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Cell Biology
The World of Cells
Getting Substances
In and Out of Cells
Like most substances, water diffuses from regions where it is in high concentration to regions where it is in low concentration.

all the water molecules are moving very fast and in all directions

One important region of high water concentration is just outside the cell where the fluid is mainly water containing few dissolved substances. On the other hand, water is at a lower concentration inside the cell because of the presence of sugars, amino acids, proteins and organelles as well as other substances. Water, therefore, diffuses from the high concentration outside the cell to the low concentration inside the cell, passing easily through the cell membrane.

The cell membrane, however, is a very selective barrier and is not permeable to many of the other substances found at high concentration inside the cell. It is said to be semipermeable; water can move across it freely but other materials cannot. As a result there is a net flow of molecules into the cell as water diffuses in and other materials are prevented from moving out.

when water molecules bump into the barrier they can pass through it to the other side, without being stopped

The passage of water across a semipermeable membrane from a dilute solution to a more concentrated solution is called osmosis. If a cell does not compensate for the effects of osmosis it will gradually swell up with water and burst.

How do cells compensate for the effects of osmosis? Cell walls surrounding plants, fungi and most prokaryotes are strong and rigid. Such cells can only expand until the swollen cell presses up against this unmovable barrier. Fresh water protozoa contain pumps. These organelles, called contractile vacuoles, specialize in collecting the excess water and expelling it from the cell.

In multicellular organisms like ourselves, each cell is surrounded by fluid which contains salts, sugars, etc. at the same concentration as those found inside the cell. Osmosis is not a problem, therefore, unless the balance of materials outside the cells is disturbed as it is in some medical conditions.

more water molecules are hitting the barrier on the outside, so more water molecules move from the outside to the inside

Experiment in Osmosis

A glass filter funnel, filled with a strong solution of molasses is covered with a semipermiable membrane and inverted into a beaker of water. Water molecules move through the membrane and into the solution of molasses. The level of the water in the funnel can be measured and the rate of uptake calculated.

© 2001, Professor John Blamire