B3.2 - The endocrine system - coordination and control

What is a hormone?

A chemical substance produced by a gland and carried in the blood, which alters the activity of one or more target organs

What is a target organ?

An organ which responds to a specific hormone

What is the endocrine system?

The collection of glands in the body that produce and secrete hormones directly into the blood [DRAW: body outline showing pituitary (brain), thyroid (neck), adrenal glands (above kidneys), pancreas, ovaries (female), testes (male) — each labelled with hormone produced]

What does the pituitary gland produce and what does it do?

Produces many hormones; acts as a "master gland" controlling other endocrine glands

What does the thyroid gland produce and what does it do?

Produces thyroxine; regulates metabolic rate and is important for growth and development

What does the adrenal gland produce and what does it do?

Produces adrenaline; prepares the body for a fight or flight response

What does the pancreas produce and what does it do?

Produces insulin; lowers blood glucose levels

What does the testis produce and what does it do?

Produces testosterone; main male sex hormone — stimulates development of secondary sexual characteristics in males

What does the ovary produce and what does it do?

Produces oestrogen and progesterone; oestrogen controls the menstrual cycle, progesterone maintains pregnancy

How do hormones travel to target organs?

Endocrine glands have a good blood supply → hormones are secreted directly into the blood → blood carries them around the body → hormones reach target organs

How do hormones affect only specific target organs?

Target organ cells have complementary receptors on the cell membrane or inside the cell — only cells with the correct complementary receptor can respond to that hormone

What is the role of the liver in hormonal control?

The liver regulates hormone levels in the blood by transforming or breaking down hormones that are in excess

What are the two ways chemical messages can be sent around the body?

(1) Nervous system — electrical impulses, very fast, short-lived. (2) Endocrine system — chemical hormones via blood, slower, longer-lasting

What are the key differences between the nervous system and the endocrine system?

Nervous: made of nerves/brain/spinal cord; electrical impulse; very fast; short-lived effect. Endocrine: made of glands; chemical hormone; slower; longer-lasting effect [DRAW: comparison table with rows: Made up of / Type of message / Speed / Length of effect — two columns Nervous vs Endocrine]

[HT] What is thyroxine and where is it produced?

A hormone produced by the thyroid gland (located in the neck); it stimulates the basal metabolic rate (BMR) — the speed at which chemical reactions occur in the body

[HT] What is the basal metabolic rate (BMR)?

The speed at which chemical reactions occur in the body when it is at rest; thyroxine stimulates BMR and is important for growth and development

[HT] What is negative feedback?

A control mechanism where a change away from the ideal/set point triggers a corrective response to bring conditions back to normal [DRAW: cycle diagram — conditions change from set point → change detected → corrective mechanisms activated → conditions returned to set point → corrective mechanisms switched off]

[HT] How is thyroxine regulated by negative feedback?

(1) Pituitary gland releases TSH (thyroid stimulating hormone). (2) TSH stimulates thyroid to release thyroxine. (3) If thyroxine too high → less TSH released → less thyroxine produced. (4) If thyroxine too low → more TSH released → more thyroxine produced

[HT] What is adrenaline and where is it produced?

A hormone produced by the adrenal glands (above the kidneys); known as the fight or flight hormone — prepares the body for danger

[HT] What are the effects of adrenaline on the body?

Increases blood glucose concentration → more respiration in muscles; increases heart rate and breathing rate → more O₂ and glucose to muscles; diverts blood flow away from non-essential parts; dilates pupils to allow more light in; breaks down stored glycogen to glucose in liver and muscle cells

What is testosterone and what are its effects?

Main male reproductive hormone produced by the testes; stimulates: sperm production, growth of body hair, growth of penis and testes, muscle development, deepening of the voice

What is oestrogen and what are its main roles?

Main female reproductive hormone produced by the ovaries; involved in the menstrual cycle; causes secondary sexual characteristics at puberty — widening of hips, body hair growth, breast development, beginning of menstrual cycle

What is progesterone and what is its main role?

A female reproductive hormone produced by the ovaries; responsible for maintaining the lining of the uterus during the menstrual cycle and in pregnancy

What is FSH and what does it do?

Follicle stimulating hormone — produced by the pituitary gland; stimulates the maturation of an egg in the ovary

What is LH and what does it do?

Luteinising hormone — produced by the pituitary gland; responsible for stimulating ovulation (release of the egg) once it has matured

[HT] What are the four main stages of the menstrual cycle?

(1) Menstruation — days 1–7, lining of uterus breaks down. (2) Follicle/egg matures — days 1–13. (3) Ovulation — day 14, egg released. (4) Maintenance of uterus lining — days 14–28, ready for possible implantation [DRAW: uterus lining thickness across 28-day cycle showing menstruation dip, build-up, ovulation at day 14, maintenance phase]

[HT] What role does FSH play in the menstrual cycle?

FSH (from pituitary) stimulates the development of a follicle in the ovary → the follicle produces oestrogen → oestrogen causes growth/repair of the uterus lining and stimulates LH release

[HT] What role does oestrogen play in the menstrual cycle?

Causes growth and repair of the uterus lining after menstruation; at high enough levels, stimulates the release of LH from the pituitary gland, triggering ovulation (around day 14)

[HT] What role does LH play in the menstrual cycle?

Surge in LH at around day 14 triggers ovulation — the release of the egg from the follicle; also stimulates formation of the corpus luteum

[HT] What is the corpus luteum and what does it do?

The empty follicle left after ovulation; it produces progesterone, which maintains the thickness of the uterus lining

[HT] What happens hormonally if the egg is NOT fertilised?

Corpus luteum breaks down → progesterone levels drop → uterus lining breaks down → menstruation occurs (period); FSH and LH production resumes to begin a new cycle

[HT] What happens hormonally if the egg IS fertilised?

Corpus luteum continues to produce progesterone, preventing the uterus lining from breaking down; once the placenta develops, it secretes progesterone throughout pregnancy [DRAW: graph showing FSH, LH, oestrogen, and progesterone levels across 28-day cycle with ovulation spike at day 14]

[HT] Examiner trap — what is the relationship between oestrogen and FSH?

High oestrogen INHIBITS FSH production (negative feedback) — this is how oral contraceptives prevent ovulation; don't say oestrogen "stops" FSH entirely, say it inhibits/reduces it

What is hormonal contraception and how does it work?

Uses hormones to inhibit FSH production so no eggs mature — prevents ovulation; also affects uterus lining and cervical mucus to further reduce chance of pregnancy

What are the two types of oral contraceptive pill?

(1) Combined pill — contains oestrogen and progesterone; inhibits FSH preventing maturation/release of eggs; also thickens cervical mucus. (2) Mini pill — progesterone only; has fewer side effects

How do injection, implant, and skin patch contraceptives work?

Slowly release progesterone to inhibit egg maturation and release; last for months or years; pill must be taken at the same time every day to be effective, whereas these methods do not

What is an IUD (intrauterine device) and how does it work?

Inserted into the uterus; plastic IUD prevents implantation of an embryo and releases a hormone; copper IUD produces a spermicidal environment preventing sperm from reaching any eggs

What are barrier methods of contraception and examples?

Prevent sperm from reaching the egg. Condom: worn over penis, prevents sperm entering vagina, protects against STIs. Femidom: inserted into vagina. Diaphragm: rubber cap over cervix entrance

What are natural methods of contraception?

Abstinence (100% effective); natural family planning — avoiding sex during fertile period of the cycle when ovulation occurs, requires monitoring cervical secretions, body temperature, and menstrual cycle length

What are surgical methods of contraception?

Vasectomy — sperm duct cut/tied, no sperm in semen; very effective but difficult to reverse. Female sterilisation (tubal ligation) — fallopian tubes cut or tied; very effective but difficult to reverse

What are the main considerations when evaluating contraception methods?

Reliability (success rates vary); side effects (hormonal methods can cause headaches, acne, mood changes); human error (must be used correctly); invasiveness of insertion; frequency of use; protection against STDs (only condoms)

[HT] What are two medical treatments for infertility using hormones?

(1) Hormones to promote egg production — FSH given to stimulate ovulation (may release multiple eggs → risk of multiple births). (2) IVF — eggs harvested, fertilised in vitro with sperm, embryos placed back into uterus

[HT] What are the steps of IVF?

(1) FSH and LH given to female to stimulate egg production. (2) Eggs harvested from ovaries. (3) Eggs fertilised with sperm in a petri dish in vitro. (4) Embryos formed and screened. (5) Several embryos implanted into uterus to increase chance of one developing

[HT] What are the drawbacks of IVF?

Relatively expensive; not all couples can afford it; several embryos are implanted so risk of multiple births (twins/triplets) is high; emotionally and physically demanding

What is a tropism?

A directional growth response made by a plant in response to a stimulus; named after the stimulus (e.g. phototropism = response to light, gravitropism = response to gravity)

What is a positive tropism vs a negative tropism?

Positive tropism — growth towards the stimulus; negative tropism — growth away from the stimulus

What are the tropisms of shoots and roots in response to gravity?

Shoots: negative gravitropism (grow away from gravity, i.e. upward). Roots: positive gravitropism (grow towards gravity, i.e. downward)

What are the tropisms of shoots and roots in response to light?

Shoots: positive phototropism (grow towards light). Roots: negative phototropism (grow away from light)

What are auxins and what do they do?

Plant hormones produced at the tips of shoots and roots; coordinate directional growth responses (tropisms) by controlling cell elongation

How do auxins affect cell elongation differently in shoots vs roots?

In shoots: auxin promotes cell elongation (more auxin = more growth). In roots: auxin inhibits cell elongation (more auxin = less growth) — this is the key examiner trap!

How does gravitropism work in shoots and roots?

When horizontal: gravity pulls auxin to the lower side of shoot/root. In shoots: lower side has more auxin → more elongation → shoot grows upward (negative gravitropism). In roots: lower side has more auxin → less elongation → root grows downward (positive gravitropism) [DRAW: seedling horizontal — auxin pooling on lower side; shoot curves up, root curves down with arrows showing auxin concentration]

Examiner trap — why do shoots curve UP and roots curve DOWN in gravitropism?

Same mechanism (auxin pools on lower side) but OPPOSITE effect: auxin promotes elongation in shoots (lower side grows more → curves up) but INHIBITS elongation in roots (lower side grows less → curves down)

How does phototropism work in shoots?

Light shines from one side → auxin distributed unevenly, moving to the shaded side → shaded side has more auxin → more cell elongation on shaded side → shoot bends towards the light [DRAW: shoot in unidirectional light — auxin concentrated on shaded side, shaded cells elongate more, shoot tip bends toward light source]

[HT] What other plant hormones exist and what do they control?

Gibberellins — germination, fruit ripening, flowering. Ethene — fruit ripening, cell separation. Gibberellins and ethene knowledge is HT only

[HT] How are auxins used commercially by humans?

As selective weedkillers — auxins applied to crops disrupt growth of broad-leaved weeds (more affected than grass); as rooting powders — auxin applied to cuttings promotes root growth for cloning plants

[HT] How is ethene used commercially?

Used in food industry to control ripening of fruit during storage and transport; production of ethene can be inhibited with chemicals to delay ripening; can be applied to unripe fruit to ripen quickly

[HT] How are gibberellins used commercially?

Promoting dormancy/breaking dormancy — gibberellins end dormancy after exposure to cold; increasing fruit size — higher gibberellin levels promote development of larger fruit; promoting flowering regardless of season

[HT] Examiner tip — gibberellins and ethene tier?

Knowledge of gibberellins and ethene is required for Higher Tier candidates only — if you see these on a foundation paper, something's wrong!

[6-MARKER] Compare the nervous system and the endocrine system as methods of communication in the body

(1) Nervous system uses electrical impulses; endocrine uses chemical hormones. (2) Nervous: carried by neurones; endocrine: carried in the blood. (3) Nervous: very fast transmission; endocrine: slower. (4) Nervous: short-lived, localised effect; endocrine: longer-lasting, widespread effect. (5) Nervous: made up of nerves, brain, spinal cord; endocrine: made up of glands. (6) Both systems coordinate responses to changes in the environment

[6-MARKER] Explain how hormones control the menstrual cycle [HT]

(1) FSH released from pituitary gland → stimulates follicle development in ovary.

(2) Developing follicle secretes oestrogen → oestrogen causes uterus lining to thicken/repair.

(3) High oestrogen → stimulates LH surge from pituitary.

(4) LH surge → triggers ovulation (egg release, ~day 14).

(5) Empty follicle becomes corpus luteum → secretes progesterone.

(6) Progesterone maintains uterus lining; if no fertilisation, corpus luteum breaks down → progesterone drops → menstruation