CONTROL IN CELLS AND ORGANISMS

Negative and Positive Feedback – Remember it! & Test it!

Negative and Positive Feedback – Remember it! & Test it!

  • feedbackHomeostasis relies on negative feedback mechanisms.
  • Positive feedback this tends to be very dangerous as it can lead to complete control breakdown.
  • The oestrus cycle is the process in which a female mammal produces eggs.
  • The pituitary gland secretes luteinising hormone (LH) and follicle stimulating hormone (FSH).
  • The ovaries: they secrete progesterone and oestrogen.
  • Using negative and positive feedback loops, hormones affect the release of each other.
  • The pituitary glands release FSH which stimulates the development of a Graafian follicle.
  • The developing follicle secretes oestrogen which stimulates the reformation of the endometrium wall in the uterus.
  • Oestrogen also travels to the pituitary gland where it inhibits FSH release. As the follicle develops, however, and the blood oestrogen levels increase, this rise stimulates the pituitary gland to secrete LH and FSH.
  • A sudden increase in LH causes the follicle to burst and release the ovum. The remaining follicle is stimulated further by LH to form into the corpus lutuem which then proceeds to release progesterone.
  • The uterus is stimulated by the progesterone to complete the endometrium wall. This hormone also travels to the pituitary gland and inhibits FSH and LH release which then inhibits the ovaries from releasing progesterone and oestrogen.
  • Over the following 10 days prostaglandins secreted by the ovaries causes the corpus luteum to degenerate. The level of progesterone in the blood decreases and menstruation begins. This also allows the pituitary gland to start releasing FSH again.
  • If an egg is fertilised then an embryo can implant itself in the uterus and releases HCG (human chorionic gonadotrophin) which prevents the corpus luteum degenerating.

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TEST IT!

1.

A number of hormones control ovulation in mammals

a) How is the concentration of LH in the blood controlled by negative feedback?

b) How is the relationship between LH and oestrogen an example of positive feedback?

c) How does progesterone prevent ovulation?

2.

The oesrus cycle is the process in which a femal mammal produces eggs. In humans the menstrual cycle uses negative and positive feedback loops

a) As the follicle develops it secrete oestrogen. What does this inhibit in the pituitary gland?

b) The blood oestrogen levels rises with the follicle’s development. What does this stimulate the pituitary gland to release?

c) What roles does progesterone place in the menstrual cycle?

ANSWERS

1.

a) As the follicle develops a rise in blood oestrogen stimulates the pituitary gland to secrete LH. A sudden increase in LH then causes the follicle to burst. The ovum is released into the oviduct and the remaining follicle is stimulated further by LH to form into the corpus luteum which then proceeds to release progesterone. The uterus is stimulated by the progesterone to complete the endometrium wall. This hormone also travels to the pituitary gland and inhibits FSH and LH release which then inhibits the ovaries from releasing progesterone and oestrogen.

b) As the follicle develops the blood oestrogen levels increase which stimulates the pituitary gland to secrete LH.

c) It inhibits the secretion of FSH and LH. FSH stimulates follicle development and LH stimulates ovulation.

2.

a) FSH release

b) FSH and LH

c) It simulates the uterus to complete the endometrium wall and inhibits FSH and LH release in the pituitary gland.