Food Biochemistry and Food Processing (2 edition)

BLBS102-c27 BLBS102-Simpson March 21, 2012 13:25 Trim: 276mm X 219mm Printer Name: Yet to Come

27 Biochemistry of Fruits 539

Glucose-1-phosphate

ADP-glucose

pyrophosphorylase

UDP-glucose

pyrophosphorylase

ADP-glucose

Starch synthase

Amylopectin

Branching

enzyme

α-1-4-Glucan

primer

α-Glucosidase

α-Glucosidase Glucose

Glucose

Glucose-1-phosphate

Metabolic pool

Photosynthesis

Carbohydrate metabolism in fruits

Maltose

Maltose

(Amylose)

Hexokinase

Dextrins

α-Glucosidase

β-Amylase

H 2 O

α-Amylase

β-Amylase

AT P

AT P

ADP

UDP-glucose

Fructose-6-

phosphate

Sucrose

phosphate

synthase

Sucrose-6-phosphate

Phosphatase Pi

Sucrose

Starch

UDP-glucose

UDP-glucose

pyrophosphorylase

Starch

phosphorylase

Invertase Sucrose

synthase

Fructose

Glucose

+ fructose

PPi

PPi

PPi

UTP

UTP

UDP

Figure 27.2.Carbohydrate metabolism in fruits. UDP, Uridine diphosphate; UTP, Uridine triphosphate.

enzymes which cleaves theα-1,6-linkages in amylopectin and

releases linear units of the glucan chain.

In general, starch is confined to the plastid compartments of

fruit cells, where it exists as granules made up of both amylose

and amylopectin molecules. The enzymes that catabolise starch

are also found in this compartment and their activities increase

during ripening. The glucose-1-phosphate generated by starch

degradation (Fig. 27.2) is mobilised into the cytoplasm where

it can enter into various metabolic pools such as that of gly-

colysis (respiration), pentose phosphate pathway (PPP) or for

turnover reactions that replenish lost or damaged cellular struc-

tures (cell wall components). It is important to visualise that

the cell always tries to extend its life under regular develop-

mental conditions (the exceptions being programmed cell death

which occurs during hypersensitive response to kill invading

pathogens, thus killing both the pathogen and the cell/tissue;

formation of xylem vessels, secondary xylem tissues, etc.), and

the turnover reactions are a part of maintaining the homeostasis.

The cell ultimately succumbs to the catabolic reactions during

senescence. The compartmentalisation and storage of chemical

energy in the form of metabolisable macromolecules are all the

inherent properties of life, which is defined as a struggle against

increasing entropy.

The biosynthesis and catabolism of sucrose is an important

part of carbohydrate metabolism. Sucrose is the major form of

transport sugar and is translocated through the phloem tissues

to other parts of the plant. It is conceivable that carbon dioxide

fixed during photosynthesis in leaf tissues may be transported

to the fruits as sucrose during fruit development. Sucrose is

biosynthesised from glucose-1-phosphate by three major steps

(Fig. 27.2). The first reaction involves the conversion of glucose-

1-phosphate to UDP-glucose by UDP-glucose pyrophospho-

rylase in the presence of UTP (Uridine triphosphate). UDP-

glucose is also an important substrate for the biosynthesis of