2.3 Passive and Active Transport

Core Explanation

The Transport Spectrum: Passive → Active

Transport processes fall along a spectrum defined by two axes: (1) whether the solute moves down or against its electrochemical gradient, and (2) whether the process is directly or indirectly coupled to an energy source.

                    MOVES DOWN GRADIENT          MOVES AGAINST GRADIENT
                    (ΔG < 0, spontaneous)        (ΔG > 0, requires energy)
                    ─────────────────────        ──────────────────────────
NO PROTEIN          Simple diffusion             — (impossible)
                    
CHANNEL             Channel-mediated             — (impossible, channels
                    facilitated diffusion          are passive pores)

CARRIER             Carrier-mediated             Primary active transport
                    facilitated diffusion        (direct ATP hydrolysis)

COUPLED             —                            Secondary active transport
                                                 (coupled to downhill ion flow)

Simple Diffusion

The unassisted movement of a molecule through the lipid bilayer, driven solely by its concentration gradient (for uncharged solutes) or electrochemical gradient (for ions, though negligible for bare bilayer). Rate depends linearly on concentration difference. No protein is involved. No saturation. Example: O₂ entering respiring mitochondria; steroid hormones crossing the plasma membrane.

Facilitated Diffusion: Channels vs. Carriers

Facilitated diffusion uses transport proteins, but the solute still moves down its gradient. No metabolic energy is consumed. The two mechanisms differ fundamentally:

Channel-Mediated Facilitated Diffusion

Ion channels are transmembrane proteins that form water-filled pores. Key features:

• Selectivity: Determined by the narrowest part of the pore — the selectivity filter. In K⁺ channels, backbone carbonyl oxygens precisely mimic the hydration shell of a K⁺ ion, allowing dehydrated K⁺ to pass while excluding smaller Na⁺ (which cannot shed its hydration shell efficiently in the wider filter).
• Speed: Up to ~10⁸ ions per second — near the diffusion limit.
• Gating: Channels open and close in response to stimuli (voltage, ligand binding, mechanical force), but when open, they are passive pores.
• No saturation: Flux increases linearly with concentration. Unlike carriers, channels do not saturate because they do not undergo a conformational cycle per substrate.
• Examples: Voltage-gated Na⁺ and K⁺ channels (action potentials), nicotinic acetylcholine receptor (ligand-gated cation channel), aquaporins (water-specific channels — note: strictly speaking, aquaporins are channels, not carriers, though textbooks sometimes blur this).

Aquaporins deserve special mention. They form tetramers, with each monomer containing an independent water pore. A key structural feature is the two NPA (Asn-Pro-Ala) motifs that orient water molecules and a narrow aromatic/arginine (ar/R) constriction that excludes protons (H₃O⁺) and other ions. AQP1 can conduct ~3 × 10⁹ water molecules per second per subunit.

Carrier-Mediated Facilitated Diffusion

Carriers (also called transporters or permeases) bind their substrate on one side, undergo a conformational change, and release it on the other side. Key features:

• Specificity: Carriers recognize specific substrates via binding sites — analogous to enzyme–substrate binding.
• Saturation kinetics: Because each transport cycle involves a finite number of carrier molecules undergoing a conformational change, the transport rate saturates at high substrate concentrations. The kinetics follow Michaelis–Menten-like behavior: V = V_max[S] / (K_m + [S]). K_m reflects the affinity; V_max reflects the number and turnover rate of carriers.
• Slower than channels: Typical turnover is ~10² to 10⁴ molecules per second — because the carrier must physically cycle between conformations.
• Can be inhibited competitively: A structural analog can occupy the binding site without being transported.
• Example: GLUT1 (glucose transporter). GLUT1 alternates between an outward-open conformation (glucose binding site exposed to the extracellular side) and an inward-open conformation (binding site exposed to the cytoplasm). Glucose binds on one side, the carrier flips, glucose dissociates on the other side. Net flux direction is determined by the glucose concentration gradient.

Primary Active Transport

Primary active transporters use an energy source — almost always ATP hydrolysis — to pump solutes against their electrochemical gradient. They are effectively ATP-powered pumps.

Na⁺/K⁺ ATPase (The Sodium–Potassium Pump)

This is the most important pump in the animal kingdom. It is a P-type ATPase (it forms a phosphorylated intermediate during its cycle).

Structure: An αβ heterodimer. The α-subunit (~100 kDa) contains the ATP-binding site, the phosphorylation site (Asp369), and the ion-binding sites. The β-subunit is a glycoprotein required for proper folding and membrane targeting. In some tissues, a small γ-subunit (FXYD protein) modulates activity.

Stoichiometry: 3 Na⁺ out, 2 K⁺ in, 1 ATP hydrolyzed per cycle.

Electrogenicity: Because 3 positive charges leave and only 2 enter, each cycle generates a net outward current — the pump is electrogenic. It directly contributes ~5–10 mV to the resting membrane potential.

The Transport Cycle (Post–Albers Scheme):

• E1 state (high affinity for Na⁺, ATP-binding site faces cytoplasm):
  1. Three cytoplasmic Na⁺ ions bind to high-affinity sites on the E1·ATP form.
  2. ATP is hydrolyzed; the γ-phosphate is transferred to a conserved aspartate residue, forming the E1~P phosphoenzyme intermediate. Na⁺ becomes occluded (trapped inside the protein).
  3. A conformational change to the E2 state releases the three Na⁺ ions to the extracellular space and exposes high-affinity K⁺-binding sites.

• E2 state (high affinity for K⁺, facing extracellular side):
  1. Two extracellular K⁺ ions bind to high-affinity sites.
  2. K⁺ binding triggers dephosphorylation of the aspartyl-phosphate (E2~P → E2).
  3. Dephosphorylation triggers a conformational change back to E1, releasing two K⁺ ions into the cytoplasm. The cycle resets.

Why this matters: The Na⁺ gradient created by the pump is the primary ion-motive force of animal cells. It drives secondary active transport, maintains cell volume (osmotic balance), and establishes the resting membrane potential for electrical excitability. The pump runs continuously — it does not "turn off" while the cell is alive.

Pharmacology: Ouabain and related cardiac glycosides (digoxin/digitalis) bind to the extracellular face of the α-subunit in the E2 state and inhibit the pump. In cardiac myocytes, partial inhibition of the Na⁺/K⁺ ATPase raises intracellular Na⁺, which reduces the driving force for the NCX (Na⁺/Ca²⁺ exchanger), causing Ca²⁺ to accumulate and increasing cardiac contractility — the mechanism by which digitalis treats heart failure.

Ca²⁺ Pumps: SERCA and PMCA

Calcium is a powerful signaling ion — cytoplasmic [Ca²⁺] is maintained at ~100 nM, roughly 10,000-fold lower than extracellular [Ca²⁺] (~1–2 mM) and even lower relative to the ER lumen (~0.5–1 mM). This steep gradient must be actively maintained.

SERCA (Sarco/Endoplasmic Reticulum Ca²⁺-ATPase):

• Located in the ER/SR membrane.
• Pumps 2 Ca²⁺ ions from the cytoplasm into the ER lumen per ATP hydrolyzed.
• In muscle, SERCA pumps Ca²⁺ back into the sarcoplasmic reticulum to terminate contraction (muscle relaxation).
• Phospholamban (in cardiac muscle) inhibits SERCA; phosphorylation of phospholamban by PKA relieves this inhibition — a key mechanism for β-adrenergic acceleration of cardiac relaxation.

PMCA (Plasma Membrane Ca²⁺-ATPase):

• Located in the plasma membrane.
• Pumps 1 Ca²⁺ out of the cell per ATP (some reports suggest 1 Ca²⁺:1 ATP; the stoichiometry is still debated for some isoforms, but it is lower capacity than SERCA).
• Activated by calmodulin: when cytoplasmic Ca²⁺ rises, Ca²⁺–calmodulin binds to PMCA and stimulates its activity — a negative feedback loop.
• High affinity but low capacity: PMCA fine-tunes resting Ca²⁺ levels; SERCA handles bulk Ca²⁺ clearance.

Secondary Active Transport

Secondary active transporters do not directly hydrolyze ATP. Instead, they harness the electrochemical gradient of one solute (typically Na⁺ in animal cells, H⁺ in plants and bacteria), established by a primary pump, to drive the uphill movement of another solute. The energy is indirect — hence "secondary."

Symport (Cotransport): Both solutes move in the same direction across the membrane.

• SGLT1 (Na⁺/glucose symporter): In the intestinal epithelium and kidney proximal tubule, SGLT1 couples the downhill entry of 2 Na⁺ ions to the uphill uptake of 1 glucose molecule. The Na⁺ gradient (maintained by the basolateral Na⁺/K⁺ ATPase) provides the driving force. This is how we absorb glucose from the gut lumen even when gut glucose is low.
• Na⁺/amino acid symporters: Multiple families absorb different classes of amino acids using the same principle.

Antiport (Exchange): Solutes move in opposite directions.

• NCX (Na⁺/Ca²⁺ exchanger): Transports 3 Na⁺ into the cell (downhill) while pumping 1 Ca²⁺ out (uphill). The 3:1 stoichiometry makes NCX electrogenic. In cardiac myocytes, NCX is the primary Ca²⁺ extrusion mechanism during diastole.
• NHE (Na⁺/H⁺ exchanger): Exchanges extracellular Na⁺ for intracellular H⁺ — critical for pH regulation.

The critical insight: If you poison the Na⁺/K⁺ ATPase with ouabain, secondary active transport grinds to a halt — not because the transporters are directly inhibited, but because the Na⁺ gradient collapses, removing the driving force. Secondary active transport is energetically coupled to primary active transport.

Comparison: Transport Mechanisms

Detailed Mechanism: SGLT1 Symport as a Worked Example

1. Outward-facing conformation: The transporter is open to the extracellular side. Extracellular Na⁺ (~145 mM) is high; the SGLT1 Na⁺-binding sites have an affinity such that Na⁺ binding is favored.
2. Na⁺ binding: Two Na⁺ ions bind cooperatively. This induces a conformational change that increases the affinity of the glucose-binding site.
3. Glucose binding: Even at low luminal glucose, the Na⁺-bound transporter captures glucose efficiently.
4. Occlusion: The external gate closes, trapping Na⁺ and glucose within the protein. The transporter is now in an occluded state.
5. Conformational transition to inward-facing: The protein shifts to expose the binding sites to the cytoplasm. This transition is thermodynamically favored because the combined free energy of Na⁺ binding and the Na⁺ gradient is coupled to the conformational change.
6. Release: Cytoplasmic Na⁺ is low (~12 mM), so Na⁺ dissociates. Na⁺ dissociation reduces glucose-binding affinity, causing glucose to dissociate into the cytoplasm.
7. Reset: The empty transporter returns to the outward-facing conformation. The cycle repeats.

Key point: ATP is NOT consumed by SGLT1. But SGLT1 depends on the Na⁺ gradient actively maintained by the Na⁺/K⁺ ATPase. This is the essence of secondary active transport.

Energy Accounting

For the Na⁺/K⁺ ATPase, the free energy of ATP hydrolysis under cellular conditions is roughly −50 to −55 kJ/mol. The pump moves 3 Na⁺ and 2 K⁺ per cycle, doing the work of moving ions against their combined electrochemical gradients:

ΔG = 3 × (RT ln([Na⁺]_out/[Na⁺]_in) + zFV_m) + 2 × (RT ln([K⁺]_in/[K⁺]_out) + zFV_m)

This roughly matches −50 kJ/mol under typical cellular conditions. The pump operates near thermodynamic equilibrium — it is reversible: under extreme artificial gradients, it can synthesize ATP from ADP and P_i.

Experimental Evidence

• Skou (1957): Discovered the Na⁺/K⁺ ATPase in crab nerve membranes. Nobel Prize in Chemistry, 1997.
• Patch clamp (Neher and Sakmann, Nobel Prize 1991): Allowed direct measurement of single-ion-channel currents, proving that channels flicker between open and closed states.
• Crystal structure of SERCA (Toyoshima et al., 2000): The first high-resolution structure of a P-type ATPase, revealing the structural basis of the E1 → E2 transition and Ca²⁺ occlusion.
• GLUT1 structure (Deng et al., 2014): Crystallized in inward-open, outward-open, and occluded conformations, visually confirming the alternating-access model.

Disease and Clinical Relevance

• Digitalis (digoxin) therapy for heart failure: Inhibits Na⁺/K⁺ ATPase → ↑ intracellular Na⁺ → ↓ NCX-driven Ca²⁺ extrusion → ↑ intracellular Ca²⁺ → ↑ cardiac contractility.
• GLUT1 deficiency syndrome: Haploinsufficiency of GLUT1 → impaired glucose transport across the blood–brain barrier → infantile seizures, developmental delay, microcephaly. Treated with a ketogenic diet (bypasses glucose transport by providing ketone bodies).
• Glucose–galactose malabsorption: Mutations in SGLT1 → inability to absorb glucose and galactose from the gut → severe neonatal diarrhea. Treated with a fructose-based diet (fructose is absorbed by GLUT5, not SGLT1).
• Brody myopathy: Mutations in SERCA1 (fast-twitch muscle isoform) → impaired Ca²⁺ reuptake into the SR → delayed muscle relaxation after contraction (pseudomyotonia).
• Oral rehydration therapy: The most lifesaving application of transport physiology. In cholera, the toxin activates CFTR, causing massive Cl⁻ and water secretion. Giving a solution of glucose + Na⁺ exploits SGLT1 — glucose-stimulated Na⁺ absorption drags water along osmotically, rehydrating the patient even as secretion continues. This simple insight has saved tens of millions of lives.

Common Misconceptions