Ion transport across membranes

Lipid Bilayer as a Barrier

• Core = hydrophobic fatty acid tails

• Acts as an energy barrier to:

  • Charged species (Na⁺, K⁺, Cl⁻)

  • Polar molecules

Permeability Ranking

Highest → Lowest permeability:

1. Small nonpolar (O₂, CO₂)

2. Small uncharged polar (urea, ethanol)

3. Large uncharged polar (glucose)

4. Ions (Na⁺, K⁺, Ca²⁺) almost impermeable

Reason:

• Ions require desolvation → extremely energetically costly

Free Energy of Transport (ΔG)

For ions:

ΔG = RTln⁡([ion]in/[ion]out) + zFΔψ

Terms:

• RT ln(...) → chemical gradient (ΔGc)

• zFΔψ → electrical gradient (ΔGe)

Interpretation:

• ΔG < 0 → spontaneous (down gradient)

• ΔG > 0 → requires energy (active transport)

Electrochemical gradient = total driving force

membrane potential origin

• Unequal distribution of ions

• Selective permeability (especially K⁺)

If membrane permeable only to K⁺

1. K⁺ diffuses out (high → low)

2. Leaves behind negative charge

3. Creates electrical gradient pulling K⁺ back in

➡ Equilibrium reached when:

• Electrical force = chemical force

Nernst Equation (Equilibrium Potential)

The equilibrium voltage for one ion:

Eion = (RT/zF) ln⁡([ion]out/[ion]in)

Meaning

• Voltage where net ion movement = 0

• Each ion has its own equilibrium potential

Key Insight

• If membrane permeable to one ion → membrane potential = that ion’s Nernst potential

Channel structure

• Form continuous aqueous pore

• Often highly selective (size + charge filter)

  • e.g. K⁺ channel excludes Na⁺ despite similar size

    • Based on hydration shell + pore geometry

Channel mechanism

• No binding cycle required

• Ions flow rapidly down gradient

• Rate: up to 10⁸ ions/sec

• No conformational change per ion

Channel gate types

• Voltage-gated

• Ligand-gated

• Mechanically gated

Channel functional roles

• Action potentials (Na⁺, K⁺)

• Ca²⁺ signalling

• Osmoregulation (Cl⁻ channels)

Carriers

Core Principle: Alternating Access Model

• Binding site exposed:

  • First to one side

  • Then to the other

• Never open to both sides simultaneously

• ~10²–10⁴ molecules/sec

• Uniporters (Facilitated Diffusion)

• Symporters

• antiporters

• Symporters + antiporters = secondary active transport

  • Energy source = ion gradient (NOT ATP directly)

Why are carriers slower than channels

• Each cycle transports:

  • 1–few molecules

• Requires conformational change

Uniporters (Facilitated Diffusion)

• Transport single molecule

• Down concentration gradient

• No energy input

Example:

• GLUT1 (glucose uptake)

Symporters

• Same direction transport

• One solute moves:

  • Down gradient (driving ion)

• Second solute:

  • Up gradient

Example

Na⁺ + glucose symporter:

• Na⁺ gradient drives glucose uptake

Antiporters

• Opposite directions

Example

Na⁺/Ca²⁺ exchanger:

• Na⁺ in → Ca²⁺ out

Pumps (Primary Active Transport)

• Use ATP hydrolysis:

  • ATP → ADP + Pi

• Drives conformational changes

Key Property

• Move ions against electrochemical gradient

• Electrogenic: Unequal charge movement → voltage generated

Na⁺/K⁺ ATPase

1. Bind 3 Na⁺ (inside)

2. ATP phosphorylates pump

3. Conformational change → Na⁺ released outside

4. Bind 2 K⁺ outside

5. Dephosphorylation → returns to original state

6. K⁺ released inside

Net Effect

• 3 Na⁺ out / 2 K⁺ in

• Creates:

  • Na⁺ gradient

  • K⁺ gradient

  • Negative membrane potential

Energy Coupling in Transport

Primary Active Transport

• Direct ATP use

Secondary Active Transport

• Uses stored energy in ion gradients

Energy Cascade

ATP → Na⁺ gradient → glucose uptake

Key Insight: Gradients are a form of stored energy

Animal Cell transport (Na⁺ Economy)

Central Pump

• Na⁺/K⁺ ATPase

Why Na⁺?

• Strong inward electrochemical gradient

Uses

• Nutrient uptake (symport)

• Ca²⁺ removal (antiport)

• Electrical signalling

Key Point: Na⁺ gradient powers MANY processes

Plants & Fungi transport (H⁺ Economy)

Central Pump

• H⁺ ATPase

Generates

• Proton gradient + membrane potential

Uses

• Nutrient uptake:

  • K⁺, phosphate, sulfate via H⁺ symport

Special Role

• pH regulation

• Salinity tolerance (Na⁺/H⁺ antiport)

Bacteria transport

• No ATP pump for H⁺ initially

Instead:

• Electron transport chain pumps H⁺

Uses

• Nutrient uptake

• ATP synthesis (chemiosmosis)

Endomembrane transport

V-type H⁺ Pumps

• Acidify organelles

Functions

• Vesicle loading (neurotransmitters)

• Waste storage (vacuoles)

Electrogenic

• Net charge movement

• Alters membrane potential

Electroneutral

No net charge change

Transport Rates vs Abundance

Channels

• Fast → few needed

Pumps

• Slow → many required

Gradient dissipation (channels/carriers) > gradient generation (pumps)

Therefore:

• Pumps must be abundant

Amino Acid Uptake

1. Na⁺/K⁺ pump:

   • Uses ATP

   • Creates Na⁺ gradient

2. Na⁺ gradient:

   • Drives symporter

3. Symporter:

   • Imports amino acids

Conclusion: Transport systems work together, not independently

Channel vs Transporter vs Pump

Channels:

• Fast

• Passive energy

• Pore

Transporter:

• Medium

• Passive/secondary energy

• Conformational

Pump:

• Slow

• ATP

• Conformational