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Endocrine coordination: Class 12 Biology
Hormones and endocrine glands, the hypothalamus and pituitary, thyroid, parathyroid, adrenal and pancreatic hormones, sex hormones, feedback control, mechanisms of hormone action and disorders caused by too much or too little hormone.
Key points
Hormones are chemical messengers made by endocrine glands. Endocrine glands have no ducts, so they release hormones straight into the blood. The blood carries each hormone to target cells, which have specific receptors for it. Hormones act in very small amounts, and their effects are slower but longer lasting than nerve impulses.
Chemically, hormones are of two main kinds. Peptide and protein hormones (insulin, ADH, growth hormone) and amine hormones such as adrenaline are water soluble. They bind to receptors on the cell surface membrane. Steroid hormones (oestrogen, testosterone, cortisol) and thyroxine are lipid soluble or can cross the membrane, and they act inside the cell.
Mechanism of action. A water soluble hormone is the first messenger. It binds to a surface receptor, which activates an enzyme (adenylate cyclase) that makes cyclic AMP inside the cell. Cyclic AMP is the second messenger and starts a chain of enzyme reactions, so a small signal is greatly amplified. A steroid hormone enters the cell, binds to a receptor in the cytoplasm or nucleus, and the complex switches particular genes on or off.
The hypothalamus links the nervous and endocrine systems. It makes releasing and inhibiting hormones that reach the anterior pituitary through a small blood portal system. It also makes ADH and oxytocin, which are stored in and released from the posterior pituitary. The pituitary gland sits below the brain in a hollow of the skull and is often called the master gland.
Hormones of the pituitary gland:
- Growth hormone (GH): stimulates growth of bones and muscles and protein synthesis. Too little in childhood causes dwarfism. Too much causes gigantism, and in adults acromegaly.
- Thyroid stimulating hormone (TSH): makes the thyroid release thyroxine.
- ACTH: stimulates the adrenal cortex to release cortisol.
- FSH and LH: control the ovaries and testes (gametes and sex hormones).
- Prolactin: stimulates milk production.
- ADH (posterior lobe): increases water reabsorption in the kidney. Too little causes diabetes insipidus, with large volumes of dilute urine.
- Oxytocin (posterior lobe): causes contraction of the uterus at birth and milk ejection.
The thyroid gland in the neck makes thyroxine, which contains iodine. Thyroxine controls the basal metabolic rate and is needed for normal growth and brain development. Lack of iodine in the diet stops thyroxine formation, TSH stays high and the gland enlarges, giving a goitre. Too little thyroxine in infants causes cretinism. Too much (hyperthyroidism) causes a fast heartbeat, weight loss and nervousness.
The parathyroid glands, behind the thyroid, release parathormone, which raises blood calcium by releasing calcium from bone and increasing its absorption by the intestine and kidney. The thyroid also releases calcitonin, which lowers blood calcium. The two hormones act as antagonists to keep calcium steady.
The adrenal glands lie above the kidneys. The medulla releases adrenaline and noradrenaline in fear or excitement (the fight or flight response): the heart beats faster, blood glucose rises and blood flow shifts to the muscles. The cortex releases cortisol (raises blood glucose and helps in long stress), aldosterone (keeps sodium and water in the body) and small amounts of sex hormones.
The islets of Langerhans in the pancreas contain beta cells that make insulin and alpha cells that make glucagon. Insulin lowers blood glucose by helping cells take up glucose and by converting it to glycogen in the liver and muscles. Glucagon raises blood glucose by breaking glycogen down. In diabetes mellitus insulin is lacking or cells do not respond to it, so glucose stays high and appears in urine.
The sex hormones are made mainly by the gonads. The testes secrete testosterone, which causes male secondary sexual characters such as a deep voice and facial hair. The ovaries secrete oestrogen and progesterone, which control the menstrual cycle, breast development and pregnancy. The pineal gland releases melatonin, which helps to control sleep and daily rhythms.
Most hormones are controlled by negative feedback. A rise in thyroxine, for example, inhibits the hypothalamus and pituitary, so less TSH is released and thyroxine falls back. A few systems use positive feedback, such as oxytocin during birth: contractions cause more oxytocin release, which causes stronger contractions until the baby is born.
Study tip: Make a table of each gland with its hormones, target organs, effect and the disorder from too much or too little. Practise drawing the feedback loop for blood glucose and thyroxine as flow diagrams, because examiners often ask for an explanation of a loop in steps.
Definitions and flashcards
Say the meaning first, then open the card to check.
Hormone
A chemical messenger made by an endocrine gland and carried in the blood to target cells.
Target cell
A cell that has specific receptors for a particular hormone.
Second messenger
A molecule such as cyclic AMP that passes the signal on inside a cell after a hormone binds at the surface.
Master gland
The pituitary, so called because many of its hormones control other endocrine glands.
Goitre
Enlargement of the thyroid gland, usually caused by lack of iodine.
Antagonistic hormones
Hormones with opposite effects, such as insulin and glucagon.
Negative feedback
A process in which the product of a pathway reduces its own further production.
Diabetes mellitus
A disorder in which blood glucose stays high because of lack of insulin or poor response to it.
Short questions with model answers
These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.
Why is the pituitary gland called the master gland? (2 marks)
Because several of its hormones, such as TSH, ACTH, FSH and LH, control other endocrine glands (thyroid, adrenal cortex, ovaries and testes). The pituitary itself is controlled by the hypothalamus.
State two differences between nervous and hormonal coordination. (2 marks)
Nerve impulses travel along neurones and act quickly for a short time on a precise target. Hormones travel in the blood, act more slowly, and their effects last longer and may reach many target cells.
How do insulin and glucagon control blood glucose? (3 marks)
After a meal blood glucose rises and beta cells of the islets of Langerhans release insulin. Insulin makes liver and muscle cells take up glucose and store it as glycogen, so the level falls. When glucose falls between meals, alpha cells release glucagon, which makes the liver break glycogen down to glucose and release it into the blood. The two hormones work against each other by negative feedback and keep the level steady.
Describe how a water soluble hormone and a steroid hormone act on a cell. (3 marks)
A water soluble hormone such as adrenaline cannot cross the membrane. It binds to a surface receptor, which activates adenylate cyclase to make cyclic AMP, the second messenger, which activates enzymes and causes the response. A steroid hormone is lipid soluble, so it passes through the membrane, binds to a receptor inside the cell and the complex acts on DNA to switch genes on or off.
Explain the causes and effects of goitre. (3 marks)
Thyroxine needs iodine. If the diet lacks iodine, not enough thyroxine is made. Negative feedback on the pituitary is lost, so TSH keeps stimulating the thyroid, which grows larger and forms a swelling in the neck called a goitre. The person may also have a low metabolic rate. It is prevented by using iodised salt.
Describe the role of the hypothalamus in endocrine control. (5 marks)
The hypothalamus is part of the brain and joins the nervous and endocrine systems. It detects changes such as blood concentration and temperature. It makes releasing and inhibiting hormones that travel in a short portal system to the anterior pituitary and control the release of its hormones, such as TSH and growth hormone. It also makes ADH and oxytocin, which pass along nerve fibres to the posterior pituitary, where they are stored and released into the blood. In this way the hypothalamus controls growth, metabolism, water balance, reproduction and the stress response, and it takes part in negative feedback loops.
Common mistakes
- Saying that ADH is made by the posterior pituitary. It is made in the hypothalamus and only stored and released by the posterior pituitary.
- Mixing up insulin and glucagon, or saying insulin raises blood glucose.
- Writing that hormones travel in ducts. Endocrine glands are ductless and use blood.
- Confusing diabetes insipidus (lack of ADH, dilute urine) with diabetes mellitus (high blood glucose).
- Forgetting that steroid hormones act inside the cell while peptide hormones act through surface receptors and second messengers.