Explanation of Osmosis.

Theory

Osmosis

If a dilute solution is separated from a concentrated solution by a partially permeable membrane water diffuses across the membrane from the dilute solution to the concentrated solution. This process is known as osmosis.

A partially permeable membrane is porous but allows water to pass through it more rapidly than dissolved substances.

Because a dilute solution contains more water molecules than a concentrated solution there is a diffusion gradient. This means that the water moves from the dilute to the concentrated solution. In living cells the cell membrane is partially permeable and the cytoplasm and vacuole (in plant cells) contain certain dissolved substances. Because of this water tends to diffuse into cells by osmosis if they are surrounded by a weak solution, like fresh water, if the cells are surrounded by a stronger solution, like sea water, then the cells will lose water by osmosis.

Explanation of Osmosis

When a substance such as sugar dissolves in water it attracts some of the water molecules and stops them moving. This, in effect, increases the concentration of water molecules. The sugar molecules on the right have captured some of the water molecules. There are more free water molecules on the left so diffusion will go more rapidly from left to right.

The partially permeable membrane does not act like a sieve in this case. The sugar molecules can diffuse from right to left, it just does this a lot slower because the molecules are larger and surrounded by a cloud of water molecules.

Artificial permeable membrane is made from cellulose acetate in sheets or tubes and used for dialysis rather than osmosis. Their pore size can be adjusted by manufacturers so that they do not let larger molecules through at all.

Cell membrane behaves like artificial permeable membrane. The partial permeability may depend on the pores in the cell membrane but the process is far more complex than an artificial membrane and depends on things such as the structure of the membrane and living processes in the cytoplasm. The cell membrane contains proteins and lipids.

Water Potential

The water potential of a solution is a measure of whether it is likely to lose or gain water molecules from another solution. A dilute solution, with its high proportion of free molecules, has a higher water potential than a concentrated ...

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Ross Robertson

10th of May 2013

In this report of an experiment to investigate the effect of changing sugar concentration on the process of osmosis, the writer gives us a good account of the theory of osmosis but does not make it clear as to the aims of the investigation. Unlike most investigations involving a range of sucrose solutions and potato tissue, this one does not go on to find the water potential of the potato tissue. Instead, it restricts itself to demonstrating the process of osmosis and showing that water leaves living tissue into solutions with lower water potentials. The report (and indeed the whole investigation) would have benefited from clearer aims, together with a discussion of previous work carried out by other students (accessible online). The data collected here is unusual and deviates from most of the data collected in similar experiments on potato tissue. However, a direct comparison is difficult owing to the use of ounces rather than grams in the method. 3 stars

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