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CLICK ON THE SLIDE SHOW BELOW TO CONTROL IT! Pause, Play, Forward, Reverse, or SKIP! ENJOY!!

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In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. The term aqueous solution is when one of the solvents is water. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent.

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Tonicity is a measure of the effective osmotic pressure gradient, as defined by the water potential of two solutions separated by a semipermeable membrane. In other words, tonicity is the relative concentration of solutes dissolved in solution which determine the direction and extent of diffusion.

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Osmosis - GCSE Science Required Practical

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Osmosis () is the spontaneous net movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, in the direction that tends to equalize the solute concentrations on the two sides.

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The most notable observation here is that 6% salt solution is hypertonic but 6% sugar solution is hypotonic. The outcome of this later trial is the most surprising of them all

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Tonicity is a measure of the effective osmotic pressure gradient, as defined by the water potential of two solutions separated by a semipermeable membrane. In other words, tonicity is the relative concentration of solutes dissolved in solution which determine the direction and extent of diffusion.

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Saline, also known as saline solution, is a mixture of sodium chloride in water and has a number of uses in medicine. Applied to the affected area it is used to clean wounds, help remove contact lenses, and help with dry eyes.

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The Natural Sugar Concentration in Potatoes

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With their high potassium and low sodium content, potatoes are a beneficial food for managing your blood pressure levels. While potatoes are high in carbohydrates, most of these carbohydrates are in ...

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Remember that osmosis is defined a net movement of particles of solvent along its concentration gradient, across a selectively permeable membrane, until equilibrium is established.

In this experiment the solvent particles are water particles, the selectively permeable membrane are the cell membranes of the cells in the potato tissue. The concentration gradient is the result of the difference in the concentration of solute in the cytoplasm of the potato cells and the solutions.

Apparatus & Materials

3 Potato strips (5 x 1 x 1) cm (or potato cylinders/cores of equal length e.g. 5 cm diameter, made with a cork borer)

2 Petri dishes with equal volumes of solutions A and B ; A = sucrose or salt solution (hypertonic solution); B = distilled water ( hypotonic solution)

1 empty petri dish (control)

Stop watch or timing device

Procedure

1. Observe each strip by feeling it, noting whether it is turgid or flaccid. Record this.

2. Weigh and measure each potato strip, recording the initial mass and length

3. Place a strip in each petri dish, taking care not to mix the strips. Start the timer.

4. Remove the strips after 15 minutes and dab on tissue.

5. Weigh each strip and measure each potato strip, recording the final mass and length.

6. Observe each strip by feeling it, noting whether it is turgid or flaccid. Record this.

7. Perform % difference calculations for the mass and length using the formula:

8. (final - initial) x 100% / initial

Record your results in a table in your book like the one below:

TABLE SHOWING RESULTS OF EXPERIMENT
TO DEMONSTRATE OSMOSIS USING POTATO STRIPS

Discussion

Define osmosis and identify the semi-permeable membrane in the experiment

Osmosis is the movement of solvent particles e.g. water along its concentration gradient - from high to low concentration - via a semi-permeable membrane. In this experiment the semi-permeable membrane is the cell (surface) membrane.
Strips in Salt Solution

Account for the changes - if any - in the mass and length of the strips in water, salt solution and air.

Strips in water

Description - Turgid

Mass - The increase in mass of the potato strip in water is due to the movement of water molecules into the plant cells via osmosis. Water molecules in the petri dish are at a higher concentration than water molelcules in the cytoplasm of cells and so move along their concentration gradient into the cells.
As more water molecules are present in cells - and water has mass - the final mass of the potato strip will be greater than the first.

Length - the decrease in length is also due to the movement of water molecules via osmosis. More water molecules occupy more space - volume - and push against the cell membrane and the proximate cellulose cell wall. The push of the water molecules result in expansion in all dimensions, including length. The expansion is limited - and regulated by the cellulose cell is inelastic. The push/force acting upon the surface area of the rigid cell produces turgor pressure (recall pressure is force acting per unit area), and hence strips are turgid.

Mass - the decrease in the mass of the potato strip is due to the movement of water molecules outside of the plant cells. Water molecules in the cytoplasm are at a higher concentration than in the salt solution in the petri dish and so move along their concentration gradient out of the cells. Water molecules have mass and so the decreased number of water molecules results in decreased mass.

Length - the decrease in the number of water molecules results in loss of volume. The turgor pressure exerted by the water molecules against the cell membrane and consequently the cellulose cell wall is less. The cell may even shrink if the turgor pressure dminishes further.

Strips in Air

Description - Slightly flaccid. This strip as the control and theoretically does not change in texture. However in reality it loses water by evaporation via the cut surfaces and becomes dehydrated. By the end of the experiment the strip in air is slightly flaccid, but certainly not as much as the strip in salt solution.

Mas - mass changes are minimal as minimal water was lost.

Length - Length changes were minimal as minimal water was lost.

study guide

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Answers to osmosis discussion questions: "Why is there are change in mass, length and texture in potato strips immersed in water or salt/sugar(hypertonic solution)? What does turgid or flaccid mean?

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A cork borer, often used in a chemistry or biology laboratory, is a metal tool for cutting a hole in a cork or rubber stopper to insert glass tubing. Cork borers usually come in a set of nested sizes along with a solid pin for pushing the removed cork (or rubber) out of the borer.