Question

Which of the following best describes the movement of substances across a cell membrane?

Answer 1

That sounds like a definition of "diffusion."

Answer 2

its osmosis not diffusion

Answer 3

You know what? I believe you're right.

Answer 4

I think you should stay with your answer InYourHead. Remember diffusion is the potential to move any species into a equilibrium. Osmosis however is only the potential to move water (and water only!) into a equilibrium. In other words the diffusion of water is called osmosis. Now the question ask "substances" so diffusion is right in this matter.

Answer 5

You pay some very fine attention to detail. Thanks for shedding light on that for me. =)

Answer 6

Well you obviously knew it sense you wrote it in the first place ;)

Answer 7

osmosis move water molecules across a cell membrane diffusion dont do that
. the question didnt give the substance name so its osmosis

Answer 8

Hmm when looking more into it, you can say it is osmosis, however we need to get a few things stright.
Diffusion is known by being a potential determined for the concentration of species. The force it self is driven by "random" motion (brownian motion ((so not that random anyway)). Diffusions random movements is caused by the particle to move in almost any direction due to the collision, which it is exposed to by other molecules.
The potential can be seen as a unrealized force and is typically called the chemical potential.
The eqaution that discribe the change in the chemical potential over a membrane is the the following:
\[\Delta F _{diffusion}=RT \ln(C _{2})-RT \ln(C _{1})\]
By calling the single potentials for μ we can rewrite the equation to the following:
\[\Delta F _{diffusion}=\mu _{2}-\mu _{1}\]
We make the following notations for the equation:
\[ \Delta F _{diffusion}<0 \rightarrow \mu _{2}<\mu _{1} \rightarrow (1)\]
\[\Delta F _{diffusion}=0 \rightarrow \mu _{2}=\mu _{1} \rightarrow (2)\]
\[\Delta G _{diffusion}>0 \rightarrow \mu _{2}>\mu _{1} \rightarrow (3)\]
(1): Passive transport into the cell.
(2): The cells concentration equals the environment. So no netto transport.
(3): Passive transport out of the cell.

This equation works for all molecules provided that purines or other channels are present and open.*
On account of the mathematical setup, we can conclude that the osmosis is a consequence of diffusion **

In addition I like to quote J. B. Reece from Campbells Biology: "Molecules have a type of energy called thermal energy (heat), due to their constant motion. One result of this motion is diffusion, the movement of molecules of any substance so that they spread out evenly into the available space. ... The diffusion of free water acreoss a selectively permeable membrane, whether artificial or cellular, is called osmosis."

Now lets take a look at what Reece wrote. Acording to her description of diffusion, molecules well spread out into any "available space" well acording to the equation I set up before this can be done for any molecule provided that purines or other channels are present and open. So "available space" is in my eyes inside and outside the cell.
When we look upon osmosis Reece write that it is strictly for water as well as the involvement of a membrane of any type.

* The equation don't work for changed molecules then we need another equation by adding the change potential into the equation given by the following:
\[\Delta F _{charge}=zF (\Psi _{2}-\Psi _{1})\]
** Compendium of Thermodynamics and Electrophysiology, N. J. Willumsen (Nov 2011)

Answer 9

Correction where I wrote Delta G I ment Delta F, and where i wrote change potential I ment charge potential.