Sunday, 25 January 2015


Improper functioning of immune system can cause comfort, disease or even death. These disorders may involve:

Figure 1

1. Hypersensitivity or Allergy: Allergy means inappropriate and excessive response to common antigens. Substances causing allergic reaction are called allergens. The common allergens are dust, pollens, mould, spores, fabrics, feathers, fur, plants, bacteria, foods, heat, cold, sunlight. Parthenium flower is a common allergen in India.

Allergy mostly affects the skin and the mucous membrane. Hay fever affects the mucous membranes of the nose, eyes and upper respiratory tracts. In asthma, the lower portions of the respiratory system are severely affected. In eczema, the skin becomes red, followed by the appearance of minute blisters.

During allergic reaction there is increased release of histamine from mast cells. It causes marked dilation of all the peripheral blood vessels and the capillaries become highly permeable so that large amounts of fluid leak out from the blood into the tissues. The blood pressure decreases drastically often resulting in the death of the individual within a short time. Spleen is called the shock organ of allergy.

The exact nature of the substance of which a person is hypersensitive must be known before he can be properly treated. Some forms of allergy are mentioned below:

Figure 2
a) Hay fever: In this allergic form, there is swollen, rendered, running eyes and nose. The drugs called antihistamines are of major importance in the treatment of this allergic disorder.
Figure 3

b) Asthma: It is the sudden spasm of tissue surrounding respiratory tract causing narrowing of respiratory tract. The tissue surrounding the respiratory tubes in the lungs swell up and compress the tubes. Hence there is difficulty in breathing.

c) Anaphylactic shock: It is an allergic reaction involving all the tissues of the body and occurs in a few minutes after the injection of an antigen such as penicillin. Such a reaction is very serious. Histamines released from ruptured from mast cells causes marked dilation of all the arteries so that a large amount of fluid is passed from the blood to the tissues and there is a drastic fall in blood pressure. The affected person may become unconscious and the individual may die within a short time.

2. Autoimmunity: Antibodies are produced against antigens but sometimes it may also happen that the immune system of the body goes off the track and starts behaving against the 'own body' or 'self'. This leads to a variety of diseases known as autoimmune diseases. This type of diseases depends on which type of 'self-antigens' is involved. The nature of autoimmune diseases depends on the autoantigens involved. For example, if the autoantigens are produced against acetylcholine receptors (Myasthenia gravis); if the autoantigens are liver cells, then it results in chronic hepatitis, etc. Other autoimmune diseases are insulin-dependent diabetes, Addison's disease, ulcerative colitis and rheumatoid arthritis.

3. Immuno deficiencies

a) Sever combined immuno deficiency (SCID): Sometimes new born children are without T-cells and B-cells. These children are highly susceptible to various infections. The most serious disorder of this type is a congenital disease known as severe combined immuno deficiency (SCID) in which both B-cells and T-cells are not present in the body. Such children are highly susceptible even to minor infections. In developed countries like U.S.A. such children are kept alive by keeping them in germ-free environments called isolation suits.

Figure 4
b) Acquired immune deficiency syndrome (AIDS): It is a disorder of cells mediated immune system of the body. There is a reduction in the number of helper T-cells which stimulate antibody production by B-cells. This results in the loss of natural defense against viral infection.

4. Graft rejection: Grafts of a kidney, heart, lung, liver, etc. from one human to another always (unless donated by an identical twin) are seen by the recipient's immune system as antigenic and elicit an immune response. If unchecked, this response will eventually lead to destruction of the graft. Both CD4+ CD8+ T cells participate in graft rejection. They responding to differences between donor and host of their class II and class I histocompatibility molecule.

5. Graft-versus-host disease: Grafts of bone marrow are used to provide, or restore, a source of blood cells of the recipient.

Figure 5
Figure 6

If there is any histocompatibility differences between donor and recipient (and there always are some, unless the patientown marrow is used or that of an identical twin), then the cells of the donor will mount an immune response against the tissues of the recipient. Fortunately, graft-versus-host disease can usually be controlled with immunosuppressive drugs.

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Monday, 19 January 2015

Water Absorption and Pathway of Water Across the Root

Water absorbing structure of the plant is root hair zone. Root hair is tubular prolongation of epiblema cells. Root hairs are unicellular, short lived and arranged in an acropetal manner. Root hairs are found in zone of cell maturation. During transplantation the root hairs are removed, that is why, the plant remains wilted in the new habitat. The cell wall of the hair is made up of two layers. The outside wall is of pectin which dissolves in water, so that root hair surface becomes slimy and sticky. The inner wall is made up of cellulose. They are about 10 micro m. There OP is higher (3-8 atm) as compared to soil solution (less than 1 atm). Many forest trees, shrubs and some conifers have scantly root hairs so they make association with the fungi, called mycorrhiza. Orchid roots have a specific type of tissue for absorbing environmental moisture; this tissue is called velaman tissue.

Movement of water from root hair cell to xylem may occour by two possible paths :

1. Apoplast pathway: In this method, water passes from root hair cell to xylem through the walls of intervening cells without crossing any membrane or cytoplasm. The apoplastic movement of water beyond cortex is blocked due to the presence of casparian strips in the endodermal cells. Major movements of water through cortical cells occur by this method, as cortical cells offer least resistance.

Figure: Pathway of water movement in the root

2. Symplast Pathway: In this method, water passes from cell to cell by crossing plasma membrane; therefore it is also known as transmembrane pathway. This may occur by two methods:

(i) Non Vacuolar Symplast Pathway: In this method, water passes adjacent cells through plasmodesmata. It does not enter into the vacuoles.

(ii)  Vacuolar Symplast Pathway: In this method, water passes the tonoplast, surrounding the vacuole.
 This pathway offers a lot of resistance. Beyond cortex (through endodermis and pericycle) water is forced to move through symplast pathway. Terms Apoplast and Symplast were proposed by "Munch".

Mechanism of Water Absorption

(1) Passive absorption of water: In actively transpiring plants, absorption of water takes place due to the forces developed at the transpiring surface of the plant ( i.e., transpiration pull). In this type, the cells of the root do not play any part, and it does not consumes energy, hence it is known as passive absorption. Thus in passive absorption, water is just pulled through the roots. This is the most common (96%) and rapid method of water absorption. Generally, water is absorbed by the root hairs when the osmotic concentration of their sap is high. This is made possible by transpiration taking place in the aerial parts of the plant. It continuously removes water from the sap of the root hairs which, in turn, are in contact of the soil water.
In actively transpiring plants, water loss from mesophyll cells occurs and increases their osmotic concentration. It also results in the increase of their DPD. As a result, water from neighboring cells enters in them by osmosis. These cells in turn have now increased their osmotic concentration or lowered their water potential. Hence, water enters into them by osmosis from other adjacent cells. In this way mesophyll cells draw water from one another along the suction pressure gradient or DPD till it reaches the xylem of the leaf. Once water is drawn from xylem of the leaf, the entire water column in the xylem of the leaf, stem and the root is lifted. The movement of water is apoplastic. In this way, water is absorbed by the root hair due to illusion pressure deficit gradient produced by transpiration that develops in the leaf. Root simply acts as a path of water.

(2) Active absorption of water: Although a very small amount of water (4 %) is absorbed by active mechanism, it involves an expenditure of metabolic energy which comes from the respiring cells of the root. Roots are actively involved in this method, so it is absorption by the roots. Water absorption from soil and its inward movement is OP dependent or independent (OP of the root hairs is higher than soil solution, OP of cortical cells is higher than root hairs). Passage of water from living cells to xylem channel requires accumulation of solute in xylem which is an energy dependent process. Hence, pumping of water in xylem channel is active. This creates a positive pressure in xylem called root pressure. Certain evidences also favors non-osmotic absorption of water, requiring energy.

Factors affecting water absorption

(1) Available soil water: Absorption of water is more, if the amount of available water is more. Rate of water absorption decreases, if the amount of soil water is below permanent wilting percentage or beyond field capacity.
(2) Soil air: Absorption of water takes place at a rapid rate in well aerated soil. Oxygen deficiency retards the growth of roots, thus inhibiting of water. In the soil, if all the air spaces are filled with water the condition is known as water logging of soil. Such soil is physiologically dry soil.
(3) Concentration of soil solution: If the soil solution is highly concentrated due to the presence of salts, it will inhibit the water absorption. It is also kind of physiological dryness.
(4) Soil temperature: An increase in soil temperature up to about 30 Celsius favors water absorption. At higher temperatures, water absorption is decreased and at 0 Celsius it is almost checked. 

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Saturday, 17 January 2015


Traditionally plant kingdom has been divided into two sub-kingdoms named as PHANEROGAMAE and CRYPTOGAMAE.

(a) CRYPTOGAMAE (phaneros-visible, gamos-marriage): All flowering plants which bear seeds are included in this group. They are also known as spermatophytes (sperma-seed, phyton- plant), since they produce seeds. These seed bearing plants are further divided into divisons gymnospermae and angiospermae. Gymnosperms have naked ovules or seeds. The angiosperms (covered seed) include all the flowering plants which produce seed and have ovules in ovary or fruit.

(b) PHANEROGAMAE (kryptos-concealed, gamos-marriage): All non flowering plants such a algae, fungi, lichen, moses and ferns are included in this sub-kingdom. The cryptogams are further classified into three divisions- THALLOPHYTA, BRYOPHTA and PTERIDOPHYTA.

A. Early attempts for classification
Aristole and other Greek philosophers divided living organisms into two groups: plants and animal,    Aristole also divided plants into 3 groups. HERBS, SHRUBS and TREES. Charaka (The father of Ayurveda), in 1st century A.D., listed 200 kinds of animals and 340 kinds of plants in his ' CHARAKA SAMHITA '.

B. Artificial System of Classification
It is based on a few morphological character of vegetative nature for grouping of organisms. e.g. early systems of classification by Aristole, Theophrastus, Pliny, Bauhin , Linnaeus etc. THEOPHRASTUS proposed the first system of artificial classification of plants on the basis of habit and classified plants into herbs, shrubs, undershrubs and trees. CAROLUS LINNAEUS (1707-1778) proposed the artificial system of classification based exclusively on nature and number of stamens and carpels. It was called as sexual system of classification. Classification by Linnaeus consisted of 24 classes, in which 23 classes were of flowering plants ( Phanerogamia ) and 24th class had flowerless plants ( Cryptogamia ). Details of this classification were published in Genera Plantarum (1737). Total 24 classes given by Linnaeus were : Monandria, Diandria  , Triandria , Tetrandria , Pentandria , Hexandria , Heptandria , Octandria , Enneandria , Decandria , Dodecandria , Iconsandria , Polyandria , Didynamia , Tetradynamia , Monadelphia ,Diadelphia , Syngensia , Gynandria , Monoecia , Polygamia and Cryptogamia.
(i) This system was based on one or a few characters, hence the diverse animals or plants were placed into limited number of groups.
(ii) Natural affinities and phylogenetic relationships were not considered.
(iii) The artificial system gave equal weight age to vegetative and sexual characters. This is not acceptable and since we know that often the vegetative characters are more easily affected by environment.

C. Natural System of Classification
Also known as HORIZONTAL system of classification or 2 D system.  Organisms in this system are classified on the basis of the natural affinities. This system uses as many taxonomic characters as possible to group organisms. This classification is mainly based on forms, relationship realizing all information available at the time of collection of plants. This also considers internal features like ultra structure, anatomy, embryology, and phytochemistry. Common natural systems were proposed by- John Ray, de Candolle, Bentham and Hooker,etc. George Bentham and J.D. Hooker gave most important natural system of classification of angiosperms and published it in three volumes of 'George Plantarum'. They described 202 families. In this system, description of plants was based on their detailed studies and dissections. This system is followed in all British Commonwealth countries including India.

D. Phylogenetic System of Classification
The term phylogeny was given by Lamarck and concept of phylogeny by E. Haeckel. Phylogeny is the evolutionary history of the organism. This system is also called '3D' or vertical system. In this system, plants are classified according to their evolutionary and genetic affinities. Organisms belonging to same texas are believed to have a common ancestor and may be represented in the form of family tree called Cladogram. A.W. Eichler modified Bentham and Hooker's system of classification by placing gymnosperms in the beginning. He is also called as the pioneer in phylogenetic system of classification. Adolph Engler and Karl A.E. Prantl, two german botanists, adopted their system in "Die naturalichen Pflanzen familien" (1887-1915).

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Thursday, 15 January 2015


(1) Two Kingdom Classification: It was given by Linnaeus. Traditionally all the organisms of the world were divided into two kingdoms - the animal kingdom (Animalia) and the plant kingdom (Plantae). The major creation of classification was the presence or absence of cell wall. Other Criteria’s were locomotion, modes of nutrition to external stimuli etc.

Short comings of two-kingdom system of classification: Two-kingdom system of classification presented too many inconsistencies. This system did not distinguish between the eukaryotes and prokaryotes, unicelled and multicelled organisms and photosynthetic and non-photosynthetic organisms. There are few organisms like Chlamydomonas, Euglena and the slime moulds which have been claimed by both zoologists and botanists (organisms which share characteristics of both animals and plants). Since there are certain organisms that do not fall naturally into either plant or animal kingdom, it was proposed that a new kingdom is to be established to accommodate such organisms.

(2) Three Kingdom Classification: Haeckel, a German zoologist (1866), suggested that a third kingdom Protista, be created to include all unicellular microorganisms. This includes a wide variety all unicellular, mostly aquatic eukaryotes like - Fungi, Protozoa, Algae, Bacteria and Slime moulds. Thus, he proposed three kingdoms, namely - Plantae, Protista and Animalia.

(3) Four Kingdom Classification: Copeland(1956) gave four kingdom of classification and included Monera as fourth kingdom. Copeland originally called it as kingdom 'Mychota'. It was called 'Monera' by Daugherty and Allen. Kingdom Monera includes all the prokaryotic organisms i.e., eubacteria (including cyanobacteria, formerly known as blue-green algae) and archaebacteria. The actinomycetes (filamentous bacteria) are also included in this kingdom.

(4) Five Kingdom Classification: According to five-kingdom concept proposed by R.H. Whittaker (1969), the organisms are divided into five kingdoms namely Monera, Protista, Fungi, Plantae, Animalia, on the basis of the following criteria:
(a) Complexity of cell structure: prokaryotic vs eukaryotic organization of cells.
(b) Complexity of body organization: unicellularity vs multicellularity; simple multi-cellular forms to complex multi-cellular forms.
(c) Modes of nutrition: Autotrophic vs heterotrophic (parasitic or saprobic or ingestive organisms). It was the major criteria of this classification system.
(d) Reproduction.
(e) Phylogenetic or evolutionary interrelations.

(5) Six Kingdom Classification: Carl Woese proposed six kingdom classification. These six kingdoms are Kingdom-Archaebacteria, Kingdom-Eubacteria, Kingdom-Protista, Kingdom-Fungi, Kingdom-Plantae and Kingdom-Animalia. He separated the archaebacteria from eubacteria on the basis of some major differences such as the absence of peptidoglycan in the cell walls of the former and the occurrence of branched chain lipids (a monolayer instead of a phospholipid bilayer) in the membrane.
Based on the sequence of 16S ribosomal RNA genes, Woese found that the six kingdoms naturally cluster into three main categories. He called these categories as domains of life. These domains are Bacteria, Archae and Eukarya and are believed to have originated from common ancestor called progenote.

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