Re: Why do plants transport carbohydrates as sucrose?

First, lets make some things clear. The concept of the "beginning of a 
metabolic pathway" is a didatic concept. This means that it was created by 
teachers just to make their lives easier. For the plant, however, the 
reactions follow each other ina continuous way. In some plants you could 
say that starch is the beginning of respiration. In other systems, other 
carbohydrates like xylose, mannose, etc. are important substrates for 
respiration.

Another important thing is that not all plants transport sucrose. Some 
transport raffinose, others fructans, but thats another story.

Answering your question (finally), take a look at the reactions in 
respiration. You will find that fructose is just below glucose in the 
pathway. This means that fructose can enter in the respiration as easy as 
glucose. In fact, in the cell, many sugars are comming in and out of this 
pathway: xylose, mannose, galactose, ribose, etc. All of them can be easily 
converted in each other.

This brings another question. Why not transport these sugars free (as 
monosaccharides)? Why does the plant link them together (as sucrose, for 
example) just to split them in their final destiny? The key is a property 
of the free monosaccharides called "reducin power". Free monosaccharides 
are highly reactive when free in the cell. The rise of their concentration 
in the plant can lead to a series of inespecific chemical reactions 
(glycosilations) that could kill the plant. In the case of sucrose, all of 
the aldehydes (the reactive part of the sugars) are hiden (linked to each 
other). This makes sucrose a very stable and convenient way to transport 
sugars. This is also the case for others oligosaccharides such as rafinose 
anf fructans, but does not occur in maltose, which is much more reactive 
than sucrose.

The details of this are very complicated and involves a lot of organic 
chemistry. I will not explain further here but if you are really 
interested, mail me directly at m_thorn@hotmail.com.

Bye,
Marco.