Carbohydrates in Plants Flashcards
How is carbon stored in plants?
Sucrose and starch, using the calvin cycle
Sucrose?
Most abundant and widespread disaccharide in plants, with a non-reducing configuration ideal for transport
Produced from photosynthesis so starting point of all further synthesis etc
= major form of translocated carbon
Sugar translocation?
Continuous supply needed for metabolism in all tissues, even when supplies are discontinuous (light, water etc)
Role of tuber?
Underground storage; filled with sucrose from photosynthesis in the main plant during summer, converted to starch for storage and dormancy during winter, converted back to sucrose in early spring for sprout growth and then shrivels in late spring as plant can photosynthesise.
Sucrose synthesis?
Occurs in cytosol, linked to photosynthesis;
Calvin cycle in chloroplasts exchanged triose phosphate with inorganic phosphate to export TP to cytosol
TP –F16BP – F6P –sucroseP – sucrose
(full in notes)
Enzymes of sucrose synthesis?
F16BPase
Sucrose phosphate synthase
Sucrase phosphate phosphatase
What is sucrose synthesis regulated by?
Demand
Rate of carbon dioxide assimilation
i.e. highest in photosynthetic and gluconeogenic tissues
Feed forward; CO2 assimilation in chloroplast either +/-
Feed-backward; sucrose demand from non-photosynthetic tissues +/- rate
When is starch formed?
In photosynthetic cells where photosynthetic rate exceeds sucrose export (triose phosphate – starch stored in chloroplast short term)
Sucrose exported to non-photosynthetic cells converted to starch
Starch forms; amylose and amylopection
Amylose- a1-4 glycan residues
Amylopection, larger with a1-4 and a1-6 forming branches every 20-25 units
Found in water-insoluble granules, majority amylopection
Synthesis of starch
Sucrose broken to hexose phosphates — ADP-glucose by ADP-glucose phosphorylase
Starch synthase adds glucosyl unit from end of ADP-glucose to non-reducing end of glycan chain (with a1-4 bond) = starch
Starch branching enzymes
Class I and II
Both hydrolyse 1-4 glucosyl bond, transfer to a hydroxyl group and introduce the 1,6 linkage branch point between two adjacent chains
I does longer chains than II
Starch breakdown
Amylases regulate this
A-amylase digests starch granule to release chains into cytoplasm
Debranching enzymes act upon them
Linear chains converted to glucose/G1P
What are fructans?
Storage organs of onions, barley, oats etc
Water soluble
Non reduing
5-300 unit polymers joined to single glucose molecule
Fructosyltransferase?
Takes fructans from Glc-Fru (i.e. sucrose) to leave glucose and make kestose (Glc-Fru-Fru), used commercially as low-cal sweetener
Long term sucrose storage
e.g. in sugar cane/sugar beet
This form is broken down in vivo by two pathways;
Sucrose + UDP – UDP-glucose + fructose
Catalysed by sucrose synthase (reverse of normal synthesis)
Sucrose + H2O — glucose + fructose
Catalysed by invertases, irreversible, linked to pH