Hi guys,
I was having trouble with this question in my biozone workbook and was wondering if someone could explain how they got the answer ??
The answers were
1a urea
1b sucrose
1c urea, glycerol, glucose, sucrose
Thanks 
I am not entirely sure where you are confused (when asking a question next time it would be great to point this out) but I would imagine that its probably confusing about what the absorbance reading indicates. As the information at the top of the page explains, molecules diffusing into red blood cells will draw water into the cells and may cause them to lyse - to break apart. A solution of these red blood cells will normally be coloured, and hence will absorb specific wavelengths of light, however when these red blood cells are lysed (such as when a molecule that readily diffuses into the cell is introduced to the solution) the cellular material seperates out from the solution and leaves the solution (essentially) colorless, meaning now the solution will no longer absorb those specific wavelengths. This is what the graph in the question is representing - the absorbance of the red blood cell solution when different molecules have been added. An absorbance of ~0 would indicate that the solution is colourless (no light is being absorbed by the solution) whereas absorbance readings like those for glucose throughout the experiment would indicate the solution is colored and is absorbing light. Remembering that the solution losing colour and thus absorbing less light indicates that red blood cells are being lysed, we can understand that the reductions in absorbance reading for certain molecules indicate that lysis of the red blood cells is occuring. Remember also that this lysis is caused by the diffusion of molecules into the red blood cell. Putting this together we can see from the graph that the fastest a solution is decreasing in absorbance, the faster the red blood cells are lysing, and the faster that particular molecule is diffusing into the cell.
Here we see urea's absorbance reading dropping very quickly which indicates very quick lysis and a great rate of ureas diffusing into these red blood cells. We can also see glycerol's absorbance drop, but this occurs much slower (looking at the x-axis showing time in seconds) which suggests that glycerol is infact diffusing into the red blood cells and causing them to lyse, however is not doing so at a rate as great as the other two - the glycerol molecules are simply taking longer to cross the membrane than the urea molecules. Looking at glucose we can see that its absorbance remains essentially constant right until about the 700 second mark where it seems it begins dropping slightly in absorbance, suggesting that very gradually some red blood cells are lysing, and thus indicating that glucose is diffusing across the membrane, however at a very slow rate. Then turning to sucrose we see that the absorbance reading doesn't drop at all, suggesting that no lysis is detected, which itself indicates that either sucrose isn't diffusing into these red blood cells, or that the rate at which it is diffusing is so low that within 840s, no lysis has occured but may if the experiment occured over a longer time period.
Hopefully this helps you understand the question, however please let me know if there is still something you don't understand
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