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978-0-521-68056-1 - NSSC Biology Module 3
Ngepathimo Kadhila
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Unit 1
Sexual reproduction
in humans
Every hour, the human population increases by 9 000! That is 150
per minute or 2.5 per second! During a woman’s lifetime, she will
release about 400 eggs, each one a potential human being. In a
man’s lifetime, he can produce over 800 000 million sperm! A man
produces from 100 to 200 million sperm at each ejaculation.
In this unit you will learn about the male and female reproductive
systems and how a baby develops inside the uterus. You will also
look at the sex hormones and how they affect what you look like.
We will consider some of the methods of contraception which are
available to couples who wish to space out the births of their children. We also look at ways of increasing fertility in those couples
who wish for children but may be unable to conceive. At the end
of the unit we look at sexually transmitted diseases such as gonorrhoea and syphilis and the transmission of HIV.
This unit is divided into five sections:
A
Male and female human reproductive systems
B
The role of the sex hormones
C
Fertilisation and development of the fetus
D
Birth control and fertility
E
Sexually transmitted diseases
In this unit you will learn to:
• describe the structure and function of the male and female reproductive
systems
• describe how the gametes are formed and relate their structure to their
functions
• describe the role of FSH, oestrogen, progesterone and LH in the menstrual
cycle and pregnancy
• describe the roles of the sex hormones, oestrogen and testosterone in the
development of secondary sexual characteristics
• describe the processes of sexual intercourse, fertilisation and implantation
Glossary
FETUS – the embryo after it has
become recognisable and its
parts distinctly formed; at birth,
and after, the fetus is termed the
baby or infant.
• describe the development of the FETUS
• describe the role of the placenta, amnion and amniotic fluid
• describe the exchange of substances between the blood supply of the
mother and that of the fetus
• explain the importance of ante-natal care
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Cambridge University Press
978-0-521-68056-1 - NSSC Biology Module 3
Ngepathimo Kadhila
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NSSC Biology
• describe the process of birth
• compare the advantages of breast milk over bottled milk
• describe examples of the four main types of birth control
• discuss some of the means of increasing fertility, including test-tube fertilisation and embryo transfer
• describe the symptoms and effects of gonorrhoea and syphilis
• describe methods of transmission of HIV and how it can be prevented from
spreading
• plot data about increase in height in humans
• discuss the increased vulnerability of Namibians to other illnesses due to the
increased prevalence of HIV/AIDS
• outline the socio-economic consequences of the HIV/AIDS pandemic for Namibia.
A Male and female reproductive systems
Hint
One sperm, many sperm. The
sperm are also called
spermatozoa.
In this section we shall look at the male and female human reproductive systems. They both produce gametes and deliver them to
the optimum location for fertilisation. After fertilisation, the
female system on its own protects and nourishes the zygote as it
develops. The testes produce the male gametes, the sperm. The
ovaries produce the female gametes, the ova or eggs. They also produce the male and female sex hormones.
The male reproductive system
The male reproductive system produces the male gametes, called
sperm, and delivers them close to the cervix of the uterus in the
female. How does it do this? Figure 1 shows us the structure of the
male reproductive organs.
seminal
vesicle
bladder
prostate gland
Cowper’s gland
penis
Hint
The vas deferens is often called
the sperm duct.
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epididymis
erectile
tissue
sperm duct (vas deferens)
urethra
testis
scrotum
Figure 1 The structure of the male reproductive organs
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Cambridge University Press
978-0-521-68056-1 - NSSC Biology Module 3
Ngepathimo Kadhila
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Module 3 Unit 1
3
The sperm are made in the testes which hang outside the body in
a bag of skin called the scrotum. This keeps the testes about 2 ˚C
cooler than normal body temperature which would inhibit sperm
production. You can see the scrotum and one testis in Figure 1.
Hint
One testis, two testes.
ACTIVITY 1
The testes consist of many tightly coiled tubules where the sperm
are made. The sperm are passed to the epididymis, which lies on
the top of the testes. Sperm mature in the epididymis and become
mobile. The epididymis leads to the sperm duct, the vas deferens.
The sperm then travel through the sperm duct and secretions are
added from different glands, such as the seminal vesicles and
prostate gland. For example, secretions from the seminal vesicles
provide energy for the sperm. The final liquid is called the semen
and passes into the urethra which opens at the tip of the penis.
Spend about 5 minutes on this activity.
Complete the spaces 1 to 6 in the table to show the structure
and function of the parts of the male reproductive system.
structure
function
testes
1
2
bag of skin outside the body which holds testes
at lower temperature than the rest of the body
penis
3
4
tube through which sperm travel from the testes to
the urethra
urethra
5
6
both secrete nutrients and enzymes to stimulate
sperm; fluid from these glands, together with sperm,
is called semen
The female reproductive system
Hint
One ovum, many ova.
Ovum = egg.
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The female reproductive system produces the female gametes,
the ova. It provides a suitable site for semen to be deposited and
for the ovum to be fertilised. After fertilisation it provides a place
for the zygote to develop into a fetus. The muscular walls of the
uterus expel the fetus at the end of pregnancy.
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978-0-521-68056-1 - NSSC Biology Module 3
Ngepathimo Kadhila
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NSSC Biology
Figure 2 shows the female reproductive organs.
funnel-shaped end
of oviduct
oviduct
ovary
uterus lining
muscular uterus wall
cervix
vagina
Figure 2 The structure of the female reproductive organs
Hint
The oviduct is sometimes
described as the egg tube. The
uterus is often described as the
womb.
ACTIVITY 2
The ovaries produce the ova, the female gametes. The ovum
travels down the oviduct to the uterus, a muscular organ richly
supplied with blood vessels. This is where the fetus will develop. At
the neck of the uterus is the cervix, a narrow passage that leads to
the vagina, a muscular tube. A thick mucous membrane lines the
muscular walls of the vagina. The vagina opens to the outside at
the vulva.
Spend about 5 minutes on this activity.
Complete the spaces 1 to 5 in the table to show the structure
and function of the parts of the female reproductive system.
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structure
function
ovary
1
2
narrow tube through which ova travel to uterus
uterus
3
4
ring of muscle at the neck of the uterus
vagina
5
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Cambridge University Press
978-0-521-68056-1 - NSSC Biology Module 3
Ngepathimo Kadhila
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Module 3 Unit 1
5
Structure of the gametes
Structure of the sperm
Hint
You also read about these terms
in Module 2 Unit 8.
Figure 3 shows a sperm, the male gamete. The sperm is able to
swim by moving its long tail from side-to-side. Most of the head of
the sperm contains the genetic material, which has the haploid
number of chromosomes, 23.
length 0.05 mm
head
middle piece,
containing
mitochondria to
release energy
for swimming
tail, which produces
swimming movements
nucleus, containing chromosomes
vesicle containing
enzymes that
digest their way
into the ovum
Figure 3 The structure of a sperm
The front of the head has a vesicle which contains enzymes. The
sperm uses these enzymes to digest its way into the ovum. The
middle piece, or body, of the sperm contains mitochondria, which
releases energy by oxidising the nutrients in the semen.
Structure of the ovum
Figure 4 shows an ovum, the female gamete. The ovum is also a
single cell and has the haploid number of chromosomes, 23.
diameter 0.1mm
cell membrane
layer of jelly
nucleus containing
chromosomes
cytoplasm
containing yolk
Figure 4 The structure of an ovum
The ovum is much larger than the sperm. The ovum can be larger
because it does not move on its own. It also needs greater reserves
of energy, especially in the form of fatty substances called lipids, for
the development of the zygote. The layer of jelly around the ovum
ensures that only one sperm can penetrate. This is discussed later
in the section on fertilisation.
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Ngepathimo Kadhila
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NSSC Biology
Formation of gametes
The formation of the gametes differs in males and females; but in
both cases, haploid gametes are formed by meiosis.
Figure 5 summarises the steps that occur in the formation of
gametes for both males and females.
The cells in a human
body each contain
46 chromosomes
body
cell
meiosis
body
cell
In sexual reproduction,
cells in the testes and
ovaries divide by
meiosis, producing
gametes with half
the number of
chromosomes
meiosis
sperm
23
23
ovum
fertilisation
Male and female
gametes fuse
to form a zygote
which has the full
number of chromosomes
46
zygote
Figure 5 Stages in gamete formation in males and females
Formation of sperm
This process of sperm production begins at puberty, the time when
a boy begins to develop into a man. It is stimulated by two hormones, the follicle-stimulating hormone (FSH) and
testosterone. FSH is released from the pituitary gland in both
males and females.
Glossary
SPERMATIDS – immature sperm
cells
LUMEN – the cavity in a tubular
organ
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Sperm are produced in the testes. Special cells line the coiled
tubules in the testis. These cells divide by mitosis to produce
diploid primary spermatocytes. These primary spermatocytes
then divide by meiosis to produce haploid secondary spermatocytes. These divide to produce haploid SPERMATIDS . Spermatids
move into the LUMEN of the convoluted tubules as shown in
Figure 6.
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978-0-521-68056-1 - NSSC Biology Module 3
Ngepathimo Kadhila
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Module 3 Unit 1
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Figure 6 shows how sperm are made.
a A section through
a testis
tube in epididymis,
where sperm
are stored
tube where sperm
are made
sperm duct
b A section through
a sperm tube
lumen
wall of tubule
cell dividing by meiosis
to make sperm cells
spermatids
Figure 6 How sperm are made
Hint
Figure 3 showed the structure of
a sperm.
In the lumen the spermatids attach themselves to nutritive Sertoli
cells. Here they develop into mature sperm cells which have a
head, mid-piece and a tail. Once they have developed, they swim
freely in the lumen of the tubule. From there the sperm go to the
epididymis where they are stored. The sperm may stay in various
parts of the male reproductive tract up to five weeks after they are
formed.
Formation of ova
Ova are produced in the ovary. The outer layer of the ovary is germinal epithelium. Some germinal epithelium cells migrate to the
interior of the ovary where they may produce primary follicles.
These follicles produce one ovum each. In each follicle, diploid
cells divide by mitosis and develop into diploid primary oocytes.
This process is usually completed when the female baby is born.
The primary oocytes then undergo the first stage of meiosis. In
this state they then wait until puberty when a girl begins to
develop into a woman.
Once a month, the primary oocytes divide by meiosis to form
haploid secondary oocytes. Each secondary oocyte is contained in a
swollen follicle, called a Graafian follicle as shown in Figure 7.
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NSSC Biology
Figure 7 shows how ova are made in the ovary.
2 The space round the developing
ovum gets bigger and fills with
fluid. A Graafian follicle is formed
1 Some of the epithelium cells
divide to make cells which will
develop into ova
epithelium
of ovary
developing
ovum
4 The empty Graafian follicle becomes
a corpus luteum
3 Once a month a Graafian follicle
bursts and the ovum is released into
the oviduct. This is called ovulation
Figure 7 How ova are made
The Graafian follicle, and its development into the corpus luteum,
also produce hormones. These will be covered in greater detail in the
section on the menstrual cycle. The Graafian follicle migrates to the
surface of the ovary and bursts to release the secondary oocyte. This
is the process of ovulation. The ovum is now floating freely at the
entrance to the oviduct.
ACTIVITY 3
Spend about 10 to 15 minutes on this activity.
Look at Figure 8 which shows the gametes.
diameter 0.1 mm
length 0.05 mm
head
sac containing
enzymes
membrane
nucleus
nucleus
middle
piece
cytoplasm containing yolk
jelly coat
tail
Figure 8
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Cambridge University Press
978-0-521-68056-1 - NSSC Biology Module 3
Ngepathimo Kadhila
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Module 3 Unit 1
9
Answer these questions in your notebook.
1 Write down three differences between a sperm and an ovum.
2 Suggest why the ovum is larger than the sperm.
3 Why is it important that a very large number of sperm should
be present in the semen?
4 Why is it an advantage for the testes to be situated in the
scrotum outside the body? Can you think of any disadvantages?
5 By drawing a line, match the words in the left-hand column
with the correct descriptions in the right-hand column. The
first one has been done for you.
a testis
male sex cells
b sperm
male sex organ in an animal
c ovary
the fusion of a male and female gamete
d fertilisation
female sex organ in an animal
e zygote
female sex cell
f gamete
a fertilised ovum develops into this
g ovum
the biological name for a sex cell
h embryo
a fertilised ovum
6 Cover up Figure 8 and draw labelled diagrams to show the
structure of a human ovum and a human sperm.
B The role of the sex hormones
The female hormones are progesterone and oestrogen. The
male hormone is testosterone. The function of testosterone is
dealt with in the next section. Let us see how progesterone and
oestrogen are connected with the menstrual cycle.
Hormones and the menstrual cycle
The menstrual cycle is normally a 28-day cycle. Changes in the
levels of oestrogen and progesterone in the blood control the menstrual cycle. The pituitary gland produces hormones which
stimulate the ovaries to produce oestrogen and progesterone. In
this way the amount of oestrogen and progesterone in the blood is
controlled.
The menstrual cycle starts with menstruation, the flow of blood
caused when the lining of the uterus breaks down and passes out
of the body. The ovaries produce little oestrogen or progesterone at
this time.
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978-0-521-68056-1 - NSSC Biology Module 3
Ngepathimo Kadhila
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NSSC Biology
Figure 9 shows the changes that take place in the uterus during
the menstrual cycle.
1 A follicle develops inside
the ovary. The uterus lining
is repaired and begins to
thicken.
2 Ovulation
The follicle bursts, releasing
an ovum from the ovary.
Fertilisation could take place.
4 Menstruation
As the ovum has not
been fertilised, the
thick uterus
lining is not needed.
It breaks down, and
is gradually lost
through the vagina.
corpus luteum
3 The follicle develops
into a corpus luteum. The
lining of the uterus
becomes thicker and well
supplied with blood
capillaries, ready to
receive the embryo if the
ovum is fertilised.
Figure 9 The menstrual cycle
Look at the top of Figure 9 and see where the follicle develops
inside the ovary. Follow the numbered arrows round to ‘Ovulation’
and ‘Menstruation’ as you read the points below.
• During menstruation a Graafian follicle begins to develop
in the ovary. A week after menstruation this follicle is producing oestrogen. Oestrogen stimulates the repair and
growth of the uterus lining, which begins to thicken.
• Usually around day 14 of the cycle, ovulation occurs and
an ovum is released from one ovary. The ovum travels
along the oviduct to the uterus. If sperm are present in the
oviduct as a result of intercourse then the ovum becomes
fertilised. An unfertilised ovum is passed out of the body.
• After ovulation the Graafian follicle develops into a corpus
luteum. It no longer produces oestrogen but begins to produce progesterone.
• Progesterone maintains the uterus lining as a thick and
spongy layer, which is ready to accept a fertilised ovum.
• If the ovum is not fertilised, the corpus luteum gradually
shrinks and produces less progesterone. The low level of
this hormone causes the uterus lining to disintegrate and so
menstruation begins again.
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