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CLASS 10TH BIOLOGY NOTES
HOW
DO WE ORGANISM REPRODUCE ?
CHAPTER 8th (NCERT)
Contact to:
ASHISH
MATHUR (5 yrs experience)
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Reproduction
Reproduction
is an integral feature of all living beings. The process by which a living
being produces its own like is called reproduction.
Importance of Reproduction:
Reproduction
is important for each species, because this is the only way for a living being
to continue its lineage. Apart from being important for a particular
individual, reproduction is also important for the whole ecosystem.
Reproduction helps in maintaining a proper balance among various biotic
constituents of the ecosystem. Moreover, reproduction also facilitates
evolution because variations come through reproduction; over several
generations.
Types of
Reproduction:
There are two main types, viz.
asexual and sexual reproduction.
Asexual Reproduction: When
a single parent is involved and no gamete formation takes place; the method is
called asexual reproduction. No meiosis happens during asexual reproduction.
Sexual Reproduction: When
two parents are involved and gamete formation takes; the method is called
sexual reproduction. Meiosis happens during gamete formation; which is an
important step of sexual reproduction.
Reproduction
in Simple Organisms
Binary Fission: Most
of the unicellular animals prefer this method for reproduction. These organisms
reproduce by binary fission; especially when conditions are favourable, i.e.
adequate amount of food and moisture is available. Binary fission is somewhat
similar to mitosis. The mother cell divides into two daughter cells; and each
daughter cell begins its life like a new individual. The parent generation
ceases to exist, after binary fission. Amoeba is a very good example of the
organism which reproduces by binary fission.
Fig: Binary Fission in
Amoeba
Multiple Fission: When
conditions become unfavourable, i.e. food, moisture, proper temperature, etc.
are not available; this is the preferred mode of reproduction by unicellular
organisms. The organism develops a thick coating around itself. This is called
cyst. The cyst helps the organism to tide over the bad phase. The nucleus
divides into several nuclei and each daughter nucleus is surrounded by a membrane.
All metabolic activities stop in the organism, after cyst formation. When
favourable conditions return, the cyst dissolves or breaks down; releasing the
daughter nuclei. The daughter nuclei; in turn; grow into new individuals.
Plasmodium and entamoeba undergo cyst stage, when they are not in the body of
their prime host, i.e. humans.
Fig: Multiple Fission
Budding: Yeast
is an example of unicellular organism which reproduces by budding. Hydra is an
example of multicellular organism which reproduces by this method.
Fig: Budding in Yeast
Budding in Yeast: A
small bud grows at any end of the yeast cell. Nucleus gets elongated and a part
of it protrudes into the bud. The nucleus then divides into two nuclei. One of
the nuclei goes into the bud. The bud grows to certain extent and gets detached
from the mother cell.
Fig: Budding in Hydra
Budding in Hydra: A
bud grows anywhere on the main body of hydra. The bud grows to a certain size
and gets detached from the mother hydra. This develops further to grow into a
new individual.
Fig: Fragmentation
Fragmentation: Reproduction
by fragmentation is seen in filamentous algae, e.g. spirogyra. The filament of
spirogyra divides into many pieces and each piece develops into a new
individual.
Regeneration: Some
simple organisms can regenerate a new individual from a part which has been
accidentally cut off. Planaria shows good example of reproduction by
regeneration. If a planaria is cut into several pieces; each piece develops
into a new individual.
Spore Formation: Most
of the fungi, bryophytes and pteridophytes reproduce by this method. Spores are
produced in special spore-bearing organs; called sporangium. When spores
mature; the sporangium bursts open to release them.
Advantages of Spore
Formation: In fact, spores give certain survival benefits to the
organisms which reproduce by spores. Spores can be disseminated through air and
water or even through some other carriers; like animals. This helps an organism
to spread its presence to a wider geographical area. Spores can also remain
dormant for a long time, till favourable conditions are found. Scientists
consider spores are precursors of seeds.
Vegetative Propagation: Vegetative
propagation is a special case, as it happens in higher plants; which otherwise
have the capability to reproduce sexually. When a vegetative part of a
flowering plant reproduces a new plant, it is called vegetative propagation.
Some examples of vegetative propagation are given below.
Tuber of Potato: The
potato tuber is a modified stem. Many notches can be seen on the surface of
potato. These are called ‘eyes’ of potato. Each ‘eye’ of a potato can give rise
to a new potato plant. Farmers utilize this capability of potatoes to grow
potato more quickly; which is not possible by using the seeds of potato.
Modified roots of Carrot and
Sweet Potato: Carrot and sweet potato are examples of modifications of
roots; for food storage. These roots can give rise to new plants; when kept
under the soil.
Rhizomes of Ginger and
Turmeric: Rhizomes are examples of modified underground stems for food
storage. These contain nodes, internodes and scaly leaves. When buried under
the soil, the rhizomes produce new plants.
Leaf of Bryophyllum: Leaves
of bryophyllum have notches on the margin. If a leaf is put under the soil,
small saplings grow from the notches on the leaf margin.
Artificial Vegetative
Propagation: Man has used artificial vegetative propagation to grow many
plants. This has enabled farmers and horticulturists to grow many plants in
shorter duration and has helped them to earn more profit. Artificial vegetative
propagation has also helped in developing many new varieties of plants. Stem
cutting, layering and grafting are the preferred means of artificial vegetative
propagation.
Advantages of
Asexual Reproduction:
- The
organism does not have to depend on another organism for carrying out
reproduction; because a single parent is needed.
- It
takes less time than sexual reproduction and hence more number of
offspring can be produced in shorter time.
- The
offspring are exact clones of their parent.
- Desirable
characteristics can be easily incorporated into plants with artificial
vegetative propagation.
Disadvantages
of Asexual Reproduction:
- As a
single parent is involved, so there is negligible chance of variation.
- In
most of the cases in simple organisms, the parent generation ceases to
exist after asexual reproduction.
- Asexual
reproduction cannot give rise to biodiversity which is important for a
healthy ecosystem.
Sexual Reproduction and Variations:
As
discussed earlier, sexual reproduction involves two parents and gamete
formation. Gametes are special cells which are formed after meiosis. There are
two types of gametes, viz. male and female gametes. The number of chromosomes
is haploid in the gametes. When gametes fuse during fertilization, the number
of chromosomes becomes diploid. This is important for maintaining the unique
identity of a particular species which reproduces by sexual method.
In sexual
reproduction, the offspring gets sets of genes from two different individuals.
This leads to subtle variation through subsequent generations. These variations
accumulate over thousand of generations and finally may give rise to a new
species. That is how all complex organisms have evolved from a common ancestor.
DNA
Replication:
DNA replication is the process by
which DNA makes a copy of itself. DNA replication happens during the S – phase
(synthesis phase) of the cell cycle. This is important because the daughter
cells would need additional copies of the DNA. The process of DNA replication
is a foolproof process, yet some alterations do take place. These alterations
may lead to some variations in the characters of the daughter cells.
Sexual
Reproduction in Flowering Plants:
Flower is a modified leaf which
bears special organs and plays the role of the reproductive system in plant.
Structure of a
typical Flower:
A typical flower is composed of
four distinct whorls, viz. calyx, corolla, androecium and gynoecium.
Calyx: The
outermost whorl of the flower is called calyx. It is composed of green
leaf-like structures; called sepals.
Corolla: The
second whorl of the flower is called corolla. It is composed of colourful
leaf-like structures; called petals. Petals are colourful so that insects and
birds can be attracted; to assist the flower in pollination.
Androecium: This
is the third whorl in the flower. It is composed of stamens. Stamen is made of
a slender stalk and anthers on top. Anthers produce the pollen-grains. Pollen
grains are the male gametes.
Gynoecium: This
whorl is at the centre of the flower. It is composed of a swollen base; called
ovary. A slender style stands upright on the ovary. It has a flat top; called
stigma. Ovules are inside the ovary. Ovules are the female gametes.
Pollination: The
pollen grains need to be transferred to the stigma so that fertilization can
take place. The transfer of pollen grains from anther to the stigma is called
pollination. If the pollen grains from the same flower or the same plant are
transferred to the stigma; it is called self pollination. If pollen grains from
a different plant are transferred to the stigma; it is called
cross-pollination. Cross pollination is better; from the perspective of
variations. Many agents help plants in cross pollination, e.g. insects,
animals, air, water, etc. Insects are the main pollinators for the plant
kingdom.
Fertilisation:
The fusion of male and female
gametes is called fertilization. The product of fertilization is called zygote.
Zygote undergoes several rounds of mitosis and develops into an embryo.
Subsequently, the embryo develops into a new individual.
Fertilization
in flowering plant:
After landing at the stigma,
pollen grains absorb moisture and germinate. A pollen grain develops a pollen
tube; which penetrates through the tissue of the style and reaches the ovule.
Pollen nuclei are transferred through the pollen tube. After fertilization,
zygote is formed; which finally develops into the embryo.
Changes in flower; After
Fertilization: The calyx and corolla wither and fall off and so do the
stamens. The ovary turns into the fruit. The embryo turns into seed. Once the
seed becomes mature, fruit dries up so that dispersal of seeds can take place.
Structure of Seed: A
seed contains an embryo, some reserve food and is enclosed by a protective
covering; called seed coat. The reserve food is stored in the cotyledons. The
embryo has two pointed parts. The upper part is called plumule which gives rise
to the shoot system. The lower part is called radicle which gives rise to the root
system. Cotyledons supply food when the embryo needs it during germination.
Seed germination is the process by which the embryo in the seed kick-starts a
new life.
Male Reproductive System:
The male
reproductive system in human beings is composed of following parts:
Testis: There
is a pair of testes; which lie in a skin pouch; called scrotum. Scrotum is
suspended outside the body; below the abdominal cavity. This helps in
maintaining the temperature of testes below the body temperature. This is
necessary for optimum sperm production. Testis primarily serves the function of
sperm production. Sperms are the male gametes. Apart from that, testis also
produces testosterone. Testosterone is also called the male hormone, as it is
responsible for developing certain secondary sexual characters in boys.
Vas Deferens: Vas
deferens is the tube which carries sperms to the seminal vesicle.
Seminal Vesicle: This
is the place where sperms are stored. Secretions from the seminal vesicle and
prostate gland add up to make the semen.
Penis: It
is a muscular organ which serves the genitor-urinary functions. The urethra
works as the common passage for urine as well as for sperms.
Female
Reproductive System:
The female reproductive system in
human beings is composed of following parts:
Uterus: This
is pear-shaped hollow muscular organ. Uterus is the place where the embryo gets
implanted and develops into a newborn baby. The wall of the uterus provides
safety and nutrition to the growing foetus.
Fallopian Tubes: One
fallopian tube comes out from each side at the top of the uterus. The fallopian
tubes end in finger-like structures; called flimbriae. Fertilization happens in
the fallopian tube.
Ovary: There
are two ovaries; one near each fallopian tube. Ovary produces the eggs or the
female gametes. All the eggs are produces by the ovary when the female child is
still in the womb. One egg matures in each ovulation cycle and is released from
the ovary. The egg is caught by the flimbriae and transferred to the fallopian
tube.
Vagina: The
cervix (mouth of the uterus) opens into the vagina. Vagina is a muscular
tube-like organs; which serves as the passage for the sperms and also as the
canal during the child birth.
Puberty
Human beings are complex animals
and hence there is a distinct phase in their life cycle which marks the onset
and attainment of sexual maturity. This period is called puberty. It usually
starts at around 10 – 11 years of age in girls and at around 12 – 13 years of
age in boys. It usually ends at around 18th year of age in girls and at around
19th year of age in boys. Since the years during puberty end in ‘teens’; hence
this phase is also called teenage.
Changes in Boys during
Puberty: The boys suddenly grow in height dramatically. Voice becomes
deep and the Adam’s apple becomes prominent. Shoulders become broad and body
becomes muscular. Facial hairs begin to grow. Hairs also grow under the armpit
and in the pubic region.
Changes in Girls during
Puberty: The voice becomes thin. Shoulders and hip become rounded.
Breasts get enlarged. Hairs grow under the armpit and in the pubic region.
Sexual Dimorphism: The
physical dissimilarities in the male and female of a species which give them
different appearances is called sexual dimorphism.
Secondary Sexual Characters: Features
which highlight sexual dimorphism are called secondary sexual characters.
Menstruation: Menstruation
is a trait which is unique to humans and some primates. During each ovulation
cycle, the uterus prepares itself in anticipation of a possible pregnancy. The
uterine wall develops an additional lining. When the egg is not fertilized, it
gets disintegrated and so does the additional lining in the uterine wall. The
fragments of disintegrated tissues are shed; along with blood. This is observed
in the form of bleeding through the vagina which can last from 3 to 7 days. The
whole sequence of events during an ovulation cycle is called menstrual cycle.
The bleeding which occurs for few days is called menstruation. The first
menstrual flow is called menarche and the last menstrual flow (which happens in
the late 40s) is called menopause.
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