2.1 endocrine system

function of the endocrine system

1. To secrete hormones

2. keeping internal body environment consistent (homeostasis)

exocrine glands

secrete chemical substances into ducts that bring the secretion to the body surfaces/cavities

endocrine glands

secretes hormones into the extracellular fluid to surrounding cells, making up the gland

hormones

- chemicals (secreted by endocrine glands) that are transported throughout the body in the blood

- only able to influence cells that carry correct receptor for hormone

hormones can...

1) activate certain genes in the nucleus so a particular enzyme is produced

2) change the shape/structure of the enzyme to turn it 'on'/'off'

3) change rate of transcription/translation to change rate of production of an enzyme

steroid hormones

- Lipid-soluble → can pass through cell membranes

- Travel in blood bound to transport proteins

- Diffuse into target cells when they arrive

- Bind to intracellular receptors (cytoplasm or nucleus)

- Hormone-receptor complex binds to DNA → activates or inhibits gene transcription

- Leads to protein synthesis

- ⏳ Effect: slow onset but long-lasting

- Examples: oestrogen, progesterone, cortisol, aldosterone

protein and amine hormones

- Water-soluble → cannot cross the cell membrane

- Bind to receptor proteins on the cell membrane

- Hormone-receptor binding activates a second messenger inside the cell

- Second messenger activates enzymes → causes the cell response

- ⚡ Effect: fast-acting but short-lasting

- Example: Insulin binds to membrane receptors → increases glucose uptake by cells

hormone receptors

- Limited number of receptors on each cell membrane

- When all receptors are occupied, the cell response cannot increase further

- This is called saturation (adding more hormone has no extra effect)

- Different cells have different numbers/types of receptors → causes different sensitivities to hormones

- Example: insulin binding to its receptor increases glucose uptake, but once all receptors are bound, more insulin won't increase uptake further.

enzyme amplification

- One hormone molecule can activate thousands of molecules

- Causes a cascade effect along the metabolic pathway

- Each step multiplies the number of reacting molecules (100s-1000s more each step)

- Can produce millions or billions of enzyme molecules

- 📈 Result: a very small stimulus produces a very large effect

hormone clearance

- Hormones must be inactivated after their effect

- Done by breaking down the hormone molecules

- Some breakdown occurs in target cells, but mainly in the liver and kidneys

- Degraded hormones are excreted in bile or urine

control of hormone secretions

- Hormone levels must be tightly regulated to maintain homeostasis

- Oversecretion or undersecretion causes abnormal body function

- Usually controlled by negative feedback loop