Lecture 17: Glucose transport Flashcards
What are the main functions of epithelial tissues?
Epithelial tissues consist of cells arranged in continuous sheets, form boundaries between organs and the external environment, undergo rapid renewal, and are subject to physical breakdown and injury.
What is the role of tight junctions in epithelial cells?
Tight junctions restrict intercellular space, welding cell membranes together, and allow selective passage of certain ions while holding epithelial cells together at their luminal edges.
What are the two types of transport across epithelial cells?
Transcellular transport, which occurs through the cell, and paracellular transport, which occurs between cells.
How do epithelial cells mediate absorption and secretion?
Epithelial cells can either absorb substances from the lumen into the bloodstream or secrete substances from the bloodstream into the lumen, depending on the physiological needs.
What is glucose absorption in the intestine and kidney?
Glucose is absorbed in the intestine and kidney through specific transport proteins, and conditions like glucose/galactose malabsorption syndrome and glucosuria can affect this process.
What happens in glucose/galactose malabsorption syndrome?
This syndrome affects the absorption of glucose and galactose in the intestine, leading to gastrointestinal symptoms and nutrient deficiencies.
What is glucosuria in the kidney?
Glucosuria is the presence of glucose in the urine, often due to the kidney’s inability to reabsorb glucose effectively, which can occur in conditions like diabetes.
What are the two main functions of tight junctions?
Tight junctions act as a barrier, restricting the movement of substances through intercellular spaces
As a fence, preventing membrane proteins from diffusing between adjacent cells.
What are the two distinct membrane domains created by tight junctions in epithelial cells?
The two distinct membrane domains are:
Apical (luminal) membrane facing the lumen
Basolateral membrane adhering to the basement membrane and interfacing with the blood.
How do tight junctions influence transcellular and paracellular transport?
Tight junctions create distinct membrane domains that allow different transport proteins to be inserted, facilitating either transcellular or paracellular transport, or both.
What governs paracellular transport in epithelial tissues?
Paracellular transport is governed by the laws of diffusion and the tightness of the junctions, with higher electrical resistance indicating more restrictive junctions.
What are the characteristics of leaky and tight epithelia?
Leaky epithelium has low electrical resistance and allows paracellular transport to dominate, while tight epithelium has high electrical resistance and relies more on transcellular transport.
How does tight junction resistance change along the GI tract and kidney?
Tight junction resistance changes from proximal to distal, with leaky epithelia in the proximal region allowing bulk transport and tight epithelia in the distal region being hormonally controlled.
What types of transport do epithelial cells use for transcellular transport?
Epithelial cells use primary and secondary active transport, often in combination with passive diffusion through ion channels, to facilitate transport across epithelial tissues.
What is the difference between absorption and secretion in epithelial transport?
Absorption refers to the transport of substances from the lumen to the blood, while secretion refers to the transport of substances from the blood to the lumen.
What are the key steps in transepithelial transport?
Entry and exit steps: the entry step for absorption is the (lumen to cell to blood) apical but for secretion is the basolateral membrane (basolateral membrane to cell to lumen
Electrochemical gradient: is the entry or exit step passive or active? (one is passive, other is active)
Maintaining electroneutrality: movement of a positive or negative ion will attract a counter ion (naturally wants to maintain neutrality)
The role of osmosis: net movement of ions will establish a difference in osmolarity that will cause water to flow by osmosis
How does the electrochemical gradient affect transepithelial transport?
The entry or exit step can be passive or active, with the Na/K pump creating an electrochemical gradient that drives sodium influx and glucose absorption.
What is the role of osmosis in transepithelial transport?
Net movement of ions establishes a difference in osmolarity, causing water to flow by osmosis, which is essential for maintaining fluid balance.
How do aquaporins influence water movement in epithelial cells?
Cells express different aquaporin isoforms, allowing for varying permeability to water, with most water moving through water channels.
What is the process of glucose absorption in the small intestine?
- Tight junctions divide cells into apical and basolateral membrane domains
- Na-pump sets up ion gradient (primary transport example)
- The sodium glucose symporter (SGLT) uses the energy of the Na gradient to actively accumulate glucose above its concentration gradient
- Facilitative glucose transporter (GLUT) mediates glucose exit across the basolateral membrane via passive diffusion down its gradient
- Na+ is removed via the glucose exits via the basolateral Na-pump
- The transport of Na+ and glucose across the epithelium induces paracellular Cl- and water influxes, maintaining the osmolarity of the blood.
What is the significance of oral rehydration therapy?
Oral rehydration therapy utilizes glucose to enhance Na+ absorption, which helps retain Cl- and water, saving lives in dehydrated individuals, especially children with diarrhea.
What is glucose-galactose malabsorption syndrome?
This syndrome results from a mutation in the glucose symporter, preventing glucose reabsorption, leading to increased osmolarity in the intestinal lumen and causing diarrhea.
How does increased osmolarity in the intestinal lumen affect water movement?
Increased osmolarity draws water into the intestinal lumen, resulting in pronounced diarrhea due to the efflux of water.
What is the primary treatment for glucose-galactose malabsorption?
The primary treatment is to remove glucose and galactose from the diet and use fructose as a carbohydrate source, utilizing the GLUT5 transporter that is specific to fructose.
