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Digestion & Absorption

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Catalytic Proteins

Enzymes selectively accelerate each step in metabolism (enzymes are regulated)

Catabolic Pathways - release energy by breaking down complex molecules to simpler molecules (energy is stored in cell for future use)

Anabolic Pathways - consume energy to build complicated molecules from simpler compounds (energy from catabolic pathways is used)

Structures within the cell bring order to metabolic pathways

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A team of enzymes for several steps of a metabolic pathway may be assembled together as a multienzyme complex

The product from the first can then pass quickly to the next enzyme until the final product is released 

Overview of GI System

  • Movement

  • Secretion

  • Digestion

  • Absorption

  • Elimination

  • Nutrient production by intestinal bacteria

Digestion = hydrolysis

- break bonds by adding water

- break large molecules in foods into smaller molecules

Absorption = transport or transfer of the products of digestion into blood

- blood will carry molecules to cells

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Tightly controlled and regulated

GI Structure

GI stretches from mouth to anus

Common Features (not the mouth)

  • concetric circles of tissues

  • same layers regardless of location

    • muscles​

    • cells (secretion)

    • blood vessels

    • nerves

    • emphasis varies with respect to which layer functions in which location

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Mouth

  • Glands produce saliva

    • saliva contains enzymes that break down starch and lipid​s and mucus that lubricates food

    • contains lyzozyme that kills bacteria

  • chewing breaks food into smaller pieces

    • increases surface area, allowing enzymes to have greater access​

  • tongue has taste receptors

  • food enters the stomach via esophagus

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Esophagus

  • very muscular

  • forces food into stomach

  • at the top of the esophagus is a valve-like flap called the epiglottis (prevents food from entering trachea) 

  • food also gets down esophagus by muscular contractions (peristalsis) and gravity 

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Stomach

  • 2-3 distinct regions

    • upper​ - fundus​​

      • stretchy

      • storage

    • middle - body

      • empty ≈ 2oz​

      • full ≈ 1-1.5L

    • lower - pylorus

      • mix and propel​

  • processes swallowed food in a preliminary manner for delivery into SI

    • secretes acids and enzymes​

  • food usually stays in for 2-3 hours

    • solids take longer than liquids​

    • fatty meal takes longer than protein or CHO

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Small Intestine

  • small because of diameter, not length

  • three regions

    • duodenum (first foot)​

    • jejunum (next 4 feet)

    • ileum (last 5 feet)

  • most digestion completed in the jejunum with help from enzymes (from intestinal cells and pancreas)

    • muscular contractions mix food​

  • attaches to colon (large intestine)

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  • increases surface area to facilitate digestion and absorption of nutrients 

  • mucosa are folded into villi -finger - like projections

    • villi have microvilli that make up the brush border membrane of the absorptive cells​

    • increases surface area 600x

  • the absorptive cells that have this brush border membrane are called enterocytes

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Accessory Organs

LIVER

  • makes bile (needed for lipid digestion)

GALLBLADDER

  • stores/concentrates bile

  • bile duct connected to SI

PANCREAS

  • exocrine - secretes through ducts

    • secretes HCO3 (to neutralize H+)​

    • secretes enzymes

    • enter via pancreatic duct

  • endocrine - secretes directly into blood

    • insulin and glucagon​

    • both made in the islets of langerhans

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Colon (large GI)

  • measures 3.5ft

  • separated into 5 sections

  • contains bacteria, which may digest some escape CHO and fiber

    • synthesizes vitamin B12 and K (absorbed in colon)​

  • little digestion happens here

    • 95% occurs in SI​

  • contents remain for 24-72 hours

  • reabsorption of water and compaction of chyme into feces and storage of fecal matter

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Regulation of Digestion (neural and hormonal)

NEURAL

  • faster

  • nerves line walls of GI tract

  • exist in networks or plexus

  • receive messages from stimuli

    • food​

    • pressure

    • acid

  • response

    • contraction​

    • secretion

HORMONAL

  • most sustained

  • via regulatory pipes

  • 4 main ones

    • each has a main function​

    • 1 made in stomach (gastrin)

    • 3 made in duodenum

      • CCK​

      • secretin

      • GIP

  • In response to chyme

    • release peptides -> blood -> target organ -> action​

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ROLE OF HORMONES​

  • gastrin

    • chyme -> stomach cells -> release gastrin -> parietal cells -> HCl

  • secretin

    • stimulates pancreas to release HCO3​

    • stimulates liver to secrete bile to gallbladder

    • HCO3 counteracts the acidity of chyme and raises the pH so enzymes can function properly

  • CCK

    • released in response to ​

      • partially hydrolyzed fat

      • proteins

      • acidic pH of chyme

    • stimulates gall bladder to contract and release bile

    • stimulates release of a secretion from the pancreas

      • contains digestive enzymes and proenzymes​

      • these enzymes work best at neutral pH, where HCO3 comes in

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Major Digestive Enzymes

MOUTH

  • salivary amilase

    • secreted by salivary glands​

    • works on starch and glycogen

    • end-product = maltose

  • lingual lipase

    • acts on short chain and medium chain TGs​

    • end-product = FAs and monoglycerides

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PANCREAS

  • enzymes made and secreted by the pancreas

  • ​active​

    • pancreatic amylase​

      • acts on starch, glycogen

    • pancreatic lipase

      • acts on TGs and cholesterol​

  • inactive

    • packaged in pancreas as zymogens​

    • CCK stimulates

    • inactive zymogens secreted into SI

      • activated in SI​

      • digest proteins

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STOMACH

  • pepsin​

    • activated by gastric acidity​

    • pepsinogen -> pepsin

    • works on protein

    • end product = large peptide fragments and peptones

  • gastric lipase

    • acts on short chain and medium chain TGs​

    • end-product = FAs and monoglycerides

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INTESTINE

  • Enterokinase

    • involved in protein digestion (activates trypsin)​

  • digestion completed by intestinal enzymes at brush border membrane of enterocytes

  • aminopeptidase

    • peptides -> amino acids​

  • disaccharidases

    • work on sucrose, maltose and lactose​

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Overview

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©2023 by Syracuse University Dr.Margaret Voss

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