Chapter 41 Animal Nutrition

 

I. Overview

A.      Nutritionally adequate diet

1.       Provides chemical energy for cellular work

2.       Raw materials for biosynthesis

3.       Essential nutrients in prefabricated form

B.      Digestion

1.       Enzymatic hydrolysis of macromolecules into monomers that can be absorbed across cell membranes

C.      Gastrovascular cavities

1.       Digestive sacs in which some extracellular digestion takes place before food particles are phagocytized by cells lining the cavity

D.      Alimentary canals

1.       One-way tracts with specialized regions for mechanical breakdown of food, storage, digestion, absorption of nutrients, elimination of wastes

 

NUTRITIONAL REQUIREMENTS

II. Homeostatic mechanisms manage an animal’s fuel

A.      Monomers of carbohydrates, fats, proteins as fuel for cellular respiration (first two used preferentially

B.      Consumption of more calories than needed

1.       Excess calories stored in liver and muscles as glycogen

2.       Adipose tissue as fat when glycogens stores are full

3.       Level of glucose in blood carefully regulated by pancreatic hormones

C.      Undernourishment

1.       Diet insufficient in calories

a)       Body breaks down own proteins for energy

(1)     Causing irreversible damage

D.      Overnourishment (Obesity)

1.       Excess fat calories in diet are readily converted into fat stores

2.       Diet rich in fats overrides feedback mechanisms that regulate body weight

III. Animal’s diet must supply nutrients and carbon skeletons for biosynthesis

A.      Animals can fabricate most of the organic molecules they need from carbon skeletons and organic nitrogen from food

B.      Essential nutrients

1.       Animal requires but cannot make

2.       Varies from species to species

3.       Malnourishment

a)       Diet missing from one or more essential nutrients

C.      Essential amino acids

1.       Eight of 20 in adult human

2.       Protein deficiency

a)       Develops from a diet that lacks one or more essential amino acids

b)      Produces retarded physical and mental development in children

3.       Meat, eggs, cheese

a)       Contain complete proteins with all essential amino acids

4.       Most plant proteins incomplete

a)       Reliance on single staple, such as corn, can result in protein deficiency

D.      Essential fatty acids

1.       Able to make most of fatty acids needed

2.       Linoleic acid, and unsaturated fatty acid used to make some membrane        phospholipids, is required in human diet

3.       Deficiencies rare

E.      Vitamins

1.       Essential organic molecules required in small amounts in diet

2.       13 essential in humans identified

3.       Water soluble

a)       B-complex (function as coenzymes)

b)      Vitamin C (production of connective tissue)

4.       Fat soluble

a)       Vitamin A (incorporated into visual pigments)

b)      Vitamin D (aiding calcium absorption/bone formation)

c)       Vitamin E (protect phospholipids in membranes from oxidation)

d)      Vitamin K (required for blood clotting

F.      Minerals

1.       Inorganic nutrients needed in small amounts

2.       Vetebrated required relatively large quantities of calcium and phosphorous for bone construction

a)       Calcium also needed for normal nerve and muscle function

b)      Phosphorous need for ATP and nucleic acids

3.       Iron

a)       Component of cytochromes and hemoglobin

4.       Other minerals function as cofactors of enzymes

a)       Iodine

(1)     Needed by vertebrates to make the metabolism-regulating thyroid hormones

b)      Sodium, Potassium, Chlorine

(1)     Important in nerve function and osmotic balance

 

 

FOOD TYPES AND FEEDING MECHANISMS

IV. Most animals are opportunistic feeders

A.      Herbivores eat autotrophs

B.      Carnivores eat animals

C.      Omnivores consume both autotrophs and animals

D.      Eat foods from different categories when they are available

V. Diverse feeding adaptations have evolved among animals

A.       Suspension-feeders

1.       Aquatic animals, sifting small food particles from water

B.       Substrate-feeders

1.       Live in or on food, eating their way through it

C.      Deposit-feeders

1.       Substrate-feeders that consume decaying organic matter

D.      Fluid-feeders

1.       Suck fluids from a living plant or animal hose

E.       Bulk-feeders

1.       Eating relatively large pieces of food

VI. Four main stages: Ingestion, digestion, absorption, elimination

A.      Ingestion

1.       Act of eating

B.      Digestion

1.       Splits macromolecules into monomers by enzymatic hydrolysis (addition of water molecule when the bond between monomers is broken)

2.       Mechanical fragmentation often precedes chemical digestion

C.      Absorption

1.       Monomers and small molecules absorbed into cells

D.      Elimination

1.       Undigested remainder eliminated

 

VII. Digestion occurs in specialized compartments

A.      Intracellular digestion

1.       In food vacuoles

a)       Food vacuoles fuse with enzyme-containing lysosomes

(1)     Digestion occurs safely within a membrane-enclosed compartment

2.       Typical of protozoa and sponges

B.      Extracellular digestion

1.       Within a separate compartment of the body that connects to the external environment

C.      Gastrovascular cavities

1.       Function in both digestion and transport of nutrients throughout body

D.      Complete digestive tracts (Alimentary canals)

1.       Two openings, mouth and anus

2.       Specialized regions allowing for the sequential digestion and absorption of nutrients

 

THE MAMMALIAN DIGESTIVE SYSTEM

 

VIII. OVERVIEW

 

            A.  Consists of the alimentary canal and accessory glands

           

            B.   Rhythmic waves of muscular contraction called peristalsis push food through the tract

 

            C.   Ringlike vaves called sphincters regulate the passage of material between some          segments

 

            D.    Accessory glands

 

                        1.    Salivary glands, pancreas, liver with its gall bladder, secrete digestive enzyme                    into the alimentary canal through ducts

 

IX. THE ORAL CAVITY, PHARYNX, AND ESOPHAGUS INITIATE FOOD PROCESING

A.      The Oral Cavity

1.       Physical and chemical digestion begins in mouth

a)       Teeth grind food to expose a greater surface area to enzyme action

b)      Presence of food in oral cavity triggers release of saliva

(1)     Mucin

(a)     Glycoprotein that protects the mouth lining from abrasion and lubricates food for swallowing

(b)    Buffers to neutralize acidity, antibacterial agents, salivary amylase (begins the hydrolysis of starch and glycogen into polysaccharides and maltose)

c)       Tongue tastes and manipulates food

d)      Pushes food ball (bolus) into pharynx for swallowing

B.      The Pharynx

1.       Intersection leading to both the esophagus and trachea

2.       During swallowing, top of windpipe moves up so that its opening is blocked by cartilaginous epiglottis

C.      The Esophagus

1.       Food moves down through the narrow, flexible esophagus to the stomach

2.       Food squeezed along by a wave of smooth muscle contraction called peristalsis

X. THE STOMACH STORES FOOD AND PERFORMS PRELIMINARY DIGESTION

A.      Expandable stomach stores food so we do not have to eat constantly

1.       Epithelium lining the stomach secretions                 

a)       Gastric juice

(1)     Fluid containing hydrochloric acid that breaks down food tissues, kills bacteria

b)      Pepsin

(1)     Enzyme that hydrolyzes specific peptide bonds in proteins

(2)     Synthesized and secreted in an inactive form called pepsinogen

(3)     Activated by hydrochloric acid and by pepsin itself (example of positive feedback)

2.       Mucous coating secreted by the epithelium protects the stomach lining from digestion

a)       Gastric ulcers, mainly caused by bacteria, worsens when lining is eroded faster than it can be regenerated

3.       Smooth muscles mix the contents of the stomach

a)       Acid chyme is the nutrient broth produced by the action of the stomach and its secretions on ingested food

b)      Stomach usually closed off by two sphincters

(1)     One at cardiac orifice (prevents backflow into esophagus)

(2)     Pyloric sphincter

(a)     Regulates passage of acid chyme into intestine

XI. THE SMALL INTESTINE IS THE MAJOR ORGAN OF DIGESTION AND ABSORPTION

B.      Overview

1.       Most enzymatic hydrolysis of macromolecules and nutrient absorption into blood takes place in small intestine, longest section of alimentary canal

2.       Digestive juices (from pancreas, liver, gallbladder, and gland cells of intestinal wall) mixed with the chyme in the duodenum (first section of small intestine)

a)       Pancreas produces hydrolytic enzymes and a bicarbonate-rich alkaline solution that offsets the acidity of the chyme

(1)     Liver produces bile, stored in gallbladder until needed

(a)     Bile aids in digestion of fats and contains pigments that are by-products of the breakdown of red blood cells in the liver

C.      Enzymatic Action in the Small Intestine

1.       Digestion of starch and glycogen into disaccarides is continued by pancreatic amylases

a)       Disaccharidases

(1)     Enzymes specific for hydrolysis of different disaccharides

(2)     Built into the membranes and extracellular matrix of epithelial cells

(3)     Facilitates sugar absorption through intestinal wall

2.       Protein digestion

a)       Trypsin and chymotrypsin

(1)     Enzymes specific for peptide bonds adjacent to certain amino acids

b)      Carboxypeptidase

(1)     Splits amino acids off the free carboxyl end

c)       Aminopeptidase

(1)     Works from the amino end

d)      Dipeptidases

(1)     Attached to intestinal epithelium and split small peptides

e)      Enteropeptidase

(1)     Intestinal enzyme that activates the inactive pancreatic protein-digesting enzymes

3.       Nucleases

a)       Group of enzymes that hydrolyze DNA and RNA into their nucleotide monomers

b)      Other enzymes dismantle nucleotides

4.       Digestion of fats

a)       Aided by bile salts

(1)     Coat or emulsify tiny fat droplets so they do not coalesce

(2)     Leaving greater surface area for lipase to hydrolyze fat molecules

5.       Most digestion is completed while the chyme is still in the duodenum

6.       Jejunum and ileum are regions of the small intestine specialized for nutrient absoprtions

D.      Absorption of Nutrients

1.       Circular folds of the small intesting lining are covered with fingerlike projection called villi

a)       On which the epithelial cells have microscopic extensions called microvilli

(1)     Brush border of microvilli creates a huge surface area adapted for absorption

2.       Core of each villus has a net of capillaries and a lymph vessel called a lacteal

a)       Nutrient Absorption

(1)     Absorbed across the spithelium of the villus and then across the single-celled epithelium of the capillaries or lacteal

(2)     Transport may be passive by diffusion or active by pumping against a gradient

3.       Glyceral and fatty acids are absorbed by epithelial cells

a)       Recombine to form fats and are mixed with cholesterol and coated with proteins to make tiny globules called chilomicrons

(1)     Packages are transported by exocytosis out of the epithelial cells and into a lacteal

(a)     Then transported by the lymphatic system to veins near the heart

4.       Absorbed amino acids and sugars enter capillaries

a)       Nutrient-laden blood from small intestine is carried directly to the liver by the large hepatic portal vessel

b)      Liver interconverts molecules and regulates the nutrient content of the blood

XII.  HORMONES HELP REGULATE DIGESTION

A.      Sight, smell, or taste of food sends a nervous message from brain to the stomach that initiates secretion of gastric juice

1.       Food then stimulates the stomach wall to release the hormone gastrin into the circulatory system

a)       Stimulates further secretion of gastric juice

(1)     If pH of the stomach contents becomes too low, release of gastrin inhibited

B.      Regulatory hormones produced by the duodenum coordinate the release of digestive secretions

1.       Secretin

a)       Released by cells in the intestinal wall in response to the acidic pH of the chyme

b)      Stimulates the pancrease to release bicarbonate

2.       Cholescystokinin (CCK)

a)       Produced in response to amino acids or fatty acids

b)      Stimulates gallbladder contraction and the release of pancreatic enzymes

3.       Enterogastrone

a)       Released by the duodenum caused a fat-rich chyme

b)      Hormone that inhibits peristalsis in the stomach

(1)     Slowing the entry of the chyme into the duodenum

XIII.  RECLAIMING WATER IS A MAJOR FUNCTION OF THE LARGE INTESTINE

A.      Small intestine leads into the large intestine (colon)

1.       At a T-shaped junction with a sphincter

a)       A pouch called the cecum, with a fingerlike extension, the appendix, attaches at this junction

2.       Colon finishes the reabsorption of the large quantity of water secreted with digestive enzymes into the digestive tract

B.      Escherichia coli and other mostly harmless bacteria live on organic material in the feces

1.       Some bacteria produce vitamins absorbed by host

a)       Several B vitamins

b)      Vitamin K

C.      Feces contain cellulose, other undigested ingredients of food, salts excreted by the colong, and a large proportion of intestinal bacteria

D.      Feces are stored in rectum

1.       Voluntary and involuntary sphincter between the rectum and anus control the elimination of feces

EVOLUTIONARY ADAPTATIONS OF VERTEBRATE DIGESTIVE SYSTEMS

XIV.  STRUCTURAL ADAPTATIONS OF THE DIGESTIVE SYSTEM ARE OFTEN ASSOCIATED WITH DIET

 

A.      Dentition

1.       Type and arrangement of teeth

2.       Correlates with diet

B.      Herbivores have longer alimentary canals

1.       Plant material is more difficult to digest than meat

C.      Specialized structures, such as the spiral valve in shark’s intestine, functionally increase intestinal length by providing addition surface area

XV.  SYMBIOTIC MICROORGANISMS HELP NOURISH MANY VERTEBRATES

D.      Special fermentation chambers

1.       Many herbivorous mammals

2.       Filled with symbiotic bacteria and protozoa

a)       Often housed in cecum

b)      Digest cellulose into simple sugars

c)       Produce a variety of essential nutrients

E.      Ruminants

1.       Elaborate system

a)       Several stomachs

b)      Regurgitation

c)       Rechewing of cud

d)      Digestion of their symbiotic bacteria to maximize the nutrient yield of their grass or hay diet