496 Part V: Fruits, Vegetables, and Cereals
The enzymes that catabolize starch are also found in
this compartment, and their activities increase dur-
ing ripening. The glucose-1-phosphate generated by
starch degradation (Fig. 21.2) is mobilized into the
cytoplasm, where it can enter into various metabolic
pools such as that of glycolysis (respiration), the
pentose phosphate pathway, or turnover reactions
that replenish lost or damaged cellular structures
(cell wall components). It is important to visualize
that the cell always tries to extend its life under reg-
ular developmental conditions (the exceptions are
programmed cell death, which occurs during a hy-
persensitive response to kill invading pathogens,
thus killing both the pathogen and the cell/tissue;
formation of xylem vessels, secondary xylem tis-
sues, etc.), and the turnover reactions are a part of
maintaining the homeostasis. The cell ultimately
succumbs to the catabolic reactions during senes-
cence. The compartmentalization and storage of
chemical energy in the form of metabolizable ma-
cromolecules are 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 translo-
cated through the phloem tissues to other parts of
the plant. It is conceivable that photosynthetically
fixed carbon from leaf tissues may be transported to
the fruits as sucrose during fruit development. Su-
crose is biosynthesized from glucose-1-phosphate
by three major steps (Fig. 21.3). The first reaction
involves the conversion of glucose-1-phosphate to
UDP-glucose, by UDP-glucose pyrophosphorylase
in the presence of UTP (uridine triphosphate). UDP-
glucose is also an important substrate for the biosyn-
thesis of cell wall components such as cellulose.
UDP-glucose is converted to sucrose-6-phosphate
by the enzyme sucrose phosphate synthase, which
utilizes fructose-6-phosphate during this reaction.
Finally, sucrose is formed from sucrose-6-phosphate
by the action of phosphatase with the liberation of
the inorganic phosphate.
Even though sucrose biosynthesis is an integral
part of starch metabolism, sucrose often is not the
predominant sugar that accumulates in fruits. Su-
crose is further converted into glucose and fructose,
which are characteristic to many ripe fruits, by the ac-
tion of invertase. Alternatively, glucose-1-phosphate
can be regenerated from sucrose by the actions of
sucrose synthase and UDP-glucose pyrophosphory-
lase. As well, sugar alcohols such as sorbitol and
mannitol are major transport and storage compo-
nents in apple and olive, respectively.
Biosynthesis and catabolism of starch has been
extensively studied in banana, where prior to ripen-
ing, it can account for 20–25% by fresh weight of the
pulp tissue. All the starch-degrading enzymes,-
amylase,-amylase,-glucosidase, and starch phos-
phorylase, have been isolated from banana pulp. The
activities of these enzymes increase during ripening.
Concomitant with the catabolism of starch, there is
an accumulation of the sugars, primarily sucrose,
glucose, and fructose. At the initiation of ripening,
sucrose appears to be the major sugar component,
which declines during the advancement of ripening,
with a simultaneous increase in glucose and fructose
through the action of invertase (Beaudry et al. 1989).
Mango is another fruit that stores large amounts of
starch. The starch is degraded by the activities of
amylases during the ripening process. In mango, glu-
cose, fructose, and sucrose are the major forms of
simple sugars (Selvaraj et al. 1989). The sugar con-
tent is generally very high in ripe mangoes and can
reach levels in excess of 90% of the total soluble
solids content. Fructose is the predominant sugar in
mangoes. In contrast to the bananas, the sucrose lev-
els increase with the advancement of ripening in
mangoes, potentially due to gluconeogenesis from
organic acids (Kumar and Selvaraj 1990). As well,
the levels of pentose sugars increase during ripening,
which could be related to an increase in the activity
of the pentose phosphate pathway.GlycolysisThe conversion of starch to sugars and their subse-
quent metabolism occur in different compartments.
During the development of fruits, photosynthetically
fixed carbon is utilized for both respiration and bio-
synthesis. During this phase, the biosynthetic pro-
cesses dominate. As the fruit matures and begin to
ripen, the pattern of sugar utilization changes. Ri-
pening is a highly energy intensive process. And this
is reflected in the burst of respiratory carbon dioxide
evolution during ripening. As mentioned earlier, the
respiratory burst is characteristic of some fruits,
which are designated as climacteric fruits. The post-
harvest shelf life of fruits can depend on their inten-
sity of respiration. Fruits such as mango and banana
possess high levels of respiratory activity and are
highly perishable. The application of controlled
atmosphere conditions having low oxygen levels