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Lecture #15 Digestion & Nutrition Nutrition • an animal’s diet must satisfy three nutritional needs – 1. chemical energy for cellular processes – 2. organic building blocks for macromolecules – 3. essential nutrients • body function depends on the chemical energy derived from food – energy is used to produce ATP • food also provides the building blocks for biosynthesis – food provides organic carbon and organic nitrogen • materials an animal cannot synthesize = essential nutrients Essential Nutrients • • • • • four classes of essential nutrients 1. essential amino acids: 2. essential fatty acids 3. vitamins 4. minerals Essential Nutrients • four classes of essential nutrients • 1. essential amino acids: 20 amino acids required by animals to make proteins – most animals have the enzymes required to make half of these – the other half must be taken in through their food – adult humans require 8 amino acids in their diet (infants require 9 – including histamine) – complete proteins of meat, eggs and cheese are complete – they provide all the essential amino acids needed in their appropriate proportions – plant proteins are incomplete Vegetarian? Don’t worry you’re covered Essential Nutrients • 2. essential fatty acids: fatty acids that contain one or more double bonds and are unsaturated – omega-3 and omega-6 families of fatty acids – deficiencies in fatty acids are rare Essential Nutrients • 3. vitamins: organic molecules with diverse functions – 13 vitamins identified for humans – classified as water soluble and fat soluble • many water-soluble can function as co-enzymes – e.g. vitamin B1 • the fat-soluble vitamins can act as hormones – e.g. vitamin D – deficiencies result in a wide variety of diseases distinct chemicals despite sharing a similar name Essential Nutrients • 4. minerals: inorganic nutrients – diverse functions from being co-factors in reactions to functioning in osmotic balance – ingesting large amounts can disturb homeostasis – excess salt = hypertension – excess iron = liver damage and failure Food processing • four stages – – – – 1. ingestion 2. digestion 3. absorption 4. elimination Small molecules Pieces of food Mechanical digestion Chemical digestion Nutrient (enzymatic hydrolysis) molecules enter body cells Undigested material Food INGESTION DIGESTION • digestion occurs in specialized compartments ABSORPTION ELIMINATION – prevents the animal from digesting itself • compartments can be – intracellular – digestion within the cell • within food vacuoles • occurs following phagocytosis or pinocytosis • sponges use intracellular digestion entirely – extracellular – digestion outside the cell • seen in most animals • digestion occurs in extracellular compartments continuous with the outside of the body • can be followed by absorption and continued intracellular digestion – e.g. hydra • allows for the ingestion and digestion of much large pieces of food then what can be taken in via phagocytosis/pinocytosis • simplest compartment – gastrovascular cavity – also plays a role in the distribution of the digested and absorbed nutrients – other cells of the gastrodermis engulf the smaller food pieces and continue digestion intracellularlly – waste is expelled out the same way the food enter – via the mouth • most animals possess a digestive tube or alimentary canal – continuous tube from mouth to anus Extracellular Digestion Mouth Tentacles Gastrovascular Food cavity Epidermis Mesoglea Gastrodermis Nutritive muscular cells Flagella Gland cells Food vacuoles Mesoglea Digestive Tract • also called the alimentary canal • starts with a mouth pharynx – – – – – esophagus stomach small intestine large intestine rectum anus • accessory glands (shown in green) can provide additional enzymes and digestive hormones • many specializations associated with these structures in animals • the pharynx can often be used like the stomach in many animals • other animals is subdivide their gut into fore-, mid and hind-gut regions Salivary glands Mouth Esophagus Gallbladder Liver Pancreas Stomach Small intestines Large intestines Rectum Anus A schematic diagram of the human digestive system Digestive Anatomy • Mouth---bite, chew, swallow • Pharynx and esophagus---transport • Stomach----mechanical disruption; absorption of water & alcohol • Small intestine--chemical & mechanical digestion & absorption • Large intestine----absorb electrolytes & vitamins (B and K) • Rectum and anus---defecation • Accessory glands – liver, gallbladder and pancreas Mammalian Digestion • food enters the mouth where it is mechanically and chemically digested • food is swallowed and travels by peristalsis down the esophagus • enters the stomach – chemical and mechanical digestion • enters the small intestine chemical and mechanical digestion and absorption • enters the large intestine absorption Mammalian Digestion • food enters the mouth where it is mechanically and chemically digested – – – – digestion of carbohydrates and fats mechanical digestion: teeth chemical digestion: saliva containing amylase and lipase mixing with saliva turns the ground up food into a bolus • bolus is swallowed and travels by peristalsis down the esophagus – reflex that it controlled at the level of the medulla oblongata – peristalsis = series of wavelike contractions in smooth muscle – swallowing means you can’t breathe! Bolus of food Tongue Epiglottis up Pharynx Esophageal sphincter contracted Glottis Larynx Trachea Esophagus To lungs Relaxed muscles Contracted muscles Sphincter relaxed To stomach Stomach The Stomach • enters the stomach chemical and mechanical digestion – digestion of proteins and fats – absorption of water and some drugs – mechanical digestion: three layers of smooth muscle to churn food – chemical digestion: production of gastric juice – food mixes with gastric juice to become chyme The Stomach Esophagus Sphincter Stomach Sphincter 10 m – chemical digestion is through gastric juice: principally water and mucus – stomach is lined with a gastric mucosa that folds in to form gastric glands – glands secrete the HCl and the enzymes pepsin & lipase – production of H+ and Cl- ions by the parietal cells of the gastric gland – production of gastric lipase & pepsinogen by the chief cells of the gastric gland Small intestine Folds of epithelial tissue Epithelium Gastric pits on interior surface of stomach 3 Pepsinogen Pepsin 2 Gastric gland HCl Chief cell Mucous cell Chief cell Parietal cell 1 Cl H Parietal cell The Small Intestine • enters the small intestine chemical and mechanical digestion PLUS absorption – digestion and absorption of carbs, fats & proteins plus nucleic acids – small intestine is lined with finger-like structures called villi – increases absorptive surface area Vein carrying blood to liver Villi Microvilli (brush border) at apical (lumenal) surface Lumen Epithelial cells Blood capillaries Epithelial cells Muscle layers Villi Intestinal wall Large circular folds Basal surface Lacteal Key Nutrient absorption Lymph vessel The Small Intestine • enters the small intestine chemical and mechanical digestion PLUS absorption – each villus is covered with cells called absorptive cells – create a mix of enzymes called brush-border enzymes • sucrase, maltase, lactase, aminopeptidase, dipeptidase, enterokinase – food is digested as the chime flows over these absorptive cells = most digestion is done in the duodenum The Small Intestine and the Pancreas – SI is also the site for the secretion of pancreatic juice – mixes with the chyme in the duodenum • pancreatic amylase, lipase and 4 proteases • made as inactive proteases: trypsinogen, chymotrypsinogen, proelastase, procarboxypeptidase • trypsin must be activated by the brush-border enzyme enterokinase before it can work • trypsin activates the other three proteases The Small Intestine – nutrients are also absorbed by the absorptive cells as they travel through the SI = jejunum and ileum • breakdown of carbs in the mouth and SI monosaccharides • breakdown of proteins and peptides in the stomach and SI amino acids • once absorbed into the absorptive cells – digestion stops • no intracellular digestion – monosaccharides & amino acids directly absorbed by the absorptive cells and transferred into the venous blood leaving the villus Fat Absorption • but the absorption of fats is different • breakdown of fats/triglycerides in mouth, stomach and SI monoglyceride and 2 fatty acids • fatty acids & glycerol absorbed into the absorptive cells and then transferred into the lacteal of the villus – recombined into triglycerides in the absorptive cell and combined with proteins, cholesterol and phospholipids to form a chylomicron enters the lacteal – chylomicrons eventually transferred to the blood via the subclavian veins LUMEN OF SMALL INTESTINE Triglycerides Epithelial cell Fatty acids Triglycerides Phospholipids, cholesterol, and proteins Chylomicron Lacteal Monoglycerides The Large Intestine • enters the large intestine or colon for absorption of water and salts • NO HUMAN ENZYME BASED DIGESTION!!! The Large Intestine • lined with absorptive cells – absorb water and salt – mainly NaCl – most water is absorbed by the SI – the last liter of water is reclaimed by the LI – absorption of water is via osmosis and accompanies the active pumping of Na+ and Clinto the absorptive cells The Large Intestine • leftover, undigested food = feces • digestion may take place in the colon through the action of bacterial enzymes – mostly from E. coli – by products are carbon dioxide, methane, hydrogen sulfide, nitrogenous and sulfurous compounds – some bacteria produce vitamin K, B7 and B9 in exchange • terminal portion of the LI = rectum – storage of feces until expelled via defecation Accessory glands • Liver – numerous functions in digestion – storage of iron & copper – storage of fatty acids – production of LDL and HDL – main digestive function – production of bile Bile – bile: water, cholesterol, bilirubin and salts • produced by hepatocytes • secreted into the duodenum • emulsification of fats – breakdown of larger fats into smaller triglycerides • PLUS – the breakdown of TGs into monoglyceride (glycerol + 1 fatty acid) and 2 fatty acids • bile + monoglycerides or bile + fatty acids form into micelles • excess bile stored in the gallbladder Fat globule Bile salts Fat droplets coated with bile salts Micelles made up of fatty acids, monoglycerides, and bile salts Epithelium of small intestine Epithelium of lacteal Lacteal Accessory glands • Pancreas – exocrine and endocrine functions in digestion – exocrine: production of pancreatic juice – endocrine: production of insulin, glucagon & somatostatin • glucose balance Digestive Hormones • production of digestive hormones by the stomach and small intestine • stomach: – gastrin – by the G cells of the stomach lining • stimulates production of gastric juice and encourages emptying of the stomach • also relaxes the sphincter between the ileum and cecum (ileocecal) Digestive Hormones • small intestine: – gastric inhibitory peptide (enterogastrone) – antagonist to gastrin – CCK – by the enteroendocrine cells of the SI (presence of fatty acids) • CCK stimulates the release of pancreatic juice and bile (synthesis and increased gallbladder contraction) • also slows the emptying of the stomach • decreases gastric juice production – secretin – by the enteroendocrine cells of the SI • secretin stimulates the release of bicarbonate from the pancreas – neutralizes chime • decreases gastric juice production • decreases gastrin production and release • increases pepsinogen Digestive Feedback systems • emptying of the stomach: – production of gastrin – stimulates emptying – production of GIP/enterogastrone and CCK – inhibits emptying • pancreatic juice production: – secretin and CCK – stimulation of production • bile production: – CCK – stimulation of secretion Key Liver Enterogastrone Gallbladder Stimulation Inhibition Gastrin CCK Stomach Pancreas Secretin Duodenum CCK Appetite control • satiation center = hypothalamus • ghrelin – made by the stomach wall – triggers feelings of hunger – stimulates appetite when the stomach is empty – levels increase in dieters! • CCK – increases satiation (also nausea and anxiety) • insulin – secreted by the pancreas in response to increased glucose levels – suppresses appetite when released in a slow, steady manner – surges of insulin can result in feelings of hunger quicker! • leptin – produced by adipose tissue – suppresses appetite – as body fat levels drop, so does leptin production and appetite may increase • some animals have developed a complex mutalistic association with these bacteria • allow for the digestion of plant-based materials by herbivores – e.g. digestion of cellulose by herbivores • herbivores and many insects (e.g. termites) – house populations of bacteria in fermentation chambers in their alimentary canal • location of these bacteria depends on the animal species – horses and other herbivorous mammals – house them in the caecum – rabbits and some rodents – LI + caecum Digestion Adaptations – ruminants – deer, sheep and cattle • specialized adaptations to their gut • stomach has four chambers: rumen, reticulum, abomasum & omasum • rumen – chewed grass first enters rumen where it encounters bacteria = bolus is formed (20 to 80 quarts of saliva produced a day!) – largest of the four chambers – 25 gallons – storage area for food – digestion of cellulose starts here Intestine Rumen Reticulum Abomasum Esophagus Omasum • reticulum – honeycombed in structure – bolus moves into the reticulum & the bacteria continue to digest – ruminant periodically regurgitates and re-chews parts of the bolus (called “the cud) in its mouth to continue physically breaking down the grass » because cattle don’t spend much time actually chewing the first time – chemical digestion of cellulose continues in reticulum Intestine Rumen Reticulum Abomasum Esophagus Omasum • omasum – when the cud is re-swallowed, the bolus will eventually end up here – water is removed • abomasum – “true stomach” – cud moves into the abomasum containing the ruminant’s own digestive enzymes = protein digestion • food then moves into the intestine for absorption – 20X length of animal Intestine Rumen Reticulum Esophagus Abomasum Omasum Windows into the Cow Gut • farmers install “windows” (i.e. fistula) in some cows • allows them to see and sample the food being digested in the cow’s stomach • cow is not bothered by this fistula • also a port for the administration of medicines or beneficial microbes Digestive Adaptations to the Jaw • dental adaptations: • 1. carnivores – large, pointed incisors for biting and large canines for ripping – jagged pre-molars and molars for shredding Incisors Premolars Carnivore • 2. herbivores – broad, large premolars and molars for grinding – modified incisors and canines for biting plant material – some herbivores have no canines • 3. omnivores – mixture of diets so teeth show both kinds of adaptations Molars Canines Herbivore Omnivore • amphibian digestion and nutrition: – – – – most are carnivores on a wide variety of invertebrates larvae are herbivorous – feed on plants and algae true tongue first appears in the amphibian many salamanders are relatively unspecialized in their feeding methods – using only their jaws to capture prey • some can “flick” tongues out at incredible speeds https://www.youtube.com/watch?v=zcWxAfl0okE • amphibian digestion and nutrition: – but the anurans (e.g. frogs) have more advanced specializations • use their tongue and jaws to “flip and grab” its prey • tongue attaches at the anterior margin of the jaw and folds back into the oral cavity • the tongue is flicked out at the prey • the tongue and prey are flipped back into the mouth • may take approx. 0.05 to 0.15 seconds for the complete flip and grab • the prey is pressed up against the teeth and the muscles of the mouth push it into the esophagus https://www.youtube.com/watch?v=MFlHHG57V2U https://www.youtube.com/watch?v=CmoVURYBc4Q https://www.youtube.com/watch?v=wXqK5QulbJ8 liver stomach heart intestine liver gallbladder intestine stomach Bullfrog diet • reptile digestion and nutrition: – most are carnivores – tongues of turtles and crocodilians are non-protrusible and aid in swallowing – tongue of some lizards is sticky – for prey capture • https://www.youtube.com/watch?v=cwAHjl9PZH4 • some demonstrate incredible speeds and accuracy https://www.youtube.com/watch?v=eN8L2ESXMPM • reptile digestion and nutrition: – snakes possess many unique adaptations for eating • tongue is not for eating – sensory • modifications to the skull - lower jaw can spread apart to allow for swallowing of whole prey • quadrate bone at the back of the skull acts as a double hinge – allows for the lower jaw to be unhinged and thrust forward • the snake then “walks” the food into its mouth • glottis is far forward so the snake can breathe while slowly swallowing • food movement is produced by muscular contractions of the body wall – NOT the digestive system itself http://www.youtube.com/watch?v=T9COATjmaHg • fish: digestive & nutrition – – – – – most fish are predators usually swallow their prey whole some fish have teeth – capture and hold prey some teeth are modified to crush the shells of molluscs and arthopods often use a suction force that is generated by the closing of the operculum and the opening of the mouth – generates a negative pressure that sweeps water into the mouth containing their prey SWIM BLADDER LIVER INTESTINE Gills HEART STOMACH TESTES • birds: digestion and nutrition – large appetites to support a very high metabolic rate (flight) – bills and tongues are modified for a variety of feeding habitats and food sources – in many birds – the esophagus is associated with a pouch = crop • storage structure • allow the bird to quickly ingest food and digest it later • also used to store food for regurgitation to young – stomach is modified into two regions • 1. proventriculus – secretes gastric juices to initiate digestion • 2. ventriculus – or gizzard – mechanical digestion – some birds will swallow pebbles and sand to aid in digestion in this region – bulk of digestion and absorption occurs in the small intestine • • • • aided by secretions from the pancreas and liver contain bacteria for cellulose digestion undigested food is eliminated through the cloaca however owls will expel their “pellets” through the mouth