Chapter 2 of 5 - Physiology Course
Membrane Transport & Resting Potential
Every action potential, synapse, and secretory event starts with gradients across the lipid bilayer. This chapter links diffusion, carriers, pumps, and the electrochemical forces that set the resting potential.
Board questions often test whether you can separate passive movement (down electrochemical gradients) from primary active transport (ATP hydrolysis on the transporter) and secondary active transport (ion gradients as the immediate energy source). The diagrams below compress those distinctions into exam-style sequences you can redraw from memory.
The Lipid Bilayer as a Barrier
Small nonpolar molecules (O2, CO2) cross freely. Polar molecules and ions need channels or transporters. Conductance describes how easily an ion species crosses; permeability is often dominated by open leak channels for K+ and Na+ at rest.
The electrochemical gradient combines chemical (concentration) and electrical (charge) driving forces. Ions move toward equilibrium when the net driving force is zero - hence distinct equilibrium potentials for Na+, K+, Ca2+, and Cl-. Drugs and toxins frequently act by shifting conductance (opening or blocking channels) or by disabling pumps, which secondarily collapses gradients needed for secondary active transport in kidney and gut epithelia.
Passive Transport
- Simple diffusion - movement down a chemical gradient through the bilayer or through pores.
- Facilitated diffusion - carrier or channel-mediated movement still down electrochemical gradient (for example GLUT transporters, aquaporins).
Water follows osmotic gradients; in many tissues, aquaporins determine how rapidly cells or epithelial layers can equilibrate tonicity. Glucose enters many cells through GLUT family transporters that do not consume ATP directly - a contrast with SGLT-mediated uptake in kidney and intestine, which is coupled to the inward Na+ gradient established by the Na/K pump.
If movement is down the combined electrical and chemical gradient without direct ATP coupling on that step, it is passive; if ATP or a pre-built ion gradient powers the step, classify as primary or secondary active transport.
Solute must cross membrane
Polar, charged, or large molecules usually need proteins.
Down electrochemical gradient?
If yes → passive: simple diffusion, channel, or facilitated carrier.
ATP hydrolysis coupled?
If yes → primary active (Na/K-ATPase, H+-ATPase, Ca-ATPase).
Uses ion gradient (often Na+)?
If yes → secondary active: symport or antiport.
Na/K-ATPase maintains low intracellular Na+ and high intracellular K+, powering secondary transport and setting baseline gradients.
Adenosine triphosphate (ATP)
Hydrolysis of ATP to ADP + Pi provides the free energy for primary active transport, including the Na/K pump.
Formula
C10H16N5O13P3
Mol. Weight
507.18 g/mol
Active Transport
Primary active transport couples ATP hydrolysis to vectorial ion movement. Secondary active transport (cotransport or exchange) uses the Na+ gradient as a battery - critical in kidney tubules and intestinal absorption.
Because the Na/K-ATPase is electrogenic (three positive charges out, two in per cycle), it makes a small direct contribution to the negative intracellular voltage while chiefly maintaining the steep Na+ and K+ gradients. Inhibiting the pump (for example with cardiac glycosides in clinical use) raises intracellular Na+, which can slow secondary transporters such as the Na/Ca exchanger - one link between membrane biophysics and cardiac contractility that appears in integrated physiology questions.
Excitable cells add voltage-gated channels for action potentials; this sequence explains the baseline before depolarization.
Na/K-ATPase sets ion asymmetry
Low intracellular Na+, high intracellular K+ (relative to ECF).
Leak channels set resting permeability
Often dominant K+ leak → Vm biased toward EK.
Goldman-style combination
If PNa or PCl rise, Vm shifts away from EK toward ENa or ECl.
Resting Vm near steady state
Pump rate matches passive leak; metabolism supplies ATP.
Quick Check
Which process directly consumes ATP to move ions against their electrochemical gradients?
Fill in the Blank
At rest, many excitable cells have high permeability to________, so resting potential is closest to that ion's equilibrium potential (unless other conductances dominate).
Resting Membrane Potential (Conceptual)
The Nernst equation gives the equilibrium potential for one ion at a given inside/outside concentration ratio. The Goldman-Hodgkin-Katz formulation combines relative permeabilities of Na+, K+, and Cl- to predict membrane voltage - the conceptual backbone for understanding depolarization, repolarization, and excitability.
You do not need to memorize logarithmic forms for most Step-style items, but you should state the direction of change: raising extracellular K+ depolarizes resting Vm; blocking Na+ channels typically leaves Vm less sensitive to ENa unless other conductances change. In epithelia, asymmetric transporter expression between apical and basolateral membranes generates transepithelial ion flux that underpins absorption and secretion - the same gradient logic applied at the tissue scale.
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