3.2.1 Principles of hormonal coordination and control

The endocrine system

1. The endocrine system is the set of glands that controls body processes by releasing chemicals into the bloodstream.
2. An endocrine gland has no duct, so the chemical it makes passes straight out of the gland cells into the capillaries running through the gland.
3. That is what separates an endocrine gland from an exocrine gland such as a salivary gland, which releases its product down a tube.
4. The chemicals released by endocrine glands are hormones, and the blood then distributes them around the whole body.
5. Because the blood reaches every organ, a single gland can control organs that lie a long way from it.
6. Endocrine control suits processes that must be held steady or changed gradually, such as growth, development, and the concentration of substances dissolved in the blood.

How hormones reach their target

1. A hormone is made inside an endocrine gland and passes into the blood plasma.
2. The heart pumps that plasma through every organ, so the hormone is carried past cells throughout the body.
3. Only cells carrying a receptor with a shape complementary to the hormone are able to bind it.
4. Cells without the matching receptor are unaffected even though the hormone reaches them in the blood.
5. The organ whose cells carry the matching receptors is the target organ for that hormone.
6. Binding of the hormone to its receptor changes what the target cell does, for example by switching a chemical reaction on or off.
7. Hormones are broken down slowly, mostly in the liver, so the response continues for a while after the gland stops secreting.

Nervous and hormonal control

1. Nervous coordination uses electrical impulses carried along neurones, while hormonal coordination uses chemicals carried in the blood.
2. An impulse travels along a neurone far faster than blood carries a hormone, so nervous responses happen in a fraction of a second.
3. A hormonal response takes seconds, hours or even years, because the hormone must first be secreted and then circulated.
4. A neurone ends at one precise place, so a nervous response acts on a single muscle or gland.
5. A hormone reaches every organ, so its effect can be spread across several organs at once.
6. A nervous response stops quickly once the impulses stop, because the transmitter substance at the synapse is broken down.
7. A hormonal response lasts longer because the hormone remains in the blood until it is broken down.
8. Speed makes the nervous system suited to danger and reflexes, while duration makes the endocrine system suited to growth, development and pregnancy.

The main endocrine glands

1. The pituitary gland lies at the base of the brain and is known as the master gland.
2. The thyroid gland in the neck secretes thyroxine, which sets the rate of the chemical reactions taking place in the body.
3. The adrenal glands sit on top of the kidneys and secrete adrenaline when you are frightened or under stress.
4. The pancreas secretes insulin and glucagon, which together control the concentration of glucose in the blood.
5. The ovaries secrete oestrogen and progesterone, which control the changes of puberty and the menstrual cycle.
6. The testes secrete testosterone, which controls sperm production and the changes of puberty.
7. Every one of these glands works the same way, secreting its hormone into the blood rather than down a duct.

The pituitary as master gland

1. The pituitary gland receives signals from the brain about conditions inside the body.
2. It responds by secreting hormones of its own into the blood.
3. Several of those hormones act on other endocrine glands rather than on ordinary body tissue.
4. Pituitary hormones act on the thyroid gland, on the adrenal glands and on the ovaries or testes.
5. Each of those glands then secretes its own hormone, so control passes along a chain of two glands.
6. Because the first step in so many chains happens in the pituitary, it sets the output of much of the endocrine system.

3.2.2 Roles of thyroxine and adrenaline

Thyroxine and metabolic rate

1. The thyroid gland lies in the neck, just below the voice box, and secretes thyroxine into the blood.
2. Thyroxine molecules contain iodine, which the body can only obtain from the diet.
3. Thyroxine sets the basal metabolic rate, which is the rate at which the body's chemical reactions run while at rest.
4. It raises the rate of aerobic respiration in cells, so more energy is transferred from glucose every second.
5. A high concentration of thyroxine speeds up these reactions, raising the heart rate, increasing body heat and causing weight loss.
6. A low concentration slows the reactions, causing tiredness, weight gain and a feeling of being cold.
7. Thyroxine is also needed for normal growth and for the development of the brain during childhood.

Negative feedback control

1. The concentration of thyroxine in the blood is held close to a set point by negative feedback.
2. The pituitary gland secretes thyroid stimulating hormone, known as TSH, into the blood.
3. TSH travels to the thyroid gland and stimulates it to secrete thyroxine.
4. When the thyroxine concentration rises above the set point, thyroxine acts on the pituitary and inhibits the release of TSH.
5. Less TSH reaches the thyroid, so the thyroid secretes less thyroxine and the concentration falls back towards the set point.
6. When the thyroxine concentration drops below the set point, that inhibition is lifted and the pituitary secretes more TSH.
7. More TSH stimulates the thyroid, so thyroxine output rises and the concentration climbs back towards the set point.
8. The result is a concentration that fluctuates slightly around the set point rather than drifting away from it.

Adrenaline and fight or flight

1. The adrenal glands sit one on top of each kidney and secrete adrenaline into the blood.
2. Release is triggered by nervous impulses sent from the brain when it detects fear, danger or stress.
3. Adrenaline increases the heart rate and the force of each beat, so blood is pumped around the body faster.
4. It increases the breathing rate and the depth of each breath, so more oxygen enters the blood.
5. Together these changes deliver oxygen and glucose to the muscles and the brain more quickly.
6. Adrenaline makes the liver break down stored glycogen into glucose, raising the blood glucose concentration.
7. Blood vessels supplying the muscles widen while those supplying the gut narrow, diverting blood to where it is needed.
8. The pupils widen, letting more light into the eye so that vision improves.
9. All of these changes together prepare the body either to fight the danger or to run from it.
10. Adrenaline is not controlled by negative feedback, because it is released as a response to a stimulus and stops once the danger has passed.

Comparing the two hormones

1. Thyroxine is secreted continuously and its concentration is held steady, while adrenaline is secreted in short bursts.
2. Thyroxine release is controlled by negative feedback from the pituitary, while adrenaline release is triggered by nervous impulses from the brain.
3. Thyroxine controls a long-term process, the overall rate of the body's reactions, over months and years.
4. Adrenaline controls a short-term response that lasts minutes, preparing the body for immediate action.
5. Both hormones are secreted into the blood and both raise the rate at which energy is transferred, so the difference lies in the timing and the trigger.

3.2.3 Hormones in human reproduction

Sex hormones at puberty

1. Puberty begins when the pituitary gland starts to secrete more of the hormones that act on the ovaries and testes.
2. Those hormones make the ovaries or testes become active and secrete sex hormones of their own.
3. The sex hormones circulate in the blood and act on tissues throughout the body.
4. They bring about secondary sexual characteristics, which are the physical changes other than the reproductive organs themselves.
5. In males these include a deeper voice, facial and body hair, broader shoulders and more muscle.
6. In females these include breast development, wider hips and the start of the menstrual cycle.
7. The reproductive organs also mature, so sperm production begins in males and eggs begin to be released in females.
8. Because the hormones stay in the blood for years, these changes happen gradually and are permanent.

Testosterone and oestrogen

1. Testosterone is secreted by the testes and is the main male reproductive hormone.
2. It stimulates the testes to produce sperm from puberty onwards.
3. It also brings about the male secondary sexual characteristics.
4. Oestrogen is secreted by the ovaries and is the main female reproductive hormone.
5. It brings about the female secondary sexual characteristics at puberty.
6. During each cycle it makes the lining of the uterus grow thicker and richer in blood vessels.
7. A thick lining is needed so a fertilised egg can implant and be supplied with food and oxygen.
8. Both hormones are secreted from puberty onwards, but oestrogen output rises and falls within each cycle.

Stages of the menstrual cycle

1. The menstrual cycle lasts about $28$ days, though the length varies from person to person.
2. Day $1$ is the first day of menstruation, when the uterus lining breaks down and passes out through the vagina.
3. Menstruation usually lasts about $4$ days.
4. From roughly day $4$ to day $14$ the lining of the uterus is rebuilt until it is thick and well supplied with blood.
5. Over the same period an egg matures inside a follicle in one of the ovaries.
6. Ovulation happens at about day $14$, when the mature egg is released from the ovary into the oviduct.
7. From day $14$ to day $28$ the thick lining is maintained so it is ready to receive a fertilised egg.
8. If the egg is not fertilised the lining breaks down again, and the next cycle starts at day $1$.

The four hormones of the cycle

1. Four hormones control the cycle, two secreted by the pituitary gland and two by the ovaries.
2. FSH is secreted by the pituitary and makes an egg mature inside a follicle in the ovary.
3. FSH also stimulates the ovaries to secrete oestrogen.
4. LH is secreted by the pituitary and triggers ovulation at about day $14$.
5. Oestrogen is secreted by the ovaries and thickens the uterus lining in the days before ovulation.
6. Progesterone is secreted by the ovaries after ovulation and maintains the thickened lining.
7. When progesterone output falls, the lining can no longer be maintained and menstruation begins.

3.2.4 Interactions of reproductive hormones

FSH and the start of the cycle

1. At the end of a cycle the concentrations of oestrogen and progesterone in the blood are low.
2. With little inhibition acting on it, the pituitary gland secretes FSH into the blood.
3. FSH is carried in the blood to the ovaries, which are its target organs.
4. In an ovary, FSH makes a follicle containing an immature egg grow and the egg inside it mature.
5. FSH also stimulates the ovary to secrete oestrogen, so the oestrogen concentration starts to rise from about day $4$.
6. The rise in oestrogen is therefore a direct consequence of the FSH released at the start of the cycle.

Oestrogen inhibits FSH

1. Oestrogen from the ovary travels in the blood to the uterus, where it makes the lining grow thick and rich in blood vessels.
2. Oestrogen also travels to the pituitary gland, where it inhibits the release of FSH.
3. Less FSH means no further follicles are stimulated, so usually only one egg completes its development.
4. At the same time oestrogen stimulates the pituitary to secrete LH.
5. Because a rise in oestrogen causes a further rise in LH, this part of the cycle is an example of positive feedback.
6. The LH concentration therefore climbs steeply and reaches a sharp peak at about day $14$.

LH triggers ovulation

1. The sharp peak in LH acts on the mature follicle in the ovary.
2. The follicle bursts and releases the egg into the oviduct, which is ovulation, at about day $14$.
3. The empty follicle stays behind in the ovary and develops into a yellow structure of dividing cells.
4. That structure secretes progesterone, so the progesterone concentration rises from day $14$ onwards.
5. Oestrogen output drops slightly just after ovulation, then rises again as the empty follicle develops.

Progesterone maintains the lining

1. Progesterone acts on the uterus and maintains the thick lining, keeping it ready to receive a fertilised egg.
2. Progesterone also acts on the pituitary gland, where it inhibits the release of both FSH and LH.
3. With FSH inhibited, no new follicle matures, so a second ovulation cannot happen in the same cycle.
4. If the egg is fertilised and implants in the lining, progesterone stays high and the lining is kept in place.
5. If the egg is not fertilised, the structure in the ovary breaks down and progesterone and oestrogen concentrations fall.
6. Once progesterone is low, the lining is no longer maintained and it breaks down as menstruation.
7. Low progesterone and oestrogen also stop inhibiting the pituitary, so FSH rises again and the next cycle begins.

3.2.5 Hormones in contraception

How hormonal contraception works

1. Hormonal contraception uses oestrogen, progesterone or both to interrupt the menstrual cycle.
2. Oestrogen taken continuously keeps its concentration in the blood permanently high.
3. A permanently high oestrogen concentration inhibits the release of FSH from the pituitary gland.
4. Without FSH no follicle is stimulated, so no egg matures and ovulation cannot happen.
5. Progesterone thickens the mucus produced at the cervix, forming a plug that sperm cannot swim through.
6. Progesterone also inhibits the release of FSH and LH, which is a second route to preventing ovulation.
7. Some methods also make the uterus lining too thin for a fertilised egg to implant.

Hormonal methods available

1. The combined pill contains oestrogen and progesterone and is taken daily.
2. Used exactly as directed it is over $99\%$ effective, but with typical use effectiveness falls to about $91\%$.
3. The progesterone only pill avoids oestrogen and so suits people at risk of blood clots, but it must be taken at the same time each day.
4. A skin patch releases oestrogen and progesterone through the skin and is replaced once a week.
5. An implant placed under the skin of the upper arm releases progesterone slowly for up to $3$ years.
6. An injection of progesterone lasts about $3$ months before it must be repeated.
7. An intrauterine system placed inside the uterus releases progesterone and lasts between $3$ and $5$ years.
8. Side effects can include headaches, changes in mood and a small rise in the risk of blood clots.
9. No hormonal method gives any protection against sexually transmitted infections.

Non-hormonal methods

1. A condom is a barrier method worn over the penis that traps sperm so they never enter the vagina.
2. Condoms are about $98\%$ effective when used correctly, though typical use gives closer to $82\%$.
3. A condom is the only common method that also reduces the spread of sexually transmitted infections.
4. A diaphragm is a barrier fitted over the cervix and used with spermicide, giving roughly $92\%$ effectiveness.
5. Spermicide is a chemical that kills or immobilises sperm, and used on its own it is only about $71\%$ to $82\%$ effective.
6. A copper intrauterine device releases copper ions that are toxic to sperm and also prevent implantation, giving over $99\%$ effectiveness for up to $10$ years.
7. Sterilisation means cutting or tying the oviducts in females or the sperm ducts in males, and it is over $99\%$ effective.
8. Sterilisation must be treated as permanent, because reversing the operation is difficult and often fails.
9. Natural methods avoid intercourse in the days around ovulation, which needs careful record keeping and gives between $76\%$ and $99\%$ effectiveness.
10. Abstinence from intercourse is the only method that prevents pregnancy completely.

Comparing the methods

1. Judge a method on its effectiveness, whether it is reversible, whether it protects against infection, its side effects, its cost and how easy it is to use.
2. The implant, injection, intrauterine device and intrauterine system are the most effective in practice, because they do not depend on the user remembering anything.
3. Pills are highly effective in theory but less so in practice, since a single missed pill can allow a follicle to mature.
4. Only condoms reduce the transmission of sexually transmitted infections, so they are often used alongside a hormonal method.
5. Barrier methods have no hormonal side effects, which suits people who cannot take oestrogen.
6. Sterilisation is the most permanent option and is unsuitable for anyone who may want children later.
7. Some people avoid hormonal and surgical methods for religious or ethical reasons and choose natural methods instead.
8. Cost differs too, since condoms are bought repeatedly while an implant is fitted once and lasts for years.

3.2.6 Hormones in reproductive technologies

Causes of infertility

1. Infertility has several causes, and only some of them can be treated with hormones.
2. If the pituitary gland secretes too little FSH, no follicle is stimulated and no egg matures.
3. If it secretes too little LH, an egg may mature but ovulation never happens.
4. Both of these causes are hormonal, so giving the missing hormone can restore the cycle.
5. Blocked oviducts stop sperm reaching the egg, and hormones cannot clear the blockage.
6. A low sperm count or poorly swimming sperm also prevent fertilisation, and again hormones do not help.
7. Treatment is chosen to match the cause, so the first step is always to find out which part of the process is failing.

Fertility drugs

1. A fertility drug contains FSH and LH, the same hormones the pituitary gland normally secretes.
2. The drug is given by injection over the first part of the cycle.
3. FSH stimulates follicles in the ovaries so that eggs mature.
4. LH is then given to trigger ovulation, releasing the mature eggs into the oviducts.
5. The natural cycle is restored, so the couple can conceive through intercourse in the usual way.
6. Because the dose stimulates more than one follicle, several eggs can be released at once.
7. That raises the chance of twins or triplets, which carries greater risk for the mother and the babies.

Steps in IVF

1. FSH and LH are given first, so several eggs mature in the ovaries at the same time.
2. The mature eggs are collected from the ovaries through a fine needle.
3. Each egg is mixed with the father's sperm in a dish in the laboratory, where fertilisation takes place.
4. The fertilised eggs are kept in an incubator and allowed to divide into embryos.
5. When each embryo is a small ball of cells, one or two are inserted into the uterus through the cervix.
6. Progesterone is often given so the uterus lining is maintained and the embryo can implant.
7. A pregnancy test taken about two weeks later shows whether implantation was successful.
8. Any remaining embryos can be frozen and stored for a later attempt.

Evaluating IVF

1. IVF allows couples who could not conceive naturally to have a child of their own, which is its main benefit.
2. Success rates are low, at roughly $32\%$ of cycles for patients under $35$ and falling below $5\%$ above the age of $43$.
3. Several attempts are often needed, so the emotional strain of repeated failure is considerable.
4. The hormone injections have physical side effects including abdominal pain, vomiting and, rarely, a dangerous over-response of the ovaries.
5. Transferring more than one embryo raises the chance of multiple births, which brings a higher risk of premature birth and low birth mass.
6. Each cycle is expensive, often over £5000\pounds5000£5000 privately, and funding on the NHS varies between areas.
7. Spare embryos are frozen or destroyed, and some people object to this because they regard an embryo as a potential life.
8. A judgement therefore weighs the chance of a much wanted child against the cost, the risks and the ethical objections.

3.2.7 Plant hormones in growth and coordination

Tropisms in plants

1. A plant cannot move, so it responds to a stimulus by changing the direction in which it grows.
2. A tropism is a growth response in which the direction of growth is set by the direction of the stimulus.
3. A response towards the stimulus is positive, and a response away from the stimulus is negative.
4. Phototropism is a growth response to light, and shoots show positive phototropism while roots show negative phototropism.
5. Gravitropism is a growth response to gravity, and roots show positive gravitropism while shoots show negative gravitropism.
6. Growing shoots towards light raises the light the leaves receive, so the rate of photosynthesis increases.
7. Growing roots downwards takes them into the soil, where they absorb water and mineral ions and anchor the plant.

Auxin and cell elongation

1. Auxin is a plant hormone made in the tip of a growing shoot or root.
2. It then moves back from the tip into the region of cells just behind it.
3. In a shoot, auxin makes the cell walls looser so the cells take in more water and elongate faster.
4. Faster elongation on one side of a shoot makes that side longer, which bends the shoot the other way.
5. In a root, a high auxin concentration has the opposite effect and inhibits cell elongation.
6. Because auxin is made in the tip, cutting the tip off removes the source and the growth response stops.
7. Auxin is only redistributed after it has been made, so the size of the response depends on how much reaches each side.

Phototropism in shoots

1. A shoot lit from one side receives more light on one side than the other.
2. Auxin made in the tip moves across towards the shaded side, giving an unequal distribution of auxin.
3. The shaded side therefore has a higher auxin concentration than the lit side.
4. Cells on the shaded side elongate more than the cells on the lit side.
5. The shaded side grows longer, so the shoot bends towards the light.
6. Once the shoot faces the light, auxin is shared evenly again and the shoot grows straight.

Gravitropism in roots and shoots

1. A germinating seed is buried in soil, so it has no light to respond to and uses gravity instead.
2. When a root or a shoot lies horizontally, gravity makes auxin collect on its lower side.
3. In a shoot, the extra auxin on the lower side makes those cells elongate faster.
4. The lower side of the shoot grows longer, so the shoot curves upwards, away from gravity.
5. In a root, the extra auxin on the lower side inhibits elongation instead.
6. The upper side of the root therefore elongates faster, so the root curves downwards, towards gravity.
7. The same unequal distribution produces opposite results because auxin promotes elongation in shoots and inhibits it in roots.
8. The outcome is a shoot that reaches the light and a root that reaches water, whichever way up the seed happens to land.

3.2.8 Effects of plant hormones

Auxins and plant growth

1. Auxin controls growth by changing how much the cells behind a shoot or root tip elongate.
2. Auxin made in the shoot tip travels down the stem and reaches the side buds lower down.
3. At the concentration that reaches them, auxin inhibits those side buds from growing, which is apical dominance.
4. The main shoot therefore grows tallest and reaches the light before its neighbours shade it out.
5. If the shoot tip is removed, auxin no longer reaches the side buds and they start to grow.
6. The plant then produces many side shoots and becomes short and bushy rather than tall.
7. Auxin also stimulates the growth of new roots from the cut end of a stem.
8. The effect depends on concentration, because a low auxin concentration promotes root growth while a high one inhibits it.

Gibberellins

1. Gibberellins are plant hormones made in seeds, young leaves and roots.
2. They end seed dormancy, which is the resting state that stops a living seed germinating.
3. Gibberellin makes the seed produce enzymes such as amylase inside the seed.
4. Amylase breaks the stored starch down into sugars such as maltose and glucose.
5. Those sugars are used in respiration to release the energy the embryo needs to grow, so germination begins.
6. Gibberellins also promote stem elongation, so treated plants grow taller than untreated ones.
7. They promote flowering, and can make a plant flower without the day length it would normally need.
8. They also increase the size of fruit, because larger cells make a larger fruit.

Ethene and fruit ripening

1. Ethene is unusual among plant hormones because it is a gas.
2. It is released by ripening fruit and by ageing plant tissue.
3. Being a gas, ethene spreads through the air and reaches fruit that is not touching the source.
4. Ethene switches on enzymes that convert stored starch into sugars, making the fruit taste sweeter.
5. Other enzymes break down the cell walls, which is what makes a ripe fruit soft.
6. Further enzymes break down green chlorophyll and reveal or make other pigments, so the fruit changes colour.
7. Ripening fruit releases still more ethene, so ripening spreads from one fruit to the whole crate.
8. Ethene also controls cell division in growing parts of the plant.

Flowering and leaf fall

1. Gibberellins promote flower opening, so treated plants flower earlier or at an unusual time of year.
2. Ethene speeds up the ageing of an open flower, so the petals wilt and are shed sooner.
3. Leaf fall, called abscission, is controlled by the balance between auxin and ethene.
4. A healthy leaf makes auxin, and that auxin keeps the leaf attached to the stem.
5. As autumn approaches the leaf makes less auxin, so the balance shifts towards ethene.
6. Ethene makes a weak abscission layer of cells form across the base of the leaf stalk.
7. The layer breaks and the leaf falls, sealing the scar so water is not lost from it.
8. Shedding leaves cuts water loss in winter, when frozen soil makes it hard for the roots to replace that water.

3.2.9 Uses of plant hormones

Selective weedkillers

1. A selective herbicide contains a synthetic auxin, a chemical that behaves like the plant's own auxin.
2. The herbicide is sprayed across a whole field, so it lands on the crop and the weeds together.
3. Broad leaved weeds such as dandelions and thistles have a large leaf surface area, so they absorb far more of the spray.
4. The high auxin concentration inside them makes them grow uncontrollably fast.
5. Growing that fast uses up their stored food faster than photosynthesis can replace it, so the weeds die.
6. Narrow leaved crops such as wheat, barley and grass catch far less spray on their upright leaves and survive.
7. Removing the weeds reduces competition for light, water, mineral ions and space, so the crop yield rises.
8. Repeated use can select for resistant weeds, and spray drifting onto hedgerows reduces the biodiversity around the field.

Rooting powder for cuttings

1. A cutting is a short length of stem cut from a parent plant.
2. The cut end is dipped in rooting powder, which contains auxins at the concentration that promotes root growth.
3. The auxin stimulates the cells at the cut end to develop into new roots.
4. Once rooted, the cutting can absorb water and mineral ions and grow into a complete plant.
5. Every plant produced this way is a clone of the parent, so a desirable variety is reproduced exactly.
6. The method is faster and more reliable than growing from seed, and it works on species that root poorly on their own.
7. Because the plants are genetically identical, a whole crop of them can be wiped out by the same disease.

Producing seedless fruit

1. A flower normally has to be pollinated and fertilised before its ovary develops into a fruit.
2. The developing seeds release hormones, and those hormones make the ovary wall grow into the fruit.
3. Spraying auxins or gibberellins onto unpollinated flowers supplies those hormones from outside.
4. The ovary develops into a fruit even though no fertilisation has happened, which is parthenocarpy.
5. Because there was no fertilisation, the fruit contains no seeds.
6. Seedless grapes, and some tomatoes and cucumbers, are produced this way.
7. Customers will pay more for fruit without seeds, so the grower earns a higher price for the same crop.
8. No seeds also means no way of growing the next generation from them, so growers must buy new stock or take cuttings.

Altering seed dormancy

1. Many seeds stay in dormancy until they have had a period of cold or several months in storage.
2. Dormancy protects a wild seed from germinating in autumn, when the seedling would be killed by winter.
3. For a grower, dormancy is a nuisance, because a sown batch germinates over several weeks rather than all at once.
4. Treating the seeds with gibberellin ends dormancy, so they germinate straight away.
5. The whole batch then germinates at the same time, giving an even crop that can be harvested on one date.
6. Gibberellin is used the same way on barley grains in malting for the brewing industry.
7. It makes every grain produce amylase together, and that amylase converts the stored starch into sugars.
8. The sugars are then fermented by yeast, so treating the grain shortens the whole process.

Controlling fruit ripening

1. Fruit is picked while it is still unripe and hard, because ripe fruit bruises easily in transit.
2. It is then stored in cool air that is low in oxygen and from which ethene has been removed.
3. Those conditions slow respiration and stop the ripening reactions, so the fruit stays unripe for weeks.
4. Ethene gas is released into the store a few days before the fruit is needed.
5. The ethene switches on the ripening enzymes, so the fruit softens, sweetens and changes colour together.
6. Timing the ripening this way means fruit reaches the shelves ready to eat rather than already spoiled.
7. It also lets fruit be shipped long distances, so supermarkets can sell it all year round.
8. The stores need refrigeration and gas control, so the method costs energy and money.