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School biology: Puberty, menstrual cycle, fertility control, contraception, IVF

Hormone systems - menstrual cycle - controlling fertility - use of hormones - methods of contraception - IVF fertility treatments

 Doc Brown's school biology revision notes: GCSE biology, IGCSE  biology, O level biology,  ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old students of biology

 This page will answer many questions e.g.  What are hormones?  How does the menstrual cycle work?  How does In Vitro Fertilisation' (IVF) work?  How can hormones be used to control fertility?  What are FSH and LH hormones and what do they do?  Describe various methods of contraception and their effectiveness.

Sub-index for this page

(a) A brief reminder about hormones

(b) Puberty and the female menstrual cycle

(c) Using hormones to control female fertility

(d) Non-hormonal methods of preventing pregnancy

(e) Contraceptive evaluation - comparing methods of contraception

(f) Methods of increasing fertility - increasing the chance of pregnancy


See also Hormone system - Introduction to the endocrine system  gcse biology revision notes

For plants see Hormone control of plant growth and uses of plant hormones  gcse biology revision notes



(a) A brief reminder about hormones

(see also endocrine system and introduction to homeostasis for other notes and links)

Hormones are produced by and excreted from endocrine glands and are transported by the blood to their target organs

Know and understand that many process within the body are coordinated and controlled by chemical substances called hormones.

Hormones, being directly released into the blood, are quite rapidly carried to all parts of the body BUT only affect the function of particular cells.

Hormones effectively act as 'chemical messages' to trigger particular biochemical reactions.

Their effects are relatively long-lasting compared to e.g. the nerve impulses and responses of reflex arc,

but both the nervous system and hormones also help us to control conditions inside our bodies.

Know that hormones are used in some forms of contraception and in fertility treatments.

The pituitary gland produces the hormones FSH and LH which are important control chemicals in the female menstrual cycle as are the hormones oestrogen and progesterone.

The oestrogen, progesterone and testosterone are steroid hormones - along with others that control glucose levels in the blood, water balance in the body and protein metabolism - they are all very 'busy' molecules!

See also

Cell division - mitosis, meiosis, sexual/asexual reproduction

Inherited characteristics and human sexual reproduction


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(b) Puberty and the female menstrual cycle

Sex hormones are present in the foetus, but sex is determine by the X and Y chromosome pairings.

So, but if the foetus has a Y chromosome (from the XY chromosome pairing), then male characteristics develop, and if it has the XX chromosome pair, female characteristics develop.

Sexual reproduction cannot happen without the intervention of several sex hormones.

When your body reaches puberty it starts to release sex hormones that trigger the formation of secondary sex characteristics - exemplified by the obvious physical changes to the male and female bodies!

e.g. in men, facial hair develops. The principal reproductive male sex hormone is testosterone, produced in the testes and also stimulates sperm production and is important for the development of the male reproductive system.

e.g. in women, breasts develop. The principal reproductive sex hormone is oestrogen, produced in the ovaries. Apart from promoting physical changes e.g. breast development, oestrogen is also involved with other hormones in the menstrual cycle concerning female fertility (described next, starting with the summary diagram below).

The menstrual cycle is the reproductive cycle in women and summarised in the diagram below.

Note that the 4 hormone level graphs are NOT to scale, BUT,

 the graph trends and timing of peaks are important, and crucial to, understanding the menstrual cycle.

A simplified description of four stages in the 28 day menstrual cycle (need to x-reference with the above graph)

Four hormones control the monthly (~28 days) menstrual cycle.

Take it that 'level' means the same as 'concentration'.

The concentration of these hormones fluctuate in the cycle and they interact with each other to promote or inhibit the release of other hormones.

Follicle stimulating hormone (FSH) which causes a follicle (egg + surrounding cells) to mature in one of the ovaries.

Luteinising hormone (LH) stimulates the release of a mature egg from an ovary at day 14.

Oestrogen and progesterone are involved in the maintenance of the lining of the uterus Womb) i.e. to ensure it is renewed to receive a fertilised egg.

(More notes on the hormones after the four basic stages of the menstrual cycle have been described).

By convention, if the woman is not pregnant, the menstrual cycle begins with a period

Stage 1 - the menstruation starts - the bleeding starts on day 1 as the uterus lining breaks down for 4 days promoted by the decrease in progesterone level - known as having a 'period' or menstruation.

The mucous lining of the uterus wall that thickens in the menstrual cycle is called the endometrium.

Stage 2 - over days 5 to 14 the uterus lining builds up again ('repaired') to give a thickish spongy layer of tissue full of blood vessels, potentially ready to receive a fertilised egg for implantation - this is promoted by the rise in oestrogen level.

Stage 3 - around day 14 an egg is developed and released from an ovary follicle in the process called ovulation - this is facilitated by the increase in the FSH and LH hormone - they peak at ~day 13, and the egg is released at ~day 14, hardly a coincidence!

Eggs mature in the ovaries and are released about once every 28 days - ovulation.

Ovulation is the release of a mature egg from an ovary follicle which typically happens once during each menstrual cycle ~day 14 in the cycle. The egg cell lives for up to 24 hours after being released, if the egg cell is not fertilised, the egg cell dies and the menstrual cycle progresses to the next phase

Ovarian follicles are small sacs filled with fluid and an immature egg, that are found inside a woman’s ovaries. They secrete hormones which influence stages of the menstrual cycle and when women begin puberty. Each has the potential to release an egg for fertilisation. Follicles and their size and status are a vital part of assessing fertility and fertility treatment (see later section on treating infertility).

After ovulation the residue cells from the follicle form a structure called the corpus luteum.

Stage 4 - the thickened uterus tissue wall lasts for around 14 days. The uterus wall is maintained by the presence of increased levels of progesterone. If no fertilised egg settles on the uterus wall by day 28, the spongy uterus lining breaks up and the whole menstrual cycle repeats itself.

 

Hormonal control of the menstrual cycle (link in with stages 1 to 4 on the menstrual cycle diagram above)

Several sex hormones are involved in the menstrual cycle of a woman and hormones are involved in promoting the release of an egg: The monthly release of an egg from a woman’s ovaries and the changes in the thickness of the lining of her womb are controlled by hormones secreted by the pituitary gland and by the ovaries. You now need to know, as described below, the function of various hormones that control the different stages of the menstrual cycle.

(a) The ovaries produce the hormone oestrogen, which causes the lining of the uterus to grow and thicken. It also triggers the release of LH (luteinising hormone), which causes the release of an egg, and inhibits further release of FSH (follicle-stimulating hormone) so that only one egg is released in each cycle.

Note that the level of oestrogen rises to a peak through Stage 2, stimulating the growth and completion of the spongy uterus lining.

The high level of oestrogen stimulates a surge in the LH concentration.

(b) The follicle stimulating hormone (FSH) is secreted by the pituitary gland (in the brain) and causes an egg to mature in one of the ovaries in a structure called a follicle. It also stimulates the ovaries to produce hormones including oestrogen,

Note that the level of FSH rises to a peak ~day13-14 in Stage 3 when the spongy uterus lining is complete and ready for an egg delivery.

(c) The luteinising hormone (LH), is also secreted from the pituitary gland, and stimulates the release of an egg from the ovary around day 14 in the cycle (Stage 3 ovulation).

LH causes the follicle to rupture and a mature egg is released.

LH also indirectly stimulates progesterone production.

LH further stimulates the remains of the follicle to develop into a structure called a corpus luteum, which secretes progesterone which maintains the physical integrity of the uterus lining.

Note that the level of LH also rises to a peak ~day13-14 in Stage 3 when the uterus lining is complete and so an egg release is stimulated.

(d) Progesterone is produced in the ovaries by the remains of the follicle (the corpus luteum) after ovulation (its level peaks in the middle of Stage 4). Progesterone maintains the lining of the uterus wall in the 2nd half of the cycle (see diagram) and when its level falls, the uterus lining breaks down. It also inhibits the release of LH and FSH (described above).

Note that the level of progesterone rises to a peak in Stage 4 to maintain the spongy uterus lining in case a fertilised egg settles on it, as well as inhibiting the secretion of LH and FSH so that the cycle can be completed.

If pregnancy occurs, the progesterone level stays high to maintain the uterus lining.

As the progesterone level falls and there is a low level of oestrogen, the lining of the uterus begins to break down as the menstrual cycle repeats itself - it is a low progesterone level that allows the FSH hormone level to rise and trigger the cycle to start again.

(e) A change in one hormone concentration can be used to predict the change in another.

e.g. an increase in oestrogen when progesterone concentrations are low, would be followed by an increase in LH concentration.

 

Pregnancy

If a fertilised egg lands on the uterus lining and becomes implanted, the woman becomes pregnant.

If so, the level of progesterone stays at a high level to maintain the lining of the uterus during pregnancy.


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(c) Using hormones to control female fertility

There are several reasons for reducing fertility leading to pregnancy by some method of contraception e.g.

The rising population of the world - we must try to achieve sustainable populations - but there are ethical issues here about controlling the freedom control the size of family size a couple can have - a question of human rights.

Family planning to enable to control when their children are born - a welcome lifestyle choice for many people in the 21st century - can wait until they can afford to raise a family.

Pregnancy can occur if a sperm reaches an ovulated egg.

The idea of contraception is to prevent pregnancy happening by one means or another.

Know and understand about the uses of hormones in controlling fertility.

Hormones are a key factor in fertility, and can be used to decrease fertility - to decrease the chance of pregnancy.

Oral contraceptives

You can take oral contraceptives that contain hormones to inhibit FSH production so that no eggs mature. noting that .

Oestrogen is used to prevent the release on an egg and the basis of one method of contraception.

Under normal circumstances in the menstrual cycle, oestrogen stimulates the release of an egg, BUT, if taken every day, keeping this hormone level high, inhibits the production of FSH and after some time both egg development and egg release are permanently stopped (as long as oestrogen continues to be taken).

Progesterone also reduces fertility by stimulating the production of thick cervical mucous, preventing sperm getting through the entrance to the uterus (the cervix) to reach an egg.

Progesterone also

(i) thins the lining of the uterus reducing the chances of a fertilised egg being implanted,

and (ii) prevents egg maturation - ovulation by inhibiting the production of FSH and LH (this doesn't apply to some types of mini-pill).

Oral contraceptives eg 'the combined contraceptive pill' may contain BOTH synthetic oestrogen and progesterone to inhibit egg maturation ie prevent egg release, high and sustained levels of oestrogen inhibit FSH production and egg development is stopped permanently as long as the contraceptive is taken.

This 'pill' is sometimes referred to as the 'combined oral contraceptive pill'.

It is taken as a cycle of 21 days of taking the pill daily, followed by no pill for 7 days.

This contraception method is over 99% effective in preventing pregnancy.

However, there are side effects e.g. headaches, nausea and it doesn't protect women from sexually transmitted diseases.

In the first birth-control pills contained too large amounts of oestrogen which resulted in some women suffering significant side effects eg headaches, nausea, irregular bleeding, fluid retention and blood clots.

Birth-control pills now contain a much lower dose of oestrogen, or only contain the hormone progesterone.

Progesterone-only pills (e.g. the mini-pill oral contraceptive) lead to fewer side effects and can be as effective as oestrogen pills.

This synthetic progesterone mini-pill must be taken every day.

They are useful for older women or women who can't take other contraceptive pills e.g. due to some underlying medical condition such as high blood pressure.

 

 

Other hormone contraception treatments

(a) The contraceptive patch contains oestrogen and progesterone (same as combined pill). It is usually 5 cm x 5 cm and is stuck on the skin and is 91-99% effective.

Typically, the patch is worn on the skin in a 4 week cycle - its replaced once a week for 3 weeks and then no patch for 1 week.

(b) A contraceptive implant is inserted under the skin and releases a continuous dose of progesterone. This stops the ovaries releasing an egg, it thickens the cervix mucous, so hindering the sperm swimming to the egg and so stops the implantation of a fertilised egg in the uterus. A contraceptive implant can last for three years and is 99% effective.

(c) A contraceptive injection of only progesterone can have a effect for 2-3 months.

A few general comments on these hormonal methods of contraception (a) to (c)

Advantages:

Convenience - you don't have to remember to take the pill every day, but you must remember to change the patch or implant or receive the next injection when appropriate.

High percentage of effectiveness in preventing pregnancy.

Disadvantages - mainly side effects:

With any hormonal contraceptive treatment you may get temporary side effects at first, including headaches, nausea, breast tenderness, mood changes, heavy and irregular periods.

The patch may cause skin irritation.

(d) An intrauterine device (IUD) is a T-shaped device that is inserted into the uterus to kill sperm which prevents the implanting of a fertilised egg.

There are two main types of IUD (sometimes called an intrauterine system (IUS)..

(i) Plastic IUDs releasing progesterone (hormonal method as above and can be effective for 3-5 years).

(ii) Copper IUDs that kill sperm and prevent them surviving in the uterus, and so can never fertilise a released egg (non-hormonal method, effective for up to 10 years.

See more in next section on other methods of non-hormonal contraception).

 


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(d) Non-hormonal methods of preventing pregnancy

 

Barrier methods to stop the sperm ever meeting an egg, hence stop fertilisation of egg and therefore no pregnancy.

All such devices must be carefully fitted in place before sexual intercourse takes place.

(i) Condoms are worn over the penis during intercourse to stop sperm entering the vagina.

Female condoms are worn inside the vagina.

Note that condoms are the only form of contraception that protect you against sexually transmitted diseases.

(ii) A diaphragm is a shallow flexible plastic cup or dome shaped device that fits over the opening of cervix making a barrier to the entrance to the uterus.

The diaphragm prevents sperm reaching an egg and it can be used in conjunction with a spermicide that kills sperm.

 

Killing the sperm

(i) It is possible to use a spermicide on its own as a contraceptive, but is only 70-80% effective, much less than the other contraception methods previously described.

(ii) Copper IUDs that kill sperm and prevent them surviving in the uterus, and so can never fertilise a released egg (non-hormonal method).

 

Sterilisation - requires a surgical procedure

This involves cutting or tying tubes in the reproductive system ...

(i) female - the fallopian tubes (oviducts) which connect the ovaries to the uterus, are cut, sealed or blocked by an operation so that eggs cannot be released into the uterus.

(ii) male (vasectomy) - the sperm duct tube that connects the testes with the penis is cut, sealed or tied so that sperm cannot be injected into the vagina.

These are surgical procedures with a permanent effect -there is only a very small chance of the tubes joining up again.

Successful surgery is 100% effective, but not reversible.

 

'Natural' methods and the menstrual cycle

This involves knowing when the woman is most fertile within the 28 day menstrual cycle.

Therefore the idea is to avoid sexual intercourse during these fertile periods.

At around 14 days when the egg is released (ovulation) there is a slight increase in temperature.

By estimating when ovulation might occur, by avoiding sexual intercourse at that time, you reduce the chance of fertilising an egg.

However, it is not very effective, because eggs and sperm can live for several days and a woman's cycle can be irregular, but it is used by people who think that barrier and hormonal methods are unnatural (or considered wrong by some religious teachings).

The man can also remove his penis before ejaculation to prevent sperm entering the uterus, but that requires a good deal of self control.

 

Abstinence

No sexual intercourse at all, is the only truly 100% guaranteed pregnancy prevention.

 


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(e) Contraceptive evaluation - comparing methods of contraception

A comparison barrier, natural, surgical and hormonal methods of contraception

Generally speaking, if used correctly, most hormonal methods are more effective at preventing pregnancy than barrier methods, and have the advantage of a couple not having to think about contraception each time they have sexual intercourse - which you have to think of using a barrier method.

Examples of advantages and disadvantages of some methods of contraception.

Barrier methods are generally free from side-effects - though rare allergic reactions have been reported.

Condoms are widely available and protect against sexually transmitted diseases like HIV,

BUT, they can slip off and must be withdrawn after ejaculation to avoid spillage.

Diaphragms can be put in just before sex and there are no health risks,

BUT, diaphragms must be left in for several hours after sex (sperm survive for a time) and some people are sensitive to spermicide.

IUD methods have the advantage of working immediately and can be left in place for 3-5 years if hormonal or up to 10 years if copper.

In using natural methods things must be done carefully - natural cycles can be 'mistimed'.

Hormonal contraception methods can have unpleasant side effects such as nausea, headaches, acne and mood changes.

AND, unlike the use of condoms, hormonal methods do NOT protect against sexually transmitted diseases.

Long-lasting methods of contraception lasting for weeks, months or years might be preferable than short-term methods, if only for convenience and less thinking to do.

Successful surgery, both male and female is 100% effective, no need to think about using a contraceptive, but it is irreversible.

Types of contraceptives and typical success rates in preventing pregnancy

Condoms 98%,  diaphragm 92-96%,  implants 99%,  IUD >99%, oral contraceptives 91-99%,  sterilisation >99%

I can't find any figures for other contraception strategies.

 

You should get, and take, good medical advice from practitioners e.g. doctor or nurse about your strategy for contraception.

 


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(f) Methods of increasing fertility - increasing the chance of pregnancy

This is the complete opposite of contraception.

The desire for children is a very powerful motivation in human nature but to be infertile means you cannot reproduce naturally.

Unfortunately some women are infertile, unable to get pregnant and reproduce naturally, but, help is at hand from various hormonal treatments to greatly improve their fertility and ultimately deliver a baby.

Hormones are a key factor in fertility, and can be used to increase fertility - to increase the chance of pregnancy.

Using hormones to increase fertility

Some women have too low a level of FSH (follicle-stimulating hormone) to stimulate the eggs to mature in the ovary. Therefore no ovulation, no eggs are released and pregnancy cannot happen.

FSH and LH (luteinising hormone) can be administered as a fertility drug to stimulate ovulation.

The method obviously improves fertility - the chances of getting pregnant, but there are 'negatives'.

(i) The treatment is expensive and doesn't always work.

(ii) Sometimes too many eggs are stimulated at the same time resulting in multiple pregnancies e.g. twins or triplets.

Women can be infertile due to lack of ovulation (none or irregular ovulation), so another similar approach is to use the medication clomifene (clomiphene).

Clomifene works by causing more FSH and LH to be released by the body - this in turn stimulates eggs to mature and be released (ovulation).

If you know when the woman is more likely to ovulate, a couple can have intercourse during the treatment period to increase the chances of the woman becoming pregnant.

However, there are some unpleasant side effects from using clomifene e.g. pelvic pain and hot flushes.

 

IVF treatment - In Vitro Fertilisation

What is IVF?

IVF stands for 'In Vitro Fertilisation', is an option for treating infertility in which eggs are fertilised outside the woman's body by sperm from the father.

Why choice of IVF?

Sometimes giving FSH and LH in a ‘fertility drug’ to a woman whose own level of FSH is too low to stimulate eggs to mature doesn't always work.

If women are unable to get pregnant with hormonal treatment they may try IVF treatment.

IVF is an example of Assisted Reproductive Technology (ART), defined as a fertility treatment involving the handling of eggs (usually fertilised) outside of the body.

The IVF procedure

IVF involves giving a mother FSH and LH to stimulate the maturation of several eggs where they would not have been released - this must be done before egg collection, and it also means more than one egg can be collected.

In IVF treatment, after the potential mother has been treated with FSH and LH, eggs are collected from her ovaries and fertilised in the laboratory by sperm from the father.

The mixture is monitored microscopically, over a period of 16-20 hours, to look for successful fertilisation.

IVF treatment can also involve a technique called Intra-Cytoplasmic Sperm Injection (ICSI), where the sperm is directly injected into an egg.

This may be applied if the sperm count of the man is low.

In either case, in a laboratory, the fertilised eggs grow and develop into embryos in a incubator.

At the stage when the fertilised eggs are just tiny balls of cells (~100), one or two embryos are inserted into the mother’s uterus (womb) to attempt a pregnancy.

The initial success rate is not that high, 20-30% of each cycle of treatment

The chance of pregnancy decreases with each successive round and the cost increases too.

However, the cumulative effect of three full cycles of IVF treatment can give an over 45% chance of pregnancy.

IVF techniques are more successful if the woman has (i) been pregnant before, (ii) younger, (ii) has a BMI of between 10 and 30, (iv) has a low alcohol and caffeine intake and doesn't smoke.

Advantages and disadvantages of IVF treatment - the 'pros and cons'

IVF has enabled many mothers to get pregnant and give birth to healthy children - a wonderful application of medical science and technology.

BUT it doesn't always work for an infertile couple and there other downsides too.

About half of the embryos produced by IVF have an incorrect number of chromosomes - genetically defective.

Also, about 1/3rd of the normal embryos selected, will not implant in the uterus.

Sadly, tragically, there are raised incidences of premature births, stillbirths, low birth-weights and infant deaths.

It is possible for several eggs to be simultaneously matured resulting in multiple pregnancies eg twins, triplets and more develop from the growth of more than one embryo into a baby!

This increases the risk for mother and babies - multiple pregnancy puts extra strain on the mother - there is an increased risk of an unsuccessful birth e.g. miscarriage or stillbirth.

IVF has a low success rate, averaging 26% in the UK. It ranges from 29% for women under 35, down to 23% or less for women over 35. Age is an important factor and fertility decreases as a woman gets older.

This low success rate makes the whole process very stressful both physically and mentally - the situation is emotionally draining, especially if IVF treatment fails several times.

Women can suffer side-effects from the hormone treatment which can be physically and emotionally demanding e.g. abdominal pain, dehydration, vomiting - an overlap of side effect symptoms from hormonal contraception

BUT, most infertile couples would regard IVF treatment is worth the risk.

Advances in IVF techniques

Microscope techniques have become more advanced and improved the success rate of IVF fertility treatment.

This involves using special micro-tools to manipulate sperm and eggs under the microscope.

You can now remove single cells from the embryo for genetic testing to check if the genome will facilitate a healthy embryo-baby.

With a microscope and camera, you can also use time lapse imaging of the embryo growing in the incubator, allowing you to continuously monitor the growth of the embryos to see which are the most likely to result in a successful pregnancy

 

Objections to IVF treatments - ethical issues

Most objections centre around the ethical issues of IVF.

(i) IVF procedures often result in unused embryos being destroyed and some people consider this unethical - immoral, because you have destroyed a potential human life.

Is right that the bundle of cells of the embryo are arbitrarily used or discarded? and destroyed.

Are the inevitable losses of potential life, albeit genetically defective, worth the price of successfully developed embryos?

(ii) The genetics and genetic testing of embryos before implantation in the mother's womb raises the ethical issue of preferential choice of characteristics of the baby e.g. choice of gender, eye colour or even whether you allow a child to be born with disabilities.

With advances in microscopic and genetic techniques it is possible to screen the embryos for abnormalities e.g. genetic defects, this means we are making a life or death decision on the test outcome.

Couples, where one is a normal fertile women, could use IVF to select, and allow/not allow to develop, a child with particular characteristics e.g. sex. Many would regards this as unethical and a misuse of IVF.

(iii) Some people consider that IVF as just getting round natural obstacles to getting pregnant

But IVF is not natural, and some consider it unnatural, because it replaces the physical and emotional relationship of conceiving by intercourse with a 'synthetic' laboratory technique.

 

Evaluation of IVF

You might be expected to evaluate the benefits of, and the problems that may arise from, the use of hormones to control fertility, including In Vitro Fertilisation (IVF) and might be given information data to work on.


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See also Hormone system - Introduction to the endocrine system  gcse biology revision notes

For plants see Hormone control of plant growth and uses of plant hormones  gcse biology revision notes


General HUMAN BIOLOGY revision notes

Introduction to the organisation of cells => tissues => organs => organ systems (e.g. in humans)

Examples of surfaces for the exchange of substances in animal organisms   gcse biology revision notes

See also Enzymes - section on digestion and synthesis  gcse biology revision notes

The human circulatory system - heart, lungs, blood, blood vessels, causes/treatment of cardiovascular disease

Homeostasis - introduction to how it functions (negative feedback systems explained)  gcse biology revision notes

Homeostasis - control of blood sugar level - insulin and diabetes  gcse biology revision notes

Homeostasis - osmoregulation, ADH, water control, urea and ion concentrations and kidney function, dialysis

Homeostasis - thermoregulation, control of temperature  gcse biology revision notes

The brain - what the different parts do and the dangers if damaged gcse biology revision notes

An introduction to the nervous system including the reflex arc  gcse biology revision notes

Hormone systems - Introduction to the endocrine system - adrenaline & thyroxine hormones  gcse biology revision

Hormone systems - menstrual cycle, contraception, fertility treatments  gcse biology revision notes

Respiration - aerobic and anaerobic in plants and animals.  gcse biology revision notes

Keeping healthy - communicable diseases - pathogen infections   gcse biology revision notes

Keeping healthy - non-communicable diseases - risk factors for e.g. cancers   gcse biology revision notes

Keeping healthy - diet and exercise  gcse biology revision notes

Keeping healthy - defence against pathogens, infectious diseases, vaccination, drugs, monoclonal antibodies

See also Culturing microorganisms like bacteria - testing antibiotics/antiseptics  gcse biology revision

Food tests for reducing sugars, starch, proteins and lipids  gcse biology revision notes

The eye - structure and function - correction of vision defects  gcse biology revision notes

Optics - lens types (convex, concave, uses), experiments, ray diagrams, correction of eye defects (gcse physics)


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