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.