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Lecture 9

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BIOL 370
Dinu Nesan

Lecture 9: Introduction to Digestion - Digestion is a highly complicated process o Associated subordinate/symbiotic organisms o Multiple interacting cell types & molecules - Different nutrients undergo different digestive pathways o Proteins vs. Carbohydrates vs. Lipids - Digestive systems increase in size & specialization as they increase in complexity o Structure-function relationships are present in almost all areas  Teeth, mouths, internal tissues - Digestion: o Assimilation: Process of nutrient acquisition, digestion, & absorption (GI tract - external) o Egestion: Excretion of undigested food - Energy content: o Protein & carbohydrates = 4 kcal/gm; Fat = 9 kcal/gm o Some food is indigestible (e.g. fibre), energy is lost in feces or urine o Some energy is spend in digesting the food  Specific dynamic action (SDA): Increase in metabolic rate during digestion (thermal energy) - Digestive enzymes: Converts macromolecules into absorptive forms o Lipases: Break down triglyceride & phospholipids into FA o Proteases: Break down proteins into polypeptides o Amylase: Break down polysaccharides into oligosaccharides o Nucleases: Break down DNA into nucleotides, then nucleosides, & N bases for absorption - Symbiotic Organisms & Digestion (3) o Enterosymbionts: Lives within the lumen of GI tract (e.g. cecum)  E.g. Birds eats wax in beehives, bacteria within the bird gut break down the wax into shorter C units that can be absorbed by the bird o Exosymbionts: Cultivated outside of the body  E.g. Leaf-cutter ants feed leaf fragments to fungi cultivated by the ant colony. Ants consume both the fungi & the plant material that has been partially degraded by the fungi o Endosymbionts: Grows in interstitial spaces or within host cells  E.g. Cyanelles are cyanobacteria living in association with sponges, uses photosynthesis to produce Carbon skeletons that are taken up by the animal cells - Transport across membranes: o Facilitated diffusion (favorable) o Active transport (ATP, non-favorable) o Vesicles: Pinocytosis “drinking” vs. Phagocytosis “eating” vs. Exocytosis “expulsion” - Carbohydrate Digestion o Polysaccharides: Glycogen, starch, cellulose o Disaccharides: Sucrose, lactose, maltose  Monosaccharides: Glucose, fructose, galactose  Break down occurs at the mouth & SI, absorbed by enterocytes of the gut o Low glucose level:  Apical GLUT-5 takes in fructose & SGLT-1 “Na /Glucose co-transporter” takes in glucose  Basolateral fructose exists via GLUT-5 & glucose exists via GLUT-2 o High glucose level:  GLUT-2 in vesicles are transported to apical surface to increase glucose uptake - Protein Digestion o Protein  Dipeptides  AA absorbed by epithelial cells via AA/Na cotransporter o Some proteins can be transported across epithelial cells via endocytosis & exocytosis - Lipid Digestion o Fats  FAs & Monoglycerides, lipids diffuse across membrane into enterocytes o GI tract secretes bile to emulsify lipids into small droplets “micelles” o Short chain FA & glycerol are transported into blood stream o Triglyceride are transported to the lymph as chylomicrons - Sensing Food “Different sense receptors” o Gustatory “Blood” & Olfactory “Nose” detects chemical stimuli o Other receptors detects energy emitted/reflected “Light – Eagle”, “Electricity – Knifefish, detect muscle activity of potential prey” - Feeding mechanisms o Filter feeding: depends on random encounters “Coral reefs” o Food absorption from body wall “Endoparasites – Tape worm” o Endocytosis – Phagocytosis & pinocytosis “Protoozoans, sponges” - Symbiosis w/ autotrophic microbes “Inorganic carbon” o Photosynthetic:  E.g. Reef-building corals: Dinoglagellates are endosymbiotic, they provide food energy & facilitate the formation of calcareous skeleton, gains protection o Chemosynthetic:  E.g. Hydrothermal-vent communities rich in sulfur, provides food for sulfur-oxidizing bacteria & worm species - Symbiosis w/ heterotrophic microbes “organic carbon” o E.g. Fermenting microbes living in anaerobic environments (gut/lumen)  Rumen microbes can synthesize essential vitamins & amino acids  Microbes fermentative breaks down compounds that animals cannot digest  E.g. Cellulolytic bacteria contains cellulase enzyme that breaks down cellulose, resulting in C2 & methane production  Microbes permit waste nitrogen from the animal’s metabolism to be recycled - Sponges: Ingest via phagocytosis o Digestion occurs intracellularly in endocytic vacuoles - Cnidaria: Ingest by bringing food into the gastrovascular cavity o Digestion occurs extracellularly (enzymes) then intracellularly (endocytic vacuoles) - Feeding structures: Specialized mouthparts for obtaining & processing food o Siphons are attachment organs found in endoparasitic worms  suckling (e.g. suckers & hooks) o Tongues:  Radula – Rasping tongue in snails to grind & cut chunks of food  proboscis – tubelike tongue to obtain nectar o Beaks & Jaws  E.g. bird beaks are composed of bones covered by overlapping epidermal scales, morphology is diverse & reflect various feeding strategies, also used for vocalization, defense, & grooming  E.g. snakes have very mobile upper jaw & very strong neck muscles to crush the egg, egg shell are vomited out o Teeth (Mammal only, 4 types)  Incisors, canines, premolars & molars (shape of teeth reflects the type of diet)  First 2 for piercing & tearing, middle for tearing & grinding, last for grinding - Digestive system & Evolution o Istreasing anatomical & functional specialization o 1 – 2 way gut: Food enters & leaves the gut through a single opening (e.g. flatworm)  Simple internal sac, may have diverticula “more branching” to increase SA o 2 – 1 way gut “GI”: Specialized regions, passing through compartments regulated by sphincters  Mouth, pharynx & esophagus: Mechanical breakdown of food  Stomach: Acidic compartment  Small Intestine: Digestion & absorption  Large Intestine: Re-absorption of water & salt  Anus: Release of indigestible material - Surface Area: Nutrients are hydrolyzed in GI tract lumen & taken up by cells lining the gut o More SA: Increase gut length & Increase in surface undulations “Circular folds  Villi  MV” - Specialized compartments: Increase digestive efficiency, every compartment have functional specializations o pH, Enzymes, types of secretory & absorptive cells o Complexity of gut morphology varies across taxa “reflects complexity of the diet & ease of digestion” Lecture 10: Digestive Physiology & Regulation - Digestive tissues & locations are highly specialized for their various functions o Stomach vs. Intestines vs. Mouth o Structure-function relationships - Digestion is under complex neural & endocrine control o
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