Osmosis…

One of my favourite SF classroom moments was when discussing density. I said that osmium was the densest metal, and therefore the densest material that one would be likely to meet on Earth, at 22.59 g/cm³ (with iridium as a close second on 22.56 g/cm³). A voice chimed up from the front asking whether osmium was smelly because osmé is the Greek for smell. “No” I said, “Metals don’t smell.” At which point another voice piped up from the back of the room with, “Um, I don’t know how I know this but osmium tetroxide is volatile and has a strong smell.” 27 seconds later Wikipedia had confirmed that osmium’s most common oxide was indeed the yellowy, volatile (meaning turning easily into a gas), smelly osmium tetroxide. I was, as they say, well and truly sussed!

Osmium has little to do with osmosis because this word derives from the Greek ōsmos meaning push. Osmosis describes the tendency for two solutions, either side of a semi-permeable membrane, to equalise out their concentrations. That sentence may need a little explaining. A semi-permeable membrane is a thin layer that has holes in it large enough to allow solvent molecules to pass through (usually water) but not the larger solute molecules. Look at the diagram below (from Wikipedia).

On the left you can see two solutions, of different concentrations, separated by a thin membrane. The more concentrated solution is on the left and the more dilute one is on the right. Over time, as a result of the random movement of the water molecules, the concentrations of the two sides balance out. The effect of this is to increase the volume of the liquid on the left. But you can see from the diagram that the concentrations of the two liquids are now equal (isotonic). This process doesn’t require any energy input although it is relying on the kinetic energy already available to the water molecules.

Some examples…

• Osmosis keeps plant cells ‘inflated’ with water pressure giving them their strength. Water enters the root hair cells by osmosis.
• Fruit can be preserved in sugar (jam) because any microorganism landing on its surface has all its water pushed out of it by osmosis. The sugar solution is more concentrated (hypertonic) than the cytoplasm of the organism (hypotonic).
• Fresh water and salt water fish cannot switch environments because fresh water would be too dilute for salt water fish and their cells would fill up with water and burst, whereas freshwater fish placed in the sea would lose water to their surroundings and their cells would shrivel. Either way you end up with dead fish! Fish like salmon that start life in fresh water then travel to the Arctic Ocean to feed, have a change in their biochemistry as they transition from fresh to ever saltier water. They would not survive being picked up in one environment and immediately deposited into the other.

The particles in a gas or a liquid are moving around randomly, bumping into each other and changing direction. The hotter they are, the faster they move (the greater their kinetic energy). You can smell osmium tetroxide because the molecules enter the air and travel to your nose by diffusion. The movement of water molecules across a semi-permeable membrane is another example of diffusion.

There is a good page on osmosis at Focus eLearning. You will need to log in with the link and then click here to go to the bit I want you to see. There is also an experiment film from Twig below – stick with it!

I have the full version of Osmosis Jones, which we will watch after the exams. I think I will enjoy it though judging by this clip. It seems to be about my level! It has someone called Chris Rock in it – I am sure I have heard that name.

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Questions…

1. What is the formula for calculating density?
2. What is the density of (a) water (b) aluminium (c) iron (d) copper?
3. Name three features you might find in a plant cell that you would not find in an animal cell?
4. Name a specialised plant cell.
5. Name a specialised animal cell.
6. What word describes fish that migrate between salt and fresh water? (Some research required!)
7. How does turning fruit into jam preserve the fruit?
8. How does putting salt on beef stop the beef from going off?