What is the Alternation of Generation and its Significance?

ALTERNATION OF GENERATIONS:

• In the life history of liverworts, mosses and hornworts there are two distinct multicellular phases or generations.

• These generations are haploid gametophyte and diploid sporophyte, which regularly alternate with each other.

• The gametophyte is the dominant generation because it is more conspicuous.

• It produces gametes called spermatozoids and eggs, therefore called gamete producing generation.

• A haploid spermatozoid fuses with a haploid egg to produce a diploid oospore.

• The oospore does not produce the gametophyte directly but produces a totally different plant called the sporophyte.

• The sporophyte is a less conspicuous generation, which is usually differentiated into foot, seta and capsule (also called sporogonium).

• Spores develop within the capsule by reduction division (meiosis) from spore mother cells.

• The sporophyte produces spores and is, therefore, called spore-producing generation.

• The spore on germination does not develop into a sporophyte but gives rise to the gametophyte.

• Thus in the life history of a bryophytic plant, the two generations, the gametophyte and the sporophyte, regularly alternate with each other.

• The phenomenon of alternation of gametophyte and sporophyte in the life history of a plant is called alternation of generations.

• It should be noted that the gametophyte or haploid stage begins within spores and ends at gametes, whereas the sporophyte being with oospore and ends at spore mother cell.

THE SIGNIFICANCE OF ALTERNATION OF GENERATIONS:

• During the formation of spores from a spore mother cells by meiotic division reshuffling of genes occurs.

• As a consequence, a great variety of spores with different genetic make-up are produced.

• These spores, in turn, produce gametophytes with different genetic combinations.

• The gametophytes with better genetic make-up will have a better chance of survival in the environment where they occur.

• On the other hand, the gametophytes with less advantageous characteristics will be eliminated.

• There is no reshuffling of genes during gametogenesis in the gametophyte as gametes are produced after mitosis.

• The oospore developing after fertilization now has a new genetic make-up as compared to the parent.

• This genetic variation passes to the new sporophyte which on maturity once again produces further genetic recombination which is transferred to the gametophyte.

• In this natural process, the sporophyte thus provides a large amount of genetic variability and nature selects the best genetic combinations.

• In the long run, this will allow the populations to become increasingly better adapted to their environment.

Explain the different types of reproduction in Fungi ?

REPRODUCTION:

(Except imperfect fungi in which sexual reproduction has not been observed).


Asexual Reproduction:

Asexual reproduction takes place by

(i) Spores               (ii) Conidia               (iii) Fragmentation and                 (iv) Budding

(i) Spores:

• Spores are produced inside the reproductive structures called sporangia, which are cut off from the hyphae by complete septa.

• Spores may be produced by sexual or asexual process, are haploid, non-motile and not needing water for their dispersal, are small, produced in very large number and dispersed by wind to great distance and cause wide distribution of many kinds of fungi, including many plant pathogens.

• When spores land in a suitable place, they germinate, giving rise to new fungal hyphae.

• Spores may also be dispersed by insects and other small animals and by rain splashes.

• Spores are a common means, of reproduction in fungi.

(ii) Conidia:

• Conidia (singular conidium) are non-motile.

• Asexual spores which are cut off at the end of modified hyphae called conidiophores, and not inside the sporangia, usually in chains or clusters.

• These may be produced in a very large number,

• Can survive for weeks and cause repaid colonization of new food.

(iii) Fragmentation:

Fragmentation is simple breaking of mycelium of some hyphal fungi, each broken fragment rise to a new mycelium.

(iv) Budding

Unicellular yeasts reproduce by budding (an asymmetric division in which tiny outgrowth or bud is produced which may separate and grow or By simple, relatively equal cell division.

Sexual Reproduction:

Details of sexual reproduction vary in different groups of fungi but fusion of haploid nuclei and meiosis is common to all.

When fungi reproduce sexually, hyphae of two genetically different but compatible mating types come together, their cytoplasm fuse followed by nuclear fusion.

(i) Karyogamy and Plasmogamy:

In two of the three main groups of fungi (Basidiomycetes, Ascomycetes) fusion of nuclei (Karyogamy) does not take place immediately after the fusion of cytoplasm (Plasmogamy); instead, the two genetic types of haploid nuclei from two individuals may coexist and divide in the same hyphae for most of the life of the fungus. 

(ii) Dikaryotic (Heterokaryotic):

Such a fungal hypha/cell having 2 nuclei of different genetic types is called dikaryotic (also heterokaryotic) hypha/cell.

(iii) Haploid Sexual Spores:

Different groups of fungi produce different types of haploid sexual spores, such as basidiospores and ascopores, subsequent upon meiosis in zygote.

These spores may be produced by their characteristics structure/fruiting bodies such as basidia/basidiocarps and asci/ascocarps.

Difference between the Chlorophyll A and Chlorophyll B: 

Ans.        CHLOROPHYLL A  AND  CHLOROPHYLL B:

 IMMUNITY: 

(1) Definition:

(Ability of individual to resist against the micro-organisms is called immunity).

The capacity to recognize the intrusion (disturbance) of any material foreign to the body and to mobilize cells and cell products to help remove the particular sort (particular types) of foreign material with greater speed and effectiveness" is called immunity.

(a) In animals in addition to physical barriers (skin + mucous membranes).

(b) Phagocytes,

(c) There is a third mechanism, to defend their bodies against the foreign invaders; this is the Immune system.

(2) Components of Immune System:

The components of immune system include the:

(i) Lymphocytes (B and T) and the

(ii) Antibodies which are special type of proteins.

These antibodies are immunoglobulins which are synthesized by vertebrates, in response to antigens; and immobilize it, or sets in motion events that ultimately cause its destruction.

(3) Antigen:

Antigen or immunogen is a foreign substance, often a protein which stimulates the formation of antibodies

(4) Antibodies:

Antibodies are specific proteins i.e. cause the destruction of the antigen, which stimulated their production.

Antibodies are Manufactured:

Antibodies are manufactured in B-Lymphocytes, then secreted into the lymph and blood where they circulate freely.

(5) Cell-Mediated Response:

T-cells recognize antigen then combat micro-organisms and/or effect the rejection of foreign tissues (in case of tissue transplant such as kidneys).

This called cell-mediated response.

(6) Humoral Immune Response:

B-cells recognize antigen and form plasma cell clone.

These plasma cells synthesis and liberate antibodies into the blood plasma and tissue fluid.

Here antibodies attach to the surfaces of bacteria and speed up their phagocytosis, or combine with and neutralize toxins produced by micro-organisms, by producing antitoxins.

This is called humoral Immune response.

When we get vaccination, against a specific disease (antigen), we become immune to that infection or disease.

If we get vaccination against, Polio, Smallpox, measles, mumps etc., once in our life time, we are protected or become immune to that infection in our future life.

(7) Types of Immunity:

(i) Active Immunity:

Most of the vaccines consist of a non-virulent, mutant strains of the poliovirus.

The use of vaccines, which stimulate the production of antibodies in the body, and making a person immune against the disease or infection, is called active immunity.

(a) Artificially Induced Active Immunity:

But this active immunity has been achieved by artificially introducing, antigens in the body, so it is called artificially induced active immunity.

(b) Naturally Induced Immunity or Auto Immune Response:

But, when a person is exposed to an infection (antigen)-becomes ill, and in most cases survives then this immunity, developed against that disease is called naturally induced immunity or auto immune response.

Antiserum is a serum containing antibodies.

(ii) Passive Immunity:

In contrast to active immunity, in which case antigens are introduced to stimulate the production of antibodies, by artificial or natural method; antibodies are injected in the form of antisera, to make a person immune against a disease. This is called passive immunity.

Antigen-Antibody Complexes:

In body, antigen-antibody complexes are formed which are taken up by phagocytes and destroyed.

The patient is spared (treat or afford something) the complications (or possibly death) caused by the infection or venom.

Passive immunity response is immediate, but not long lasting.

Because no time is taken for the production of sufficient level of antibodies, (as antibodies are being injected) and after the level of antibodies is reduced or they are used up-No more antibodies production is there.

Combat Active Infections:

 The method of passive immunization is used to combat active infections of:

(i) Tetanus,                    (ii) Infectious hepatitis,                    (iii) Rabies, 

(iv) Snake bite venom etc.

In the case of snake bite venom passive immunity is produced by the antitoxins-so the serum is called anti-venom serum.

AIDS (Acquired Immune Deficiency Syndrome) is a disease caused by a virus.

Affected suffer deficiency in their immune system of the body, and the immune system collapses.

Thus the AIDS victim often succumbs (die) to a bacterial disease or cancer, that under normal circumstances, the immune system can overcome.

There is no known cure of the disease it can be spread by blood transfusions and by close contact with the infected persons.

STRUCTURE OF PROTEINS:

Definition:

Proteins are polymers of amino acids, the compounds containing carbon, nitrogen, oxygen and hydrogen.

The number of amino acids varies from a few to 3000 or even more in different proteins.

Abundance:

Proteins are the most abundant organic compounds in cells. They are over 50% of total dry weight of the cell.

Amino Acids:

About 170 types of amino acids are present in cells and tissues.

Of these, about 25 types are involved in the formation of proteins. However, most of the proteins are made of 20 types of amino acids.

Structure of Amino Acid:

All the amino acids have an amino group (-NH₂) and a carboxyl group (-COOH) attached to the same carbon (alpha carbon). The general formula of a amino acid is a follows:

Formation of Protein Molecule by Peptide Linkage:

Amino acids are linked to form polypeptide. The polypeptides may assemble to form proteins.

The amino group of one amino acid reacts with the carboxyl group of another amino acid and a water molecule is released. Here OH is released from carboxyl group of one amino acid while H is released from amino group of next amino acid.

The linkage between C of carboxyl group of one amino acid and N of amino group of next amino acid is called peptide bond

Example: 

Glycine and alanine combine to form glycylalanine.

Glycylalanine has two amino acids and is called dipeptide.

A dipeptide has an amino group at one end and a carboxyl group at the other end.

So both reactive sites are available for the formation of further peptide bonds. As a result tripeptides, tetrapeptides, pentapeptides and polypeptides etc can be produced.

Structure of Proteins:

Each protein has specific properties which are due to:

(i) Number of amino acids,

(ii) Kinds of amino acids.

(iii) Specific sequence of amino acids and.

(iv) The shape of protein molecule.

As a result protein structure is formed which is organized at four levels.

(1) Primary Structure of Protein:

It includes the number, kind and sequence of amino acids in a protein molecule.

Size of protein Molecule:

The size of a protein molecule is determined by the type of amino acids and the number of amino acids in that protein molecule.

Sequence of Amino Acids:

There are over 10,000 proteins in the human body. These are formed by the specific arrangements of 20 types of amino acids.

The sequence of amino acids is determined by the order of nucleotides in the DNA.

(2) Secondary Structure:

It is the coiling of primary polypeptide chains.

The chains in a protein molecule are not flat. They usually coil into α helix, or into β-pleated sheet.

(i) α-Helix:

It is most common structure in which the basic polypeptide chain is spirally arranged.

It is very uniform geometric structure with 3.6 amino acids in each turn of the helix.

The helical structure is kept by the formation of hydrogen bonds among amino acid molecules in spiral turns.

(ii) β-Pleated Sheets:

These are formed by the fold backs of the polypeptide (e.g. silk).

(3) Tertiary Structure:

In the tertiary structure, a polypeptide chain bends and folds upon itself forming a globular shape.

Structural Maintenance:

It is maintained by three types of bonds:

(a) Ionic bonds,                   (b) Hydrogen bonds and                   (c) Disulfide (-S-S-) bonds

EVOLUTION OF SEED HABIT:

Dominancy of Seed Plants Over the Non-seed Plants:

A review of the kingdom Plantae indicates that the seed-pants (spermatophytes) predominate over non-seed vascular plants.

Development of Seed Habits:

One of the most significant events in the history of land plants was the development of seed habit.

Change in Reproductive Structure:

It was an important change in the reproductive system of the vascular plants which occurred approximately 390 million years ago.

First Seed Appear in Late Devonian:

First complete seeds appeared approximately 365 million years ago during late Devonian times.

Seed is Fertilized Ovule:

Technically a seed may be defined as a fertilized ovule.

Ovule is Immature Seed:

Integument Indeshiscent:

An ovule is an integumented indehiscent (not discharge or ripe) megasporangium. 

Integuments are specialized protective coverings around megasporangium which vary in number.

Spermatophytes:

All seed-producing plants are called spermatophytes.

Various steps involved in the evolution of seed habit are as follows.

Steps Involved in Evolution of Seed Habit:

Summary:

(i) Evolution of Heterospory.

(ii) Retention and germination of megaspore within the megasporangium.

(iii) Development of protective layers around megasporangium.

(iv) Reduction to a single functional megaspore per sporangium.

(v) Development of an embryo sac within the sporangium.

(vi) Modification or distal end of megasporangium for pollen capture.

In Detail:

(i) Evolution of Heterospory:

• Primitive vascular land plants produced one kind of spores, a condition called homospory.

• All groups of land plants up to pteridophytes are homosporous.

• During the early phase of evolution, some plant groups started producing two different types of spores.

• The microspores produced inside microsporangia germinated to form male gametophyte or microgametophyte.

• The megaspores germinated to form female gametophyte or megagametophyte.

(ii) Retention and Germination of Megaspore within the Megasporangium:

• During the usual reproductive cycle in the heterosporous vascular land plants, the megaspores used to be shed and dispersed soon after their formation in order to germinate into the female gametophyte.

• However, in some plants (e.g. Selaginella) the megaspore was not allowed to escape from megasporangium immediately after its formation.

• In other, the megaspore(s) was/were permanently retained within the megasporangium.

• Here, within the confines of the megasporangium wall, the megaspore germinated to form egg containing female gametophyte.

(iii) Development of Protective Layers Around Megasporangium.

(iv) Reduction to a Single Functional Megaspore per Sporangium.

• Each megaspore mother cell within a megasporangium used to produce four functional megaspores.

• These megaspores germinated to produce four variable female gametophytes.

• There was a competition for space and food among the four gametophytes.

• Soon the early vascular plants adopted a new strategy i.e., only one megaspore was selected for further development into a healthy female gametophyte while the remaining three were aborted.

(v) Development of an Embryo Sac within the Sporangium.

• The single healthy megaspore retained with the megasporangium germinated to form an egg containing female gametophyte called an embryo sac.

(vi) Modification or Distal end of Megasporangium for Pollen Capture.

• When most of the structural and functional changes leading to the development of seed habit were completed.

• Another important modification took place in the megasporangium which was now integumented, indehiscent and permanently attached to the sporophyte.

• The distal end of the megasporangium became modified for capturing pollen (microspore containing male gametophyte).

• Pollen after being trapped in the distal cavity of the megasporangium produced pollen tube which carried male gametes deep into the embryo sac to fertilize the egg, forming a zygote, which forms an embryo.

• The megasporangium (ovule) after fertilization is transformed into a seed, the integuments becoming the second coats (testa).

• The seed offers maximum degree of protection to a developing embryo under the unfavorable terrestrial environment.

• The development and evolution of seed habit was a great success and giant leap which ultimately enable plants to colonize land permanently.