Unit 5: Transporters in the Biological system

Biological membranes are lipid bilayers largely impermeable to ions and polar solutes, so cells rely on integral membrane proteins to move material across them. This unit examines the two protein classes that accomplish this — channels and transporters — the physics of voltage-gated ion channels, the light-driven pumps and channels used in optogenetics, and the motile appendages (cilia and flagella) whose beating depends on molecular motors.

I. Orientation: Membrane Transport Fundamentals

Transport across a membrane is governed by the electrochemical gradient, which combines the concentration gradient with the transmembrane voltage.

• Electrochemical potential: the free energy of moving a mole of ion S across the membrane.

Δμ(S) = RT·ln([S]in/[S]out) + zF·Vm

• R, T, F: gas constant, absolute temperature, Faraday constant.
• z: ion valence; Vm: membrane potential (Vin − Vout).
  • Passive vs active: passive transport dissipates a gradient (ΔG < 0); active transport builds one against ΔG > 0 using an energy source (ATP, light, or a coupled ion gradient).
  • Selectivity: proteins discriminate by ion size, charge, and hydration energy; e.g., K⁺ channels conduct K⁺ ~1000× better than Na⁺.
  • Flux limit: channels pass ~10⁶–10⁸ ions/s (near diffusion limit); transporters cycle at ~10²–10⁴ ions/s because each cycle needs a conformational change.

II. Channels and Transporters in Biological System

A. Definition and Distinction

Both are membrane proteins, but they differ in mechanism, speed, and thermodynamic capability.

• Ion channel: a gated aqueous pore; when open it provides a continuous path so ions flow down their electrochemical gradient.
  • Gating: channels open/close in response to voltage, ligand, mechanical stress, or temperature.
  • No conformational cycle per ion: ions stream through, giving very high throughput.
• Transporter (carrier/permease): binds solute on one face, undergoes a conformational change, and releases it on the other — the alternating-access model.
  • Never open to both sides at once: this prevents the pore-like leak that channels permit.

B. Channels and Transporters in Biological System

Transporters are further classed by their energetics.

• Uniporter: moves a single solute down its gradient (facilitated diffusion), e.g., GLUT1 for glucose.
• Primary active transporter (pump): uses chemical energy directly.
  • Na⁺/K⁺-ATPase: exports 3 Na⁺, imports 2 K⁺ per ATP hydrolysed — a P-type ATPase.
• Secondary active transporter: harnesses an ion gradient set up by a pump.
  • Symporter (cotransport): solutes move the same direction, e.g., SGLT carries Na⁺ + glucose inward.
  • Antiporter (exchange): opposite directions, e.g., Na⁺/Ca²⁺ exchanger expels Ca²⁺ using inward Na⁺.
• Distinguishing readout: channels show single-channel current steps in patch-clamp; transporters show no unitary events but steady, saturable flux (Michaelis–Menten kinetics with a Km and Vmax).

III. Functional Properties of Voltage-Gated Ion Channels

A. Purpose and Principle

Voltage-gated channels (Nav, Kv, Cav) sense the membrane field and open to carry the ionic currents underlying the action potential; they are the molecular basis of electrical excitability.

• Domain architecture: four homologous domains (or four subunits), each with six transmembrane segments S1–S6.
  • S4 voltage sensor: bears repeated positively charged arginines/lysines that move outward when the membrane depolarises.
  • S5–S6 + P-loop: line the pore and form the selectivity filter.

B. Functional Properties of Voltage-Gated Ion Channels

The channel's behaviour is defined by three coupled properties.

1. Voltage sensing and activation: depolarisation drives outward S4 movement, producing a measurable gating current that precedes ionic flow; the open probability follows a Boltzmann curve.

Popen = 1 / (1 + exp[(V½ − Vm)·zg·F / RT])

• V½: voltage of half-maximal activation; zg: effective gating charge.
  2. Inactivation: many channels close a second gate even while depolarised.
• Nav fast inactivation: an intracellular IFM motif ("ball and chain") plugs the pore within ~1 ms, ensuring the Na⁺ current is transient and setting the action-potential refractory period.

• Selectivity: the K⁺ filter uses backbone carbonyl oxygens (the TVGYG signature) to mimic water's hydration shell, so dehydrated K⁺ passes but the smaller Na⁺ is destabilised and excluded.
• Conductance and driving force: single-channel current is ohmic locally.

i = γ·(Vm − Erev)

• γ: single-channel conductance (pS); Erev: reversal (Nernst) potential.
  • Applications: targets of local anaesthetics (lidocaine blocks Nav), antiepileptics, and toxins (tetrodotoxin blocks Nav; charybdotoxin blocks Kv).

IV. Ion Pumping and Ion Channel Rhodopsins and Their Use

A. Definition and Principle

Rhodopsins are seven-transmembrane proteins carrying a retinal chromophore that isomerises on absorbing light; this conformational trigger either pumps an ion or opens a channel, converting photons into transmembrane charge movement.

• Chromophore switch: all-trans ⇌ 13-cis retinal isomerisation on illumination drives the transport cycle.
• Two functional classes: active pumps (ion pumping rhodopsins) versus passive light-gated channels (channelrhodopsins).

B. Ion Pumping Rhodopsins

Microbial rhodopsins that use light energy to move ions against their gradient — true primary active transporters.

• Bacteriorhodopsin (BR): from Halobacterium salinarum; pumps one H⁺ outward per absorbed photon, building a proton-motive force for ATP synthesis.
  • Photocycle: intermediates J→K→L→M→N→O; proton release occurs around the M state.
• Halorhodopsin (HR): pumps Cl⁻ inward, hyperpolarising the cell.
• Optogenetic use: expressing HR or the H⁺-pump archaerhodopsin (Arch) lets light silence neurons by hyperpolarisation.

C. Ion Channel Rhodopsins

Light-gated ion channels that passively conduct ions when open — the depolarising tools of optogenetics.

• Channelrhodopsin-2 (ChR2): from Chlamydomonas reinhardtii; blue light (~470 nm) opens a non-selective cation pore (H⁺, Na⁺, K⁺, Ca²⁺), depolarising the cell within milliseconds.
  • Kinetics: fast opening and closing let single spikes be evoked by light pulses.
• Their use — optogenetics: neurons are made light-sensitive by targeted expression.
  1. Excitation: ChR2 + blue light fires action potentials.
  2. Inhibition: HR/Arch + yellow-green light suppresses firing.
• Advantages: millisecond temporal precision and cell-type specificity (via promoters) that electrical stimulation cannot match; used to map neural circuits and model disorders.

V. Cilia and Flagella: Structure and Movement

A. Definition and Principle

Cilia and flagella are microtubule-based motile organelles projecting from the cell surface; their beating moves cells through fluid or moves fluid across cell layers, powered by the motor protein dynein.

• Shared core: both are built on the axoneme and enclosed by the plasma membrane; flagella are longer and fewer, cilia shorter and numerous.

B. Cilia and Flagella: Structure

The defining feature is the highly conserved microtubule scaffold.

• 9 + 2 axoneme: nine outer doublet microtubules surrounding a central pair.
  • Doublet: a complete A-tubule (13 protofilaments) fused to an incomplete B-tubule (10–11).
  • Central sheath and radial spokes: link the central pair to each doublet, coordinating the beat.
• Dynein arms: inner and outer axonemal dynein arms project from each A-tubule toward the neighbouring B-tubule — the force generators.
• Nexin links: elastic bridges tie adjacent doublets, converting sliding into bending.
• Basal body: a 9 + 0 triplet structure (like a centriole) anchoring the axoneme and templating its growth.

C. Cilia and Flagella: Movement

Motion arises from ATP-driven dynein "walking," constrained by the axoneme's crosslinks.

• Sliding-filament mechanism: dynein heads hydrolyse ATP and step along the adjacent B-tubule toward its minus end, trying to slide doublets past one another.
• Sliding → bending: because nexin links and the basal body resist free sliding, the shear is converted into localised bending; alternating activation of dynein on opposite sides propagates the bend.
• Beat patterns:
  1. Ciliary beat: asymmetric — a stiff power stroke followed by a curved recovery stroke, moving fluid parallel to the surface (e.g., airway mucus clearance).
  2. Flagellar beat: symmetric propagating sine waves that drive the cell forward (e.g., sperm propulsion).
• Regulation: the central-pair/radial-spoke system and Ca²⁺ signals switch which dyneins are active, controlling beat direction and frequency.
• Significance: defects cause primary ciliary dyskinesia — chronic respiratory infection, situs inversus, and infertility — underscoring that structure and motor function are inseparable.