Transport Along and Across GI Tract PART 2 Flashcards
Describe the motor activity in the small intestine in the fed state.
- Mixing contractions, i.e segmentation, which mixes and stirs the contents with enzymes, and prevents an unstirred layer formation.
- Peristaltic contractions - move the contents in an oral to anal direction
Describe motor activity in the small intestine in the fed state with reference to the nervous system.
- Usually a reflex mediated by ENS
- Sympathetic innervation inhibits motility
- Parasympathetic innervation stimulates motility
What two emotions can decrease intestinal motility?
PAIN
FEAR
Describe storage in the large intestine.
- Occurs whilst water is absorbed from the contents.
- Intensive mixing and slow movements of waste and indigestible material occur aborally.
What are the purpose of fermentation chambers in the large intestine?
Hydrolysis of fibre and indigestible nutrients, leading to faeces formation.
Describe the motility of the large intestine. PART 1
- Segmental contractions mix the contents - using taenia coli longitudinal muscle.
- Peristalsis: slow in the large intestine in comparison to the small intestine, moves the contents towards the anus; distention initiates contraction.
Describe the motility of the large intestine. PART 2
Mass movement: powerful contraction of the mid-transverse colon that sweeps the colon contents into the rectum (responsible for colonic evacuation).
SUMMARY - What are the three main features of motility in the large intestine?
Fermentation
Intensive mixing and slow aboral flow
How does the hypothalamus detect food in the gut? PART 1
- Outer muscle layer of the gut is covered by the serosa, which is continuous with the mesentery containing blood vessels and nerve fibres.
- Sensory neurones are connected to the mucosal chemoreceptors, which detect different chemical substances in the gut lumen
How does the hypothalamus detect food in the gut? PART 2
- Sensory neurons also connected to stretch receptors, which respond to the tension in the gut wall, caused by the food and chyme.
FPP RECAP: Describe the sympathetic ganglia and where they are found.
- Found along length of sympathetic trunk
- Distinguished as cervical, thoracic, lumbar, and sacral - based on position on spinal cord
Describe carbohydrate (CHO) digestion and absorption.
- Can only be absorbed in the form of monosaccharides
- Complex carbohydrates reduced to disaccharides by amylase
- Brush border enzymes convert disaccharides to monosaccharides (eg. glucose)
Describe the transport of galactose across the apical membrane.
- Actively transported by the luminal glucose carrier system
- Competitive inhibitor of glucose transport.
Describe the transport of fructose across the apical membrane.
- No effect on the absorption of glucose and galactose.
- Absorbed by a carrier-mediated, facilitated diffusion system - energy is not required.
For active transport, where is the sodium-potassium ATPase pump found?
Basolateral membrane.
Describe the movement of fructose from the intestinal lumen to the blood and various tissues.
- After apical transport mediated by GLUT5, fructose is transported across the basolateral membrane by GLUT2.
How is GLUT2 different to GLUT5?
- GLUT2 transports fructose, glucose and galactose. GLUT5 is only specific to fructose
- GLUT2 is sensitive to phloretin and cytochalasin B. GLUT5 is not.
What is the role of GLUT2?
- Fructose uptake across hepatic plasma membrane into liver
- Fructose uptake across basolateral membrane of intestinal/renal epithelial cells
Describe protein digestion.
- Polypeptides produced by action of pepsin.
- Polypeptides, di- and tri- peptides produced by action of pancreatic proteases.
- Di-peptidases in the brush border complete digestion to amino acids.
Describe protein absorption.
Hydrolytic digestive products are absorbed intact across the intestinal mucosa and into the blood.
Describe amino acid transport.
- Transported on a sodium-coupled carrier system.
- Separate carriers for different types of amino acids.
Describe di/tripeptide transport.
Carrier system using an inwardly directed H+ gradient.
Give examples of dietary lipids.
- Triglycerides(TGs) - make up majority of dietary lipids.
- Phospholipids
- Cholesterol
- Vitamins (A,D,E,K)
Generally, describe the digestion of lipids.
- Dietary TGs are broken down into simpler units to facilitate absorption.
- In the mouth, salivary lipase digests a small fraction of the TGs.
- Most dietary TGs are digested in the small intestine.
What must happen to triglycerides before digestion?
Dissolved in the aqueous phase
In detail, describe the digestion of lipids. PART 1
- Facilitated by emulsification and micelle formation
- Gastric lipase breaks down approximately 10-30% of fats, the remainder is digested by pancreatic lipase.
In detail, describe the digestion of lipids. PART 2
- Lipase action requires the emulsification of TGs by bile salts (which dissolve TGs in water).
- Pancreatic lipase binds to the surface of the small emulsion particles.
Describe micelles.
- Lipid molecules that arrange themselves in a spherical form in aqueous solutions.
- Micelle formation is response to the amphipathic nature of fatty acids
FLC RECAP: What does it mean for a fatty acid to be amphipathic?
Contain both hydrophilic and hydrophobic regions (polar head group and long hydrophobic chain).
Describe mixed micelle formation.
- Simple lipid molecules diffuse into the lipophilic core of the simple bile micelle and form mixed micelle.
What are some of the simple lipid molecules used in mixed micelle formation?
- Cholesterol, phospholipids, fatty acids, 2-monoglycerides, fat-soluble vitamins, and lyso-lecithin
What happens to the mixed micelle following formation?
Carry the lipids that are absorbed by intestinal microvilli
Summarise the digestion and transport of lipids. PART 1
- Dietary fat composed of TGs is emulsified by the action of bile salts into a suspension of fat droplets.
- Lipid emulsion prevents the fat droplets from coalescing and increases surface area available for attack by pancreatic lipase.
Summarise the digestion and transport of lipids. PART 2
- Lipase hydrolyses TGs into monoglycerides and free fatty acids.
- Water-insoluble products are carried in the interior of water-soluble micelles, to the luminal surface of the small intestine epithelial cells.
- Micelles formed by bile salts and other bile constituents
Summarise the digestion and transport of lipids. PART 3
- When a micelle approaches the absorptive epithelial surface, the MGs and fatty acids leave the micelle and passively diffuse through the lipid bilayer of the luminal membranes.
Summarise the digestion and transport of lipids. PART 4
- Monoglycerides and free fatty acids are resynthesised into TGs in the epithelial cells.
- TGs aggregate and are coated with a layer of lipoprotein to form water-soluble chylomicrons
Summarise the digestion and transport of lipids. PART 5
- Chylomicrons extrude through the basal membrane of the cells by exocytosis.
- Chylomicrons are unable to cross the basement membrane of blood capillaries, so instead they enter the central lacteals.
Describe disorders of fat digestion/ absorption.
Gallstones, pancreatitis, Crohn’s Disease, and liver disease can lead to fat malabsorption (steatorrhoea or fat-diarrhoea = excess fat in the faeces).
Where do bile salts go following lipid digestion?
→ Recycled by enterohepatic circulation
How do bile salts work?
→ They make the lipid molecules much smaller
→ Enzymes from the pancreas can break down the lipid and get to the core
What does the duodenum detect?
Lipids which must be emulsified
How is pepsin secreted and how is it activated?
→ Secreted as a zymogen (pepsinogen)
→ HCl converts it into pepsin
List the equations for the breakdown of the following?
SUCROSE
LACTOSE
GLUCOSE OLIGOMERS
Sucrose (+ sucrase) → Glucose + fructose
Lactose (+lactase) →Glucose + galactose
Glucose oligomers (+glucoamylase) → glucose
Describe the way glucose and galactose are transported.
→ On the basolateral side Na+ is transported into the blood
→ Gradient is created for Na+ to come into the lumen
→ Driving force of Na+ allows glucose to come with it
What is the glucose/galactose co-transporter called?
SGLT-1
What are adaptations of the small intestine to absorb food?
→ Epithelial folds
→ Villi + microvilli
→ Increase SA
What is absorbed when it enters the upper small intestine?
→ Dietary nutrients
→ Water
→ Electrolytes
The intestines contain crypt cells. What is kept within these cells?
Stem cells
What are the two modes of transport across enterocytes?
→ Transcellular
→ Paracellular
