Plant Physiology for 5th Semester Botany Major NEP 2020
If lower- level botany focused on classifying green life and learning it's anatomy, this semester shifts the focus inside the cell.Plant Physiology moves beyond what plants look like to examine how they actually work. It breaks down the chemical mechanisms, energetic balances, and cellular systems that transform sunlight,soil,and air into living tissue.
How to Use This Blog
Each post breaks down complex biochemical cycles into bite- sized summaries, clear metabolic diagrams, key lab protocol overviews , and exam-focused conceptual questions.
Bookmark this page, keep your lab notebook ready, and let's decide the functional science of plants together!
Syllabus of BT- 501C
Unit III : Plant Physiology I
Chapter 1 : Plant Water Relationship
•Water Potential & it's components
•Pathways of water Movement
•Symplast
•Apoplast
• Ascent of Sap
•Cohesion -Tension Theory
Chapter 2 : Transpiration
•Definition of Transpiration
• Types of Transportation
•Mechanism of Stomatal movement
•Antitranspirants
Chapter 3: Minerals Nutrition
• Micro nutrients & Macro nutrients
•Mechanism of mineral absorption
•Physiological roles and deficiency symptoms of Macro nutrients.
Chapter 4: Photosynthesis
• Photosynthetic pigments
• Structure of PSI & PS II
• Light Phase of Photosynthesis
•Absorption spectrum
•Action spectrum
•Red drop
•Emerson Effect
•Dark phase of photosynthesis
•C3 Cycle , C4 cycle
•CAM / Crassulaceun acid metabolism .
•Photorespiration
Unit IV : Plant Physiology II
Chapter 5 Cellular Respiration
•Aerobic & Anaerobic respiration
•Glycolysis
•Oxidative Decarboxylation
•TCA Cycle
•Mitochondrial Electron Transport
•Pentose Phosphate Pathway
Chapter 6 Nitrate Assimilation
• Biological Nitrogen Fixation
• Nodule formation
• Role of Nitrogenase enzyme
•Role of Leg Hb
• Functions of nif gene and nod gene
Chapter 7.Plant Hormones
•Chemical nature, Source, Physiological Role of:
•Auxin
• Gibberellin
•Cytokinin
• Functions of Absecic acid
•Functions of Ethylene
Chapter 8 Photoperiodism
• Photoperiodic classification of plants
•Phytochrome : chemical nature, mode of action, role in flowering
•Floral Induction: role of Florigen
Chapter 9 Seed Dormancy
•Types of Seed Dormancy
•Methods of breaking seed dormancy
Chapter 1
1.3. Ascent of Sap
The phenomenon of movement of "xylem sap" (the whole liquid content of water and dissolved minerals) against the force of gravity is called ascent of sap.
1.3.1.The Cohesion -Tension Theory
There are many theories regarding the "ascent of sap" ,among them "The Cohesion -Tension Theory" is more authentic.
The Cohesion -Tension Theory was proposed by Dixon and Joly in 1894. They proposed that the water ascentin trees in exclusively due to transpiration pulled from continuous water columns in the xylem conduit running from the roots to the leaves.
The Theory is based on:
(a) Cohesive and Adhesive propertiy of water molecules which form a continuous water columns in the xylem.
(b) Transportation pull or tension exerted on this water column.
Cohesion:
A water molecule is attached to other water molecule by hydrogen bonds and show Cohesive property.
Adhesion
Water molecule stick to the xylem wall by adhesion.
Transportation
Loss of water in the form of vapour through stomata and others.
Mechanism
(i) The mesophyll cells lose water vapour through transpiration.
(ii) As a result of this , diffusion pressure deficit ( DPD) increases or water potential decreases (~30 bars) in the mesophyll cells.
(iii) Hence, water is sucked from adjoining mesophyll cells and ultimately from xylem which are connected to the root hairs via pericycle, endodermis and cortex.
(iv) Water inside the xylem form a continuous column due to cohesion and adhesion. The xylem cells are placed one above another with perforated ends to form a tube from the sub-stomatal cavity to the root hair.
(v) The water column is pulled by transpiration pull and it never be broken.
(vi) This pull creates a pressure gradient to cause osmosis passively from the soil through xylem which is connected to root hairs via pericycle, endodermis and cortex cells to replace the lost water volume.
Objection
Copeland (1902) believed that air bubbles in xylem can break the tensile strength by embolism or cavitation but Scholander et. al., (1957) strongly opposed it.
Chapter 2
2.1.Definition of Transpiration
Transpiration is defined as water loss in the form of vapour from the internal tissues of aerial parts of the plant body through stomata,lenticels,etc. under the influence of sunlight and other factors .
2.2. Antitranspirants
A plant utilised 1% of absorbed water in metabolism, the rest transpired. Moreover, during drought plants show wilting which reduces 50% of plant yield. In horticulture practices some chemicals in the form of antitranspirants are used to reduce the rate of transpiration without hampering other metabolic process like photosynthesis and growth.
2.2.1.Types of Antitranspirants :
(i) Stomatal closing type: These chemical is like phenylmercuric acetate(PMA) and absecic acid (ABA) induce closer of stomata to cut down the rate of transpiration. On application of PMA at very low concentrations, stomata closed partially for 2 weeks.
Drawbacks: Due to closure of stomata the processes like respiration and photosynthesis also become inhibited on application of such chemicals as stomata constitute common pathways for both evaporation and exchange of gases.
(ii) Film forming types: Colourless plastics and low viscocity waxes and silicon oils act as physical barrier on leaf surface on application by forming a thin film. It reduces transpiration but at the same time allow diffusion of oxygen and carbon dioxide.
2.2.2.Mechanism:
(i) Antitranspirants reduce absorption of solar energy by leaves and lower leaf temperature, as a result transpiration reduced.
(ii) Antitranspirants form a thin transparent layer on leaves to hinder the escape of water vapour from the leaves.
2.2.3.Effect on field crops: Fuahring,1973, sprayed antitranspirants on sorghum leaves under limited irrigation conditions which resulted in 5 % to 17% increase in yield.
2.3. Types of Transpiration
2.4. Stomatal Closing & Opening
2.4.1. K(+) Malate Hypothesis of opening and closing of stomata
A: Closed stomata
B: Open stomata
The role of K(+) ion in stomatal opening was first discovered in 1943 by Imamura and suppported and elaborated by Fujino(1967), Levitt (1974), Raschke(1975),Noggle-Firtz(1976) .
2. In presence of light PEP of guard cells reacts with carbon dioxide in presence of PEP carboxylase to form C4 substrates OAA.
Light Phase: The light dependent reactions of photosynthesis occurs in grana thylakoids to produce NADPH and ATP which are utilised in the Dark phase .
Photo courtesy: Wikipedia
Fig. Red drop of photosynthesis yield
Hence, Emerson observed that when both and long wavelengths of red light acted on at the same time the rate of photosynthesis (Z) was higher than the sum of photosynthetic rates obtained by using short and long wavelengths separately (X+Y).
Courtesy: Plant Physiology by P.L. Kochhar and H.N.Krishnamoorty,p- 207
Fig: Action spectrum of photosynthesis
Transfer of e from water to PS II:
Fig. Cyclic e flow and ATP synthesis
. Fig: Kranz anatomy in Maize leaf
(v) The CO2 is taken up by RuBP (the final acceptor of CO2) and enters the Calvin cycle, producing PGA.
Significance:
Photograph of Kalanchoe daigremontiana
5.1.Aerobic Respiration
Fig: Schematic representation of Glycolysis
(1) 1st Phosphorylation: Glucose reacts with ATP in presence of Hexokinase and Mg(2+) , thus producing Glucose 6-Phosphate.
(5) Dehydrogenation & Phosphorylation:
(6) Dephosphorylation: 1,3- di PGA looses the P of C1 position to ADP; as a result 3-PGA and ATP are produced. The reaction is catalysed by Phosphoglyceric transphosphorylase and Mg (2+) .
(7) Isomerisation: 3-PGA is changed to its isomer 2-PGA by the enzyme phosphoglycero mutase by an intermolecular shift in the position of phosphate group.
(8) Dehydration: 2-PGA loses a molecule of water and changes to Phosphoenol Pyruvic Acid by enzyme anolase and Mg(2+).
(9) Dephosphorylation: In the final reaction PEP dephosphorylated to Pyruvic Acid by the enzyme Pyruvate Kinase , the P is accepted by ADP and converted to ATP by substrate level phosphorylation.
This reaction requires Mg(2+) and K(+) as cofactors.
Fig. Schematic representation of Krebs cycle
Chemical nature of the reactions:
•During oxidative phosphorylation , reduced coenzymes like , NADH + H(+) and FADH2 release protons into the matrix and pass high - energy electrons down a chain of sequential electron carriers (ETC) embedded in the inner mitochondrial membrane.
• Finally, in terminal oxidation, electrons and protons are transferred to molecular oxygen ---- the ultimate electron acceptor -- forming water.
Fig: A simplified representation of terminal oxidation and oxidative phosphorylation.
Fig: Schematic representation of ETC
Fig: ATP synthesis by inner mitochondrial membrane particles.
Fig: Pentose Phosphate Pathways ,Source : A Handbook of Plant Biosynthetic Pathways ; by Dr Debabrata Chaudhari, Dr Dilip Kumar Kar & Dr Soma Halder; New Central Book Agency (P) Ltd,pp 144
(ii) Due to oxidation of Glucose 6-Phosphate by NADP+ and NAD-dependent enzyme glucose -6-phosphate dehydrogenase to form 6-phosphoglucono lactone and NADPH + H+. It is called the Commited,rate- limiting step of the PPP.
(Courtesy: Plant Physiology; P L Kochhar and H N Krishnamoorthy ; Atma Ram Publication, pp 408 ,7th Revised Edition,1985)
Fig: Behaviour of LDP and SDP under varied photoperiods.
Photograph : Xanthium strumarium ( Courtesy: Google)
Fig.Absorption coefficient of the red and far- red light absorbing forms of phytochrome .
Fig. Pr and Pfr conversion in red and far- red light.
Fig: Chemical Structure of Phytochrome
Fig: A flowering twig of Spinach
Fig: Stages of formation of flowering hormone in SDPs.
Fig: Sequence of events leading to to a photoperiodic response
Fig: Schematic representation of types of seed dormancy ( based on M.G. Nikolaeva)
Source : TNAU Agritech, https://agritech.tnau.ac.in
The main reasons for the development of seed dormancy are discussed below:-
Commercial cultivars and agriculturist use a number of methods to break down the seed dormanancy , and few are discussed below:
Mechanism:
In Day time:
1. Levitt (1974) observed that proton (H+) uptake by guard cells takes place with the help of ATP and results into increase of pH. At increased pH ,starch is hydrolysed to form Phosphoenol Pyruvic Acid (PEP).
2. In presence of light PEP of guard cells reacts with carbon dioxide in presence of PEP carboxylase to form C4 substrates OAA.
3.The OAA is then converted to malic acid and NAD(+) by the activity of enzymes malate dehydrogenase.
4. Malic acid is then dissociated into H(+) and Malate(-)ion.
5.In the next step H(+) effluxed from the guard cells due to the activity of H(+) ion pump of guard cell membrane.
6. The malate (-)ion remained in the cell and the cell interior becomes negatively charged and pH of the cell increased avobe 7.0 which helps in more malate synthesis.
7.To balance the charge of the cell interior k(+) ion passively influxed into the guard cells from the adjacent subsidiary cells and epidermal cells. Thus the concentration of K+ ion increases from 50 mM to 300 mM in the guard cells.
8.The K(+) rushes to the cell vacuole where it combines with malate (-) ion to form K-malate( potassium malate).
9.The k-malate induces endosmosis of water from the adjacent cells into the guard cells. As a result turgor pressure (TP) of the guard cell increases which leads to opening of stomata.
The sequence of events is as follows:
Light ➞ Malic acid synthesis ➞ Dissociation of malic acid into H(+) --> and malate (-)ions --> Influx of k (+) ions and efflux H(+) ions --> Formation of K- malate in vacoules--> Osmosis of water from adjacent cells -- > Increase of TP of guard cells --> Stomata opens
AT NIGHT-
In the absence of light, the sequence of events of the day time is reversed .
1. During night, photosynthesis stops , carbon dioxide accumulated in guard cells.
2. As a result, the pH of the guard cells decrease.
3. At low pH , k- malate dissociates into K+ ion and malate(-) ion .
4. K+ion efflux from guard cells into the subsidiary cells and H+ ion infflux into the guard cells.
5. Then water comes out of the guard cells ,
and TP of the guard cells decreases which results into the closure of stomata.
Chapter 3
In pure water plants can not grow, in addition to water plants require a number of mineral elements. These are taken up as inorganic ions from the soil solution. de Saussure ,1804, demonstrated that the inorganic mineral elements contained in plant ash are obtained from the soil via root system. He showed that nitrogen and inorganic mineral elements of soil were essential to the growth and development of the plant. The sources of these inorganic requirements are minerals, the elements are known as mineral nutrients and the nutrition is called mineral nutrition.
3.1 Framework Elements:
Carbon, hydrogen and oxygen are called as the "Framework Elements" , because these male up cellulose, lignin, eaxes, reserve foods, most of the protoplasm , in short 90% or more of the dry matter of plant materials.
3.2 Criteria for essentiality:
According to Arnon and Stout (1939) , an element is essential if,
(i) The element is required for growth and reproduction ; without it the plant fails to complete flowering and seed formation.
(ii) The element should not be replaced by any other elements.
(iii) The element is directly involved in plant metabolism.
3.3 Macronutrients
There are 9 elements which required relatively in a large amount for growth & development of green plants. If the symbols are arranged in a line along with iron (Fe) , they can be used to remember the elements like:
C HOPK'NS CaFe Mg
"See Hopk' ns cafe, mighty good" . These nine elements ,with the exception of iron, called as macronutrients , because they are needed in relatively large amounts ( more than 100 ppm).
Nutrients & Concentration in dry matters:
C - 450,000 ppm
H - 60,000 ppm
O - 450,000 ppm
P - 2,000 ppm
K- 10,000 ppm
N- 15,000 ppm
S- 1,000 ppm
Ca - 5,000 ppm
Fe- 100 ppm
Mg - 2,000 ppm
3.3.1 Functions of Essential Macronutrients & Their Deficiency Symptoms
Carbon, Hydrogen and Oxygen : Although these three are not minerals in true sense, but included in the list as no plant life is possible without these elements. They are termed as framework elements .
Phosphorus: Plants absorb Phosphorus in the form of phosphate ions, H2PO4(-) & HPO4(2-).
Physiological role:
(i) P is a constituent of both phospholipids or glycerol phosphatides or phosphoglycerides , nucleic acids, coenzymes , ATP, etc.
(ii) It is necessary for mitochondrial function .
(iii) P is essential for the glycolytic reactions.
(iv) P is essential for all the sugar phosphates involved in photosynthesis.
(v) It is more abundant in growing and storage regions of plants.
(vi) It is necessary for cambial activity, nitrate reduction and protein formation.
Deficiency Symptoms
(i) Plants dark green, often red or purple colours appear, lower leaves yellow, drying to dark green, stalks become short, slender.
(ii) Growth retarded, dead patches appear on leaves.
(iii) Petioles, fruits and leaves fall early.
Potassium (K) : It is absorbed as K(+) ion.
Physiological Role :
(i) K(+) ion plays vital role in opening and closing of stomata.
(ii) K(+) is essential for the functioning of about 40 different enzymes ( Evans & Sorger , 1996 ).
(iii) K (+) acts as essential co- factor of the respiratory enzymes.
(iv) Under water stress conditions K(+) being absorbed to prevent loss of water.
Deficiency Symptoms
(i) Mottled chlorosis followed by nacrosis at the tip and margin of the older leaaves.
(ii) Growth of internodes retarded.
(iii) Apical dominance appears to be lacking.
Nitrogen : Plants absorb nitrogen from the soil in the form of NO3(-) and NH4(+) ions.
Physiological Role :
It is an important constituent of purines, pyrimidines, porphyrin molecule, cytochromes, and coenzyme like NAD(+), NADPH(+) ,FAD, etc.
Deficiency Symptoms :
(i) Yellowing of leaves followed by stunting in the growth os all parts of the plant.
(ii) Stem, petiole, and lower leaf surface turn purplish due to anthocyanic formation.
(iii) Flowering is suppressed or delayed.
Sulphur : Plant roots absorb in the form of sulphate ions , SO4(2-) .
Physiological Role
(i) S is a basic component of cysteine and methionine.
(ii) It takes part in the synthesis of vitamins, biotin, thiamine and CoA.
Deficiency Symptoms
(i) Terminal buds chlorotic or wilted, without necrotic spots.
(ii) Young leaf vains chlorotic.
A key to Nutrient Deficiency Symptoms
Older leaves affected
A. Affect mostly over whole plant, lower leaves dry up and die .
(i) Plants light green, lower leaves yellow, drying to brown, stalks become short and slender. -> N deficient
(ii) Plants dark green, often red or purple colours appear, lower leaves yellow, drying to dark green, stalks become short and slender. -->P deficient
B. Affects mostly localized, chlorosis, lower leaves do not dry up but become chlorotic, leaf margins cupped.
(i) Leaves chlorotic, necrotic spots, stalks slender.-> Mg deficient
(ii) Chlorotic leaves, necrotic spots small and between veins or near leaf tips and margins, stalks slender.-> K deficient
(iii) Necrotic spots large and general, leaves thick, stalks short.-> Zn deficient
Young leaves affected
A. Terminal buds die, distortion and necrosis of young leaves.
(i) Young leaves hooked, then die at tips. -> Ca deficient
(ii) Young leaves light green at the bases, die back from base, leaves twisted.-> B. deficient.
B. Terminal buds remain alive but chlorotic or wilted, without necrotic spots.
(i) Young leaves wilted, without chlorosis, stem tip weak. -> Cu deficient
(ii) Young leaves notcwilted, chlorosis occurs.
• Small spots, veins remain green.-> Mn deficient
•No necrotic spots, veins green -> Fe deficient.
• No spots, veins chlorotic ->S deficient
Adapted from Diagnostic Techniques for Soils and Crops. American Potash Inst, Washington D.C. , 1948
4.1. Phases of Photosynthesis :
Photosynthesis occurs in two phases---Light phase and Dark phase.
Dark phase : The light independent reactions of photosynthesis takes place in the stroma of chloroplast.
4.2.What is Hill Reaction?
4.3. What is a Hill Reagent?
A Hill reagent is an artificial electron acceptor used in vivo biochemical experiments to study light phase of photosynthesis.
e.g., Potassium Ferrioxalate , Potassium Ferriocyanide, 2,6-dichlorophenolindophenol (DCPIP),etc.
NADP (+) is a natural Hill reagent which accepts during photolysis of water.
4.4. Red drop & Emerson Effect:
Q. What is Emerson Effect ? 3/TU-2012
(i) Emerson and Lewis(1943) measured quantum yield at different wavelengths of light during photosynthesis.
(ii) The Oxygen molecules released per quantum of light absorbed is called quantum yield.
(iii) Emerson et.al., (1957)noticed that when a plant was shifted from short wavelength (650nm) to wavelength longer than 680 nm (red region) quantum yield dropped suddenly.
(iv) This sudden fall in the photosynthesis yield beyond red region of spectrum is called red drop or long red wavelength drop-off.
(v) They further observed that photosynthetic rate can be restored if simultaneously shorter wavelength is provided.
(vi) Let, photosynthetic rate of simultaneous shorter and longer wavelength is Z, and the photosynthetic rate at shorter wavelength is X, and photosynthetic rate at longer wavelength is Y.
(vii) Emerson et.al. observed that Z > X+ Y , and this photosynthetic enhancement is called Emerson Enhancement Effect or Emerson Effect.
Photosynthesis at 700 nm = 10 unit. (X)
Photosynthesis at 653 nm = 43.5 unit. (Y)
-------------------------------------------------------------------------
Total ( beams at seperate) = 53.5 unit (X+Y)
When wavelength used simultaneously:
Photosynthesis at (653+700) nm = 72.5 unit (Z).
4.5. Absorption spectrum
The graphical representation of amount of light of different wavelengths absorbed by a pigment is called absorption spectrum.
The absorption spectra of the chlorophyll a and chlorophyll b differ somewhat from each other:-
(i) The maximum absorption occurs in the blue end of the spectrum:-
449 nm in chl.a and 453 nm in chl.b
(ii) Second maximum absorption occurs in the red end of the spectrum:-
660 nm in chl.a and 642 nm in chl.b
(iii) Virtually no absorption occurs in the green- yellow region. Green rays are reflected so, chlorophyll solution and leaves appear green to the eye.
(iv) Bacteriochlorophyll absorps most in the infrared and the blue violet parts of the spectrum.
The light absorbed by chl.b, chl.c and chl.d is passed onto chl.a to use in photosynthesis.
4.6. Action spectrum
The graphical representation of the rate of photosynthesis at different wavelengths of light is called action spectrum.
It shows that maximum photosynthesis occurs in blue- violet and red part of the light. However, sufficient photosynthesis occurs in the mid part of the light spectrum where the carotenoids are active.
4.7. Light Reactions of Photosynthesis:
The formation of ATP in presence of light is called photophosphorylation and there are two distinct types of such reactions -- one, in which only ATP is formed (Cyclic ) and other in which NADPH is formed alongwith ATP formation and oxygen liberation ( Non-cyclic).
4.7.1.Non-Cyclic photophosphorylation:
It was illustrated by Robin Hill and Fay Bendal in 1960 by "Z-scheme" in which electrons transferred from (i) water to PS II, (ii) from PS II to PS I, and (iii) PS II to NADPH+. It happens when light of shorter wavelengths and longer wavelengths are illuminated at the same time on the green pigmets of plants.
Fig:Hill & Bendal's Z-scheme of light reaction
Photolysis of water :
(i) Splitting of water occurs on the inner side of thylakoid membrane to produce (H)+ ions /protons and (OH)- ions/ hydroxyl ions in presence of Mn++, Cl- and liberating oxygen as a biproduct at light wavelength of less than 680 nm. Reactions of photolysis of water are given below:-
(ii) Simultaneously, PS II also absorbes photons and excited to release electrons from P680 and get ready to receive electrons from OH- of ionised water.
(iii) The exited electrons are transferred to PQ ( plastoquinone).
Transfer of e from PS II to PS I :
(iv) Next electrons are transferred to PS I via Cyt f and PC ( plastocyanin) as PS I also liberated electrons after being exited by absorbing photon from light wavelength above 680 nm.
(v) When electrons transferred via Cyt.f , ATP synthesized by phosphorylation of ADP.
Transfer of e from PS I to NADPH+:
(vi) The electron liberated from P700 of PS I is accepted by FD ( Ferredoxin) .
(vii) At last, extruded electron is received by NADP+ with the help of NADPH reductase and reduced to NADPH through H+ ion which was released during photolysis of water.
The electrons released from water never came back to water or to PS II but used to reduce NADP+, hence called Non-cyclic.It involves both PS II and PS I , both short wavelengths and long wavelengths , production of NADPH , oxygen and ATP.
4.7.2. Cyclic Photophosphorylation:
(i) When there is only longer light wavelength and photosynthetic cells are exposed to red light, PS I is activated but not the PS II.
(ii) In absence of shorter wavelength process of photolysis of water and consequently the evolution of oxygen as well as formation of H+ ions and reduction of NADPH+ are stopped.
(iii) The supply of electrons to PS I are not available hence, the electron released from excited PS I is first accepted by a primary acceptor , then passed to Ferredoxin (FD) .
(iv) Then electron moves down to Cyt b6f complex , then to PC and finally returns to P700 of PS I, thus the cycle completes.
(v) ATP synthesized during the journey of electron from FD to Cyt b6 and from Cyt b6 to Cyt. f . At this moment more H+ are formed inside the thylakoid lumen and less in the stroma, this a proton motive force creates which resulted in the fusion of ADP and iP to form ATP.
According to another view, the electron is transferred from FD to PQ then subsequently to PS I via cytochrome complex and PC.
(vi) The electron which was released from PS I earlier is again returns to PS I , producing ATP so, it is termed Cyclic Photophosphorylation.
If, after a time gap short light wavelength is provided , PS II is activated, the non cyclic photophosphorylation is resumed, ATP, NADPH and oxygen are produced by splitting water.
Questions and Answers
(1) Why cyclic electron flow generates ATP but no NADPH ? 2/TU-2012
In cyclic electron flow electrons never transfer to terminal acceptor molecule NADP(+) ,rather rerouted back to cyt b6f complex.
• The electrons released from photo excited PS I pass to FD (Ferredoxin) , then from FD to Cyt b6f complex bypassing FNR enzyme.
• FNR ( Ferredoxin -NADP + reductase enzyme) is catalyst used in the formation of NADPH.
• In cyclic electron flow FNR is not involved as well as proton i.e. H+ is not formed by the photolysis of water and more importantly NADP+ never acts as an electron acceptor, hence , NADP is not synthesized.
• ATP Production :
Cyt b6 f complex shuttled electrons back down to the PSI via PC ( plastocyanin) , it pumps more H+ into thylakoid lumen and less in stroma .As a result a proton motive force creates which resulted in fusion of ADP and iP to form ATP.
The reactions involved in reductive fixation of carbon dioxide into photosynthetic products through a series of enzymatic steps using ATP and NADPH produced by light reactions are termed as Dark Reactions. In green plants there are three pathways for CO2 fixation, these are:-
(i) Calvin Cycle ( ii) Hatch - Slack cycle and (iii) CAM pathways
4.8.1. Calvin Cycle or C3 Cycle:
Malvin Calvin, Bensen and Bassham and their associates found that 3C compound 3- PGA (Phosphoglyceric acid ) is the first stable product of the dark phases while reduction of carbon dioxide takes place in the stroma of the chloroplasts with the help of the assimilatory power of the cell ( NADPH2 and ATP), produced in the earlier light reaction.
Since , PGA is a 3C compound and the first stable product of the Calvin cycle it also termed as C3 cycle.
Phases of Calvin cycle: Calvin cycle is divided into three phases: carboxylation, glycolytic reversal ,and regeneration of RuBP.
Fig: Calvin Cycle of Photosynthesis
1. Carboxylation: Benson concluded that the dark phases of photosynthesis began with the combination of CO2 to a 5C sugar Ribulose 1-5, bisphosphate ( RuBP), resulting in highly unstable 6C compound 2-Carboxy 3-keto 1,5- biphosphoribotol which was immediately hydrolysed to 2 molecules of PGA.
2. Glycolytic Reversal:
The reactions of this phases are reversal of glycolysis process.
(i) PGA is first phosphorylated to 1,3-diPGA by ATP ( the cofactors produced in the earlier light reaction) with the help of enzyme triose phosphate kinase.
(ii) Further diphosphoglyceric acid is reduced by NADPH ( the cofactor produced in the earlier light reaction) to Glyceraldehyde 3-P with the help of enzyme triose phosphate dehydrogenase.
This is the primary energy-storing chemical reaction in the biosphere.
(iii) A part of the Glyceraldehyde 3-P so formed is converted to its isomer Dihydroxy acetone-3-P with the enzyme phospho triose isomerase.
(iv) Next, one molecule of each isomers react each other to condense Fructose1,6-diP with the help of enzyme aldolase.
(v) Fructose1, 6-diP loses one P-group, forms Fructose 6-Phosphate with the help of enzyme phosphatase.
(vi) Subsequently, Fructose 6-Phosphate is changed to its isomer Glucose 6-Phosphate .
P
(vii) At last, Glucose 6-Phosphate produces a hexose sugar Glucose.
As glucose is a 6C compound, six turns of Calvin cycle are required to synthesise it's one molecule.
3.Regeneration of RuBP
The reactions and enzymes involved in the regeneration of RuBP are given below:
(i) 6C compound F-6-P and 3C compound 3-PGAld interact and transketolase catalysed it , and resulted into 4C sugar E-4-P and 5C sugar Xylulose -5-P.
(ii)4C sugar E-4-P condensed by aldolase with triose PGAld (3C) to form (7C) Sedoheptulose 1,7- diP.
(iii)Next 7C compound Sedoheptulose 1,7-diP loses a molecule of P and converted to 7C compound S 7-P .
(iv) S 7- P (7C) reacts with Glyceraldehyde 3-P (3C,triosephosphate) to produce 5C sugars Xylulose 5-P and Ribose 5-P .
(v)Next Xylulose 5-P is converted to Ribulose 5-P by epimerase.
(vi) Ribose 5-P also converted to Ribulose 5-P by an isomerase.
(vii) At last Ribulose 5-P picks up a P from ATP to become RuBP .
This cycle is called Calvin Cycle after Malvin Calvin who worked out its details. In this cyclic reduction of 6 molecules of carbon dioxide, 18 molecules of ATP and 12 molecules of NADPH are consumed.
4.8.2. C4 Cycle or Hatch Slack Pathway
Discovery: In 1965 , Kortschak, Hartt and Burr observed 80% of radioactive 14CO2 is found in 4-carbon dicarboxilic acids( malic acid and aspartic acid) and only 10% of it in Phospho Glyceric Acid (PGA) during an experiment on photosynthesis.
In 1967, M.D.Hatch and C.R. Slack explained that radioactive malic acid appeared in photosynthetic cells much earlier than PGA in plants like Maize, Sugarcane, Sorghum, Amaranthus , Panicum,etc.
These plants are termed as C4 plants and have Kranz anatomy where vascular tissue is surrounded by large bundle sheath (BS)cells contain agranal large chloroplasts with RUBP carboxylase ,and undifferentiated mesophyll cells with normal chloroplast with grana and PEP carboxylase enzyme. Plasmodesmata connects a BS cell to a mesophyll cell.
Steps:
Fig: C4 Cycle in Sugarcane leaf cells
Inside the mesophyll cells:
(i) In mesophyll cells , CO2 reacts with H2O and rapidly converts into HCO3- and protons . The reaction is catalysed by the enzyme carbonic anhydrase .
(ii) Subsequently HCO3- fixed onto PEP , a 3C compound, directly yielding OAA (Oxalo acetic acid) ,a 4C compound , catalysed by the enzyme PEPcarboxylase.
(iii) The unstable oxaloacetate (OAA) is soon reduced to 4-C malic acid by enzyme malic dehydrogenase in presence of the coenzyme NADPH + H+. In some plants OAA converts to aspartic acid by reductive amination.
Malic acid or aspartic acid is translocated to BS cells through plasmodesmata.
Inside the BS cells:
(iv) In BS cells malic acid is decarboxylated or aspartic acid is deaminated to form pyruvic acid and CO2.
Pyruvic acid is sent back to mesophyll cells.
Inside the mesophyll cells:
(vi) Pyruvic acid reacts with ATP under the influence of phosphopyruvic diakinase to regenerate PEP ( primary CO2 acceptor) and thus completing the cycle.
(i) C4 plants are more efficient than C3 plants because PEP carboxylase is more active in CO2 fixation than RuBP carboxylase .
(ii) C4 plants perform photosynthesis even when stomata are closed ( from CO2 produced in respiration).
(iii) Even at very low concentration of CO2 a C4 plant can continue photosynthesis ( 10 - 50 ppm).
(iv) Hatch- Slack plants show greater increase in dry weight than the Calvin plants.
(v) Hatch - Slack plants can utilize higher light intensities than Calvin plants and the rate of photosynthesis at optimum light intensities may be twice that of Calvin plants.
Model Question :
• C4 plants are photosynthetically more efficient than C3 plants, explain. 3 marks
Hints: Write the above discussed significance.
Q. Discuss the metabolic pathways of CAM.
8/TU-2012
4.8.3. CAM is a photosynthetic adaptation to periodic water supply, occurring in plants in arid regions ( e.g., cacti) or in tropical epiphytes (e.g., orchids & bromeliads) .
CAM plants close their stomata in day time to prevent transpiration, and open their stomata in night when the air temperature is low to collect atmospheric CO2, converting it into Oxaloacetate (OAA) and subsequently storing it as malic acid in vacuoles.The following day ,this malic acid is decarboxylated to release CO2 internally to run Calvin cycle in chloroplast for carbohydrate synthesis in stroma . CAM is named after Crassulacrae, the family of succulent plants, in which this type of metabolism was first discovered.
Mechanism:
Processes behind open stomata in night:
(1)CAM plants take up atmospheric CO2 predominantly at night. CO2 enters the mesophyll leaf tissue through open stomata at night.
(2)Then CO2 reacts with PEP to form C4 substrates OAA. This carboxylation of PEP is catalysed by the enzyme Phosphoenol Pyruvate Carboxylase ( PEP carboxylase ) .The precursor (PEP) is being generated via glycolysis or soluble sugars formed during the previous day .
(3) Subsequently unstable OAA is reduced to Malic Acid by the enzyme malic dehydrogenase in the presence of the coenzyme NADPH2 and stored in the central cell sap vacuole increasing the acidity.
Malate (-) ion moves passively into the vacuole down an electrochemical gradient through a specific channel, following the active transport of protons via a vacuolar H(+) - ATPase pump.
Processes behind closed stomata at day time:
(4) The following day in presence of light , malic acid is remobilized from the vacuole and transported to chloroplast for decarboxylation to form CO2 and Pyruvic acid and subsequently resulting in the decrease of acidity. With respect to the carboxylation process CAM plants are of two types:
(i) NAD(P)-ME- type species: where the decarboxylation is brought about by NAD- dependent -mitochondrial/ cytoplasmic Malic enzyme (ME).
(ii) PEPCK-type species:where cytosolic ME serves decarboxylation.
(5) Regenerated CO2 is refixed via the C3 photosynthesis( Calvin - Benson Cycle ) by RubisCo as CO2 concentration turns 60- fold(upto 10,000 ppm) as compared to the atmospheric CO2 level and fixed to RuBP then carbohydrates are reformed.This process also suppresses photorespiration .
(6) The pyruvic acid is again converted to PEP, then PEP is converted to triose by gluconeogenesis ,hexose phosphate and subsequently into starch.
(7) In the following night, starch is converted to PEP to fix CO2 behind the closed stomata to regenerate the cycle again.
CAM is found in 33 families and 328 genera comprising monocots and dicots ( Smith and Winter,1996). e.g., Kalanchoe daigremontiana.
Q. Mention the relationship of C4 and CAM pathways.
4/TU-2012
There is a close biochemical and evolutionary relationship between C4 and CAM pathways. Both act in the same direction to concentrate carbon dioxide for bypassing transpiration & photorespiration which were found in the C3 plants.
Similarly:
• Common enzyme: Both C4 and CAM use PEPcarboxylase enzyme for initial carbon fixation:
• PEPcarboxylase has a very high affinity for bicarbonate negative ions ( HCO3 -) .
• PEPcarboxylase can't bond oxygen, avoids at the initial capture step.
• Common biochemical reactions
Carboxylation Reaction:
(i) The primary carboxylation in both CAM and C4 pathways are same:
•3C compound PEP reacts with HCO3(-) to form 4C compound OAA.
• OAA is then reduced to malic acid with the help of enzyme Malate dehydrogenase.
Decarboxylation reaction
In both cases maic acid is then decaboxylated to give Pyruvate and CO3 . This CO2 is used by RuBisco to run Calvin Cycle.
Regeneration of PEP
Newly formed 3C compound Pyruvic Acid is then converted back to PEP in both cases.
Core Divergence:
Malate formation
In C4 plants malate formation takes place in the mesophyll cells in day time.
Whereas, in CAM malate formation takes place in night time in the mesophyll cells and stored in large central vacuoles as stomata opens.
Decarboxylation
In C4 plants decarboxylation of malate takes place in Bundle sheath cells in presence of RuBisco. Whereas, in CAM decarboxylation of malate takes place in the same mesophyll cells .
Spatial separation vs Temporal Separation
•C4 plants separate the two steps across two distinct cells : mesophyll cells and bundle sheath cells. So the separation is Spatial.
•CAM plants separates the two steps across time( night vs day) within single mesophyll cell. So the separation is Temporal.
Conclusion
•C4 plants prevent oxygenation only across different cells, but CAM solve both extreme water loss and oxygenation actress time.
Chapter 5
A stepwise enzymatically controlled reactions of complete oxidation of glucose to give rise ATP, carbon dioxide and water in expance of Oxygen ic called aerobic respiraion.
5.2.Glycolysis
Glycolysis is the common step to both aerobic as well as anaerobic modes of respiration. It is also called EMP pathway as it was discovered by Emboden, Meyerhof and Parnas ( all three were German scientists).
( Courtesy: Modern's abc of Biology pp 591)
(2) Isomerisation: (Reversible reaction) Glucose 6-P changes to its isomer Fructose 6-P under the influence of the enzyme Phosphogluco isomerase .
(3) 2nd Phosphorylation: Fructose 6-P is further phosphorylated to Fructose-1,6- di P with the help of ATP. The reaction is catalysed by enzyme Phosphofructo kinase and Mg(2+) as a cofactor.
(4) Cleavage: ( Reversible reaction)
At last, Fructose -1, 6-diP splits up into two molecules of triosephosphates, DHAP and 3-PGAld respectively under the influence of the desmolytic enzyme Aldolase.
Glyceraldehyde 3-P loses hydrogen to NAD and the H acceptor is reduced to NADH + H(+) ; the substrate molecule accepts iP from cytosol and converted into 1,3 -di PGA .The reaction is catalysed by Triose phosphate dehydrogenase enzyme.
The direct synthesis of ATP from metabolites is called substrate level phosphorylation .
This is the end of the common phase of both aerobic and anaerobic respiration.
The net reaction
Summary:
• Site : Cytoplasm
• 1 molecule of 6 C compound Glucose is reduced to 2 molecules of 3 C compound Pyruvic acid.
• Decarboxylation reaction absent, hence release of carbon dioxide is nil.
• Energy used : 2 molecules of ATP
• Net gain in energy: 2 ATP + 2 NADH + H(+)
Q .Glycolysis is an aerobic process--Explain.
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5.3. Oxidative Decarboxylation of Pyruvate
Chemically, this process is an oxidative decarboxylation :
• Oxidation : 3C pyruvic acid is (oxidised ) dehydrogenated , transferring hydrogen to reduce NAD (+) into NADH + H(+).
• Decarboxylation : A molecule of carbon dioxide is removed , converting the 3C
pyruvic acid into highly reactive 2C acetyl CoA.
5.4. TCA Cycle / Krebs Cycle
The step-wise degradation of pyruvic acid takes place in the Krebs cycle, also called the Citric Acid Cycle or Tricarboxylic Acid (TCA) Cycle. It derives these names because it's initial intermediate,citric acid, as well as several subsequent intermediates in the series , are tricarboxylic acids.
Hans Krebs discovered ( 1937, 1940 ) this cyclic event and awarded Nobel prize in 1953. It occurs inside the mitochondria .
(1) Condensation ,(2) Dehydration,(3) Hydration,(4) Dehydrogenation,(5) Decarboxylation ,(6) Dehydrogenation & Decarboxylation , (7) Formation of ATP( in plants) ot GTP( in animals), (8) Dehydrogenation, (9) Hydration ,and (10) Dehydrogenation.
Analysis:
(1) Since 2 molecules of acetyl CoA are formed from 1 molecule of glucose, the Krebs cycle must rotate twice for each glucose molecule that is oxidised.
(2) In the Krebs cycle, 4 pairs of hydrogen atoms are removed from intermediate substrates by dehydrogenase enzymes.
(3) Specifically, 3 pairs of hydrogen atoms are used to reduce 3 molecules of NAD(+) to NADH + H(+) , while 1 pair is used to reduce 1 molecule of FAD to FADH2.
(4) Finally, Oxaloacetate is regenerated and combines with another molecule of acetyl CoA to re- enter the Krebs cycle,repeating the process.
Energy production in Krebs Cycle
(1) The Krebs cycle produces only 2 GTP ( or ATP) molecules per glucose molecule vie substrate level phosphorylation.
(2) A large section of energy is captured in reduced coenzymes,like ,NADH + H(+) and FADH2 , which must pass through the electron transport chain ( ETC) to generate ATP.
(3)The 6 NADH and 2 FADH2 produced during two turns of the Krebs cycle generate 22 ATP molecules in the ETC .
5.5. Electron Transport Chain/System
•As electrons travel through this chain, a proton gradient is established to synthesize ATP.
• Finally, in terminal oxidation, electrons and protons are transferred to molecular oxygen ---- the ultimate electron acceptor -- forming water.
• This is called oxidative phosphorylation because oxygen is consumed in this process of ATP formation.
ETC is a set of seven electron carriers in a specific sequence along inner mitochondrial membrane: (i) NAD, (ii) FMN, (iii) Ubiquinone, (iv) Cyt b, (v) Cyt c, (vi) Cyt a ,and (vii) Cyt a3.
ATPs from NADH2: Oxidation of one molecule of NADH2 resulted into the formation of three molecules of ATP at different sites:
• The first is at NAD - FAD junction.
• The second is at the Cyt b-Cyt c junction, and
• The third at the Cyt-a Cyt a3 junction.
ATPs from FADH2 : FADH2 formed in the oxidstion of Succinate yields on oxidation only 2 molecules of ATP because it lacks the first site (NAD-FAD junction) .
Five Enzyme Complexes: There are five types of enzyme complexes in the mitochondrial membrane. Complexes I, II ,III, and IV are concerned with concerned with electron transport system and Vth is linked with conservation and transfer of energy along with synthesis of ATP.
Complex I : NADH/ NADPH : CoQ reductase
Complex II : Succinate : CoQ reductase
Complex III: Reduced CoQ : Cyt c reductase
Complex IV : Cyt c oxidase, and
Complex V: ATP ase
The electron pathway for NADH is complexes I,III and IV and for FADH2 the path consists of complexes II, III and IV. Energy released at the complexes I, III and IV is used to generate ATP.
Mechanism of Oxidative Phosphorylation
1. Development of the Proton Gradient
During the transfer of electrons along the ETC, protons are pumped from the mitochondrial matrix into the intermembrane space:
•Complexes I,III and IV facilitae the transfer of electrons and pump 3 pairs of protons into the intermembrane space.
•Complexes II, III and IV ( when electrons enter via complex II) pump 2 protons into the intermembrane space.
Because protons accumulate in the intermembrane space at a much higher concentration than in the matrix, an electrochemical proton gradient ( proton - motive force) is established across the inner mitochondrial membrane.
2 . Proton Flow and ATP Synthesis
•Proton Translocation : The established gradient drives protons to flow back from the intermembrane space into the mitochondrial matrix.
•The Pathways: Protons pass through specialized channels within the inner mitochondrial membrane ( oxisomes).
•ATP Formation: The proton flux activated ATP synthetase ( at the head of F1 particle), driving the phosphorylation of ADP and iP into ATP.
5.6. Hexose Monophosphate Shunt
What is PPP ?
Pentose Phosphate Pathway is an alternative pathway of direct oxidation of glucose in cell cytoplasm.
Other names of PPP
(i) Since the PPP diverged from glycolysis at Glucose- 6-P level, it has been named the Hexose Monophosphate Shunt.
(ii) Since Pentose Phosphates play an important role in the cyclic reaction sequence, it is referred as the Pentose Phosphate Cycle.
(iii) Since phosphogluconate is a key intermediate of PPP, it also referred as the Phosphogluconate Pathway.
(iv) PPP is also termed as Warburg - Dickens - Horecker Pathway after the biochemists who were the principal investigators.
Sequence of reactions of PPP
(i) In the beginning Glucose is phosphorylated to Glucose-6-P by ATP with hexokinase .
Oxidative PhaseIt is an irreversible phase that generates NADPH and Pentose / 5C sugars.
(iii) Next 6-phosphoglucono lactone is hydrolysed by lactonase to 6- phospho gluconic acid.
(iv) 6-phospho gluconic acid is then oxidised to an unstable compound 3-keto -6-phospho gluconic acid and NADH+ H+ by enzyme dehydrogenase.
Immediately the unstable compound is converted to Ribulose 5-P by decarboxylation at carbon 1 atom by decarboxylase enzyme.
This terminates the first phase, i.e., the oxidation process.
Non-oxidative phase
6 molecules of Ribulose 5-P ( produced by the oxidative decarboxylation of 6 molecules of Glucose) undergo a series of changes to give rise to 5 molecules of Glucose 6-Phosphate. The events are given below:
(i) Ribulose 5-P is then converted to other pentoses, either (a) Xylulose 5-Phosphate,
or (b) Ribose 5-Phosphate by epimerase and isomerase respectively.
•First Transketolase reaction
(ii) Next Xylulose 5-P and Ribose 5-P react in presence of transketolase where transfer of a glycolaldehyde group from Xylulose 5-P to Ribose 5-P takes place yielding :
(a)Glyceraldehyde 3-P, and
(b) Sedoheptulose 7-P
•Transaldolase reaction
(iii) The products of the above reaction react with each other in presence of Transaldolase to produce :
(a) Fructose 6-P, and
(b) Erythrose 4-P
•Second Transketolase reaction
(iv) Xylulose 5-P donates a glycolaldehyde to Erythrose 4-P in presence of transketolase, thus produce:
(a) Fructose 6-P,and
(b) Glyceraldehyde 3-P.
•Gluconeogenesis
(v) Next reverse order of Glycolysis sequence take place:
• Glyceraldehyde 3-P left over in the 2nd transketolase reaction isomerises to Dihydroxyacetone-3-P with the help of enzyme Triose phosphate isomerase.
• Dihydroxyacetone-3-P and Glyceraldehyde 3-P react in presence of Aldolase , thus produce Fructose 1,6- bisphosphate .
• Then Fructose 1,6- bisphosphate removes a Phosphate at carbon atom 1 by the help of phosphatase and Fructose 6-P produced.
• At last Fructose 6-P isomerises to Glucose-6-P with the help of enzyme Hexose Phosphate Isomerase.
For the operation of the cycle and to oxidise Glucose completely to carbon dioxide through PPP, 6 molecules of Glucose 6-Phosphate enter cycle, 5 of the 6 molecules are recovered, while 1 is completely degraded to carbon dioxide and NADH- bound hydrogen.
Significance/Importance of PPP
(i) PPP is an alternative pathway of aerobic respiraion which occurs in the cytosol and plastids( such as chloroplasts and amyloplasts).
(ii) It produces NADPH+ H(+) for various Biosynthetic reactions ( e.g., lipid and secondary metabolic pathways).
(iii) It produces a number of tetroses and pentoses for the synthesis of nucleosides, nucleotides, nucleic acids, lignin, Indole 3-acetic acid, anthocyanins and a number of compounds.
Questions and Answers
1. Where do the reactions of the PPP take place inside a plant cell?
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In the cytosol and plastids ( chloroplast and amyloplasts)
2. What is the alternative name for the PPP ?
3. What are the two major phases of the PPP?
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Hexose Monophosphate Shunt, or
The Pentogluconate Pathway
Oxidative phase
An irreversible phase that generates NADPH and Pentose ( 5C) sugars from Glucose 6-Phosphate as a substrate.
Non- oxidative phase
A reversible series of sugar- phosphate interconvertions that happens following Transketolase reactions (1st & 2nd) , Transaldolase reaction and Gluconeogenesis,this allows carbon skeletons to recycle back into Glycolysis.
4. State the primary physiological functions of PPP in plants.
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( i) Production of NADPH, which is essential for reductive biosynthesis ( like lipid and secondary metabolic synthesis).
(ii) Generation of Ribose 5-Phosphate for nucleotide and nucleic acids.
5. How do PPP differs from Glycolysis?
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(i) Electron Acceptor :
Glycolysis uses NAD(+) to produce NADPH to use for respiration / ATP generation , whereas PPP uses NADP(+) to produce NADPH (+) for biosynthesis.
(ii) ATP Production
Glycolysis yields a net gain of 2 ATP directly, whereas the PPP does not produce ATP directly.
(iii) Primary Objective
Glycolysis catabolises glucose for energy, while PPP focuses on generation of reducing NADPH and carbon skeletons.
6. State the main regulatory mechanism of the PPP.
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The PPP is primarily regulated by the cellular NADPH/ NADPH(+) ratio. If level of NADPH is high the it inhibits thev Glucose 6-Phosphate dehydrogenase enzyme which slow down the PPP when reducing power is abundant.
7. Which specific coenzyme required for hydrogen/ electron acceptor in the PPP ?
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NADP(+) [ Nicotinamide Adenine Dinucleotide Phosphate].
8. What are the roles of Transketolase enzyme and Transaldolase enzyme in the PPP?
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Transketolase
Transfer of 2C ketol group between sugar- phosphates.
Transaldolase
Transfer of 3C Dihydroxy acetone unit, facilitating the interconvertion of sugar skeleton in the non- oxidative phase.
9. State the significance of non- oxidative phase of PPP.
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(i) Carbon Interconvertion:
It permits the interconvertion of 3-,4-,5-,6-,&7- C sugars without generating ATP or NADPH.
(ii) Glycolytic Link
It converts excess pentose phosphate back into glycolytic intermediates like Fructose 6-P and Glyceraldehyde 3-P.
(iii) Metabolic flexibility
It allows the cell to balance ist requirements for the NADPH and Ribose 5-P independently.
10. Why PPP sometimes called " shunt"?
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Because it branches off from Glycolysis at Glucose- 6-P and can rejoin Glycolysis via sugar intermediates, bypassing early steps of glycolytic breakdown.
11. Which one is the "commited rate- limiting" step of the PPP?
12. Mention the NADPH forming steps found in the PPP.
13. State the decarboxylation reaction found in the PPP.
14. Describe the 1st and 2nd Transketolase reactions involved in the PPP.
15. Describe the reverse order of reactions involved in the PPP.
16. "Ribulose 5-P is the most important intermediate in the PPP" ,explain.
Answer
Ribulose 5-P is the most important intermediate in the PPP because:
(i) Ribulose 5-P can be isomerised to Ribose 5-P which provides Ribose units for the synthesis of RNA and of the nucleotide coenzymes.
(ii) It can react with ATP and give Ribulose 1,5- diphosphate which plays an important role in the Calvin Cycle of Photosynthesis.
LA Type questions
16. Describe the Oxidative phase of the PPP.
17. Describe the non-oxidative phase of the PPP.
Chapter 6
What is Nitrate Assimilation?
Chapter 7
What are Phytohormomes?
Chapter 8
8.1. Photoperiodism
According to W.W. Garner and H.A. Allard (1920, 1923) , "Photoperiodism is the physiological response of an organism ( such as flowering in plants) to the relative lengths of light and dark( the photoperiod).
The observation:
•Garner and Allard observed that a mutant of tobacco ( Nicotiana tabacum) , commonly called Maryland Mammoth did not flower in the long day of summer, early fall and killed by frost in the field during winter.
• But in greenhouse they flowered and set seeds.When the seeds from these plants were sown, the mutant remained vegetative in the field , but flowered in the green house.
•Next, Garner and Allard cultured the experiment in green house and dark chamber in controlled temperature and light period and observed that ( a) they can be flowered in summer by reducing the day length ,and (b) be kept in a vegetative state during winter by the artificial increase of the day length.
From the above findings, Garner and Allard coined the terms " photoperiod" and " Photoperiodism" .
8.2. Critical Day Lenght (CDL):
The precise amount of daily light needed to induce 🌺 flowering, acting as a minimum limits for LDPs and maximum limits for SDPs is called as CDL .
Or,
CDL is the specific duration of light exposures that initiates flowering in photoperiodic plants:
•For LDPs: The minimum hours of light needed.
•For SDPs: The maximum hours of light allowed.
Example:
(i) Maryland Mammoth variety of tobacco (SDP) : 12 hours L: 12 hours D .It produces 🌺 when photoperiod is shorter than 12 hours.
So, CDL is 12 hours .
(ii) Xanthium strumarium (SDP) : 15.5 hours L: 8.5 hours D. This plant also produces flower 🌺 when photoperiod is less than 15.5 hours.
Hence, CDL is 15.5 hours
(iii) Hyoscyamus niger ( Henben ,LDP ) : 11 hours L : 13 hours D. It induced to flower 🌺 when photoperiod given is more than 13 hours.
Hence, CDL is 11 hours.
8.2.1. Classification of Plants on the basis of photoperiod
Garner and Allard classified plants in three types on the basis of photoperiod, viz., SDPs , LDPs and DNPs. Subsequently, two pother types were recognised :- LSDP and SLDP.
Short -Day Plants ( SDPs):
The plants which flowers when the day length ( photopariod) is shorter than a CDL ( Critical Day Length) . If the photoperiod is excess of CDL , the SDPs will remain vegetative. Example:
• Nicotiana tabacum cv. Maryland Mammoth: 12 hours L : 12 hours D
• Xanthium strumarium : 15.5 hours L: 8.5 hours D
• Glycine max ( Biloxi var. of soybean) 13.5 hours L : 10.5 hours D
Long -Day Plants ( LDPs):
The plants which require a photoperiod of longer than a CDL for flowering are called LDPs. In the length of light period is shorter than the CDL, the plants remain vegetative. Example:
• Hyoscyamus niger ( Black henben) 11 hours L: 13 hours D
• Beta vulgaris ( Sugar beet) : 12 to 14 hours L : 10 to 12 hours D
• Hordeum vulgare ( Winter barley) : 12.5 hours L: 11.5 D
• Spinacea oleracea ( Spinach) 13 to 14 hours L : 10 to 11 hours D
Day -Neutral Plants (DNPs) :
The plants which are indifferent to the photoperiod with respect to the flowering behaviour and will flower over almost any photoperiod.
Example:
Cucumis sativus ( cucumber) , Lycopersicum esculentum (tomato) and Impatiens balsamina ( balsam), etc.
LSDP ( Long short- day plants):
The plants which require to flower a long photoperiod followed by a short photoperiod.
Example: Bryophyllum dalgremontianum and Cestrum nocturnum.
SLDP ( Short long- day plants):
The plants which are induced to flower when short photoperiods are succeeded by long photoperiods .
Example: Scale - cereal var. Petkus ( winter the) and Iberis durandii ( Candytuft).
8.3.Phytochrome
The term Phytochrome was coined by Warren Butler in 1959 and defined it as a bluish bili protein ( chromoprotein) pigment that acts as an optical photoreceptor in plants .
Phytochrome exists in two interconvertible forms:-
Pr ( Red absorbing): The biologically inactive, stable form that absorbs red light( ~660nm) and converts to Pfr .
Pfr ( Far-red absorbing): The biologically active , unstable form that absorbs far - red light (~730nm) and revarts back to Pr .
Under normal daylight condition, the amount of Pr and Pfr are almost equal. When exposed to red light, inactive Pr rapidly converts into active Pfr . But during the dark period or exposed to far-red light Pfr revarts back to Pr. This phenomenon is called "pigment shift" or Metabolic Dark Conversion.
8.3.1.Chemical Nature of Phytochrome
(i)As soluble chromoprotein, phytochrome consists of an apoprotein combined with a chromatophore.
(ii)The molecular mass of a phytochrome is 250 kDa , consisting of two identical 125 kDa apoprotein subunits.
(iii) Each subuntit has one N- terminal domain and another C- terminal domain.
N-terminal domain acts as photosensory part having chromatophore binding pocket , PAS and GAF sub-domains.
C- terminal domain acts as regulator having histidine kinase-related sub-domains that mediate downstream signalling.
(iv) The chromatophore is a linear tetrapyrrole, and its ring attaches to apoprotein through a thioether- linkage to a cysteine residue at GAF domain of N-terminal .
(v) The molecular mass of a chromatophore is ~ 0.58 kDa . It is similar to phycocyanin and termed as phytochromobilin.
8.3.2.Mode of action of Phytochrome for flowering
Molecular mechanism of phytochrome for flower induction process is described below:
Conversion of Pr into Pfr and entry of Pfr into the nucleus
(i) During day light , red light converts inactive Pr into active Pfr.
(ii) Next the active Pfr moves into the nucleus from cytosol through nuclear pore and cluster together to form nuclear photobodies.
(iii) These photobodies serve as central processing hub where TFs and regulatory proteins intercepted and modified.
(iv) Pfr also directly bonds to PIF proteins to form a complex.
(v) The Pfr- PIFs complex binds to and turns on G-box of the promoter:
C - G
A - T
C - G G-Box
G - C
T - A
G - C
It triggers immediate phosphorylation and polyubiquitination of PIFs marking for rapid degradation via the 26S proteasome pathway.
This clearing of PIFs from the chromatin modified the transcriptional landscape, freeing up the molecular mechanism to focus on flowering promoting genes
External Coincidence Model was proposed by Erwin Bunning in 1936 ; then developed by Colin S Pittendrigh and Minis in 1964 which explains the Role of Phytochrome in CO Regulation .
Role of phyA:
• Phytochrome A (phyA) is activated by Far- Red light during dawn and dusk . It inhibits the protein degradation mechanism, COP1-SPA complex, that normally destroy CO.
• Thus the accumulation of CO promotes flowering in LDs.
Role of phyB :
•Phytochrome B ( phyB) is activated by Red light, prominent during midday sun. In the early morning, active phyB marks the CO protein for rapid degradation of CO protein via ubiquitin- proteasome pathway.
•It prevents premature or accidental accumulation of CO during wrong times of the day, delaying flowering.
Key steps of CO Regulation :
• In Arabidopsis ,CONSTANS (CO) gene plays a central role in the induction of flowering by LDs.
• CO encodes a nuclear protein containing zinc fingers, and activates the transcription of floral regulators such as FLOWERING LOCUS T ( FT) in the light.
• The mRNA level of CO is tightly controlled by the circadian clock and shows a striking temporal pattern of expression.
• Under LDs CO mRNA peaks before dusk and stay high until the following dawn, whereas under SDs CO mRNA peaks during the night.
•Under these conditions, when CO mRNA expression coincides with the exposure of plants to light, photoreceptors enhance the level and activity of the CO protein .
•CO first binds to NF-Y complex then the complex binds to FT promoter to triggers flowering by activating FT transcription which is translated into the synthesis of FT protein Florigen .
•cry1, cry2 and phyA promote flowering and stabilize the CO protein , whareas phyB delay flowering and promotes the degradation of CO.
•Therefore, the coincidence between CO mRNA and exposure to light is required to promote flowering.
Source:
(1)https://bioone.otg ; Phytochrome signalling Mechanism ; by J. Li, G Li ,H Wang and X W Deng , The Arabidopsis Book. 2011(9): (2011)
(2) Google Search
8.4.Role of Phytochrome in flowering
While exact molecular mechanism remains under study , phytochrome regulates flowering directly depending on the plant photoperiodic response type:
8.4.1. LDPs( Long- Day Plants) : The active Pfr form promotes flowering, whereas the inactive Pr inhibits .
Case Study: Spinach ( A typical LDP)
If the long night is interrupted by a flash of light (red, 660nm), the plant will bloom as the amount of Pr formed in night will be again converts into Pfr .
Few examplrs of LDPs: Black henben, Radish, Sugar beet, Hibiscus,Petunia, Lettuce, Wheat, Barley,etc.
•SDPs ( Short -Day Plants): The active Pfr form inhibits flowering, whereas the Pr form promotes it.
•DNPs( Day -Neutral Plants): Phytochrome have no effect on flower Induction.
8.5. Florigen and flowering:
Florigen , a hypothetical hormone, is made in the companion cell of leaf and tranlocated to the shoot apical meristem through the sieve element to trigger flowering. M. Chailakhyan in 1936 postulated the existence of Florigen.
According to Gregory (1948) carbon dioxide in high light intensity is converted in the leaf to "A", which in further illumination converted to "X". In dark , "A" is converted to "B" which is finally converted to " C", i.e. a flowering hormone.
Lang (1957 ) discovered that GA caused LDPs to flower, but it failed to SDPs flower under long days , thus concluded that GA is not florigen. Brian in 1958 used Lang's paradox showed that high level of GA-like hormone is required to active florigen ,while for SDPs the level of GA- like hormone required to be low .
Florigen just initiates flowering process, whereas flower- bud development depends on auxin concentration , water and ptotosynthate.
In 2007 Oliver Corbesier et.al and S. Takayam et.al. independently proposed that florigen is not a hormone, rather a protein encoded by the gene FLOWERING LOCUS ( FT) ,a member of PEBP ( phosphatidylethanolamine -binding protein in Arabidopsis thaliana and by the gene Hd3a gene in Oryza sativa respectively.
8.6. Vernalization and flowering
Soviet agronomist Trofim Lysenko in 1928 proposed that a wide range of flowering plants require cold treatment to trigger flower and termed the phenomenon as jarovization and translated into " Vernalization " ( from Latin vernum , meaning spring)
Definition: Vernalization is that biological process by which certain plants are stimulated to flower through exposure to low temperature (1°C to 7°C) , followed by subsequent exposure to long days(15 hours) at an optimal temperature by synthesizing a hypothetical hormone vernalin .
Apical meristem is the locus of Vernalization ( Chroboczek, 1934; Purvis et. al. 1940) although work of Wellensik ( 1961 ; 1962) on Lunaria indicated dividing cells of leaves receive the vernalization stimulus.
Mechanism of Vernalization
•During cold treatment a substrate names as precursor "A" is converted to intermediate product "B" .
•If the low temperature is immediately followed by high temperature, "B" degraded.
• If the treatment of low temperature is maintained,the "B" is converted to a stable product "C" .
• Product "C" is the flower initiating hormone called Vernalin.
Devernalization:
Thompson and Miller ,1929; Miller ,1929 and Chroboczek ,1934 reported that the vernalized seeds completely revert to the normal state if treated at 35°C (Gregory and Purvis, 1937) in absence of oxygen ( anaerobic conditions). This phenomenon is called devernalization.
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Chapter 9
Introduction
Sometimes, a 100% healthy and viable seed fails to grow/ germinate , even when planted in rich soil with optimum moisture content and warm sunlight. This temporary halt in growth and development is known as seed dormancy.
Dormancy acts as a survival mechanism in nature. It prevents the seed from germinating during drought or extreme cold , which are evil for the newly grown seedlings. When the time is the best for survival, the seed sprouts into a new seedling.
According to Amen (1963) dormancy is the endogenously controlled but environmentally imposed and temporarily suspension of growth accompanied by reduce metabolic activity and relatively independent of ambient environmental factors.
According to Simpson (1965) seed dormancy is the failure of viable seed to germinate at specific length of time in a set of environmental conditions, later evoke germination when the restrictive state of seed is removed naturally or artificially.
For your Exam Paper👇
Definition : Seed Dormancy is a physiological condition of a viable seed which fails to germinate even when optimal, favorable environmental conditions, such as adequate moisture, temperature, and oxygen are provided.
Causes of Seed Dormancy
There are two main reasons for the development of dormancy:
•1. Imposed Dormancy ( Quiescence)
Causes: Absence of favourable environmental conditions.
Nature: Temporary suspension of growth.
Example: Low temperature or insufficient moisture content. The seed resumes growth immediately once favourable conditions prevails.
•2. Innate Dormancy ( Rest)
Causes : Internal factors within the seed.
Nature: The seed fails to germinate even ideal environmental conditions are provided.
Example: Hard and impermiable seed coat, presence of chemical germination inhibitors, rudiment embryo. Internal physiological changes are required to break this dormancy.
Seed dormancy is basically of two types :
(i) Imposed seed dormancy : It is due to the absence of favourable environmental conditions.
(ii) Innate seed dormancy : It is due to the internal factors within the seed coat. It is further classified into:
(a) Exogenous (Innate )seed dormancy ( Not associated with embryo):
This type seed dormancy is caused by external factors surroundings the embryo --- such as seed coat or fruit structure ---rather than any defect or malformation within the embryo itself.
It is further classified into 3 types
• Physical (Exogenous)Seed Dormancy
Seed dormancy can be caused by hard, fibrous seed or fruit coat containing one or more impermiable layers of palisade cells that block water absorption. In some cases, these layers also restrict gaseous exchange.
e.g. Legumes/ pulses with hard seed coats, and stone fruits ( drupes) like peaches and olives.
• Chemical (Exogenous ) Seed Dormancy:
Seed germination can sometimes be prevented by chemical inhibitors -- such as abscisic acid, phenols, or coumarins -- located within the seed coat , pericarp , or surroundings fruit tissues ( commonly seen in tomatoes, citrus fruits and grapes).
• Mechanical (Exogenous) Seed Dormancy:
In certain plants-- such as Alnus ( alder) , stone fruit, or Termenalia -- a hard or tough seed coat or fruit coat creates mechanical resistance. This physical barrier restricts the growth and emergence of the protruding radicle, resulting in seed dormancy.
(b) Endogenous ( Innate) Seed Dormancy :
This type of dormancy occurs when germination - inhibiting factors are present within the embryo or its immediate internal tissues. In such cases, even when external environmental conditions -- such as moisture, temperature, and oxygen --are ideal, the seed still fails to germinate due to these internal barriers.
It is further classified into 3 types:
• Morphological seed dormancy : In some cases , seed are shed with an immature, underdeveloped, and undifferentiated embryo . This embryo requires time to grow and fully differentiate before germination can occur, resulting in dormancy.
e.g. In many members of the Apiaceae family, the tiny embryo must elongate inside the mericarp prior to radicle formation.
• Physiological seed dormancy:This type of seed dormancy is caused by chemical inhibitors -- such as abscisic acid or low metabolic activities within the embryo.
It is further classified into 3 types
• Non-Deep : ( PD) : In some cases seeds possess fully developed,water permiable embryo and seed coat , but having some physiological inhibition mechanism within the embryo to create dormancy.
e.g. seed dormancy in apples, pears, cherries, etc temperature fruits, and Alpine plants like Primula glorindae, Euphorbia jolkinii, etc.
This dormancy often disappears naturally through dry after-ripening or seasonal cold/ warm periods. Application of GA3 @ 10 to 1000 mg/ L can break such dormancy.
Embryo excised from these seed can produce normal seedlings.
•Intermediate : If the internal chemical or metabolic in the embryo is moderately strong, than it creates intermediate physiological dormancy which can be removed by cold or warm stratification ( chilling at 1-10°C) over several weeks, or by GA3 application.
The embryo can produce normal seedlings.
• Deep : When the physiological block, which creates dormancy, is intense and even surgically removed embryo can not provide healthy seedling than the type of dormancy is called deep physiological seed dormancy.
GA3 treatment cannot solve the problem, but needs 3-5 months moist , cold stratification to break the dormancy.
•Morphophysiological (MPD) : When the embryo is morphologically immature and at the same time internally have physiological mechanism which prevents immediate growth due to the presence of high level of abscisic acid , the seed resulted to face MPD.
It requires warm followed by cold or cold followed by warm stratification to break the dormancy.
(c) Combined / primary combinational seed dormancy:
If a seed shed with dual - barrier condition --such as a physical barrier and a physiological barrier it resulted in PY + PD type dormancy.
Here, the (PY) , i.e. physical component is seed coat is fully the impermiable for water , and the (PD) is high concentrations of abscisic acid in the embryo to make it chemically asleep.
Classification at a glance
(i) Water Impermiability of seedcoats ( Hard -Seededness)
Many mature seeds--particularly in legumes/ Fabaceae family ( e.g., alfalfa, clovers,etc) and aquatic plants ( e.g., water lotus) -- have seedcoats which are completely impermiable to water.
Causes of Impermiability :
•The seedcoat contains thick, compact cell layers and waxy cuticle that block water absorption.
• The hilum remains tightly closed , acting as a physical barrier preventing moisture from reaching the embryo.
Once, The hilum opens , water penetrate the embryo to break the dormancy to initiate germination.
(ii) Mechanically Resistant Seedcoats
In certain plant species, the seedcoat consists of thick, hard, or " stony" layers that act as a physical barrier. This mechanical resistance restricts the enlargement and expansion of the embryo inside,keeping the seed dormant, even if water , oxygen, and light are present.
e.g., common in Capsella , Brassica, Alisma plantago , etc. When natural decay weakens the barrier, the seeds germinate by breaking the dormancy.
(iii) Seedcoat impermiable to Oxygen
In certain plant family --such as Poaceae and Astersceae - the seedcoat or surrounding fruit wall restricts the exchange of gases. This limitation prevents adequate oxygen from reaching the embryo, resulting in seed dormancy.
e.g., the Xanthium fruit contains two seeds -;a lower seed and an upper.The lower seed germinates readily in the spring. The upper seed remains dormant because it's seed coat severely restricts oxygen entry.
Over time, through natural dry storage or microbial breakdown, the seedcoat becomes permiable to oxygen to trigger germination.
(iv) Growth inhibitors and seed dormancy
Endogenous seed dormancy can be caused by the presence of growth inhibitors in the seed tissues like -- embryo, endosperm,or fruit pulp. These chemicals like abscisic acid (ABA), coumarins, naringenin, specific polypeptides, ferric chloride,and even tomato juice block vital enzymatic pathways and arrest physiological development of the embryo to make it dormant.
When these hlinhibitors leached out by water, temperature change ,or application of GA resulted in the seed germination.
(v) Rudiment Embryo
In certain plant species like Ginkgo, Ilex opaca, Faxinus excelsior and many orchids the embryo does not develop as rapidly as surroundings tissues.When the seed matured, the embryo still remains rudimentary. The seed remains dormant till the embryo is completely developed.
(vi) Physiologically Dormant Embryo
In certain plants like apple, peach, iris, dogwood, hemlock ,and pines seeds are shed with morphologically mature embryo but which is still physiologically immature or dormant. Even if water, oxygen , and temperature are ideal, the seed cannot germinate immediately.
In the after- ripening period the biochemistry of the embryo changes , breaks the inhibitors, and synthesise growth hormones for the germination.
(A) Scarification is the forceful weakening of an impervious seed coat without harming it's internal tissues, allowing water and oxygen to enter the embryo to break dormancy and initiate germination.
Natural scarification occurs when a seed falls to the ground and is subjected to freezing winter temperatures, microbial activity, or passage through the digestive tracts of animals. These natural processes help break exogenous or physical dormancy.
Forced scarification is performed by cultivars according to their needs for commercial purpose.
(i) Acid / Chemical Scarification involves soaking seeds in conccentrated sulphuric acid ( approximately 100 ml per kg of seeds) for 2 to 3 minutes to dissolve tough seed coat while leaving embryo intact.
e.g., Cotton seeds, teak seeds
Note: Insufficient soaking time fails to break dormancy, while excessive time can kill the embryo.
(ii) Mechanical scarification involves physically altering a tough seed coat by rubbing it against sandpaper or chipping it to create microscopic openings for gas and water entry, all without harming the embryo. For large seeds, a knife or file sometimes used to nick the seed coat.
e.g., Leguminous seeds
(iii) Hot water treatment (Thermal shock ) is performed by placing seeds in a container and pouring boiling water ( 75°C to 100°C) over them, then allowing the seeds to soak until the water cools to room temperature. This process softens the hard seed coat and melts waxy deposits, allowing oxygen and water to enter to initiate the germination.
e.g., Dalbergia, Cassia, Leucaena --- soaking time 10-15 minutes.
(iv) Dry heat treatment is done by exposing seeds to high dry temperature for a specific period to create micro fissures in the seed coat for the entry of oxygen and water into the embryo.
(B) Stratification involves exposing seeds to a controlled period of moist , cold, or warm temperatures to lower or increase biochemicals within the embryonic tissues, thereby stimulating the natural seasonal conditions that trigger germination. This process is applied to break physiological and morphological dormancy.
(i) Cold Stratification ( Chilling) : Seeds ( such as many flower and leguminous seeds) are placed in a moist medium -- like vermiculite, pest, or sand -- and refrigerated or incubated at 0°C to 5°C for weeks to months. These cold, moist conditions alter the hormonal balance within the embryonic tissues ( adjusting ABA and GA levels) to create a favourable environment for germination.
(ii) Warm Stratification: Certain seeds , such as paddy , are maintained at temperatures of 40°C to 50°C with around 15% moisture content to foster a morphological after-ripening environment that breaks dormancy. Similarly, oil palm seeds undergo warm conditioning ( often around 39°C to 40°C ) typically lasting about 2 months ( 60 to 80 days) to successfully overcome internal dormancy mechanisms.
(iii) Altering Temperature: Many weed and wild plant species have evolved to mimic natural diurnal cycles, using daily temperature fluctuations ( such as 20°C for 16 hours altering with 40°C for 8 hours) during inhibition to break seed dormancy. These thermal shifts act as ecological cues that trigger enzymatic activities, repair cellular damage , and stimulate metabolic pathways necessary for germination.
(iv) Seed Leaching ( Hydro - Soaking) : Soaking seeds in running water-- or changing the water frequently ( every 12 hours) -- for a duration of 3 days to wash away ( leach) germination inhibitors located in the seed coat or embryonic tissues, ensuring uniform and successful germination.
e.g.,
• Coriander: Leached out coumarins.
• Sunflower: Leaches out hydrocyanic acid.
(v) Fire Treatment: Many fire- following plant species of the family Fabaceae and Proteaceae , plants of regular wildfire prone ecosystem,etc. require high temperature ( 70°C to 12O°C) to crack , soften, or rupture the tough seed coat. Moreover, karrikins ( a class of plant growth regulators found in smoke) play a massive role to wake the dormant seeds.
(C) Light-Mediated Methods:
Many photoblastic seeds need exposure to red light of 660 nm wavelength to convert inactive form of Phytochrome red ( Pr) to the biologically active Phytochrome far red ( Pfr) . This Pfr form triggers the signal transduction pathways required for germination.
(D)Chemical ( Hormonal) Treatment: Endogenous dormancy due to the present of inhibitors can be broken by the following chemicals :
(i) GA and Cytokinin are used at the rate of 100 to 1000 ppm to induce the synthesis of hydrolytic enzymes like alpha- amylase, proteases ,etc, to break down physiological dormancy.
(ii) 2% Potassium Nitrate is used to break the dormancy of light required seed such as oats, barley and tomatoes.
(iii) Thiourea may be used to break dormancy for both light and chilling treatment requiring seeds like lettuce.
(iv) Ethrel can be used to reduce the amount of ABA present in the pericarp of the fruit to germinate the seeds of that specific plant species.
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The author is working as a faculty member of Ramkrishna Mahavidyalaya, Kailashahar , Unakoti Tripura since 1997.
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