Osmosis in plant tissue
Definition
Osmosis
The net movement of water molecules from a dilute solution to a concentrated solution through a partially permeable membrane.
- The cells inside a potato hold a solution of sugars and salts, and that solution has its own concentration.
- Put potato tissue in a solution more dilute than its cells and water moves in by osmosis, so the tissue gains mass and feels firm.
- Put it in a solution more concentrated than its cells and water moves out, so the tissue loses mass and goes floppy.
- If the outside solution matches the concentration inside the cells, water still crosses the membrane but there is no net movement, so the mass does not change.
- That no-change point is what the core practical is designed to find, because it reveals the concentration inside the potato cells.
Key Idea
A change in mass is the visible evidence of water moving by osmosis, which is why mass is the quantity you measure.
Turgid and flaccid cells
Definition
Turgid
Describes a plant cell that has gained water by osmosis until its contents press firmly against the cell wall.
- As water enters a plant cell the vacuole swells and pushes the cytoplasm and membrane out against the cell wall.
- The rigid cell wall resists that push, so the cell becomes firm rather than bursting, and the pressure helps hold a stem or leaf up.
- As water leaves, the vacuole shrinks and the contents stop pressing on the wall, so the cell becomes soft and the plant wilts.
- If a great deal of water is lost, the membrane pulls away from the wall altogether, which is called plasmolysis.
Definition
Flaccid
Describes a plant cell that has lost water by osmosis so its contents no longer press against the cell wall and the cell becomes soft.
Note
An animal cell has no wall, so in a dilute solution it swells until it bursts and in a concentrated one it shrivels, which is why the words turgid and flaccid are used for plant cells only.
Practical
Osmosis in potatoes
- Aim: to find how the concentration of a sucrose solution affects the mass of potato tissue, and from that work out the concentration inside the potato cells.
- Apparatus: one potato, cork borer and knife, white tile, ruler, a balance reading to 2 decimal places, paper towels, boiling tubes and rack, waterproof marker, sucrose solutions from 000 up to about 550 g/dm3550\,\text{g/dm}^3550g/dm3, forceps, stop clock and eye protection.
- Method:
- Cut cylinders from a single potato with a cork borer, then trim them all to the same length on a white tile, typically 3 cm3\,\text{cm}3cm.
- Blot each one gently with a paper towel to remove the surface liquid.
- Weigh each cylinder and record its initial mass to 2 decimal places.
- Label one boiling tube per concentration and put 20 cm320\,\text{cm}^320cm3 of the right solution in each.
- Drop one cylinder into each tube, start the clock and leave them for at least 30 minutes so the water movement reaches equilibrium.
- Lift each cylinder out with forceps, blot it in exactly the same way as before, and reweigh it.
- Variables: you change the sucrose concentration and you measure the percentage change in mass, while keeping the length and diameter of the cylinders, the volume of solution, the temperature, the time and the blotting the same.
- Results: in dilute solutions the cylinder gains mass and feels firm because water has moved in and the cells are turgid, and in concentrated solutions it loses mass and bends easily because water has moved out and the cells are flaccid.
- Maths: work out the percentage change in mass for each tube, plot it against concentration, draw a line of best fit, and read off the concentration where the line crosses zero, because there the solution matches the inside of the cells.
- Watch out: blot every cylinder the same number of times because leftover surface water is the biggest single error here, use one potato throughout since two potatoes can differ in internal concentration, and leave the tubes long enough for the movement to finish.
- Safety: cut on a tile with the blade moving away from your fingers because a cork borer slips easily, and wear eye protection.
Calculating percentage change in mass
- Subtract the initial mass from the final mass, divide by the initial mass, then multiply by 100100100.
- percentage change in mass=final mass−initial massinitial mass×100\text{percentage change in mass} = \dfrac{\text{final mass} - \text{initial mass}}{\text{initial mass}} \times 100percentage change in mass=initial massfinal mass−initial mass×100
- A positive answer means the tissue gained water, and a negative answer means it lost water.
- Percentage change is used rather than raw change in mass because the cylinders never start at exactly the same mass.
- A gain of 0.4 g0.4\,\text{g}0.4g means far more to a small cylinder than to a large one, so percentages make the results comparable and allow class data to be pooled.
Common Mistake
Divide by the initial mass rather than the final mass, and keep the minus sign on a loss, because a lost negative sign turns a decrease into an increase.
Example
- A cylinder in distilled water starts at 4.20 g4.20\,\text{g}4.20g and ends at 4.62 g4.62\,\text{g}4.62g.
- 4.62−4.204.20×100=0.424.20×100=+10%\dfrac{4.62 - 4.20}{4.20} \times 100 = \dfrac{0.42}{4.20} \times 100 = +10\%4.204.62−4.20×100=4.200.42×100=+10%
- The gain shows water moved into the cells, so the distilled water was more dilute than the cell contents.
- A cylinder in concentrated sucrose starts at 4.50 g4.50\,\text{g}4.50g and ends at 4.05 g4.05\,\text{g}4.05g.
- 4.05−4.504.50×100=−0.454.50×100=−10%\dfrac{4.05 - 4.50}{4.50} \times 100 = \dfrac{-0.45}{4.50} \times 100 = -10\%4.504.05−4.50×100=4.50−0.45×100=−10%
- The loss shows water moved out, so that solution was more concentrated than the cell contents.
Hint
Give percentage change to the same number of decimal places as the data allows, usually one, and always include the sign.
Reading the graph
- Put concentration on the horizontal axis and percentage change in mass on the vertical axis.
- The vertical axis needs values above and below zero, because the results include gains and losses.
- Draw a line of best fit through the points rather than joining them dot to dot.
- Where the line crosses the horizontal axis the percentage change is zero, so there is no net movement of water.
- That concentration equals the concentration of the solution inside the potato cells, which is the conclusion the practical is built around.
- To pin the crossing point down more precisely, use more concentrations clustered near it and repeat each one.
Exam technique
- Draw the read-off lines on the graph in pencil when you find the crossing point, because examiners credit the working as well as the value.
- The mark most often dropped is the conclusion, so state that at that concentration the solution and the cell contents are equal.
- Circle an anomalous point and leave it out of the line of best fit rather than forcing the line through it.
Common Mistake
- The potato gains mass because water enters, not because sucrose enters, since sucrose cannot cross the membrane.
- Zero percentage change means no net movement rather than no movement at all.
- Blotting is a controlled variable and not a source of the change, so blot identically rather than skipping it.
- Osmosis is passive, so a potato does not use energy to take water in.
Self review
- Write the equation for percentage change in mass.
- A cylinder goes from 3.00 g3.00\,\text{g}3.00g to 2.70 g2.70\,\text{g}2.70g, so what is the percentage change?
- Why is percentage change used instead of change in mass?
- What does the point where the line of best fit crosses the x-axis tell you?
- Explain why all the cylinders should be cut from the same potato.
