What is the role of tight junctions? (3)
- HOLD epithelial cells together at luminal edges
- BARRIER to substances in intracellular space
- FENCE; prevent proteins from diffusing in plane of bilayer
What is the difference between transcellular and paracellular transport?
Transport between lumen and blood
- T: across apical and basolateral membrane
- P: through lateral intercellular space
What are the characteristics of epithelial tissue? (5)
- Cells arranged in continuous sheets
- Cells sit on basement membrane
- Form boundary between organs or external environments
- Constant rapid renewal
Describe paracellular transport (4)
- Does not require transport protein
- Governed by laws of diffusion and tightness of junctions
- Can measure electrical resistance (higher = tighter)
- Relates to leaky epithelium
Describe transcellular transport
- Uses PAT & SAT with passive diffusion through channels
- Can be absorption/secretion
- Relates to tight epithelium
What is absorption?
What is the entry step?
Transport from lumen to blood
Entry step: apical membrane
What is secretion?
What is the entry step?
Transport from blood to lumen
Entry step: basolateral membrane
What are the rules of tranepithelial transport? (4)
- Entry/exit steps: secretion or absorption?
- Electrochemical gradient: passive or active?
- Electroneutrality: movement of +/- will attract another ion
- Osmosis: movement of ions will cause change in conc = water flow
What is membrane permeability to water (Pw)?
- Mediated by aquaporins (integral membrane protein channels for water)
- Cells have different aquaporin = different Pw
How does transepithelial transport work? (3)
- PAT sets up ion gradients (Na/K ATPase)
- Entry step (often SAT)
- Exit step (often passive diffusion)
What are the steps of glucose absorption? (5)
- Na-pump (PAT) in APICAL sets up ion gradients
- Na-glucose symporter (SGLT) uses energy of gradient to accumulate glucose above conc gradient
- Glucose transporter (GLUT) mediates glucose exit across BASOLATERAL membrane DOWN gradient
- Na taken up via glucose exits via basolateral Na pump
- Induced paracellular Cl and water fluxes
What is oral rehydration therapy? (2)
- Sugar, salt, water solution
- Stimulates isotonic fluid absorption
What is glucose-galactose malabsorption syndrome?
What does this result in?
- Mutation in glucose symporter in small intestine
- Glucose accumulates in intestine lumen (increases osmolarity = water efflux = diarrhea)
What is the treatment for glucose-galactose malabsorption?
Why does this work?
- Use fructose as carb instead of glucose
- This uses other transporter (GLUT5)
What is glucose reabsorption in the kidney?
Glucose in plasma is filtered and need to be reabsorbed or it will appear in urine = gylcosuria
What is glucosuria?
“Glucose in the urine”
- Glucose symporter cannot absorb glucose fast enough = appears in urine
- Common cause is diabetes
