Parts of Homeostasis and response

• Oestrogen can be used to prevent the release of an egg = a contraceptive. If oestrogen is taken every day to keep the level permanently high, it inhibits the production of FSH, and after a while egg development and production stay stopped.
• Progesterone also reduces fertility e.g. by stimulating the production of thick mucus which prevents any sperm from getting through and reaching an egg.
• The pill is an oral contraceptive containing oestrogen and progesterone-known as the combined oral contraceptive pill. It's over 99% effective at preventing pregnancy, but it can cause side effects like headaches and nausea and it doesn't protect against STDs.
• There's also a progesterone-only pill - it has fewer side effects than the pill and is just as effective. 
• Contraceptive patch-contains oestrogen and progesterone. It's a small patch that's stuck to the skin. Each patch lasts one week.
• Contraceptive implant-inserted under the skin of the arm. Releases a continuous amount of progesterone, which stops the ovaries from releasing eggs, makes it hard for sperm to swim to the egg, and stops any fertilised eggs implanting in the uterus. Can last for three years. 
• Contraceptive injection also contains progesterone. Each dose lasts 2-3 months. 
• An IUD is a T-shaped device that is inserted into the uterus to kill sperm and prevent implantation of a fertilised egg. Two main types-plastic that releases progesterone and copper that prevent the sperm from surviving in the uterus. 

Hormones can be used to increase fertility:

Some women have levels of FSH that are too low to cause their eggs to mature. This means that no eggs are released and the women can't get pregnant. 

The hormones FSH and LH can be given to women in a fertility drug to stimulate ovulation. 

Pros: It helps a lot of women to get pregnant when previously they couldn't. 

Cons: It doesn't always work-some women may have to do it many times, which can be expensive. Too many eggs could be stimulated, resulting in unexpected multiple pregnancies. 

• IVF involves collecting eggs from the woman's ovaries and fertilising them in a lab using the man's sperm. FSH and LH are given before egg collection to stimulate several eggs to mature. 
• IVF treatment can also involve a technique called Intra-Cytoplasmic Sperm Injection (ICSI), where the sperm is injected directly into an egg. Useful if a man has a very low sperm count. 
• The fertilised eggs are then grown into embryos in a laboratory incubator. 
• Once the embryos are tiny balls of cells, one or two of them are transferred to the woman's uterus to improve the chance of pregnancy. 

Pros-Fertility treatment can give an infertile couple a child. 

Cons- Multiple births can happen if more than one embryo grows into a baby-these are risky for the mother and babies. The success rate of IVF is low-the average success rate in the UK is about 26%. This makes the process incredibly stressful and often upsetting, especially if it ends in multiple failures. As well as being emotional stressful, the process is also physically stressful for the woman. Some have strong reactions to the hormones e.g. abdominal pain, vomiting and dehydration. 

• Adrenaline is a hormone released by the adrenal glands, which are just above the kidneys. 
• Adrenaline is released in response to stressful or scary situations-your brain defects fear or stress and sends nervous impulses to the adrenal glands, which respond by secreting adrenaline. 
• It gets the body ready for 'fight or flight' by triggering mechanisms that increase the supply of oxygen and glucose cells in the brain and muscles. For example, adrenaline increases heart rate. 
• Thyroxine is a hormone released by the thyroid gland in the neck. 
• It plays an important role in regulating the basic metabolic rate-the speed at which chemical reactions in the body occur while the body is at rest. Thyroxine is also important for lots of other processes in the body, such as stimulating protein synthesis for growth and development.
• Thyroxine is released in response to TSH, which is released from the pituitary gland. 
• A negative feedback system keeps the amount of thyroxine in the blood at the right level-when the level of thyroxine is higher than normal, the secretion of TSH from the pituitary gland is inhibited. This reduces the amount of thyroxine released from the thyroid gland, so the level in the blood falls back to normal. 

• Auxin is a plant hormone that controls growth near the tips of shoots and roots. 
• It controls the growth of a plant in response to light (phototropism) and gravity (gravitropism or geotropism). 
• Auxin is produced in the tips and moves backwards to stimulate the cell elongation process which occurs in the cells just behind the tips. 
• If the tip of a shoot is removed, no auxin is available and the shoot may stop growing.
• Extra auxin promotes growth in the shoot but inhibits growth in the root, producing the desired result.

• KILLING WEEDS-Most weeds growing in fields or crops or in a lawn are broad-leaved, in contrast to grasses and cereals which have very narrow leaves. Selective weedkillers have been developed using auxins, which only affect the broad-leaved plants. They totally disrupt their normal growth patterns, which soon kills them, whilst leaving the grass and crops untouched.
• GROWTH FROM CUTTINGS WITH ROOTING POWDER-A cutting is a part of a plant that has been cut off it. Normally, if you stick cuttings in the soil they won't grow, but if you add rooting powder, which contains auxins, they will produce roots rapidly and start growing as new plants. This enables growers to produce lots of clones of a really good plant very quickly.
• GROWING CELLS IN TISSUE CULTURE-Tissue culture can be used to grow clones of a plant from a few of its cells. To do this, hormones such as auxins need to be added to the growth medium to stimulate the cells to divide to form both roots and shoots. 

Gibberellin stimulates seed germination, stem growth and flowering. Its uses include: 

• CONTROLLING DORMANCY-Lots of seed won't germinate until they've been through certain conditions. This is called dormancy. Seeds can be treated with gibberellin to alter dormancy and make them germinate at times of year that they wouldn't normally. It also helps to make sure all the seeds in a batch germinate at the same time.
• INDUCING FLOWERING-Some plants require certain conditions to flower, such as longer days of low temperatures. If these plants are treated with gibberellin, they will flower without any change in their environment. It can also be used to grow bigger flowers.
• GROWING LARGER FRUIT-Seedless varieties of fruit often do not grow as large as seeded fruit. However, if gibberellin is added to these fruits, they will grow larger to match the normal types. 

• Ethene is a gas produced by ageing parts of a plant. It influences the growth of the plant by controlling cell division. It also stimulates enzymes that cause fruit to ripen.
• Commercially, it can be used to speed up the ripening of fruits-either while they are still on the plant or during transport to the shops.
• This means that fruit can be picked while it's still unripe (and therefore firmer and less easily damaged). The gas is then added to the fruit on the way to the supermarket so that it will be perfect just as it reaches the shelves.
• Ripening can also be delayed while the fruit is in storage by adding chemicals that block ethene's effect on the fruit or reduce the amount of ethene that the fruit can produce. Alternatively, some chemicals can be used to react with ethene to remove it from the air.