Adam Macierzanka [email protected]
Transport of food/nutrients in the gastrointestinal tract
Transport of food along the GI tract, Roles of the individual parts of the
GI tract, Transport across the GI tract mucosa Role of the mucus layer, Role of the chemical compounds e.g.
bile salts, phospholipids, Absorption of nutrients.
The digestive system includes many organs, and each organ has many various regions. The main tract through the digestive system (from the mouth, through the stomach and intestines) is called the gastrointestinal tract (GI) or the alimentary canal.
The Mouth
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breaking large pieces into small pieces, resulting in a massive increase in surface area, which is where digestive enzymes work,
warming or cooling of the food, softening of food and transformation into a size conducive to
swallowing, lubrication of food by mixing it with saliva.
Mastication (grounding, chewing) is the first step in the breakdown of complex foods and serves several functions, including:
Lubrication and binding: the mucus (mucins) in saliva is effective in binding masticated food into a slippery bolus that slides easily through the esophagus without inflicting damage to the mucosa.
Coats the oral cavity and esophagus: food basically never directly touches the epithelial cells of those tissues.
Solubilizes dry food: in order to be tasted, the molecules in food must be solubilized.
Oral hygiene: Saliva contains lysozyme, an enzyme that lyses many bacteria and prevents overgrowth of oral microbial populations.
Initiates digestion: Saliva contains an alpha-amylase which can begin to digest dietary starch into maltose.
Saliva is a watery substance in the mouths secreted by the salivary glands. Human saliva is composed of 99.5% water; the other 0.5% consists of electrolytes, mucus, enzymes, and antibacterial compounds.
adapted from: R. Bowen, Colorado State University ©
Mouth → Stomach
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The Stomach
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The wet mass of broken up food and digestive chemicals is called chyme.
• It serves as a short-term storage reservoir, allowing a meal to be consumed quickly and dealt with over an extended period of time.
• In the stomach a substantial enzymatic digestion is initiated, particularly of proteins by pepsin. Low pH (pH 2-3).
• Contractions of gastric smooth muscle mix food with gastric secretions, resulting in liquefaction of food, a prerequisite for delivery of the ingesta to the small intestine.
• As food is liquefied, it is slowly released into the small intestine for further processing.
The Stomach
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MRI images of the human stomach immediately prior to (left) and immediately after (right) consumption of a liquid meal. Also shown is a schematic representation of the process in which the area of the green region (gastric secretion) is the same in both shapes. A. R. Mackie, A. N. Round, N. M. Rigby, A. Macierzanka; Food Digestion (2012) 3:8–15
The Stomach
The thicknesses of the loosely adherent and firmly adherent mucus layers in vivo in the rat gastrointestinal tract (from C. Atuma et al., Am. J. Physiol. Gastrointest. Liver 280 (2001) G922–G929).
The Stomach
Possible role of an extracellular lining of zwitterionic phospholipids in generating the hydrophobic barrier of the stomach to luminal acid.
From: L.M. Lichtenberger, Annu. Rev. Physiol., 1995, 57, 565–583, and L. M. Ensign, R. Cone, J. Hanes; Advanced Drug Delivery Reviews, 2012, 64, 557-570.
Phospholipid Interactions Protect the Milk α-Lactalbumin from Pepsinolysis during in vitro Digestion
SDS-PAGE analysis of α-La subjected to gastric digestion over 120 min in the absence (A) or presence (B) of phosphatidylcholine (PC) F. J. Moreno et al., J. Agric. Food Chem. 2005, 53, 9810-9816
© R. Lentle, and Springer (Food Digestion journal website)
The Stomach
The Stomach
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The Small Intestine
The average length in an adult human male is 7 m.
It is approximately 2.5–3 cm in diameter.
Consists of: • Duodenum • Jejunum • Ileum Copyright © WordPress
The small intestine is the main site for digestion and absorption of nutrients.
Enzymatic Transport into the Duodenum
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Most digestion occurs in the upper portion of the small intestine, called the duodenum.
Absorption of nutrients
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The structure of the small intestine, in place to aid absorption of nutrients, consists of three levels: on the macro-scale, folds or plicae intrude into the lumen; on the meso-scale villi cover the surface; and on the micro-scale microvilli cover the villi on a cellular level.
• The surface area of the intestine is well suited to its function: the structural adaptations of mucosal folds, villi and microvilli give a total absorptive area for the small intestine of 200-500 m² (600-fold increase over the projected surface area of a smooth-surfaced tube of equivalent length). The small intestine normally absorbs far in excess of body needs – the major portion of this organ can be removed without deleterious effect.
Absorption of nutrients
• The major function of the small intestine is absorption of nutrients, water, electrolytes, etc.
• ~ 90% of nutrients from ingested food are absorbed in the small intestine
About five million villi (up to 1mm long) line the small intestine
Dorling Kindersley, ©2011; © DK Medical Encyclopaedia
Intestinal Motility
• contractions of small intestine serve three functions: – to mix chyme with intestinal juice, bile, and pancreatic juice
• to neutralize acid • digest nutrients more effectively
– to churn chyme and bring it in contact with the mucosa for contact digestion and nutrient absorption – to move residue toward large intestine
Intestinal Motility Segmentation & Peristalsis
Segmentation – the movement in which stationary ringlike constrictions appear in several places along the intestine – they relax and new constrictions form elsewhere – most common kind of intestinal contraction – pacemaker cells in muscularis externa set rhythm of segmentation • contractions about 12 times per minute in the duodenum • 8 to 9 times per minute in the ileum
Peristalsis – The Migrating Motor Complex
• gradual movement of contents towards colon • peristaltic wave begins in the duodenum, travels 10 to 70 cm and dies out • followed by another wave starting further down the tract
• migrating motor complex – successive, overlapping waves of contraction
• move chyme toward colon over a period of two hours • ileocecal valve usually closed
• food in stomach triggers gastroileal reflex that enhances segmentation in the ileum and relaxes the valve
• as cecum fills with residue, pressure pinches the valve shut (prevents reflux of cecal contents into the ileum)
The terminal ileum
Silva A C et al. Radiographics 2007;27:1039-1054
©2007 by Radiological Society of North America
Food bolus propelled through the intestine via peristaltic contractions
Mucins adhere to the food, wrapping it in a ‘blanket’ of mucus.
The shear thinning properties of the mucus allow the bolus to pass without perturbing the epithelium.
Enzymes that can pass through the mucus will begin to digest the food, extracting nutrients.
PERISTALSIS
from L. M. Ensign, R. Cone, J. Hanes; Advanced Drug Delivery Reviews, 2012, 64, 557-570.
BOLUS
Pig small intestine (jejunum)
Mucus Lamina propria
Lumen
Pig small intestine (jejunum)
The thicknesses of the loosely adherent and firmly adherent mucus layers in vivo in the rat gastrointestinal tract (from C. Atuma et al., Am. J. Physiol. Gastrointest. Liver 280 (2001) G922–G929).
Glycoprotein MUCIN monomer consisting of glycosylated regions flanked by regions with relatively little glycosylation.
from R. Bansil, B. S. Turner; Current Opinion in Colloid & Interface Science 11 (2006) 164 – 170
The cellular assembly of mucin MUC2, showing the main domains in the primary sequence. The monomer shows the highly variable main tandem repeat region also known as the proline, threonine, and serine (PTS)-rich region in yellow. Also shown in various colors are the terminal domains that are so similar to vWF, all of which are disulfide-rich. In the Golgi and granule, the heavy glycation of the PTS regions are shown in green. This also highlights the formation of dimers, trimers, and higher oligomers. A. R. Mackie, A. N. Round, N. M. Rigby, A. Macierzanka; Food Digestion (2012) 3:8–15
Small intestinal MUC2 mucins form trimers
Biomacromolecules (2012) 13, 3253
Andrew Round University of east Anglia
AFM data
250 nm
Columnar epithelial cells
Goblet cell
VILLUS
20
0 μ
m
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Electron micrograph of a goblet cell from the small intestine secreting mucus to the extracellular space.
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20
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Mucin monomer length (nm)
Small intestinal mucins (MUC2) form a network
A. N. Round et al., Biomacromolecules 2012, 13, 3253−3261
250 nm
A correct expansion of mucin granules requires a fast pH increase and Ca2+ removal (mucins are packed inside the goblet cell granule by Ca2+ -dependent cross-linking).
A lamellar model for the mucus barrier
AFM data show unpacking
and expansion of MUC2 mucin granules
MUC2 mucin forms porous
flat lamellar networks, linked
by covalent trimeric bonds
rather than by physical entanglement
There is little interaction between
lamellae. In vivo this predicts the
existence of transient channels forming between lamellae
Andrew Round et al. Biomacromolecules (2012) 13, 3253
Surface of the small intestine
50 um 100 um
100 um
Mucin DNA
Numerous pores that open into tubular glands on the floor of the small intestine between the bases of the villi. – similar to gastric glands – in upper half, have enterocytes and goblet cells like the villi – in lower half, dominated by dividing stem cells
• life span of 3 to 6 days • new epithelial cells migrate up the
crypt to the tip of the villus where it is sloughed off
– a few Paneth Cells are clustered at the base of each crypt
• secrete lysozyme, phospholipase, and defensins (defensive proteins that resist bacterial invasion of the mucosa)
Intestinal Crypts
The intestinal epithelium undergoes continuous self-renewal to maintain tissue homeostasis (stability) and eliminate damaged cells. The epithelial cells are constantly renewed through a process in which stem cells generated in the crypts migrate to the tip of the villi and ultimately peel off into the lumen. from H. Ashida et al., Nature Chemical Biology, 8, 36–45 (2012)
The epithelial cells shedding
Surface of the small intestine
J. Hanes et al., Adv. Drug Delivery Rev., 61 (2009) 158
Administration of particles to a mucosal
surface
Is the mucus layer a continuous homogenous barrier?
If not how does that affect transport of particulates?
TRANSPORT through the intestinal mucus
Mucin DNA Merge
31.25 x 31.25 μm
Confocal microscopy of the porcine small intestine mucus
The small intestinal mucus contains mucin and DNA
Mucin DNA Merge
62.5 x 62.5 um
Mucus from the small intestine of piglet
PIG vs. PIGLET
Transport of probe particles (latex beads) in the intestinal mucus
Enzymatic Transport into the Duodenum
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Food particulate covered with bile salts (negative charge)
- - - -
- - - -
Diffusion of solid particles (500 nm) coated with BS; 37oC)
100 μm
Mucus
bile salt
Maldano-Valderrama J., Wilde P., Macierzanka A., Mackie A.R. (2011) The role of bile salts in digestion. Advances in Colloid and Interface Science, 165 36-46
The role of BILE SALTS (BS) in the transport through the small intestine mucus
Diffusion of solid particles in the small intestine mucus
Ensemble mean-squared displacements (MSD)
- Distance travelled by the diffusing particles
+BS ζ-potential -50 mV
-BS ζ-potential -15 mV
Diffusion was driven by high negative charged provided by bile salts (BS) adsorbed to latex beads
The role of BILE SALTS (BS) in the transport through the small intestine mucus
CONFOCAL MICROSCOPY (diffusion of latex beads)
All three sizes of latex particles show the same viscosity distribution in
the presence of bile salts
A. Macierzanka et al., Soft Matter (2011) 7 8077-8084
In vitro gastro-duodenal digestion (proteolysis) of NaCas stabilised O/W emulsion in the presence or absence of bile salts (BS).
Transport of post-digestion emulsion droplets
Gastro-duodenal emulsions (GDEs), +/- BS
In vitro gastro-duodenal digestion (proteolysis) of NaCas stabilised O/W emulsion in the presence or absence of bile salts (BS).
Transport of post-digestion emulsion droplets
(A) Gastro-duodenal emulsion (GDE) droplet size distributions
(B) ζ-potential of GDEs
(C) GDE droplets interacting with intestinal ex vivo mucus in the presence or
absence of BS adsorbed onto the surface of droplets. Images 1 and 2 show
droplets in the absence of BS after 15-min and 3-h incubation, respectively.
Image 3 shows emulsion droplets in the presence of BS (30-min incubation).
A. Macierzanka et al., Soft Matter (2011) 7 8077-8084
Bile salts are key in transport of food particulates through mucus
Transport of post-digestion emulsion droplets
Interaction of bacteria with the ex vivo
intestinal mucus GFP E. coli (non-flagellated)
+ Bile Salts ζ ≈ -19mV
10 min
30 min
2.5 h
mucus
100 μm
- Bile Salts ζ ≈ -18mV
Weak negative charge of bacteria cells - no diffusion into the mucus!
Food Digestion, 2012, in press
Mucus: + Bile Salts, ζ ≈ -11mV - Bile Salts, ζ ≈ -10mV
Bacteria:
Fluorescence recovery after photobleaching (FRAP)
Aggregated networks of seed storage globulins from peanut and soybean render them resistant to proteolysis
Gelation of peanut 7S at 100 oC
4 min
15 min
30 min
Confocal microscopy (FRAP method): diffusion of FITC-labelled pepsin in the protein gels
Peanut 7S gel:
D = 0.49 ± 0.14 μm2/s
Soybean 7S gel:
D = 0.75 ± 0.11 μm2/s
Digestion kinetics can be altered by structuring of colloids !
Slow diffusion of pepsin:
Peanut Soybean
Large majority of nutrients from the diet are absorbed through the epithelial cells lining the small intestine. In addition, the intestine absorbs water and electrolytes, thus playing a critical role in maintenance of body water and acid-base balance.
Absorption in the small intestine
Columnar Epithelial Cells (Enterocytes)
Goblet cell
Villus
20
0 μ
m
Microvilli
40 μm
© AnatomyBox © Gwen V. Childs
• The epithelial cells of the villi transport nutrients from the lumen of the intestine into capillaries (amino acids and carbohydrates) and lacteals (lipids).
• Each villus has a network of capillaries and fine lymphatic vessels (lacteals) close to its surface.
• The absorbed substances are transported via the blood vessels to different organs of the body where they are used to build complex substances such as the proteins required by our body.
• The food that remains undigested and unabsorbed passes into the large intestine. adapted from: Rhoades, R. and Pflanzer R., Eds. The Gastrointestinal System.
Human Physiology, 2nd Ed. Saunders College Publishing; 1992: 777-822
Absorption of nutrients
• The small intestine must absorb massive quantities of water. A normal person takes in 1 – 2 liters of dietary fluid every day. Another 6 – 7 liters of fluid is received by the small intestine as secretions from salivary glands, stomach, pancreas, liver and the small intestine itself.
• By the time the ingesta enters the large intestine,
approximately 80% of this fluid has been absorbed. • The absorption of water is absolutely dependent on
absorption of solutes, particularly sodium.
Absorption of Water and Electrolytes
• Sodium is absorbed into the cell by several mechanisms, but mainly by cotransport with glucose and amino acids - this means that efficient sodium absorption is dependent on absorption of these organic solutes.
• Absorbed sodium is rapidly exported from the cell via sodium pumps - when a lot of sodium is entering the cell, a lot of sodium is pumped out of the cell, which establishes a high osmolarity in the small intercellular spaces between adjacent enterocytes.
• Water diffuses in response to the osmotic gradient established by sodium - in this case into the intercellular space. It seems that the bulk of the water absorption is transcellular, but some also diffuses through the tight junctions.
• Water, as well as sodium, then diffuses into capillary blood within the villus.
Absorption of Water and Electrolytes
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The key concepts involved in assimilation of carbohydrates (such as starch, lactose and sucrose) are:
• Polysaccharides and disaccharides must be digested to monosaccharides prior to absorption. • The final enzymatic digestion that releases monosaccharides is
conducted by integral membrane enzymes of absorptive enterocytes (so-called "brush border hydrolases").
Absorption of Monosaccharides
Epithelial cell
Microvilli • Fructose enters the cell from the intestinal lumen via facilitated diffusion. • Glucose generated by digestion is
absorbed in the small intestine only by co-transport with sodium using the sodium-dependent hexose transporter.
The transport by the sodium-dependent hexose transporter involves a series of conformational changes induced by binding and release of sodium and glucose: 1. The transporter is initially oriented facing into
the lumen - at this point it is capable of binding sodium, but not glucose.
2. Sodium binds, inducing a conformational change that opens the glucose-binding pocket.
3. Glucose binds and the transporter reorients in the membrane such that the pockets holding sodium and glucose are moved inside the cell (two sodium ions are transported for each glucose).
4. Sodium dissociates into the cytoplasm, causing glucose binding to destabilize.
5. Glucose dissociates into the cytoplasm and the unloaded transporter reorients back to its original, outward-facing position.
Absorption of Glucose
adapted from: R. Bowen, Colorado State University ©
Alan R. Mackie and Adam Macierzanka; Current Opinion in Colloid & Interface Science 15 (2010) 102–108
The key concepts involved in assimilation of dietary proteins are: • Through the action of gastric and
pancreatic proteases, dietary proteins are hydrolysed predominantly into medium and small peptides (oligopeptides).
• Oligopeptides are further
hydrolysed by the brush border peptidases to free amino acids and very small peptides.
Absorption of Amino Acids and Peptides
Epithelial cell
Microvilli
• There is virtually no absorption of peptides longer than four amino acids. However, there is abundant absorption of di- and tripeptides in the small intestine. These small peptides are absorbed into the epithelial cell by cotransport with H+ ions via a transporter called PepT1.
• Once inside the cell, most of di- and tripeptides are digested into amino
acids by cytoplasmic peptidases and exported from the cell into blood.
Absorption of Peptides
• The mechanism by which amino acids are absorbed is conceptually identical to that of monosaccharides (i.e. sodium-dependent transporters).
• Thus, absorption of amino acids, like that of monosaccharides, contributes
to generating the osmotic gradient that drives water absorption.
Absorption of Amino Acids
In order for lipids to be absorbed, two processes must occur:
• Large aggregates of dietary triglycerides, which are virtually insoluble in an aqueous environment, must be broken down physically and held in suspension - a process called emulsification.
• Triglyceride molecules must be
enzymatically digested to yield monoglyceride and fatty acids, both of which can efficiently diffuse or be transported into the enterocyte
Absorption of Fats/Lipids (Triglycerides)
Absorption of Fats/Lipids (Triglycerides)
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• The major products of lipid digestion - fatty acids (FA) and 2-monoglycerides (MG) - enter the enterocyte by simple diffusion or via a specific fatty acid transporter protein in the membrane.
• FA and MG are transported into the endoplasmic reticulum, where they are used to synthesize triglyceride. Continuing in the Golgi, triglyceride is packaged with cholesterol, lipoproteins and other lipids into particles called chylomicrons.
Absorption of Fats/Lipids (Triglycerides)
• Chylomicrons are transported first into the lymphatic vessel (lacteal) that penetrates into each villus. Chylomicron-rich lymph then drains into the lymphatic system, which rapidly flows into blood.
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• The ileocecal valve of the ileum (small intestine) passes material into the large intestine at the cecum.
• Partly digested food moves through the cecum into the colon, where water and some nutrients and electrolytes are removed.
• The colon is responsible for reabsorption of 1.0 to 1.5 liters of electrolyte-rich fluid entering it each day from the proximal intestine.
• The remaining material, solid waste called stool, moves through the colon, is stored in the rectum, and leaves the body through the anal canal and anus.
The Large Intestine
• The large intestine is about 1.5 m long.
• It absorbs mainly water and minerals and produces and eliminates fecal matter.
Copyright © Nasir Alar; AlarGastro.com
versus
The large intestine primarily absorbs water, and compacts and dries out the fecal bolus: villi would be a hindrance to movement of the semi-solid fecal mass, and would likely be injured by its passage. Hence there are no villi in the large intestine, and in addition, there are numerous goblet cells whose secretions (mucins) act as lubrication for the moving material.
Copyright © Thomas Caceci
The Large Intestine
Section of large intestine showing goblet cells secreting mucus to the extracellular space of the intestinal crypt. Goblet cell are in larger numbers than in the small intestine.
Epithelial cell Goblet cell
Johansson et al., PNAS 2008, 105, 15064–15069
Johansson et al., PNAS 2008, 105, 15064–15069
The thicknesses of the loosely adherent and firmly adherent mucus layers in vivo in the rat gastrointestinal tract (from C. Atuma et al., Am. J. Physiol. Gastrointest. Liver 280 (2001) G922–G929).
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