Endocrine Basics

What is endocrinology?

The study of hormones and endocrine glands.

How does the endocrine system communicate?

Through hormones that travel in blood or lymph to distant target cells.

How does the endocrine system differ from the nervous system?

Endocrine signaling is slower but longer-lasting; nervous signaling is fast and short-lived.

What are the pure endocrine organs?

Pituitary, pineal, thyroid, parathyroid, adrenal glands (cortex + medulla).

Which organs contain a large proportion of endocrine cells?

Pancreas, thymus, gonads, hypothalamus.

Name four organs that contain some endocrine cells.

Heart, kidneys, digestive tract, skin.

What is the embryological origin of endocrine cells?

Epithelial tissue.

What is a hormone?

A long-distance chemical messenger that travels through blood or lymph.

What is an autocrine signal?

A chemical that acts on the same cell that secretes it.

What is a paracrine signal?

A chemical that affects nearby cells.

Are autocrines and paracrines part of the endocrine system?

No, they are local chemical messengers.

What are the two major classes of hormones?

Amino acid–based hormones and steroid hormones.

What is the possible third class of hormones?

Eicosanoids (usually classified as paracrines).

Which hormones are water-soluble?

Amino acid–based hormones (except thyroid hormone).

Where are receptors for water-soluble hormones located?

On the plasma membrane.

Why must water-soluble hormone receptors be on the membrane?

These hormones cannot enter the cell.

Which hormones are lipid-soluble?

Steroid hormones and thyroid hormones.

Where are lipid-soluble hormone receptors located?

Inside the cell (cytoplasm or nucleus).

What is a target cell?

A cell that has specific receptors for a hormone.

Can a hormone affect different target cells differently?

Yes, depending on receptor type and intracellular signaling.

List five ways hormones alter target cell activity.

• Change membrane permeability

• Activate/deactivate enzymes

• Stimulate protein synthesis

• Induce secretion

• Stimulate mitosis

What hormones use second-messenger systems?

Amino acid–based hormones (except thyroid hormone).

What is the first messenger?

The hormone itself.

What is the second messenger in the cAMP pathway?

Cyclic AMP (cAMP).

Outline the steps of the cAMP pathway.

Hormone → receptor → G protein → adenylate cyclase → cAMP → protein kinases → cell response.

What enzyme breaks down cAMP?

Phosphodiesterase (PDE).

Why do signaling cascades amplify signals?

One hormone can activate many downstream molecules.

Which enzyme is activated in the PIP₂ pathway?

Phospholipase C.

What does PIP₂ split into?

DAG and IP₃.

What does DAG do?

Activates protein kinases.

What does IP₃ do?

Releases Ca²⁺ from intracellular stores.

What role does calmodulin play?

Calcium binds calmodulin to activate enzymes.

Which second messenger is used by some hormones instead of cAMP?

cGMP.

Which hormone works without a second messenger?

Insulin.

What type of receptor does insulin use?

Tyrosine kinase receptor.

Which hormones use direct gene activation?

Steroid hormones and thyroid hormone.

Outline the steps of direct gene activation.

Hormone enters cell → binds receptor → complex enters nucleus → binds DNA → transcription → protein synthesis.

What feedback mechanism controls most hormone levels?

Negative feedback.

What happens when blood hormone levels fall too low?

Hormone secretion increases.

What happens when hormone levels are too high?

Hormone secretion is inhibited.