Everything You Know About Osmosis Is (Probably) Wrong

Osmosis - the flow of a solvent across a semipermeable membrane from a region of lower to higher solute concentration - is a well-developed concept in physics and biophysics. The problem is that, even though the concept is important to plant and human physiology, osmosis is understood in biology and chemistry in a much simpler - and often incorrect - way.

"A textbook in 1951 offered the first coherent telling of the whole theory."

Though physicists have had this complete and correct explanation since the 50s, chemistry and biology never caught up. Why? One reason is because the incorrect theory is much easier. "The thermodynamic explanation can be pretty dense, and features entropy, which can be scary for people," he says. "The correct theory would be harder to teach at an introductory level, although I'm working with a textbook author who plans to spread the word."

Kramer lays out the common misconceptions:

(1) "The first misconception is that osmosis is limited to liquids," he says. "But it works just fine for gases, too."

(2) "Another misconception that osmosis requires an attractive force," he says. "It doesn't. When water fills the bag of sugar, it's not because the sugar is pulling the water in. That's not part of the explanation."

(3) "A misconception is that osmosis always happens down a concentration gradient," he says. "When you dissolve something in water, the water doesn't necessarily get more diluted. Depending on the substance, it can get more concentrated."

(4) "Anther misconception is that you don't need to invoke a force to explain why the water flows into the bag. It's thought that, like diffusion, it's a spontaneous process," he says. "But, in fact, there is a force. It's complicated how it happens, but it turns out that the membrane - or the bag, in the familiar lab demonstration - exerts a force that pushes the water in."

While authors in physics and biophysics have generally settled on the correct understanding of osmosis, these ideas have not penetrated into the fields of chemistry and biology.

Here's my take

1. It's easy to grasp that osmosis occurs in gases.

2. I never imagined the sugar pulled the water across. It was always explained to me that the water molecules diffused across the membrane by random motion, while the sugar molecules couldn't.

3. I don't understand this. I thought it was called reverse osmosis when pressure forced water to cross against a concentration gradient. How could the water not get more dilute if you dissolve something else in it? This sounds logically impossible to me. Anyway just dissolving isn't the same as something crossing a membrane in one direction. This isn't clear to me.

4. How does the membrane exert a force that pushes the water across? Do the edges of the pores attract the water molecules, pulling them into the pores? This isn't explained.

Does anybody have access to either of the articles mentioned at the bottom? Or perhaps a better understanding, to clear up my confusions?

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Replies to This Discussion

I think the force of osmosis comes from the free energy.  The free energy involves both enthalpy - which is from the energy contained in the system - and the entropy.  G = H - TS, where G is the free energy, H is the enthalpy, T is the temperature and S is the entropy. 

Increasing the entropy S lowers the free energy, so there's a force that comes from the change in entropy.

Osmosis increases the entropy, and the force in osmosis comes from this increase in entropy, I think. 

Thanks. I hadn't thought about the increase in entropy.

Dear me, talk about a straw man, I did my first biochemistry and molecular biology courses in the 80s and never was taught those. Only crummy universities would teach that. (rolls eyes)




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