If the environment is hypertonic (having a higher concentration of solute than the cell), water diffuses out of the cell. This explains why bacterial cells dehydrate and die in concentrated salt water and why foods can be stored in a salt solution without spoiling.
If the environment is hypotonic (having a lower concentration of solute than the cell) water diffuses into the cell. Plants do not wilt in a hypotonic environment. Water diffuses into the plant cell, the cell swells and presses against the cell wall, and the plant stiffens. Expansion of the cell is controlled by the resistive pressure of the cell wall, which increases as the cell distends. This pressure, called turgor pressure, prevents water from continuing to enter the cell.
In animal cells there is no rigid cell wall. Without a mechanism to counteract osmosis, water would diffuse into animal cells until they burst. In one-celled organisms, a structure called the contractile vacuole pumps water from the cell through the membrane to the environment to maintain equilibrium. In humans, excess water is excreted as urine and sweat. Unlike diffusion, in which the cell passively accepts or loses substances, these evacuative processes are active and require the cell to expend energy.
The process of osmosis is not limited to water and cell membranes--many other substances undergo osmosis as well. For example, pyridine (a liquid extracted from coal tar) will diffuse through a rubber membrane into a solution of sugar and pyridine.
As any solvent diffuses through a membrane into a fixed volume of solution, the pressure within the fixed volume increases. The osmotic pressure of a solution is the external pressure that must be applied to the solution to prevent the diffusion of solvent from pure solvent into the solution.
Osmosis – My Understanding
Research
Osmosis is the passage of water through a ‘semipermeable Membrane’ (meaning the cell is permeable to some molecules and not others from an are of low concentration salt or sugar to an area or region of high salt or sugar concentration. If the environment has a lower concentration of salt or sugar than the cell the water from continuing to enter the cell. If water were withheld from a plant the plant would wilt and die. This is why plants survive better, where the water they take in through osmosis is pure.
Through this investigation, I am going to try to find out the point of equilibrium in a potato. Equilibrium is where the concentration of salt in the water (or environment ) is equal to the concentration of the salt in the potato. I will know where the point of equilibrium is because when this certain amount of water is added to the water the potato will not gain or lose any mass.
This investigation is a piece or evidence to prove osmosis
These diagrams explain the process of Osmosis;
Jade Hill _ Osmosis Investigation
Finding the Point Of Equilibrium In a Potato
Aim
The aim of this investigation is to find the point of osmotic eqauallibrium in a potato (where the concentration of salt in the potato is equal to the concentration of salt in the water and the potato does not gain or lose or lose any mass when this amount is added).
Prediction
I predict if you put the potato chip in a strong salt solution it will lose mass because the concentration of salt surrounding the potato is grater than the salt concentration in the potato. There for the water will diffuse out of the potato into the salt solution, this will make the potato chip flaccid.
If I put the potato chip in a weak salt solution the potato will not lose or gain water because the concentration of salt in the potato will be equal to the concentration of salt in the solution. The potato will reach equilibrium so if the potato loses four water molecules it will gain four water molecules.
If I put the potato chip in water the mass will increase because the concentration of salt in the potato will be stronger than the concentration of salt in the water. Water will diffuse into the potato chip making it turgid and fat.
Chips I may not have been acurate enough. When I complete my full experiment, I will have to make sure I am accurate as I can be.
Evaluation
I now know that the point of equilibrium is somewhere between 1% and 5% (although this may vary with different potato’s) so I am going to test the potato in solutions of 1%, 2%, 3%, 4% and 5% salt water. When I think I have found the point of equilibrium I can find a more exact point by high lighting the two percentages it lies between and narrowing down the percentage for example, if the mass of the potato increases at 2% and decreases at 3% I will know the point of equilibrium is between these figures. Using this I could test at 2.5%, if the mass then decreases I know that the point of equilibrium is between 2% and 2.5% and so on.
To make the experiment more accurate I am going to leave the chips to soak in the water for 69 hours and 30 minutes over the weekend to give the potato’s a chance to loss or gain mass.
The investigation
Factors Effecting the Experiment
The independent variables affecting the experiment are;
The volume of salt solution used
The time the potato chip is left in the salt solution – leaving it In too long may cause some of the water to evaporate making the salt solution more concentrated.
Using different water - we are using distilled tap water for this experiment.
∙. The temperature of the solution- all of the beakers of solution with chips left in them must be left in the same area to keep them at the same temperature (room temperature) this needs to be ensured for a number of reasons ; evaporation occurs much quicker at high temperatures, the high the concentration the more chance of diffusing between the potato chip and the solution and the volume of water used is also effected by the temperature of the water.
∙. The potatoes must be placed into the beakers at the same time because otherwise the some potato’s will have more time in the salt solution and may increase in mass more than the others.
∙. It its important to use one brand of potato because different brands may have different osmotic potentials, the test could be even more accurate if we are able to use one potato for the whole experiment, like wise they must all be taken out at the same time.
∙. After the potato chips have been left in the solution it is important to dry off the excess water from them before weighing them as excess water could effect the end weight of the potato.
Equipment
Beakers (100ml)
Top Pan Balance (for accuracy when weighing the potato’s before and after the experiment)
Salt (table salt)
Measuring cylinders (to measure the water before creating the salt solutions)
Stop Clocks (accurate timing will be needed to make give reliable results)
Paper towels (to remove excess water from the surface of the water)
Knife (needs to be clean and sharp to give a clean cut on the chips)
Method for making salt solutions
1. With measuring cylinder measure out 100cc of water at 20*c (room temperature to give accurate results
2. Pour water into glass beaker
3. Weigh out salt (1gm, 2gm, 3gm, 4gm and 5gms) using the top pan balance for accuracy.
4. Stir solution three times to dissolve the salt in the water.
Method For Experiment
Cut potato chips to desired mass ( in my case 40mm), measuring accurately. This can be done using the potato chip cutter and lining the chip up along side a ruler measuring the length of it.
1. Weigh out the salt portions (once for each percentage).
2. Measure out water required, this must be done very accurately for a fair result.
3. Once the salt and water is ready put the salt into the water at the same time, stir the water and salt three times to help dissolve it.
4. Cut 15 chips at 4cm.
5. Place all the chips in the water at the same time (using some assistance) to ensure a fair test and start timer.
6. Leave the beaker in a cool dry place for the 69 hours and 30 minutes.
7. After 69 hours and 30 minutes take each potato out of the water at the same time and wipe off excess water on paper towels by rolling the chips three times (this is again to ensure fair results are gained).
8. Weigh up the potato chips and record the results, this needs to be done very accurately (to ensure accuracy is achieved a top pan balance is used).
Diagram for method