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akhpad
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Source: OG 12th Ed
All the cells in a particular plant start out with
the same complement of genes. How then can
these cells differentiate and form structures as
different as roots, stems, leaves, and fruits? The
answer is that only a small subset of the genes in a
particular kind of cell are expressed, or turned on,
at a given time. This is accomplished by a complex
system of chemical messengers that in plants
include hormones and other regulatory molecules.
Five major hormones have been identified: auxin,
abscisic acid, cytokinin, ethylene, and gibberellin.
Studies of plants have now identified a new class of
regulatory molecules called oligosaccharins.
Unlike the oligosaccharins, the five well-known
plant hormones are pleiotropic rather than specific;
that is, each has more than one effect on the
growth and development of plants. The five have
so many simultaneous effects that they are not
very useful in artificially controlling the growth of
crops. Auxin, for instance, stimulates the rate of
cell elongation, causes shoots to grow up and roots
to grow down, and inhibits the growth of lateral
shoots. Auxin also causes the plant to develop a
vascular system, to form lateral roots, and to
produce ethylene.
The pleiotropy of the five well-studied plant
hormones is somewhat analogous to that of certain
hormones in animals. For example, hormones from
the hypothalamus in the brain stimulate the anterior
lobe of the pituitary gland to synthesize and release
many different hormones, one of which stimulates
the release of hormones from the adrenal cortex.
These hormones have specific effects on target
organs all over the body. One hormone stimulates
the thyroid gland, for example, another the ovarian
follicle cells, and so forth. In other words, there is a
hierarchy of hormones.
Such a hierarchy may also exist in plants. Oligosaccharins
are fragments of the cell wall released
by enzymes: different enzymes release different
oligosaccharins. There are indications that pleiotropic
plant hormones may actually function by activating
the enzymes that release these other, more specific
chemical messengers from the cell wall.
Q
The passage suggests that which of the following is a function likely to be performed by an oligosaccharin?
(A) To stimulate a particular plant cell to become part of a plant's root system
(B) To stimulate the walls of a particular cell to produce other oligosaccharins
(C) To activate enzymes that release specific chemical messengers from plant cell walls
(D) To duplicate the gene complement in a particular plant cell
(E) To produce multiple effects on a particular subsystem of plant cells
I understood that oligosaccharin stimulates the plant cell but what about other thing in option A.
OA: A
All the cells in a particular plant start out with
the same complement of genes. How then can
these cells differentiate and form structures as
different as roots, stems, leaves, and fruits? The
answer is that only a small subset of the genes in a
particular kind of cell are expressed, or turned on,
at a given time. This is accomplished by a complex
system of chemical messengers that in plants
include hormones and other regulatory molecules.
Five major hormones have been identified: auxin,
abscisic acid, cytokinin, ethylene, and gibberellin.
Studies of plants have now identified a new class of
regulatory molecules called oligosaccharins.
Unlike the oligosaccharins, the five well-known
plant hormones are pleiotropic rather than specific;
that is, each has more than one effect on the
growth and development of plants. The five have
so many simultaneous effects that they are not
very useful in artificially controlling the growth of
crops. Auxin, for instance, stimulates the rate of
cell elongation, causes shoots to grow up and roots
to grow down, and inhibits the growth of lateral
shoots. Auxin also causes the plant to develop a
vascular system, to form lateral roots, and to
produce ethylene.
The pleiotropy of the five well-studied plant
hormones is somewhat analogous to that of certain
hormones in animals. For example, hormones from
the hypothalamus in the brain stimulate the anterior
lobe of the pituitary gland to synthesize and release
many different hormones, one of which stimulates
the release of hormones from the adrenal cortex.
These hormones have specific effects on target
organs all over the body. One hormone stimulates
the thyroid gland, for example, another the ovarian
follicle cells, and so forth. In other words, there is a
hierarchy of hormones.
Such a hierarchy may also exist in plants. Oligosaccharins
are fragments of the cell wall released
by enzymes: different enzymes release different
oligosaccharins. There are indications that pleiotropic
plant hormones may actually function by activating
the enzymes that release these other, more specific
chemical messengers from the cell wall.
Q
The passage suggests that which of the following is a function likely to be performed by an oligosaccharin?
(A) To stimulate a particular plant cell to become part of a plant's root system
(B) To stimulate the walls of a particular cell to produce other oligosaccharins
(C) To activate enzymes that release specific chemical messengers from plant cell walls
(D) To duplicate the gene complement in a particular plant cell
(E) To produce multiple effects on a particular subsystem of plant cells
I understood that oligosaccharin stimulates the plant cell but what about other thing in option A.
OA: A
