Mamta Singh

teacher | Biology Chemistry Physics


I am an experienced teacher with the bio background and having good command on Chemistry and Physics as well. I give tutions for students preparing for Medical Entrance Exams, Boards preparing for Class IX -XII.




5-7 Years

Teaching Classes


Current Organization

K V Mahavidhyalaya

Sexual Reproduction: Post-Fertilisation Events

Post-Fertilisation Events

Events taking place after fertilisation process are called post-fertilisation events.


# The haploid gametes fuse to form a diploid zygote in all organisms.

# In external fertilisation, a zygote is formed in an external medium, and in internal fertilisation, a zygote is formed inside the individual.

# The development of a zygote depends upon the life cycle of an organism. In some organisms, the zygote does not develop immediately and develops a thick wall around itself. This wall is resistant to damage and desiccation.


# It is the process of development of the embryo from the zygote. 

# The zygote undergoes cell division and differentiation.

# Cell division increases the number of cells of the embryo, and cell differentiation helps the cells undergo
modifications to form specialized tissues and organs.

# Animals can be grouped into two categories based on how and where the development of the zygote takes place. These categories are:

1. Oviparous − The fertilized egg is covered by a calcareous shell and is released into the outside environment. The development takes place inside the egg and the young one hatches out (example: birds and reptiles).

2. Viviparous − The development of the zygote takes place inside the female body, and the developed young one is delivered outside (example: mammals, including humans). 

# In flowering plants, the zygote is formed inside the ovule.
Zygote → Develops into → Embryo
Ovule → Develops into → Seed
Ovary → Develops into → Fruit → Contains → Seeds → Disperse and germinate to form new plants

Sexual Reproduction: Fertilisation Events

Fertilisation is the most important event in sexual reproduction.

This process is also called syngamy and leads to the formation of the zygote. However, in some organisms, zygote formation takes place without fertilisation, and is known as parthenogenesis (occurs in rotifers, honeybees and some lizards).

In most aquatic organisms and amphibians, Fertilisation takes place outside their body (in the water), and is termed as external fertilization. Their eggs and offspring are highly vulnerable to predators and this threatens their survival up to adulthood.

In most terrestrial organisms, fertilisation is internal, i.e., it takes place inside the female body. In this process, the male gamete is motile and reaches the female gamete to fuse with it, thereby forming zygote. Male gametes are produced in large numbers.

Reproduction : Gamete Transfer

Gametes need to be transferred for their fusion. 

In most organisms, the male gametes are motile, while the female gametes are non-motile, and the male gametes need a medium for their movement. A large number of male gametes do not make it to the female gamete, and hence, several thousands of male gametes are produced to overcome this loss.

In angiosperms, the pollen grain carries the male gamete and the ovule carries the female gamete. Pollen grains are produced in the anther and need to be transferred to the stigma for Fertilisation to occur. This is easy in monoecious plants as both the anther and the stigma are present close by; in 
dioecious plants, it takes place by pollination.

Reproduction : Process of formation of Gametes [Male and Female]

# Gametes are haploid.

# Some organisms (like algae) are almost similar (homogametes) and cannot be categorized as male and female gametes.

# Some others gametes are morphologically and physiologically different (heterogametes), and are of two types - antherozoid or sperm (male gamete) and egg or ovum (female gamete).

# Some organisms are both, the sexes are present in the same individual (monoecious or homothallic), and
in others, they are present in two individuals (dioecious or heterothallic).

# In a unisexual flower, the male flower is called staminate and the female flower is called pistillate. Gamete formation takes place by cell division.

# In haploid parents, it is by mitosis; in diploid parents, it is by meiosis, with specialized cells called
meiocytes undergoing meiosis.

Sexual Reproduction

Sexual Reproduction involves in the formation of male and female gametes in either the same individual or two individuals. These gametes fuse to form a zygote which develops into a new individual. Offsprings are not identical to other one or to the parents.

Two stages of organisms: 
1. Juvenile phase − Period of growth; non reproductive
2. Vegetative phase or reproductive phase

In non-primate mammals like rats, sheep, dogs, cows, tigers, etc, the cyclic change in the activities of the ovaries and the oviduct is called the oestrus cycle.

In primates like monkeys, apes and humans, it is called the menstrual cycle.

Few mammals are called continuous breeders because they can reproduce while some are called seasonal breeders because they can reproduce only in the favorable seasons.

Events in Sexual Reproduction : 
1. Pre-Fertilization events
2. Fertilisation events
3. Post-Fertilisation events

Asexual Reproduction

sexual Reproduction
1. In this type, a single parent can produce offspring.
2. The produced offspring are clones of each other (i.e., identical to each other and to the parent).
3. It is commonly seen in unicellular organisms belonging to Protista and Monera.
4. Here, the cell division itself is the mode of reproduction.

Means of Asexual Reproduction
1. Binary Fission − In this process, cell divides into halves and each half develops into an adult. (example: Amoeba, Paramecium).
2. Budding − In this process, cell pldivides unequally to form buds, which remain attached to the
parent initially, and then detach and develop into a mature cell (example: yeast).

Formation of specialized structures
Conidia − (Example: Penicillium)
Gemmules − (Example: Sponges)
Buds − (Example: Hydra)
Zoospores − Microscopic, motile spores (Example: Algae)

Vegetative propagation − It means asexual reproduction in plants.
Runner − (Example: Gladiolus)
Rhizome − (Example: Ginger)

Tuber − (Example: Potato)

Bulb − (Example: Onion)

Reproduction in Organisms

Reproduction is a biological process where all living things give rise to new living things. It transmits the hereditary material from parents to offsprings.

There are two types of Reproduction :
1. Asexual − Only one individual is involved
2. Sexual − Two individuals (male and female) are involved 




Shapes and size of orbitals

An orbital is the region of space around the nucleus within which the probability of finding an electron of given energy is maximum (90–95%). The shape of this region (electron cloud) gives the shape of the orbital. It is basically determined by the azimuthal quantum number l, while the orientation of orbital depends on the magnetic quantum number (m). Let us now see the shapes of orbitals in the various subshells.

s–orbitals: These orbitals are spherical and symmetrical about the nucleus. The probability of finding the electron is maximum near the nucleus and keep on decreasing as the distance from the nucleus increases. There is vacant space between two successive s–orbitals known as radial node. But there is no radial node for 1s orbital since it is starting from the nucleus.

The size of the orbital depends upon the value of principal quantum number(n). Greater the value of n, larger is the size of the orbital. Therefore, 2s–orbital is larger than 1s orbital but both of them are non-directional and spherically symmetrical in shape.

p–orbitals (l =1): The probability of finding the p–electron is maximum in two lobes on the opposite sides of the nucleus. This gives rise to a dumb–bell shape for the p–orbital. For p–orbital l = 1. Hence, m = –1, 0, +1. Thus, p–orbital have three different orientations. These are designated as px,py & pz depending upon whether the density of electron is maximum along the x y and z axis respectively. As they are not spherically symmetrical, they have directional character. The two lobes of p–orbitals are separated by a nodal plane, where the probability of finding electron is zero.

The three p-orbitals belonging to a particular energy shell have equal energies and are called degenerate orbitals.


d–orbitals (l =2): For d–orbitals, l =2. Hence m=–2,–1,0,+1,+2. Thus there are 5 d orbitals. They have relatively complex geometry. Out of the five orbitals, the three (dxy, dyz,dzx) project in between the axis and the other two  lie along the axis.

Types of Quantum Numbers

1.  Principal quantum number (n):

It tells the main shell in which the electron resides and the approximate distance of the electron from the nucleus. It also tells the maximum number of electrons a shell can accommodate is 2n2, where n is the principal quantum number.

Shell                                           K L M N

Principal quantum number (n)    1 2 3 4

Maximum number of electrons   2 8 18 32

2. Azimuthal or angular momentum quantum number (l):

This represents the number of subshells present in the main shell. These subsidiary orbits within a shell will be denoted as 1,2,3,4,... or s,p,d,f... This tells the shape of the subshells. The orbital angular momentum of the electron is given as

for a particular value of ‘n’

For a given value of n values of possible l vary from 0 to n – 1.

3. The magnetic quantum number (m):

An electron due to its angular motion around the nucleus generates an electric field. This electric field is expected to produce a magnetic field. Under the influence of external magnetic field, the electrons of a subshell can orient themselves in certain preferred regions of space around the nucleus called orbitals.

The magnetic quantum number determines the number of preferred orientations of the electron present in a subshell. The values allowed depends on the value of l, the angular momentum quantum number, m can assume all integral values between –l to +l including zero. Thus m can be –1, 0, +1 for l = 1. Total values of m associated with a particular value of l is given by 2l + 1.

4. The spin quantum number (s):

Just like earth not only revolves around the sun but also spins about its own axis, an electron in an atom not only revolves around the nucleus but also spins about its own axis.

Since an electron can spin either in clockwise direction or in anticlockwise direction, therefore, for any particular value of magnetic quantum number, spin quantum number can have two values, i.e., +1/2 and –1/2 or these are represented by two arrows pointing in the opposite directions, i.e.,  and .

When an electron goes to a vacant orbital, it can have a clockwise or anti clockwise spin i.e., +1/2 or –1/2. This quantum number helps to explain the magnetic properties of the substances.

Planck’s Quantum Theory

While heating black body, it emits thermal radiations of different wavelengths or frequency. Max Planck put forward a theory to explain these radiations, known as Planck’s quantum theory. The main points of quantum theory are 

i) Substances radiate or absorb energy discontinuously in the form of small packets or bundles of energy.

ii) The smallest packet of energy is called quantum. In case of light, the quantum is known as photon.

iii) The energy of a quantum is directly proportional to the frequency of the radiation . E (or) E = h where  is the frequency of radiation and h is Planck’s constant having the value 6.626  10–27 erg – sec or 6.626  10–34 J–sec.

iv) A body can radiate or absorb energy in whole number multiples of a quantum h, 2h,3h...........nh. where n is a positive integer.

Biological Classification – Reproduction in Bacteria

Bacteria reproduce mainly by Binary fission.


During unfavourable conditions, they produce spores.


They also reproduce by a sort of sexual reproduction by DNA transfer from one bacterium to the other which is referred to as Conjugation.




These bacteria completely lack a cell wall. They are the smallest living cells known and can survive without oxygen.


Kingdom Protista:

The cell body contains a well-defined nucleus and other membrane-bound organelles.

Presence of flagella or cilia. Protists reproduce asexually and sexually by cell fusion and results in formation of a zygote.


Kingdom Fungi:

These Organisms are heterotrophic (Saprophytes). Fungi are cosmopolitan and occur in air, water, soil and on animals and plants.


Kingdom plantae:

Includes all eukaryotic chlorophyll containing organisms. They are autotrophic. They have a cell


Kingdom Animalia:

They either directly or indirectly depend on plants for food (Classified as Herbivores and Carnivores).


Viruses, Viroids and Lichens:

Some acellular organisms like Viruses, Viroids and Lichens are not included in the five kingdom classification of Whittaker.

Biological Classification – Eubacteria (True Bacteria)

Eubacteria are characterised by the presence of a rigid cell wall which is made up of Peptidoglycan, and motile bacteria possess a flagellum which is used for locomotion.


Eubacteria includes the following types:


A. Cyanobacteria


Cyanobacteria (BGA) (blue-green algae) have chlorophyll-a similar to green plants which are found in chromatophores. Hence cyanobacteria are photosynthetic autotrophs.

·  The cyanobacteria are unicellular and exist as colonial or filamentous, marine or terrestrial forms.

·  The colonies of cyanobacteria are generally surrounded by a gelatinous sheath.

·  They form algal blooms in polluted water bodies (Eutrophication).


B. Nitrogen fixing bacteria


These bacteria fix atmospheric nitrogen in specialized cells called heterocysts, example Anabaena & Nostoc. (Heterocyst provide anaerobic condition required for N2 fixation).


C. Chemosynthetic autotrophic bacteria


·  They oxidize various inorganic substances such as nitrates, nitrites and ammonia and use the released energy for their ATP production.

·  These bacteria play a major role in bio-geochemical cycles by recycling nutrients like nitrogen, phosphorous, iron and Sulphur.


D. Heterotrophic bacteria


·  They are the mostly decomposers.

·  Some help in making curd from milk, production of antibiotics, fixing nitrogen in legume roots, etc.

·  Some are pathogens causing damage to human beings, crops, farm animals and pets.

·  Tuberculosis, Typhoid, Cholera and tetanus are some of the diseases caused by different bacteria.

Biological Classification – Archaebacteria (Primitive Bacteria)

Bacteria which live in harsh habitats such as extreme salty areas (halophiles), hot springs (thermoacidophiles) and marshy areas (methanogens) come under Archaebacteria, they are also referred to as Extremophiles.


Archaebacteria have a different cell wall structure which is made of pseudomurein and this feature is responsible for their survival in extreme conditions.


Methanogens are present in the gut of several ruminant animals such as cows and buffaloes and they are responsible for the production of methane (biogas) from the dung of these animals.

Biological Classification - Kingdom Monera

The organisms in kingdom monera include only Bacteria.


Bacteria are omnipotent and occupy different types of habitat, some are found to exist even in extreme habitats such as hot springs, deserts, snow and deep sea or as parasite in or on the surface of organisms.


Based on their modes of Nutrition, Bacteria are classified as Autotrophs and Heterotrophs. Some of the bacteria are classified as autotrophic if they synthesize their own food from inorganic substrates. They may be photo-autotrophic or chemo-autotrophic.


The majority of bacteria are heterotrophs which do not synthesize their own food but depend on other organisms or on dead organic matter for food.


Bacteria are further classified Based on their shape:

I.  Spherical – Coccus

ii.  Rod-shaped – Bacillus

iii. Comma-shaped – Vibrium

iv. Spiral – Spirillum


Biological Classification – Kingdom Concept

There are two kingdom classification concepts proposed by Carolus Linnaeus and R.H. Whittaker respectively.


Two kingdom classification:


·         Carolus Linnaeus proposed the two kingdom classification and is known as the Father of Taxonomy.

·         Linnaeus classified organisms as Plantae (Plants - autotrophs, have cell wall, do not move) & Animalia (Animals - heterotrophs, have no cell wall, can move).

·         Later Linnaeus found that the two kingdom classification was not sufficient because in that

Ø  Both Prokaryotes & Eukaryotes were grouped together.

Ø  Heterotrophs & Autotrophs were together.

Ø  There was no difference between unicellular and multi cellular Organisms.

Ø  Simple organisms were placed along with higher organisms.


Five kingdom classification:


·         Proposed by R.H. Whittaker (1969).

·         The kingdoms defined by him were named Monera, Protista, Fungi, Plantae and Animalia.


Main criteria for classification:


·         Based on the complexity of cell structure, organisms are classified as prokaryotes/ Eukaryotes.

·         Based on the Body organization, organisms are classified as Unicellular/ Multicellular.

·         Based on the mode of nutrition, organisms are classified as autotrophic / heterotrophic /

·         holozoic/Parasitic/Symbiotic.

·         Based on Life style (Producers / Consumers / Decomposers)


Biological Classification – Systems of Classification

Biological Classification is the process of grouping living organisms into convenient categories based on simple characters. It was first proposed by Aristotle who divided organisms into 2 groups: Plants and animals. He further classified animals into anaemia (without RBC) and enaima (with RBC). Later Theophrastus classified plants into herbs, shrubs and trees.


Types of Biological Classifications:

·  Kingdom Monera

·  Archaebacteria

·  Eubacteria

·  Reproduction in Bacteria


Systems of Classifications are as follows:


Artificial System of Classification:

·  t is one of the earliest systems of classification based on one or few easily observable characters.

·  These classifications focussed only on one characteristic and classified organism. For example, Linnaeus classified plants based on floral characters only and was called the Linnaeus sexual system.

·  Gaius Plinius Secundus, known as Pliny the Elder classified animals on the basis of their habitat, e.g. land, air and water.


Natural System of Classification:

·  The most important and last of natural systems for classification of seed plants was developed by Bentham (1800 - 1884) and Hooker (1817 - 1911).

·  Natural classifications are based on a large number of characters. This enabled an understanding of the relationships amongst plants as they exist in nature.

·  They classified plants as Gymnosperms (non – flowering) and Angiosperms (flowering). Angiosperms were further classified as monocots and dicots.

·  Initially, natural classifications were based mainly on morphological features and overall similarities using as many taxonomic characters as possible. This helped in placing the closely related taxa together or close to one another.

·  Natural classifications analyse the overall similarity for determining the relationship amongst the taxa.


Phylogenetic classification:

·  After Charles Darwin’s publication of “Origin of Species” (1859) there was a profound change in the outlook of taxonomists.

·  The most significant aspect of the theory of evolution was the concept that “species are not static entities but are the products of evolution”.

·  Therefore, all later classifications were mostly based on the course of evolutionary descent. These tried to reflect the evolutionary sequence /relationship between different groups of organisms and were usually constructed on the basis of natural classification.

·  Such classifications are considered as phylogenetic because they are based on evolutionary history.


Modern system of classification:

·  The, modern classifications are broad based, using data obtained from a number of branches such as morphology, anatomy, embryology, phytochemistry, ultrastructure etc. Hence, this is also referred as Phenetic relationship.

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