Chapter 6: Cell Signalling

Question 1

3 general ways that cells communicate with one another

Answer 1

1. gap junctions
2. cell to cell binding
3. extracellular chemical messengers

Question 2

📝 What are Gap Junctions?

Answer 2

Special protein tunnels (connexons) that connect neighbouring cells.

✅ Key Functions:

• Allow ions & small molecules to move between cells.
• Help synchronize neural communication.
• Enable cells to contract together as a unit (e.g., heart muscle).

Question 3

📝 What is Cell-to-Cell Binding?

Answer 3

A surface molecule on one cell binds to a surface molecule on another cell.

✅ Key Functions:

• Important during development to guide cell interactions.
• Leukocytes (white blood cells) use this to recognize foreign invaders.
• Helps the immune system detect and respond to threats.

Question 4

What are Extracellular Chemical Messengers?

Answer 4

cell releases a chemical messenger into the extracellular fluid (ECF) to communicate.

Key Functions:

• Most common method of cell communication.
• The messenger diffuses through ECF and may reach many cells.
  Examples: Hormones & neurotransmitters send signals in this way.

Question 5

Extracellular chemical messenger:

Answer 5

molecule that is release by a cell, enters extracellular fluid and then binds to a receptor on or in its target cell to cause a response

Question 6

1. Hormones (endocrine signaling)

Answer 6

• Carried by the blood to distant target cells
• Secreted by a cell into in interstitial fluid and diffused into the bloodstream to distribute to target cells as they have the correct responders to the hormone

Example: Glucagon (secreted by certain cells of the pancreas)

Question 7

2. Neurotransmitters (synaptic signaling)

Answer 7

• Extracellular chemical messengers that are released from a neuron into a synapse in order to reach a nearby target cell
• Synaptic signalling: form of short distance communication
• Neuron sends the message is called: presynaptic neuron
• Cell that received the message is called: postsynaptic neuron

Example: Dopamine (emotions)

Question 8

postsynaptic vs synaptic neuron

Answer 8

Neuron sends the message is called: presynaptic neuron

Cell that received the message is called: postsynaptic neuron

Question 9

3. Local Mediators (paracrine & autocrine signaling)

Answer 9

Extracellular chemical messengers that act on nearby target cells without entering the bloodstream
- paracrine and autocrine

Question 10

Paracrine

Answer 10

local mediators that act on neighboring cells

Question 11

Autocrine

Answer 11

those that act on the same cell that secreted them

Question 12

Local signaling

Answer 12

cell signalling that occurs through local mediators, type of short distance communication

Question 13

Cytokinesis

Answer 13

group of local mediators, molecules that regulate many cell functions including cell growth and differentiation

Question 14

Nitric Oxide

Answer 14

type of local mediator, is the gas which is released by endothelial cells lining blood vessels

Question 15

Eicosanoids

Answer 15

molecules released by many cells of the body In response to chemical or mechanical stimuli
1) Prostaglandin
2) Leukotrienes
3) Thromboxane

Question 16

steps of extracellular messenger communication:

Answer 16

S → Secretion (Release the message into interstitial fluid or blood)

T → Transport (Deliver the message using neurotransmitters & local mediators)

B → Binding (Find the target cell and bind to the receptor)

S → Signal Transduction (Trigger a response)

Question 17

Growth factors:

Answer 17

substances that play important roles in tissue development, growth and repair
- Nitrogenic substances because they cause growth by stimulating cell division

Question 18

Water-soluble Extracellular Messengers

Answer 18

solubility: in water
- move freely in plasma
- more common
- act faster

receptor location: on the cell membrane
examples: peptide/protein hormones, amine hormones, neurotransmitters

Question 19

lipid soluble extracellular messengers

Answer 19

solubility: in fats/lipids
- more slow
- longer-lasting effects on the cell

needs to transport proteins to transport in blood/plasma

receptor location: inside cytoplasm or nucleus
cellar action: alters gene expression
example: steroid hormones, thyroid hormones

Question 20

Transport proteins – 3 functions:

Answer 20

1) Make lipid soluble hormones temporarily water soluble
2) Retard passage of small hormone molecules through the filtering mechanism in the kidneys
3) Provide a ready reserve of hormones already present

Question 21

Receptor

Answer 21

chemical messengers influence their target cells by binding to specific protein receptors

• After given a messenger, the receptor undergoes confirmational change and is activated
• Role in cell signalling: detect a signal molecule (extracellular messenger) and then initiate the signal transduction process
• When chemical messenger leaves receptor = response comes to an end

Question 22

What is specificity in messenger-receptor binding?

Answer 22

A receptor only binds to one extracellular messenger or a small group of related messengers based on shape compatibility

Question 23

What does affinity mean in receptor binding?

Answer 23

Strength of binding between a messenger and receptor.

High affinity: Strong binding, low concentration needed for response.
Low affinity: Weak binding, high concentration needed for response.

Question 24

What happens when receptor saturation increases?

Answer 24

More messengers bind to receptors, leading to a greater cellular response.

Levels of Saturation:

Full saturation: 100% of receptors bound.
Half saturation: 50% of receptors bound.

Question 25

What is competition in receptor binding?

Answer 25

When multiple messengers are structurally similar and bind to the same receptor, they compete for binding

Question 26

Agonist

Answer 26

substance that binds to and activates a receptor

Question 27

Antagonist

Answer 27

a substance that binds to and blocks a receptor, preventing the exertion of effects

Question 28

where receptors are located: 1) Plasma membrane receptors

Answer 28

water-soluble extracellular messengers are unable to pass through the hydrophobic interior of the plasma membrane, their receptors are located in the plasma membrane

Question 29

where receptors are located: 2) Intracellular receptors

Answer 29

* Either in cytosol or in nucleus * lipid-soluble extracellular messengers are able to pass through the plasma membranes of their target cells.

Question 30

Down-Regulation

Answer 30

- If an extracellular messenger is present in excess, the number of target-cell receptors may decrease - Some receptors are removed from the plasma membrane via endocytosis and then degraded in lysosomes - Makes the target cell LESS SENSITIVE to the messenger

Question 31

Up-Regulation:

Answer 31

- The increase in the number of receptors occurs by the insertion of additional receptors into the plasma membrane from internal stores via exocytosis - Makes the target cell MORE SENSITIVE to messenger

Question 32

What is signal transduction?

Answer 32

The process where a signal molecule (messenger) is converted into a cellular response

Question 33

What is a signal transduction pathway?

Answer 33

A sequence of events that carries and processes the signal inside the cell.

Question 34

What are the key components of signal transduction?

Answer 34

1. Signaling pathway – Chain of events leading to a response 2. Effector protein – The protein that triggers the cellular response 3. Relay proteins – Proteins that carry the signal from the receptor to the effector protein

Question 35

What is the difference between a first messenger and a second messenger?

Answer 35

First messenger: The extracellular signal (e.g., hormones, neurotransmitters) Second messenger: The intracellular signal that continues the pathway inside the cell

Question 36

What is a protein kinase?

Answer 36

An enzyme that phosphorylates (adds a phosphate group) to activate or deactivate target proteins in the signalling pathway

Question 37

What is the main difference between the nervous system and the endocrine system?

Answer 37

Nervous system → Uses electrical signals (fast, short-lasting effects) Endocrine system → Uses hormones in the bloodstream (slow, long-lasting effects)

Question 38

How does the nervous system send signals?

Answer 38

Through graded potentials (short signals in dendrites) and action potentials (long signals along axons).

Question 39

How does the endocrine system send signals?

Answer 39

By releasing hormones into the bloodstream, which travel to distant target cells