An investigation of the glucose concentration of the cell sap in potato cells
In this experiment I intend to investigate the effects of osmosis on
potato cells. Specifically, I intend to use my knowledge of osmosis to
investigate the glucose concentration of the cell sap in potato cells.
Osmosis is a method by which water levels on either side of a semi
permeable membrane may balance themselves. It occurs between regions
of high water concentration and low water concentration. As it is a
special method of diffusion exclusive to water, it also allows dilute
and concentrated solutions to balance their strengths. It does this by
preventing large non-water particles from passing through the
semi-permeable membrane due to their size, in order that the water
levels on either side may equate, regardless of the solution's other
Osmosis is passive, that is to say it does not require energy in order
to be performed. This is because it occurs solely between regions of
high and low water concentration.
The direction and speed of osmosis depends on the two solutions'
osmotic potential. This is the measure of the pressure by which the
water molecules of a solution diffuse across a semi-permeable
membrane. A solution's potential is defined by its concentration. The
more soluble a solution has in it, the lower its water concentration
and the lower its osmotic potential. In the example below sugar
molecules represent the soluble.
Cell sap is contained in the vacuole of a cell and is a part glucose,
part water solution. The membrane of the cell acts as a semi-permeable
membrane across which osmosis occurs (see below). In the case of
osmosis into and out of the vacuole, it occurs across two membranes -
the plasmalemma and the tonoplast. When osmosis occurs, water may
enter or leave the vacuole according to the concentration of the
substances inside and outside the potato cell. In the example below we
see what would happen if a plant cell were surrounded by pure water.
As pure water has the highest water concentration possible, osmosis
will invariably occur into the cell vacuole, which by definition
contains a solution lower water concentration.
The plant cell will take in as much water as is needed to make
the water levels on either side of the membrane equal. As the glucose
molecules cannot pass out of the cell to equate the water levels, the
volume of cell sap within the cell and the weight of the cell itself
will going to increase. Consequently, it is vital that the cell is
able to accommodate some expansion. This is made possible by the
support lent to it by the slightly elastic cellulose cell wall. It
will support a turgid cell enough to prevent it from bursting (see
left). It is turgid cells such as this that provide a plant with its
Animal cells on the other hand, do not have supporting cell walls,
just cell membranes. As plants are always surrounded by water, it is
vital for them to have cell walls, but animal cells, surrounded by...