What Even Is a Hormone?
What two features must a cell have in order to respond to a hormone?
The cell must express the appropriate hormone receptor, and the receptor must be functional and coupled to an intracellular signalling pathway.
Which structure links the nervous system to the endocrine system?
hypothalamus
What type of hormones most commonly use G-protein–linked receptors?
Protein and peptide hormones
What molecule is the precursor for all steroid hormones?
Cholesterol
What enzyme converts T4 into biologically active T3?
Deiodinase 2 (Dio2)
What is meant by endocrine pleiotropy?
The ability of a single hormone to have multiple effects in different tissues, depending on receptor type and intracellular signalling mechanisms.
What is the key functional difference between the anterior and posterior pituitary?
The anterior pituitary synthesizes and secretes its own hormones, whereas the posterior pituitary stores and releases hormones synthesized in the hypothalamus.
What happens immediately after a hormone binds to a GPCR?
The receptor activates an associated G-protein, initiating intracellular signalling.
Where are steroid hormone receptors typically located?
In the cytosol or nucleus of target cells.
What is receptor internalization, and what can happen afterward?
Receptor internalization is the uptake of receptor–hormone complexes by endocytosis; receptors may be recycled back to the membrane or degraded.
Why can two individuals with the same hormone concentration show different physiological responses?
Because hormone effects depend on receptor number and affinity, binding globulins, enzymes, and intracellular signalling pathways, not just circulating hormone concentration.
Name two hormones released from the posterior pituitary and where they are synthesized.
Oxytocin and vasopressin (ADH), both synthesized in the hypothalamus.
Name two secondary messengers involved in GPCR signalling and their general role
cAMP and IP₃/DAG. they transmit and amplify the signal inside the cell, activating protein kinases
What role do inhibitory proteins play in steroid receptor function?
They keep the receptor inactive until hormone binding causes their dissociation, allowing receptor activation.
Why is receptor recycling important
It maintains receptor availability and prevents loss of hormone sensitivity
What are binding globulins, and why are they important for hormone action?
Binding globulins are plasma proteins that bind hormones, increasing their solubility and influencing transport, availability, and metabolic clearance, thereby regulating hormone action.
Explain how hypothalamic releasing hormones reach the anterior pituitary.
They are secreted into the hypothalamic–pituitary portal blood system, which directly transports them to the anterior pituitary.
Explain how signal amplification occurs in intracellular signalling pathways
One hormone–receptor interaction activates many G-proteins, each generating many second messenger molecules, producing a large cellular response from a small initial signal.
Why do steroid hormone effects tend to be slower but longer-lasting?
Steroid hormones regulate gene transcription, leading to new protein synthesis rather than rapid modification of existing proteins.
Why is the free hormone hypothesis considered an oversimplification
Because binding globulins likely regulate transport, clearance, and tissue-specific delivery, not just hormone inactivity.
Is having too much or too little hormone a good or bad thing? Explain using trade-offs.
Both too much and too little hormone can be harmful. Hormones involve trade-offs: levels that optimize one function (e.g. growth or reproduction) may increase costs such as disease risk or reduced survival.
Why is the pituitary often incorrectly called the “master gland”?
Because pituitary hormone secretion is regulated by the hypothalamus, meaning it does not function independently.
Tell the full story of GPCR signalling from hormone binding to cell response
A hormone binds a membrane GPCR, activating a G-protein. The G-protein stimulates production of second messengers such as cAMP or IP₃/DAG, which activate protein kinases. These kinases modify enzymes, ion channels, or transcription factors, leading to changes in cell function and sometimes gene expression.
Compare classical (genomic) and rapid (non-genomic) steroid hormone action
Classical action involves intracellular receptors acting as transcription factors, whereas rapid action involves membrane-associated receptors that activate second messenger pathways without directly altering gene transcription.
How can enzymes, receptors, and binding globulins regulate hormone action independently of secretion?
Enzymes can activate or inactivate hormones locally, receptor number and sensitivity can change, and binding globulins influence hormone availability and clearance, all independently of hormone secretion rates.