Lecture 19: Animal Nutrition
1) Introduction
As a group, animals exhibit a great variety of
nutritional adaptations.
For example, the snowshoe hare of the northern
forests, obtains all their nutrients from plants alone.
Hares and rabbits have a large intestinal pouch
housing prokaryotes and protists that digest cellulose.
For any animal, a nutritionally adequate diet is
essential for homeostasis, a steady-state balance in body functions.
A balanced diet provides fuel for cellular work
and the materials needed to construct organic molecules.
A nutritionally adequate diet satisfies three
needs:
fuel (chemical energy) for all the cellular work
of the body;
the organic raw materials animals use in
biosynthesis (carbon skeletons to make many of their own molecules);
essential nutrients,
substances that the animals cannot make for itself from any raw material
and therefore must obtain in food in prefabricated form.
The flow of food energy into and out of an
animal can be viewed as a budget, with the production of ATP accounting for
the largest fraction by far of the energy budget of most animals.
ATP powers basal or resting metabolism, as well
as activity, and, in endothermic animals, temperature regulation.
Nearly all ATP is derived from oxidation of
organic fuel molecules - carbohydrates, proteins, and fats - in cellular
respiration.
The monomers of any of these substances can be
used as fuel, though priority is usually given to carbohydrates and fats.
Fats are especially rich in energy, liberating
about twice the energy liberated from an equal amount of carbohydrate or
protein during oxidation.
When an animal takes in more calories than it
needs to produce ATP, the excess can be used for biosynthesis.
This biosynthesis can be used to grow in size or
for reproduction, or can be stored in energy depots.
In humans, the liver and muscle cells store
energy as glycogen, a polymer made up of many glucose units.
Glucose is a major fuel molecule for cells, and
its metabolism, regulated by hormone action, is an important aspect of
homeostasis.
If glycogen stores are full and caloric intake
still exceeds caloric expenditure, the excess is usually stored as fat.
The human body regulates the use and storage of
glucose, a major cellular fuel.
(1) When glucose levels rise above a set point,
(2) the pancreas secretes insulin into the blood.
(3) Insulin enhances the transport
of glucose into body cells and stimulates the liver and muscle cells to store glucose
as glycogen, dropping blood glucose levels.
(4) When glucose levels
drop below a set point, (5) the pancreas secretes glucagon into the blood.
(6) Glucagon promotes the
breakdown of glycogen and the release of glucose into the blood, increasing the
blood glucose levels.
In mammals, a hormone called leptin,
produced by adipose cells, is a key player in a complex feedback mechanism
regulating fat storage and use.
A high leptin level
cues the brain to depress appetite and to increase energy-consuming muscular
activity and body-heat production.
Conversely, loss of body fat decreases leptin levels in the blood, signaling the brain to increase
appetite and weight gain.
These feedback mechanisms regulate body weight around a fairly rigid set point in some individuals and over a relatively wide range in others.
Animals require 20 amino acids to make proteins.
Most animals can synthesize half of these if
their diet includes organic nitrogen.
Essential amino acids must be obtained
from food in prefabricated form.
Eight amino acids are essential in the adult
human with a ninth, histidine, essential
for infants.
The same amino acids are essential for most
animals.
Vitamins are organic molecules required
in the diet in quantities that are quite small compared with the relatively
large quantities of essential amino acids and fatty acids animals need.
While vitamins are required in tiny amounts -
from about 0.01 mg to 100 mg per day - depending on the vitamin, vitamin
deficiency (or overdose in some cases) can cause serious problems.
Minerals are simple inorganic nutrients,
usually required in small amounts - from less than 1 mg to about 2,500 mg per
day.
Iron is a component of the cytochromes
that function in cellular respiration and of hemoglobin, the oxygen binding
protein of red blood cells.
While sodium, potassium, and chloride have a
major influence on the osmotic balance between cells and the interstitial
fluids, excess consumption of salt (sodium chloride) is harmful.
The average
Excess consumption of salt or several other
minerals can upset homeostatic balance and cause toxic side effects.
For example, too much sodium is associated with high blood pressure, and excess iron causes liver damage.
2) Mammalian digestion
The general principles of food processing are
similar for a diversity of animals, including the mammalian system which we
will use as a representative example.
The mammalian digestive system consists of the
alimentary canal and various accessory glands that secrete digestive juices
into the canal through ducts.
Peristalsis, rhythmic waves of
contraction by smooth muscles in the walls of the canal, push food along.
Sphincters, muscular ringlike
valves, regulate the passage of material between specialized chambers of the
canal.
The accessory glands include the salivary
glands, the pancreas, the liver, and the gallbladder.
The stomachs second line of defense against
self-digestion is a coating of mucus, secreted by epithelial cells, that
protects the stomach lining.
Still, the epithelium is continually eroded, and
the epithelium is completely replaced by mitosis every three days.
Gastric ulcers, lesions in the stomach lining,
are caused by the acid-tolerant bacterium Heliobacter
pylori.
Ulcers are often treated with antibiotics.
The liver performs a wide variety of important
functions in the body, including the production of bile.
Bile is stored in the gallbladder until needed.
It contains bile salts which act as detergents
that aid in the digestion and absorption of fats.
Bile also contains pigments that are by-products
of red blood cell destruction in the liver.
These bile pigments are eliminated from the body
with the feces.
Specific enzymes from the pancreas and the
duodenal wall have specific roles in digesting macromolecules.
The digestion of starch and glycogen, begun by
salivary amylase in the oral cavity, continues in the small intestine.
Pancreatic amylases hydrolyze starch, glycogen,
and smaller polysaccharides into disaccharides.
A family of disaccharidases hydrolyze each disaccharide into
monomers.
Maltase splits maltose into two glucose
molecules.
Sucrase splits
sucrose, a sugar found in milk, into glucose and fructose.
These enzymes are built into the membranes and extracellular matrix of the intestinal epithelium which is
also the site of sugar absorption.
Digestion of proteins in the small intestine
completes the process begun by pepsin.
Several enzymes in the duodenum dismantle
polypeptides into their amino acids or into small peptides that in turn are
attacked by other enzymes.
Trypsin and chymotrypsin attack peptide bonds adjacent to
specific amino acids, breaking larger polypeptides into shorter chains.
Dipeptidase,
attached to the intestinal lining, split smaller chains.
Carboxypeptidases
and aminopeptidase split off one amino acid
from the carboxyl or amino end of a peptide, respectively.
Most digestion occurs in the duodenum.
The other two sections of the small intestine,
the jejunum and ileum, function mainly in the absorption of
nutrients and water.
To enter the body, nutrients in the lumen must
pass the lining of the digestive tract.
The small intestine has a huge surface area -
300 m2, roughly the size of a tennis court.
Hormones released by the wall of the stomach and
duodenum help ensure that digestive secretions are present only when needed.
When we see, smell, or taste food, impulses from
the brain initiate the secretion of gastric juice.
Certain substances in food stimulate the stomach
wall to release the hormone gastrin into the
circulatory system.
As it recirculates, gastrin stimulates further secretion of gastric juice.
If the pH of the stomach contents becomes too
low, the acid will inhibit the release of gastrin.
A major function of the colon is to recover
water that has entered the alimentary canal as the solvent to various digestive
juices.
About 7 L of fluid are secreted into the lumen
of the digestive tract of a person each day.
Over 90% of the water is reabsorbed, most in the
the small intestine, the rest in the colon.
Digestive wastes, the feces, become more
solid as they are moved along the colon by peristalsis.
Movement in the colon is sluggish, requiring 12
to 24 hours for material to travel the length of the organ.
Diarrhea results if insufficient water is
absorbed and constipation if too much water is absorbed.
Living in the large intestine is a rich flora of
mostly harmless bacteria.
One of the most common inhabitants of the human
colon is Escherichia coli, a favorite research organism.
As a byproduct of their metabolism, many colon
bacteria generate gases, including methane and hydrogen sulfide.
Some bacteria produce vitamins, including biotin, folic acid, vitamin K, and several B vitamins, which supplement our dietary intake of vitamins.
3) Evolutionary considerations
Dentition, an animals assortment of teeth, is
one example of structural variation reflecting diet.
Particularly in mammals, evolutionary adaptation of teeth for processing different kinds of food is one of the major reasons that mammals have been so successful.
Nonmammalian
vertebrates generally have less specialized dentition, but there are
exceptions.
For example, poisonous snakes, like
rattlesnakes, have fangs, modified teeth that inject venom into prey.
Some snakes have hollow fangs, like syringes,
other drip poison along grooves in the tooth surface.
All snakes have another important anatomic
adaptation for feeding, the ability swallow large prey whole.
The lower jaw is loosely hinged to the skull by
an elastic ligament that permits the mouth and throat to open very wide for
swallowing.
The length of the vertebrate digestive system is
also correlated with diet.
In general, herbivores and omnivores have longer
alimentary canals relative to their body sizes than to
carnivores, providing more time for digestion and more surface areas for
absorption of nutrients.
Vegetation is more difficult to digest than meat
because it contains cells walls.