Chapter 37 Notes

 

Animal Nutrition

 

Lecture Notes:

 

Animals are heterotrophic and, like all heterotrophs, must rely on organic compounds in their food to supply energy and the raw materials for growth and repair.

Ÿ A variety of nutritional adaptations have resulted from natural selection during evolutionary development of animals.

 

I.  Diets and feeding mechanisms vary extensively among animals

 

Animals usually ingest other organisms.

Ÿ The food organism may be either dead of alive.

Ÿ The food organism may be ingested whole or in pieces.

Ÿ Some parasitic animals (i.e. tapeworm) are exceptions.

 

Animals are categorized based on the kinds of food they usually eat and their adaptations for obtaining and processing food items.

Ÿ  Herbivores eat autotrophic organisms (plants, algae, and autotrophic bacteria).

Ÿ  Carnivores eat other animals.

Ÿ  Omnivores eat other animals and autotrophs.

 

The varied diets exhibited by animals are accompanied by a variety of mechanisms used to obtain food.

Ÿ  Suspension-feeders sift small food particles from the water.

̃            Many are aquatic animals such as clams and oysters (trap food on gills) and baleen whales (strain food from water forced through the screen-like plates on their jaws).

Ÿ   Substrate-feeders live on or in their food source and eat their way through food.

̃            Leaf miners (larvae of various insects) tunnel through the interior of leaves.

Ÿ  Deposit-feeders are a type of substrate-feeder that ingests partially decayed

      organic materials along with their substrate.

̃            Earthworms ingest soil and their digestive systems extract the organic materials.

Ÿ  Fluid-feeders suck nutrient-rich fluids from a living host.

̃            Aphids ingest the phloem sap from plants; leeches and mosquitoes suck blood from animals; hummingbirds and bees ingest nectar from flowers.

Ÿ  Bulk-feeders eat relatively large pieces of food.

̃            Most animals; they possess various adaptations to kill prey or tear off pieces of meat or vegetation.

II.  Ingestion, digestion, absorption, and elimination are the four main stages of food processing

 

Food processing involves four stages: ingestion, digestion, absorption, and elimination.

 

Ingestion is the first stage and is the act of eating.

 

Digestion is the second stage and is the process of breaking down food into small molecules the body can absorb.

Ÿ Organic food material is composed of macromolecules (proteins, fats, carbohydrates) that are too large to cross the membranes and enter an animal’s cells.

Ÿ  Digestion enzymatically cleaves these macromolecules into component monomers that can be used by the animal (polysaccharides to simple sugars; proteins to amino acids; fats to glycerol and fatty acids).

Ÿ  Digestion uses enzymatic hydrolysis to break bonds in macromolecules.

̃            Hydrolytic enzymes catalyze the digestion of each class of macromolecules by adding water.

̃            The chemical digestion is usually preceded by mechanical fragmentation (e.g. chewing) that increases the surface area exposed to digestive juices.

̃            Occurs in a specialized compartment where the enzymes are contained so they don’t damage the animal’s own cells.

 

Absorption is the third stage of the process and involves the uptake of the small molecules resulting from digestion.

 

Elimination is the fourth, and the last, stage where undigested material passes out of the digestive compartment.

 

III. Digestion occurs in food vacuoles, gastrovascular cavities, and alimentary canals

 

A.  Intracellular Digestion in Food Vacuoles

 

Food vacuoles are the simplest digestive compartments.

Ÿ They are organelles where a single cell digests its food without hydrolytic enzymes mixing with the cell’s cytoplasm.

Ÿ  Protozoa have food vacuoles that form around food by endocytosis.

Ÿ  Hydrolytic enzymes are secreted into the food vacuole and digestion occurs; this is referred to as intracellular digestion.  (See Campbell, Figure 37.5).

Ÿ  Sponges differ from other animals in that all digestion is by the intracellular mechanism. (See Campbell, Figure 37.6).

 

Extracellular digestion occurs within compartments that are continuous via passages with the outside of the body.

Ÿ At least some hydrolysis occurs in most animals by this mechanism.

 

B.  Digestion in Gastrovascular Cavities

 

Most animals that have simple body plans possess a gastrovascular cavity.

            Gastrovascular cavity = Digestive sac with a single opening.

Ÿ  Function in both digestion and nutrient distribution.

 

Digestion is Hydra involves both extracellular and intracellular digestion (See Campbell, Figure 37.7a)

Ÿ Hydra is a carnivore which captures prey.

Ÿ Food items are immobilized by stings from nemacysts on the tentacles.

Ÿ  Tentacles then force prey through the mouth into the gastrovascular cavity.

Ÿ  Specialized gastrodermal cells secrete digestive enzymes that fragment the soft tissues of the prey into tiny pieces.

Ÿ Some gastrodermal cells also possess flagella whose movement prevents settling of food particles and distributes them through the cavity.

Ÿ The small pieces are phagocytized by nutritive gastrodermal cells and surrounded by food vacuoles.

Ÿ  Hydrolysis is completed by intracellular digestion.

Ÿ  Undigested materials are expelled from the gastrovascular cavity though the single opening.

 

The combination of extracellular and intracellular digestion that occurs in most animals permits these organisms to feed on larger prey items.

Ÿ  Phagocytosis is limited to microscopic food.

Ÿ  Extracellular digestion begins the digestive process by breaking-down large food items into smaller particles.

 

Extracellular digestion may even begin outside of the animal’s body as in planarians.

Ÿ  Planarians possess a gastrovascular cavity that is extensively branched.

Ÿ  Associated with the cavity is a muscular, tubular pharynx that can be everted ventrally though the mouth.

Ÿ The pharynx penetrates the prey (small invertebrates and dead organic matter) and releases digestive juices.

Ÿ The partially digested food is sucked into the gastrovascular cavity where digestion continues.

Ÿ  Digestion is completed by intracellular digestion in cells that phagocytize the tiny particles.

 

C. Digestion in Alimentary Canals

 

Animals with body plans more complex cnidarians and platyhelminths have alimentary canals.

 

     Alimentary canal = Digestive tube running between two openings: the mouth (where food is initially ingested) and the anus (where undigested wastes are egested).  (See Campbell, Figure 37.8)

Ÿ Since food moves in one direction along the tube, the tube can be organized into specialized regions that carry out digestion and absorption of nutrients in a stepwise fashion.

Ÿ The unidirectional passage of food and the specialization of function for different regions make the alimentary canal more efficient.

 

 

IV.  A tour of the mammalian digestive system

 

The digestive system in mammals include the alimentary canal and accessory glands that secrete digestive juices into the canal through ducts.

Ÿ The digestive tract has a four-layered wall: the lumen is lined by a mucous       membrane (mucosa), then a connective tissue layer followed by a layer of smooth       muscle, and outermost is a connective tissue layer attached to the membrane of the       body cavity.

Ÿ  Peristalsis (rhythmic smooth muscle contractions) pushes food along the tract.

Ÿ  Sphincters (modifications of the muscle layer into ring-like valves) occur at some       junctions between compartments and regulate passage of materials though the       system.

Ÿ The accessory glands are: three pairs of salivary glands, the pancreas, the liver,       and the gall bladder.

 

Refer to Figure 37.9 for orientation of the specialized compartments in the human system.

 

A.  The Oral Cavity

 

Physical and chemical digestion begin in the oral cavity.

Ÿ  Chewing breaks down large pieces of food into smaller pieces.

Ÿ This makes food easier to swallow and increases the surface area available for enzyme action.

 

The presence of food in the oral cavity stimulates the salivary glands to secrete saliva into the oral cavity.

Ÿ Saliva contain mucin (protects mouth from abrasion and lubricates food), buffers       that neutralize acids, antibacterial agents, and salivary amylase - an enzyme that       hydrolyzes starch and glycogen to the disaccharide maltose or small       polysaccharides.

 

The tongue manipulates food during chewing and forms it into a bolus which is swallowed.

Ÿ The tongue also tastes the food and pushes the bolus to the pharynx.

 

 

 

B.  The Pharynx

 

The pharynx is an intersection for both digestive and respiratory systems.

Ÿ The movement of swallowing moves the epiglottis to block the entrance of the windpipe (the glottis).

Ÿ This directs food through the pharynx and into the esophagus. (See Campbell, Figure 37.10)

 

C. The Esophagus

 

The esophagus is a muscular tube which conducts food from the pharynx to the stomach.

Ÿ Peristalis moves the bolus along the esophagus to the stomach.

Ÿ The initial entrance of the bolus into the esophagus in voluntary (swallowing); once in, the peristalsis results from involuntary contraction of the smooth muscles.

Ÿ Salivary amylase remains active as the bolus moves though the esophagus.

 

D. The Stomach

 

The stomach is a large, saclike structure located just below the diaphragm on the left side of the abdominal cavity.  It functions in:

Ÿ Food storage.

̃  The stomach has an elastic wall with rugae - folds that can expand to accommodate up to 2 liters of food.

̃        Storage capacity permits periodic feeding (meals).

Ÿ Churning.

̃  The stomach has longitudinal, vertical and diagonal muscles that contract the stomach in churning movements that mix the food.

̃        Stomach contents are mixed every 20 minutes.

̃        Churning, mixing, and addition of stomach acid convert food to a nutrient broth called acid chyme.

§ The passage of acid chyme into the small intestine is regulated by the pyloric sphincter at the bottom of the stomach.

§ The pyloric sphincter relaxes at intervals and permits small quantities of chyme to pass.

Ÿ   Secretion.  Gastric secretion is controlled by nerve impulses and the hormone gastrin.  The stomach epithelium contains three types of secretory cells.

̃      Mucous cells secrete:

§ mucin - a thin mucus that protects the stomach lining from being digested.

§ gastrin - a hormone produced by the stomach.  Gastrin is released into the bloodstream and its action is to stimulate further secretion of gastric juice (HCl and pepsin).

̃      Cheif cells secrete:

§ pepsinogen, an inactive protease or zymogen that is the precursor to pepsin.  Zymogen = An inactive form of a protein-digesting enzyme.

̃      Parietal cells secrete:

§ HCl

Ÿ Protein digestion.  Both components of gastric juice, HCl, and pepsin, are involved with protein digestion:

̃      HCl - provides acidity (pH 1-4) which:

§   kills bacteria.

§   denatures protein.

§   starts the conversion of pepsinogen to pepsin.  Newly formed pepsin can also catalyze this reaction:

 

                        Pepsinogen   HCl ®  Pepsin

̃      Pepsin - splits peptide bonds next to some amino acids.

§   Does not hydrolyze protein completely

§   Is an endopeptidase that splits peptide bonds located within the polypeptide chain.

 

E.  The Small Intestine

 

NOTE:  A discussion of hormonal control of digestion includes several examples of positive and negative feedback.  This is a good place to reinforce the concepts learned in Chapter 36 and to make a transition to the topic of the endocrine system (Chapter 41).

 

The human small intestine is about 6 m long in length and is the site of most enzymatic hydrolysis of food and absorption of nutrients.

Ÿ   Remember, only limited digestion of carbohydrates occurs in the oral cavity and esophagus (by salivary amylase) and of proteins in the stomach (by pepsin).

 

The pancreas, liver, gall bladder, and small intestine all contribute to the digestion that occurs in the small intestine.  Their products are released into the duodenum, the first 25 cm of the small intestine.

Ÿ The pancreas produces:

̃      hydrolytic enzymes that break down all major classes of macromolecules - carbohydrates, lipids, proteins, and nucleic acids.

̃      bicabonate buffer that helps neutralize the acid chyme coming from the stomach.

Ÿ The liver performs many functions including the production of bile, which:

̃  is stored in the gall bladder.

̃      does not contain digestive enzymes.

̃      contains bile salts which emulsify fat.

̃      contains pigments that are byproducts of destroyed red blood cells.

 

Hormonal control of digestion involves the following regulatory hormones:

Ÿ   Gastrin - Released from the stomach in response to presence of food; stimulates the stomach to release gastric juice (HCl and pepsin).

Ÿ   Secretin - Released from the duodenum in response to acid chyme entering from the stomach; signals the pancreas to release bicarbonate buffer to neutralize acid chyme.

Ÿ   Cholecystokinin­ (CKK) - Released from the duodenum in response to chyme entering from the stomach; signals the gall bladder to release bile and the pancreas to release pancreatic enzymes into the duodenum.

Ÿ   Enterogastrone - Released from the duodenum in response to the presence of fat in the chyme; inhibits peristalsis in the stomach and slows digestion.

 

Carbohydrate digestion:

Ÿ Begins with the action of salivary amylase in the mouth.

Ÿ Begins again in the duodenum where pancreatic amylase hydrolyze starch and glycogen into disaccharides.

Ÿ   Disaccharides attached to the surface of the duodenal epithelium hydrolyze disaccharides into monosaccharides.

̃      Each disaccharide has its own disaccharidase.  For example, maltose is hydrolyzed by maltase, sucrose by sucrase, and lactose by lactase, etc.

̃      Since the disaccharidases are on the surface of the epithelium, the final breakdown of carbohydrates occurs where the sugars will be absorbed.

 

Protein digestion involves the efforts of teams of enzymes:

Ÿ Pepsin, and endopeptidase, begins protein digestion in the stomach.

Ÿ The pancreas secretes proteases in the form of zymogens that will be activated only in the lumen of the duodenum by the intestinal enzyme, enteropeptidase.

̃      Enteropeptidase converts trypsinogen to trypsin

 

                         Trypsinogen  enteropeptidase  > Trypsin

 

̃      Trypsin catalyzes the conversion of more trypsinogen to trypsin.

̃      Trypsin catalyzes the conversion of the other zymogens.

 

                        Procarboxypeptidase  trypsin  > Carboxypeptidase

 

                        Chymotrypinogen    trypsin  >  Chymotrypsin

 

Ÿ Trypsin and chymotrypsin (endopeptidases) digest large polypeptides into shorter chains by breaking internal peptide bonds adjacent to certain amino acids.

Ÿ   Carboxypeptidase (exopeptidase) splits amino acids, one at a time, off the end of a polypeptide that has a free carboxyl group.

Ÿ The lining of the small intestine also secretes protein-digesting enzymes, aminopeptidasei and dipeptidases.

̃      Aminopeptidase beings at the end of a polypeptide that has a free amino acid group and splits off one amino acid at a time.

̃      Dipeptides attached to the intestinal lining split small polypeptides.

Ÿ Since the protein-digesting enzymes from the pancreas and small intestine break bond in specific areas of the polypeptide, protein digestion to amino acids is a combined effort from all of these enzymes.

 

Nucleic acid digestion also involves teams of enzymes.

Ÿ   Nucleases hydrolyze DNA and RNA into nucleotides.

Ÿ Other hydrolytic enzymes (nucleotidases and nucleosidases) break nucleotides into nucleosides and nitrogenous bases, sugars, and phosphates.

 

Fat digestion occurs only in the small intestine, so most fat in food is undigested when it reaches the duodenum.  If fat is present in chyme,

Ÿ The duodenum secretes entergastrone, a hormone that inhibits peristalsis in the stomach, slowing the entry of food into the duodenum.

Ÿ In response to CCK, the gall bladder releases bile into the upper duodenum.  Bile salts emulsify the insoluble fat by coating fat droplets and thus preventing them from coalescing.

Ÿ   Emulsification produces many small fat droplets that collectively have a large surface area exposed for digestion.

Ÿ   Pancreatic lipase, secreted into the duodenum, hydrolyzes fats into the building blocks, glycerol, and fatty acids.

 

Macromolecules are completely hydrolyzed as peristalsis moves the digestive juice-chyme mixture through the duodenum.  See Table 37.1 for a summary of digestion.

Ÿ The remaining areas of the small intestine, the jejunum and ileum, are specialized for absorption of nutrients.

 

Nutrients resulting from digestion must cross the digestive tract lining to enter the body.  While a small number of nutrients are absorbed by the stomach and large and intestine, most absorption occurs in the small intestine.

Ÿ Large folds in the walls are covered with projections called villi, which in turn have many microscopic microvilli; this results in a surface area for absorption of about 300 m2.  (See Campbell, Figure 37.14)

̃      This brush border (microvilli surface) is exposed to the lumen of the intestine.

Ÿ   Penetrating the hollow core of each villus are capillaries and a tiny lymph vessel called a lacteal.

Ÿ Nutrients area absorbed by diffusion or active transport across the 2 cell-thick epithelium and into the capillaries pr lacteals.

̃      Amino acids and sugars enter the capillaries and are transported by the blood.

̃      Absorbed glycerol and fatty acids are recombined in epithelial cells to form fats; most are coated with proteins to form chylomicrons which enter the lacteals.

Ÿ   Capillaries and veins draining nutrients away from the villi converge into the hepatic portal vein, which leads directly to the liver.

̃      Here various organic molecules are used, stored, or converted to different forms.

̃      Blood flows at a rate of about 1 L per minute through the hepatic vessel.

 

 

F.  The Large Intestine

 

The large intestine, or colon, connects to the small intestine at a T-shaped junction containing a sphincter; the blind end of the T is called the cecum.

Ÿ The appendix is a fingerlike extension of the cecum and is composed of lymphoid tissue.

Ÿ The colon is about 1.5 m long and is the shape of an inverted “U”.  Its major function is water re-absorption.

Ÿ Feces (wastes of the digestive tract) are moved through the colon by peristalsis.

̃      Intestinal bacteria live on organic material in the feces and some produce vitamin K which is absorbed by the host.

̃  An abundance of salts in excreted by the colon lining.

̃      Feces are stored in the rectum and pass through the two sphincter (one involuntary, one voluntary) to the anus for elimination.

 

V.  Vertebrate digestive systems exhibit many evolutionary adaptations associated with diet

 

The digestive systems of vertebrates are variations on a common plan, and many adaptations associated with diet are found.

 

Variation in the dentition (assortment of teeth) of mammals reflects the animal’s diet.  See Campbell, Figure 37.15.

Ÿ   Carnivores (i.e. dogs and cats) generally have pointed canines and incisors.

̃      These teeth are used to kill prey and rip away pieces of flesh.

̃      Premolars and molars are jagged and used to crush and shred the food.

Ÿ   Herbivores (i.e. cows) have teeth with broad, ridged surfaces that are used for grinding vegetation.

̃      Incisors and canines are modified for biting off pieces of vegetation.

Ÿ   Omnivores (i.e. humans) have relatively unspecialized dentition.

̃      They are adapted to eat both vegetation and meat and teeth similar to those of both herbivores and carnivores are found.

 

Non-mammalian vertebrates typically have less specialized dentition, although there are exceptions.

Ÿ   Poisonous snakes have fangs which are teeth modified to inject venom into prey.

̃      Some fangs are hollow, others are grooved.

̃      Snakes can also swallow very large prey items due to loosely hinged lower jaw-skull articulation.

 

A correlation is also found between length of the vertebrate digestive system and diet.

Ÿ   Herbivores and omnivores have longer alimentary canals than carnivores relative to size.

Ÿ The cell walls in vegetation make it more difficult to digest than meat, and nutrients are less concentrated.

Ÿ The longer tract allows for more time for digestion and provides a greater surface area for absorption.

Ÿ The functional length of an alimentary canal may be increased by increased folding as with the spiral valve structure in sharks.

 

Special fermentation chambers are present in the alimentary canals of many herbivores.

Ÿ   Symbiotic bacteria and protozoa present in these chambers produce cellulase which can digest the cellulose.  (Animals do not produce cellulase.)

Ÿ The microorganisms digest cellulose to simple sugars and convert the sugars to nutrients essential to the animal.

Ÿ The microorganisms may be housed in the cecum (i.e. horses), cecum and colon (i.e. rabbit), or the much more elaborate structure found in ruminants.

 

VI.  An adequate diet provides carbon skeletons for biosynthesis and essential nutrients

 

A nutritionally adequate diet provides an animal with: fuel (chemical energy) for cellular respiration, raw organic materials for biosynthesis, and essential nutrients which must be obtained in prefabricated form.

 

A.  Food as Fuel

 

Chemical energy is obtained from the oxidation of complex organic molecules.

Ÿ   Monomers from any of the complex organic molecules can be used to produce energy, although those from carbohydrates and fats are used first.

Ÿ   Oxidation of a gram of fat liberates 9.5 kcal, twice that of a gram of carbohydrate or protein.

 

The basal energy requirements of an animal must be met to sustain their metabolic functions.

Ÿ When an animal takes in more calories than are consumed, the liver and muscles store the excess in the form of glycogen; further excess is stored in adipose tissue in the form of fat.

Ÿ When the diet is deficient in calories, glycogen stored in the liver and muscles is utilized first and fat is then withdrawn from adipose tissues.

Ÿ An undernourished person or animal is one whose diet is deficient in calories.

̃  If starvation persists, the body begins to breakdown its own proteins as a source of energy.

̃  The breakdown of the body’s own proteins can cause muscles to atrophy and can result in the consumption of the brain’s proteins.

 

Obesity (overnourishment) is a greater problem in the United States and other developed

countries than undernourishment.

Ÿ It increases the risk of heart attack, diabetes, and other disorders.

 

B.  Food for Biosynthesis

 

Heterotrophs cannot use inorganic materials to make organic molecules; they must obtain organic precursors for these molecules from the food they ingest.

 

Given a source of carbon and nitrogen, heterotrophs can fabricate a great variety of organic molecules by using enzymes to rearrange the molecular skeletons of precursors acquired from food.

Ÿ A single type of amino acid can supply the nitrogen necessary to build other amino acids.

Ÿ Fats can be synthesized from carbohydrates.

Ÿ The liver is responsible for most of the conversion of nutrients from one type of organic molecule to another.

 

 

C.  Essential Nutrients

 

An animal’s diet must include essential nutrients in addition to providing fuel and carbon skeletons.

 

Essential nutrients = Chemicals an animal requires but cannot synthesize.

Ÿ Vary from species to species.

Ÿ An animal is malnourished if its diet is missing one or more essential nutrients.

Ÿ   Includes essential amino acids, essential fatty acids, vitamins, and minerals.

 

Essential amino acids are those that must be obtained in the diet in a prefabricated form.

Ÿ Most animals can synthesize about half of the 20 kinds of amino acids needed to make proteins.

Ÿ Human adults can produce 12, leaving 8 as essential in the diet.  (Human infants can only produce 11.)

Ÿ Protein deficiency results when the diet lacks one or more essential amino acids.

̃  The syndrome known as kwashiorkor is a form of protein deficiency in some parts of Africa.

Ÿ The human body cannot store essential amino acids, thus a deficiency retards protein synthesis.

̃      This is most frequent in individuals, who for economic or other reasons, have unbalanced diets.

 

Essential fatty acids are those unsaturated fatty acids that cannot be produced by the body.

Ÿ An example in humans in linoleic acid which is required to produce some of the phospholipids necessary for membranes.

Ÿ Fatty acid deficiencies are rare as most diets include sufficient quantities.

 

Vitamins are organic molecules required in the diet in much smaller quantities (0.01 - 100 mg/day) than essential amino acids or fatty acids.

Ÿ Many serve a catalytic function as coenzymes are parts of coenzymes.

Ÿ Vitamin deficiencies can cause very severe effects as shown in Table 37.2.

Ÿ Water-soluble vitamins are not stock-piled in the body tissues; amounts ingested in excess of body needs are excreted in the urine.

Ÿ Fat-soluble vitamins (vitamins A, D, E, and K) can be held in the body; excess amounts are stored in body fat and may accumulate over time to toxic levels.

Ÿ If the body of an animal can synthesize a certain compound, it is not a vitamin.

̃  A compound such as ascorbic acid is a vitamin for humans (vitamin C) and must be included in our diets; it is not a vitamin in rabbits where the normal intestinal bacteria produce all that is needed.

 

Minerals are inorganic nutrients required in the diet in a small quantities ranging from 1 mg to 2500 mg per day, depending on the mineral.

Ÿ Some minerals serve structural and maintenance roles in the body (calcium, phosphorous) while others serve as part of enzymes (copper) or other molecules (iron).

Ÿ Refer to Table 37.3 for the mineral requirements of humans.