Carbohydrates in Plants Flashcards

1
Q

How is carbon stored in plants?

A

Sucrose and starch, using the calvin cycle

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2
Q

Sucrose?

A

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

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3
Q

Sugar translocation?

A

Continuous supply needed for metabolism in all tissues, even when supplies are discontinuous (light, water etc)

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4
Q

Role of tuber?

A

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.

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5
Q

Sucrose synthesis?

A

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)

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6
Q

Enzymes of sucrose synthesis?

A

F16BPase
Sucrose phosphate synthase
Sucrase phosphate phosphatase

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7
Q

What is sucrose synthesis regulated by?

A

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

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8
Q

When is starch formed?

A

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

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9
Q

Starch forms; amylose and amylopection

A

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

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10
Q

Synthesis of starch

A

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

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11
Q

Starch branching enzymes

A

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

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12
Q

Starch breakdown

A

Amylases regulate this
A-amylase digests starch granule to release chains into cytoplasm
Debranching enzymes act upon them
Linear chains converted to glucose/G1P

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13
Q

What are fructans?

A

Storage organs of onions, barley, oats etc
Water soluble
Non reduing
5-300 unit polymers joined to single glucose molecule

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14
Q

Fructosyltransferase?

A

Takes fructans from Glc-Fru (i.e. sucrose) to leave glucose and make kestose (Glc-Fru-Fru), used commercially as low-cal sweetener

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15
Q

Long term sucrose storage

A

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

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16
Q

Applications and uses of plant materials

A

Fibres/proteins = Animal/ pet food
Starches, gluten, sweeteners = Food/drink
Starches = Paper/board
Pharma, cosmetics, bio-applications in industry

This depends on amylose/amylopectin balance

17
Q

GM/Crispr?

A

GM potatoes and Crispr-Cas9 maize lack amylose, through deletion of granule-bound starch synthase gene = only amylopectin formed, with soluble starch synthase

18
Q

Why do we use chemicals from photosynthetic organisms?

A

Low energy input
Low organic input (sugars)
Minimal waste as biodegradable
Sugars can make a range of metabolites

19
Q

Biofuel?

A

Sucrose, starch and cellulose all converted to bioethanol/biobutanol by fermentation

Tricylglycerol oils from alage = biodiesal

20
Q

Fermentation?

A

Ethanol, via yeast/bacteria
sucrose – glucose + fructose
Glucose — ethanol (+ATP)

21
Q

Fermentation in sugar cane?

A

Crushed, wash out soluble sugar
Ferment with yeast = 5-12% ethanol
Distill and concentrate = 80-95% ethanol
=petrol replacement

22
Q

Fermentation in maize starch?

A
Maise flour + water + a-amylase enzyme
Liquified, saccharification with glucosidase enzyme
Ferment with yeast = 5-12% ethanol
Distill and concentrate = 80-95% ethanol
=petrol replacement
23
Q

Cellulose

A

Inert, insoluble abundant fibres
Diet industry - intestinal health, fibre, satiation
Fuel - bioethanol