Key Concepts Animal Nutrition Why eat? Eat what? Design of digestive systems Processing steps and their hormonal control Challenge of herbivory Animals are heterotrophs, obtain nutrition from other organisms herbivores carnivores omnivores detritivores frugivores What do animals get from food? 1. Energy from chemical bonds 2. C skeletons (ex. acetyl, some amino acids) 3. Minerals - macro- and micronutrients 4. Vitamins Extracting these substances is an engineering problem Different diets need different processing machinery Carnivores large canines, slashing premolars Herbivores sharp incisors and molar grinding surfaces Omnivores relatively unspecialized teeth, to do a little of everything
vegetarian mosquito larva has brushy mouthparts to produce currents vegetarian mosquito larva has brushy mouthparts to produce currents carnivorous mosquito larva has sorry brushes and large nasty jaws carnivorous mosquito larva has weak brushes and large nasty jaws gut=long tube continuous space with outside of organism extracellular digestion specialization of different regions Specialized sections esophagus stomach small intestine large intestine Tissue layers are similar along its length AND in different animals From the inside out - mucosa submucosa muscle layers Esophagus Storage? Humans In other animals, can be used for food storage. Honeypot ants enormous storage capacity
Stomach Extra muscle Preliminary digestion of protein Some absorption of small molecules Gastric glands contain 3 types of secretory cells mucus hydrochloric acid pepsinogen Mucus protects cell surfaces Hydrochloric acid dissolves extracellular matrix, kills most bacteria, low ph (2) zymogens Pepsinogen? Pepsinogen is an inactive form of a protein-digesting enzyme, pepsin Zymogen activation - they are activated when their catalytic action is appropriate trypsinogen trypsin Pepsinogen is an inactive form of a protein-digesting enzyme, pepsin How is pesinogen activated? By low ph Pepsin works well at low ph
The sequence of digestion along the long tube is coordinated by hormones food hormone secretion initiates digestion Gastric mucosal cells secrete a hormone, gastrin, in response to food in the stomach product inhibits food gastrin secretion into bloodstream food gastrin secretion stomach secretes gastric juices including HCl low ph (very acid) Each hormone can stimulate more than one response stomach secretes gastric juices low ph (very acid) increase stomach movement moves mix into small intestine Most digestion and absorption takes place small intestine
The duodenum is a receiving center. LIVER Structure of the intestine It receives material from the stomach, the liver, and the pancreas. PANCREAS large circular folds villi muscle layers Villi microvilli capillaries lymph duct (lacteal) Surface area of small intestine Tube 3 cm x 6 meters, SA ~0.6 m 2 Circular folds add ~ 3x Villi add ~ 10x Microvilli add ~ 20x 600 x TOTAL SURFACE AREA = 360 m 2 What is this S.A. for? agents of digestion Increased surface area for absorption - general feature of animal digestive systems Liver bile which emulsifies fats Pancreas enzymes to digest carbos, proteins and fats
Protein Carbohydrate Amino Acids Monosaccharides The products of both protein and carbohydrate digestion are water soluble This means they CANNOT passively cross cell membranes (lipid bilayer), but they CAN dissolve in the blood Actively transported Once they reach the blood they are carried in solution. Fat and the products of fat digestion are NOT water soluble This means they CANNOT dissolve in the intestine or in blood, but they CAN passively cross cell membranes Digestion of fats Transport of fat across wall 1. bile salts stabilize small fat droplets 2. lipase cuts fats into fatty acids and monoglycerides 3. bile salts are recycled 1. Fatty acids are lipid soluble. They can pass through cell membrane 2. In the intestinal cell, fatty acids are repackaged into chylomicrons 3. Chylomicrons are transported out of cell into lymph system
Control of digestion in the SMALL INTESTINE by hormones food delivered from stomach hormone secretion initiates nutrientspecific digestion Examples from the small intestine: 1. Cholecystokinin 2. Secretin mix enters the small intestine fats and low ph proteins cholecystokinin (CKK) released by intestinal cells muscle movement secretin released by intestinal cells cholecystokinin (CKK) released by intestinal cells muscle movement chyme enters the small intestine fats and low ph proteins secretin released by intestinal cells 1. gall bladder releases bile salts 2. pancreas secretes digestive enzymes pancreas secretes bicarbonate which neutralizes acid 1. gall bladder contracts to release bile salts 2. pancreas secretes digestive enzymes pancreas secretes bicarbonate NEXT... the large intestine, where water and ions are reabsorbed vestigial * cecum If evolution is so clever, why do we have appendices?
Remember that... Plants have tough cell walls. Different diets require different processing machinery These requirements are ALSO reflected in the design of the digestive system Cellulose: most animals cannot digest (without help from other kingdoms) It isn t easy being an herbivore What to do, gutwise? 1. Longer guts for longer processing time 2. Help from bacteria and protists that have CELLULASE or other useful enzymes 3. Chew well Carnivore Vegetarian Ruminant herbivores Cattle Sheep Goats Deer Giraffes Antelope and their relatives, the camels and llamas
Ruminant herbivores and their relatives, the camels and llamas, have multiple stomach compartments bacterial fermentation aiding digestion cud-chewing Ruminants have 4 stomach compartments 1. rumen 2. reticulum 3. omasum 4. abomasum Other cellulose eaters with symbionts