Chapter 41: Animal Nutrition

Nutritional Requirements

• Animal diet satisfies three needs: fuel for cellular work of body, organic raw material used for biosynthesis, and essential nutrients that the animal cannot make for itself from raw material

Homeostatic mechanisms manage an animal’s fuel

• ATP accounts for largest part of energy budget of animals
• ATP comes from oxidation of organic fuel molecules (carbohydrates, proteins, fats) in cellular respiration

Glucose Regulation as an Example of Homeostasis in Nutrition

• Surplus calories are used for biosynthesis
• If animal isn’t growing/reproducing, energy is stored in depots
• In glycogen in liver and muscle cells
• More surplus? Store it in fat!
• During heavy spending of energy, body uses liver glycogen, then muscle glycogen, then fat

Caloric Imbalance

• Undernourishment – deficient in calories
• Body begins breaking down its own proteins, muscles shrink, brain becomes protein-deficient
• Overnourishment – excessive food intake
• Intake of fat = stored, intake of carbohydrates = increased rate of carbohydrate oxidation

Obesity

• Human body imposes limits on weight gain and loss
• Hormone called leptin, produced by adipose cells, controls this
• High leptin levels (from increase in adipose tissue)= lower appetite and increase energy-consumption
• Lower leptin levels (from loss of body fat) = increase appetite and weight gain

An animal’s diet must supply essential nutrients and carbon skeletons for biosynthesis

• To grow and maintain itself, an animal must obtain organic precursors (carbon skeletons) from food
• Need source of organic carbon (sugar) and organic nitrogen (amino acids from proteins)
• Can make carbohydrates, proteins, lipids from these
• Essential nutrients – materials that must be obtained in preassembled form because animal cannot make them from raw material
• Malnourished – missing one or more essential nutrients

Essential Amino Acids

• Essential amino acids – obtained from food in prefabricated form
• 8 essential amino acids: tryptophan, methionine, valine, threonine, phenylalanine, leucine, isoleucine, lysine
• Protein deficiency – most common type of malnutrition
• Meat = most reliable source of essential amino acids

Essential Fatty Acids

• Essential fatty acids – unsaturated fatty acids that animals can’t make
• E.g. linoleic acid makes phospholipids in membranes

Vitamins

• Vitamins – organic molecules required in the diet, small compared with the other essential nutrients
• 13 essential vitamins
• Water soluble vitamins
• Vitamin B – coenzymes in metabolic processes
• Vitamin C – connective tissue production
• Overdose is harmless because they come out in urine
• Fat-soluble proteins
• Vitamin A – eye pigments
• Vitamin D – calcium and bone
• Vitamin E and C protect phospholipids in membranes from oxidation
• Overdose is harmless because they are deposited in body fat, might become toxic

Minerals

• Simple organic nutrients required in small amounts
• Humans and vertebrates need large amounts of calcium and phosphorous
• Calcium: bone maintenance and construction, nerves and muscles
• Phosphorous – ATP and nucleic acids
• Iron, magnesium, zinc, copper, manganese, selenium etc
• Too much salt (sodium chloride) upsets homeostatic balance

Food Types and Feeding Mechanisms

• Herbivores – eat autotrophs
• Carnivores – eat other animals
• Omnivores – consume animals as well as plants or algal matter

Diverse feeding adaptations have evolved among animals

• Suspension feeders – sift small food particles from water (whales, clams and oysters etc)
• Substrate feeders – live in or on their food (maggots etc)
• Deposit feeders – eat through dirt (earthworms)
• Fluid feeders – suck nutrient-rich fluid from living host (mosquitoes, leeches, bees etc)
• Bulk feeders – eat large pieces of food

Overview of Food Processing

Four main stages of food processing: ingestion, digestion, absorption, and elimination

• Ingestion – eating
• Animals must deal with packaged food that have large polymers, proteins, carbohydrates
• Polymers are too large to pass through membranes
• Macromolecules that make up animal are not identical to those of food
• Digestion – process of breaking down food into molecules small enough for the body to absorb
• Cleaves macromolecules to component monomers
• Polysaccharides and disaccharides à simple sugars
• Fats à glycerol and fatty acids
• Proteins à amino acids
• Nucleic acids à nucleotides
• Breaks down monomer bonds by enzymatic hydrolysis
• Absorption – animal’s cells absorb small molecules from digestive compartment
• Elimination – undigested material passes out of digestive compartment

Digestion occurs in specialized compartments

Intracellular Digestion

• Food particles are engulfed by endocytosis and digested within food vacuoles, fused with lyosome

Extracellular Digestion

• Extracellular digestion – breakdown of food outside cells
• Occurs in compartments that are connected to outside of animal’s body
• Advantage: animal can eat large prey than phagocytosis
• Gastrovascular cavities – simple body plans with digestive sacs and single openings
• Complete digestive tracts/alimentary canals – digestive tubes with two openings, anus and mouth
• Can have specialized regions
• Can eat additional food before previous is digested

The Mammalian Digestive System

• Peristalsis – rhythmic waves of contraction by smooth muscles in wall of cana, pushes food along tract
• Sphincters – closes tubes between segments to regulate passage of material
• Accessory glands – secrete digestive juices
• Salivary glands
• Pancreas
• Liver
• Gall bladder – stores digestive juices

The oral cavity, pharynx, and esophagus initiate food processing

The Oral Cavity

• Food in oral cavity causes saliva
• Slippery glycoprotein mucin which protects mouth from abrasion and lubricates food for easier swallowing
• Neutralizes acid in mouth, kills bacteria
• Salivary amylase – enzyme that hydrolyzes starch and glycogen
• Tongue shapes food into a bolus, a ball

The Pharynx

• Aka, the throat – opens to esophagus and windpipe (trachea)
• When we swallow, top of windpipe moves up so that its opening – glottis – is blocked by a cartilaginous flap, the epiglottis

The Esophagus

• Conducts food down to stomach by peristalsis

The stomach stores food and performs preliminary digestion

• Stomach is in upper abdominal cavity
• Has folds and elastic wall
• Secretes gastric juice – digestive fluid that mixes with food by churning action of smooth muscles of wall
• Acidic gastric juice disrupts cells in food, kills bacteria
• Pepsin – enzyme that begins hydrolysis of proteins
• Secreted in inactive form – pepsinogen, from chief cells located in gastric pits
• Parietal cells secrete hydrochloric acid which converts pepsinogen to pepsin
• Mucus from epithelial cells protect stomach lining
• Mixing and enzyme action turns food into acid chime
• Opening between stomach and small intestine is pyloric sphincter, regulates passage of chime

The small intestine is the major organ of digestion and absorption

• Responsible for enzymatic hydrolysis of food macromolecules and absorption of nutrients
• First 25 cm = duodenum
• Acid chime mixes with digestive juices
• Juices from pancreas, liver, gallbladder, and gland cells of intestinal wall
• Pancreas – hydrolytic enzymes and alkaline solution rich in bicarbonate; the bicarbonate offsets acidity of chime on the stomach
• Liver produces bile, which aid in digestion and absorption of fats

Enzymatic Action in the Small Intestine

• Carbohydrate Digestion:
• Begins with salivary amylase in oral cavity
• Pancreatic amylase hydrolyzes starch, glycogen, and polysaccharides to disaccharides
• Monomers then absorbed into blood
• Protein Digestion
• Begins with pepsin in stomach
• Several enzymes in duodenum break polypeptides to component amino acids
• Trypsin and chymotrypsin – break large polypeptides into small peptides (pancreas)
• Dipeptidase – split small peptides
• Carboxypeptidase – splits off one amino acid at a time, starting with the end that has free carboxyl group (pancreas)
• Aminopeptidase – works in opposite direction (epithelium)
• Enteropeptidase – triggers activation of these enzymes
• Nucleic Acid Digestion
• Nucleases – team of enzymes that hydrolyze DNA and RNA into component nucleotides – nucleosides, nitrogenous bases, sugars, phosphates
• Fat Digestion
• Problem: fat molecules are insoluble in water, hydrolysis won’t work
• Bile salts coat fat droplets and keep them from coalescing – emulsification
• Lipase – enzyme that hydrolyzes fat molecules
• Jejunum and ileum – absorbs nutrients and water

Absorption of Nutrients

• Villi – large circular folds in small intestine lining – has fingerlike projections with microvilli that are exposed to intestinal lumen
• Increases rate of nutrient absorption by increasing surface area
• In core of each villus is net of capillaries and a small vessel of lymphatic system – lacteal
• Amino acids and sugars are absorbed into blood categories
• Fatty acids and glycerol absorbed into lymphatic system (form small globules called chylomicrons)
• Capillaries and veins converge into hepatic portal vessel – leads to liver

Digestive Efficiency and Cost

• Digestion and absorbing a meal requires energy they got from food

Hormones help regulate digestion

• Hormones released ensure that digestive secretions only present when needed
• Gastrin – produced by cells in stomach lining when food reaches stomach/when smell or see food
• Enters blood stream and stimulates other cells of the stomach to produce gastric juices
• Enterogastrones – secreted by wall of duodenum – stimulates cells in the wall to release secretin
• Secretin stimulates pancreas to produce bicarbonate
• Cholecystokinin (CCK) – produced in response to presence of fats – stimulates the release of bile and pancreatic enzymes

Reclaiming water is a major function of the large intestine

• Large intestine – colon – connected to small intestine at the T-shaped function where there is a sphincter
• One arm of T-shaped junction is cecum
• Humans have appendix – dispensable but may help with immune system
• Intestine reabsorbs water along with nutrient
• Wastes of digestive tract – feces – becomes more solid as they move along colon (due to reabsorption of water)
• Diarrhea – too little water reabsorbed, so feces watery
• Constipation – too much water absorbed, so compacted feces
• Has bacteria that helps with digestion of cellulose, especially E. coli, and produce vitamins
• End of colon = rectum, where feces are stored and excreted through anus

Evolutionary Adaptations of Vertebrate Digestive Systems

Structural adaptations of digestive systems are often associated with diet

• Special teeth and large, expandable stomachs for carnivores
• Herbivores and omnivores have larger alimentary canals than carnivores
• More vegetation to digest, so longer canals = longer time

Symbiotic microorganisms help nourish many vertebrates

• Some herbivores have special fermentation chambers with symbiotic bacteria and protists that can digest cellulose into sugars/other compounds that the animal can absorb