OCR GCSE Biology A: Gateway Science (J247)

B3.2 Coordination and control — the endocrine system

Your endocrine system is the body's slow, chemical communication network. Instead of using electrical impulses like the nervous system, it sends hormones — chemical messengers — through the bloodstream to target organs. OCR Gateway Science Biology A (J247) topic B3.2 wants you to know the major endocrine glands (pituitary, thyroid, adrenals, pancreas, ovaries and testes), the hormones they secrete, and how those hormones keep the body in balance. You'll need to explain blood glucose control with insulin and glucagon, the differences between Type 1 and Type 2 diabetes, the four hormones of the menstrual cycle, the fight-or-flight response, and how negative feedback regulates thyroxine.

Why this matters

Homeostasis means keeping internal conditions roughly constant despite changes outside the body. Blood glucose, body temperature, water content and ion concentration are all controlled this way. The endocrine system is one of the two control systems that make homeostasis possible — the nervous system is the other. Hormones are made and released by endocrine glands, travel in blood plasma, and only affect target cells that have the right receptors. A useful comparison: nervous signals are fast, short-lasting and travel along neurones; hormonal signals are slower to start but last much longer and travel in blood. Most hormonal control loops are negative feedback loops: a change in a variable triggers a response that reverses the change, restoring the original level. This is the key idea behind blood-glucose control, thyroxine and the menstrual cycle. Insulin and glucagon both come from the pancreas and both act on the liver, but they push glucose in opposite directions. In reproduction, the pituitary releases FSH and LH which act on the ovaries, while the ovaries release oestrogen and progesterone which act on the uterus and feed back on the pituitary. Understanding which gland releases which hormone — and what that hormone does to its target organ — unlocks almost every exam question on this topic.

How to learn this topic

Build on what you already know

Cell structure and the idea of receptors on cell membranes (from B1).
Circulatory system — blood transports substances around the body (from B2).
Basic idea of homeostasis (introduced earlier in B3).
What an enzyme is — useful when discussing glycogen ↔ glucose conversion.

Define hormone, endocrine gland and target organ; compare hormonal vs nervous coordination.
Map the major glands of OCR J247 B3.2 onto a human body diagram and pair each with its hormone.
Build the blood-glucose negative-feedback loop using insulin and glucagon, with glycogen storage in the liver.
Contrast Type 1 (no insulin produced) with Type 2 (cells stop responding to insulin) diabetes and their treatments.
Trace the menstrual cycle: FSH → oestrogen → LH surge → ovulation → progesterone → lining maintained or shed.
Apply hormonal knowledge to the fight-or-flight response (adrenaline) and to reproduction (FSH/LH).
Generalise negative feedback to thyroxine and practise 4–6 mark exam answers in OCR style.

Key terms

hormone: A chemical messenger secreted by an endocrine gland into the bloodstream that travels to a specific target organ. (OCR mark schemes want 'chemical messenger' and 'travels/transported in the bloodstream'.)
endocrine gland: A gland that secretes hormones directly into the blood (e.g. pituitary, thyroid, pancreas).
pituitary gland: The 'master gland' in the brain that secretes several hormones (FSH, LH, ADH, growth hormone, TSH); many of these stimulate other glands.
insulin: A hormone released/secreted by the pancreas when blood glucose is too high. Causes the liver and body cells to take up glucose and convert it to glycogen. (Be precise: insulin lowers blood glucose; glucose is converted to glycogen.)
glucagon: A hormone released/secreted by the pancreas when blood glucose is too low. Travels in the blood to the liver, where glycogen is converted to glucose. (Spell it 'glucagon' (not glycogen) — these are commonly confused.)
glycogen: The storage form of glucose, kept in the liver and muscle cells; can be converted back into glucose when blood glucose falls.
pancreas: The gland below the stomach that detects changes in blood glucose and secretes both insulin and glucagon.
liver: The target organ for insulin and glucagon; site of glycogen storage and breakdown.
Type 1 diabetes: A condition in which the pancreas fails to produce insulin; treated with insulin injections. (Insulin must be supplied from outside the body.)
Type 2 diabetes: A condition in which body cells stop responding to insulin; strongly linked to obesity; usually managed with a carbohydrate-controlled diet and exercise. (Diet can keep blood glucose at a normal level.)
negative feedback: A control loop in which a change in a variable triggers a response that reverses the change, restoring the original level. (Use the phrase 'negative feedback reverses the initial stimulus' for credit.)
thyroxine: A hormone produced by the thyroid gland that increases metabolic rate; controlled by negative feedback via the pituitary.
adrenaline: A hormone released/secreted by the adrenal gland in times of stress; carried in the bloodstream; increases heart rate and converts glycogen into glucose, preparing the body for fight or flight.
FSH: Follicle-stimulating hormone, released/secreted by the pituitary gland, that causes an egg to mature in the ovary and stimulates oestrogen production.
LH: Luteinising hormone, released/secreted by the pituitary gland, whose surge around day 14 causes ovulation.
oestrogen: A hormone produced in the ovaries that thickens the uterus lining and triggers the LH surge.
progesterone: A hormone produced in the ovaries after ovulation that maintains the uterus lining; falling progesterone causes menstruation.

Notes

What the endocrine system is

The endocrine system is a network of glands that secrete chemicals called hormones directly into the bloodstream. The blood then carries each hormone to a target organ where it produces a specific effect. Because hormones travel in blood rather than along neurones, their effects are slower to start but longer-lasting than nervous responses. Only cells with the matching receptor respond to a given hormone, which is why a hormone released into the whole bloodstream only acts on certain organs.

The major glands you need for OCR J247 B3.2 are:

Pituitary gland — in the brain. The 'master gland' because it secretes several hormones (FSH, LH, ADH, growth hormone, TSH) and many of these tell other glands to release their own hormones.
Thyroid gland — in the neck. Releases thyroxine, which controls metabolic rate.
Adrenal glands — one sitting on top of each kidney. Release adrenaline in response to fear or stress.
Pancreas — below the stomach. Releases insulin and glucagon to control blood glucose.
Ovaries (in females) — release oestrogen and progesterone, the main menstrual cycle hormones.
Testes (in males) — release testosterone, which controls sperm production and male secondary sexual characteristics.

Blood glucose homeostasis

Blood glucose concentration is monitored by the pancreas and kept within a narrow normal range by negative feedback.

When blood glucose is too high (e.g. after a sugary meal):

The pancreas detects the rise in blood glucose.
Insulin is released/secreted by the pancreas.
Insulin causes the liver and body cells to absorb glucose from the blood; in the liver, glucose is converted to glycogen.
Negative feedback returns blood glucose to normal.

When blood glucose is too low (e.g. during exercise or fasting):

The pancreas releases/secretes glucagon.
Glucagon travels in the blood to the liver.
In the liver, glycogen is converted to glucose and released into the blood.
Blood glucose concentration increases back to normal — negative feedback reverses the initial stimulus.

Both hormones come from the pancreas and both act on the liver, but they push glucose in opposite directions. This is a textbook negative-feedback loop.

Diabetes

Diabetes is the condition in which blood glucose cannot be properly controlled.

Type 1 diabetes — the pancreas fails to produce insulin (the insulin-producing cells are destroyed, usually as a child). Blood glucose rises uncontrollably after meals. Treatment: insulin injections — insulin must be supplied from outside the body. Diet and exercise also matter, but injections are essential.
Type 2 diabetes — the body cells stop responding to insulin (insulin resistance). The pancreas still makes insulin, but cells ignore it. Strongly linked to obesity. Treatment: a carbohydrate-controlled diet and regular exercise — diet can keep blood glucose at a normal level for many sufferers; some also need medication.

A common misconception is that eating sugar causes diabetes. Type 1 is autoimmune and unrelated to diet; Type 2 is linked to obesity, which has many causes.

Hormones in human reproduction

Four hormones run the menstrual cycle, two from the pituitary and two from the ovaries. All four travel/are transported in the bloodstream to their target organs.

FSH (follicle-stimulating hormone) — released/secreted by the pituitary gland. Causes an egg to mature in a follicle inside the ovary and stimulates the ovaries to produce oestrogen.
Oestrogen — produced in the ovaries. Thickens the lining of the uterus, ready to receive a fertilised egg. Oestrogen inhibits FSH and triggers the release of LH.
LH (luteinising hormone) — released/secreted by the pituitary gland. A sharp surge of LH around day 14 causes ovulation — the mature egg is released from the ovary.
Progesterone — produced in the ovaries (from the empty follicle, the corpus luteum). Maintains the uterus lining during the second half of the cycle. If no fertilisation happens, progesterone falls and the lining is shed (menstruation).

OCR J247 places strong emphasis on naming the source gland for each hormone and on the idea that hormones are transported in the bloodstream to target organs.

Contraception and fertility (overview)

Hormonal contraceptives such as the combined pill use oestrogen and/or progesterone to inhibit FSH so eggs do not mature. Barrier methods (condoms, diaphragms) physically block sperm. Some couples use IVF (in-vitro fertilisation), in which the mother is given FSH and LH to mature several eggs at once; eggs are collected, fertilised in the lab and embryos implanted into the uterus.

Adrenaline and fight or flight

Adrenaline is released/secreted by the adrenal gland in times of fear or stress. It is carried in the bloodstream to its target organs (heart, liver, muscles). Its main effects:

Increases heart rate — boosting oxygen and glucose delivery to muscles and brain.
Converts glycogen into glucose in the liver — raising blood glucose for energy.
Diverts blood flow to the brain and muscles, preparing the body for fight or flight.

Adrenaline is a short-term emergency hormone and is broken down quickly once the threat is over.

Thyroxine and negative feedback

Thyroxine is produced by the thyroid gland and increases the basal metabolic rate. It is controlled by negative feedback via the pituitary: when thyroxine levels are high, this is detected by the pituitary/hypothalamus, which reduces release of TSH and so reduces thyroxine production. When thyroxine levels are low, more TSH is released and thyroxine production rises. In both cases, negative feedback reverses the change and brings thyroxine back to the set point.

Exam tips

Always name the gland AND the hormone in any 4–6 mark answer (e.g. 'pancreas releases/secretes glucagon', not just 'glucagon is released').
OCR loves the phrase 'travels/transported in the bloodstream' — use it whenever you describe a hormone reaching its target organ.
For negative feedback, finish with 'negative feedback reverses the initial stimulus' or 'returns blood glucose to normal' — examiners look for that wording.
Spell glycogen and glucagon carefully — they look almost identical but are completely different molecules.
Type 1 = no insulin made; Type 2 = cells don't respond to insulin. Remember the treatment matches the cause.
When asked about adrenaline, mention 'fight or flight' and at least two physical effects (increases heart rate, converts glycogen into glucose).
For the menstrual cycle, learn which gland makes each hormone: pituitary → FSH + LH; ovaries → oestrogen + progesterone.

Mark-scheme phrasing

Common misconceptions

Worked example

Question:

Answer:

Frequently asked questions

Are hormones faster or slower than nerve impulses?

Slower to start, but their effects last much longer. Nerve impulses are fast and short; hormones are slow and persistent.

Why is the pituitary called the 'master gland'?

Because many of the hormones it secretes (e.g. FSH, LH, TSH) act on other endocrine glands and tell them to release their own hormones.

What is the difference between glycogen and glucagon?

Glycogen is a storage molecule made of many glucose units, kept in the liver. Glucagon is a hormone from the pancreas that triggers glycogen to be converted to glucose.

Why does Type 2 diabetes get worse with obesity?

Excess body fat is linked to cells becoming less sensitive to insulin (insulin resistance), so even when the pancreas releases insulin, body cells do not absorb glucose properly.

Does the LH surge cause ovulation directly?

Yes — a sharp rise in LH from the pituitary gland around day 14 of the cycle triggers the mature egg to be released from the follicle in the ovary.

Why is negative feedback called 'negative'?

Because the response opposes — that is, is the negative (opposite) of — the original change. A rise triggers a fall, a fall triggers a rise. This keeps the variable close to a set point.