L5 Transport energetics and organic solute transport I Flashcards
what’s active transport?
active transport: net flux of the solute across the membrane occurs against an opposing electrochemical potential
chemical potential equation
same formula with outside
often replace concentrations with activities (A)
electrochemical gradient Δ𝜇̃ formula (energy in J)
> 0 → ion moves inward
< 0 → ion moves outward
electrochemical gradient Δ𝜇̃ formula (in Volts)
Nerst Equation formula
ion with z=1 and at equilibrium
cotransport and counter-transport mechanisms
- cotransport: carrier binds at least 2 different substrates at different sites before transporting them across the membrane in the same direction
- counter-transport: carrier binds one molecule on each side + transports them through membrane in opposite directions
primary active transport characteristics + main pumps in kidney cells (3)
- directly uses ATP hydrolysis for “uphill” transport
- in kidney cells :
Na⁺/K⁺-ATPase : 3 Na⁺ out / 2 K⁺ in (maintains low intracellular Na⁺) ; 44.13 kJ/mol = 10.5 kcal/mol
H⁺-ATPase: pumps protons into the tubular lumen to acidify urine, especially in the collecting duct ; more acidic urine = more energy required
H⁺/K⁺-ATPase: found in the outer medullary collecting duct
secondary active transport characteristics + Na+/H+ exchanger mechanism
- uses energy from another solute’s “downhill” movement
- example: Na⁺/H⁺ exchanger (proton secretion driven by Na⁺ gradient) - large inward concentration gradient favoring Na⁺ influx into the cell provides the driving force for proton secretion from the cell to the lumen
how are membrane vesicles studied? (2 methods) what does it show? (3)
used to study carrier-mediated transport in isolation
- brush-border vesicles prepared from apical membrane of proximal tubule by making homogenized slices to renal cortex so brush border breaks off and forms small membrane vesicles
- vesicles separation
- load vesicles with solution to impose [ ] gradient
- are ideal for studying substrate specificity, stoichiometry, and coupling
- measure radiotracer uptake rates → shows initial overshoot in transport before gradient collapses and allows characterization of substrate specificity and stoichiometry of transporters
basolateral vesicles are harder to prepare (lack brush border)
what is transport affected by?
- pH differences
- Na⁺ availability
epithelial cell structure (2 domains and key features)
epithelia: continuous sheets lining organs (kidney, gut, etc.), structural asymmetry
apical / luminal side:
- faces the lumen of the tubule
- has brush border (dense microvilli for increased surface area)
basolateral / peritubular side:
- faces the interstitial space and capillaries
key features:
- tight junctions: separate apical and basolateral domains
- in proximal tubule: leaky (allow paracellular transport) - low transepithelial electrical resistance (6Ω·𝑐𝑚2)
- in distal nephron: tight (restricts ion movement) - high transepithelial electrical resistance (200Ω·𝑐𝑚2)
- gap junctions: allow communication between cells
- epithelial polarity: ensures directional transport (e.g., Na⁺ reabsorption from lumen to blood)
- heterogenous cell types in collecting duct: principal cells secrete K+ ; intercalated cells secrete H⁺ or HCO₃⁻
what did experiments in Necturus gallbladder shows on ion diffusion pathways?
- 96% of ion diffusion occurs via paracellular route (through tight junctions)
- voltage scanning detects current leaks above cell borders
- epithelial heterogeneity: principal cells secrete K⁺ ; intercalated cells secrete H⁺ or HCO₃⁻
- tight junctions can be: selective or non-selective, regulated dynamically
filtrate-to-plasma ratio (F/P ratio)
+ how does osmolality vary in the proximal tubule and why?
- indicates [solute] in the filtrate relative to plasma
- F/P = 1 → same concentration in filtrate as in plasma
- despite ~2/3 of water being reabsorbed in the proximal tubule, osmolality stays relatively constant because water follows solute reabsorption, maintaining balance
how does [Na+] concentration vary throughout the proximal tubule? why?
Na⁺ concentration remains nearly constant throughout the proximal tubule because water and sodium are reabsorbed at similar rates
Glucose Titration Curve characteristics
- _______ vs _______
- filtered glucose = ___ × ___ [glucose]
- reabsorption is saturable due to _______
- Tm (___ ___): _______
- threshold: plasma [glucose] (~___–___ mg/dL or ~___ mM) at which glucose starts appearing in the urine
- ____: gradual, non-sharp transition between full reabsorption and appearance in urine ; may be due to: _______, _______, _______
Glucose Titration Curve characteristics
- filtered glucose load vs reabsorbed/excreted glucose
- filtered glucose = GFR × plasma [glucose]
- reabsorption is saturable due to a limited number of glucose transporters
- Tm (transport maximum): plateau where all transporters are saturated
- threshold: plasma [glucose] (~180–200 mg/dL or ~10 mM) at which glucose starts appearing in the urine
- splay: gradual, non-sharp transition between full reabsorption and appearance in urine ; may be due to: nephron-to-nephron variability, delayed saturation of carriers, lack of balance between filtration and reabsorption
glucose falls dramatically in the ___ ___ ___ (almost to 0) → affects ___ ___
glucose is filtered at the ___ and reabsorbed in the ___ ___ (trans-___ reabsorption), especially the ___ segment
glucose reabsorption is done via specific ___ ___ transport:
- powered by ___ gradient from the ___ ATPase on the ___ side (electrogenic → depolarizes ___ membrane + generates lumen-___ transepithelial potential)
transport - apical membrane
___ ___ : use Na⁺ to bring glucose into the cell :
- S1 & S2: SGLT___ (___ Na⁺ : 1 glucose)
- S3: SGLT___ (___ Na⁺ : 1 glucose) → stronger Na⁺ gradient, reabsorbs glucose at lower concentrations
transport - basolateral exit:
- ___ transporters (facilitated diffusion) transport glucose out (primarly ___)
membrane transporters and inhibition - apical side:
- Na⁺/K⁺ ATPase (basolateral side) maintains Na⁺ gradient
- ___ inhibits SGLT → no glucose uptake
membrane transporters and inhibition - basolateral side:
- GLUT2 allows glucose to ___
- ___ also inhibits GLUT2 but doesn’t affect Na⁺ coupling
- Na⁺/K⁺ ATPase keeps Na⁺ low inside the cell, essential for glucose uptake
transport can be studied using:
- _______ (e.g., α-methyl-glucose)
- ___ ___
glucose falls dramatically in the early proximal tubule (almost to 0) → affects transepithelial voltage
glucose is filtered at the glomerulus and reabsorbed in the proximal tubule (trans-tubular reabsorption), especially the S1 segment
glucose reabsorption is done via specific secondary active transport:
- powered by Na⁺ gradient from the Na⁺/K⁺ ATPase on the basolateral side (electrogenic → depolarizes apical membrane + generates lumen-negative transepithelial potential)
transport - apical membrane
SGLT cotransporters : use Na⁺ to bring glucose into the cell :
- S1 & S2: SGLT2 (1 Na⁺ : 1 glucose)
- S3: SGLT1 (2 Na⁺ : 1 glucose) → stronger Na⁺ gradient, reabsorbs glucose at lower concentrations
transport - basolateral exit:
- GLUT transporters (facilitated diffusion) transport glucose out (primarly GLUT2)
membrane transporters and inhibition - apical side:
- Na⁺/K⁺ ATPase (basolateral side) maintains Na⁺ gradient
- phlorizin inhibits SGLT → no glucose uptake
membrane transporters and inhibition - basolateral side:
- GLUT2 allows glucose to exit
- phloretin also inhibits GLUT2 but doesn’t affect Na⁺ coupling
- Na⁺/K⁺ ATPase keeps Na⁺ low inside the cell, essential for glucose uptake
transport can be studied using:
- non-metabolizable analogs (e.g., α-methyl-glucose)
- radioactive tracers
how does transepithelial voltage vary across the epithelium and down the proximal tubule?
how does it influence solute fluxes across the epithelium in superficial nephrons vs in juxtamedullary nephrons
the basolateral membrane remains unaffected, so the net voltage across the epithelium becomes negative
voltage reverses around 25% down the proximal tubule:
- glucose and amino acid reabsorption stops
- chloride concentration increases in the tubule → creates a diffusion potential as Cl⁻ diffuses out of the tubule
- leaves the lumen electropositive compared to plasma
→ strongly influence solute fluxes across the epithelium because tight junctions are very leaky, especially in superficial nephrons
- juxtamedullary nephrons have low chloride conductance at the junctions, so this effect is less pronounced there.
