Biology-Notes

Chapter-8: How Do Organisms Reproduce?

Grade-X
Reproduction: It is the process of producing new individuals of the same species by existing
organisms of a species, i.e., parents.

Importance of reproduction-
 It ensures the continuity of life on earth
 It plays an important role in evolution by transmitting favourable variations of the
organisms from one generation to the next.
Importance of DNA copying in the process of reproduction
 Characteristics of parent organisms are transmitted to its offspring’s and at the
same time occasional variations are produced in the organisms.
Importance of variation
 It helps the species of various organisms to survive and flourish even in
adverse conditions.
 It provides stability to the populations of various species by preventing them
from getting wiped out during adverse conditions.
Modes of reproduction
1. Asexual reproduction
2. Sexual reproduction
Differences between asexual &sexual reproduction
Asexual reproduction Sexual reproduction
1.It involves the production of an 1. It involves the production of an
offspring from a single parent with out offspring
the fusion of gamets from two parents with the fusion of
2. Simplest method of reproduction. gametes.
3. New organisms ,do not show much 2. Advanced method of reproduction
variations 3. New organisms show variations
Modes of asexual reproduction
 1. Binary fission – it is the division of one cell into two similar or identical cells.
 In this method, the nucleus first divides into two, followed by the division of the
cytoplasm.
 The cell finally splits into two daughter cells.
 In plants, binary fission can be seen in bacteria, yeast and euglena.
 Among animals, amoeba and paramecium reproduce through binary fission. (Refer
Figure 8.1 Page no.129)

2. Multiple fission – in multiple fission, many individuals are formed from a single individual.
 The nucleus of the cell divides repeatedly, producing many nuclei.
 Each nucleus is surrounded by a small amount of cytoplasm and many daughter cells are
produced within the cyst.
 The cyst breaks up under favorable conditions and small offspring are liberated.
E.g.: In plants – algae
In animals – plasmodium

3. Regeneration
It is the ability of the fully differentiated organism to give rise to new individual organism from
its body parts.
E.g. – Hydra and Planaria. (Refer Figure 8.3 Page No.131)
 4.Budding
In budding, a small part of the body of the parent grows out as a bud which then detaches
and becomes a new organism.
Eg : hydra reproduces by budding using the regenerative cells .(Refer Figure 8.4 Page
No.131)

VEGETATIVE PROPAGATION –
In vegetative propagation new plants are obtained from the parts of old plants like stems,
roots and leaves without the help of any reproductive organ. The different types of vegetative
propagation are:

.Plants reproduce through stem , roots and leaves.

STEM
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.

a) Cutting – in this type of propagation any part of the plant root, stem or leaf is cut and
buried partly in the moist soil. Eg: rose plant, chrysanthemum, grapes are propagated by
means of cutting.

b) Layering – the adventitious roots are produced in the branch of the stem before the plant
is detached from the parent plant. The branch of stem is called layer. This process is
utilized in the propagation of plant and the phenomenon is called layering. The process is
used in the propagation of plants like lemon, guava, hibiscus, bougainvillea, jasmine ,
raspberry , strawberry and many ornamental plants .
c) Grafting – in this method of reproduction, two plants of closely related varieties are
joined together so that they live as one plant.
 The portion of a plant that is grafted on the other plant is called scion and the plant in
which grafting is performed is called the stock.
 This method is applied to improve varieties of fruits like mango, apple, peas, citrus
and guava.

ADVANTAGES OF VEGETATIVE PROPAGATION :

 Vegetative propagation is a cheaper, easier and more rapid method of propagation in
plants than growing plants from their seeds .
 The traits or characters of the parent plant are preserved by vegetative propagation.
  Better quality of the plants can be maintained by this method.
 It results in propagation of those plants which do not produce viable seeds or produce
seeds with prolonged period of dormancy.
 The plants generated from vegetative means require less time to grow and have the
advantage of being more uniform and genetically similar to the parent stock.

DISADVANTAGES OF VEGETATIVE PROPAGATION :

 Vegetative propagation induces over – crowding
 There is no genetic variation so there is less adaptability to the environment.
 The disease of the parent plant gets transferred to the offspring.
 The plants lose vigor.
 New characters can neither be introduced nor undesirable characters be eliminated.

TISSUE CULTURE
It is the production of new plants from isolated plant cells or small pieces of plant tissue in a
synthetic medium of culture solution.
 This technique is also known as micro propagation in vitro because it takes place outside
the body of the parent plant in a test ube using an artificial environment.
 Micro propagation technique is being used for the production of ornamental plants like
orchids, dahlia and carnation.

SPORE FORMATION
While a slice of bread is kept in moist dark place for a few days spore of Rhizopus present in air
settle on the bread to form new fungus plants of Rhizopus.

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: ( Refer Diagram – Fig,8.7 pg 134)

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: (Refer DIAGRAM - Fg, 8.8 – pg, 135)

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: (Refer Diagram - Fig.8.9 Pg. 135)

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.

Reproduction in Human Beings

Male Reproductive System: Fig. 8.10 Pg. 137)
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:Fg. 8.11 pg. 137
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.
. 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

Functions of placenta:-
.The Embroyo gets nutrition fromthe mother's blood with the help of a special
tissue called PLACENTA.
It provides a large surface area for glucose and oxygen to pass from the mother to the embryo.
Similarly the wastes from developing embryo are removed to mother's blood through placenta.
The child is born as a result of rhythmic contractions of the muscles in the uterus. after Nine
months (36 weeks) of development inside mother's womb. It is also called Gestation Period.

The sexual cycle in a woman continues upto the age of 45 to 50 years. After that the ovary do not
release egg. This stage is called Menopause. It a also marks the end of menstruation in the
woman.

REPRODUCTIVE HEALTH

Reproductive Health means a total well-being in all aspects of reproductive, ie., physical
emotional, social and behavioural.

Contraception :
It is the avoidance of pregnancy. It can be achieved by Methods of contraception

Methods of contraception
1. PHYSICAL BARRIER -To prevent union of sperm & egg. Use of condoms, Diaphragm &
cervical caps.
2.SURGICAL METHOD-
Also called sterilization in Vasectomy, the vas deferens of male is blocked to prevent
sperm transfer.
In Tubectomy, the fallopian tube of female is blocked to prevent egg to reach uterus.
Copper-T or loop is placed in uterus to prevent pregancy.

3.CHEMICAL METHOD-Oral contraceptive (OCs) - changes the hormonal balance to

check the egg release in females. OCs cause side effect.
Healthy society needs a balanced sex ratio that can be achieved by educating the people
to avoid malpractices like female foeticide & pre-natal sex determination.

Sexually Transmitted Disease (STDs)- STDs are communicated during unsafe sexual
contact.

1.VIRAL STDs- Eg. H.I.V. – AIDS Warts

2.Bacterial STDs Eg. Syphilis & Gonorrhoea

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

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.