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PLANT GROWTH REGULATORSR

PLANT GROWTH REGULATORSR

POINTS TO REMEMBER :

Definition of Growth

Growth is defined as “an irreversible permanent increase in size of an organ or its part or even of an individual cell.”

In other words, Growth is the most fundamental and conspicuous characteristics of living beings and is accompanied by several metabolic processes that occurs at the expense of energy. These metabolic processes may be catabolic or anabolic. In case of plants, seed germinates, develops into seedling and later it takes the shape of an adult plant are different stages of growth. Plants displays indefinite growth.

On the other hand, animals show uniform and fixed growth.  

Characteristics of Growth


Plant Growth is generally Indeterminate –Plants possess the ability of growth throughout their life. This is due to the presence of meristems at certain locations in their body and these meristems have the ability to divide and self –perpetuate.


Growth is Measurable – At cellular level, Growth is the consequence of increase in protoplasm and this increase is difficult to measure. Growth, in plants, is measured via different methods like increase in dry weight, volume, cell number, volume or increase in fresh weight.



The following diagram represents the location of root apical meristem, shoot apical meristem and vascular cambium. The arrows display the direction of growth of cells and organs. 



The Growth of Plants has three phases:


Formative Phase – Cell division is the basic event in the growth of plant. All cells are the result of division of pre-existing cells. Mitosis is the type of cell division that happens during growth and includes both quantitative and qualitative division of cells. This division is carried out in two steps – Division of Nucleus, which is referred as Karyokinesis and division of cytoplasm referred as Cytokinesis. In case of higher plants, an increase of cells is carried out in meristematic region, whereby some daughter cells retain this meristematic activity while some enter in the next phase of growth, i.e. the phase of cell enlargement.


Cell Enlargement and Cell Differentiation – At this stage, the size of tissues and organs is increased and this enlargement occurs by forming Protoplasm, Hydration (absorbing water), developing vacuoles and then adding new cell wall to make it permanent and thicker.


Cell Maturation – At this stage, the enlarged cells acquire specific size and forms as per their location and role. Thus, several cells are differentiated from simple and complex tissues which perform different functions.

Phases of growth :

• The period of growth is generally divided into three phases
  • Meristematic.
    
  • Elongation.
  • Maturation.
• Root apex and shoot apex represent the meristematic phase of growth.
• The cells of this region are rich in protoplasm, possesses large conspicuous nuclei.
• Their cell walls are primary in nature, thin and cellulosic with abundant plasmodesmatal connection.
• The cells proximal to that region are the phase of elongation.
• Increased vacuolation, cell enlargement and new cell wall deposition are the characteristic of the cells in this phase.
• Further away from the zone of elongation is the phase of maturation.
• The cells of this zone attain their maximal size in terms of wall thickening and protoplasmic modifications.

 Experiment to Study Phases of Growth

In order to study the phases of Growth, Germinate few seeds of peas in moist saw dust. Select the couple of seedlings with 2 – 3 cm of length, wash them and blot the surface water. Then, mark the radicles from tip to base with 10 – 15 point at interval of 2 mm via water proof ink. After drying of ink, place those seedlings on moist blotting paper and allow them to grow for 1 – 2 days. Finally measure the intervals between the marks and we can clearly observe the different phases of growth.

Following diagram shows the phases of growth in root. A is the marked radicle of seedling at the beginning of experiment and B is the condition of seedling after 48 hours. We can clearly identify zone of cell formation, cell elongation, cell differentiation and zone of matured cells. 



“The increased growth per unit time is termed as Growth Rate. Thus, the rate of growth is expressed mathematically.” An organism can produce cells in several ways and display Geometric as well as Arithmetic Growth.Following diagram shows both types of growth in plants:






The following diagram displays the various stages of embryo development showing both Geometric and Arithmetic Phases. Here dark blue blocks represent the cells capable of division while light blue blocks represents the cells that have lost the capacity to divide:



Thus, in Arithmetic Growth, only one daughter cell continues to divide while other differentiates and matures. The following graph represents the length of an organ against time, whereby a linear curve is obtained. We can clearly observe the constant linear growth against time t.



In Mathematical Terms, Growth Rate is expressed as:

Lt = Lo + rt

Where, Lt = length at time “t”

Lo = length at time “zero”

r = growth rate or elongation per unit time.

Now focusing on Geometrical Growth, In majority of systems, Initial Growth is slow and is referred as lag phase. Then, it increases rapidly at an exponential rate referred as log phase or exponential phase. The growth of plant slows down in cases of limited nutrient supply and results in stationery phase. When we plot the growth against time, it results in S-Curve or Sigmoid Curve. Following graph represents an idealized sigmoid growth curve typical of cells in culture and many higher plants and plant organs.



The above sigmoid curve is the characteristic of living organism growing in natural environment and is typical for all cells, tissues and organs. The exponential growth in expressed as:

W1 = Woert

Where,

W1 = final size (weight, height, number etc.)

Wo = Initial size at the beginning of the period

r = relative growth rate and the measure of the ability of plant to produce new plant material

t = time of growth

e = base of natural logarithms  

Types of Growth

There are five types of Growth:


Primary and Secondary Growth: “The mitotic divisions of meristematic cells present at the root and shoot apex increases the length of the plant body. This is referred as Primary Growth and the Secondary meristem that results in an increase in diameter of the body of plant is called as Secondary Growth.”


Unlimited Growth: This is the stage, when root and shoot of plant continuously grow from germination stage to death and throughout the entire lifespan.


Limited Growth: This is the stage, when fruits, leaves and flowers stop growing after attaining certain size. It is also called determinate type of Growth.


Vegetative Growth: The Growth of Plant before flowering in called Vegetative Growth. This Growth includes producing of stems, leaves and branches.


Reproductive Growth: At this stage, plants start flowering, which is the reproductive part of the plant.   



Factors Affecting Plant Growth


External Factors: The Growth of Plant primarily depends on habitat in which it is growing. Along with this, external factors also play an integral role in the growth of plants. It includes availability of Oxygen, Water and Nutrients followed by Temperature and Light.


Temperature plays important role in the growth of plants. The minimum, optimum and maximum temperature varies and from species to species. As the temperature increases above minimum, growth is accelerated until the optimum temperature is attained, when the growth gets slower and is completely retarded. Effect of duration for which a plant is exposed to certain temperature also varies amongst different species. For Example: The plant shows good growth when it is exposed to 86°F for a short duration and the same temperature has negative impact if maintained for longer duration.


Light also affect the growth and development of plant. Several factors of light like light intensity, duration of light and quality of light influences several physiological processes like movement of stomata, chlorophyll synthesis, temperature of aerial organs, formation of anthocyanin, absorption of minerals streaming of protoplasm and rate of transpiration. Intensity of light also influences plant growth and the variation in intensity has significant impact on growth pattern. Most ornamental plants and crops, such as Peas, Corn, Tobacco and Peas makes stocky and vigorous growth will full sun and thus, is also called “Sun Plant.”


Difference in wave length of light also effects the growth of plant. Several experiments have proved that plants that has full spectrum of visible light shows proper development and increase in dry weight. Plants grown in violet and blue light tend to dwarf, while plants in red light are taller and spindly.


Duration of light also affects the plant growth as it affects the rate of photosynthesis. For instance, during winters when days are short, the growth is very slow, while, it increases during summers when the days are longer.


The plants with lesser availability of oxygen show retarded growth while it is vice versa in the presence of ample of oxygen. It is important to note that plants in flooded areas, results in deficiency of soil aeration which on the other hand, results in poor plant growth.


Water is very important for plants and inadequate water results in poor growth. Plants grow well only in the presence of optimum water. Plants respond to deficiency of moisture as well. For instance, peppers, spinach and radishes wilt and cease to grow when the percentage of water in soil is lower.


Soil nutrients, their quantity and nature also affect the growth of plant. For Luxuriant Growth, it is important to have adequate amount of nutrients.




External Factors: The Growth of Plant primarily depends on habitat in which it is growing. Along with this, external factors also play an integral role in the growth of plants. It includes availability of Oxygen, Water and Nutrients followed by temperature and light.These factors include growth regulators, C/N ratio and genotype and genetic factor.


There are several classes of growth regulators. Some promote the growth like Auxins, Florigen, Cytokinins, Gibberellins, etc., while some are growth inhibitors like ethylene, abscisic and chlorocholine chloride.


The ratio of carbohydrates and nitrogen also govern the growth of plants. Presence of more carbohydrates as compared to nitrogen facilitates vegetative growth, fruiting and flowering while presence of more nitrogenous compounds results in poor vegetative growth.


Following diagram shows the percentage contribution of various factors in the growth of plants. According to it, the percentage of mineral particles is 45% and air & water is 25%.


Genotypes are responsible for controlling all the metabolic activities, growth and development of plant. Expression of genes in the correct sequence is controlled by two things, i.e. environment and genes. These genes are located in chromosomes and transcribe information to m-RNA that translates in enzyme and structural protein.

• 

Condition of growth :

• Water, oxygen and nutrients as very essential element for growth.
• Turgidity of cells helps in extension growth.
• Water also provides the medium for enzymatic activities needed for growth.
• Oxygen helps in releasing metabolic energy essential for growth activities.
• Nutrients are required by plants for synthesis of protoplasm and act as source of energy.

Differentiation, dedifferentiation and redifferentiation :

• The cells derived from root apical and shoot apical meristems and cambium differentiate and mature to perform specific functions.
• This act of maturation is termed as differentiation.
• During differentiation major changes takes place in their cell wall and protoplasm.
• Differentiated tracheary element cells loose their protoplasm, develop a very strong, elastic lignocellulosic secondary cell walls.
• The living differentiated cells, that by now have lost the capacity to divide can regain the capacity of division under certain condition is dedifferentiation.
• Development of interfascicular cambium and cork cambium from fully differentiated parenchymatous cells is the example of dedifferentiation.
• Cells produced by the dedifferentiated tissues again loose the capacity to divide and mature to perform specific function is called redifferentiation.
  
  
  
  

PLANT GROWTH REGULATORS :
Characteristics :

• The plant growth regulators are small, simple molecules of diverse chemical composition.
• They could be:
  • Indole compounds (indole-3-acetic acid, IAA);
  • adenine derivatives (N6-furfurylamino purine, kinetin)
  • derivatives of carotenoids (abscisic acid,ABA)
  • terpenes (gibberellic acid, GA₂)
  • Gases (ethylene, C₂H₄)
• One group of PGRs are involved in growth promoting activities such as cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting and seed germination. These are called plant growth promoters, e.g. auxin, gibberellins and cytokinin.


• Another group of PGRs play important role in plant responses towards to wounds and stresses of biotic and abiotic origin. They involved in inhibitory responses like dormancy and abscission, e.g. abscisic acid.

 Discovery of plant growth regulators :

• Auxin was isolated by F.W. Went from tips of oat seedlings.


• The ‘bakane’ (foolish seedling) a disease of rice seedlings, was caused by a fungal pathogen Gibberalla fujikuroi.


• E. Kurosawa reported the appearance of the symptom of the disease in uninfected rice seedlings when treated with sterile filtrate of the fungus. The active substance was later identified as Gibberellic acid.


• Skoog and Miller identified and crystallized the cytokinesis promoting active substance that they termed as kinetin.


• During mid 1960s three different kinds of inhibitors purified, i.e. inhibitor-B abscission II and dormin. Later all the three proved to be chemically identical and named as Abscisic acid (ABA).


• Cousinsdiscovered a gaseous PGR called ethylenefrom ripened orange.

Physiological effect of plant growth regulators :

Auxin :

• The term auxin is applied to indole-3-acetic acid
• Generally produced by growing apices of the stems and roots.
• IAA and IBA have been isolated from plants.
• NAA and 2, 4-D (2, 4-dichlorophenoxyacetic acid) are synthetic auxin.
• Promote rooting in stem cutting.
• Promote flowering.
• Inhibit fruit and leaf drop at early stages.
• Promote abscission of older mature leaves and fruits.
• The growing apical bud inhibit the growth of lateral bud, the phenomenon is called apical dominance.
• Auxin induces parthenocarpy.
• Used as herbicides.
• Controls xylem differentiation.
• Promote cell division.
  

Gibberellins :

• Ability to cause an increase in length of axis is used to increase the length of grapes stalks.
• Gibberellins cause fruits like apple to elongate and improve its shape.
• Delay senescence
• GA3 is used to speed up the malting process in brewing industry.
• Gibberellins promote to increase length of stem in sugar cane.
• Promote early seed production.
• Promote bolting (internodes elongation) in beet, cabbages.

Cytokinins :

• Cytokinins have specific effects on cytokinesis.
• Zeatin isolated from corn-kernels and coconut milk.
• Promote cell division.
• Help to produce new leaves, chloroplast in leaves, lateral shoot growth
• Promote formation of adventitious shoot.
• Cytokinins help to overcome apical dominance.
• Promote nutrient mobilization.
• Delay senescence.

Ethylene :

• Ethylene is a simple gaseous PGR.
• Synthesized in the tissue undergoing senescence and ripening fruits.
• Promote horizontal growth of seedling.
• Promote swelling of axis and apical hook formation in dicot seedlings.
• Promote senescence and abscission of plant organs like leaf and flower.
• Increase rate of respiration during ripening of fruits, called respiratory climactic.
• Breaks seed and bud dormancy.
• Initiate germination.
• Promote rapid internodes elongation.
• Promote root growth and root hair formation.
• Used to initiate flowering and for synchronizing fruit-set.
• Induce flowering in mango.
• The source of ethylene is ethephon.
• Promote female flower in cucumbers thereby increasing the yield.

Abscisic acid :

• Regulates abscission and dormancy.
• Acts as general plant growth inhibitor and an inhibitor of plant metabolism.
• Inhibit seed germination.
• Stimulates the closure of stomata and increases the tolerance of plants to various kinds of stresses, hence called as stress hormone.
• Important role in seed development, maturation and dormancy.
• Inducing dormancy, ABA helps seeds to withstand desiccation and other factors unfavourable for growth.
• Acts as antagonist to Gas.

PHOTOPERIODISM :

• Some plants require a periodic exposure to light to induce flowering.
• Response of plants in terms of day/night in relation to flowering is called photoperiodism.
• Long day plant: plant requires the exposure to light for a period exceeding critical period.
• Short day plant: plant requires the exposure to light for a period less than critical period.
• Day neutral plant: there is no such correlation between exposure to light duration and induction of flowering response.
• The site of perception of light/dark duration is the leaves.
  
• 
  

VERNALISATION :

• Vernalisation: There are plants for which flowering is either quantitatively or qualitatively dependent on exposure to low temperature.
• It prevents precocious reproductive development late in the growing season.
• Vernalisation refers to the promotion of flowering by a period of low temperature.

Respiration In Plants

RESPIRATION IN PLANTS

POINTS TO REMEMBER :

Growth :

• An irreversible permanent increase in size of an organ or its parts or even of an individual cell.
• Growth is accompanied by metabolic process that occurs at the expense of energy.

Plant growth is generally is indeterminate :

• Plants retain the capacity of unlimited growth throughout their life.
• This ability is due to the presence of meristems at certain locations in their body.
• The cells of such meristems have capacity to divide and self-perpetuate.
• The product eventually looses the capacity to divide and differentiated.
• Apical meristems responsible for primary growth of the plants and principally contribute to the elongation of the plants along their axis.
• The lateral meristem, vascular cambium and cork cambium appears later and responsible for the increase in the girth.

Phases of growth :

• The period of growth is generally divided into three phases
  • Meristematic.
  • Elongation.
  • Maturation.
• Root apex and shoot apex represent the meristematic phase of growth.
• The cells of this region are rich in protoplasm, possesses large conspicuous nuclei.
• Their cell walls are primary in nature, thin and cellulosic with abundant plasmodesmatal connection.
• The cells proximal to that region are the phase of elongation.
• Increased vacuolation, cell enlargement and new cell wall deposition are the characteristic of the cells in this phase.
• Further away from the zone of elongation is the phase of maturation.
• The cells of this zone attain their maximal size in terms of wall thickening and protoplasmic modifications.

Condition of growth :

• Water, oxygen and nutrients as very essential element for growth.
• Turgidity of cells helps in extension growth.
• Water also provides the medium for enzymatic activities needed for growth.
• Oxygen helps in releasing metabolic energy essential for growth activities.
• Nutrients are required by plants for synthesis of protoplasm and act as source of energy.

Differentiation, dedifferentiation and redifferentiation :

• The cells derived from root apical and shoot apical meristems and cambium differentiate and mature to perform specific functions.
• This act of maturation is termed as differentiation.
• During differentiation major changes takes place in their cell wall and protoplasm.
• Differentiated tracheary element cells loose their protoplasm, develop a very strong, elastic lignocellulosic secondary cell walls.
• The living differentiated cells, that by now have lost the capacity to divide can regain the capacity of division under certain condition is dedifferentiation.
• Development of interfascicular cambium and cork cambium from fully differentiated parenchymatous cells is the example of dedifferentiation.
• Cells produced by the dedifferentiated tissues again loose the capacity to divide and mature to perform specific function is called redifferentiation.

PLANT GROWTH REGULATORS :
Characteristics :

• The plant growth regulators are small, simple molecules of diverse chemical composition.
• They could be:
  • Indole compounds (indole-3-acetic acid, IAA);
  • adenine derivatives (N6-furfurylamino purine, kinetin)
  • derivatives of carotenoids (abscisic acid,ABA)
  • terpenes (gibberellic acid, GA₂)
  • Gases (ethylene, C₂H₄)
• One group of PGRs are involved in growth promoting activities such as cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting and seed germination. These are called plant growth promoters, e.g. auxin, gibberellins and cytokinin.


• Another group of PGRs play important role in plant responses towards to wounds and stresses of biotic and abiotic origin. They involved in inhibitory responses like dormancy and abscission, e.g. abscisic acid.

Discovery of plant growth regulators :

• Auxin was isolated by F.W. Went from tips of oat seedlings.


• The ‘bakane’ (foolish seedling) a disease of rice seedlings, was caused by a fungal pathogen Gibberalla fujikuroi.


• E. Kurosawa reported the appearance of the symptom of the disease in uninfected rice seedlings when treated with sterile filtrate of the fungus. The active substance was later identified as Gibberellic acid.


• Skoog and Miller identified and crystallized the cytokinesis promoting active substance that they termed as kinetin.


• During mid 1960s three different kinds of inhibitors purified, i.e. inhibitor-B abscission II and dormin. Later all the three proved to be chemically identical and named as Abscisic acid (ABA).


• Cousinsdiscovered a gaseous PGR called ethylenefrom ripened orange.

Physiological effect of plant growth regulators :

Auxin :

• The term auxin is applied to indole-3-acetic acid
• Generally produced by growing apices of the stems and roots.
• IAA and IBA have been isolated from plants.
• NAA and 2, 4-D (2, 4-dichlorophenoxyacetic acid) are synthetic auxin.
• Promote rooting in stem cutting.
• Promote flowering.
• Inhibit fruit and leaf drop at early stages.
• Promote abscission of older mature leaves and fruits.
• The growing apical bud inhibit the growth of lateral bud, the phenomenon is called apical dominance.
• Auxin induces parthenocarpy.
• Used as herbicides.
• Controls xylem differentiation.
• Promote cell division.

Gibberellins :

• Ability to cause an increase in length of axis is used to increase the length of grapes stalks.
• Gibberellins cause fruits like apple to elongate and improve its shape.
• Delay senescence
• GA3 is used to speed up the malting process in brewing industry.
• Gibberellins promote to increase length of stem in sugar cane.
• Promote early seed production.
• Promote bolting (internodes elongation) in beet, cabbages.

Cytokinins :

• Cytokinins have specific effects on cytokinesis.
• Zeatin isolated from corn-kernels and coconut milk.
• Promote cell division.
• Help to produce new leaves, chloroplast in leaves, lateral shoot growth
• Promote formation of adventitious shoot.
• Cytokinins help to overcome apical dominance.
• Promote nutrient mobilization.
• Delay senescence.

Ethylene :

• Ethylene is a simple gaseous PGR.
• Synthesized in the tissue undergoing senescence and ripening fruits.
• Promote horizontal growth of seedling.
• Promote swelling of axis and apical hook formation in dicot seedlings.
• Promote senescence and abscission of plant organs like leaf and flower.
• Increase rate of respiration during ripening of fruits, called respiratory climactic.
• Breaks seed and bud dormancy.
• Initiate germination.
• Promote rapid internodes elongation.
• Promote root growth and root hair formation.
• Used to initiate flowering and for synchronizing fruit-set.
• Induce flowering in mango.
• The source of ethylene is ethephon.
• Promote female flower in cucumbers thereby increasing the yield.

Abscisic acid :

• Regulates abscission and dormancy.
• Acts as general plant growth inhibitor and an inhibitor of plant metabolism.
• Inhibit seed germination.
• Stimulates the closure of stomata and increases the tolerance of plants to various kinds of stresses, hence called as stress hormone.
• Important role in seed development, maturation and dormancy.
• Inducing dormancy, ABA helps seeds to withstand desiccation and other factors unfavourable for growth.
• Acts as antagonist to Gas.

PHOTOPERIODISM :

• Some plants require a periodic exposure to light to induce flowering.
• Response of plants in terms of day/night in relation to flowering is called photoperiodism.
• Long day plant: plant requires the exposure to light for a period exceeding critical period.
• Short day plant: plant requires the exposure to light for a period less than critical period.
• Day neutral plant: there is no such correlation between exposure to light duration and induction of flowering response.
• The site of perception of light/dark duration is the leaves.

VERNALISATION :

• Vernalisation: There are plants for which flowering is either quantitatively or qualitatively dependent on exposure to low temperature.
• It prevents precocious reproductive development late in the growing season.
• Vernalisation refers to the promotion of flowering by a period of low temperature.

PHOTOSYNTHESIS IN HIGHER PLANTS

POINTS TO REMEMBER :

• Photosynthesis: Photosynthesis is an enzyme regulated anabolic process of manufacture of organic compounds inside the chlorophyll containing cells from carbon dioxide and water with the help of sunlight as a source of energy.

Historical Perspective :

• Joseph Priestley (1770) : Showed that plants have the ability to
  
  take up CO₂ from atmosphere and release O₂.
• Jan Ingenhousz (1779) : Release of O₂ by plants was possible only in sunlight and only by the green parts of plants.
• Theodore de Saussure (1804) : Water is an essential requirement for photosynthesis to occur.
• Julius Von Sachs (1854) : Green parts in plant produce glucose which is stored as starch.
• T. W. Engelmann (1888) : The effect of different wavelength of light on photosynthesis and plotted the first action spectrum of photosynthesis.
• C. B. Van Niel (1931) : Photosynthesis is essentially a light dependent reaction in which hydrogen from an oxidisable compound reduces CO₂ to form sugar. He gave a simplified chemical equation of photosynthesis.
• Hill (1937) : Evolution of oxygen occurs in light reaction.
• Calvin (1954-55) :  Traced the pathway of carbon fixation.
• Hatch and Slack (1965) : Discovered C4 pathway of CO₂ fixation.

Site for photosynthesis :

• Photosynthesis takes place only in green parts of the plant, mostly in leaves.
• Within a leaf, photosynthesis occurs in mesophyll cells which contain the chloroplasts.
• Chloroplasts are the actual sites for photosynthesis.
• The thylakoids in chloroplast contain most of pigments required for capturing solar energy to initiate photosynthesis.
• The membrane system (grana) is responsible for trapping the light energy and for the synthesis of ATP and NADPH. Biosynthetic phase (dark reaction) is carried in stroma.

Pigments involved in photosynthesis:

• Chlorophyll a : (Bright or blue green in chromatograph). Major pigment, act  as  reaction  centre,  involved  in  trapping  and  converting  light  into chemical energy.
• Chlorophyll b : (Yellow green)
• Xanthophylls : (Yellow)
• Carotenoid : (Yellow to yellow-orange)
• In the blue and red regions of spectrum shows higher rate of photosynthesis.

What is light reaction?

• Light reactions or the ‘Photochemical ‘phase includes light
  absorption, splitting of water, evolution of oxygen and formation of high energy compound like ATP and NADPH.
• Light Harvesting Complexes (LHC) : 
  
   

• The light harvesting complexes are made  up  of  hundreds  of  pigment  molecules  bound  to  protein  within  the photosystem I (PSI) and photosystem II (PSII).
• Each photosystem has all the pigments except one molecule of chlorophyll ‘a’ forming a light harvesting system (antennae).
• The reaction centre (chlorophyll a) is different in both the photosystems.
• Photosystem I (PSI) : Chlorophyll ‘a’ has an absorption peak at 700 nm (P700).
• Photosystem II (PSII) : Chlorophyll ‘a’ has absorption peak at 680 nm (P680).

Process of photosynthesis :

• It includes two phases - Photochemical phase and biosynthetic phase.
• Photochemical phase (Light reaction) : This phase includes - light absorption, splitting of water, oxygen release and formation of ATP and NADPH.
• Biosynthetic phase (Dark reaction) : It is light independent phase, synthesis of food material (sugars).

The electron transport :

• In photosystem centre chlorophyll a absorbs 680 nm wavelength of red light causing electrons to become excited and release two electrons from the atomic nucleus.
• These electrons are accepted by primary electron acceptor i.e. ferredoxin.
• The electron from the ferredoxin passed to electron transport system consisting cytochromes.
• The electron moved in down hill in terms of redox potential by oxidation-reduction reactions.
• Finally the electron reached photosystem-I.
• Simultaneously electron released from photosystem-I is accepted by electron acceptor.
• Electron hole created in PS-I is filled up by the electron from PS-II.
• Electron from PS-I passed down hill and reduce NADP into NADPH⁺ + H⁺.

Photolysis of water :

• PS-II loose electrons continuously, filled up by electrons released due to photolysis of water.
• Water is split into H⁺, (O) and electrons in presence of light and Mn²⁺ and Cl^-.
• This also creates O₂ the bi-product of photosynthesis.
• Photolysis takes place in the vicinity of the PS-II.
• 2H₂O → 4H⁺ + O₂ + 4e^-.

Photophosphorylation :

• The process of formation of high-energy chemicals (ATP and NADPH).

Non Cyclic photophosphorylation :

• Two photosystems work in series – First PSII and then PSI.
• These two photosystems are connected through an electron transport chain (Z. Scheme).
• ATP and NADPH + H⁺ are synthesized by this process.  PSI  and  PSII  are  found  in  lamellae  of  grana,  hence  this  process  is carried here.

 Cyclic photophosphorylation : 

• Only PS-I works, the electron circulates within the photosystem.
• It happens in the stroma lamellae (possible location)because in this region PS-II and NADP reductaseenzyme are absent.
• Hence only ATP molecules are synthesized.

Chemiosmotic Hypothesis : 

• Chemiosmotic hypothesis explain the mechanism of ATP synthesis in chloroplast.
• In photosynthesis, ATP synthesis is linked to development of a proton gradient across a membrane.
• The protons that are produced by the splitting of water are accumulated inside of membrane of thylakoids (in lumen).
• As the electron moves through the photosystem, protons are transported across the membrane.
• NADP reductase enzyme is located on the stroma side of the membrane, along with electrons from the acceptor it removes H+ from the stroma during reduction of NADPH + H⁺.
• This creates proton gradients across the thylakoid membrane as well as a measurable decrease in pH in the lumen.
• ATPase has a channel that allows diffusion of protons back to stroma across the membrane. 
• This releases energy to activate ATPase enzyme that catalyses the formation of ATP.

Biosynthetic phase in C3 plants :

• ATP  and  NADH,  the  products  of  light  reaction  are  used  in  synthesis  of food. The first CO₂ fixation product in C3 plant is 3-phosphoglyceric acid or PGA.
• In some other plants the first stable product is an organic acid called oxaloacetic acid a 4-C compound hence is called C4 plants.

The Calvin cycle :

• The CO₂ acceptor molecule is RuBP (Ribulose bisphosphate).
• The cyclic path of sugar formation is called Calvin cycle on the name of Melvin Calvin, the discoverer of this pathway. Calvin cycle proceeds in three stages:


• Carboxylation : 
  • Carboxylation is the fixation of CO₂ into a stable organic intermediate.
  • CO₂ combines with Ribulose 1, 5 bisphosphate to form 3 PGA in the presence of RuBisCo enzyme.
• Reduction : 
  • These are a series of reactions that lead to the formation of glucose.
  • 2 molecules of ATP for phosphorylation and two of NADPH for reduction per CO₂ molecule fixed.
  • The fixation of six molecules of CO₂ and 6 turns of the cycle are required for the formation of one molecule of glucose.
• Regeneration :
  • Regeneration of the CO₂ acceptor molecule RuBP is crucial if the cycle is to continue uninterrupted.
  • Regeneration steps required one ATP for phosphorylation to form RuBP.
• Hence for every CO₂ molecule entering the Calvin cycle, 3 molecules of ATP and 2 molecules of NADPH are required.
  
  
  

The C₄ pathway :

• Plants that are adapted to dry tropical regions have the C₄ pathway.
• C₄ oxaloacetic acid is the first CO₂ fixation product.
• These plants have special type of leaf anatomy, they tolerate higher temperatures.
• The leaf has two types of cells: mesophyll cells and Bundle sheath cells (Kranz anatomy).
• Initially CO₂ is taken up by phosphoenol pyruvate (PEP) in mesophyll cells and changed to oxaloacetic acid (OAA) in the presence of PEP carboxylase.
• Oxaloacetate is reduced to malate/asparate that reaches into bundle sheath cells.
• In the bundle sheath cells these C₄ acids are broken down to release CO₂ and a 3-carbon molecule i.e. pyruvic acid.
• The CO₂ released in the bundle sheath cell enters the C3 cycle because these cells are rich in enzyme Ribulose bisphosphate carboxylase-oxygenase (RuBisCO).
• The pyruvate formed in the bundle sheath cell transported back to the mesophyll cell, get phosphorylated to form phosphoenol pyruvate.

Photorespiration:

• The light induced respiration (evolution of CO₂) in green plants is called photorespiration. 
• Active site of RuBisCO has active site for both O₂ and CO₂.
• In  C3  plants  some  O₂  binds  with  RuBisCo  and  hence  CO₂ fixation  is  decreased. 
• In  this  process  RuBP  instead  of  being  converted  to  2 molecules  of  PGA,  binds  with  O₂   to  form  one  molecule  of  PGA  and phosphoglycolate.
• In the photorespiratory pathway there is neither synthesis of sugar, nor of ATP. Rather it results in the release of CO₂ with utilization of ATP.
• In the photorespiratory pathway there is no synthesis of ATP or NADPH.
• Therefore photorespiration is a wasteful process.
• In C₄ plant photorespiration does not occur:
  • RuBisCO enzyme is present in the bundle sheath cells.
  • Primary carboxylation is takes place in the mesophyll cell by PEP carboxylase.
  • CO₂ supplied to bundle sheath cell by C₄ acid intermediate.
  • Hence C₄ plants are photosynthetically more efficient than C3 plant.

Law of Limiting Factors :

• If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value. It is the factor which directly affects the process if its quantity is changed.
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