osmosis in biology

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Osmosis : is the movement of water molecules or solvent molecules from a region of lower concentration (pure water or pure dilute solution) to a region of higher concentration through a semi-permeable membrane.
It can also be defined as the movement of water molecules from a hypotonic solution to a hypertonic solution through a semi-permeable membrane.top↑

Hence, hypotonic is the region of lower concentration, e.g pure water.
While hypertonic is the region of higher concentration, e.g concentrated copper sulphate.

In the experiment to demonstrate diffusion in liquids, you observed that the molecules of copper (ii) tetraoxosulphate (vi) (copper sulphate solution) diffused evenly throughout the solution and in demonstration for diffusion in gases, you also observed that, the molecules of ammonia (NH3) diffuses through the water containing one or two drops of hydrochloric acid.top↑

Conditions For Diffusion

In organisms, diffusion usually take place across membranes. For example,
1) The diffusion of water molecules from the soil into the cells of the root hairs take place across the Cell Membrane
2) The diffusion of water from one cell into a neighbouring cell in human being takes place across the cell membranes
3) Another example is the diffusion of carbon dioxide from the cytoplasm of amoeba through the cell membrane into the water in its external environment.

Results Of Diffusion On A Cell

When diffusion occurs through cell membranes, there are three possible results which depends on the type of molecules diffusing and the type of cell membranes.top↑

These three results are;

1) when diffusion occurs through cell membrane, some membranes do not allow the passage of molecules through them. For example, a rubber membrane will not allow the passage of water molecules or copper sulphate molecules. This type of membranes that do not allow passage of either solvent or solute molecules through them are called impermeable membrane.

2) Some membranes such as filter paper will allow the passage of all molecules through them, these type of membranes that allows full passage of molecules through them are called Fully Permeable membrane.

3) And the last result, some membranes will allow the passage of water molecules but not of solute molecules e.g copper sulphate solution. These type of membrane that allows the passage of one type of molecules but not other types are called Semi-permeable membrane.top↑

There are two examples of semi-permeable membrane, which are non-living and living semi-permeable membrane.

Examples of non-living semi-permeable membrane which are permeable to water but not solvent e.g copper sulphate solution are visking tube, parchment and cellophane.

Examples of living semi-permeable membrane are sheep or pig bladder, yam tuber, irish potato, cell membrane, unripe pawpaw and tonoplast, etc.

Note : Osmosis is a special type of diffusion, that is, it is the diffusion of water molecules or solvent molecules through semi-permeable membrane.top↑

Osmosis Process

Osmosis involves diffusion of molecules through a semi-permeable membrane. This semi-permeable membrane is very much more permeable to water or solvent molecules than to the solute molecules.

I believe you know what a solute and solvent are? Where, here is the definition.
Solute are something (it may be water of liquid) that dissolves solvent, while Solvent are substances ( it may be in liquid or gaseous form) that dissolves in solutes.
For example, when you dissolves NaCl ( common house salt) into water, water is the solute and the salt is the solvent.

A living cell is usually surrounded by a medium which can be described in three ways.
These three ways are exosmosis, endosmosis and isotonic.top↑

a] if the medium is more concentrated than the cellular fluid, it is described as hypertonic, hence, exosmosis is when the movement of water molecules moves out of the cell into the surrounding medium. That is, exosmosis is the movement of water molecules out of a living cell.

b] if the medium is less concentrated than the fluid in the cell, it is described as hypotonic. In this case, water molecules move into the living cell. The movement of water molecules into the living cell is called endosmosis.

c] if the surrounding medium has the same concentration as the fluid in the cell, it is described as isotonic. Hence, there is no movement of water molecules to either side.top↑

So therefore, isotonic is when there is no net movement of water molecules from a living cell to its environment.

Demonstration Of Osmosis

Materials needed for the experiment are : piece of cellophane materials, four beakers of same size, rubber bands, four thistle funnels, candles, stirring rod, three retort stands, pipette and granulated sugar or crushed sugar cubes.


1) In one of the beakers, add about 200 cm³ of water and add the granulated sugar or crushed sugar gradually and stir. On dissolving, add more sugar to the solution and stir, repeat until no more sugar will dissolve.

Note The reason why too much sugar is added to the solution is to have a concentrated sugar solution.top↑

2) Secure a piece of the cellophane material over the end of each of the thistle funnels, seal the edge of the thistle funnels with wax from the candle in order to make it air tight.
This might not be necessary if the rubber band holding the cellophane material to the thistle funnel is made very tight.

3) Label the beakers A, B, and C.
Pour water into beakers A and B and pour some of the concentrated sugar solution into the labeled C beaker.

4) Run into one of the thistle funnels a quantity of the same water held in beakers A and B.
Stand this thistle funnel in a beaker A, held up by retort stand.

5) Set up the other two funnels similarly, but in beakers B and C, in place of water, run the concentrated sugar solution into the funnels. Ensure that the three thistle funnels are well dipped into the liquid in the beaker to the same level.top↑
Note : The level of liquid in each of the three thistle funnels must be the same.

6) Mark the position of the liquid in each funnel and allow the three beaker and funnels to stand for one hour. Then observe and record your observations.


If you carry out the demonstration accurately, you will observe that, the level of the solution rose up in thistle funnel B due to the net movement of water molecules into solution in it. Read osmotic pressure to understand the observations better.

Osmotic Pressure And Osmotic Potential

Osmotic pressure is the pressure that causes osmosis to take place.

When a solution is separated from water or solvent by a semi-permeable membrane, osmosis occurs
On one side of the semi-permeable membrane, there are water molecules only and on the other side of the semi-permeable membrane, there are both water and solute molecules which are in constant motion.
Both water and solute molecules hit the semi-permeable membrane but only water molecules can pass through it.top↑
Since there are more water molecules in the water than in solution, more water molecules will diffuse from water through the semi-permeable membrane into the solution than from the solution into the water, as a result of this movement, there will be net movement of water molecules into the solution.

Osmotic Pressure Definition

Osmotic pressure is defined as the amount of pressure needed to prevent the passage of water into a solution when separated from it by a semi-permeable membrane.
Osmotic pressure can also be defined as increases with the concentration of the solution. For example,
A 50% solution has higher osmotic pressure than 30% solution of the same substance.top↑

A solution of sugar in a beaker has the capacity to bring about osmosis, if and when it is separated from water by a semi-permeable membrane while in the beaker, and while it is not separated from water by a semi-permeable membrane, the solution is said to be osmotic potential,

Osmotic Potential

so simply;
Osmotic potential is when a solution is capable of undergoing osmosis but the solution is not yet undergoing it.

You will recall that in our previous class, we observe that, diffusion of petrol from one corner of a room continues to spread out until the petrol molecules are equally distributed in all parts of the room.
Hence, osmosis can be considered that, it will continue to spread until water molecules are equally distributed on both sides of the semi-permeable membrane.top↑

Demonstration Of Osmosis

During the demonstration of osmosis, the pressure on the solution increases the speed of water molecules in the solution and their rate of diffusion from the solution through the semi-permeable membrane into the water.
When equal numbers of water molecules move in both directions in unit time, the rise in the column of the solution will stop.
The excess pressure represented by the column of the solution in the thistle funnel over the solution in the beaker is the measure of the osmotic pressure of the solution.

Demonstration Of Osmosis In Living Tissue

Materials needed for the experiment are : unripe pawpaw fruits or four irish potato tubers, boiling water, sugar solution, three beakers and a knife.top↑


1) Prepare a concentrated solution as did in the above experiment.

2) Cut each pawpaw fruit into two to produce four halves of which only three will be used, " so you can eat the fourth one". If you are using Irish potatoes (not sweet potatpes) in lieu of pawpaw, scoop out a hole in each tuber, then peel the tuber and slice the bottom off so that it can fit into the beaker, you can also use yam tuber as the substitute.

3) peel the pawpaw fruit and slice the bottom off so that it can fit into the beaker.

4) Into one of the half pawpaw or scooped out potato tuber, add water to the pawpaw and labeled it as A. In another half pawpaw, pour some of the concentrated sugar solution and labeled it as B.top↑

5) Place the other half of the pawpaw in the boiling water for one to two minutes. The pawpaw or the yam tuber is placed inside the boiling water in order to kill the cells of the pawpaw or potato, that is, to make it permeable. Withdraw the pawpaw from the boiling water and allow it to cool.

6) Pour some of the concentrated sugar solution into this treated pawpaw or yam tuber and labeled it as C, then place each specimen in a beaker of water for about two to three hours.


There should be a rise of liquid level only in B and not in A or C.


In this experiment, the cells of the raw Irish potato or pawpaw act together as a semi-permeable membrane allowing the passage of water but not sugar, that is, the solute.
In B, water diffuses through the potato cells down a concentration gradient of water molecules into the sugar solution, hence, the level in B therefore rises.top↑

In C, you killed the pawpaw or the Irish potato tuber cells when you placed it inside the boiling water. Boiling made the cells fully permeable to both water and sugar, allowing the concentration gradients to be removed by the movement of water molecules in and sugar molecules out.
Hence, there is no rise in level as seen in C.

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