REPRODUCTION
What is reproduction?
All living organisms die of old age or are killed by another organism, so if there are no more new individuals, the entire
species will go extinct. Reproduction is the replication and passing on of genes in the creation of new individuals of a
species to replace those that die.
Each species reproduces in a different way, but reproduction itself can only be sexual or asexual.
Asexual reproduction
Asexual reproduction involves only one parent. The parent organism simply produces new cells through mitosis, which
then grow into a new organism that is identical to the parent.
Amoeba reproduce asexually, through binary fission. Just like cells in
other organisms that undergo mitosis when they get too large and
the surface area to volume ratio becomes too small. With amoeba,
in good conditions, division can take place every 2-3 days.
Some plants can reproduce asexually by vegetative propagation.
This is type of asexual reproduction that many flowering plants use to produce offspring. New plants are formed from
an outgrowth of the original plant. These outgrowths can be runners, plantlets or storage organs.
Plantlet
New
plantlet
Bud
Runners
Plantlets
Storage organ. (ginger)
Nodes
Cutting/Grafting
Adventitious roots
There are also artificial methods of vegetative propagation employed by garderners and farmers to increase healthy
stock. These methods also produce plants that are just like their parents. Cuttings are one such method that can
produce many young plants quickly. A shoot is cut off from the parent plant and placed in a nutritious solution, water or
moist dirt where it will grow roots in a few days, after which it is planted in the soil. Grafting is used to grow flowering
or fruit plants but not on its own roots, because its current rooting system is not ideal. To do this, a piece of the plant is
grafted (cut and stuck) onto the root stock of another sort of tree. After some time the two will grow together. The new
plant will have all the features of its parent,but its size will be determined by the host tree. Many fruit trees inculding
citrus and mangoes have been grafted in this way.
Sugarcane is grown from stem cuttings, a stem is cut into sections with a few buds on it and then planted. These buds
grow into identical versions of the parent plant (clones) so all the sugarcane in a field is usually identical and become
harvet ready at the same time.
Tissue culture is another artificial vegetative propagation method. It involves the growth of plant material like cells,
tissues or organs in a suitable medium in sterile lab conditions. This is particularlly useful for growing clones with
advantages like disease resistance, seasonal tolerance, high yield and the propagation of species that cannot be
naturally obtained. It has been usde for bananas in the Caribbean.
Parthogenesis
Some organisms can reproduce on their own.
Sometimes, an egg will develop into a new organism without being fertilized. Known as parthogenesis. Organisms like
Aphids (greenfly) can do this, when they have plenty food, the female produces a large number of eggs that are not
fertilized but develop inside her into young aphids and are born fully developped. Drone bees are reproduced in the
same manner, from unfertilized eggs.
SEXUAL REPRODUCTION
Sexual reproduction involves the fertilization of a female egg by a male sperm. In sexual reproduction, the parent
organism produces sex cells or what we call gametes.
When two of these gametes fuse, its called fertilization. The new cell formed is called a zygote. The zygote undergoes
mitosis (cell division) multple times to grow into a new organism.
Gametes are different from ordinary cells as they contain half the number of chromosomes as regular cells.
Why?
So that when the two gametes fuse, they can together contain the correct number of chromosomes as a normal cell. If
they were to contain the regular amount of chromosomes and fuse, the new organism would end up with twice as
many chromosomes as a regular cell. Having more or less than the regular amount of chromosomes can lead to
mutations.
Humans have 46 chromosomes in each cell of the body, except in human egg and sperm cells; these have 23
chromosomes each. When the egg (23 chromosomes) and sperm (23 chromosomes) fuse, the resulting zygote then as
a full normal compliment of 46 chromosomes. The 46 chromosomes in an ordinary human cell are of 23 different kinds.
i.e there are 2 of each kind. The pair of one kind are called homologous chromosomes. A cell with the full number of
chromosomes is called a diploid cell. The gametes are called haploid, because they only have half the number of
chromosomes.
How are gametes made?
Gametes are made through a process known as gametogenesis. This involves ordinary cells dividing, for example,
human sperm are made when cells in the testis divide. Because gametes must have half the number of chromosomes,
mitosis will not be enough. During gametogenesis, cells divide in a different way, known as meiosis.
MEIOSIS IN AN ANIMAL CELL
In many organisms there are 2 types of gametes: male and female. In humans the male gametes are usually smaller and
mobile, as they search for the female gamete which is larger but isn’t as mobile. i.e male gametes move and females
stay still. In plants the male gametes are found inside the pollen grain, however they cannot move on their own, they
must be carried to female gamete by the pollen tube.
In most cases, only one organism can produce one kind of gamete; males produce male gametes and females produce
female gametes, this determines the sex of the organism. All mammals, then, are either male or female.
However, there are instances where some organisms are capable of producing both types of gametes by themselves.
Earthworms and slugs are an example of this. These types of organisms are called hermaphrodites. Many flowering
plants are also hermaphrodites.
SEXUAL REPRODUCTION IN MAMMALS
Female reproductive organs
The female gametes called eggs or ova, are made in the two ovaries. Leading away from the ovaries are the oviducts,
they’re also called Fallopian tubes. They don’t directly connect to the ovaries, but have a funnel shape near the ovaries
to catch the realeased ova. The two oviducts lead to the womb , known as the uterus. Made of muscle that forms thick
walls; its relatively small, about the size of your fist, but it can expand enormously when the woman is pregnant. At the
base of the uterus there is a narrow opening, guarded by muscles. This is called the neck of the womb or cervix. The
cervix leads to the vagina, which opens to the outside. The opening from the bladder, the urethra, runs in front of the
vagina, while the rectum is behind it. The 3 tubes open very much separately to the outide of the body.
The male reproducive organs
The male gametes called spermatozoa or sperm are made in two testes. These are found outside of the body in two
sacs called the scrotum. The sperm are carried away from each testis in a tube called the sperm duct also called the vas
deferens. The vasa deferentia (plural) join up with the urethra just below the bladder. The urethra continues
downwards and opens at the tip of the penis. The urethra is capable of carrying both urine and sperm at different
times. Where the vasa deferentia joins to the urethra, there is a gland called the prostate gland. This serves to produce
a fluid in which the sperm swim. Just behind the prostate gland are the seminal vesicles, which also secrete a fluid.
The ovaries make the eggs!
Epithelium
Eggs are made from cells in the outer layer or
epithelium of the ovary. A small space with
liquid forms around the cells, together called a
follicle. This happens in females even before
birth; at birth there are already thousands of
follicles inside her ovaries. At puberty some of
these follicles begin developing. Usually only
one at a time. The cell inside the follicle grows
larger and moves to the edge of the ovary.
Now called a Graafian follicle, its about 1cm in
width and bulges from the outside of the
ovary. Inside it, the cell is undergoing meiosis.
However only one of the cells becomes an
egg. The Graafian follicle burst and the egg is
released out of the ovary into the oviduct. This
is the process of ovulation and occurs once a
month in humans. The empty graafian follicle
becomes a corpus luteum.
The testes make sperm!
Tube in
epididymis that
stores sperm
The testes contain thousands of narrow coil like
tubules that make the sperm. Developed from
cells in the walls of the tubules which divide by
meiosis. Unlike eggs, sperm are made
continuously from puberty onward. It is a
sensitive process, particullary to heat. If its too
hot the cells in the tubules will not develop into
sperm. This is why the testes are located outside
of the body, where they are cooler.
How does the sperm get to the egg?
Mating introduces sperm into the vagina, the sperm must then travel, using their tails, up through the cervix, uterus and
into the oviduct where it can then enter the egg i.e fertilize it. The sperm can only travel at a rate of about 4mm per
minute, so it takes a while and many will not reach, because chemicals in the uterus dissolve the sperm. Before the
sperm can reach though, the egg that has left the ovary and entered the oviduct is being wafted towards the entrance
of the oviduct by cilia which line the oviduct. The cilia rhythmically beat to sweep the egg along. This is very slow
process and so muscles in the funnel also push it along by peristalsis.
If the egg isn’t fertlizied by a sperm within 8-24 hours after ovulation, it dies. By this time, it would have only travelled
a short distance in the oviduct. So a sperm as to reach almost near the top of the oviduct for successful fertilization to
occur. When a man is sexually excited, blood is pumped into spaces inside the penis, that makes it erect. To bring the
sperm as close as possible to the egg, the man’s penis is placed inside the vagina of the woman. This is known as sexual
intercourse. Sperm are pushed out of the penis into vagina by muscles in the walls of the tubes containing sperm
contract rhythmically. The wave of contractions begin in the testes, travels along the vasa deferentia and into the penis.
The sperm are squeezed along and out through the urethra into the vagina. A process called ejaculation. The fluid
produced by the prostate gland and seminal vesicles with the sperm is called semen. The ejaculation deposits millions
of sperm to the top of the vagina near the cervix, so there is a good chance that some will reach the egg. However only
one sperm can enter the egg. Onle the head of the sperm goes in; the tail is left outside. The nucleus of the sperm then
fuses with the nucleus of the egg (fertilization). As soon as one sperm enters the egg cell, the membrane of the egg
becomes impenetrable, so no other sperm can enter. All the unsuccessful sperm will die.
Sperm head entering egg cell.
Egg membrane becomes impenetrable.
Implantation of zygote
The final stage of early embryo development is the implantation of the zygote into the endometrial lining of the uterus

The zygote breaches the jelly coat that was surrounding it and preventing its attachment to the
endometrium

Digestive enzymes are released which degrade the endometrial lining, while autocrine hormones
released from the zygote trigger its implantation into the uterine wall
Only once the zygote is embedded within the uterine wall can the next stage of embryogenesis occur

The growing embryo will gain oxygen and nutrients from the endometrial tissue fluid, ensuring its
continued development

The entire process (from fertilization to implantation) takes roughly 6 – 8 days
Implantation in the Endometrium
Zygote
Endometrial lining of
the uterus
When a zygote becomes implanted in the endometrial lining it begins to secrete human chorionic gonadotropin (hCG)

hCG promotes the maintenance of the corpus luteum within the ovary and prevents its degeneration
As a consequence of this, the corpus luteum survives and continues to produce both oestrogen and progesterone

Oestrogen inhibits FSH (follicle stimulating hormone) and LH (luteinizing hormone) production by the
pituitary gland, preventing the release of more eggs from the ovaries

Progesterone also functions to maintain the endometrium (which is nourishing the embryo) and
thicken the cervix
The levels of hCG are maintained for roughly 8 – 10 weeks while the placenta is being developed

After this time, the placenta becomes responsible for progesterone secretion and nourishing the
embryo

At this point the corpus luteum is no longer required and begins to degenerate as hCG levels drop
The Role of hCG and Progesterone During Early Pregnancy
THE PLACENTA
It functions as the life support system for the foetus, serving two key functions:

It facilitates the exchange of materials between the mother and foetus

It secretes hormones to maintain the pregnancy after the corpus luteum has degenerated
Structure of the Placenta

The placenta is a disc-shaped structure that nourishes the developing foetus

It is formed from the development of the trophoblast upon implantation and eventually invades the
uterine wall

Maternal blood pools via open ended arterioles into intervillous spaces within the placenta called
lacunae

Chorionic villi extend into these pools of blood and mediate the exchange of materials between the
foetus and the mother

Exchanged material is transported from the villi to the foetus via an umbilical cord, which connects the
foetus to the placenta

Upon birth, the placenta is expelled from the uterus with the infant – it is then separated from the
infant by severing the umbilical cord (the point of separation becomes the belly button)
Overview of the Structure of the Placenta
Material Exchange

The chorionic villi extend into the intervillous space (lacuna) and exchange materials between the
mother and foetus

Chorionic villi are lined by microvilli to increase the available surface area for material exchange

Foetal capillaries within the chorionic villi lie close to the surface to minimise diffusion distance from
blood in the lacunae

Materials such as oxygen, nutrients, vitamins, antibodies and water will diffuse from the lacunae into
foetal capillaries

Foetal waste (such as carbon dioxide, urea and hormones) will diffuse from the lacunae into the
maternal blood vessels
Exchange of Materials between Foetal and Maternal Blood
Foetus
GESTATION
This is the period of time during which the embryo develops into a fetus and then into a baby. It is not fully understood
how the cells in the zygote arrange themselves to develop and form a baby, but they divide my mitosis and after about
11 weeks start to take shape. (foetus). After this the foetus grows for 9 months before being ready to be born. In this
period of time the foetus is completely dependant on the mother as a source of nutrition and oxygen., which is passed
from mother to foetus thorugh the placenta.
AMNIOTIC SAC
The foetus develops in a fluid-filled space that is encased by an amniotic sac
 The amniotic sac separates the foetus from the mother and hence functions as a barrier against
infection
The amniotic sac is filled with a protective liquid called amniotic fluid, which serves a number of key roles:

The fluid is largely incompressible and good at absorbing pressure, and so protects the foetus from
impacts to the uterus

The fluid also creates buoyancy so that the foetus does not have to support its own weight while a
skeletal system develops

Finally, amniotic fluid prevents the dehydration of foetal tissues
The amniotic sac ruptures when a woman comes to term during labour – this is known as
her ‘waters breaking'
Ultrasound of the Amniotic Sac
CHILD BIRTH
The process of childbirth is called parturition and occurs via positive feedback under hormonal control
Positive feedback involves a response that reinforces the change detected (it functions to amplify the change)

In the case of childbirth, fetal growth eventually causes stretching of the uterine walls, which is
detected by stretch receptors

This triggers the release of hormones (oxytocin) that induce uterine muscles to contract, further
reducing space in the womb

This causes more stretching and hence more contraction until the origin stimulus (the foetus) is
removed (i.e. birth)
The Role of Positive Feedback in the Birth Process
Hormonal Control
The chemical regulators of the birthing process include oxytocin, oestrogen, progesterone and prostaglandin

After 9 months, the baby is fully grown and stretches the walls of the uterus – placing a strain on both
mother and infant

This stress induces the release of chemicals which trigger a rise in the levels of estrogen (estriol in
particular)

Estriol prepares the smooth muscle of the uterus for hormonal stimulation by increasing its sensitivity
to oxytocin

Estriol also inhibits progesterone, which was preventing uterine contractions from occurring while the
foetus developed

Now that the uterus is primed for childbirth, the brain triggers the release of oxytocin from the
posterior pituitary gland

Oxytocin stimulates the uterine muscles to contract, initiating the birthing process (it also inhibits
progesterone secretion)

The foetus responds to this uterine contraction by releasing prostaglandins, which triggers further
uterine contractions

As the uterine contractions trigger the release of chemicals that cause further contractions, a positive
feedback loop ensues

Contractions will stop when labour is complete and the baby is birthed (no more stretching of the
uterine wall)
Hormonal Regulation of the Birth Process
Mammals care for their young
Although it developed for 9 months, a human baby is very much helpless in the first few years of its life. Usually both
parents help to care for it.
During pregnancy the mammary glands in the mother’s breasts became larger. After the birth of the baby, these glands
begin to make milk. This process is known as lactation. Lactation happens in all mammals, but not other species.
The milk contains all the nurtients necessary for the baby as well as antibodies which build the immune system and help
fight infections. In addition to being fed, babies need to be kept warm, due to their large surface area to volume ratio,
because they lose heat quickly. They also need lots of attention and emotional care. In humans this comes with the
addtion of teaching the baby and young child how to take care of itself and live within a society. Humans do this for a
much longer time than any other animal.
The menstrual cycle
Usually , once per month in an adult woman, one egg is released into the oviduct. Before the egg is released though, the
uterus becomes thick and spongy, ready to receive a fertilized egg in it’s thickened lining. It becomes full of many tiny
blood vessels that would supply the embryo with nutrients and oxygen. However if the egg isn’t fertilized, it dies by the
time it reaches the uterus. No implantation occurs and it continues downward into the vagina. Because the thick spongy
layer is no longer required, it beings to break down, gradually disintegrating and slowly excreted out through the vagina.
This is called menstration, or the period as it last for about 5 days. After menstration the uterus walls rebuild to be able
to receive another fertilized egg. The entire cycle is controlled by hormones.
SEXUAL MATURITY
This is reached at puberty, it usually a few years earlier for girls than boys. It’s during the period of adolescence, marked
by sperm production in males and in females,ovulation. During adolescenes the secondary sexual charateristics begin to
develop. In males these include facial hair growth, deepening of the voice, muscular development and pubic hair. In
females they include pubic hair growth, breast development and the widening of the pelvic girdle.
All these changes are brought on by horomones; in boys its androgens, most important being testosterone and in
females its oestrogens.
Sexual maturity starts with social responsibility. The changes that occur bring a heightened sense of personal awareness
and increased attraction between the sexes, which of course is natural. Sexual maturity, however, doesn’t mean that
individual is an adult, mostly due to the fact that the emotional development has not been completed. Despite all this,
its still a time for young persons to take on more social responsibilities and become concerned about issues surrounding
them. Issues like:
-
Birth control
Responsible vs irresponsible parenthood
Promiscuity and its dangers, including cervical cancer development
Sexually transmitted diseases and infections and their effects.
Veneral diseases
Sexually transmitted diseases are referred to as veneral diseases, due to their historical naming after the greek goddess
Venus who was the the goddess of love and sex. Today they are more commonly referred to as sexually transmitted
diseases/infections (STDs/STIs) and there are more than 12 of them. They occur most frequently in persons within the
age gap of 14-24 years old.
Anyone having sexual intercourse or sexual contact with an infected person can become infected. The chances of
becoming infected increases with the number of sexual partners you have. Even more important than the number is the
the type of partner you choose. Individuals that engage in sex indiscriminately or with casual acquaintances have a
much higher probablility of getting an STD/STI, than someone who is more discrete and careful about their choice.
Some of the STDs are: Gonorrhoea, syphilis, Herpes genetalis and AIDS.
BIRTH CONTROL
There are many methods of preventing pregnancy, this is
what we will discuss when we refer to birth control.
Family planning: many countries have organizations set
up to advise people on how to plan their family, i.e how
many kids they should have or when to have them. This
helps to ensure the quality of life for those children and
their parents. At the front line though, family planning
organizations, are there to inform persons on the
different methods of birth control so that people can
prevent unwanted pregnancies.
There are many ways to prevent the fertilization of an
egg.
The use of contraceptives is one of the first. A man can
cover his penis with a condom, which catches the
released sperm when ejaculated, preventing it from
entering the vagina. There are also female condoms that are inserted into the vagina before sexual intercourse. Women
can also use a diaphram or rubber cap cover the cervix that prevent sperm from getting into the oviduct.
Sterilization: If a couple decides to not have any or any more children, one or both of the partners may be sterilized. A
surgery is done to prevent the egg or sperm from leacing the body. In men its called a vasectomy (cutting and tying the
vasa deferentia) and in women, they have their oviducts cut and tied. This is much more difficult, invasive and
complicated operation that isn’t always reversible.
The pill: contraceptive pills contain hormones similar to those produced during the menstrual cycle that prevent the
release of the egg from the ovary. There are many types and each woman has to find the one that suits her.
The coil: this is small spring-shaped device that is inerted into the uterus by a doctor. It disrupts the development of the
uterus lining in preparation for receiving a fertilized egg. This is a reliable method of birth control. Once inserted the
device is called a intra-uterine device (IUD).
Injectable hormones: new methods of brith control are being discovered all the time. Once such method is the
injection of contraceptive hormones, which work similar to “the pill” but must be taken at weekly intervals. Another
type is the “morning after pill”, which can be taken orally and prevents the implantation of the fertilized egg. However
these methods are not sustainable and long terms effects can prevent the normal fucntioning of the body and may even
prevent women from ever becoming pregnant.
The safe period: The safe period or rhythm method, is one of the most common methods of birth control. However this
is not reliable as it depends on a detailed knowledge of the female’s menstrual cycle. While fertilization can only take
place after ovulation and while the egg is still in the female reproductive system, meaning its unsafe to have
unprotected sex 3-4 days before or after ovulation, the menstrual cycle length may vary and there is a measure of
unpredictability. Making this method the most unreliable.
PLANT REPRODUCTION
Understanding:
• Success in plant reproduction depends on pollination, fertilization and seed dispersal
Plants can reproduce in a number of different ways:

Vegetative propagation (asexual reproduction from a plant cutting)

Spore formations (e.g. moulds, ferns)

Pollen transfer (flowering plants – angiospermophytes)
Sexual reproduction in flowering plants involves the transfer of pollen (male gamete) to an ova (female gamete)

This involves three distinct phases – pollination, fertilization and seed dispersal

The transfer of pollen grains from an anther (male plant structure) to a stigma (female plant structure)

Many plants possess both male and female structures (monoecious) and can potentially self-pollinate

From an evolutionary perspective, cross-pollination is preferable as it improves genetic diversity

Fusion of a male gamete nuclei with a female gamete nuclei to form a zygote

In plants, the male gamete is stored in the pollen grain and the female gamete is found in the ovule
Pollination:
Fertilisation:
Seed dispersal:

Fertilisation of gametes results in the formation of a seed, which moves away from the parental plant

This seed dispersal reduces competition for resources between the germinating seed and the parental
plant

There are a variety of seed dispersal mechanisms, including wind, water, fruits and animals

Seed structure will vary depending on the mechanism of dispersal employed by the plant
Plant Reproduction Stages
Understanding:
• Most flowering plants use mutualistic relationships with pollinators in sexual reproduction
Cross-pollination involves transferring pollen grains from one plant to the ovule of a different plant

Pollen can be transferred by wind or water, but is commonly transferred by animals (called
pollinators)
Pollinators are involved in a mutualistic relationship with the flowering plant – whereby both species benefit from the
interaction

The flowering plant gains a means of sexual reproduction (via the transference of pollen between
plants)

The animal gains a source of nutrition (plants secrete a sugar-rich substance called nectar to attract
pollinators
Common examples of pollinators include birds, bats and insects (including bees and butterflies)

Flowers may be structured to optimise access for certain pollinators (e.g. tube-shaped flowers for
birds with long beaks)
Cross-Pollination
Flowers are the reproductive organs of angiospermophytes (flowering plants) and develop from the shoot apex

Changes in gene expression trigger the enlargement of the shoot apical meristem

This tissue then differentiates to form the different flower structures – sepals, petals, stamen and
pistil
The activation of genes responsible for flowering is influenced by abiotic factors – typically linked to the seasons

Flowering plants will typically come into bloom when a suitable pollinator is most abundant

The most common trigger for a change in gene expression is day/night length (photoperiodism)
Flowers are the reproductive organs of angiospermophytes (flowering plants) and contain male and female structures

Most flowers possess both male and female structures (monoecious), but some may only possess one
structure (dioecious)
Structure of a Flower
Flower Structures
The male part of the flower is called the stamen and is composed of:

Anther – pollen producing organ of the flower (pollen is the male gamete of a flowering plant)

Filament – slender stalk supporting the anther (makes the anther accessible to pollinators)
The female part of the flower is called the pistil (or carpel) and is composed of:

Stigma – the sticky, receptive tip of the pistil that is responsible for catching the pollen

Style – the tube-shaped connection between the stigma and ovule (it elevates the stigma to help catch
pollen)

Ovule – the structure that contains the female reproductive cells (after fertilisation, it will develop into
a seed)
In addition to these reproductive structures, flowers possess a number of other support structures:

Petals – brightly coloured modified leaves, which function to attract pollinators

Sepal – Outer covering which protects the flower when in bud

Peduncle – Stalk of the flower
Simplified Flower Drawing