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Botany
Dr. M. Chandraiah
1
Q: If a ring of bark is removed from an actively
growing plant, what will happen and why? (2 M)
A: The part of the bark above the ring on the stem
swells in a week as the food is accumulated in
the tissues there, because food was not
allowed to pass down due to the absence of
phloem. It indicates that phloem is responsible
for the conduction of food.
I. Plant Physiology
Q: What is meant by plasmolysis? How is it
practically useful to us?
A: The outside movement of water from the cell
during which plasma membrane shrinks is
called plasmolysis or exosmosis or negative osmosis or Anosmosis or reverse
osmosis.
★ It occurs when a cell is kept in hypertonic
solution as follows.
1. Water is first lost from the cytoplasm.
2. Then water from the vacuole is lost.
3. Protoplast shrinks.
4. Vacuole becomes smaller.
5. Plasma membrane separates from the cell
wall, first in the corners. It is called incipient
plasmolysis.
★ The cell that looses water is said to be plasmolysed or flaccid cell. In such cells cell sap
becomes hypertonic.
6. Hypertonic solution is seen between cell wall
and plasma membrane.
★ It occurs when there is water stress or in
saline environments.
Cell wall
Cell wall
plasma
membrane
Nucleus
vacuole
hypertonic
solution
plasma
membrane
vacuole
cytoplasm
corner
Incipient plasmolysis
Plasmolysed cell
★ The principle of plasmolysis is useful in making 1) Jellys 2) Pickles 3) Raisins 4) Jams.
Q: Stomata are turgor operated valves.
Explain.
A: Stoma is an elliptical pore in the epidermis.
It is protected by 2 kidney shaped guard
cells. The inner wall of each guard cell,
towards the pore is thick and outer wall
towards the subsidiary cells is thin, convex
and elastic. Guard cells have chloroplasts.
The cell walls have cellulosic microfibrils oriented radially.
According to Levitt's K+ Pump Theory
active Efflux of H+ leads to passive influx of K+
and Cl −. Water potential value of guard cells
decreases. Endosmosis occurs. Guard cells
become turgid. Outer thin walls of guard cells
Exercises (2 Marks)
Q: Compare imbibing capacity of Pea seeds
and Wheat seeds.
A: Pea seeds store proteins in its cotyledons.
Wheat seeds store starch in the
endosperm. The imbibing capacity of proteins is very high when compared to the
starch (carbohydrates).
Q: In general in a plant which path of water
movement is more and why?
A: There are 2 kinds of distinct pathways of
water movement in the plants.
1) Apoplast Pathway
2) Symplast Pathway
Apoplastic movement of water occurs
through the cell walls and intercellular places.
It does not involves any crossing the cell membranes. Whereas symplast pathway occurs
through the cytoplasm, plasmamembrane and
plasmodesmata. Hence Apoplast path of water
movement in the plants is faster.
Q: Why Pinus seeds fail to germinate in the
absence of Mycorrhiza?
A: Mycorrhiza is a symbiotic association of
fungi with roots of higher plants. It absorbs
How is plasmolysis useful?
expand outwardly and become more convex. It
forces the inner walls into a crescent shape.
Stoma opens. At night, Efflux of K+ and Cl−
occurs. In turn influx of H+ occurs water potential increases. Exosmosis occurs. Guard cells
become flaccid. Stoma closes. Thus the turgidity of the guard cells controls the movement of
stomata. Hence we can say that stoma is a turgor operated valve.
Q: Explain pressure flow hypothesis of translocation of sugars in plants.
A: Girdling experiment proves that food materials are translocated through phloem. Food
is conducted in the form of sucrose. It
occurs between source (Leaf) and sink
(Root). Source is one but sink may be any
non-photosynthetic organ. So the conduction of food is multidirectional. Munch
(German) proposed Mass flow/ Pressure
flow hypothesis to explain translocation of
Sugars. It occurs according to Turgor
Pressure gradient (from higher T.P. to lower
t.p.). It is explained in 2 steps:
1. Phloem Loading: Sucrose moves into sieve
tube cells through companion cells, by active
transport. The solution in phloem becomes
hypertonic. Water moves from Xylem moves
into Phloem by osmosis. ψ in phloem
increases. T.P. becomes more. Then sugars
move from source to sink.
2. Phloem Unloading: At sink sucrose is
unloaded. It results increase in Mass at sink.
Sucrose is converted to starch or utilised in
sink. ψ increases. It pushes the water in the
Xylem towards source.
Q: ''Transpiration is a necessary evil". Explain.
A: Though excessive transpiration retards
growth of the plant and plants show permanent wilting, transpiration is beneficial to
plants in many ways like
1) It helps in passive absorption of minerals
and water.
2) It provides necessary force to pull the
water during ascent of sap.
3) It regulates the temperature of the plant
body.
4) It gives cooling effect.
Hence transpiration is a necessary evil.
Q: A gardener forgot to water a potted plant for
a day in summer. What will happen to the
plant? Do you think it is reversible? Explain.
A: The plant shows wilting. It is reversible. If
the plant is again watered, it becomes normal. That wilting showed by the plant is temporary. If the plant is not watered, it
becomes wilted permanent.
Q: Explain the type of molecular movement
which is highly selective and requires special membrane proteins, but does not
require any metabolic energy.
A: Hydrophilic substances or molecules are
transported through the membrane proteins
called Porins. The movement is highly
selective. Porins are specific, special proteins present in the membrane which help in
the facilitated transport without the expenditure of energy. It may be uniport, symport or
antiport. It may be shown as follows.
Carrier Proteins
1)
Sink
Xylem
H+
A
Inside
Phloem
Source
Transport in Plants
4 Marks Questions
Pure
Water
water, minerals and supplies to the roots.
Thus fungus takes shelter and food from
the roots and in turn supplies water and
minerals. The roots of the host have no root
hairs. So host needs the help of fungus.
Water, Minerals
⎯⎯⎯⎯⎯⎯→ Roots
Fungus ←⎯⎯⎯⎯⎯⎯
Shelter and food
Pinus seeds cannot germinate and establish without mycorrhiza as seeds have obligate
association with mycorrhiza.
Q: Which structures do you think the Pinus
plant does not possess due to which its
seeds fail to germinate?
A: Pinus plant does not possess root hairs. So
it can not absorb water on its own.
Q: What do you think is the driving force for
ascent of sap?
A: Transpiration pull is the driving force ascent
of sap.
Q: Why do stomata close under water stress
conditions?
A: When there is insufficient water in the soil,
plants are subjected to water stress. Then
Abscisic Acid (ABA) is produced more and
it induces the stomatal closure.
2)
A
H+
3)
A
B
Uniport
Outside
Na+
Antiport
B
Symport
Plasma membrane
Q: How are stomata distributed in a typical
monocot plant?
A: A typical monocot leaf has stomata on
upper and lower epidermis. Such leaf is
said to be Amphistomatic. The number of
stomata is same on either side. So it can
also be called isobilateral leaf.
Q: In what form the sugars are transported
through Phloem?
A: Sugars are transported through Phloem in
the form of sucrose.
Q: The inward movement of water into a plant
begins either as symplast or apoplast. How
does it conclude before entering into
xylem?
A: The movement of water through the root
layers from epidermis to cortex (endodermis) may be apoplastic or symplastic. But
ultimately it is symplastic in the endodermis
and pericycle, before entering into xylem.
Q: Why does the root endodermis transports
ions in one direction only?
A: As mineral ions are dissolved in water the
root endodermis transports ions in one
direction only.
1) Molecules move through the carrier independently in one direction. Ex: H+
2) Two molecules move in opposite direction.
Ex: Inward movement of H+
Outward movement of Na+
3) Two molecules move in the same direction.
Ex: 1) H+, NO3− 2) H+, Cl − 3) H+, PO4−
Q: How does most of the water move within a
healthy plant body and by which path?
A: Water moves within a healthy plant body
takes place through Xylem by apoplast pathway.
Q: Transpiration and Photosynthesis, a compromise. Explain.
A: Surface of the lamina is related to the rate of
transpiration and photosynthesis.
1) If the lamina is large the rate of transpiration
is more. If transpiration is more it may cause
insufficient supply of water which further
affects photosynthesis.
2) Lamina is reduced to reduce the rate of transpiration. In such case, rate of photosynthesis will also reduce as the lamina is narrow.
Thus, plant has to survive by making a compromise between transpiration and photosynthesis.
Q: Do different species of plants growing in the
same area show the same rate of transpiration at a particular time. Justify your comment.
A: The same area may be inhabited by different species belonging to mesophytes, xerophytes, dicots and monocots. If there is a
pond, hydrophytes also grow there.
Though they grow in the same area the rate
of transpiration is not same because
1) Mesophytes- The rate of transpiration is
more
2) Xerophytes- The rate of transpiration or
amount of transpiration is less.
3) Hydrophytes show very less or no transpiration.
2 Marks Questions
Q: Explain what will happen to plant cell if it is
kept in a solution having higher water potential?
A: Endosmosis
occurs. Water
Solution with higher Ψ
Vacuole
passes from
Cell
solution
of
A
higher water
potential into
the cell. The
Vacuole (increases in size)
size of the
vacuole
direction of osmosis
B
increases.
The cell becomes turgid.
Q: Pressure potential in plant systems can be
negative. Elaborate.
A: Pressure potential in plant systems can be
some times negative - when the plants loose
water by transpiration inspite of insufficient
water in the soil.
Q: How does ABA bring about the closure of
stomata under water stress conditions?
A: Under water stress conditions ABA (natural
antitranspirant) closes the stomata by
1) changing the permeability of plasma
membrane
2) driving out K+ ions out of guard cells.