Synopsis
Explain how membrane structure, electrochemical gradients, transport proteins and intracellular compartments determine cell volume, excitability, secretion and common clinical disturbances.
- The phospholipid bilayer is selectively permeable: small non-polar molecules cross readily, while ions and most polar solutes require channels, carriers or vesicular transport.
- Passive flux follows an electrochemical gradient and does not directly consume metabolic energy; facilitated diffusion is saturable because a finite carrier population changes conformation.
- Primary active transport couples solute movement directly to an energy source, whereas secondary active transport uses a gradient previously created by another pump.
Reasoning priorities
Determine the chemical direction predicted for each transported species.
Write intracellular and extracellular concentrations separately, then add membrane voltage for ions; concentration alone can misstate net electrochemical drive.
Worked reasoning
A model cell contains 300 mOsm/kg of effectively non-diffusible intracellular solute. It is transferred from isotonic fluid into 250 mOsm/kg fluid whose added solutes cannot cross the membrane; water can equilibrate freely.
- Compare effective extracellular and intracellular particle concentrations at the moment of transfer: 250 outside versus 300 inside creates an inward water-driving gradient.
- Because the extracellular solutes are non-diffusible, they maintain the osmotic difference rather than rapidly entering and abolishing it.
- Water moves into the cell until the concentrations approach equilibrium, so cell volume increases; no new intracellular particles are required for swelling.
- The final qualitative outcome is cellular swelling, with lysis possible if volume reserve and regulatory mechanisms are exceeded.
- Verify by reversing the premise: if the external solute were freely permeant, osmolality could still be 250 initially but sustained tonicity and final volume prediction would differ.