Transcript Lecture
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: 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 – includes 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 • e.g. corn is deficient in tryptophan and lysine • 2. essential fatty acids • 3. vitamins • 4. minerals Essential Nutrients • four classes of essential nutrients • 2. essential fatty acids: fatty acids that contain one or more double bonds and are unsaturated • 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 • the fat-soluble vitamins can act as a hormones – deficiencies result in a wide variety of diseases • e.g. vitamin C = scurvy • e.g. vitamin D = rickets 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 ABSORPTION ELIMINATION • digestion occurs in specialized compartments – prevents the animal from digesting itself • compartments can be – intracellular – digestion within the cell • within food vacuoles • occurs following phagocytosis or pinocytosis – 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 • 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 – e.g. hydra – gland cells of the gastrodermis lining the GV cavity secrete digestive enzymes into the cavity – other cells of the gastrodermis engulf the smaller food pieces and continue digestion intracellularlly • 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 – e.g. crop of birds – e.g. gastric caeca of insects • other animals is subdivide their gut into fore-, mid and hind-gut regions rather than esophagus, SI, LI 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 – – – – 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 – peristalsis – series of wavelike contractions in smooth muscle Bolus of food Epiglottis up Tongue Pharynx Esophageal sphincter contracted Esophagus Glottis Larynx Trachea To lungs To stomach Relaxed muscles Contracted muscles Sphincter relaxed Stomach Mammalian Digestion • bolus enters the stomach – chemical and mechanical digestion Sphincter Stomach Sphincter 10 m – digestion of proteins and fats – mechanical digestion: three layers of smooth muscle to churn food – chemical digestion: production of gastric juice – food mixes with gastric juice to become chyme Esophagus 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 Mammalian Digestion • bolus enters the stomach – chemical and mechanical digestion • activation to pepsin accomplished by exposure to HCl Sphincter Stomach Sphincter 10 m – gastric juice: principally water + HCl + pepsin + gastric lipase – stomach is lined with a gastric mucosa that forms gastric glands – glands secrete the HCl and the enzymes – production of H+ and Cl- ions by the parietal cells of the gastric gland – production of pepsinogen by the chief cells of the gastric gland Esophagus 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 Mammalian Digestion • 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 – 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 flows over these absorptive cells = most digestion is done in the duodenum Vein carrying blood to liver Villi Microvilli (brush border) at apical (lumenal) surface Epithelial cells Blood capillaries Epithelial cells Muscle layers Villi Intestinal wall Large circular folds Basal surface Lacteal Key Nutrient absorption Lymph vessel Lumen Mammalian Digestion • enters the small intestine - chemical and mechanical digestion PLUS absorption – SI is also the site for the secretion of pancreatic juice – mixes with the chyme in the duodenum • pancreatic amylase, lipase and 4 proteases • 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 Mammalian Digestion • small intestine - chemical and mechanical digestion PLUS absorption – 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 for absorption • breakdown of proteins and peptides in the stomach and SI amino acids for absorption • 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 Mammalian Digestion • small intestine - chemical and mechanical digestion PLUS absorption • 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 to form a chylomicron enters the lacteal – chylomicrons eventually transferred to the blood via the subclavian veins LUMEN Triglycerides OF SMALL INTESTINE Epithelial cell Fatty acids Monoglycerides Triglycerides Phospholipids, cholesterol, and proteins Chylomicron Lacteal Mammalian Digestion • enters the large intestine or colon – for absorption of water and salts • 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 Cl- into the absorptive cells Mammalian Digestion • leftover, undigested food = feces – becomes more and more solid as water is reclaimed through the LI • digestion may take place through the action of bacterial enzymes – mostly from E.coli – by products create carbon dioxide, methane 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 Fat globule Bile salts • Liver – numerous functions – storage of iron & copper – storage of fatty acids – production of LDL and HDL – main digestive function – production of bile Fat droplets coated with bile salts Epithelium of small intestine Epithelium of lacteal Micelles made up of fatty acids, monoglycerides, and bile salts Lacteal Accessory glands – bile: water, cholesterol, bilirubin and salts • produced by hepatocytes & secreted into the duodenum • emulsification of fats – breakdown into smaller triglycerides & breakdown of TGs – A. monoglyceride (glycerol + 1 fatty acid) – B. 2 fatty acids • bile + monoglycerides or bile + fatty acids = micelles • excess bile stored in the gallbladder Fat globule Bile salts Fat droplets coated with bile salts Epithelium of small intestine Epithelium of lacteal Micelles made up of fatty acids, monoglycerides, and bile salts 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 – gastrin – by the G cells of the stomach lining • stimulates production of gastric juice and encourages emptying of the stomach – gastric inhibitory peptide – 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) • 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 Digestive Feedback systems • emptying of the stomach: – gastrin – stimulates emptying – 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 • 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 • 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 • 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 Digestion Adaptations Intestine Rumen – horses and other herbivorous mammals – house them in the caecum – rabbits and some rodents – LI + caecum Reticulum Abomasum Esophagus Omasum – ruminants – deer, sheep and cattle • stomach has four chambers: rumen, reticulum, abomasum & omasum • rumen – chewed grass first enters where it encounters bacteria = bolus is formed • reticulum – some of the bolus moves into the reticulum & the bacteria continue to digest – part of the bolus (called “the cud) is regurgitated into the mouth to be chewed again • omasum – when the cud is re-swallowed, it ends up here • abomasum – cud moves into the abomasum containing the ruminant’s own digestive enzymes Ruminants & Digestion IntestineRumen Reticulum Abomasum Esophagus Omasum Digestive Adaptations • 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 Molars Canines Herbivore • 3. omnivores – mixture of diets so teeth show both kinds of adaptations 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 – 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 • 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 – snakes possess many unique adaptations for eating • tongue is not for eating – sensory • quadrate bone at the back of the skull acts as a double hinge – allows for the jaws to be 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 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) – in many birds – the esophagus is associated with a pouch = crop • storage structure • allow the bird to quickly ingest food and digest it later – stomach is modified into two regions • 1. proventriculus – secretion of gastric juices • 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 – undigested food is eliminated through the cloaca