REPRODUCTION
asexual reproduction:
 A process resulting in genetically identical offspring from one parent
 Population can be increased rapidly/very fast – takes less energy
 Limited genetic variation – prone to extinction – can’t adapt – overpopulation
sexual reproduction:
 A process involving the fusion of the nuclei of two gametes (sex cells) to form
a zygote (fertilized egg cell) and the production of offspring that are
genetically different from each other
 Fertilization: the fusion of gamete nuclei
 Increased genetic variation
 adapt easily
 takes longer time + isolated member of species cannot reproduce
 Gametes:
o
A gamete is a sex cell
o Contain half the number of chromosomes – only one copy – cells have 46
o
chromosomes – gametes have 23 chromosomes
Haploid & diploid cells:
 The nuclei of gametes are haploid – only half – 23 chromosomes
 Nucleus of zygote is diploid – 23 pairs of chromosomes – stays diploid
throughout pregnancy
SEXUAL REPRODUCTION IN PLANTS
 pollination and fertilization:
o
o
pollination is the ways in which pollen grains can be transferred from an anther to a
stigma
fertilization:
 occurs when a pollen nucleus fuses with an ovum nucleus in the ovule
 pollen needs to grow a pollen tube to reach female nucleus
 nucleus inside pollen grain slips down the tube as it grows down the style
towards the ovary
 ovary contains one or more ovules – each contain an ovum with the female
nucleus
 once nuclei fuse and fertilization has occurred – zygote is formed
 zygote starts to divide and eventually forms a seed within the ovule
 self-pollination & cross-pollination:
o
o
cross pollination – pollen from one plant – lands on another plant of same species –
improves genetic variation
occasionally – pollen can land on its own stigma – self-pollination – reduces genetic
variation
the flower - reproductive organ in plants
structures →
sepal: protects unopened flower
petals: brightly coloured – to attract insects
anther: produces and releases the male sex cell (pollen grain)
stigma: top of the female part of the flower which collects the pollen grains
ovary: produces the female sex cell (ovum)
ovule: contains the female sex cells
Insect-pollinated flowers:
wind-pollinated flowers:
 factors affecting germination:
o water allows the seed to swell up and the enzymes in the embryo to start
working so that growth can occur
o oxygen is used so that energy can be released for germination
o warmth as germination improves as temp rises (up to a point) as the reactions
taking place are controlled by enzymes
SEXUAL REPRODUCTION IN HUMANS
In Males
Structures –
1. prostrate gland: produces fluid called semen that provides sperm cells with nutrients
2. sperm duct: sperm passes through the sperm duct to be mixed with fluids produced
by the glands before being passed into the urethra for ejaculation
3. urethra: tube running down the centre of the penis that can carry out urine or semen,
a ring of muscle in the urethra prevents the urine and semen from mixing
4. testis: lies in a bag of skin (scrotum) and produces sperm (male gamete) and
testosterone (hormone)
5. scrotum: sac supporting the testes outside the body to ensure sperm are kept at a
temperature slightly lower than the body temperature
6. penis: passes urine out of the body from the bladder and allows semen to pass into the
vagina of a woman during sexual intercourse
In females
Structures –
1. oviduct: connects the ovary to the uterus and is lined with ciliated cells to push the
released ovum down it. Fertilization occurs in the oviduct
2. ovaries: contains ova (female gametes) which will mature and develop when
hormones are released
3. uterus: muscular bag with a soft lining where the fertilized egg (zygote) will be
implanted to develop into a fetus
4. cervix: ring of muscle at the lower end of the uterus to keep the developing fetus in
place during the pregnancy
5. vagina: muscular tube that leads to the inside of a woman’s body – place where males
penis enters during sexual intercourse and sperm are deposited
gametes:
sperm cell
 has a flagellum tail – so it can swim to the egg
 contains enzymes in head region (acrosome) – to digest through the jelly coat and cell
membrane of the egg cell when it meets it
 contains many mitochondria – provided energy from respiration so it can move back
and forth (locomotion)
 produced every day in huge numbers – around 100 million per day
 very small in size (45 micrometers)
egg cell
 its cytoplasm has a store of energy – needed for dividing the zygote after fertilization
 has a jelly like coating – that changes after fertilization – forms an impenetrable
barrier to prevent other sperm from entering the egg cell
 larger in size (0.15 millimeters)
 incapable of movement
 thousands of immature eggs in each ovary (female is born with them) – only one is
released each month
pregnancy
 fertilization occurs in oviduct – zygote travels towards uterus (takes about 3 days) –
during this time zygote will divide to form a ball of cells known as an embryo
 in the uterus – embryo embeds itself in the thick lining (implantation) and continuous
to grow and develop
 9 month gestation periodo
o
o
o
o
Major development of organs takes place within first 12 weeks – during this time
embryo gets nutrients from the mother by diffusion through the uterus lining
After this point when all organs are in place – placenta is formed – embryo is now a
fetus
Remaining gestation period is used by fetus to grow bigger in size
Fetus is surrounded by an amniotic sac – contains amniotic fluid (made by mothers
blood plasma) – protects the fetus by cushioning it from bumps to the mothers
abdomen
The umbilical cord joins the fetus’s blood supply to the placenta, for the exchange of
nutrients and removal of waste products
 The placenta & umbilical cord
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o
o
o
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Fetus develops and grows by: gaining glucose, amino acids, fats, water and oxygen it
needs from the mothers blood – through umbilical cord
The mothers blood absorbs dangerous waste (urea+Co2) from the fetus’s blood in the
placenta (depends on the size of molecule) – diffuses out
Movement of all molecules across the placenta occurs by diffusion due to difference
in conc gradients
Placenta acts as a barrier to prevent toxins and pathogens from entering the fetus’s
blood – however not all are prevented
Placenta has a large surface area and a thin wall for efficient diffusion
Birth:
 Amniotic sac breaks – releasing amniotic fluid
 Muscles in uterus wall contract to push baby out while cervix dialates
 Baby exits mother through vagina
 Umbilical cord cut after birth (no nerves only 2 blood vessels)
 Placenta pushed out through contractions (known as afterbirth)
HUMAN SEXUAL HORMONES
 Secondary sexual characteristics are changes that occur during puberty which are
controlled by the release of hormones
 Female secondary sexual characteristics:
o
o
o
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Breasts develop
Body hair growth
Menstrual cycle begins
Hips widen
 Male secondary sexual characteristics:
o
o
o
o
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Growth of penis and testes
Growth of facial and body hair
Muscle develops
Voice breaks/deepens
Testes start to produce sperm