Transport Across Membranes

Transport across a membrane is important as without it, our bodies would not be able to function.

Types of transport for small metabolites and ions:

  • Non-mediated transport = diffusion across the membrane to reach equilibrium (very slow)
  • Mediated transport = membrane proteins facilitate the transport across membranes
    • Passive transport = flow to diffuse a concentration gradient
    • Active transport = flow against a concentration gradient (requires energy)

Free Energy of Transport:

To calculate the free energy of a metabolite we use a basic equation:

A(outside) <—> A(inside), where A is an arbitrary metabolite that we are trying to push across a cell.

To determine the energy required to transport, we use the following formula:

Where R (gas constant) = 8.314 J/mol * K

ℱ = Faraday’s constant – 96,485 C/mol

ΔGA= partial molar free energy (free energy for taking A outside to A inside)

ZA = Charge of A (if the charge is zero then this term drops out)

ΔΨ = electrical potential difference across a membrane in volts =
Ψ(inside)-Ψ(outside)

FYI: The first part of the formula describes the chemical potential energy while the second part describes the electrical potential energy.

Note: Volts MUST be in in volts, not mV – otherwise your units will not cancel out.

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