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Which of the following is LEAST suited for long-distance phloem transport of photo-assimilated carbon in plants?

Question: Which of the following is LEAST suited for long-distance phloem transport of photoassimilated carbon in plants?

  1. Reducing sugars
  2. Mannitol
  3. Galactosyl-sucrose oligosaccharides
  4. Non-reducing sugars
Answer: 1. Reducing sugar

  • Facts about the Phloem transportation:

Explanation:

  1. Reducing agent: A reducing agent, also known as a reductant, is a substance that donates electrons to another substance in a chemical reaction, thus reducing the oxidation state of that substance. During this process, the reducing agent itself becomes oxidized, losing electrons.
  2. Reducing sugar: A reducing sugar is one that contains a free aldehyde (-CHO) or ketone (-C=O) group, which enables it to act as a reducing agent.
Examples:

  • Monosaccharides: Glucose, Fructose, Galactose
  • Disaccharides: Maltose, Lactose
3. Non-reducing sugar: A non-reducing sugar does not have a free aldehyde or ketone group because it is involved in a glycosidic bond, which prevents it from reducing other compounds.
Examples:
  • Disaccharides: Sucrose
  • Polysaccharides: starch, cellulose, glycogen.
The chemical composition of the substances transported in the phloem: 
Translocated solutes encompass both organic and inorganic compounds. Phloem sap delivers the majority of the inorganic and organic substances essential for plant growth. The main organic solutes include carbohydrates, amino acids, organic acids, hormones, RNAs, and certain secondary metabolites that play a role in defense and protection.
The carbohydrates that are translocated are primarily non-reducing sugars and sugar alcohols, such as sorbitol and mannitol. The most common translocated sugar is sucrose, a disaccharide comprised of one glucose molecule and one fructose molecule.
Some plant families, in addition to sucrose, also translocate oligosaccharides from the raffinose family. Members of this family include raffinose (a trisaccharide), stachyose (a tetrasaccharide), and verbascose (a pentasaccharide). All of these are nonreducing sugars.
Non-reducing sugars are the primary compounds transported in phloem because they are less reactive than reducing sugars.
Reducing sugars, like glucose, are highly reactive. While animals can manage to transport glucose in their blood due to its relatively low concentration, glucose levels cannot be tolerated in phloem, where sugar concentrations are significantly higher.

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