Osmosis in plant cells and animal cells

Divergent Skills

Osmosis in plant cells

Osmosis in living system refers to the movement of water molecules across the partially permeable cell surface membrane. The cell wall is fully permeable.
As the large central vacuole occupies most of the space in a plant cell, the water potential of the cell sap is considered to be the water potential of the plant cell.
When a plant cell is immersed in a solution of higher water potential relative to its cell sap, water molecules enter the cell by osmosis.
The vacuole increases in size and the expanded cell contents exert pressure on the cell wall.
The cellulose cell wall of a plant cell is strong and rigid.
The cell wall exerts an opposing pressure on the cell contents, preventing the entry of more water. This prevents the cell from overexpanding and bursting.
At this point, the plant cell is very firm or turgid. Turgor pressure provides mechanical support for many non-woody plants.

When a plant cell is immersed in a solution with a lower water potential relative to its cell sap, water diffuses out of the cell into the solution by osmosis.
The vacuole shrinks and the cell stops exerting pressure on the cell wall. The cell becomes limp or flaccid. If it is placed in a solution with a higher water potential at this point, turgidity can be restored.
If more water leaves the cell, the vacuole and cytoplasm shrink to such an extent that the cell surface membrane pulls away from the cell wall. The phenomenon in which the cell surface membrane pulls away from the cell wall is called plasmolysis. This can be lethal if the cell is not quickly transferred to a solution with a higher water potential relative to its cell sap.

Osmosis in animal cells

When and animal cell is immersed in a solution with a higher water potential relative to its cytoplasm, water diffuses into the cell by osmosis.
The cell swells. As more water enters the cell, it swells to such an extent that it bursts. This is because it does not have a cell wall. This process is called cytolysis.

When an animal cell is immersed in a solution with a lower water potential. relative to its cytoplasm, water diffuses out of the cell by osmosis.
The cell shrinks and become dehydrated. In red blood cells, little spikes appear on the cell surface membrane, and the cell is said to have undergone crenation. The animal cell will die if it is not removed from the solution.

Osmosis another example of passive transport but the term is only used in the context of water molecules; osmosis is the movement of water molecules across a partially permeable membrane from a region of lower solute concentration, where there is high concentration of water molecules, to a region of higher solute concentration, where the concentration of water molecules is lower.

**Before osmosis**: Two solutions are separated by a partially permeable membrane. B has a higher solute concentration than A. The soluble molecules are too large to pass through the pores in the membrane but the water molecules are small enough.

Figure 2.6 Osmosis

**At equilibrium:** As the arrows in diagram a indicate, more water molecules moved from A to B than from B to A, so the net movement has been from A to B, raising the level of the solution in B and lowering it in A. The solute concentrations in A and B are now equal.

A special case of diffusion is osmosis Figure 2.6. This is the passive movement of water across a partially permeable membrane from an area of lower solute concentration to an area of higher solute concentration.

Osmosis is the diffusion of water molecules from a region of their higher concentration to a region of their lower concentration, through a partially permeable membrane.

Osmosis is a special form of diffusion and always involves the movement of H₂O across a membrane. Osmosis is:

the movement of H₂O
across a selectively permeable membrane
down a water potential gradient

In the picture below

The concentration of sugar molecules is higher on the concentrated solution (L) and lower on the diluted one (R)
The concentration of water molecules is higher on the (R) and lower on the (L) (a lot of place is taken up by sugar molecules)

It is confusing to talk about the ‘concentration of water’, so we can say that a diluted solution (R) has a high water potential and a concentrated solution (L) has low water potential.

There is a water potential gradient between the 2 sides. The water molecules diffuse down this gradient, from a high water potential (R) to a low water potential (L).

Cell membranes

partially permeable (let some substances pass through, but not others).
separate 2 solutions: cytoplasm and solution around the cell.
if the solutions are of different concentrations, osmosis will occur.

Effect of Osmosis on plant and animal cells

When place in H₂O

Concentration of H₂O outside the cell is higher than inside it. Cells will take in H₂O by osmosis:

plant cells become turgid (swollen) but do not burst (have tough cell wall which is fully permeable).
animal cells will burst (no cell wall).

When placed in concentrated sugar or salt solutions

Concentration of H₂O inside the cell is higher than outside it. H₂O get out of the cells by osmosis:

plant cells become flaccid (soft and limp), cytoplasm is no longer pressed against the cell wall. The plant loses its firmness and begin to wilt.
animal cells shrink, become crenated.

Importance of H2O potential gradient in the uptake of H2O by plants

Enables H2O movement by osmosis

from soil to root hairs
from tissue fluid to cells
from xylem to leaf mesophyll cells

Common misconceptions

Sugar and salt do not move by osmosis. Cell membranes prevent them entering or leaving the cell.

Osmosis

If you’ve got your head round diffusion, osmosis will be a breeze.

Osmosis is a special case of diffusion, that’s all

OSMOSIS is the movement of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration.

A partially permeable membrane is just one with very small holes in it. So small, in fact, only tiny molecules (like water) can pass through them, and bigger molecules (e.g. sucrose) can’t.
The water molecules actually pass both ways through the membrane during osmosis. This happens because water molecules move about randomly all the time.
But because there are more water molecules on one side than on the other, there’s a steady net flow of water into the region with fewer water molecules, i.e. into to stronger solution.
This means the strong sugar solution gets more dilute. The water acts like it’s trying to “even up” the concentration either side of the membrane.
Osmosis is a type of diffusion – passive movement of water particles from an area of higher water concentration to an area of lower water concentration.