Re: the effect of 0.2M, 0.3M and 0.4M of s. on plant c

Hi, 

You've got it, it is the concentration of solution that make hypertonic 
solution, no matter whether it is caused by sugar or salt. Cell membrane plays an 
important role because it is semipermable: it allows the passage of solvent 
(water), but much less passage of solutes. 
This restriction of diffusion of solute can result in the establishment of 
water potential. The concept and calculation of water potential is complex 
in case of real cell, but generally speaking, water potential of a solution 
is lower than that of pure water, and higher the concentration of solute, the 
lower the water potential. Furthermore, water molecules have the tendency of 
diffusing from the location of higher water potential to the location of 
lower potential until the two locations reach equilibrium. 

Now, consider a cell in sugar solution; we suppose the water 
potential of the cell is A, and the water potential of sugar solution is B. 
If AB, then cell will 
lose water. So, if sugar solution has really high concentration, the water 
potential will be low, it will "absorb" water from cell. 

OK, then let's talk about the function of the epidermis in solution, if I do 
not misunderstand your question. In the case of plant tissues such as 
leaves, the normal epidermis cell contains cuticle and has a thick cell wall, 
that prevents the direct contact of functional mesophyll cells and the 
outside solution, thus protecting mesophyll cells from dramatic change of 
inside solution's concentration. In the case of a single plant cell, due to 
the rigidity of the cell wall, a pressure can be built up inside of the 
cell, so plant cell will not break so easily as animal cells when it absorbs 
water.

Hope this will help.



Reference: 
Salisbury & Ross, Plant Physiology, Fourth Edition, Wadworth Publishing Co., 
1991, Chapter 2 & 3