, the stomach wall may erode to the point that it
actually has a hole in it. This hole can lead to a life-threatening infection with the abdomen
or internal bleeding.
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Digestion of the large molecules occurs in the small intestine. The nutrients that result from
this digestion are absorbed into the blood from the small intestine. With a length of more
than 6 m, the small intestine is the longest organ of the alimentary canal.
Sources of Digestive Enzymes and Bile
Two large organs, the pancreas and the liver contribute to digestion in the small intestine.
The pancreas produces pancreatic juice, a mixture of digestive enzymes and an alkaline
solution rich in bicarbonate. The bicarbonate acts as a buffer to neutralize the acidity of
chyme as it enters the small intestine. The pancreas also produces hormones that regulate
blood-glucose levels.
In addition to its many other functions, the liver produces bile. Bile contains bile salts that
emulsify fats, making them more susceptible to attack by digestive enzymes. The
gallbladder stores bile until it is needed in the small intestine.
Some of these enzymes are secreted into the lumen of the small intestine; others are
bound to the surface of epithelial cells. The first 25 cm or so of the small intestine is called
the duodenum. The is where chyme squirted from the stomach mixes with bile from the
gallbladder, pancreatic juice from the pancreas juice from the pancreas, and digestive
enzymes form gland cells in the intestinal.
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Table 1 summarizes the processes of enzymatic digestion that occur in the small intestine.
All four types of large molecules (carbohydrates, proteins, nucleic acids, and fats) are
digested in the small intestine. As we discussed the digestion of each, the table will help
you keep track of the enzymes involved (shown in red).
The digestion of carbohydrates that began in the oral cavity is completed is in the small
intestine. An enzyme called pancreatic amylase hydrolyzes starch (a polysaccharide) into
the disaccharide maltose. The enzyme maltase then splits maltose into the
monosaccharide glucose.
The small intestine also completes the digestion of proteins that was begun in the stomach.
The pancreas and the duodenum secrete hydrolytic enzymes (trypsin and chymotripsin)
that completely dismantle polypeptides into amino acids.
Another team of enzymes, the nucleases, hydrolyzes nucleic acids. Nucleases from the
pancreas spilt DNA and RNA into their component nucleotides. The nucleotides are then
broken down into nitrogenous bases, sugars, and phosphates by other enzymes.
In contrast to starch and proteins, fats remain undigested until they reach the duodenum.
First, bile salts in bile cause fat globules to be physically broken up into smaller fat droplets,
a process called emulsification. When there are many small droplets, a larger surface area
of fat is exposed to lipase, a pancreatic enzyme that breaks fat molecules down into fatty
acids and glycerol.
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Absorption in the Small Intestine
Structurally, the small intestine is well suited for its task of absorbing nutrients. Its lining
has a huge surface area – roughly 300 m2, about the size of a tennis court. Diagram 6
illustrates this extensive surface area results from several kinds of folds and projections.
Around the inner wall of the small intestine are large circular folds with numerous small,
fingerlike projections called villi (singular, villus). Each of the epithelial cells lining a villus
has many tiny surface projections, called microvilli. The microvilli extend into the lumen of
the intestine and greatly increase the surface are across which nutrients are absorbed.
Some nutrients are absorbed by simple diffusion; other nutrients are pumped against
concentration gradients into the epithelial cells. Notice that a small lymph vessel (yellow)
and a network of capillaries (red, purple, and blue) penetrate the core of each villus. After
fatty acids and glycerol are absorbed by an epithelial cell, these building blocks are
recombined into fats, which are then transported into a lymph vessel. Other absorbed
nutrients, such as amino acids and sugars, pass out of the intestinal epithelium and then
across the thin walls of the capillaries into the blood.
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The liver has a strategic location in the body – between the intestines and the heart. As
indicated in Figure 11, capillaries from the small and large intestines converge into veins
that lead into the hepatic portal vein. This large vessel transports nutrients absorbed by
the intestines directly to the liver. The liver thus gets first access to nutrients absorbed from
a meal. One of its main function is to remove excess glucose from the blood and convert it
to glycogen, which is stored in liver cells. In balancing the amount of glycogen it stores
with the amount of glucose its releases to the blood, the liver plays a key role in regulating
body metabolism.
Liver cells also synthesizes plasma proteins important in blood clotting and in maintaining
the osmotic balance of the blood, as well as lipoproteins that transport fats and cholesterol
to body cells. The liver has a chance to modify and detoxify substances absorbed by the
digestive tract before the blood carries these materials to the heart for distribution. It
converts toxins such as alcohol and other drugs into inactive products that can be excreted
in urine. The combination of alcohol and some drugs is particularly harmful to the liver.
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The large intestine, or colon, is about 1.5 m long and 5 cm in diameter. As the enlargement
in Figure 12 shows, it joins the small intestine at a T-shaped junction, where a sphincter
control the passage of unabsorbed food material out of the small intestine. One arm of the
T is a blind pouch called the cecum. The appendix, a small, fingerlike extension of the
cecum, contains a mass of white blood cells that make a minor contribution to immunity.
Despite this role, the appendix is prone to infection (appendicitis).
One of the major functions of the colon is to absorb water from the alimentary canal. As
the water is absorbed, the remains of the digested food become more solid as they move
along the colon by peristalsis. These waste products, the feces, consist mainly of
indigestible plant fibers (cellulose for example) and prokaryotes that normally live in the
colon. Some of our colon bacteria, such as E. Coli, produce important vitamins, including
biotin, folic acid, several other B vitamins, and vitamin K which are then absorbed by the
colon
Feces are stored in the final portion of the colon, the rectum, until they can be eliminated.
Strong contractions of the colon create the urge to defecate. Two rectal sphincters, one
voluntary and the other involuntary, regulate the opening of the anus.
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Natural selection has favored adaptations that fit the structure of an animal’s digestive
system to the function of digesting the kind of food the animal eats
•Large, expandable stomachs are common adaptations in carnivores, which
may go long time between meals and must eat as much as they can when
they do catch prey
•The length of an animal’s digestive tract is often correlated with diet
•Herbivores and omnivores have longer alimentary canals, relative to
their body size, than carnivores. Vegetation is more difficult to digest
than meat because it contains cell walls
•Most herbivorous animals also have special chambers that house great
number of microbes – bacteria and protists
•The most elaborate adaptations for an herbivorous diet have evolved in the
mammals called ruminants, which include cattle, sheep, and deer. The
stomach of a ruminate has four chambers containing symbiotic microbes
Figure 13: Compares the digestive tract of a carnivore, the coyote, with that of an
herbivore, the koala. The two mammals are about the same size, but the koala’s intestine is
much longer and includes the longest cecum (about 2 m) of any animal of its size.
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The rate of energy consumption by the body is called metabolic rate. It is the sum of all the
energy-requiring biochemical reactions over a given time interval. Cellular metabolism
must continuously drive several processes for an animal to remain alive:
•Cell maintenance
•Breathing
•The beating of the heart
•The maintenance of body temperature (birds and mammals)
The number of kilocalories (Calories) a resting animal required to fuel these essential
processes for a given time is called the basal metabolic rate (BMR). The BMR for humans
averages 1,300-1,500 kcal per day for adult females and about 1,600-1,800 kcal per day for
adult males.
The more strenuous the activity, the greater the energy demand. Table 2 gives you an idea
of the amount of activity it takes for a 68 kg (150-lb) person to use up the kilocalories
contained in several common foods.
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Besides providing fuel and organic raw materials, an animal's diet must also supply
essential nutrients.
Undernourishment is a condition resulting from a diet that is chronically deficient in
calories
Malnourishment results form the long-term absence from the diet of one or more
essential nutrients
Our cells make fats and other lipids by combing fatty acids with other molecules, such as
glycerol. We can make most of the fatty acids we need. Those we cannot make, called
essential fatty acids, we must obtain in our diet.
Adult humans cannot make either of the 20 kinds of amino acids needed to synthesize
proteins. These eight, known as the essential amino acids, must be obtained from the diet.
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Vegetarians diets may range from avoiding meat to the vegan diet of avoiding meat and
meat by-products such as eggs, milk, cheese. Vegetarians have to know how to get all the
essential nutrients. The key to being a health vegetarian is to eat a variety of plant and
foods that together supply sufficient quantities of all the essential amino acids. Simply by
eating a combination of beans and corn, for example, vegetarians can get all the essential
amino acids as show in Figure 14.
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Vitamin – is an organic nutrient that we must obtain from our diet, but is required in
minute amounts. For example, one tablespoon of vitamin B12 can provide the daily
requirement for nearly a million people
Minerals – are simple inorganic nutrients, usually required in small amounts
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The Food and Drug Administration (FDA) requires that various types of information be
given on packed food labels, as shown in Figure 19. You’ll find ingredients listed in order
from the greatest amount (by weight) to the least. Food labels emphasize nutrients
believed to be associated with disease risks (fats, cholesterol, and sodium) and with a
healthy diet ( such as dietary fiber, protein, and certain vitamins and minerals).
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Overnourishment, consuming more food energy than the body needs for normal
metabolism, causes obesity, the excessive accumulation of fat. The complexity of weight
control in humans is evident from studies of the hormone leptin, one of the key long-term
appetite regulators in mammals.
Though fat hoarding can be a health liability today, it may actually have been advantageous
in our evolutionary past. Only in the past few centuries have large numbers of people had
access to a reliable supply of high calories food. During the times of hunting and gathering,
feast-and-famine existed and natural selection favored those individuals with a physiology
that induces them to gorge on rich, fatty food on those rare occasion when such treats
were available.
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