THE CELL ENVELOPE: THE OUTER WRAPPING OF BACTERIA:

Definition:

In bacteria the complex of layers external to the cell protoplasm is called as cell envelope. It includes capsule, slime and cell wall.

(i) Capsule:

Some bacteria produce capsule. It is tightly bound to the cell. Capsule is made up of of repeating polysaccharide units, or of protein, or of both.

(ii) Slime:

It is a loose, soluble cover of macromolecules called as slime capsule. It is present in some bacteria.

Slime provides greater pathogenicity to bacteria and protects them against phagocytosis.

(iii) Cell Wall:

Beneath the extracellular substances and (external to cytoplasmic membrane is cell wall).

It is rigid structure and determines the shape of bacterium.

Cell wall also protect the cells from osmotic lysis.

Cell wall is only absent in mycoplasmas.

Gram Stain Technique on the Basis of Differences in Cell Wall.

Christian Gram developed the technique of gram stain on the basis of differences in the cell wall.

On the basis of this technique bacteria can be divided into two groups.

(i) Gram Positive Bacteria:

These are the group of bacteria  that stained purple. They retain the primary dye due to the formation of CV-I complex in the cell wall.

(ii) Gram Negative Bacteria:

These bacteria are stained pink because they retain secondary dye in the cell wall.

Comparison of Gram Positive and Gram-Negative Cell Wall:

Difference in staining is due to difference in structure of cell walls of two groups. It is clear from the following table:

Nuclear War OR

 Ban on Nuclear Weapons

NUCLEAR WAR:

The developed countries like Russia, America, Japan and France have been making nuclear weapons for many years. America attacked the two cities of Japan, Nagasaki and Hiroshima in the Second World War with atomic bombs. It caused huge destruction in both the cities. Millions of people were wounded and killed in that bombing. Since then a serious thought has been given to the preparation of nuclear weapons. Every country considers it essential to equip itself with nuclear weapons for its security.

Nuclear energy is considered the basic armament of self-defence. In the mad race of preparing nuclear weapons, the underdeveloped countries like India, South Africa and Israel have also joined hands with the developed countries. India has already tested the capability of her nuclear weapons and has successfully exploded the atomic bomb.

 It seems that the nations of the world have forgotten the savagery of atom bombing by the Americans over the Japanese cities. The atom, despite its devastating powers, is still the favourite subject of study among scientists. They go on studying it with the sole object of killing mankind. Persistent nuclear explosions and tests in Siberia or Pacific are hailed as the greatest achievement of science.

The political rulers and statesmen of every country consider the collection of nuclear weapons as a safeguard of peace. They are mistaken in it. The piling up of nuclear weapons is an inevitable prelude to the whole destruction of mankind.

A nuclear war is an exercise is wholesale destruction. A single day's atomic bombing of the U.S.A is estimated to cause the death of about 50 million people and a serious injury to about half the number. Such a great destruction is expected to paralyse the whole steam of life. The influence of radiation will continue to haunt the world for many years. It may give rise to mysterious diseases and ailments. It may render the fertile land barren and wild.

Nobody can wish for such a gloomy and dark future. World peace can only be secured if all the countries decide to put a ban on the research, the production and preparation of nuclear weapons. No country, however powerful should be allowed to make nuclear explosion. Public opinion against the negative effects of nuclear energy should be launched by the people of world.  The existing stock of such destructive armaments must be destroyed.

A powerful organization should be set up in every country which should watch and control the production of nuclear weapons. It should mobilize the public opinion against the menace of atom bomb. People should be told that radio activity is an unchecked and uncontrollable monster. A world wide awareness against the use of nuclear weapons should be produced. The powerful and advanced countries should also be forced to stop producing the deadly weapons. This may seem to be an ideal plan to do away with nuclear weapons. But unless such drastic measure is taken, the future of mankind will continue to be dark. The statesmen, the rulers and politicians of all the countries should realize their responsibility regarding the bright future of mankind. If there is a total disarmament in all the countries, the world peace can be secured in a better way.

Atom is a Greek word which means a thing that cannot be cut or broken. It is the smallest particle of matter. It is no longer undivided and unbroken. The scientists have broken it into further smallest particles. They are called electrons, protons and neutrons. They release great power and energy. It was Albert Einstein who began to work on the theory of producing energy from atom in 1905. He explained many things. First, he showed that matter and energy are different forms of the same thing. In other words, matter can be converted into energy and vice versa. Second, a small amount of matter produces a great amount of energy. The discovery of atomic energy is one of the wonders of the modern science.

Atomic energy is a double-edged weapon. It can be utilized for destructive as well as peaceful purposes. The scientists, first of all, used it for destructive purposes. They prepared atomic bombs, hydrogen and cobalt bombs which could destroy cities and towns within the twinkling of an eye. It was in 1945, that a single atom bomb destroyed Hiroshima and Nagasaki. It killed thousands of men and women and destroyed vast areas of land. Now all the big powers possess much more atomic and dangerous hydrogen bombs. They can raze the world to ground if they are carelessly used.

Atomic energy can also be used for the welfare of mankind. The scientists in U.S.A, England, France, Canada and Germany are at work for peaceful use of energy. An atom for Peace conference was held at Geneva to think over the use of Nuclear Energy. The U.S.A has already started to use this energy to propel submarines and space rockets. The Russians have also harnessed it for space travel. At present, electricity is produced from the atomic energy in advanced countries like U.S.A, Russia and Britain. Our scientists are also trying to generate electricity from atomic energy. The scientists are also trying to find ways of using atomic power as fuel in ships, aeroplanes, motor cars etc. At present atomic reactors for boiling water exists in different countries.

Atomic energy is safely used to conquer many fatal diseases. Radium is used in the treatment of cancer. Radio-therapy is an important branch of medical science. It is used to destroy the overgrown cells of our body. The isotopes of sodium, potassium, bromine and iodine are used to known the origin of disease. The atomic energy is of immense help to alleviate human sufferings.

Nuclear energy has also brought a revolution in the field of industry. Petrol and coal are well known sources of energy and power. The supply of these two sources of power is coming to an end. The world is in search of another source of power. It is the nuclear energy. Electronic machines and computers are being used in our industry. They are used in the plastic, glass, paper, textile, rubber and cement industry.

Atomic energy is also of great help in agriculture. The barren lands can be tilled into smiling fields with the help of computerized tractors and machines. Crops can be ripened within a very short time. The yield of crops per acre can also be multiplied. In this way, the scientists are trying to solve the food problem. We can say that atomic energy can be used with full advantage for the welfare of mankind.

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: