Coordination and control – the endocrine system
What you'll learn
- How the human endocrine system uses hormones to coordinate the body.
- What adrenaline and thyroxine do, including negative feedback for thyroxine.
- How hormones control reproduction, the menstrual cycle, contraception and fertility treatment.
- How plant hormones such as auxin control growth responses.
Hormonal coordination: the big idea
Your body needs to coordinate many organs at once. The nervous system uses electrical impulses for fast, short-lived responses. The endocrine system uses hormones for responses that are often slower but longer-lasting.
Hormones, glands and receptors
- A hormone is a chemical messenger made by a gland and carried in the blood.
- An endocrine gland releases hormones directly into the bloodstream.
- A target organ or target tissue is the part of the body affected by a particular hormone.
- A receptor is a molecule on or in a cell that a specific hormone can bind to.
Hormones travel all around the body in the blood, but only cells with the correct receptors respond. This is why one hormone can have a specific effect even though it is carried everywhere.
Endocrine coordination
An endocrine response usually follows this pattern: endocrine gland releases hormone → hormone travels in blood → hormone binds to receptors on target cells → target cells produce a response.
Why only some cells respond to a hormone
- If the adrenal glands release adrenaline, the hormone is carried around the body in the blood rather than along nerves.
- A cell responds only if it has receptors that match adrenaline, making it a target cell.
- Liver cells with adrenaline receptors can respond by breaking down stored glycogen into glucose for respiration.
- A cell without the correct receptor will not respond, even if adrenaline passes it in the blood.
Hormones do not travel along nerves
Hormones are transported in the blood plasma. Nerve impulses travel along neurones; hormones do not.
Adrenaline and thyroxine
This bit is Higher Tier only.
Adrenaline
Adrenaline is released by the adrenal glands, which sit above the kidneys. It prepares the body for “fight or flight”.
Its effects include:
- increasing heart rate, so more glucose and oxygen reach muscles
- increasing breathing rate
- increasing blood flow to muscles
- causing glycogen stores in the liver to be converted into glucose
Thyroxine
Thyroxine is released by the thyroid gland in the neck. It helps control the body’s metabolic rate, which means the rate of chemical reactions in cells. It is also important in growth and development.
Thyroxine is controlled by negative feedback.
Negative feedback
Negative feedback is a control system where a change triggers responses that reverse the change, helping return the body towards a normal level.
The pituitary gland releases TSH: thyroid-stimulating hormone. TSH stimulates the thyroid gland to release thyroxine. If thyroxine gets too high, the pituitary releases less TSH, so the thyroid releases less thyroxine.
Predicting thyroxine feedback
- If thyroxine in the blood is too high, the body needs to reduce thyroid stimulation.
- The pituitary gland releases less TSH, because TSH normally stimulates the thyroid gland.
- With less TSH, the thyroid gland releases less thyroxine.
- Thyroxine level then moves back towards the normal range.
Hormones in human reproduction
Reproductive hormones control puberty, sperm production, egg maturation and the menstrual cycle.
Key hormones you need to know:
- Testosterone is made by the testes. It stimulates sperm production and male secondary sexual characteristics at puberty.
- Oestrogen is made by the ovaries. It is involved in female secondary sexual characteristics and thickening the lining of the uterus.
- Progesterone is made by the ovaries after ovulation. It maintains the lining of the uterus.
- FSH stands for follicle-stimulating hormone. It is released by the pituitary gland and stimulates an egg to mature in an ovary.
Menstrual cycle terms
- Menstruation is the loss of the uterus lining and blood at the start of the cycle.
- Ovulation is the release of an egg cell from an ovary.
- Fertilisation, sometimes called conception, is when a sperm cell nucleus joins with an egg cell nucleus, usually in the oviduct.
- Implantation is when the early embryo embeds in the uterus lining.
A textbook menstrual cycle is about 28 days, although real cycles vary. The uterus lining is lost during menstruation, then thickens again ready for a possible pregnancy.
Interactions in the menstrual cycle
The detailed interaction of FSH, LH, oestrogen and progesterone is Higher Tier only.
- FSH stimulates an egg to mature and stimulates oestrogen release.
- Oestrogen thickens the uterus lining. At first it inhibits FSH, but high oestrogen causes a surge in LH, luteinising hormone.
- LH triggers ovulation, usually around day 14.
- Progesterone maintains the uterus lining after ovulation and inhibits FSH and LH.
- If no pregnancy occurs, progesterone falls, the uterus lining breaks down, and menstruation begins again.
Interpreting hormone changes around ovulation
- If oestrogen is high just before day 14, the uterus lining has been thickened and the LH surge is about to occur.
- The LH surge triggers ovulation, so an egg is released from the ovary.
- After ovulation, progesterone rises and maintains the uterus lining for possible implantation.
- If progesterone later falls, the lining is no longer maintained and menstruation begins.
Conception and the uterus lining
Conception usually happens shortly after ovulation, not during menstruation. By then, the uterus lining is thickening or maintained, so an embryo can implant later if fertilisation occurs.
Contraception
Contraception means preventing pregnancy. You need to explain and evaluate both hormonal and non-hormonal methods.
Hormonal methods
Hormonal contraception changes hormone levels to reduce the chance of fertilisation or implantation. Examples include the pill, implant, injection, patch and hormonal IUS.
They may work by:
- preventing ovulation
- thickening cervical mucus so sperm cannot easily reach the egg
- making the uterus lining less suitable for implantation
Non-hormonal methods
Non-hormonal methods include:
- condoms, which act as a barrier and also reduce STI transmission
- diaphragms, which cover the cervix
- copper IUDs, which affect sperm survival and may prevent implantation
- sterilisation, a permanent surgical method
- natural methods, such as avoiding sex during fertile days, which are less reliable
Evaluating contraception choices
- If someone wants strong pregnancy prevention with little daily effort, a long-acting method such as an implant or IUD may be more effective because it reduces user error.
- If protection from sexually transmitted infections is important, condoms are important because hormonal methods do not block pathogens.
- A balanced recommendation might combine condoms with another effective method, while considering side effects, reversibility, personal beliefs and medical advice.
Hormones and infertility treatment
This is Higher Tier only.
Infertility means difficulty becoming pregnant. Some infertility can be treated using hormones.
In women, FSH and LH can be given to stimulate egg maturation and ovulation. In IVF, in vitro fertilisation, eggs are fertilised outside the body and then an embryo is transferred into the uterus.
Typical IVF stages are:
- hormones stimulate several eggs to mature
- eggs are collected from the ovaries
- eggs are fertilised with sperm in a laboratory
- one or more embryos are placed into the uterus
IVF can help people have a baby, but it may be expensive, emotionally stressful, and not always successful. There can also be ethical concerns about unused embryos and medical risks such as multiple births.
Plant hormones
The plant hormone content here is for separate Biology J247, not Combined Science.
Plant hormones and tropisms
- A plant hormone is a chemical that controls or regulates plant growth and development.
- A tropism is a growth response to a directional stimulus.
- Phototropism is growth in response to light.
- Gravitropism is growth in response to gravity.
- Auxin is a plant hormone that affects cell elongation.
Auxin becomes unevenly distributed in shoots and roots. This unequal distribution causes unequal growth, so the plant bends.
In shoots, auxin increases cell elongation. If light comes from one side, auxin collects on the shaded side, so that side grows faster and the shoot bends towards the light.
In roots, high auxin reduces cell elongation. If a root is horizontal, auxin collects on the lower side, so the lower side grows less and the root bends downwards.
Predicting shoot bending towards light
- Light from one side causes auxin to build up on the shaded side of the shoot.
- In shoots, more auxin causes more cell elongation, so cells on the shaded side grow longer.
- The shaded side becomes longer than the lit side, making the shoot bend towards the light.
- Bending towards light helps the plant absorb more light for photosynthesis.
Other plant hormone effects and uses
Some of this detail is Higher Tier only as well as separate Biology.
Plant hormones can control growth, germination, fruit ripening, flower opening and leaf shedding. You should know examples involving auxins, gibberellins and ethene.
People use plant hormones to control plant growth, for example:
- selective herbicides: auxin-based weedkillers can kill broad-leaved weeds but leave grass crops mostly unaffected
- rooting powder: auxins encourage roots to grow from cuttings
- seedless fruit: hormones can cause parthenocarpic fruit development, meaning fruit forms without fertilisation
- altering dormancy: gibberellins can be used to make seeds germinate after a dormant period
- fruit ripening: ethene can be used to control ripening during transport and storage
Auxin in shoots versus roots
In shoots, more auxin usually means more elongation. In roots, high auxin usually means less elongation. This difference is why shoots and roots bend in different directions.
In the exam
- For endocrine questions, link gland → hormone → blood → receptor → target organ → response.
- For menstrual cycle questions, anchor your answer around ovulation near day 14 and name the hormones causing each change.
- For contraception evaluation, compare effectiveness, STI protection, side effects, reversibility and user error before making a judgement.
- For plant tropisms, state where auxin builds up and whether that side elongates more or less.
Check yourself
- Why does adrenaline affect some target cells but not every cell in the body equally?
- How do FSH, LH, oestrogen and progesterone interact around ovulation?
- How does auxin distribution make a shoot bend towards light?
