|MadSci Network: Botany|
Since this question has many parts, I will try to address them one by one.
First off, plant cells do not take in water until they explode due to the presence of a cell wall. Water will enter the cell until it physically cannot take in anymore water, due to lack of space.
Also, much of the sugar produced in photosynthesis remains in the free space outside the cell so that the difference in concentration between the inside of the cell and the free space is not as great as if the free space had been filled with water.
The second question I will address is, "How can plants survive even if they don't transform their sugar into starch?" Plants can utilize sugar and don't need to convert it to starch. In many plants starch is the carbohydrate of storage and it is hydrolysed to glucose and fructose for transport back into the plant to be used for metabolism.
There are enzymes present in the cell and cell wall (invertases and sucrose synthase) that can hydrolyse sucrose or synthesize sucrose for use in the plant. Sugar beets and sugarcane metabolize and store sucrose differently, and I have supplied you with two papers that discuss what your question is asking.
Robert T. Giaquinta
Sucrose Translocation and Storage in the Sugar Beet.
Plant Physiol. (1979) 63, 828-832
Sugar Accumulation by Sugar-Cane Storage Tissue: the Role of Sucrose Phosphate
Biochem. J. (1964), 93, 521.
As to your question about organisms eating sugarcane and sugar beets due to their high sucrose content, all plants must deal with insects and herbivores due to their carbohydrate content. As you say, the sucrose level in both sugarcane and sugar beets are highly attractive to predation.
I have provided another link for some information on how this problem is being addressed. Many plants, besides sugarcane and sugar beets, produce fruits with a high sugar content, which would also attract herbivores.
Maria Cristina Falco, Phellippe Arthur S. Marbach, Patrícia Pompermayer, Francisco Cláudio C. Lopes and
Marcio C. Silva-Filho
Mechanisms of sugarcane response to herbivory
Genet. Mol. Biol. (2001), 24:(1-4), 113-122.
I hope that this helps a little to answer your questions.
Try the links in the MadSci Library for more information on Botany.