Words to know
hormone
A chemical messenger made by an endocrine gland and carried in the blood to target cells, where it causes a specific response.
endocrine gland
A ductless gland that releases hormones directly into the blood (compare exocrine glands, such as sweat glands, which use ducts).
target cell
A cell with the specific receptor protein that a hormone binds to.
steroid hormone
A lipid-soluble hormone made from cholesterol (e.g., cortisol, aldosterone, testosterone, estrogen) that crosses the membrane and acts on genes.
second messenger
A molecule inside the cell, such as cyclic AMP (cAMP), that carries the signal from a membrane receptor to the cell's enzymes.
tropic hormone
A hormone whose target is another endocrine gland (e.g., TSH stimulates the thyroid).
antagonistic hormones
Two hormones with opposite effects on the same variable (e.g., PTH and calcitonin on blood calcium).
Chemical text messages
The nervous system is like a phone call: a direct line from one cell to another, fast and private. The endocrine system is more like a group text sent to everyone in the body. A gland releases a hormone into the blood, and it reaches almost every cell. But only cells with the right receptor "read" the message. Those are the target cells.
In the last few lessons you already met several hormones (ADH, aldosterone, renin's product angiotensin II, ANP). In this lesson we step back and look at the whole endocrine system: its glands, the two main ways hormones act, and how the brain controls it all.
Two kinds of hormones, two ways of acting
What a hormone can do depends on its chemistry, which you studied in Biochemistry:
1. Water-soluble hormones (proteins and peptides such as insulin, glucagon and ADH, and amines such as epinephrine). Because they are polar, they cannot cross the phospholipid bilayer. They bind to a receptor protein in the cell membrane. The receptor then activates a second messenger inside the cell, often cyclic AMP (cAMP), which switches on a series of enzymes. Effects are fast (seconds to minutes).
Each step of this cascade activates many molecules of the next one, so the signal is amplified: one hormone molecule can lead to millions of product molecules. This is why hormones work at tiny concentrations.
2. Steroid hormones (made from cholesterol: cortisol, aldosterone, estrogen, progesterone, testosterone). Because they are lipids, they diffuse through the cell membrane and bind a receptor inside the cell. The hormone–receptor complex enters the nucleus and acts as a transcription factor, switching particular genes on or off, as you saw in Molecular Genetics. The cell makes new proteins. Effects are slower (hours to days) but longer-lasting.
(Thyroid hormones are an exception: they are amines, but they are lipid-soluble and act on genes like steroids.)
The master control: hypothalamus and pituitary
The pituitary gland is a pea-sized gland that hangs below the hypothalamus. It is often called the "master gland", but it takes its orders from the hypothalamus:
- Posterior pituitary: really an extension of the brain. Neurosecretory cells in the hypothalamus make ADH and oxytocin, send them down their axons, and the posterior pituitary releases them when the neurons fire.
- Anterior pituitary: true glandular tissue. The hypothalamus sends releasing hormones (and inhibiting hormones) through a tiny set of blood vessels called the portal system. These tell the anterior pituitary when to release its own hormones.
Many anterior pituitary hormones are tropic hormones: their target is another endocrine gland. For example: hypothalamus → TRH → anterior pituitary → TSH → thyroid → thyroxine → body cells. Then negative feedback closes the loop: high thyroxine inhibits both the hypothalamus and the pituitary, so less TRH and TSH are released. You will see this pattern again for cortisol (two lessons from now) and for sex hormones.
Worked exampleExample 2: Antagonistic hormones and calcium
Blood calcium (Ca²⁺) is needed for muscle contraction, nerve signals and blood clotting. Normal levels are kept by two hormones. PTH (parathyroids) raises blood calcium by releasing Ca²⁺ from bone, increasing Ca²⁺ reabsorption in the kidneys, and activating vitamin D so more calcium is absorbed from food. Calcitonin (thyroid) lowers blood calcium, mainly by reducing the release of Ca²⁺ from bone. Describe what happens after a person's blood calcium falls, and explain why PTH and calcitonin are called antagonistic.
- Stimulus: blood Ca²⁺ falls below the set point.
- Receptor/control centre: cells of the parathyroid glands detect the drop and release more PTH. Calcitonin release falls.
- Effectors: bone releases Ca²⁺; kidneys reabsorb more Ca²⁺; with active vitamin D, the intestine absorbs more Ca²⁺ from food.
- Response: blood Ca²⁺ rises back to normal, so PTH release decreases (negative feedback).
- They are antagonistic because they have opposite effects on the same variable: PTH raises blood calcium and calcitonin lowers it. Working against each other gives precise, two-way control, like having both a furnace and an air conditioner.
Low Ca²⁺ → more PTH → Ca²⁺ released from bone, saved by kidneys, absorbed from food → Ca²⁺ back to normal. PTH and calcitonin have opposite effects, so they are antagonistic.
Hands-on activityModel a signal cascade
You need: about 30 dominoes (or cards folded into tents, or small plastic cups stacked in a pyramid), a table, and a timer.
- Direct signal: set up 4 dominoes in a single line. Knock the first one. One "hormone" produces one final effect. This is like a signal with no amplification.
- Cascade: set up a branching pattern: 1 domino knocks 2, each of those knocks 2 more, and so on, for 4 levels (1 + 2 + 4 + 8 = 15 dominoes). Knock the first one. Count how many fall in the last row.
- Calculate: if each level tripled instead of doubled, how many would fall in the 4th level? (1 × 3 × 3 × 3 = 27.)
- Turning it off: in a real cell, enzymes (phosphodiesterases) break down cAMP so the signal stops. How could you add an "off switch" to your model? (For example, a person who removes some dominoes after a few seconds.)
Analyse: Draw a flow chart comparing your branching model with the second-messenger diagram above. Which part of your model is the hormone? The receptor? The second messenger? Why does a cell need a way to switch the cascade off?
What to look for: the last row of the cascade has 8 dominoes from one push. Students should connect the branching to amplification and recognize that without an off switch, one hormone molecule would keep the cell responding far too long.