Friction in diffusion transport

One may also want to call this more fancily "energy dissipation in diffusion transport".

Diffusion transport is not free.
Just as in any type of transport there needs to be energy dissipated such that
– there is a defined arrow of time
– there is a source and a target

It is just that diffusion transport dissipates the energy not along the way but at the "toll stations".
I soft nanotechnology that are typically cell membrane crossings or chemical transformations at sites fixed in space.

The "change money" problem

As "toll stations" are not interconnected in soft nanosystems and molecules can only be used up in whole chunks,
excess energy cannot be used to drive a reaction somewhere (somewhen) else.
That is soft nanosystems do not provide the opportunity of dissipation sharing.

Attaining comparability to actively driven transport

Making speed, energetic losses over speed in diffusion transport comparable the same quantities in driven machine phase transport is a nontrivial problem.

Diffusion speed sort off tapers off with distance
so what speed of actively driven transport to compare it to
to be able to compare energetic losses over distance?

The crude sketch here illustrates a perhaps sensible but naive first shot first principles approach.
Picking a sequence of spacings after which the speed is reset one can get a distance independent speed.
Tighter spacing increases the speed
as a second much more limitedly usable lever of changing the temperature.

• Changing temperature is limited to liquid range,
  also operational range, and ultimately decomposition limits
• Changing temperature changes energetic efficiency too

A bit more concretely:
Assuming a sequence of (infinitely many) coplanar membranes
with energetic toll stations acting like valves preventing back-flow.

Dimensionality

Dimensionality of the space of diffusion causes qualitative changes.
Complicatingly in molecular biology things are very rich in variants on dimensionality.

• Varying dimensionality of diffusion transport in free cell plasma of divergent bounded volume shape
• Varying dimensionality along inter-cellular membranes of different curvature and topology

Nature of transport

Complicatingly in molecular biology this highly simplified scenario
is not even the only mode of diffusion transport. There is:

• The simple cross membrane wall transport described
• transport or material via entire vesicles ah get split off and fuse in elsewhere (e.g. endo- & exocytosis)
• and on the most fancy end the active ragging of vesicles along microbiology using that goes a bit into the direction of active directed machine phase transport
• … and more

Hydrogen gradients

On the energetic efficiency side beside paying directly with ATP (or other energy molecule)
there is also the very common use of hydrogen concentration gradients.
This may push down the loss in "not getting small change back at the toll stations".
But is still the limit of being well above kT.
Which advanced gemstone based productive nanosystems may be able to avoid.
See page: Dissipation sharing Warning! you are moving into more speculative areas.

Related

• Diffusion transport
• Diffusion
• Friction

External Links

• Diffusion
• Molecular diffusion
• Brownian motion
• Brownian ratchet
• Surface diffusion
• Atomic diffusion
• Random walk

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• Diffusion equation
• Einstein relation (kinetic theory)
• Langevin equation
• Wiener process
• Fick's laws of diffusion

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• Arrhenius equation
• Eyring equation

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• Intracellular transport
• Membrane transport protein
• Membrane vesicle trafficking
• Vesicular transport adaptor protein