Intracellular signalling

Question 1

What is signal transduction?

Answer 1

The process by which an extracellular signal activates a membrane receptor, which then alters intracellular molecules, activating a cellular response

Question 2

What happens during signal transduction?

Answer 2

One form of signal is converted into another

Question 3

Why are signal transduction pathways important?

Answer 3

They allow communication between cells of different tissues or organs.

Question 4

What are the first and second messengers in signal transduction?

Answer 4

The first messenger is the extracellular signal molecule, while intracellular molecules form the second messenger system.

Question 5

Name 4 examples of extracellular signalling molecules.

Answer 5

Amines (e.g., adrenaline)
Peptides/proteins (e.g, insulin)
Steroids (e.g., hormones)
Other small molecules (e.g, amino acids, gases)

Question 6

How do small non-polar molecules like steroids signal?

Answer 6

They diffuse through the plasma membrane and bind to intracellular receptors, often acting as transcription factors for gene regulation (slow response).

Question 7

How do large polar molecules signal?

Answer 7

They bind to cell surface receptors, activating intracellular pathways for a faster response.

Question 8

What are the 4 main classes of receptors?

Answer 8

1. Ligand-gated ion channels
2. G-protein coupled receptors (GPCRs)
3. Enzyme-linked receptors
4. Nuclear receptors

Question 9

What are ionotropic receptors?

Answer 9

Receptors that form an ion channel pore, such as nicotinic acetylcholine and GABA 𝐴 receptors
​

Question 10

What are metabotropic receptors?

Answer 10

Receptors indirectly linked to ion channels through G protein-mediated signal transduction pathways, such as muscarinic acetylcholine and GABA 𝐵 receptors.

Question 11

Compare ionotropic and metabotropic receptors.

Answer 11

Ionotropic receptors directly open ion channels, while metabotropic receptors activate signalling pathways to affect channels indirectly.

Question 12

What are the components of GPCRs?

Answer 12

3 subunits:
alpha (α),
beta (β),
gamma (γ).

Question 13

What is the common structure of GPCRs?

Answer 13

7 transmembrane domains encoded by over 800 genes.

Question 14

What happens in the G protein cycle when adrenaline binds to a β-adrenoreceptor?

Answer 14

The receptor activates G protein interaction, GDP is exchanged for GTP, the α subunit activates adenylyl cyclase, and a cellular response occurs

Question 15

How does cAMP activate Protein Kinase A (PKA)?

Answer 15

cAMP binds to PKA’s regulatory subunits, releasing the catalytic subunits to phosphorylate target proteins.

Question 16

Give examples of cellular responses mediated by the cAMP/PKA pathway.

Answer 16

Kidney: Water retention by vasopressin
Cardiac muscle: Increased heart rate by adrenaline
Pancreas: Glucose release by glucagon

Question 17

How is signal transduction terminated?

Answer 17

Through signal or receptor removal/inactivation, dephosphorylation of proteins, or degradation of second messengers.

Question 18

What is the role of phosphodiesterase in signal termination?

Answer 18

PDE hydrolyses cAMP, rapidly decreasing its concentration to turn off the response.

Question 19

What happens during β-adrenoreceptor desensitisation?

Answer 19

Phosphorylation by β-ARK reduces receptor affinity for adrenaline, decreasing cellular response.

Question 20

Describe the structure of inactive PKA.

Answer 20

A tetramer with regulatory (R) and catalytic (C) subunits bound together, suppressing catalytic activity.

Question 21

How is PKA activated?

Answer 21

cAMP binding to R subunits releases active C subunits.

Question 22

What does active PKA do?

Answer 22

Phosphorylates serine/threonine residues on substrate proteins, affecting cellular functions.

Question 23

What does the nicotinic acetylcholine receptor do?

Answer 23

it forms an ion channel, allowing Na+ and Ca2+ entry upon acetylcholine binding, triggering electrical responses.

Question 24

What does the GABA 𝐴 receptor regulate?

Answer 24

It allows Cl- ion entry, serving as an inhibitory receptor important in the CNS.

Question 25

What does the muscarinic acetylcholine receptor regulate?

Answer 25

It activates signal transduction pathways via G proteins to affect ion channels indirectly.

Question 26

What is the role of protein phosphorylation in signalling?

Answer 26

Kinases transfer phosphate groups to proteins, altering their activity and triggering cellular responses.

Question 27

What is dephosphorylation, and what enzyme performs it?

Answer 27

Removal of phosphate groups by protein phosphatases, often deactivating the protein.

Question 28

How does caffeine affect signal termination?

Answer 28

Caffeine inhibits PDE, prolonging cAMP levels and cellular responses.

Question 29

Name a physiological response mediated by adrenaline through GPCRs.

Answer 29

Increased heart rate and relaxation of vascular smooth muscle.

Question 30

What is the role of CREB in the cAMP pathway?

Answer 30

Phosphorylated by PKA, CREb activates the transcription of target genes.

Question 31

What are the 3 main stages of signal transduction?

Answer 31

1. Signal molecule binds receptor.
2. Intracellular molecules relay the signal.
3. Cellular response is activated.