SSC Biology Plant Physiology Plant Physiology

Plant Physiology

Category : SSC

 

Means of Transport

 

  • Diffusion is a random movement of individual molecules from a region of higher concentration to a region of lower concentration. Diffusion rates are affected by concentration gradient, membrane permeability, temperature and pressure.
  • The substance that have a hydrophilic moiety, find it difficult to pass through membrane. The movement of such molecules are facilitate, for which proteins provide site at which such molecule cross membrane. This is called as facilitated diffusion.
  • Active transport uses energy to pump molecules against a concentration gradient.


 

Plants-Water Relations

  • Water Potential: The potential energy of water is referred to as water potential. It is measured in term of pressure.

\[{{\psi }_{w}}={{\psi }_{s}}+{{\psi }_{p}}\]

  • Osmosis: It is the diffusion of water through a semi-permeable membrane. It depends on two factors

(i) Concentration of dissolved solutes in a solution

(ii) Pressure difference.

  • Plasmolysis: If a turgid cell is placed in a solution that has more solutes, it exerts a higher osmotic pressure and water will move out.
  • Isotonic solution: When concentration of outer solution (in which cell is placed) is equal to concentration of cell sap.
  • Hypotonic solution: When concentration of outer solution is lower than concentration of cell sap.
  • Hypertonic solution: When concentration of outer solution is higher than concentration of cell sap.

 

  • Imbibition: It is a type of diffusion by which movement of water takes place along a diffusion gradient. Factors influencing the rate of imbibition are nature of imbibant, surface area of imbibant, temperature, concentration of solutes, pH of imbibant.

 

  • Cohesion Theory:
  • Proposed by Henry Dixon 1914.
  • Evaporation of water from the leaf to atmosphere decreases the water potential of the epidermal cells.

 

  • Transpiration
  • Loss of water in the form of water vapour from plant through the small pores (stomata) present on leaves is called transpiration.

 

  • The Pressure Flow or Mass Flow Hypothesis
  • It was put forward by Munch (1930). According to this hypothesis, organic substances move from the region of high osmotic pressure to the region of low osmotic pressure in a mass flow due to the development of a gradient of turgor pressure.
  • Hydroponics: The system of growing plants in soilless culture (also called solution culture or tank farming) is known as hydroponics.

 

Role of Essential Elements (Macro and Micro) and their Deficiency Symptoms         

       

S. No.

Name of element

In which form they are absorbed

Functions

Deficieny symptims

1.

Nitrogen

\[NO_{2}^{-}\],\[NO_{3}^{-}\]or\[NH_{4}^{+}\]

Major constituent of proteins, nucleic acids, vitamins and minerals.

Chlorosis (yellowing of older leaves

2.

Phosphorous

\[{{H}_{2}}PO_{4}^{2-}\]or\[HPO_{4}^{2-}\]

Constituent, of cell membrane, nucleic acids, nucleotides and some proteins.

Delay in seed germination purple or red spots on leaves

3.

Potassium

\[{{K}^{+}}\]

Involved in protein synthesis, closing & opening of stomata. Maintenance of turgidity of cells.

Chlorosis in intervernial area, loss of apical dominance

4.

Calcium

\[C{{a}^{++}}\]

Used in synthesis of cell wall (middle lamella)

Stunted growth, necrosis of meristematic regions.

5.

Magnesium

\[M{{g}^{++}}\]

Activate enzymes in respiration, photo synthesis, DNA and RNA synthesis. Constituents of the porphyrin ring of chlorophyll structure.

Chlorosis between leaf veins, necrosis on older leaves,

6.

Boron

\[BO_{3}^{3-}\] and \[{{B}_{4}}O_{7}^{2-}\]

Necessary for uptake and utilization of\[C{{a}^{2+}}\], pollen generation.

Death of root and shoot tips, abscission of flowers.

7.

Chlorine

\[C{{l}^{-}}\]

Determine solute concentration (with \[N{{a}^{+}}\]and\[{{K}^{+}}\]) and anion-cation balance in cells, essential for photolysis of water.

Stunted root growth, reduced fruiting.

 

 

Metabolism of Nitrogen

 

  • Fixation of \[{{N}_{2}}\]
  • Ammonia is rapidly converted first to nitrites (by Nitrobacter) by the process nitrification.

\[2N{{H}_{3}}+3{{O}_{2}}\to 2NO_{2}^{-}+2{{H}^{+}}+2{{H}_{2}}O\], \[2NO_{2}^{-}+{{O}_{2}}\to 2NO_{3}^{-}\]

  • Nitrate is then either available to the plant, or converted to nitrogen gas in the process of denitrification (by Pseudomonas).
  • Fixation is done by both free living e.g. Azotobacters, Clostridium, cyanobacteria like Nostoc, Anabaena and symbiotic bacteria Rhizobium.
  • Nodules act as the Site for\[{{N}_{2}}\]fixation. It contains leghaemoglobin (a pink pigment) and enzyme nitrogenase (Mo-Fe protein).
  • During this process, the \[{{N}_{2}}\]atmospheric (dinitrogen) is reduced by the addition of hydrogen atoms to ammonia.
  • \[{{N}_{2}}+8{{e}^{-}}+8{{H}^{+}}16ATP\to 2N{{H}_{3}}+{{H}_{2}}+16ADP+16Pi\]

 

  • Photosynthesis
  • It is actually oxidation reduction process in which water is oxidised and \[C{{O}_{2}}\] is reduced to carbohydrates.
  • The reduction of \[C{{O}_{2}}\]to carbohydrates needs assimilatory powers, i.e., ATP and\[NADP{{H}_{2}}\].
  • Photosynthesis occurs particularly in specialised cells called mesophyll cell. These cells contain chloroplast, which is the actual sites for photosynthesis.
  • The two forms of chlorophyll 'a' are chl a 683 \[({{P}_{680}})\] chl a 703 \[({{P}_{700}})\] with peak absorption at 683 and 703 respectively are anchored in thylakoids membranes. They are the reaction centres.
  • Electron Transport: It was first formulated by Hill (1939). It is a series of electron carrier over which electrons pass in a downhill journey releasing energy at every step that is used in generating an electrochemical proton gradient which helps in synthesizing ATP.
  • Photorespiration: Light stimulated oxidation of photosynthetic intermediates to \[C{{O}_{2}}\]is known as photorespiration. The course of photorespiration is related to chloroplasts, peroxisomes and mitochondria. This is a wasteful process & occurs in \[{{C}_{3}}\] plants.
  • Respiration: The phenomenon of breaking of the C-C bond of complex organic molecules through oxidation and releasing of energy for cellular use, is called respiration.

\[{{C}_{6}}{{H}_{12}}{{O}_{6}}+6{{O}_{2}}\to 6C{{O}_{2}}+6{{H}_{2}}O+\]Energy (2870kg)

  • Glycolysis: The scheme of glycolysis was given by Gustav Embden, Otto Meyerhof and J. Parnas, hence it is referred to as the EMP pathway. It occurs in the cytoplasm of the cell.

 

  • Fermentation: When \[{{O}_{2}}\] is limiting, NADH and pyruvic add begin to accumulate. Under this condition, plants carry out fermentation (anaerobic respiration), leading to the formation of \[C{{O}_{2}}\] and either ethanol or lactic acid.
  • Citric acid cycle or tricarboxylic acid cycle or kreb's cycle:

It occurs in mitochondrial matrix.

 

 

Growth

 

  • It can be broadly defined as "permanent and irreversible increase in size of living structure which is accompanied by an increase in dry weight and the amount of protoplasm".
  • Growth rate can show two types of increase i. e., arithmetic increase and geometrical growth.
  • Arithmetic Increase

\[{{L}_{t}}={{L}_{0}}+rt\] where, \[{{L}_{t}}=\] length at time 't’ \[{{L}_{0}}=\]length at time 'zero'; r = growth rate /elongation per unit time; t = time of growth.

  • Geometrical Growth

\[{{W}_{1}}={{W}_{0}}{{e}^{rt}}\]Where, \[{{W}_{1}}=\]Final size (weight, height, number etc.); \[{{W}_{0}}=\] Initial size at the beginning of the period; e = base of natural logarithms; r = growth rate; t = time of growth. Development is growth accompanied by differentiation.

Differentiation: The cells are derived from root apical and shoot apical meristems and cambium differentiate and mature to perform specific functions.

Dedifferentiation: The living differentiated cells loose their capacity of division. These cells may regain their capacity to divide under certain conditions.

Redifferentiation: The products of dedifferentiated cells which lose the capability to divide but mature to perform specific functions are called redifferentiated cells.

  • Plant Growth Regulators

Auxins

  • Auxins induce elongation in shoot cells and inhibition of elongation of root cells. Synthetic auxins are indole butyric acid (IBA), indole propionic acid (IPA), 2, 4- dichlorophenoxy acetic acid- (2,4 D); naphthalene acetic acid (NAA).
  • At the removal of apical bud the lateral buds grow vigorously It shows that apical bud suppresses the growth of lateral bud (axillary bud) just below it. This is known as apical dominance.
  • Initiating and promoting cell division in certain tissues such as cambium.
  • Promotes elongation of stem and coleoptile.
  • Auxin generally inhibits flowering but in case of pineapple spray of auxins induces early flowering.
  • Gibberellins
  • \[G{{A}_{3}}\] was one of the first gibberellin to be discovered and remains the most intensively studied form. All GAs are acidic in nature. They occur in various plant organs such as roots, stems, leaves, buds, immature seeds and callus tissues of higher plants.
  • The most typical and striking effect of gibberellin is on the elongation of stem. The intemodes increase in length.
  • Gibberellin causes the plants to bolt and flower.
  • Gibberellins have been found be more effective than auxins in causing parthenocarpic development of fruits e.g., tomatos, apples and pears.

 

  • Cytokinins
  • Miller in 1954 isolated the first crystals of a 'cell division inducing substance' from the autoclaved herring sperm DNA. Since this substance has specific effect on cytokinesis it is called as kinetin (a modified form of adenine).
  • Permanent cell division occurs only in presence of cytokinins. It also plays important role in causing expansion of cells.
  • Cytokinins not only breaks dormancy but also promotes the germination of seeds.

 

  • Abscisic Acid
  • Carns and Addicott (1963) isolated two substances Abscisin I and Abscisin II from the cotton balls that were responsible for accelerated abscission of leaves.
  • ABA inhibits seed germination and growth of excised embryos.
  • ABA stimulates stomatal closure by inhibiting the \[{{K}^{+}}\] uptake by guard cell.
  • ABA increases tolerance of plants to various kinds of stresses.

 

  • Ethylene
  • Ethylene is only gaseous hormone \[(C{{H}_{2}}=C{{H}_{2}})\] that is synthesized in large amounts by tissues undergoing senescence and ripening fruits.
  • Ethylene inhibits elongation of stem, causes swelling of nodes and nullifies geotropism.
  • It is highly effective in inducing fruit ripening when it is produced in large amount which coincides with respiratory climactric i.e., a brief rise to a very high level of respiration. This rise indicates the beginning of senescence and death.

 

  • Photoperiodism
  • The term photoperiodism is used by Garner and Allard, 1920 for the ability of plant to detect and respond the relative length of day and night to which the plant is exposed. The site of perception of tight/dark duration are leaves.
  • Most of the plants are short day plants that include Cosmos, Dahlia, Chrysanthemum, rice, etc. The long day plants are wheat, barley, sugar beet, larkspur, etc. Cucumber, sunflower, tobacco, tomato etc. are some examples of intermediate day plants.

 

  • Vernalisation
  • In some plants early flowering is induced by pretreatment of seeds with a certain low temperature.

 

  • Seed Dormancy
  • Dormancy may be defined as the inactive state of the seed in which the growth of the embryo is temporarily suspended for a specific length of time.

 

  • Flower
  • Flowers are highly modified shoots, bearing nodes and modified floral leaves, which are meant essentially for sexual reproduction in plant.
  • A typical mature embryo sac of angiosperm is 7-celled, 8 nucleate structure i.e., 3 antipodal cells, 3-egg apparatus cells (consists of 2 synergids and 1 egg cell) and one central cell (2 polar nuclei).

 

  • Double Fertilization
  • Fusion of male and female gametes is called fertilization.
  • One of these male gametes fuses with egg to form diploid zygote (2n) while the other fuses with two polar nuclei of the central cell to produce triploid primary endosperm nucleus (PEN) (3n). Since, the latter involves fusion of three haploid nucleus, therefore it is called triple fusion.
  • In some angiosperms, two types of fusion occur in the same embryo sac "syngamy" leading to the formation of zygote & "triple fusion" forming primary endosperm cell, this phenomenon is called "double-fertilization".
  • Endosperm is the nutritive tissue which provides nourishment to the embryo in seed plant.
  • Seed is a fertilized ovule. After fertilization ovary begins to grow and gradually matures into fruit.
  • Polyembryony - Given by Leeuwenhoek (1917) in Orange More than one embryo in a seed.

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