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Model Answer
B.Sc. FORESTRY VIth Semester
Paper – Principles of plant Physiology
Question 1: Multiple choice questioni. Impermeability is found in;
a. Cellulosic cell wall
b. Lignified cell wall
c. Cuticularized cell wall
d. None of these
ii. TP of cell increases during:a. Exosmosis
b. Plasmolysis c. Endosmosis d. Imbibition
iii. Transpiration pull causes an increase in:a. DPD
b. TP
c. WP d. Water potential
iv. Transpiration is measured by:
a. Porometer
b. Potetometer c. Potometer
d. Sonometer
v. Wilting occurs when:
a. Xylem is blocked
b. Phloem is blocked
c. Cambium is blocked d. All correct
vi. Splitting of water in photosynthesis is called:
a. Dark reaction
b. Photolysis
c. electron transfer
d. Phototropism
vii. Emerson found red drop in wavelength:
a. 660nm
b
670nm
c. 680nm
d. 680-700nm
viii. Deficiency of which element causes chlorosis:
a. Magnesium b. Calcium
c. Chlorine
d. Sodium
ix. In aerobic respiration pyruvic acid is:
a. Produced in the presence of O2
b. Produced as a byproduct of Krebs cycle
c. Broken down to Acetyle Co-A
d. Produced by break down of Acetyl Co-A
x. Femaleness is promoted in cucurbits by using:
a. IAA
b. GA
c. Cytokinin
d. Ethylene
Question 2. Distinguish between the following:- (Any three)
a. Active and passive absorption of water
b. Transpiration and guttation
c. Endosmosis and exosmosis
d. Transpiration and evaporation
Answer:
a. Active and passive absorption of water
The uptake of water by plants is called water absorption. Water is absorbed from the soil
mainly through root hairs. The absorbed water crosses a variety of cells such as cortical cells,
endodermis (passage cells), pericycle cells and xylem tubes to reach the leaves.
In 1949, Kramer proposed that water is absorbed by two mechanisms. They are
a) Active absorption
b) Passive absorption
Active absorption: When the roots absorb water by their own efforts, it is known as active
absorption. It takes place when transpiration is low and the quantity of water in the soil is high.
In this process the root cells play active role in the absorption of water.
Passive absorption: According to passive absorption, the root hair cells do not play any active
role in the absorption of water. The root hair cells remain passive during absorption of water. It
takes place, when rate of transpiration is usually high.
Difference between Active and Passive water absorption
Active absorption
1. It occurs due to the activity of roots and particularly root hairs
2. It is due to osmotic and non osmotic mechanisms
3. In active absorption the osmotic process involves diffusion pressure deficit (DPD) of the cells.
The root hairs have more DPD as compared to soil solution and therefore, water is taken in.
Water is still absorbed if soil solution has greater DPD, But under expenditure of metabolic
energy.
4. Active absorption involves symplast movement of water in root hairs. The water first enters
the cell sap and then passes from one cell to another. Such types of movement where type of
movement where living protoplasm involved, is called symplast.
5. Evidences in support of active water absorption are root pressure, bleeding and guttation.
Passive absorption
1. The passive absorption occurs due to the activity of upper part of plant, such as shoot and
leaves.
2. Passive absorption is due to the process of active transpiration in the upper part.
3. The passive absorption occurs due to the tension created in the xylem sap by transpiration pull.
4. In passive absorption water moves probably through the free spaces or apoplast of root. The
apoplast path of water movement includes cell wall and intercellular space which are fully
permeable. The water can reach up to endodermis through apoplast but it moves through the
endodermis by symplast.
5. Evidences in support of passive water absorption can be given by cutting the roots under
water. The absorption of water still occurs if all the roots are removed
b. Transpiration and Guttation
Transpiration : The loss of water from the living tissue of aerial parts of the plant is termed as
transpiration.
(i) It occurs through stomata, cuticle and lenticels.
(ii) Water is lost in the form of water vapour.
(iii) It occurs only in day time.
(iv) The stomata of leaves usually remain open during day and get closed at night.
(v) Water is pure and contains no salt.
(vi) Major loss of water takes place through stomata.
(vii) It takes place in all higher terrestrial plants.
(viii) Transpiration maintains the temperature of the plant, (ix) Root pressure is not involved.
Guttation: The loss of water in the form of liquid from the injured margins of the leaves is called
guttation. It is caused due to a positive hydrostatic pressure (root pressure) which develops in the
xylem ducts of the root.
(i) It 'occurs through hydathodes in the leaves.
(ii) It is exuded in the form of liquid.
(iii) It takes place cither in the morning or during the night.
(iv) The hydathode remains open whole day and night.
(v) Oozed out water is not pure and contains inorganic and organic substances.
(vi) It occurs through hydathodes.
(vii) It takes place mostly in herbaceous plants.
(viii) It has no relation with the temperature.
(ix) It takes place due to development of root pressure.
Endosmosis and Exosmosis:
Endosmosis- When a cell is placed in hypotonic solution or solution of lower concentration than
cell sap ex. Water, the water or hypotonic solution diffuse in to the cell and cell become
plasmolysed. This is called endosmosis. In this process the outer solution enter into the inside of
the cell results increase into turgor presser.
Fig. Endosmosis
Exosmosis- If a plant cell which is in turgid state is kept in a solution of higher concentration
(than cell sap) i.e. a hypertonic solution, water molecules diffuse out of the cell. This outward
movement of water molecules from the cell sap into the outer solution is called exosmosis. As a
result of this, the plasmalemma leaves the cell wall and the whole cytoplasm contracts in the
centre. This process is called plasmolysis and the cell is called in flaccid condition.
Fig. Exosmosis
Evaporation and Transpiration
Evaporation is the process wherein water from different bodies of water changes from a
liquid into a gas or water vapor, and it goes up into the air. This happens only if energy is present
to change the water into water vapor.
As energy is applied, water molecules collide with each other at different rates causing
molecules that are near the surface to be released into the air or atmosphere. Applying heat to
water or exposing it to the heat of the sun causes evaporation.
Transpiration, on the other hand, is the process of the release of water from plants
through the tiny openings in their leaves or stomata. Plants can control the release of water by
opening and closing the stomata which helps them survive during very hot weather.
Transpiration is dependent on the humidity or wetness of the air or atmosphere and also on how
much moisture the soil in which the plants are planted has. Water is taken in by the plants
through their roots and is carried to all its parts as nourishment.
Water that reaches the leaves is then released into the air or atmosphere to cause
transpiration. The loss of water through both evaporation and transpiration is called evapotranspiration. Together with evaporation and transpiration, precipitation and runoff, evapotranspiration is an integral part of the water cycle.
1. Evaporation is the process of the release of water into the air from open water surfaces while
transpiration is the process of the release of water into the air from plants.
2. Transpiration naturally occurs in plants while evaporation occurs when energy in the form of
heat is applied to water and changes it into water vapor.
3. Both are important to the water cycle. While the amount of water that goes through
evaporation depends on the heat that is applied to it, transpiration depends on the moisture
content of the soil on which the plant is planted and the humidity of the air.
4. The process of losing or releasing water into the air through both evaporation and transpiration
is called evapotranspiration.
Question 3. Define photoperiodism? Classify plants on the basis of photoperiodism.
Answer:
Photoperiodism is the response of the plant to the relative length and timings of light and
dark conditions.
Garner and Allard 1920 were the term photoperiodism. They were experimented on
tobacco variety and failed to produce flowers during summer but when grown in green house
during winter the plant flowered profusely. This was called as short day plant because it flowers
only under short days. Later on it was found that some plants require longer photoperiods to
induce flowering while others produce flowers under both long and short and log photoperiods.
Classification of plants on the basis of Photoperiodism
The plants fall in to the following photoperiodic classes with respect to their flowering
behavior. The classification given below is based on a 24 hour cycle of light and darkness.
1. Short day plants (SDP): such plants flower when day length is shorter than a certain critical
period. Under photoperiods longer than a critical point these plants will not flower. Eg. Coleus
blumei, Xanthium strumarium, Cannabis sativus, Kalonchoe blossfeldiana, Nicotiana tobaccum.
2. Long day Plant (LDP): These plants flower when day length is longer than a certain critical
period. Under photoperiods shorter than a critical point these plants will not flower. Eg. Beta
vulgaris, Spinacea oleracea, Plantago lanceolata, Hyscyamus niger.
3. Indeterminated (Day neutral) plants: The plants which flower over a wide range of day lengths
from relatively short photoperiods to continuous illumination. Eg. Lycopersicum esculentum,
Mirabilis, Capsicum annum and Zea mays.
4. Intermediate Plants: these plants flower only under day lengths within a certain range
usually between 12-14 hours of light but fail to flower under either longer or shorter
photoperiods. E.g. Mikania scandens, Eupatorium hyssopifolium and Phaseolus polystachus.
5. Ambiphotoperiodic Plants: Such plants remain vegetative on intermediate day length (12 to
14 hours) and flower only on shorter or longer day lengths (8- 18 hours). E.g. Melia elegans.
6. Plants requiring alternate periods of long day short day in order to flower:
a. Short long day plants: They will flower only when if a certain number of short days are
followed by certain number of long days. Eg. Triticum vulgare, Secale cereal.
b. Long short day plants: These plants flower when long photoperiods are followed by short
photoperiods. E.g. Cestrum nocturnum and Bryophyllum
Queston 4. Define essential nutrients in plants. Explain the role of nitrogen, calcium and
magnesium in plants and also indicating their deficiency symptoms.
Answer: Arnon 1938 considered an element as essential merely on the basis of its presence in
plants. The criteria as to which element should be considered as essential are as follows:
1. In its absence the plant is unable to complete a normal life cycle
2. That the element is part of some essential plant constituent or metabolite.
3. The element is specific and con not be replaced by another element.
Baron Justus von Liebig, a German scientist in the mid-19th century, showed that
nutrients are essential for plant life. He also authored the term "law of the minimum," which
states that "plants will use essential elements only in proportion to each other, and the element
that is in shortest supply—in proportion to the rest—will determine how well the plant uses the
other nutrient elements." Knowing the nutrients required to grow plants is only one aspect of
successful crop production. The elements are classified as follows:
•
The primary macronutrients: nitrogen (N), phosphorus (P), potassium (K)
•
The three secondary macronutrients: calcium (Ca), sulphur (S), magnesium (Mg)
•
The macronutrient Silicon (Si)
•
The micronutrients/trace minerals: boron (B), chlorine (Cl), manganese (Mn), iron (Fe),
zinc (Zn), copper (Cu), molybdenum (Mo), nickel (Ni), selenium (Se), and sodium (Na)
Role of Nitrogen
Of the three major nutrients, plants require nitrogen in the largest amounts. Nitrogen promotes
rapid growth, increases leaf size and quality, hastens crop maturity, and promotes fruit and seed
development. Because nitrogen is a constituent of amino acids, which are required to synthesize
proteins and other related compounds, it plays a role in almost all plant metabolic processes.
Nitrogen is an integral part of chlorophyll manufacture through photosynthesis.
Photosynthesis is the process through which plants utilize light energy to convert atmospheric
carbon dioxide into carbohydrates. Carbohydrates (sugars) provide energy required for growth
and development. The chemical equation for photosynthesis is
6CO2 + 12H2O + 672 Kcal radiant energy = C6H12O6 + 6H2O + 6O2
Deficiency
1. Nitrogen-deficient plants exhibit slow stunted growth, and their foliage is pale green.
Deficiency symptoms generally appear on the bottom leaves first. In severe cases, the lower
leaves have a “fired” appearance on the tips, turn brown, usually disintegrate, and fall off.
2. They generally exhibit yellow leaf tips, stunted growth with spindly stalks and low yields of
poor quality grain.
Role of Calcium
Calcium is a constituent of cell walls and is involved in production of new growing points and
root tips. It provides elasticity and expansion of cell walls, which keeps growing points from
becoming rigid and brittle. It is immobile within plants and remains in the older tissue
throughout the growing season. It acts as a base for neutralizing organic acids generated during
the growing process and aids in carbohydrate translocation and nitrogen absorption. Indeed,
calcium might be considered the bricks in plant assembly, without which cell manufacture and
development would not occur.
Deficiency
1. Calcium (Ca) deficiency symptoms appear in the meristem regions (new growth) of leaves,
stems, buds and roots. Younger leaves are affected first and are usually deformed. In extreme
cases, the growing tips die.
2. The leaves of some plants hook downward and exhibit marginal necrosis. Roots on calciumdeficient plants are short and stubby. In tomatoes and peppers, a black leathery appearance
develops on the blossom end of the fruit (a disorder called blossom-end rot). In such cases, the
fruit ceases to develop and eventually falls off.
Role of Magnesium
Magnesium is a constituent of the chlorophyll molecule, which is the driving force of
photosynthesis. It is also essential for the metabolism of carbohydrates (sugars). It is an enzyme
activator in the synthesis of nucleic acids (DNA and RNA). It regulates uptake of the other
essential elements, serves as a carrier of phosphate compounds throughout the plant, facilitates
the translocation of carbohydrates (sugars and starches) and enhances the production of oils and
fats.
Deficiency
1. The predominant symptom is interveinal chlorosis (dark green veins with yellow areas
between the veins). The bottom leaves are always affected first. As the deficiency becomes more
acute, the symptoms progress up the plant.
2. Chlorotic leaves generally turn red and then develop spotted necrotic areas.
3. Crops that commonly exhibit magnesium deficiency include tobacco, corn, small grains,
forages and vegetable crops.
4. On grain crops like small grains and corn, magnesium-deficient leaves have light green to
yellow stripes that run parallel with the blade. In severe cases, the entire leaf turns yellow.
Question 5. Write brief notes on the following: (Any two)
a. Daily periodicity of stomatal movement
b. Role of auxins in vegetative production of plants
c. Role of photosynthetic pigments in photosynthesis
Answer:
a. Daily periodicity of stomatal movement
Generally stomata open during the day and close during night. As soon as light is available in the
morning stomata starts to open and after sometime opens completely. Due to this complete
opening, the rate of transpiration increases. Due to this increased transpiration the turgidity of
guard cells decreases and stomata close down partially, sometimes just before noon. As a result
of partial closure of stomata the rate of transpiration decreases due to which guard cells become
turgid again. Thus the rate of transpiration increases again. As a result of increase in the rate of
transpiration the turgidity of guard cells decreases again. In the mean time, the intensity of light
starts to decrease and stomata get completely closed at sunset.
b. Role of auxins in vegetative production of plants- It is defined as an organic
compound characterized by its capacity in low concentration to induce elongation in
shoot cells and inhibition of elongation of root cells. Auxin first time came into existence
by Darwin in 1880. The movement of auxin is polar or unidirectional. It moves from the
shoot tip downward to the base of the plant.
Some common auxins are:
Indole acetic acid, Indole butyric acid (IBA), Nephhthalene acetic acid (NAA) and 2,4dichlorophenoxyacetic acid.
Role of auxin:
1. Phototropism and geotropism
2. Apical dominance
3. Control of abscission – the spray of auxin delayed fruit and leaf fall.
4. Root differentiation on stem cutting: This play very important role in propagation of
new plants. Stem, leaf and root cutting is treated in different concentration of auxin. If we
dip the lower cut end of a cutting in dilute solution of auxins very large number of roots
are developed on the cut ends due to which these cuttings develop into successful plants.
Adventitious bud differentiated into adventitious root in cutting due to the
influence of auxin and it become new plants.
5. Parthenocarpy: If flower bud is emasculated and auxin paste applied to the stigma of
the flower a seedless fruit develop.
6. Sex expression; The spray of auxins increases the number of female flowers in
cucurbits. It also induce flowering in pineapple.
c. Role of photosynthetic pigments in photosynthesis
As sunlight shines on a plant leaf the light of some wavelengths, is absorbed and
put to use in photosynthesis while the light of other wavelength is reflected back from the
leaf or transmitted through it.
Light absorbing molecules called photosynthetic
pigments in the membrane of granum absorb light mainly of blue, violet, red and orange
wavelengths.
Plant leaves are green because they contain large quantities of the pigment
chlorophyll, which absorbs most strongly in the blue and red leaving the intermediate
green wavelengths to be reflected to our eyes. It is the absorbed light that is used in
photosynthesis.
Pigment composition of higher plants and chlorophycean algae is the same and
produce true starch as their photosynthetic reserves. Chlorophyll a is the universal
photosynthetic pigment. The highest pigment in chloroplast is chlorophyll b and lightest
carotenes.
Role in photosynthesis
1. Different pigments absorb light of different wavelengths. Chlorophyll a absorb blue,
violet and red light so it participate directly in the light reactions of photosynthesis.
2. Chlorophyll b absorbs mainly blue and orange light and reflects yellow green.
3. Chloroplast also contains a family of yellow orange pigments called carotenoids
which absorb mainly blue green light. Chlorophyll a and b and the carotenoid pigments
are clustered in the thylakoid membrane of each chloroplast in assemblies of about 200300 pigment molecules.
4. This pigment serve as antenna, collecting light and transferring the energy to the
reaction center, where the chemical reaction leading to energy storage take place.
Question 6. Discuss the mechanism and significance of Hatch-Slack pathway in C4 plants.
Answer:
Till 1965 it was believed that Calvin cycle was the only path way of CO2 fixation in
photosynthesis. In 1957 Kortschak and co-workers reported synthesis of a 4-C organic acid as
the first stable product of photosynthesis in sugar cane. In 1967 two Australian scientists of the
north plant research centre, Queensland namely M,O. Hatch and C.R. slack thoroughly
investigated the complete pathway in these plants where the first stable product of CO2 fixation
was a $-C compound. This path way was known as C4 cycle because the first stable product was
a 4-C compound. It is also knows as hatch slack pathway in honour of the two scientists.
The plants which exhibit this cycle are knows as C4 plants. The common example of C4
plants are tropical grasses, sugar cane, maize cynodon etc.
The anatomy of C4 leaves is known as kanz anatomy. In this case the leaves have two
types of cells - the mesolphyll cells and the bundle sheath cells. The bundle sheath cells are
single layered and around the vascular bundles. They contain few large chloroplasts and lack
grana. On the other hand the mesophyll cells contain large number of normal chloroplasts. They
lack enzymes of Calvin cycle and donot contain starch.
Mechanism: The steps involved in the C4 path way are as follows:
1) In the mesophyll cells the C4 cycle occurs, the primary acceptor of CO2 is a 3-C compound
phosphoenol pyrvic acid. It combines with CO2 in presence of the enzyme phosphoenol
pyrvate carboxylase to form a 4-c compound oxalo acetic acid. It is the first stable
product of c4 pathway.
2) Oxaloacetic acid is then reduced to malic acid using NANDPH produced during light reaction.
The reaction is catalysed by the enzyme malic-dehydrogenase.
3) Sometimes the oxaloacetic acid is converted to aspatic acid by reaction. However aspatic
acid has no role in the cycle.
4) The malic acid formed in mesoph7ull cell is transported to bundle sheath cells where they are
decarboxylated in presence of NANDP sheath cells where they are decarboxylated in
presence of specific malic enzyme tto produce pyruvic acid.
5) The co2 so liberated by decarboxylation of malic acid is accepted by ribulose 1, 5 disphophate
and enters the Calvin cycles.
6) The pyrvic acid formed in the bundle sheath cells is transported back to mesophyll cells where
they are phosporylated in presence of ATP produced in light reaction to form phospoenol
pyruvic acid in presence of enzyme pyruvate phosphate dikinase.
Thus the phospoenol pyrvic acid is regenerated which can take part again in the cycle.
Significance:
1) The C4 plants can absorb CO2 from a low concentration of CO2.
2) It requires more light energy for photosynthesis.
3) Photorespiration does not take place.
4) The plants are better adapted to deserts.
Question 7. Define respiration. Explain the pathway of Krebs cycle starting from Acetyl CO
CO-A.
Answer:
Respiration is primarily a cellular energy yielding dissimilation process and a
phenomenal exhibited by all living organisms.
In this process high energy containing
substances, carbohydrates, proteins are broken down in stepwise manner, under enzymatic
control in to simpler substances of lower energy content. Energy is liverated at certain specific
stagess and trapped in ADP and stored in pyrophosphate bonds of ATP. It is expressed as –
C6H12O6 + 6O2 ----------------6O
---------------- 2 +6H2O+ 38ATP
It is a process of gaseous exchange whereby oxygen is usually absorbed from the
atmosphere and CO2 is usually evolved when organic matter is broken down in the cell with
consequent release of energy.
Pathway of Krebs cycle: The citric acid cycle — also known
own as the tricarboxylic acid cycle
(TCA cycle), or the Krebs cycle. It is a series of chemical reactions used by all aerobic
organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats
and proteins into carbon dioxide. In addition, the cycle provides precursors including certain
amino acids as well as the reducing agent NADH that is used in numerous biochemical reactions.
Its central importance to many biochemical pathways suggests that it was one of the earliest
established components of cellular metabolism.
The name of this metabolic pathway is derived from citric acid that is first consumed and
then regenerated by this sequence of reactions to complete the cycle. In addition, the cycle
consumes acetate (in the form of acetyl-CoA) and water, reduces NAD+ to NADH, and produces
carbon dioxide. The NADH generated by the TCA cycle is fed into the oxidative
phosphorylation pathway. The net result of these two closely linked pathways is the oxidation of
nutrients to produce usable energy in the form of ATP.
Reaction 1: Synthesis of Citric Acid
Acetyl CoA and oxaloacetic acid condense to form citric acid. The acetyl group
CH3COO is transferred from CoA to oxaloacetic acid at the ketone carbon, which is then
changed to an alcohol. The net effect is to join a 2 carbon piece with a 4 carbon piece to make
citric acid which is 6 carbons. This is just called the synthesis of citric acid. This reaction is
catalyzed by citric acid synthetase.
Reaction 2: Dehydration of an alcohol
Two steps Rx. 2 and 3) are required to isomerize the position of the -OH group on citric
acid. This first step is a dehydration of an alcohol to make an alkene. The cis-aconitic acid
remains bound to the enzyme aconitase in readiness for the next step. This reaction is catalyzed
by aconitase.
Reaction 3: Hydration to make alcohol
This reaction is a hydration reaction of an alkene to make an alcohol. The net effect of
reactions 2 and 3 has been to move the -OH group from C-3 to C-2, which is isocitric acid. This
reaction is catalyzed by aconitase.
Reaction 4: Oxidation
This is the first oxidation reaction in which an alcohol is converted to a ketone. Two
hydrogens and 2 electrons are transferred to NAD+ to NADH + H+. This is the entry point into
the electron transport chain. The product of this reaction, oxalosuccinic acid, remains attached
to the isocitrate dehydrogenase for the next step. This reaction is catalyzed by isocitrate
dehydrogenase.
Reaction 5: Decarboxylation
This is the first step where a carbon group is lost as carbon dioxide in a decarboxylation
reaction. The remaining compound now has 5 carbons and is called alpha-ketoglutaric acid. This
reaction is also catalyzed by isocitrate dehydrogenase.
Reaction 6: Oxidation, Decarboxylation, Thiol Ester Synthesis
This complex oxidative decarboxylation is guided by three enzymes in much the same
fashion as the formation of acetyl CoA from pyruvic acid. This is actually the only nonreversible step in the entire cycle and prevents the cycle from operating in the reverse direction.
This is the second oxidation reaction in which an alcohol is converted to a ketone. Two
hydrogens and 2 electrons are transferred to NAD+ to NADH + H+. This another the entry point
into the electron transport chain.
This is the second step where a carbon group is lost as carbon dioxide in a
decarboxylation reaction. Essentially, although not the exact same carbons, the two carbons from
the acetyl CoA have been converted to carbon dioxide at the end this step/.
The remaining 4 carbon group is attached to the CoA through a thiol ester high energy
bond. Notice that the final product, succinyl CoA, has 4 carbons in the succinate group at one
end of the CoA molecule. This reaction is catalyzed by alpha-ketoglutarate dehydrogenase
complex.
Reaction 7: Hydrolysis of Succinyl CoA:
Synthesis of ATP: The hydrolysis of the thioester bond (exothermic) is coupled with the
formation of ATP (Actually guanosine triphosphate is formed first but is further coupled with the
ADP to make ATP). This is the only "visible" ATP formed in the entire cycle. Succinic acid, a 4
carbon acid, is the product of this reaction. This is the start of the return to the beginning of the
cycle. This reaction is catalyzed by succinyl CoA.
Reaction 8: Oxidation
This slightly unusual oxidation reaction results in the removal of the hydrogens from
saturated alkyl carbons to form an alkene, fumaric acid. The hydrogen acceptor is the coenzyme
FAD instead of the more usual NAD+. This will be significant when the ATP is tabulated from
the electron transport chain, since this coenzyme is in the enzyme complex 2. Only 2 ATP result
from this reaction in the electron transport chain. This reaction is catalyzed by succinate
dehydrogenase.
Reaction 9: Hydration to form an alcohol
This is a simple hydration reaction of an alkene to form an alcohol. Take your pick where
you place the -OH group since it must be adjacent to a carboxylic acid group in either case and
forms malic acid. This reaction is catalyzed by fumarase.
Reaction 10: Oxidation
This is the final reaction in the citric acid cycle. The reaction is the oxidation of an
alcohol to a ketone to make oxaloacetic acid. The coenzyme NAD+ causes the transfer of two
hydrogens and 2 electrons to NADH + H+. This is a final entry point into the electron transport
chain. This reaction is catalyzed by malate dehydrogenase.
Question 8. Give a brief account of vital force theory and physical force theory of ascent of sap.
Answer: The upward movement of water from the root towards the top of the plant is known as ascent of
sap.
Vital Force Theories
According to these theories, as some early investigators believed the ascent of water was under
the control of Vital activities in the stem.
1. Westermaier 1883-84 stated that force for upward conduction of water is provided by the xylem
parenchyma cells, the tracheids and vessels simply acting as water reservoirs.
2. As per Godlewswki 1884 thought that upward movement of water was due to pumping activity of cell
of wood parenchyma and medullary rays brought about by periodic change in their osmotic pressure.
3. J.C. Bose 1923 believed that there was a layer of cells in the stem in a state of active pulsations, which
caused the rise of water. He demonstrated his theory using galvanometer. Bose believed that cellular
pulsations from cell to cell bring about the unidirectional flow of sap.
This theory was criticized by Shull and Benedict tried to apply Bose theory to 10 species of trees
and concluded that the actual rate of sap flow under condition of maximum transpiration was 8000 to
30000 times as rapid as would be possible under Bose theory.
Physical Force Theories
1. Imbibition theory- Unger 1868 stated that the upward movement of water in plants occure due to
imbibition of water by thick walls of the xylem elements.
2. Root pressure theory- It refers to positive hydrostatic pressure which sometimes develops in the xylem
sap or roots as a result of metabolic activities of roots. If well watered herbaceous plant is cut off abouve
the soil xylem sap will exude from the cut surface.
This theory was not accepted because it is widely held that root pressure is too weak a force to account for
movement of water to top of tall trees.
3. Capillary force theory- According to Christian Wolf in 1873, water rises up in the nary tubes of xylem
vessels by surface tension called capillary.
There was many objection
a. lifting power of the capillary formed by lumina is not large and can not account for the rise of water
exceeding three meters.
b. Capillary force cannot operate in plants having tracheids instead of vessels due to the presence of end
walls.
4. Transpiration Pull- Cohesive force of water theory- The most widely accepted theory for ascent of sap
is cohesive theory. This theory was proposed by Dixon and Jolly in 1894.
Transpiration pull – the water vapour move out of the plant through stomata. As a result of loss of
water from mesophyll cells the diffusion pressure deficit increase. With the increase of DPD these cells
absorb water from adjoining cells ultimately the water is absorbed from xylem elements of vascular
bundles of leaf. Since the xylem elements are filled with continuous water column a tension or pull called
transpiration pull develops at the top of the colum. This tension is transmitted down from petiole to stem
and finally to roots leading to upward movement of water.
2. Cohesion of water in xylem- xylem tracheids and trachea are long tubular structure extending from root
to leaf. Thus one end of xylem is in the root and other end in the leaf. The tubes are filled with water. The
water molecules are held together from mesophyll to root hairs because of cohesive force and thus a form
a continuous clumn. Hydrogen bonds among water molecules provide a cohesion that holds together a
chain of water that extends the entire height of the plant within the xylem.
According to this theory water ascends in the plant because of transpiration pull and this column of water
remains continuous because of cohesive force of water molecules.