I've learned that plants transform glucose into sucrose before sending it into phloem. But the process seems to be complex and energy comsuming. Why should plants do it? Is it really necessary?
Glucose, fructose and galactose are the three dietary monosaccharides. Glucose and Fructose are simple monosaccharides found in plants. A monosaccharide is the basic unit of carbohydrate and the simplest form of sugar, glucose are aldose and Fructose are ketose.
If the carbonyl is at position 1 (that is, n or m is zero), the molecule begins with a formyl group H(C=O)-, and is technically an aldehyde. In that case, the compound is termed an aldose. Otherwise, the molecule has a keto group, a carbonyl -(C=O)- between two carbons; then it is formally a ketone, and is termed a ketose. Ketoses of biological interest usually have the carbonyl at position 2. http://en.wikipedia.org/wiki/Monosaccharide
Whereas Sucrose is a disaccharide composed of glucose and fructose. A disaccharide is more complex than monosaccharide, more complex compounds like oligosaccharides and polysaccharides exists. Sucrose synthesised within the cytosol of photosynthesizing cells is then available for general distribution and is commonly trans located to other carbon-demanding centers via the phloem.
Sucrose and starch are more efficient in energy storage when compared to glucose and fructose, but starch is insoluble in water. So it can't be transported via phloem and the next choice is sucrose, being water soluble and energy efficient sucrose is chosen to be the carrier of energy from leaves to different part of the tree. Another problem exists, glucose is highly reactive and this may result in some intermediate reactions while transporting glucose. Being a complex structure, sucrose is not as much reactive as glucose. So plants uses the sucrose as a medium to transfer energy. Inside the cells, sucrose is converted back to glucose and fructose. Energy is yielded when it is needed. So plants transfer glucose and fructose in the form of sucrose in order to:
- Increase energy storage
- Efficient energy transfer
- Removing in between reactions
there is no free glucose in the photosynthesis. Stop to spread that myth. The net product is G3P. The end products of photosynthesis are sucrose and starch, but never glucose. Do you test glucose in the leaves? No... it is always for starch. ;) The G3P is converted to sucrose and other molecules, for example, thiamine. Glyceraldehyde 3-phosphate occurs as a reactant in the biosynthesis pathway of thiamine (Vitamin B1), another substance that cannot be produced by the human body. Part of sucrose is then translocated to the phloem. Starch is stored in the stroma of chloroplasts. It is also stored in the amyloplasts in the roots, stems cells after sucrose suffers a conversion to starch.