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Biology · The Endocrine System

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Biology · OCR Paper 1

The Endocrine System

Hormones and the Endocrine System

The endocrine system uses hormones released by glands into the blood, which act on target organs more slowly than nerves but for longer.

  • Hormone A chemical messenger carried in the blood.
  • Endocrine gland A gland that releases hormones into the blood.
  • Target organ An organ with receptors that respond to a particular hormone.
A human body outline with the pituitary gland, thyroid gland, adrenal glands, pancreas, ovaries and testes marked.
The endocrine glands release hormones into the blood.

Nervous control and hormonal control

  • Nervous system Electrical impulses along neurones; Very fast response; Short-lived effect
  • Endocrine system Chemical hormones in the blood; Slower response; Longer-lasting effect

The Menstrual Cycle

FSH matures an egg, LH triggers its release, oestrogen repairs the lining and progesterone maintains it.

  • FSH Follicle stimulating hormone, which makes an egg mature in an ovary.
  • LH Luteinising hormone, which stimulates ovulation.
  • Oestrogen A hormone from the ovaries that repairs the uterus lining.
A graph of the levels of FSH, LH, oestrogen and progesterone through a 28-day menstrual cycle with the thickness of the uterus lining above.
LH peaks at ovulation on day 14, and progesterone is high while the lining is maintained.

FSH and LH

  • FSH Released by the pituitary gland; Makes an egg mature in a follicle; Stimulates the ovary to release oestrogen
  • LH Released by the pituitary gland; A surge causes ovulation; Peaks around day 14

Contraception and Fertility Treatment

Hormonal contraception stops egg release, other methods block sperm or implantation, and for Higher tier hormones such as FSH and LH are also used to treat infertility.

  • Contraception A method of preventing pregnancy.
  • Combined pill A contraceptive pill containing oestrogen and progesterone.
  • Implant A small rod put under the skin that releases progesterone slowly.
A bar chart of the number of women out of 100 who become pregnant in a year when using each method of contraception.
The implant and the IUD are the most effective methods in typical use.

Hormonal and non-hormonal contraception

  • Hormonal methods Very effective when used correctly; Pill: taken daily, so easy to forget; Implant and injection last for weeks or years
  • Non-hormonal methods Condoms protect against infections; No effect on the hormones; Barrier methods are less effective if used wrongly

Plant Hormones (Separate only)

Plant hormones, such as auxin, coordinate growth, and the unequal distribution of auxin makes shoots grow towards light and roots grow down.

  • Plant hormone A chemical that regulates plant growth and development.
  • Auxin A plant hormone made in shoot and root tips that controls growth.
  • Phototropism Growth of a plant in response to light.
A shoot bending towards light because auxin collects on the shaded side, and a root bending down and a shoot bending up because auxin collects on the lower side.
Unequal auxin makes one side of a shoot or root grow faster than the other.

Three plant hormones

  • Auxin Made in shoot and root tips; Makes cells in shoots elongate; Causes phototropism and gravitropism
  • Gibberellins and ethene Gibberellins: germination, stem growth, flowering; Ethene: fruit ripening, leaf and fruit fall; Ethene is a gas