CHAPTER 15 – PLANT GROWTH AND DEVELOPMENT

CHAPTER 15

PLANT GROWTH AND DEVELOPMENT

  • Trees continue to increase in height or girth over a period of time.
  • However, the leaves, flowers and fruits of the same tree not only have limited dimensions but also appear and fall periodically and sometime repeatedly.
  • Development is the sum of two processes: growth and differentiation.

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GROWTH

  • Growth is regarded as one of the most fundamental and conspicuous characteristics of a living being.
  • Growth can be defined as an irreversible permanent increase in size of an organ or its parts or even of an individual cell.
  • Generally, growth is accompanied by metabolic processes (both anabolic and catabolic), that occur at the expense of energy. Therefore, expansion of a leaf is growth while swelling of piece of wood when placed in water is not.

Plant Growth Generally is Indeterminate

  • Plant growth is unique because plants retain the capacity for unlimited growth throughout their life.
  • This ability of the plants is due to the presence of meristems at certain locations in their body.
  • The cells of such meristems have the capacity to divide and self-perpetuate.
  • The product, however, soon loses the capacity to divide and such cells make up the plant body.
  • This form of growth wherein new cells are always being added to the plant body by the activity of the meristem is called the open form of growth.
  • The root apical meristem and the shoot apical meristem are responsible for the primary growth of the plants and principally contribute to the elongation of the plants along their axis.
  • In dicotyledonous plants and gymnosperms, the lateral meristems, vascular cambium and cork-cambium appear later in life and cause the increase in the girth of the organs in which they are active. This is known as secondary growth of the plant.

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Growth is Measurable

  • Growth, at a cellular level, is principally a consequence of increase in the amount of protoplasm. Since increase in protoplasm is difficult to measure directly, one generally measures some quantity which is more or less proportional to it.
  • Growth is, therefore, measured by a variety of parameters some of which are: increase in fresh weight, dry weight, length, area, volume and cell number.

Phases of Growth

  • The period of growth is generally divided into three phases, namely, meristematic, elongation and maturation.
  • The constantly dividing cells, both at the root apex and the shoot apex, represent the meristematic phase of growth. The cells in this region are rich in protoplasm, possess large conspicuous nuclei. Their cell walls are primary in nature, thin and cellulosic with abundant plasmodesmatal connections.
  • The cells proximal (just next, away from the tip) to the meristematic zone represent the phase of elongation. Increased vacuolation, cell enlargement and new cell wall deposition are the characteristics of the cells in this phase.
  • Further away from the apex, i.e., more proximal to the phase of elongation, lies the portion of axis which is undergoing the phase of maturation. The cells of this zone, attain their maximal size in terms of wall thickening and protoplasmic modifications.

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Growth Rates

  • The increased growth per unit time is termed as growth rate.
  • Thus, rate of growth can be expressed mathematically.
  • The growth rate shows an increase that may be arithmetic or geometrical.
  • In arithmetic growth, following mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures. The simplest expression of arithmetic growth is exemplified by a root elongating at a constant rate.
  • On plotting the length of the organ against time, a linear curve is obtained. Mathematically, it is expressed as  Lt = Lo + rt

Lt = length at time ‘t’

L0 = length at time ‘zero’

r = growth rate / elongation per unit time.

  • In geometrical growth, the initial growth is slow (lag phase), and it increases rapidly thereafter – at an exponential rate (log or exponential phase). Here, both the progeny cells following mitotic cell division retain the ability to divide and continue to do so. However, with limited nutrient supply, the growth slows down leading to a stationary phase.
  • If we plot the parameter of growth against time, we get a typical sigmoid or S-curve. A sigmoid curve is a characteristic of living organism growing in a natural environment. It is typical for all cells, tissues and organs of a plant.
  • The exponential growth can be expressed as

Wt = Woert

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

Wo = initial size at the beginning of the period

r = growth rate

t = time of growth

e = base of natural logarithms.

Here, r is the relative growth rate and is also the measure of the ability of the plant to produce new plant material, referred to as efficiency index. Hence, the final size of W1 depends on the initial size, Wo.

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  • Quantitative comparisons between the growth of living system can also be made in two ways :

(i) measurement and the comparison of total growth per unit time is called the absolute growth rate.

(ii) The growth of the given system per unit time expressed on a common basis, e.g., per unit initial parameter is called the relative growth rate.

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Conditions for Growth

  • water, oxygen and nutrients are very essential elements for growth.
  • The plant cells grow in size by cell enlargement which in turn requires water. Turgidity of cells helps in extension growth. Thus, plant growth and further development is intimately linked to the water status of the plant. Water also provides the medium for enzymatic activities needed for growth.
  • oxygen helps in releasing metabolic energy essential for growth activities.
  • Nutrients (macro and micro essential elements) are required by plants for the synthesis of protoplasm and act as source of energy.
  • In addition, every plant organism has an optimum temperature range best suited for its growth. Any deviation from this range could be detrimental to its survival.
  • Environmental signals such as light and gravity also affect certain phases/stages of growth.

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 leading to maturation is termed as
  • During differentiation, cells undergo few to major structural changes both in their cell walls and protoplasm. For example, to form a tracheary element, the cells would lose their protoplasm. They also develops a very strong, elastic, lignocellulosic secondary cell walls, to carry water to long distances even under extreme tension.
  • differentiated cells, that have lost the capacity to divide can regain the capacity of division under certain conditions. This phenomenon is termed as For example, formation of meristems – interfascicular cambium and cork cambium from fully differentiated parenchyma cells.

While doing so, such meristems/tissues are able to divide and produce cells that once again lose the capacity to divide but mature to perform specific functions, i.e., get redifferentiated.

  • Growth in plants is open, i.e., it can be indeterminate or determinate. Now, we may say that even differentiation in plants is open, because cells/tissues arising out of the same meristem have different structures at maturity.
  • The final structure at maturity of a cell/tissue is also determined by the location of the cell within. For example, cells positioned away from root apical meristems differentiate as root-cap cells, while those pushed to the periphery mature as epidermis.

DEVELOPMENT

  • Development is a term that includes all changes that an organism goes through during its life cycle from germination of the seed to senescence.

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  • Plants follow different pathways in response to environment or phases of life to form different kinds of structures. This ability is called plasticity, g., heterophylly in cotton, coriander and larkspur.

In such plants, the leaves of the juvenile plant are different in shape from those in mature plants.

  • On the other hand, difference in shapes of leaves produced in air and those produced in water in buttercup also represent the heterophyllous development due to environment. This phenomenon of heterophylly is an example of plasticity.

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  • Thus, growth, differentiation and development are very closely related events in the life of a plant. Broadly, development is considered as the sum of growth and differentiation.
  • Development in plants (i.e., both growth and differentiation) is under the control of intrinsic and extrinsic factors. The former includes both intracellular (genetic) or intercellular factors (chemicals such as plant growth regulators) while the latter includes light, temperature, water, oxygen, nutrition, etc.

PLANT GROWTH REGULATORS

Characteristics

  • The plant growth regulators (PGRs) 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, GA3) or
    • gases (ethylene, C2H4).
  • Plant growth regulators are variously described as plant growth substances, plant hormones or phytohormones in literature.
  • The PGRs can be broadly divided into two groups based on their functions in a living plant body.
  • One group of PGRs are involved in growth promoting activities, such as cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting and seed formation. These are also called plant growth promoters, e.g., auxins, gibberellins and cytokinins.
  • The PGRs of the other group play an important role in plant responses to wounds and stresses of biotic and abiotic origin. They are also involved in various growth inhibiting activities such as dormancy and abscission. The PGR abscisic acid belongs to this group.
  • The gaseous PGR, ethylene, could fit either of the groups, but it is largely an inhibitor of growth activities.

The Discovery of Plant Growth Regulators

  • Interestingly, the discovery of each of the five major groups of PGRs have been accidental.
  • Charles Darwin and his son Francis Darwin – observed phototropism in Canary grass.
  • W. Went – isolated Auxin from tips of coleoptiles of oat seedlings.
  • Kurosawa – reported the appearance of symptoms of the ‘Bakane disease’ in uninfected rice seedlings when they were treated with sterile filtrates of the fungus Gibberalla fujikuroi. The active substances were later identified as gibberellic acid.
  • Skoog and co-workers – observed callus proliferation in tobacco callus in presence of extracts of vascular tissues/yeast extract/coconut milk or DNA in addition to Auxins.
  • Skoog and Miller – identified and crystallised the cytokinesis promoting active substance and termed it kinetin.
  • Cousins – confirmed the release of a volatile substance from ripened oranges that hastened the ripening of stored unripened bananas. Later this volatile substance was identified as ethylene, a gaseous PGR
  • During mid-1960s, three independent researches reported the purification and chemical characterisation of three different kinds of inhibitors: inhibitor-B, abscission II and dormin. Later all the three were proved to be chemically identical. It was named abscisic acid (ABA).

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PHYSIOLOGICAL EFFECTS OF PLANT GROWTH REGULATORS

AUXINS

  • Auxins (from Greek ‘auxein’ : to grow) was first isolated from human urine.
  • The term ‘auxin’ is applied to the indole-3-acetic acid (IAA), and to other natural and synthetic compounds having certain growth regulating properties.
  • They are generally produced by the growing apices of the stems and roots, from where they migrate to the regions of their action.
  • Auxins like IAA and indole butyric acid (IBA) have been isolated from plants.
  • NAA (naphthalene acetic acid) and 2, 4-D (2, 4-dichlorophenoxyacetic) are synthetic auxins.
  • All these auxins have been used extensively in agricultural and horticultural practices.
  • They help to initiate rooting in stem cuttings, an application widely used for plant propagation.
  • Auxins promote flowering e.g. in pineapples.
  • They help to prevent fruit and leaf drop at early stages but promote the abscission of older mature leaves and fruits.
  • In most higher plants, the growing apical bud inhibits the growth of the lateral (axillary) buds, a phenomenon called apical dominance. Removal of shoot tips (decapitation) usually results in the growth of lateral buds. It is widely applied in tea plantations, hedge-making.
  • Auxins also induce parthenocarpy, e.g., in tomatoes.
  • They are widely used as herbicides. 2, 4-D, widely used to kill dicotyledonous weeds, does not affect mature monocotyledonous plants.
  • It is used to prepare weed-free lawns by gardeners.
  • Auxin also controls xylem differentiation and helps in cell division.

GIBBERELLINS

  • There are more than 100 gibberellins reported from widely different organisms such as fungi and higher plants. They are denoted as GA1, GA2, GA3 and so on.
  • However, Gibberellic acid (GA3) was one of the first gibberellins to be discovered and remains the most intensively studied form.
  • All GAs are acidic.
  • They have ability to cause an increase in length of axis. It is used to increase the length of grapes stalks.
  • Gibberellins, cause fruits like apple to elongate and improve its shape.
  • They also delay senescence. Thus, the fruits can be left on the tree longer so as to extend the market period.
  • GA3 is used to speed up the malting process in brewing industry.
  • Sugarcane stores carbohydrate as sugar in their stems. Spraying sugarcane crop with gibberellins increases the length of the stem, thus increasing the yield by as much as 20 tonnes per acre.
  • Spraying juvenile conifers with GAs hastens the maturity period, thus leading to early seed production.
  • Gibberellins also promotes bolting (internode elongation just prior to flowering) in beet, cabbages and many plants with rosette habit.

CYTOKININS

  • Cytokinins have specific effects on cytokinesis, and were discovered as kinetin (a modified form of adenine, a purine) from the autoclaved herring sperm DNA.
  • Kinetin does not occur naturally in plants.
  • Search for natural substances with cytokinin-like activities led to the isolation of zeatin from corn-kernels and coconut milk.
  • Since the discovery of zeatin, several naturally occurring cytokinins, and some synthetic compounds with cell division promoting activity, have been identified.
  • Natural cytokinins are synthesised in regions where rapid cell division occurs, for example, root apices, developing shoot buds, young fruits etc.
  • It helps to produce new leaves, chloroplasts in leaves, lateral shoot growth and adventitious shoot formation.
  • Cytokinins help overcome the apical dominance.
  • They promote nutrient mobilisation which helps in the delay of leaf senescence.

ETHYLENE

  • Ethylene is a simple gaseous PGR.
  • It is synthesised in large amounts by tissues undergoing senescence and ripening fruits.
  • Influences of ethylene on plants include horizontal growth of seedlings, swelling of the axis and apical hook formation in dicot seedlings.
  • Ethylene promotes senescence and abscission of plant organs especially of leaves and flowers.
  • Ethylene is highly effective in fruit ripening.
  • It enhances the respiration rate during ripening of the fruits. This rise in rate of respiration is called respiratory climactic.
  • Ethylene breaks seed and bud dormancy, initiates germination in peanut seeds, sprouting of potato tubers.
  • Ethylene promotes rapid internode/petiole elongation in deep water rice plants.
  • It helps leaves/ upper parts of the shoot to remain above water.
  • Ethylene also promotes root growth and root hair formation, thus helping the plants to increase their absorption surface.
  • Ethylene is used to initiate flowering and for synchronising fruit-set in pineapples.
  • It also induces flowering in mango.
  • Since ethylene regulates so many physiological processes, it is one of the most widely used PGR in agriculture.
  • The most widely used compound as source of ethylene is ethephon.
  • Ethephon in an aqueous solution is readily absorbed and transported within the plant and releases ethylene slowly.
  • Ethephon hastens fruit ripening in tomatoes and apples and accelerates abscission in flowers and fruits (thinning of cotton, cherry, walnut).
  • It promotes female flowers in cucumbers thereby increasing the yield.

ABSCISIC ACID

  • Abscisic acid (ABA) was discovered for its role in regulating abscission and dormancy.
  • It acts as a general plant growth inhibitor and an inhibitor of plant metabolism.
  • ABA inhibits seed germination.
  • ABA stimulates the closure of stomata in the epidermis and increases the tolerance of plants to various kinds of stresses. Therefore, it is also called the stress hormone.
  • ABA plays an important role in seed development, maturation and dormancy.
  • By inducing dormancy, ABA helps seeds to withstand desiccation and other factors unfavourable for growth.
  • In most situations, ABA acts as an antagonist to GAs.

 

  • There are a number of events in the life of a plant where more than one PGR interact to affect that event, e.g., dormancy in seeds/ buds, abscission, senescence, apical dominance, etc.
  • The role of PGR is of only one kind of intrinsic control. Along with genomic control and extrinsic factors, they play an important role in plant growth and development.
  • Many of the extrinsic factors such as temperature and light, control plant growth and development via PGR.
  • Some of such events could be: vernalisation, flowering, dormancy, seed germination, plant movements, etc.

PHOTOPERIODISM

  • Some plants require a periodic exposure to light to induce flowering. Such plants are able to measure the duration of exposure to light.
  • For example, some plants require the exposure to light for a period exceding a well defined critical duration, while others must be exposed to light for a period less than this critical duration before the flowering is initiated in them. The former group of plants are called long day plants while the latter ones are termed short day plants.
  • The critical duration is different for different plants.
  • There are many plants, however, where there is no such correlation between exposure to light duration and inducation of flowering response; such plants are called day-neutral plants.
  • It is now also known that not only the duration of light period but that the duration of dark period is also of equal importance.
  • Hence, it can be said that flowering in certain plants depends not only on a combination of light and dark exposures but also their relative durations. This response of plants to periods of day/night is termed
  • It is also interesting to note that while shoot apices modify themselves into flowering apices prior to flowering, they (i.e., shoot apices of plants) by themselves cannot percieve photoperiods.
  • The site of perception of light/dark duration are the leaves.
  • It has been hypothesised that there is a hormonal substance(s) that is responsible for flowering.
  • This hormonal substance migrates from leaves to shoot apices for inducing flowering only when the plants are exposed to the necessary inductive photoperiod.

 

VERNALISATION

  • There are plants for which flowering is either quantitatively or qualitatively dependent on exposure to low temperature. This phenomenon is termed
  • It prevents precocious reproductive development late in the growing season, and enables the plant to have sufficient time to reach maturity.
  • Vernalisation refers specially to the promotion of flowering by a period of low temperature.
  • Some important food plants, wheat, barley, rye have two kinds of varieties: winter and spring varieties.
  • The ‘spring’ variety are normally planted in the spring and come to flower and produce grain before the end of the growing season.
  • Winter varieties, however, if planted in spring would normally fail to flower or produce mature grain within a span of a flowering season. Hence, they are planted in autumn.
  • They germinate, and over winter come out as small seedlings, resume growth in the spring, and are harvested usually around mid-summer.
  • Another example of vernalisation is seen in biennial plants.
  • Biennials are monocarpic plants that normally flower and die in the second season.
  • Sugerbeet, cabbages, carrots are some of the common biennials.
  • Subjecting the growing of a biennial plant to a cold treatment stimulates a subsequent photoperiodic flowering response.

 

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CHAPTER 15 – PLANT GROWTH AND DEVELOPMENT

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CHAPTER 14 – RESPIRATION IN PLANTS

CHAPTER 14

RESPIRATION IN PLANTS

  • All the energy required for ‘life’ processes is obtained by oxidation of some macromolecules that we call ‘food’.
  • Only green plants and cyanobacteria can prepare their own food; by the process of photosynthesis they trap light energy and convert it into chemical energy that is stored in the bonds of carbohydrates like glucose, sucrose and starch.
  • In green plants too, not all cells, tissues and organs photosynthesise; only cells containing chloroplasts, that are most often located in the superficial layers, carry out photosynthesis.
  • Hence, even in green plants all other organs, tissues and cells that are non-green, need food for oxidation. Hence, food has to be translocated to all non- green parts.
  • Animals are heterotrophic, i.e., they obtain food from plants directly (herbivores) or indirectly (carnivores). Saprophytes like fungi are dependent on dead and decaying matter.
  • Ultimately all the food that is respired for life processes comes from photosynthesis.
  • Photosynthesis, of course, takes place within the chloroplasts (in the eukaryotes), whereas the breakdown of complex molecules to yield energy takes place in the cytoplasm and in the mitochondria (also only in eukaryotes).
  • The breaking of the C-C bonds of complex compounds through oxidation within the cells, leading to release of considerable amount of energy is called
  • The compounds that are oxidised during this process are known as respiratory substrates. Usually carbohydrates are oxidised to release energy, but proteins, fats and even organic acids can be used as respiratory substances in some plants, under certain conditions.
  • During oxidation within a cell, all the energy contained in respiratory substrates is not released free into the cell, or in a single step. It is released in a series of slow step-wise reactions controlled by enzymes, and it is trapped as chemical energy in the form of ATP.
  • The energy released by oxidation in respiration is not used directly but is used to synthesise ATP, which is broken down whenever and wherever energy needs to be utilised. Hence, ATP acts as the energy currency of the cell.
  • This energy trapped in ATP is utilised in various energy-requiring processes of the organisms, and the carbon skeleton produced during respiration is used as precursors for biosynthesis of other molecules in the cell.
  • Plants require O2 for respiration to occur and they also give out CO2. Hence, plants have systems in place that ensure the availability of O2. Plants, unlike animals, have no specialised organs for gaseous exchange but they have stomata and lenticels for this purpose.
  • Reasons why plants can get along without respiratory organs.
    • First, each plant part takes care of its own gas-exchange needs. There is very little transport of gases from one plant part to another.
    • Second, plants do not present great demands for gas exchange. Roots, stems and leaves respire at rates far lower than animals do. Only during photosynthesis are large volumes of gases exchanged and, each leaf is well adapted to take care of its own needs during these periods. When cells photosynthesise, availability of O2 is not a problem in these cells since O2 is released within the cell.
    • Third, the distance that gases must diffuse even in large, bulky plants is not great. Each living cell in a plant is located quite close to the surface of the plant. This is true for leaves, in stems, the ‘living’ cells are organised in thin layers inside and beneath the bark. They also have openings called lenticels. The cells in the interior are dead and provide only mechanical support. Thus, most cells of a plant have at least a part of their surface in contact with air. This is also facilitated by the loose packing of parenchyma cells in leaves, stems and roots, which provide an interconnected network of air spaces.
  • The complete combustion of glucose, which produces CO2 and H2O as end products, yields energy most of which is given out as heat.

  C6H12O6 + 6O2 ————– > 6CO2 + 6H2O + Energy

  • If this energy is to be useful to the cell, it should be able to utilise it to synthesise other molecules that the cell requires.
  • Plant cell uses is to catabolise the glucose molecule in such a way that not all the liberated energy goes out as heat. The key is to oxidise glucose not in one step but in several small steps enabling some steps to be just large enough such that the energy released can be coupled to ATP synthesis.
  • During the process of respiration, oxygen is utilised, and carbon dioxide, water and energy are released as products. The combustion reaction requires oxygen.
  • But some cells live where oxygen may or may not be available.
  • All living organisms retain the enzymatic machinery to partially oxidise glucose without the help of oxygen. This breakdown of glucose to pyruvic acid is called

GLYCOLYSIS

  • The term glycolysis has originated from the Greek words, glycos for sugar, and lysis for splitting.
  • The scheme of glycolysis was given by Gustav Embden, Otto Meyerhof, and J. Parnas, and is often referred to as the EMP pathway.
  • In anaerobic organisms, it is the only process in respiration.
  • Glycolysis occurs in the cytoplasm of the cell and is present in all living organisms.
  • In this process, glucose undergoes partial oxidation to form two molecules of pyruvic acid.
  • In plants, this glucose is derived from sucrose, which is the end product of photosynthesis, or from storage
  • Sucrose is converted into glucose and fructose by the enzyme, invertase, and these two monosaccharides readily enter the glycolytic pathway.
  • Glucose and fructose are phosphorylated to give rise to glucose-6- phosphate by the activity of the enzyme hexokinase.
  • This phosphorylated form of glucose then isomerises to produce fructose-6-phosphate. subsequent steps of metabolism of glucose and fructose are same.
  • In glycolysis, a chain of ten reactions, under the control of different enzymes, takes place to produce pyruvate from glucose.
  • ATP is utilised at two steps: first in the conversion of glucose into glucose 6-phosphate and second in the conversion of fructose 6-phosphate to fructose 1, 6-diphosphate.
  • The fructose 1, 6-diphosphate is split into dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL).
  • We find that there is one step where NADH + H+ is formed from NAD+; this is when 3-phosphoglyceraldehyde (PGAL) is converted to 1, 3-bisphosphoglycerate (DPGA). Two redox-equivalents are removed (in the form of two hydrogen atoms) from PGAL and transferred to a molecule of NAD+.
  • PGAL is oxidised and with inorganic phosphate to get converted into DPGA. The conversion of DPGA to 3-phosphoglyceric acid (PGA), is also an energy yielding process; this energy is trapped by the formation of ATP.
  • Another ATP is synthesised during the conversion of PEP to pyruvic acid.
  • Total 4 ATP molecules are directly synthesised in this pathway from one glucose molecule.
  • Pyruvic acid is then the key product of glycolysis.
  • The metabolic fate of pyruvate depends on the cellular need. There are three major ways in which different cells handle pyruvic acid produced by glycolysis. These are lactic acid fermentation, alcoholic fermentation and aerobic respiration.
  • Fermentation takes place under anaerobic conditions in many prokaryotes and unicellular eukaryotes.
  • For the complete oxidation of glucose to CO2 and H2O, however, organisms adopt Krebs’ cycle which is also called as aerobic respiration. This requires O2

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FERMENTATION

  • In fermentation, the incomplete oxidation of glucose is achieved under anaerobic conditions by sets of reactions where pyruvic acid is converted to CO2 and ethanol.
  • The enzymes, pyruvic acid decarboxylase and alcohol dehydrogenase catalyse these reactions.
  • Other organisms like some bacteria produce lactic acid from pyruvic acid.
  • In animal cells also, like muscles during exercise, when oxygen is inadequate for cellular respiration pyruvic acid is reduced to lactic acid by lactate dehydrogenase.
  • The reducing agent is NADH+H+ which is reoxidised to NAD+ in both the processes.
  • In both lactic acid and alcohol fermentation not much energy is released; less than seven per cent of the energy in glucose is released and not all of it is trapped as high energy bonds of ATP. Also, the processes are hazardous – either acid or alcohol is produced.
  • Yeasts poison themselves to death when the concentration of alcohol reaches about 13 per cent.
  • The maximum concentration of alcohol in beverages that are naturally fermented is 13%.
  • Alcoholic beverages of alcohol content greater than this concentration are obtained by distillation.

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AEROBIC RESPIRATION

  • For aerobic respiration to take place within the mitochondria, the final product of glycolysis, pyruvate is transported from the cytoplasm into the mitochondria.
  • The crucial events in aerobic respiration are:
    • The complete oxidation of pyruvate by the stepwise removal of all the hydrogen atoms, leaving three molecules of CO2
    • The passing on of the electrons removed as part of the hydrogen atoms to molecular O2 with simultaneous synthesis of ATP.
  • The first process takes place in the matrix of the mitochondria while the second process is located on the inner membrane of the mitochondria.
  • Pyruvate, which is formed by the glycolytic catabolism of carbohydrates in the cytosol, after it enters mitochondrial matrix undergoes oxidative decarboxylation by a complex set of reactions catalysed by pyruvic dehydrogenase.
  • The reactions catalysed by pyruvic dehydrogenase require the participation of several coenzymes, including NAD+ and Coenzyme A.

1

  • During this process, two molecules of NADH are produced from the metabolism of two molecules of pyruvic acid (produced from one glucose molecule during glycolysis).
  • The acetyl CoA then enters a cyclic pathway, tricarboxylic acid cycle, more commonly called as Krebs’ cycle after the scientist Hans Krebs who first elucidated it.

TRICARBOXYLIC ACID CYCLE

  • The TCA cycle starts with the condensation of acetyl group with oxaloacetic acid (OAA) and water to yield citric acid. The reaction is catalysed by the enzyme citrate synthase and a molecule of CoA is released.
  • Citrate is then isomerised to isocitrate.
  • It is followed by two successive steps of decarboxylation, leading to the formation of a-ketoglutaric acid and then succinyl-CoA.
  • In the remaining steps of citric acid cycle, succinyl-CoA is oxidised to OAA allowing the cycle to continue.
  • During the conversion of succinyl-CoA to succinic acid a molecule of GTP is synthesised. This is a substrate level phosphorylation.
  • In a coupled reaction GTP is converted to GDP with the simultaneous synthesis of ATP from ADP.
  • Also there are three points in the cycle where NAD+ is reduced to NADH + H+ and one point where FAD+ is reduced to FADH2.
  • The continued oxidation of acetic acid via the TCA cycle requires the continued replenishment of oxaloacetic acid, the first member of the cycle.
  • In addition it also requires regeneration of NAD+ and FAD+ from NADH and FADH2 The summary equation for this phase of respiration may be written as follows:

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ELECTRON TRANSPORT SYSTEM (ETS) AND OXIDATIVE PHOSPHORYLATION

  • The following steps in the respiratory process are to release and utilise the energy stored in NADH+H+ and FADH2.
  • This is accomplished when they are oxidised through the electron transport system and the electrons are passed on to O2 resulting in the formation of H2
  • The metabolic pathway through which the electron passes from one carrier to another, is called the electron transport system (ETS) and it is present in the inner mitochondrial membrane.
  • Electrons from NADH produced in the mitochondrial matrix during citric acid cycle are oxidised by an NADH dehydrogenase (complex I), and electrons are then transferred to ubiquinone located within the inner membrane.
  • Ubiquinone also receives reducing equivalents via FADH2 (complex II) that is generated during oxidation of succinate in the citric acid cycle.
  • The reduced ubiquinone (ubiquinol) is then oxidised with the transfer of electrons to cytochrome c via cytochrome bc1 complex (complex III).
  • Cytochrome c is a small protein attached to the outer surface of the inner membrane and acts as a mobile carrier for transfer of electrons between complex III and IV.
  • Complex IV refers to cytochrome c oxidase complex containing cytochromes a and a3, and two copper centres.

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  • When the electrons pass from one carrier to another via complex I to IV in the electron transport chain, they are coupled to ATP synthase (complex V) for the production of ATP from ADP and inorganic phosphate.
  • The number of ATP molecules synthesised depends on the nature of the electron donor. Oxidation of one molecule of NADH gives rise to 3 molecules of ATP, while that of one molecule of FADH2 produces 2 molecules of ATP.
  • Although the aerobic process of respiration takes place only in the presence of oxygen, the role of oxygen is limited to the terminal stage of the process. Yet, the presence of oxygen is vital, since it drives thewhole process by removing hydrogen from the system. Oxygen acts as the final hydrogen acceptor.
  • Unlike photophosphorylation where it is the light energy that is utilised for the production of proton gradient required for phosphorylation, in respiration it is the energy of oxidation-reduction utilised for the same process. It is for this reason that the process is called oxidative phosphorylation.
  • The energy released during the electron transport system is utilised in synthesising ATP with the help of ATP synthase (complex V).
  • This complex consists of two major components, F1 and F0. The F1 headpiece is a peripheral membrane protein complex and contains the site for synthesis of ATP from ADP and inorganic phosphate. F0 is an integral membrane protein complex that forms the channel through which protons cross the inner membrane. The passage of protons through the channel is coupled to the catalytic site of the F: component for the production of ATP. For each ATP produced, 2H+ passes through F0 from the intermembrane space to the matrix down the electrochemical proton gradient.

Screenshot (56)

THE RESPIRATORY BALANCE SHEET

  • The calculations of the net gain of ATP for every glucose molecule oxidised; are in reality only a theoretical exercise as these calculations can be made only on certain assumptions that:
    • There is a sequential, orderly pathway functioning, with one substrate forming the next and with glycolysis, TCA cycle and ETS pathway following one after another.
    • The NADH synthesised in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation.
    • None of the intermediates in the pathway are utilised to synthesise any other compound.
    • Only glucose is being respired – no other alternative substrates are entering in the pathway at any of the intermediary stages.

But this kind of assumptions are not really valid in a living system; all pathways work simultaneously and do not take place one after another; substrates enter the pathways and are withdrawn from it as and when necessary; ATP is utilised as and when needed; enzymatic rates are controlled by multiple means.

  • There can be a net gain of 36 ATP molecules during aerobic respiration of one molecule of glucose.
  • Comparisons of fermentation and aerobic respiration:
    • Fermentation accounts for only a partial breakdown of glucose whereas in aerobic respiration it is completely degraded to CO2 and H2
    • In fermentation there is a net gain of only two molecules of ATP for each molecule of glucose degraded to pyruvic acid whereas many more molecules of ATP are generated under aerobic conditions.
    • NADH is oxidised to NAD+ rather slowly in fermentation, however the reaction is very vigorous in case of aerobic respiration.

AMPHIBOLIC PATHWAY

  • Glucose is the favoured substrate for respiration. All carbohydrates are usually first converted into glucose before they are used for respiration.
  • Other substrates can also be respired, as has been mentioned earlier, but then they do not enter the respiratory pathway at the first step.
  • Fats would need to be broken down into glycerol and fatty acids first. If fatty acids were to be respired they would first be degraded to acetyl CoA and enter the pathway. Glycerol would enter the pathway after being converted to PGAL.
  • The proteins would be degraded by proteases and the individual amino acids (after deamination) depending on their structure would enter the pathway at some stage within the Krebs’ cycle or even as pyruvate or acetyl CoA.
  • Since respiration involves breakdown of substrates, the respiratory process has traditionally been considered a catabolic process and the respiratory pathway as a catabolic pathway.

Screenshot (57)

  • The respiratory pathway comes into the picture both during breakdown and synthesis of fatty acids. similarly, during breakdown and synthesis of protein too, respiratory intermediates form the link. Breaking down processes within the living organism is catabolism, and synthesis is anabolism. Because the respiratory pathway is involved in both anabolism and catabolism, it would hence be better to consider the respiratory pathwayas an amphibolic pathway rather than as a catabolic one.

 

RESPIRATORY QUOTIENT

  • The ratio of the volume of CO2 evolved to the volume of O2 consumed in respiration is called the respiratory quotient (RQ) or respiratory ratio.

Screenshot (58)

  • The respiratory quotient depends upon the type of respiratory substrate used during respiration.
  • When carbohydrates are used as substrate and are completelyoxidised, the RQ will be 1, because equal amounts of CO2 and O2 are evolved and consumed, respectively, as shown in the equation below :

Screenshot (59)

  • When fats are used in respiration, the RQ is less than 1. Calculations for a fatty acid, tripalmitin, if used as a substrate is shown:

Screenshot (60)

  • When proteins are respiratory substrates the ratio would be about 0.9.
  • In living organisms respiratory substances are often more than one; pure proteins or fats are never used as respiratory substrates.

 

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CHAPTER 14 – RESPIRATION IN PLANTS

CHAPTER 13 – PHOTOSYNTHESIS IN HIGHER PLANTS

CHAPTER 13

PHOTOSYNTHESIS IN HIGHER PLANTS

  • Green plants carry out ‘photosynthesis’, a physico-chemical process by which they use light energy to drive the synthesis of organic compounds.
  • Ultimately, all living forms on earth depend on sunlight for energy.
  • The use of energy from sunlight by plants doing photosynthesis is the basis of life on earth. Photosynthesis is important due to two reasons:

(a) it is the primary source of all food on earth.

(b) It is also responsible for the release of oxygen into the atmosphere by green plants.

  • chlorophyll (green pigment of the leaf), light and CO2 are required for photosynthesis to occur.

 

  • Two leaves experiment for importance of chlorophyll for starch formation – a variegated leaf or a leaf that was partially covered with black paper, and one that was exposed to light. On testing these leaves for starch it was clear that photosynthesis occurred only in the green parts of the leaves in the presence of light.
  • Half-leaf experiment for importance of CO2 for starch formation – a part of a leaf is enclosed in a test tube containing some KOH soaked cotton (which absorbs CO2), while the other half is exposed to air. The setup is then placed in light for some time. On testing for starch later in the two halves of the leaf the exposed part of the leaf tested positive for starch while the portion that was in the tube, tested negative.

 

Early Experiments

Joseph Priestley (1770)               – revealed the essential role of air in the growth of green plants

– discovered oxygen in 1774.

– Bell jar experiment

– hypothesised that Plants restore to the air whatever breathing animals and burning candles remove.

1

 

Jan Ingenhousz (1730-1799)      – showed that sunlight is essential to the photosynthesis.

Julius von Sachs (1854)              – provided evidence for production of glucose when plants grow.

                                                      – Glucose is usually stored as starch.

– showed that the green substance in plants is located in special bodies (later called chloroplasts) within plant cells.

T.W Engelmann (1843 – 1909)   – Used a prism to split light into its spectral components and then illuminated a green alga, Cladophora, placed in a suspension of aerobic bacteria. The bacteria were used to detect the sites of O2 evolution.

observed that the bacteria accumulated mainly in the region of blue and red light of the split spectrum.

described first action spectrum of photosynthesis, which resembles roughly the absorption spectra of chlorophyll a and b.

The empirical equation representing the total process of photosynthesis for oxygen evolving organisms was then understood as:

CO2 + H2O —Light –> [CH2O] + O2

where [CH2O] represented a carbohydrate (e.g., glucose, a six-carbon sugar).

Cornelius van Niel (1897-1985)  – a microbiologist

– studies of purple and green bacteria,

– demonstrated that photosynthesis is essentially a light-dependent   reaction in which hydrogen from a suitable oxidisable compound reduces carbon dioxide to carbohydrates.

2H2A + CO2Light –> 2A + CH2O + H2O

– In green plants H2O is the hydrogen donor and is oxidised to O2.

– When H2S is the hydrogen donor for purple and green sulphur bacteria, the ‘oxidation’ product is sulphur or sulphate and not O2.

– Hence, he inferred that the O2 evolved by the green plant comes from H2O, not from carbon dioxide.

– This was later proved by using radioisotopic techniques.

The correct equation, that would represent the overall process of photosynthesis is therefore:

6CO2 +12H2O —Light –> C6H12O6 + 6H2O + 6O2

 

SITE OF PHOTOSYNTHESIS

  • Photosynthesis occurs in green leaf in the chloroplasts.
  • mesophyll cells in the leaves, have a large number of chloroplasts. Usually the chloroplasts align themselves along the walls of the mesophyll cells, such that they get the optimum quantity of the incident light.
  • Within the chloroplast there is the membranous system consisting of grana, the stroma lamellae, and the fluid stroma. There is a clear division of labour within the chloroplast. The membrane system is responsible for trapping the light energy and also for the synthesis of ATP and NADPH. In stroma, enzymatic reactions incorporate CO2 into the plant leading to the synthesis of sugar, which in turn forms starch.
  • The former set of reactions, since they are directly light driven are called light reactions. The latter are not directly light driven but are dependent on the products of light reactions (ATP and NADPH). Hence, to distinguish the latter they are called, by convention, as dark reactions. However, this should not be construed to mean that they occur in darkness or that they are not light- dependent.

Screenshot (42)  HOW MANY PIGMENTS ARE INVOLVED IN PHOTOSYNTHESIS?

  • A chromatographic separation of the leaf pigments shows that the colour that we see in leaves is not due to a single pigment but due to four pigments:

Chlorophyll a (bright or blue green in the chromatogram),

chlorophyll b (yellow green),

xanthophylls (yellow) and

carotenoids (yellow to yellow-orange).

  • Pigments are substances that have an ability to absorb light, at specific wavelengths.
  • wavelengths at which there is maximum absorption by chlorophyll a, is in the blue and the red regions, also shows higher rate of photosynthesis. Hence, we can conclude that chlorophyll a is the chief pigment associated with photosynthesis.
  • These graphs, together, show that most of the photosynthesis takes place in the blue and red regions of the spectrum; some photosynthesis does take place at the other wavelengths of the visible spectrum.
  • Though chlorophyll is the major pigment responsible for trapping light, other thylakoid pigments like chlorophyll b, xanthophylls and carotenoids, which are called accessory pigments, also absorb light and transfer the energy to chlorophyll a. Indeed, they not only enable a wider range of wavelength of incoming light to be utilised for photosyntesis but also protect chlorophyll a from photo-oxidation.

Screenshot (43)

WHAT IS LIGHT REACTION?

  • Light reactions or the ‘Photochemical’ phase include light absorption, water splitting, oxygen release, and the formation of high-energy chemical intermediates, ATP and NADPH.
  • several complexes are involved in the process.
  • The pigments are organised into two discrete photochemical light harvesting complexes (LHC) within the Photosystem I (PS I) and Photosystem II (PS II). These are named in the sequence of their discovery, and not in the sequence in which they function during the light reaction.
  • The LHC are made up of hundreds of pigment molecules bound to proteins.
  • Each photosystem has all the pigments (except one molecule of chlorophyll a) forming a light harvesting system also called antennae.
  • These pigments help to make photosynthesis more efficient by absorbing different wavelengths of light. The single chlorophyll a molecule forms the reaction centre.
  • The reaction centre is different in both the photosystems.
  • In PS I the reaction centre chlorophyll a has an absorption peak at 700 nm, hence is called P700, while in PS II it has absorption maxima at 680 nm, and is called P680

Screenshot (44) THE ELECTRON TRANSPORT

  • In photosystem II the reaction centre chlorophyll a absorbs 680 nm wavelength of red light causing electrons to become excited and jump into an orbit farther from the atomic nucleus. These electrons are picked up by an electron acceptor which passes them to an electrons transport system consisting of cytochromes.
  • This movement of electrons is downhill, in terms of an oxidation-reduction or redox potential scale.
  • The electrons are not used up as they pass through the electron transport chain, but are passed on to the pigments of photosystem PS I.
  • Simultaneously, electrons in the reaction centre of PS I are also excited when they receive red light of wavelength 700 nm and are transferred to another accepter molecule that has a greater redox potential.
  • These electrons then are moved downhill again, this time to a molecule of energy-rich NADP+.
  • The addition of these electrons reduces NADP+ to NADPH + H+.
  • This whole scheme of transfer of electrons, starting from the PS II, uphill to the accepter, down the electron transport chain to PS I, excitation of electrons,transfer to another accepter, and finally down hill to NADP+ causing it to be reduced to NADPH + H+ is called the Z scheme, due to its characterstic shape. This shape is formed when all the carriers are placed in a sequence on a redox potential scale.

Screenshot (45)  Splitting of Water

  • The electrons that were moved from photosystem II must be replaced.
  • This is achieved by electrons available due to splitting of water.
  • The splitting of water is associated with the PS II; water is split into H+, [O] and electrons.
  • This creates oxygen, one of the net products of photosynthesis.
  • The electrons needed to replace those removed from photosystem I are provided by photosystem II.
  • 2H2O ——-> 4H+ + O2 + 4e
  • water splitting complex is associated with the PS II, which itself is physically located on the inner side of the membrane of the thylakoid.

Cyclic and Non-cyclic Photo-phosphorylation

  • Living organisms have the capability of extracting energy from oxidisable substances and store this in the form of bond energy.
  • Special substances like ATP, carry this energy in their chemical bonds.
  • The process of whichATP is synthesised by cells (in mitochondria and chloroplasts) is named phosphorylation.
  • Photo- phosphorylation is the synthesis of ATP from ADP and inorganic phosphate in the presence of light.
  • When the two photosystems work in a series, first PS II and then the PS I, a process called non-cyclic photo-phosphorylation occurs.
  • The two photosystems are connected through an electron transport chain, as seen earlier – in the Z scheme.
  • Both ATP and NADPH + H+ are synthesised by this kind of electron flow.
  • When only PS I is functional, the electron is circulated within the photosystem and the phosphorylation occurs due to cyclic flow of electrons.
  • A possible location where this could be happening is in the stroma lamellae.
  • While the membrane or lamellae of the grana have both PS I and PS II the stroma lamellae membranes lack PS II as well as NADP reductase enzyme.
  • The excited electron does not pass on to NADP+ but is cycled back to the PS I complex through the electron transport chain. The cyclic flow hence, results only in the synthesis of ATP, but not of NADPH + H+.
  • Cyclic photophosphorylation also occurs when only light of wavelengths beyond 680 nm are available for excitation.

Chemiosmotic Hypothesis

  • The chemiosmotic hypothesis has been put forward to explain the mechanism of synthesis of ATP.
  • Like in respiration, in photosynthesis too, ATP synthesis is linked to development of a proton gradient across a membrane.
  • This time these are membranes of the thylakoid.
  • There is one difference though, here the proton accumulation is towards the inside of the membrane, i.e., in the lumen.
  • In respiration, protons accumulate in the intermembrane space of the mitochondria when electrons move through the ETS.
  • Processes that take place during the activation of electrons and their transport to determine the steps that cause a proton gradient to develop are –

(a) Since splitting of the water molecule takes place on the inner side of the membrane, the protons or hydrogen ions that are produced by the splitting of water accumulate within the lumen of the thylakoids.

(b) As electrons move through the photosystems, protons are transported across the membrane. This happens because the primary accepter of electron which is located towards the outer side of the membrane transfers its electron not to an electron carrier but to an H carrier. Hence, this molecule removes a proton from the stroma while transporting an electron. When this molecule passes on its electron to the electron carrier on the inner side of the membrane, the proton is released into the inner side or the lumen side of the membrane.

(c) The NADP reductase enzyme is located on the stroma side of the membrane. Along with electrons that come from the accepter of electrons of PS I, protons are necessary for the reduction of NADP+ to NADPH+ H+. These protons are also removed from the stroma.

  • Hence, within the chloroplast, protons in the stroma decrease in number, while in the lumen there is accumulation of protons.
  • This creates a proton gradient across the thylakoid membrane as well as a measurable decrease in pH in the lumen.
  • This proton gradient is important because it is the breakdown of this gradient that leads to release of energy.
  • The gradient is broken down due to the movement of protons across the membrane to the stroma through the transmembrane channel of the F0 of the ATPase.
  • The ATPase enzyme consists of two parts:

one called the F0 is embedded in the membrane and forms a transmembrane channel that carries out facilitated diffusion of protons across the membrane.

The other portion is called F1 and protrudes on the outer surface of the thylakoid membrane on the side that faces the stroma. The breakdown of the gradient provides enough energy to cause a conformational change in the F1 particle of the ATPase, which makes the enzyme synthesise several molecules of energy-packed ATP.

  • Chemiosmosis requires a membrane, a proton pump, a proton gradient and ATPase.
  • Energy is used to pump protons across a membrane, to create a gradient or a high concentration of protons within the thylakoid lumen.
  • ATPase has a channel that allows diffusion of protons back across the membrane; this releases enough energy to activate ATPase enzyme that catalyses the formation of ATP.
  • Along with the NADPH produced by the movement of electrons, the ATP will be used immediately in the biosynthetic reaction taking place in the stroma, responsible for fixing CO2, and synthesis of sugars.

Screenshot (46)

WHERE ARE THE ATP AND NADPH USED?

  • The products of light reaction are ATP, NADPH and O2.
  • Of these O2 diffuses out of the chloroplast while ATP and NADPH are used to drive the processes leading to the synthesis of food, more accurately, sugars.
  • This is the biosynthetic phase of photosynthesis.
  • This process does not directly depend on the presence of light but is dependent on the products of the light reaction, i.e., ATP and NADPH, besides CO2 and H2
  • Immediately after light becomes unavailable, the biosynthetic process continues for some time, and then stops. If then, light is made available, the synthesis starts again.
  • CO2 is combined with H2O to produce (CH2O)n or sugars.
  • Melvin Calvin used radioactive 14C in algal photosynthesis studies which led to the discovery that the first CO2 fixation product was a 3-carbon organic acid or 3-phosphoglyceric acid (PGA).
  • He also contributed to working out the complete biosynthetic pathway; hence it was called Calvin cycle after him.
  • In another group of plants, first stable product of CO2 fixation is 4 carbon organic acid oxaloacetic acid or OAA.
  • The Primary Acceptor of CO2 is a 5-carbon ketose sugar – ribulose bisphosphate (RuBP).

THE CALVIN CYCLE

  • Calvin and his co-workers then worked out the whole pathway and showed that the pathway operated in a cyclic manner; the RuBP was regenerated.
  • The Calvin pathway occurs in all photosynthetic plants; it does not matter whether they have C3 or C4 (or any other) pathways.
  • The Calvin cycle can be described under three stages: carboxylation, reduction and regeneration.
  1. Carboxylation –

Carboxylation is the fixation of CO2 into a stable organic intermediate.

Carboxylation is the most crucial step of the Calvin cycle where CO2 is utilised for the carboxylation of RuBP.

This reaction is catalysed by the enzyme RuBP carboxylase which results in the formation of two molecules of 3-PGA.

Since this enzyme also has an oxygenation activity it would be more correct to call it RuBP carboxylase-oxygenase or RuBisCO.

  1. Reduction –

These are a series of reactions that lead to the formation of glucose.

The steps involve utilisation of 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 removal of one molecule of glucose from the pathway.

  1. Regeneration –

Regeneration of the CO2 acceptor molecule RuBP is crucial if the cycle is to continue uninterrupted.

The regeneration steps require one ATP for phosphorylation to form RuBP.

  • Hence for every CO2 molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH are required.
  • It is probably to meet this difference in number of ATP and NADPH used in the dark reaction that the cyclic phosphorylation takes place.
  • To make one molecule of glucose 6 turns of the cycle are required.
  • Total 18 ATP, 12 NADPH, 12 CO2 are used synthesis of for 1 molecule of glucose.

 Screenshot (47)

THE C4 PATHWAY

  • Plants that are adapted to dry tropical regions have the C4 pathway
  • Though these plants have the C4 oxaloacetic acid as the first CO2 fixation product they use the C3 pathway or the Calvin cycle as the main biosynthetic pathway.
  • C4 plants are special: They have a special type of leaf anatomy, they tolerate higher temperatures, they show a response to highlight intensities, they lack a process called photorespiration and have greater productivity of biomass.
  • The particularly large cells around the vascular bundles of the C4 pathway plants are called bundle sheath cells, and the leaves which have such anatomy are said to have ‘Kranz’ anatomy.
  • ‘Kranz’ means ‘wreath’ and is a reflection of the arrangement of cells.
  • The bundle sheath cells may form several layers around the vascular bundles; they are characterised by having a large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces.
  • This pathway also known as Hatch and Slack Pathway, is a cyclic process.
  • The primary CO2 acceptor is a 3-carbon molecule phosphoenol pyruvate (PEP) and is present in the mesophyll cells. The enzyme responsible for this fixation is PEP carboxylase or PEPcase.
  • The mesophyll cells lack RuBisCO enzyme.
  • The C4 acid OAA is formed in the mesophyll cells.
  • It then forms other 4-carbon compounds like malic acid or aspartic acid in the mesophyll cells itself, which are transported to the bundle sheath cells.
  • In the bundle sheath cells these C4 acids are broken down to release CO2 and a 3-carbon molecule.
  • The 3-carbon molecule is transported back to the mesophyll where it is converted to PEP again, thus, completing the cycle.
  • The CO2 released in the bundle sheath cells enters the C3 or the Calvin pathway, a pathway common to all plants. The bundle sheath cells are rich in an enzyme Ribulose bisphosphate carboxylase-oxygenase (RuBisCO), but lack PEPcase.
  • Thus, the basic pathway that results in the formation of the sugars, the Calvin pathway, is common to the C3 and C4
  • the Calvin pathway occurs in all the mesophyll cells of the C3 plants while in the C4 plants it does not take place in the mesophyll cells but does so only in the bundle sheath cells.

Screenshot (48)

PHOTORESPIRATION

  • RuBisCO is the most abundant enzyme in the world.
  • It is characterised by the fact that its active site can bind to both CO2 and O2 – hence the name.
  • RuBisCO has a much greater affinity for CO2 than for O2.
  • This binding is competitive. It is the relative concentration of O2 and CO2 that determines which of the two will bind to the enzyme.
  • In C3 plants some O2 does bind to RuBisCO, and hence CO2 fixation is decreased. Here the RuBP instead of being converted to 2 molecules of PGA binds with O2 to form one molecule and phosphoglycolate in a pathway called photorespiration.
  • In the photorespiratory pathway, there is neither synthesis of sugars, nor of ATP. Rather it results in the release of CO2 with the utilisation of ATP.
  • In the photorespiratory pathway there is no synthesis of ATP or NADPH. Therefore, photorespiration is a wasteful process.
  • In C4 plants photorespiration does not occur. This is because they have a mechanism that increases the concentration of CO2 at the enzyme site.
  • This takes place when the C4 acid from the mesophyll is broken down in the bundle cells to release CO2 – this results in increasing the intracellular concentration of CO2.
  • In turn, this ensures that the RuBisCO functions as a carboxylase minimising the oxygenase activity.
  • Because of absence of photorespiration in C4 plants, productivity and yields are better in these plants.
  • These plants show tolerance to higher temperatures.
Characteristics C3 Plants C4 Plants
Cell type in which the Calvin cycle takes place Both Bundle sheath
Cell type in which the initial carboxylation reaction occurs Both Mesophyll
How many cell types does the leaf have that fix Co2. Three: Bundle sheath, palisade, spongy mesophyll Two: Bundle sheath and

Mesophyll

Which is the primary Co2 acceptor RuBP PEP
Number of carbons in the primary Co2 acceptor 5 3
Which is the primary Co2 fixation product PGA OAA
No. of carbons in the primary Co2 fixation product 3 4
Does the plant have RuBisCo? Yes Yes
Does the plant have PEP Case? No Yes
Which cells in the plant have Rubisco? Mesophyll/Bundle sheath/none Mesophyll/Bundle sheath/none
Co2 fixation rate under high light conditions Medium High
Whether photorespiration is present at low light intensities Sometimes Negligible
Whether photorespiration is present at high light intensities Sometimes Negligible
Whether photorespiration would be present at low CO2 concentrations High Negligible
Whether photorespiration would be present at high CO2 concentrations Sometimes Negligible
Temperature optimum 20-25C 30-40 C

FACTORS AFFECTING PHOTOSYNTHESIS

  • The rate of photosynthesis is very important in determining the yield of plants including crop plants.
  • Photosynthesis is under the influence of several factors, both internal (plant) and external.
  • The plant factors include the number, size, age and orientation of leaves, mesophyll cells and chloroplasts, internal CO2 concentration and the amount of chlorophyll. The plant or internal factors are dependent on the genetic predisposition and the growth of the plant.
  • The external factors include the availability of sunlight, temperature, CO2 concentration and water.
  • As a plant photosynthesises, all these factors will simultaneously affect its rate. Hence, though several factors interact and simultaneously affect photosynthesis or CO2 fixation, usually one factor is the major cause or is the one that limits the rate. Hence, at any point the rate will be determined by the factor available at sub-optimal levels.
  • Blackman’s (1905) 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
  • For example, despite the presence of a green leaf and optimal light and CO2 conditions, the plant may not photosynthesise if the temperature is very low. This leaf, if given the optimal temperature, will start photosynthesising.

Light

  • It includes light quality, light intensity and the duration of exposure to light.
  • There is a linear relationship between incident light and CO2 fixation rates at low light intensities.
  • At higher light intensities, gradually the rate does not show further increase as other factors become limiting.
  • Light saturation occurs at 10 per cent of the full sunlight.
  • Hence, except for plants in shade or in dense forests, light is rarely a limiting factor in nature.
  • Increase in incident light beyond a point causes the breakdown of chlorophyll and a decrease in photosynthesis.

Screenshot (50)

Carbon dioxide Concentration

  • Carbon dioxide is the major limiting factor for photosynthesis.
  • The concentration of CO2 is very low in the atmosphere (between 0.03 and 0.04 per cent).
  • Increase in concentration upto 0.05 per cent can cause an increase in CO2 fixation rates; beyond this the levels can become damaging over longer periods.
  • The C3 and C4 plants respond differently to CO2 At low light conditions neither group responds to high Co2 conditions. At high light intensities, both C3 and C4 plants show increase in the rates of photosynthesis.
  • C4 plants show saturation at about 360 µlL-1 while in C3 plants saturation is seen only beyond 450 µlL-1. Thus, current availability of CO2 levels is limiting to the C3
  • The fact that C3 plants respond to higher CO2 concentration by showing increased rates of photosynthesis leading to higher productivity has been used for some greenhouse crops such as tomatoes and bell pepper. They are allowed to grow in carbon dioxide enriched atmosphere that leads to higher yields.

Temperature

  • The dark reactions being enzymatic are temperature controlled.
  • Though the light reactions are also temperature sensitive they are affected to a much lesser extent. The C4 plants respond to higher temperatures and show higher rate of photosynthesis while C3 plants have a much lower temperature optimum.
  • The temperature optimum for photosynthesis of different plants also depends on the habitat that they are adapted to.
  • Tropical plants have a higher temperature optimum than the plants adapted to temperate climates.

Water

  • Even though water is one of the reactants in the light reaction, the effect of water as a factor is more through its effect on the plant, rather than directly on photosynthesis.
  • Water stress causes the stomata to close hence reducing the CO2 Besides, water stress also makes leaves wilt, reducing the surface area of the leaves and their metabolic activity as well.

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CHAPTER 13 – PHOTOSYNTHESIS IN HIGHER PLANTS