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Osmosis question AS biology AQA

A group of students investigated the effect of sucrose concentration on the change in length of cylinders of tissue cut from a young carrot. They measured the initial lengths of the carrot cylinders, then placed one in each of a number of sucrose solutions. After 18hours, they removed the carrot cylinders and measured their final lengths. Some of the results are shown in the table.

(a) Young carrots store sugars in their tissues but, in older carrots, some of this is converted to starch. How would using cylinders of tissue from older carrots affect the results obtained for a sucrose solution of 0.6moldm–3? Give a reason for your answer.

Mark scheme:
Greater decrease/length smaller; More water removed; Greater difference in water potential/cell with higher/less negative water potential; Starch is insoluble/has no effect on osmosis

can someone explain why?

Reply 1

zabveniye

There is something confusing about the wording of this question … I was stuck too, at first, but after looking at it for a while, I think I can explain. :smile:

It is simple, really, once you get past the odd wording.

Water potential is the key concept here. Pure water has the highest water potential(0Ψ)– sugars, as they bind to water molecules, lower water potential. The more sugar there is within tissue, the lower its water potential will be.

Osmosis is the diffusion of water down a water potential gradient – from an area of high water potential to an area of lower water potential.

As the question explains, young carrots store sugars in their tissues.The sugar they store lowers the water potential of their tissues.

In older carrots, some of the sugars are converted to starch, so they do not have as many sugar molecules to bind to the water molecules.

Starch does not bind to water molecules (it is insoluble), and therefore does not affect water potential or osmosis.

All of this means that the water potential in tissues of old carrots > the water potential of tissues in young carrots.

In practise:

Due to the sugars they store, young carrots have a lower water potential than pure water: if placed in pure water and left for a few hours,water will move by osmosis INTO the young carrots, and their length should increase.

Older carrots also store some sugars, but a lower concentration of them than young carrots – they will have a lower water potential than pure water, but a higher water potential than young carrots. If placed in pure water and left for a few hours, water will still move by osmosis into the tissues of the old carrots, BUT less water will move into them than into the young carrots, as there is less of a difference between the water potential of tissues of older carrots and of pure water.

Now here's the important part.

I found and took a look at the results mentioned in the question you're referencing here. At all the concentrations of sucrose solution investigated, the length of the carrots placed in them decreased. This means that, unlike pure water, sucrose solution has a LOWER water potential than the tissues of the carrots, and so water will move by osmosis the other way – from the tissues of the carrots into the surrounding sucrose solution, accounting for the decrease in length in the carrots.

To use some random figures (not actual measured figures!) just to illustrate the point, say the sucrose solution has a water potential of -1000Ψ.
Pure water has a water potential of 0Ψ.
Let's say that the tissues of young carrots have a water potential of -800Ψ.
The tissues of older carrots will have a higher water potential than young carrots; say -200Ψ.

Water always moves by osmosis from an area of higher water potential to an area of lower water potential. The greater the difference in water potential, the greater the movement of water will be.

In pure water;
Some water moves by osmosis from the surrounding water (0Ψ) into the tissues of older carrots (-200Ψ.)
More water moves by osmosis from the surrounding water (0Ψ)into the tissues of younger carrots (-800Ψ.)

In sucrose solution;
Some water moves by osmosis from the tissues of younger carrots (-800Ψ)into the sucrose solution (-1000Ψ.)
More water moves by osmosis from the tissues of older carrots (-200Ψ)into the sucrose solution (-1000Ψ.)

So more water is removed from the tissues of older carrots than from those of younger carrots, and the decrease in their length is therefore greater.

I hope this clarifies things! :smile:

Feel free to ask if any of what I wrote is unclear, or you have any more questions about it at all.

Reply 2

James08836

Starch is insoluble so you are effectively dealing with, 100% water vs. 0.6mol/dm-3. This means the level of osmosis occurring will be far greater than a young carrot with dissolved sugars because there is a bigger gap and I'm sure you know that the greater the concentration difference the more osmosis occurs.