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PUPILS' EXPERIMENTS WITH OSMOSIS IN CARROT TISSUE

In this experiment, pieces of carrot will be placed into liquids of different concentrations, to show how water passes by osmosis into or out of plant tissue. The uptake or loss of water will be monitored by measuring changes in mass, using a top-pan balance.

Each pupil/pair of pupils will perform part of the experiment but these will be pooled, so that each bench will have its own set of results. These will then be pooled once again, so that it can be seen how comparable these results are to the rest of the class. Each member of the class will need to use the same balance, so you must work quickly and efficiently. It is also important to work fairly quickly to prevent the material from drying out.

You will be provided with carrots, cut into about discs about 2-3 mm thick. It is advisable to handle them carefully (perhaps with forceps), and to take them to and from the balance in a plastic Petri dish, along with your results table. All weighings will take place in a special marked dish, which may occasionally need to be wiped out and replaced for the next user. Do not put pieces of carrot directly onto the balance pan. Make sure you understand the balance's taring system (by which the weight of this container is cancelled out).

Procedure

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1) Collect a Petri-dish containing 6 carrot discs.

2) Take them to the balance, weigh them as a group but not the dish and record the result in the table provided, under "mass at start".

3) Add enough of the liquid (A,B,C,D,E or F as instructed) to cover the base of the Petri-dish (about 20 ml). Spread out the carrot discs in the liquid, and leave the dishes in a safe position on the bench for as long as possible.

4) Towards the end of the period, drain away the liquid into the sink, and blot the discs on paper towel.

5) (Making sure the balance is reading zero correctly before starting:) Re-weigh the discs but not the dish and record the result, under "mass at end".

6) Feel the texture of your carrot slices, then discard them in the colander provided and place the Petri dishes into the washing-up bowl.
What did the discs feel like?
> A - crisp and not flexible ----F flabby and easily bent

7) Calculate the start:end ratio to 2 decimal places only, (probably using a calculator):
"mass at start" divided by "mass at end". Key in the first mass, then "÷", then the second mass, then "=".

8) Place this result into the table at the appropriate place, then obtain the complementary results from others on your bench. At the end of the session, other benches' results should be available.

RESULTS TABLE

(use underline to show which are your own results)

*Table for carrot discs expt*
SOLUTION	A	B	C	D	E	F
Molarity	0.00 M	0.15 M	0.30 M	0.45 M	0.60 M	0.75 M
(i.e. strength)	(pure water)	(increasingly more concentrated sodium chloride solutions ->)
Mass at start /g	>	>	>	>	>	>
Mass at end /g	>	>	>	>	>	>
start : end ratio	>	>	>	>	>	>

start : end ratio - other benches	> > >	> > >	> > >	> > >	> > >	> > >
MEAN VALUE	>	>	>	>	>	>

Now do your best to answer these questions:

Why take all this trouble with discs? Surely a single 15 or 20g chunk of carrot would suffice?

> more surface area exposed - reliable results obtained more quickly

Why all this fuss about start:end ratio? Surely the difference between start and end is good enough?
In other words: isn't it easier to do taking away than dividing(?)

> The ratio is better because it makes allowance for the fact that larger chunks of carrot would increase (or decrease) in mass more in proportion to original mass (and the converse for smaller chunks). Then the ratio can be used to forecast what might happen in other circumstances, whereas the difference itself would be meaningless in another context.

What would a start:end ratio of 1.00 indicate?

> no change in mass - no water has entered or left cells

What would a start:end ratio of greater than 1.00 indicate?

>mass has gone down (water has been lost from cells)
If the discs have lost 0.5 g in mass, what volume of water has gone from them?

>0.5 ml or 0.5 cm³; By definition, water has a density of 1.0000 g per ml or 1.0000 g per cm³ or 1.0000 g.cm^-3

Check that you understand these questions. They show whether it was worthwhile doing the experiment.

What do these results show about the concentration of (solutes in) the cytoplasm in the carrot cells at the start of the experiment?

> Concentration of solutes in carrot cytoplasm was about the same as say B or C - equivalent to 0.15 or 0.3 molar sodium chloride

What can you say about the repeatability of this experimental results by other groups?
> Practically the same pattern was shown in each case (Start:end ratio less than 1.00 in A, crossing 1.00 then gradually increasing to above 1.00 then levelling off) so the repeatability of the results is high, and you can be confident about the conclusions.

This topic has connections with other units on:-

CELL STRUCTURE
HOW SUBSTANCES GET INTO AND OUT OF CELLS - 1 (OSMOSIS)
HOW SUBSTANCES GET INTO AND OUT OF CELLS - 2 (PLASMOLYSIS + DIALYSIS)
TRANSPORT & SUPPORT IN PLANTS

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