practical 3 Flashcards
Production of a dilution series.
Determining the water potential of potato tuber cells .The required number of potato cylinders are cut
Their initial mass is measured and recorded before placing into the solutions
They are left in the solutions for a set amount of time, usually in a water bath
They are then removed and dried
The final length and mass of each potato cylinder is then measured and recorded and produce calibration curve ,calc percentage change in mass (y) against sucrose conc
In this investigation, you were given potato cylinders taken from potatoes of the same
variety.
A)Give one reason why this was important.
B). In this investigation, you should have used potato cylinders that were all the same length.
Suggest and explain one reason why this was important. 2
a)1. (Cells/tissues) have same/similar
water potential (sugar content);
2. (So) no genetic differences/all cells
genetically the same;
b)Same/similar surface area;
2. (Otherwise) affects rate of water
uptake/water loss/osmosis/
diffusion;
Production of a dilution series.
Determining the water potential of potato tuber cells .The required number of potato cylinders are cut
They are all cut , their initial mass is measured and recorded before placing into the solutions
They are left in the solutions for a set amount of time, usually in a water bath
They are then removed and dried
The final length and mass of each potato cylinder is then measured and recorded and produce calibration curve ,calc percentage change in mass (y) against sucrose conc
a)Potato skin had been removed from the cylinders given to you.
Suggest one reason why the potato skin could have affected osmosis.
b). Describe how you would use your results for 100% blackcurrant squash to find the rate of
osmosis.
a)1. Potato skin is impermeable to water;
2. (Skin is) a different tissue;
3. Cells of skin have different water
potential (to rest of tissue);
b)1. (Find) change/difference in mass;
2. Divide by time/30 minutes;
Make a series of dilutions
of 1M sucrose solution. These should be at 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0M sucrose
. Dilute using distilled water.
how much distilled water and how much sucrose solution do we needto make 20cm of solution having conc of
i)0
ii)0.2
iii)0.4
iiii)0.6
iiiii)0.8
iiiiii)1
1)to make 0 need 20cm of distilled water and 0 of sucrose
2) (method)to make0.2 conc, use c1v1=c2v2
c1 = initial conc which is 1M
v1=amount of sucrose volume initially needed(unknown)
c2=the conc want
v2=volume want (20cm)
v1=c2v2/c1
0.2x20/1=4 cm of sucrose so to make need 16cm of water
3)0.4= suc vol= 8 water vol=12
4) 0.6=suc vol=12 water vol=8
5)0.8 suc vol= 16 water vol=4
6)1 suc vol= 20 water vol=0
A student investigated the effect of surface area on osmosis in cubes of potato.
* He cut two cubes of potato tissue, each with sides of 35 mm in length.
* He put one cube into a concentrated sucrose solution.
* He cut the other cube into eight equal-sized smaller cubes and put them
into a sucrose solution of the same concentration as the solution used for
the large cube.
* He recorded the masses of the cubes at intervals.
His results are shown in the graph.
Describe the method the student would have used to obtain the results in
the graph. (Y- TOTAL MASS IN G X-TIME IN M0 (SHOWING CHANGES IN 1 LARGE CUBE AND 8 SMALL CUBES) Start after all of the cubes of potato have been cut. Also
consider variables he should have controlled. 3
- Axes correct way round with linear scales;
Axes labelled with mol dm−3 and ratio without units;
Correct values correctly plotted and suitable curve drawn; - Accept point to point or smooth curve but no
extrapolation
NFP – 3. Graph starts just below 1.4 and finishes
just above 0.7 and looks right.
(0.8 mol dm−3 sucrose) solution has a more negative / lower
water potential than potato (cytoplasm);
OR
potato (cytoplasm) has a less negative / higher water potential than
(0.8 mol dm−3 sucrose) solution;
(therefore) water moves out (of potato) into the (sucrose)
solution by osmosis (so cells decrease in mass); - Accept sucrose solution is hypertonic / potato
cytoplasm is hypotonic
Accept water moves down a water potential
gradient
Method to ensure all cut surfaces of the eight cubes are exposed to
the sucrose solution;
Credit valid method descriptions to fulfil mp1, 2 and
3 (no explanation is required).
Method of controlling temperature;
Accept ‘at room temperature’ for method
Method of drying cubes before measuring;
Measure mass of cubes at stated time intervals;
Accept time intervals between every 5 minutes with
maximum of every 40 minutes.
Accept ‘weigh the cubes at stated time intervals’
what does a positive percentage change in mass mean in terms of water movement
A positive percentage change in mass indicates that the potato has gained water by osmosis (net movement of water from the solution into the potato) meaning the potato had a lower water potential than the solution (hence water moved form high w.p to low w.p)
