Feeding and Digestion Lecture Slides PDF

Acquiring energy: Feeding and digestion Acquisition of food critical to all aspects of life and survival Food comes in all shapes/sizes, but only some components/molecules are useful Morphology and physiology of feeding/ digestive machinery adapted to food And constrained by oth...

Acquiring energy: Feeding and digestion Acquisition of food critical to all aspects of life and survival Food comes in all shapes/sizes, but only some components/molecules are useful Morphology and physiology of feeding/ digestive machinery adapted to food And constrained by other aspects of an animal’s ecology, etc. © K. Welch – Do Not Distribute 1  Single cells, some small animals, parasites Direct absorption sugars, amino acids, etc. Endocytosis (cellular phenomenon) larger food digested particles intracellularly © K. Welch – Do Not Distribute 2 All animals/metazoans (except sponges) undergo gastrulation Blastula (or blastocyst) is a sphere of cells undifferentiated cells that forms shortly after initial bout of cell division Gastrulation is process of invagination and differentiation into 2 or 3 cell layers G astrulation in a Sea U rchin Em bryo © K. Welch – Do Not Distribute 3 All animals/metazoans (except sponges) undergo gastrulation Diploblastic Two cell layers formed (ectoderm and endoderm) E.g. Cnidarians and Ctenophores Triploblastic 3 cell layers formed (+mesoderm) Other animals © K. Welch – Do Not Distribute 4  Most multicellular animals: moving food through a digestive system of “reactors” Some foodstuffs digested extracellularly (within gut, but not within cells) Relatively simple: Only one Digestive system can be composed of multiple, orifice needed, not as efficient sometimes specialized, ‘reactor’ types © K. Welch – Do Not Distribute 5 All animals/metazoans (except sponges) undergo gastrulation Diploblastic E.g. Cnidarians and Ctenophores Only one orifice remains Triploblastic Other animals Mouth forms where Archenteron meets Ectoderm Permitting unidirectional flow © K. Welch – Do Not Distribute 6  Evolution of gut complexity (e.g. vertebrates) Commonality: digestive passage and separate secretory organs “Higher vertebrates” – increasing specialization/ complexity of compartments Increased efficiency Increased surface area Comparing components E = Esophagus L = Liver G = Gall bladder St = Stomach P = Pancreas © K. Welch – Do Not Distribute 7 Generalized alimentary (oral cavity, pharynx) canal (digestive tract) (esophagus, stomach) Vertebrates… (small intestine: duodenum, jejunum, ileum ) (large intestine/colon, + water/ion rectum/cloaca) reabsorption © K. Welch – Do Not Distribute 8 Specialization of Piercing, licking, sucking headgut to diet Darwin’s finches Specializations for food handling © K. Welch – Do Not Distribute 9 Specialization of foregut (e.g. esophageal crop) Can serve as storage organ Nectar/food metered out to rest of digestive system for gradual processing © K. Welch – Do Not Distribute 10  Mammalian stomachs (typically omnivores and carnivores) Gastric pits: Remember: mucus stomach is part pepsinogen of the “Foregut” HCl Mixing with food to form chyme Most muscular © K. Welch – Do Not Distribute 11  Q: What is the function of AKA mucus here? mucous neck cells: mucus A: Protect stomach wall from HCl, pepsin HCl pepsin (active) pepsinogen autocatalysis (inactive) © K. Welch – Do Not Distribute 12 Mammalian stomachs Digastric stomach (ruminants: cows, sheep, deer, etc.) Digestive enzyme Fermentation secretion (symbiotic microflora) © K. Welch – Do Not Distribute 13 Generalized alimentary canal (digestive tract) (oral cavity, pharynx) (esophagus, stomach) Vertebrates… (small intestine: duodenum, jejunum, ileum ) (large intestine/colon, + water/ion rectum/cloaca) reabsorption © K. Welch – Do Not Distribute 14  The intestine In the smallest animals, cilia could be enough to move food particles along the gut “reactor” system But, in larger animals, cilia aren’t enough to guarantee flow Plus, food particles need to be made small enough for enzymatic digestion to be effective Lieberkühn © K. Welch – Do Not Distribute 15  The intestine https://www.youtube.com/watch?v=Ujr0UAbyPS4 © K. Welch – Do Not Distribute 16 Some meals harder to digest than others Simple is easier Simple sugars, amino acids, etc. Complex is more costly Whole tissues, big proteins, complex fats and carbohydrates Protein/lipids harder to digest than carbohydrates (on average) In most vertebrates, metabolic rate goes up ≤2× that at rest during digestion © K. Welch – Do Not Distribute 17 Gastric secretions (Enzymes and pH) © K. Welch – Do Not Distribute 18 Carbohydrate digestion Cellulose – only symbiotic gut bacteria in some animals can break it down (e.g. ruminants) No enzymatic degradation of carbs directly in stomach © K. Welch – Do Not Distribute 19  Digestion and Absorption of Carbohydrates α-1,4 linkage α-1,6 linkage Starch/glycogen molecule Disaccharides Monosaccharides Sucrose (table sugar) sucrase glucose + fructose Lactose (milk sugar) lactase glucose + galactose Maltose maltase glucose + glucose © K. Welch – Do Not Distribute 20  Other polysaccharides? Cellulose – poly-glucose molecule with β-1,4 linkages Inulin – poly-fructose molecule Found in some plants instead of starch Vertebrates (most multicelled animals) don’t possess enzymes to break these down Must rely on symbiotic gut microflora In us, this occurs past small intestine, where absorption of monomers would occur Thus, we just get gassy © K. Welch – Do Not Distribute 21  Getting symbiotic gut flora Symbiotic protists help termites digest the cellulose and lignin (wood poly-aromatic alchohols) Termites must molt to grow Digestive tract lining also gets molted To reconstitute gut flora, https://www.youtube.com/watch?v=L79OXbA5vow termites must eat nestmates’ feces Yum! © K. Welch – Do Not Distribute 22  Absorption of Carbohydrates at the Brush Border “Cellular mediated absorption” Basolateral Na+/K+ pump sets up Na+ gradient (from lumen into absorptive cell) SGLT1 SGLT1 co-transports Na+ & glucose into cell For fructose, GLUT5 transporter moves it inward down conc. grad. GLUT2 Glucose, galactose, fructose Facilitated diffusion transported to interstitial fluid by GLUT2 (down conc. grad.) Other sugars follow similar routes © K. Welch – Do Not Distribute 23  Specialization of the midgut: flying animals Problem: “Higher” vertebrates like birds, bats, tend to have more complex digestive tracts These can be heavy (more weight to fly with) So, reduce size! But flight is energetically expensive (high metabolic rate required) So, nutrient absorption must be high Catch 22! © K. Welch – Do Not Distribute 24  Bird/bat guts are shorter Good for weight savings But, less surface area for absorption Cellular-mediated absorption, that is And average energy expenditure (FMR = field metabolic rate) is higher Price et al., 2015, Physiology © K. Welch – Do Not Distribute 25  Specialization of the midgut: hovering vertebrate nectarivores and other flying animals High rates of cellular-mediated Exceptionally high rates of transport per unit surface area paracellular glucose absorption High sucrase activity* *In nectarivores like hummingbirds, nectar bats, specifically © K. Welch – Do Not Distribute 26 Sugar uptake and usage is constrained at multiple steps © K. Welch - Do Not Distribute 27 Tracking fuel oxidation during (e.g. during exercise) CO2 glucose © K. Welch - Do Not Distribute 28 Tracking fuel oxidation during (e.g. during exercise) © K. Welch - Do Not Distribute 29 What about nectarivores? % hovering (exercise) metabolism supported by exogenous sugar after 30 min Hummingbirds/nectar bats can fuel ≤100% of hovering with ingested nectar © K. Welch - Do Not Distribute 30 Humans using glucose, other sugars… Limited capacity to use fructose during exercise Ingested separately Ingested together © K. Welch - Do Not Distribute 31 The fate of fructose in most mammals Fructose uptake/oxidation by muscles is very limited (Kristiansen et al., 1997; Zierath et al., 1995) More goes to liver (Delarue et al., 1993) Promotes lipogenesis (Caton et al., 2011) © K. Welch - Do Not Distribute 32  What fuels do nectarivores burn? ≈ 50% sucrose (glucose + fructose) ≈ 25% glucose ≈ 25% fructose What happens to component monosaccharides? © K. Welch - Do Not Distribute 33 Hummingbirds: superior sucrose users fexo (prop. of CO2 from oxidation of…) of sucrose ≤ 100% in ~30-40 min © K. Welch - Do Not Distribute 34 Hummingbirds (prob. nectar bats): glucose and fructose used equally well! fexo (prop. of CO2 from oxidation of…) of sucrose = glucose = fructose ≤ 100% in ~30-40 min © K. Welch - Do Not Distribute 35  Surface area maximization: reversible? Some snakes go months between meals Maintaining biological machinery (like digestive and support organs) is expensive Increase in surface area necessary to ensure effective absorption © K. Welch – Do Not Distribute 36  Example: the physiology of eating very, very infrequently Fasted 12 hr after meal  Burmese pythons display gut hypertrophy immediately following a meal (e.g. mouse or rat)  Runningthe machinery is also costly (hypertrophy → more blood flow, more pumping, required) There is hypertrophy and growth of NEW heart tissue © K. Welch – Do Not Distribute 37 But what about a bat digesting a complex meal? Fish-eating Myotis bat Almost exclusively eats fish and https://youtu.be/KHph-Di8IKw?si=uKVcfwmkjxLMS3VR crustaceans Complex, protein rich meals © K. Welch – Do Not Distribute 38 But what about a bat digesting a complex meal? Fish-eating Myotis bat Relatively costly to digest complex, protein-rich shrimp meal using a short Welch et al., 2015 JEB 218(8):1180 gut! © K. Welch – Do Not Distribute 39 When is a bat like a snake? Fish-eating Myotis are a bit like pythons* *with respect to costliness of digesting meal only! Most vertebrates/mammals: Exception: Infrequently feeding reptiles Scope of increase in MR ≤ 2 – and fish-eating Myotis! Secor and Diamond, 1998; Secor, 2009 © K. Welch – Do Not Distribute 40 Generalized alimentary canal (digestive tract) (oral cavity, pharynx) (esophagus, stomach) Vertebrates… (small intestine: duodenum, jejunum, ileum ) (large intestine/colon, + water/ion rectum/cloaca) reabsorption © K. Welch – Do Not Distribute 41  Hindgut: Large intestine Transverse colon Ascending Descending colon Ileocecal colon sphincter Caecum Rectum Appendix Sigmoid colon internal anal sphincter external anal sphincter © K. Welch – Do Not Distribute 42 Question Some people take fiber (cellulose, inulin, and other nutrients we can’t effectively break down) to relieve constipation. Given what you’ve learned in previous slides, can you explain how fiber can help relieve constipation? © K. Welch – Do Not Distribute 43 Blood glucose homeostasis and the insulin response Blood glucose levels rise after a meal as glucose, broken down from complex and/or simple sugars, is absorbed Chronic elevated blood glucose can lead to “glycation stress” It’s bad…consider any diabetic person you’ve known © K. Welch – Do Not Distribute 44 Blood glucose homeostasis and the insulin response Effects of insulin help to restore blood glucose homeostasis Highly conserved among vertebrates Why do you think blood glucose levels stay higher, longer on “high-starch diet”? © K. Welch – Do Not Distribute 45 Diabetes, insulin, and the new diabetes drugs © K. Welch – Do Not Distribute 46 Diabetes, insulin, and the new diabetes drugs Insulin: Wegovy/Ozempic: Take more often Take less often As much as with each meal Longer lasting effect Effect wears off Insulin cleared from circulation fairly quickly Weight loss! © K. Welch – Do Not Distribute 47 Diabetes, insulin, and the new diabetes drugs © K. Welch – Do Not Distribute 48  Wegovy is a Glucagon Like Peptide-1 receptor agonist* GLP-1 released by specialied instestinal epithelial cells in response to feeding (i.e. elevated blood sugar) GLP-1 has upstream effects on insulin secretion (and other things) But is also quickly degraded/removed from circulation Like insulin *agonist: A chemical that activates a receptor © K. Welch – Do Not Distribute 49 https://theconversation.com/wegovy-was-inspired-by-gila-monster- venom-here-are-some-other-drugs-with-surprising-origins-208630 © K. Welch – Do Not Distribute 50 Gila monster (Heloderma suspectum) Poisonous saliva Slow, not usually very active Carnivore (prefers easy to “catch” eggs and newborns/nestlings Can go months (all winter) without food Extremely infrequently eating, like a python © K. Welch – Do Not Distribute 51  Saliva contains “exendin-4” which is similar to GLP-1 in mammals, but remains in gila monster circulation MUCH longer Analysis of what made exendin-4 stay in circulation directly informed/guided development of Wegovy and Ozempic © K. Welch – Do Not Distribute 52 Post absorption processing Toxins and pharmacological agents subject to biotransformation Detoxifies (renders less toxic) these chemicals and/or makes them easier to excrete So, toxin/medicine activity declines © K. Welch – Do Not Distribute 53 © K. Welch – Do Not Distribute 54 Post absorption processing Phenylephrine rapidly biotransformed via conjugation of a sulfate group Largely happens in intestinal epithelium Leaves little active “parent compound” in circulation to have effect © K. Welch – Do Not Distribute 55 Post absorption processing Phenylephrine rapidly biotransformed via conjugation of a sulfate group Largely happens in intestinal epithelium Leaves little active “parent compound” in circulation to have effect © K. Welch – Do Not Distribute 56