Animal NutritionAnimal Nutrition

Chapter 41

Animals are HeterotrophsAnimals are HeterotrophsA nutritionally balanced diet must satisfy 3 main needs:

• Fuel or chemical energy

• Organic raw materials for biosynthesis (food containing carbon to make carbon skeletons)

• Essential nutrients that the body cannot make and must be obtained from food

Homeostatic Regulation of Cellular Fuel

Glucose is the main cellular fuel. Its storage and use is closely monitored by the animal body. After a meal, glucose and other monomers are absorbed into the blood from the digestive tract.

1. If blood glucose levels rise above a set point,

2. The pancreas release insulin, a protein hormone into the blood

3. Insulin causes the uptake of glucose by cells and stimulates liver and muscle cells to store glucose as glycogen

4. When blood glucose levels drop below a set point, the pancreas release glucagon that stimulate the liver and muscle cells to breakdown glycogen and release glucose into the blood.

Nutritional Requirements

• Essential Fatty Acids (certain unsaturated fatty acids like linoleic acid which is an omega-6 fatty acid – found in sunflower oil and others)

• Essential Amino Acids (Trp,Met,Val,Thr,Phe,Leu,Iso,Lys)

• Minerals (Ca, P, S, Cl, Na, K, F, Mn, Mg, Fe, Cu, I, Co, Se, Cr, Mo)

• Vitamins (13 needed)- Water-soluble (B1, B2, B6, B12, Niacin, Folic acid, Biotin, Vitamin C, Pantothenic acid)

- Fat-soluble (A, D, E, K)

Food Types

• Herbivores• Carnivores• Omnivores

Feeding Mechanisms

•Substrate-feedersLive on their food source and eat

through it

•Suspension-Feeders (Filter feeders)Filter food from their water, so have to

be aquatic animals

•Deposit-feedersSift through dirt and other deposits for food•Fluid-feeders

Suck nutrient-rich fluids from host

•Bulk-feedersEat large amounts and pieces – most

animals are bulk feeders.

Food-Processing

• Ingestion• Digestion• Absorption• Elimination

Where does digestion occur?

• IntracellularFood vacuoles are helped by lysosomes to break down food in the cytoplasm – simplest digestive compartments

• ExtracellularDigestive organs and gastrovascular cavities involved in digestion outside of cells. Complex animals have the alimentary canal or digestive tracts

Intracellular vs. Extracellular

All animals conduct both – Intracellular and Extracellular digestion

Extracellular• Large foods are ingested • The digestive tract will release enzymes into its

lumen to breakdown the food into monomers• These are absorbed by the individual cells that

line the tract

Intracellular• Autophagy of dead organelles by lysosomes• Phagocytosis by some cells such as

macrophages

The Mammalian Digestive System

• The oral cavity, pharynx and esophagus

• The stomach

• The small intestine

• The large intestine

The Oral Cavity

• Saliva contains an enzyme called Salivary Amylase, that breaks down starch and glycogen into maltose

• Saliva contains a slippery glycoprotein called mucin that protects lining of mouth from abrasions

• Saliva also contains anti-bacterial agents, buffers that prevent tooth decay

• Teeth mechanically break food down• Tongue tastes food and manipulates it during chewing• Tongue shapes the chewed food into a “bolus” for

swallowing

The Pharynx

• Intersection between the esophagus and trachea• During swallowing, the tracheal opening is

blocked by a cartilaginous flap called epiglottis

The Esophagus

• Guides the bolus into the stomach, with the help of involuntary smooth muscle spasms called peristalsis

• The muscles at the top of the esophagus are striated and so swallowing begins voluntarily

• Amylase continues to work on bolus on the way down

The Stomach• Located under the diaphragm, to the left of the

abdominal cavity• Large enough to hold 2 liters of food and water –

so constant eating is not needed• The inner epithelium secretes gastric juices –

which contains HCl, has pH of 2• Juice also contains pepsin (secreted as pepsinogen by

chief cells in folds of stomach lining), an enzyme that hydrolyzes proteins – works best in acidic environmentsEpithelial cells release mucous, that prevents the erosion of the stomach lining – however, it does eventually erode and mitosis replaces the cells

Bolus mixed with gastric juices is now a nutrient broth called Acid Chyme

Cardiac sphincter is the same as the gastro esophageal sphincter

Hiatal Hernia• A hernia occurs when one part of the body

protrudes through a gap or opening into another part.

• A hiatal hernia forms at the gastroesophageal sphincter, where the esophagus joins the stomach. Part of the stomach pushes through this opening causing a hiatal hernia.

• A large hiatal hernia can allow food and acid to back up into the esophagus, leading to heartburn and chest pain.

Stomach Lining

(Parietal Cells)(Parietal Cells)

(Chief Cells)

(Goblet Cells)

(Other cells near the pits secrete Gastrin)

Helicobacter pylori• Bacteria that cause gastric ulcers – lesions in the stomach lining

• H. pylori is a spiral-shaped gram-negative bacterium.

• it requires oxygen but at lower levels than those contained in the atmosphere.

• With its flagella and its spiral shape, the bacterium drills into the mucus layer of the stomach, and then can either be found suspended in the gastric mucosa or attached to epithelial cells.

• It excretes the enzyme urease, which converts urea into ammonia and bicarbonate. The release of ammonia is beneficial to the bacterium since it partially neutralizes the very acidic environment of the stomach (whose very purpose is to kill bacteria). Ammonia is, however, toxic to the epithelial cells, and with other products of H. pylori, including protease, catalase, and phospholipases, causes damage to those cells

Helicobacter pyloriHelicobacter pylori

The Small Intestine

• Workhorse of digestion and absorption• Longest section of alimentary canal• First 25 cm called the duodenum where Acid

Chyme from stomach mixes with digestive juices from pancreas, liver, gall bladder and gland cells of the intestinal lining

• The middle section of the SI is called the jejunum

• The last section of the SI is called the ileum

Small IntestineSmall Intestine

• The lumen of the small intestine receives The lumen of the small intestine receives secretions from secretions from – the pancreas and the pancreas and – LiverLiver– The small intestine itselfThe small intestine itself

• The SI The SI – completes digestion of the nutrients in chyme,completes digestion of the nutrients in chyme,– absorbs the products of digestion, and absorbs the products of digestion, and – transports the remaining residues to the large transports the remaining residues to the large

intestine.intestine.

The other players in SI digestion

Enzymatic Actions in the Duodenum1. Pancreas

- Proteases called trypsinogen, chymotrypsinogen and procarboxypepsidase are secreted into the SI lumen and then become their active forms – trypsin, chymotrypsin and carboxypepsidase. The activation of these enzymes is triggered by an SI enzyme called enteropeptidase.

These 3 activated enzymes help break down protein in the acid chyme.

- Pancreatic Lipase breaks down dietary fat into 1 monoglyceride and 3 fatty acids- Pancreatic Amylase hydrolyzes dietary starch into maltose- Bicarbonate (HCO3

- ) and Water neutralize the acidity in the acid chyme- Two nucleases breakdown DNA and RNA.

2. Small Intestine- Secondary protein hydrolyzing enzymes called aminopepsidase and dipeptidases found in the lining of the small intestine also help speed the process of protein digestion.

Summary of Protein Digestion in SI

AminopeptidaseAminopeptidaseDipeptidaseDipeptidase

Regulation of Pancreatic Secretion

• The nervous and endocrine systems regulate release of pancreatic juice.

• Secretin from the duodenum stimulates the release of pancreatic juice with a high bicarbonate ion concentration but few digestive enzymes.

• Cholecystokinin from the wall of the small intestine stimulates the release of pancreatic juice with abundant digestive enzymes.

Enzymatic Actions in the SI

3. Liver - Produces bile, which is a combination of bile salts, lecithin, *bilirubin, water and cholesterol

- Bile is stored in the gall bladder and released into the SI lumen through the common bile duct

- Bile emulsifies dietary lipids, so that pancreatic lipase can breakdown each triglyceride into fatty acids and glycerol.

*Bilirubin is the main bile pigment that is formed from the breakdown of heme in red blood cells. The broken down heme travels to the liver, where it is secreted into the bile by the liver. Bilirubin is passed in the feces and gives it the yellow-brown color. Excess bilirubin causes jaundice and stains skin, eyes and nails yellow.

Composition of Bile

• Bile is a yellowish-green liquid that hepatic cells secrete; it includes water, bile salts, bile pigments (bilirubin, biliverdin), cholesterol, and electrolytes.

• Bile pigments are breakdown products from red blood cells.

• Only the bile salts have a digestive function.

The gallbladderThe gallbladder

• The gallbladder is a pear-shaped sac lying on The gallbladder is a pear-shaped sac lying on the interior surface of the liver.the interior surface of the liver.

• It is connected to the cystic duct, which joins It is connected to the cystic duct, which joins the hepatic duct; these two ducts merge to the hepatic duct; these two ducts merge to form the common bile duct leading to the form the common bile duct leading to the duodenum.duodenum.

• A sphincter muscle controls the release of A sphincter muscle controls the release of bile from the common bile duct.bile from the common bile duct.

Regulation of Bile Release

• Bile does not normally enter the duodenum until cholecystokinin stimulates the gallbladder to contract.

• The hepatopancreatic sphincter remains contracted unless a peristaltic wave approaches it, at which time it relaxes and a squirt of bile enters the duodenum.

Functions of Bile Salts

• Bile salts emulsify fats into smaller droplets, so that pancreatic lipase can hydrolyze the triglycerides

• Aids in the absorption of fatty acids, cholesterol, and certain vitamins

The Jejunum and Ileum

• Most of the digestion in the SI occurs in the first section – the Duodenum

• The next sections of the SI – the Jejunum and the Ileum, are mainly involved in the absorption of nutrients and water

Absorption of Nutrients in the SI• Circular folds in the epithelial tissue lining of the

intestines have finger-like projections called villi.• Each epithelial cell of the villi has many microscopic

microvilli.• This increases the surface area for absorption• Capillaries and a lymphatic vessel called a lacteal

penetrate the villi – this is so that nutrients can be absorbed across the intestinal lining, across the capillaries, into the blood stream or lymphatic system.

Nutrients have to traverse only two layers of cells – one layer of intestinal epithelial cells and one layer of capillary cells

The movement of nutrients can be either active or passive transport.

Absorption in the Small Intestine

• The small intestine is the major site of absorption within the alimentary canal.

• There are digestive enzymes on microvilli, including peptidases, sucrase, maltase, and lactase, and intestinal lipase.

• Monosaccharides and amino acids resulting from carbohydrate and protein digestion are absorbed by the villi through active transport and enter blood capillaries.

• The intestinal villi also absorb water (by aquaporins) and electrolytes (by active transport).

Fat Absorption• Fatty acids are absorbed and transported differently than the

other nutrients.

1. Fats are digested into individual fatty acids in the SI lumen by bile and lipase.

2. These fatty acids enter the cells of the SI lining via microvilli

3. The endoplasmic reticula of the cells reconstruct the fats (triglycerides).

4. These fats collect in clusters that become encased in protein

5. These protein+ fat clusters are called chylomicrons.6. Chylomicrons are carried away in lymphatic lacteals until

they eventually join the bloodstream.

Microvilli

The Lymphatic System• Consists of organs, ducts, and nodes. • Transports a watery clear fluid called lymph.  • This fluid distributes immune cells and other factors throughout

the body. It also interacts with the blood circulatory system to drain fluid from cells and tissues. 

• The lymphatic system contains immune cells called lymphocytes, which protect the body against antigens (viruses, bacteria, etc.) that invade the body.  

Main functions of lymphatic system:  

• to collect and return interstitial fluid, including plasma protein to the blood, and thus help maintain fluid balance, 

• to defend the body against disease by producing lymphocytes,  • to absorb lipids from the intestine and transport them to the

blood.

Summary of Digestion

After AbsorptionAfter Absorption

• The capillaries carrying the nutrient-rich The capillaries carrying the nutrient-rich blood join to form the hepatic portal vessel blood join to form the hepatic portal vessel which carries the nutrients to the liverwhich carries the nutrients to the liver

• The blood then goes to the heart, which The blood then goes to the heart, which pumps the nutrient and oxygen to the rest of pumps the nutrient and oxygen to the rest of the body cellsthe body cells

Hormones That Regulate DigestionThe hormones that control digestion are gastrin, secretin, and

cholecystokinin (CCK):

• Gastrin (Secreted by cells in stomach lining)– causes the stomach to produce acid– necessary for the normal growth of the lining of the stomach, small

intestine, and colon.

• Secretin (An enterogastrone secreted by cells in wall of duodenum)– causes the pancreas to release juices with bicarbonate. – stimulates the stomach to produce pepsin, – stimulates the liver to produce bile.

• CCK (cholecystokinin) (An enterogastrone secreted by cells in wall of duodenum)– causes the pancreas to grow and to produce its digestive enzymes– causes the gallbladder to empty stored bile into the SI.

hormones in the GI that regulate appetite:

• Ghrelin is produced in the stomach and upper intestine in the absence of food in the digestive system and stimulates appetite.

• Peptide YY is produced in the GI tract in response to a meal in the system and inhibits appetite.

Both of these hormones work on the brain to help regulate the intake of food for energy.

Large Intestine • The large intestine absorbs water and electrolytes and forms

and stores feces.

Parts of the Large Intestine• The large intestine consists of the cecum (pouch at

the beginning of the large intestine), colon (ascending, transverse, descending, and sigmoid regions), the rectum, and the anal canal.

• The anal canal opens to the outside as the anus; it is guarded by an involuntary internal anal sphincter and a voluntary external anal sphincter muscle.

The Large Intestine

• Large population of bacteria – most harmless, some mutualistic – E.coli produce vitamin , others produce vitamin B, folic acid, biotin, etc.

• Some of these bacteria breakdown the undigested cellulose for food for themselves.

• Bacteria produce gases like methane and hydrogen sulfide• Feces contain undigested food like cellulose, bacteria, salts, bilirubin

makes it yellow!

•AKA colon•Carnivores have a small cecum ; the appendix is a part of it

Water absorption is the main task of LI (90% reabsorbed)Waste (feces) is eliminated, after being stored in the rectum

Evolutionary Adaptations of Digestive Systems

• Teeth – sharp incisors, canines vs. molars

• Cecum – long vs. short

• Four-chambered stomach – ruminants like cows and sheep can regurgitate and re-chew food – their stomachs contain cellulose-digesting bacteria.

• Symbiotic bacteria – vitamin production, cellulose digestion

Carnivores – have sharp teeth to tear flesh

Herbivores have sharp front teeth to cut vegetation and flat rear teeth then to grind it down.

Teeth that fit the diet

Carnivores – have sharp teeth to tear fleshHerbivores have sharp front teeth to cut vegetation and flat rear teeth then to grind it down.

Omnivore teeth

Ruminant Digestive Tract4-Chambered Stomach4-Chambered Stomach

THE END