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Read the passage and look at the diagram.
Hormones are chemical messengers that are transported in the blood. The endocrine system is a group of glands that secrete hormones directly into the bloodstream to control and coordinate body functions.
A gland is an organ that produces and releases hormones. These hormones travel in the blood to a target organ, where they cause a response. An example is puberty, when the sex hormones oestrogen (in females) and testosterone (in males) cause physical changes.
The pituitary gland is the 'master gland' because it releases hormones that act on other endocrine glands. It plays an important role in homeostasis and the response to stress.
Read the passage about temperature.
Body temperature must be kept close to 37°C. Maintaining a constant internal temperature is part of homeostasis, and is important because many enzymes only work properly at this temperature.
If body temperature is too high, enzymes can denature and stop working, which can be life-threatening. If body temperature is too low, enzyme activity slows down, reducing the rate of important reactions like respiration.
Body temperature is monitored and controlled by the thermoregulatory centre in the brain (the hypothalamus), which receives information from temperature receptors in the skin and the blood.
Read the passage about the responses.
When body temperature is too high: blood vessels dilate (vasodilation), so more blood flows near the skin surface and more heat is lost; and sweating increases, so the evaporation of sweat removes heat.
When body temperature is too low: blood vessels constrict (vasoconstriction), so less blood flows near the skin and less heat is lost; sweating decreases; shivering occurs (muscle contractions release heat from respiration); and hairs stand up (goosebumps) to trap a layer of insulating air.
Read the passage and look at the diagram.
A constant supply of glucose is needed for respiration. Blood glucose concentration must be regulated because glucose is soluble and affects the water potential of the blood. It is controlled by the endocrine system: the pancreas monitors blood glucose and releases hormones to control it, and the liver stores glucose as glycogen.
When you eat, your blood glucose rises: the pancreas detects the rise and secretes insulin; insulin travels to the liver; the liver absorbs the glucose and converts it into glycogen; body cells also take up glucose; and blood glucose returns to normal.
Read the passage about low blood glucose.
When you exercise or have not eaten in a while, your blood glucose falls: the pancreas detects the fall and secretes glucagon into the bloodstream; glucagon travels to the liver; the liver converts glycogen back into glucose and releases it into the blood; and blood glucose returns to normal.
Read the passage and use the table.
Diabetes is a disorder where a person cannot control their blood glucose concentration on their own. It comes in two forms:
| Type 1 | Type 2 | |
|---|---|---|
| Caused by | The immune system attacking the pancreas cells | Poor diet and obesity over a long period |
| Effect | The pancreas no longer makes insulin | The liver cannot recognise insulin in the blood |
| Consequence | Blood sugar rises | Blood sugar rises |
| Treated by | Injections of insulin | A carbohydrate-controlled diet, exercise and medication |
Read the passage about the kidneys.
Water content must be regulated to maintain a stable internal environment. Too much or too little water affects the water potential of cells and can damage them. Water is lost from the body through breathing (water vapour), sweating and urine.
The kidneys filter the blood: they remove urea (a waste product), excess ions and excess water. Useful substances such as glucose, ions and water are then selectively reabsorbed back into the blood. The remaining waste forms urine, stored in the bladder. If the body is dehydrated, more water is reabsorbed, producing a small volume of concentrated urine.
HT only - urea production: excess amino acids cannot be stored. In the liver, amino acids are deaminated to form ammonia; ammonia is toxic, so it is converted into urea, which is transported to the kidneys for excretion.
Read the passage and look at the diagrams.
All homeostatic mechanisms follow the principle of negative feedback: when the body moves from the optimum level, a change occurs to move it back; once the optimum is restored, the change is removed (switched off). It is called negative feedback because the change the body makes inhibits itself once the optimum is restored.
HT only - water content is controlled by anti-diuretic hormone (ADH). When the body is dehydrated, the pituitary gland releases more ADH; ADH increases the permeability of the kidney tubules, so more water is reabsorbed, producing a small volume of concentrated urine. When the body has too much water, less ADH is released, the tubules reabsorb less water, and a large volume of dilute urine is produced.
Read the passage about treatments.
If the kidneys become damaged by injury or disease, toxins build up and water balance is not controlled, which can be fatal. The only cure is a transplant - a healthy kidney from a donor. A disadvantage is that transplanted organs do not last forever (roughly 10 years) and, if the donor is not matched, the immune system attacks the transplanted organ.
While waiting for a donor, a patient can have dialysis to clean their blood: at hospital the blood is pumped through a filter that removes the waste products, then the clean blood is returned to the body. With regular dialysis a person can live a long, healthy life. A disadvantage is that it takes around 8 hours, several times a week.
| Water taken in (cm3) | Water lost (cm3) | ||
|---|---|---|---|
| Drink | 1450 | Urine | 1500 |
| Food | 800 | Sweat | 600 |
| Metabolic water | 350 | Faeces | 100 |
| Total | 2600 | Breath / Total | ? / 2600 |
| Substance | Plasma (g/dm3) | Filtrate (g/dm3) | Urine (g/dm3) |
|---|---|---|---|
| Protein | 70 | 0 | 0 |
| Glucose | 1 | 1 | 0 |
| Urea | 0.3 | 0.3 | 20 |
| Sodium ions | 3 | 3 | 6 |
Read the passage about puberty.
One of the key roles of the endocrine system is to control reproduction through hormones. During puberty the reproductive organs mature and begin to function.
In males, the testes produce testosterone, which stimulates male secondary sexual characteristics: growth of facial and body hair, deepening of the voice, increased muscle development, and the production of sperm.
In females, the ovaries produce oestrogen, which stimulates female secondary sexual characteristics: development of breasts, widening of the hips, growth of body hair, and the start of the menstrual cycle and ovulation.
Read the passage and look at the graph.
The menstrual cycle is controlled by hormones and prepares the body for a possible pregnancy. A typical cycle lasts around 28 days. It is controlled by four main hormones:
Follicle stimulating hormone (FSH) causes an egg cell to mature in the ovaries. Luteinising hormone (LH) stimulates the ovary to release the egg. Progesterone and oestrogen maintain the uterus lining so a fertilised egg can implant and develop into an embryo.
If the egg is not fertilised, the lining and egg are shed as a small volume of blood - this is menstruation (a 'period').
Read the day-by-day passage.
Day 1-7: the cycle begins with menstruation (the uterus lining is shed) because oestrogen and progesterone are low.
Day 7-14: FSH is released by the pituitary gland, causing an egg to mature and stimulating the ovary to produce oestrogen. Oestrogen rebuilds the uterus lining, inhibits FSH (to prevent multiple eggs maturing) and stimulates LH.
Day 14: LH triggers ovulation (the release of a mature egg).
Day 14-28: the ovary produces progesterone, which maintains the uterus lining and inhibits both FSH and LH. If the egg is fertilised, progesterone stays high; if not, progesterone and oestrogen fall, the lining breaks down, and menstruation occurs as the cycle repeats.
Read about the hormonal methods.
Contraception is any method that prevents pregnancy. Methods fall into two main categories: hormonal and non-hormonal. Some methods also help stop the spread of sexually transmitted infections.
| Hormonal method | How it works | Notes |
|---|---|---|
| Oral contraceptive (the pill) | Combined pill (oestrogen + progesterone) or progesterone-only pill stops FSH, so no eggs mature | 99% effective; taken daily; does not prevent STIs; some side effects |
| Implant / IUD | Plastic structures that release progesterone (stops FSH) | 99.95% effective; last 3-5 years |
| Injection | Progesterone injected into the body (stops FSH) | Lasts about 12 weeks |
| Patch | Contains oestrogen and progesterone, absorbed into the blood | Replaced every 7 days |
Read about the non-hormonal methods.
| Method | How it works | Notes |
|---|---|---|
| Spermicide | Kills or disables sperm | Easy to use; must be applied each time; does not prevent STIs |
| Condom | Latex sheath over the penis collects semen | Protects against STIs; ~97% effective; can break |
| Diaphragm | Rubber cup placed over the cervix | Does not affect hormones; does not prevent STIs |
| Rhythm method | Avoiding intercourse during ovulation | Inexpensive but unreliable |
| Surgical (vasectomy / tubal ligation) | Sperm ducts or oviducts cut and tied | 100% effective; permanent; risk of infection |
Use the recall points to compare methods.
All forms of contraception share one purpose - to prevent pregnancy. Only barrier methods (e.g. the condom) also protect against STIs. Surgical methods are permanent. Progesterone is the hormone common to the hormonal methods.
| Method | Pregnancies per 100 women per year | Side effects |
|---|---|---|
| Diaphragm + spermicide | 8 | Usually none; can cause bladder infection |
| Condom | 2 | None |
| Contraceptive pill | 1 | Mood swings, headaches, high blood pressure, blood clots, breast cancer |
Read about fertility drugs.
About 1 in 6 couples have difficulty conceiving. Causes include female hormone levels being too low, damaged oviducts, a lack of sperm in semen, and environmental factors such as poor diet, age and lifestyle (smoking, drinking). Modern treatments can help.
Fertility drugs contain FSH and LH. FSH stimulates eggs to mature in the ovaries; LH stimulates ovulation (the release of an egg). This increases the chance of fertilisation occurring naturally.
Read the IVF method.
IVF is a process where fertilisation occurs outside the body. The steps are:
1. The woman is given FSH and LH to stimulate the maturation of multiple eggs.
2. Eggs are collected from the ovaries.
3. The eggs are fertilised with sperm in a laboratory.
4. The fertilised eggs develop into embryos.
5. One or two embryos are implanted into the uterus.
Read about the pros and cons.
Advantage: IVF can help people who cannot conceive naturally.
Disadvantages: it is physically and emotionally stressful; success rates are relatively low; and there is an increased chance of multiple births, which can cause complications for the mother and babies. IVF is also expensive, though it is available on the NHS in the UK for free.
| Total treatments 2007 | Pregnancies 2007 | Predicted success 2008 (%) | |
|---|---|---|---|
| Clinic R | 1004 | 200 | 18-23 |
| Clinic S | 98 | 20 | 3-56 |
Read the recap and look at the diagram.
All homeostatic mechanisms follow the principle of negative feedback: when the body moves from the optimum level, a change occurs to move it back; once the optimum is restored, the change is removed. It is called negative feedback because the change the body makes inhibits itself once the optimum is restored.
Read the passage and look at the diagram.
Thyroxine is a hormone produced by the thyroid gland. It controls the basal metabolic rate (BMR) - the rate of chemical reactions in the body at rest - and is important for growth and development.
Its release is controlled by the pituitary gland (regulated by the hypothalamus), by negative feedback: if thyroxine levels are too high, the pituitary releases less TSH (thyroid-stimulating hormone), so less thyroxine is produced; if thyroxine levels are too low, more TSH is released, increasing thyroxine production.
Read the passage about adrenaline.
Adrenaline is a hormone produced by the adrenal glands, located above the kidneys. It is released in response to stress or danger and prepares the body for a 'fight or flight' response.
Adrenaline causes an increased heart rate, increased breathing rate, and increased blood flow to the muscles and brain. This increases the supply of oxygen and glucose to cells, allowing more respiration and releasing more energy for rapid action.
Adrenaline acts quickly and its effects are short-lived. It is not controlled by negative feedback in the same way as thyroxine - the adrenal glands simply stop releasing it, returning levels to a resting state.
Read the passage about tropisms.
The way a plant responds to the environment is called a tropism. There are two main types: geotropism and phototropism.
Geotropism is the plant's response to gravity: shoots grow up (negative geotropism) and roots grow down (positive geotropism).
Phototropism is the plant's response to light: shoots grow towards light (positively phototropic) and roots grow away from light (negatively phototropic).
Read the passage and look at the diagram.
Gravitropism and phototropism are controlled by the hormone auxin. Auxin is produced in the tips of shoots and roots and diffuses to other parts of the plant. It controls cell elongation (how much cells grow in length).
The effects differ: in shoots, a high concentration of auxin stimulates cell elongation; in roots, a high concentration of auxin inhibits cell elongation. In tropisms, auxin is distributed unevenly in response to light or gravity, causing unequal growth and bending.
Phototropism: light is detected by the shoot tip; auxin moves to the shaded side; auxin stimulates cell elongation, so the shaded cells grow more and the shoot bends towards the light.
Auxin has industrial uses: as a rooting powder (dipping a stem in auxin makes its cells form roots) and as a weed killer (spraying high concentrations makes weeds grow so fast they die).
Read about gibberellins and ethene.
Gibberellins are plant hormones responsible for growth and development. They initiate seed germination, and can be used to promote flowering throughout the year, increase fruit size (by promoting cell elongation), and in the brewing industry (to germinate barley for malt).
Ethene is used in the food industry to provide controlled ripening during storage, transport and display. Growers harvest fruit while green and unripe so it survives transport, then add ethene (a gas) to ripen it just before it reaches the shops.
