Metabolism of Storage Carbohydrates
Besides other roles carbohydrates are the major source of energy for all living beings. Almost 30% of the carbohydrates in plants are utilized for cell wall biosynthesis by each cell. Carbon skeleton also needs to be diverted for synthesis of defense chemicals (secondary metabolites) in order to deter herbivory. This requires a continuous flow of carbohydrates from source to sink. In autotrophic plants CO2 is fixed in green parts resulting in production of simple sugars (monosaccharides). During night, surplus photosynthates stored as transitory starch in chloroplasts are transported to other parts of the plant after being converted to soluble form (Fig. 9.1). Sugars are transported in the least reactive soluble form, primarily as sucrose from source (site of its synthesis) to sink (site of its utilization) though carbohydrates other than sucrose are also translocated, which include raffinose, verbascose, and stachyose. Starch is the primary storage form of carbohydrates. However, there are instances of sucrose being stored also, e.g., in sugarcane and beetroot. In some plants fructans are the storage form of carbohydrates. In members of grass family, starch is stored in the grains. Though carbohydrate metabolism is similar in other organisms, there are certain unique features which make plants distinct. These include their autotrophic nature and the presence of specialized class of organelles, i.e., plastids. Besides plastids, cytosol and vacuoles are also involved in the metabolism of carbohydrates (Fig. 9.2). Because of their inability to move to a safer place under unfavorable environmental conditions, plants have a flexible metabolism and many a times exhibit alternate metabolic pathways. In this chapter, the main focus will be the metabolism of storage form of carbohydrates in plants.
KeywordsAmylases Amylopectin Debranching enzyme Fructans Hexose monophosphate pool Sucrose-phosphate synthase Starch synthase Starch phosphorylase Transitory starch Triose phosphate translocator
Suggested Further Readings
- Browsher C, Steer M, Tobin A (eds) (2008) Plant biochemistry. Garland Science, Tailor & Francis Group, New York, pp 195–235Google Scholar
- Jones R, Ougham H, Thomas H, Waaland S (2013) The molecular life of plants. Wiley-Blackwell, Chichester, pp 211–213Google Scholar
- Zeeman SC (2015) Carbohydrate metabolism. In: Buchanan BB, Gruissem W, Jones RL (eds) Biochemistry and molecular biology of plants. Wiley-Blackwell, Chichester, pp 567–609Google Scholar