Osmosis

OSMOSIS

Osmosis Theory

Diagrams available in any science book

Osmosis is the diffusion of a solvent through a partially permeable membrane until there is an equal concentration on either side of the membrane. The partially permeable membrane contains pores that are very small. Because these pores are only very small, only certain molecules can pass through it. Usually though the membrane will allow all or none of the molecules through. When only one type of molecule is let through though, the membrane is said to be partially permeable. This is because it allows a selective flow. A classic example of this is Visking tubing. If a concentrated solution of sugar (glucose) is put in Visking tubing with no leaks, then when the tubing is lowered into distilled water, after a time the water molecules will have diffused into the Visking tubing until the concentration is equal. The Visking tubing is a partially permeable membrane as only water molecules can pass through it. Sugar molecules cannot pass through it, as they are too large, they contain 24 atoms (C6H12O6) whereas water molecules only have three (H2O). So, instead of the sugar molecules diffusing into the water, the opposite has happened because the sugar molecules are too large for the partially permeable membrane.

In the above diagram there are more water molecules on the left hand side of the membrane (grey blocks) than the right, so there is a concentration gradient for water molecules. They travel down the gradient from left to right. There is also a concentration gradient for sugar molecules, however they cannot travel down it, as they are too large to go through the partially permeable membrane.

When the Osmosis has occurred, the right side of the Visking tubing will have become more dilute as more water molecules have come to that side. When both sides of the Visking tubing have equal concentrations, the osmosis will have been completed. Osmosis is the diffusion of water molecules from a place where they are in higher concentration to a place where they are in lower concentration through a partially permeable membrane.

Exosmosis is rather like Osmosis except that the other solution is a surrounding solution. This can be best thought of as an animal cell in water. Inside the cell is a more concentrated solution, and outside of it is pure water. The water molecules osmose into the cell until the concentrations are equal. This is because the animal cells have a partially permeable membrane that acts like Visking tubing. The solution inside the cell is contained in the cytoplasm; it is full of proteins and other substances in water. These substances in the cytoplasm are too large to go through the partially permeable membrane so instead the water molecules come in. This situation is similar to the diagram above. In an animal cell though, as more and more water is coming in the cell membrane has to swell until eventually it bursts. In the opposite way, if the animal cell is placed in a highly concentrated solution, water from the cytoplasm will go through the partially permeable membrane and eventually it will shrink.

Plant cells do not burst in