SICM Tuition
Biology AS
If you learn the previous page, you pretty much have it covered (although there are details
still to learn). From previous lessons, we all know how much you love drawing / colouring
in…so feel free to try to copy that diagram (with the graphs) again and again and again and
again…
Right, so let’s begin from the beginning (which is always a good place to start from). The
menstrual cycle consists of a series of changes that take place in the ovaries (we’re talking
about humans here….not plants) and the endometrium (uterus lining) with objective of
releasing an egg for fertilisation and implantation.
So how does this all happen? Well, like a lot of processes in the body, this is controlled by
chemicals within ourselves. These chemicals are called hormones. Once again, from GCSE,
I’m sure this is all familiar to you.
Hormones are:
chemical substances / messengers
which are produced in an endocrine/ductless gland
and are released directly into the bloodstream
and have an effect only on particular cells (e.g. insulin – liver only)
OK. So what hormones are responsible for the beginnings of life?! FOLP. That’s right. It’s not
a real word…but “allow”. Learn it. It’s a magic word that tells you the order in which the
hormones act for the menstrual cycle.
Follicle Stimulating Hormone (FSH):
I would say stimulates the follicle but that’s just a silly definition isn’t it!
stimulates development of Graafian follicle (basically the developing egg)
stimulates production of oestrogen from the ovary
Oestrogen (labelled estradiol in the previous page as this is the most important oestrogen in
the menstrual cycle):
stimulates repair of uterus lining
stimulate production of Luteinising hormone in the pituitary
Luteinising hormone (LH):
stimulates ovulation
stimulates formation of corpus luteum
stimulates production of progesterone
Progesterone:
stimulates maintenance of uterus lining
inhibits FSH (via negative feedback) – and so inhibits menstruation.
If there is no pregnancy, progesterone stops being produced.
Note that FSH and LH are produced in the pituitary whereas the progesterone and oestrogen
are produced in the ovary.
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Biology AS
causes ovulation and
consequent development of
corpus luteum
causes development of
Graafian follicle
Follicle
stimulating
hormone
PITUITARY
Luteinising
hormone
inhibits
production of
stimulates
production of
stimulates
production of
stimulates
production of
Oestrogen
causes repair of uterus
lining
OVARY
Progesterone
maintains the lining of the
uterus wall in readiness for
the implantation of the
blastocyst
The cycle of hormones: This is slightly complicated, but it’s important for you to learn. Think
about each step in association with the diagram on page 10.
At the beginning of the cycle, FSH is produced at a reasonably high level. This causes the
release of oestrogen. At the beginning, oestrogen has a negative feedback on LH: stopping the
prodcution of LH. However, at some point, this negative feedback turn into positive feedback.
At about 14 days, the oestrogen level rises and the positive feedback causes a rise in LH.
This rise in LH causes the release of the egg from the ovary. This is called ovulation.
After ovulation, the LH level and the oestrogen level decreases (although the oestrogen is also
maintianed as it is also needed for pregnancy). However, the LH has already caused an
increase in progesterone. This progesterone prepares the uterus and fallopian tube for
fertilisation. The progesterone also inhibits FSH production. This stops another follicle (egg)
being made.
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Biology AS
Right, the next part of the syllabus says (and I quote): “describe the
transfer of male gametes leading to fertilisation”. The question
is…where do I start? When the guy buys the girl dinner? When the girl
complains about the place he’s taken her to?? Hmm…
Well, let’s start from the place where the sperm is created: the testes.
On its way out of the male, the sperm takes a journey via various tubules. These include the
rete testis, the epididymis and the vas deferens. On its way through these, fluid is added to the
sperm. This provides nutrients for the long journey it has ahead.
The journey from the testis to the oviduct (fallopian tube) is about 100 000 times its own
length! That’s like a human (about 1.5m tall) travelling 150km!
When the penis is placed within the vagina, various nerve impulses cause the sperm to be
forced out of the testes and out via the urethra in a process called ejaculation. By the time the
sperm has gone through all the tubules, it is able to move using its tail.
However, the sperm is only able to penetrate the ovum (egg) when it has reached the fallopian
tube as there are certain hormones required which are present there. The ovum is surrounded
by something called the zona pellucida which helps make sure that only one sperm can
penetrate the ovum for fertilisation.
If you remember from before, the ovum had previously been held at metaphase 2 and the
second polar body is released when fertilisation takes place. After fertilisation, the cell is
known as a zygote.
After fertilisation has occurred, the cell begins to
divide. Remember that fertilisation has restored the
diploid number. It takes about 3-4 days for the
dividing zygote to move from the oviduct to the
uterus. During this period, it receives its nutrients
from the uterine secretions.
The dividing cell continues until it is about 16 cells.
The shape then changes forming a small group of
cells which will develop into the foetus and a larger
group of cells around the foetus which will make up
the amniotic sac and placenta. This is known as the
blastocyst. The amniotic sac is filled with fluid and
protects the foetus from physical damage. At the time
of birth, this sac breaks: this is commonly known as
the “water breaking”.
During the entirety of pregnancy, progesterone levels are the highest of the four. This is vital
for maintaining the pregnancy. Progesterone has a negative feedback on FSH (and oestrogen)
ensuring that no more follicles are created. If implantation does not occur, the progesterone
levels fall, meaning that the negative feedback on the FSH is no longer present: FSH can rise
again to stimulate another follicle.
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Biology AS
Implantation
in humans, implantation of the foetus is invasive
the blastocyst attaches to the uterus and breaks down the tissue
this is the beginning of the formation of the placenta
The placenta
-
-
this is responsible for taking over the secretion of progesterone from the ovary
develops in 3rd foetal month
connects embryo to walls of uterus
produce villi which grow into uterus wall – providing a large surface area for
exchange between maternal and foetal blood
placenta joins to the embryo via umbilical cord
the umbilical cord contains an artery and a vein
artery is from the foetus – to the placenta
the vein is from the placenta – to the foetus
foetal and maternal blood supplies do not mix
exchange between blood supplies is carried out by
diffusion and active transport
-
the diffusion pathway is short – for rapid diffusion
the blood supply is separated (i.e. do not mix) because:
different blood groups may be present
the blood pressure is higher in the mother (could damage foetus)
in case of infection of mother
the blood of the mother and the foetus travel in opposite directions
-
pregnancy lasts 38 weeks. This is called the gestation period
in the last 2 weeks, the foetus should turn so the head is at the cervix
in the last few weeks, the uterus contracts irregularly
just before labour, amniotic sac bursts and fluid leaks – water breaks
labour begins – uterus contracts regularly and strongly
cervix dilates (i.e. widens)
labour (minutes or hours!) results in baby being forced through birth canal
the baby takes its first breath (due to temperature drop): lungs inflate
umbilical cord is cut. It withers away leaving a scar – belly button
shortly after birth, the placenta detaches and is expelled from the uterus
this is known as the afterbirth
Birth
During the last part of pregnancy, the level of progesterone falls and oestrogen becomes
dominant. This allows oxytocin to act in association with oestrogen causing contractions. The
contractions are what force the foetus out of the uterus, through the vagina.
Lactation (mmm…milk)
Two main points:
Prolactin causes milk synthesis
Oxytocin causes milk ejection
When the baby suckles on the breast, nerve impulses cause oxytocin to be released leading to
milk being ejected from the breast. Prolactin is also stimulated, but this is responsible for milk
synthesis rather than ejection.
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Biology AS
Syllabus checklist
Reproduction in humans
-
describe the structure and functions of the male and female reproductive
systems;
-
describe the production of gametes in oogenesis and spermatogenesis;
-
recall the events in the menstrual cycle; understand the roles of luteinising
hormone, follicle-stimulating hormone, oestrogen, progesterone;
-
describe the transfer of male gametes leading to fertilisation;
-
describe implantation; understand the functions of the placenta in relation to the
development of the foetus;
-
understand birth and lactation, and the roles of oxytocin and prolactin.
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