Reproduction: Biology O/L

Reproduction is a characteristic of all living Organisms. It is the process that makes more
of the same kind of organism. Reproduction is essential in all species to ensure that the
species does not extinct. There are two types of reproduction, Asexual and sexual
reproduction.

Asexual Reproduction:
Asexual means not sexual. This means that this kind of reproduction does not involve
sex. Asexual reproduction is the production of genetically identical offsprings from one
parent. It is simply a single organism growing a new organism from itself.

Asexual Reproduction in Bacteria:

Bacteria are tiny single-celled organisms. They reproduce by a process called binary
fission. In binary fission, one bacterium grows and exact copy of it’s DNA coil which
carries its genetic information. Then the bacterium completely divides witch one DNA coil
in the parent and one in the daughter bacterium. Each1 bacterium can undergo binary
fission once every 20 Minutes making them able to reproduce massive numbers from
one parent in very little time.

Asexual Reproduction in Fungi:

Earlier, you’ve studied that fungi are multi-cellular organisms that grow long threads
called hyphae on pieces of food. There are two types of hyphae, reproductive and
feeding hyphae. Reproductive hyphae grow vertically above the food material. At the top
of the hyphae, there is a spherical bag in which many spores are grown. This bag is
called sporangium. Spores being produced in the sporangium are reproductive
structures that can grow into another fungus. At some point, the sporangium will burst
open dispersing the spores into the air. If a spore falls on an area of favorable conditions
(food – water – air) it will germinate and grow into a new identical fungus.

Asexual Reproduction in Potato Plants:

A potato plant starts as a lateral bud (seed) under the soil. In favorable conditions, a
shoot grows out from the bud vertically, which then becomes the stem of the plant
above the soil. Roots also grow out of the bud downwards into the soil. The bud is
now a plant. The stem then grows a part of it under the soil, which is called
underground stem. Swellings start to grow from the underground stem, these
swellings are called tubers which are the new potatoes. Glucose formed by
photosynthesis in the leaves of the above ground stem is converted into sucrose and
transported down the stem to the tubers to be stored there as starch. When the
tubers are fully grown the mother plant dies and the new lateral buds form on the
tubers. These buds then grow into new potato plants identical to the mother plant.

Sexual Reproduction:
 Sexual reproduction is the process involving the fusion of haploid nuclei to form a
diploid zygote and the production of genetically dissimilar offspring.

Sexual Reproduction in Flowering Plants:

In a plant, the organs which are responsible for sexual reproduction are the flowers.
The diagram below shows a typical flower. Sex cells are called gametes. The male
gametes are produced inside the anther. And the female gametes are produced
inside the ovules. Some flowers can produce both male and female gametes. They
are called hermaphrodite. Male gametes are inside pollen grains in the anthers.

Methods of Pollination:
Pollination is the transfer of pollen grains from the male part of a flower to the female part
of a flower.

There are two methods of pollination, insect pollination and wind pollination. Some
flowers pollinate by insects while others pollinate by wind.
Insect pollinating flowers have special attractive features like brightly coloured petals,
attractive scents and sugary nectar. These features’ aim is to attract insects like bees to
come and collect their nectar. While an insect is collecting the nectar, its body will touch
the anthers. The pollen grains of insect pollinating flowers have hooks and spikes all
over them in order to stick to the bodies of the insect that touch it. When this happens,
the pollen grain sticks to the body of the insect. When the insect moves on to another
flower to collect its nectar, the pollen grain falls off the insect onto the stigma of the
flower, thus insect pollination took place.

Wind pollinating flowers however look very differently to insect

pollinating ones. This is because they do not need attractive features such as bright
coloures and scents and nectar to attract insects. Instead, their petals are green or
brown with their anthers and stigmas hanging outside the flower to be exposed to the
wind. When wind is strong enough, it will blow the pollen grains off the anther and carry it
along. At some point the wind will get weaker and the po en grain will land, if it lands on
the stigma of a flower of the same species then the pollination was successful. The
surface of pollen grains of ind pollinated plants is smooth because it does not need
spikes to catch on insects.

Self Pollination and Cross Pollination:

Self and cross pollinations are the two types of pollination.
Self-pollination: The transfer of a pollen grain from the anther to the stigma of the same
flower or the stigma of a flower of the same plant.
Cross Pollination: The transfer of a pollen grain from the anther to the stigma of a
flower of another plant of the same species.
Self pollination is considered sexual reproduction because it involves the fusion of two
gametes together even though they might come from the same flower or plant.

Fertilisation:
If a pollen grain lands on the stigma of a flower of the same pecies, fertilization takes
place. This starts by the stigma secreting chemicals that start what’s called a pollen tube.
A pollen tube is the pathway in which the pollen moves downwards to reach the ovule. If
a pollen grain falls on the stigma of a flower of a different species, the stigma will not
secrete these chemicals and fertilization
fails. At the tip of the pollen tube, enzymes are secreted that digest the tissue of the
style digging the pollen tube further down till it opens on the other end in the ovule. The
male gamete enters the pollen tube and starts its journey downwards to the ovule.

When the male gamete reaches the ovary, it enters the ovule through a whole in its
outer shell. The male gamete, which is a haploid nucleus starts fusing with the ovule
producing a diploid zygote, then develops into an embryo plant.

Seeds and Fruits:

During the early stages of fertilization, some parts of the fruit become useless, they die
and fall off. These parts include the sepals, the petals and the stamens.

The fertilized ovule becomes a seed. Inside the seed is the zygote which develops into
an embryo plant. An embryo plant has shoot called plumule and a root called radicle.
The plant makes food for the seed and brings it to the seed where it is stored in a
structure called cotyledon inside the seed. The outside layer of the ovule becomes
thicker and harder and is now called the testa. In the seed there is a whole called the
micropyle, this is the whole through which the male gamete entered the ovule. When
the seed is fully grown, it becomes dormant. That means it loses its water and
metabolic reactions inside it stop. These reactions are reactivated when the seed is
planted in favourable conditions including water and air, this is when the seed grows
into a new plant. This enables the seed to survive hostile conditions until it is put in
somewhere where it can grow into a plant.
The ovary of a flower contains many ovules. The ones that are fertilized turn into seeds
and the ovary its self turns into a fruit in which the seeds are. The function of the fruit is
to protect the seeds and to disperse them from the parent plant to colonise new area.

Seed Dispersal:
Seed dispersal is the scattering of the seeds all over the place to colonise he area. It
always the plants to grow in new places and contribute to variation of species. A plant
can use any of two methods of seed dispersal. These methods are wind dispersal and
animal dispersal.

Wind dispersal depends on the fruit having wind or parachute like structure that allows it
to float in air to be carried by wind to distant areas in order to spread the species.

The photo on the left is of a dandelion plant. Its fruit

Grows feathery threads that make it fly over long
Distances before landing on the soil.

The photo on the left is of a sycamore plant. Its fruit

Grows two wing like structures that make it spin and
Fly to distant areas before landing in the soil thus
Spreading the species.

Other plants rely on animal dispersal. In this method

The fruit has hooks all over it which catch in the fur of
Animals. The animals move from place to place taking
The fruit with it until at some point it falls somewhere
Away from the parent tree. An example of this type of

Plants is the burdock plant

Seed Germination:
A seed remains dormant until it is put in suitable conditions to start growing.
These are:
• Water
• Air (oxygen)
• Suitable temperature
• Some need Sunlight
If the seed is put in an area of these conditions it will start germinating.
Germinating includes the following steps:
1. The water activates enzymes stored in the seed
2. Enzymes start to digest the starch stored in the seed into sugars
 3. The sugars produced taken to the embryo and is used as an energy source
for growth
4. The radicle (root) grows first, fixing the seedling into the soil where it can
absorb more water.
5. The seed is now called a seedling
6. Then the shoot grows vertically.
7. Once the shoot grows the first green leaf the seedling becomes fully
independent. (before that it was depending on its storage materials) The plant is
now able to photosynthesis and make its own food

Sexual Reproduction in Humans:

In humans, the female sex cell (gamete) is called an egg or ova (Plural: Ovum). The
male sex cell in humans is called a sperm. Sexual reproduction in humans starts by a
sexual intercourse followed by the fusion of the two gametes together forming a zygote
(Fertilisation).

The Female Reproductive System:

The female reproductive system is located under the stomach. It consists of the
following parts:
• The Ovaries: There are two ovaries. One on each side. They contain follic
es where eggs are produced.
• Oviducts (Fallopian Tube): They are two tubes, one on each side connected to an
ovary. They are where fertilization occurs and they provide a pathway for the eggs
to travel to the uterus by sweeping them by cilia on its walls.
• Uterus (Womb) : Where the fetus develops
• Cervix: A muscular tissue which separates the vagina from the uterus.
• Vagina: it receives the male penis during sexual intercourse

The Male Reproductive System:

The male reproductive system is also located under

the stomach. It consists of the following parts:
• Testes: a male human has two testicles. A testes is
a male gland which produces sperms and the male
sex hormone testosterone:
• Scrotum: it is the sac which contains the testicles
• Sperm Ducts (Vas deferens): They are two muscular
tubes, each connected to a testis. They carry the
sperms from the testis to the urethra
 • Prostate Gland: It secretes a nutritive fluid to the
sperms to form a mixture called semen.
• Urethra: It is a tube inside the penis which is the
pathway of semen and urine out of the body.
• Penis: It is the male sex organ which ejaculates
semen into the vagina during sexual intercourse.
Epididymes: coiled tubes in which sperms are stored.

• Seminal vesicle: it is another gland like the

Prostate gland. It also secretes nutritive fluids for sperms to feed from
and swim in forming semen.

The testicles are inside the scrotum which hangs outside the body. This
is because testicles work best at a temperature below that of the body.

Comparing Male and Female Gametes:

Eggs are much bigger than sperms. This is because it needs space to store
nutrients on which the embryo feeds on before it reaches the uterus. Eggs
being very large makes it wasteful to make plenty of them, this is why a woman
has one egg at a time. Sperms however are much smaller and in larger
quantities to increase The chance of successful fertilization. Sperms feed on
Nutrients in the semen fluid. Eggs are unable to move By them selves, they are
swept to the uterus by cilia in the walls of the oviduct.

On the other hand, sperms have long tails which helps them swim their way to
the egg. A sperm also has a large number of mitochondria to release lots of
energy to be used in swimming.