Cell signalling & communication

Question 1

Why do cells communicate?

Answer 1

• respond to changes within their own intracellular fluid

* need to maintain homeostasis - ensure factors within internal environment are within narrow limits

Question 2

What are gap junctions?

Answer 2

In animal cells, they consist of protein lined pored in the plasma membranes of adjacent cells, allowing small molecules to pass between cells and electrical signal transmission.

Question 3

What is plasmodesmata?

Answer 3

• Cell to cell communication in plants is achieved through gaps through cell walls.
• The plasma membrane of cells is continuous and forms the lining of the plasmodesmata.

Question 4

What is the definition of signal reception?

Answer 4

Process of detection of a signal from the external environment by a target cell involving the binding of a signalling molecule to a specific receptor.

Question 5

What is the definition of signal transduction?

Answer 5

Signalling pathway in which a series of steps are triggered that is brought about by the change in receptor molecule.

Question 6

Where can receptors be located?

Answer 6

• Receptors can be located either on the plasma membrane of a specific target cell (cell surface receptors)
• Receptors can be located either in the cytosol or nucleus of the specific target cell (intracellular receptors)

Question 7

What generally occurs in signal transduction?

Answer 7

• process that converts signal from outside the cell into a response within the cell.
• signal received in one form, changed to another molecule and relayed to appropriate target within the cell that responds through an effector protein

Question 8

How does transduction of a hydrophobic signal occur?

Answer 8

• Steroid hormones are hydrophobic, able to cross the lipid bilayer of the plasma membrane through diffusion.
• inside the cell the steroid hormone binds with the specific receptor, a protein in solution either in the cytosol or the fluid component of the nucleus.

Question 9

What is the transduction pathway of a hydrophobic signal?

Answer 9

1. The binding of the steroid hormone to its specific receptor produces a change in the 3D shape of the receptor protein, exposing a region of the receptor that was previously within the molecule
2. The hormone receptor protein complex moves from the cytoplasm into the nucleus.
3. The exposed segment of the receptor protein attaches to a target DNA sequence next to certain genes and activated those genes.

Question 10

How does transduction of a hydrophilic signal occur?

Answer 10

• The peptide hormone binds to the specific receptor on the cell surface as it is hydrophilic and cannot cross the lipid bilayer of the plasma membrane.
• The signal must be transferred from receptors on the cell surface to second messengers within the cytoplasm.
• large numbers of molecules of a second messenger are produced which amplify the original signal by a factor of up to 1000 in order to transfer it from receptor to nucleus.

Question 11

Solubility in water

Answer 11

PROTEIN AND PEPTIDE HORMONES: yes as they are hydrophilic and require no assistance to travel through the blood.

STEROID AND THYROID HORMONES: no as they are hydophobic and require a carrier protein for transport by the blood

Question 12

What are the 3 basic steps of signal transduction pathway?

Answer 12

1. signalling molecule or ligand binds to a receptor
2. transduction - conveying message to cell
3. response (effects)

Question 13

What are the different types of responses that can occur in a cell?

Answer 13

• SHORT term: increase in enzyme activity

- LONG term: altered DNA transcription

Question 14

What is hormone action signalling?

Answer 14

When signalling molecules/ligands travel to the target cell via the bloodstream

Question 15

Compare paracrine and autocrine action signalling

Answer 15

Paracrine: binds to receptors on a nearby cell

Autocrine: signalling molecule binds to receptors located on the cell that secreted it.

• They both do NOT enter the blood. Only travel in the interstitial fluid.

Question 16

What type of signalling molecules bind to intracellular receptors?

Answer 16

• hydrophobic/lipid soluble signalling molecule (ie. cortisol, thyroxine, estrogen) binds to intracellular receptor
• receptor has chaperone attaches
• chaperon is released upon binding of signalling molecule
• cortisol and receptor bind to DNA and activate transcription of cellular genes

Question 17

What are the 3 main components of a membrane bound receptor?

Answer 17

• extracellular domain - binds signalling molecule
• transmembrane region - anchors it to the plasma membrane
• intracellular region - important for cell response (contains the active site for a protein/enzyme)

Question 18

What is direct transduction? Give an example.

Answer 18

• It is a direct cell response

• Acetylcholine binds to the extra-cellular response
• receptor’s 3D shape changes
• Na+ ions cross the membrane - this leads to the response of the muscle contraction

Question 19

What are the 3 types of membrane bound receptors?

Answer 19

1. Ligand - gated ion channel receptor
2. G-protein linked receptor
3. Protein kinase receptor

Question 20

Describe the pathway of a G-protein linked receptor

Answer 20

• signalling molecule binds to the the G-protein linked receptor
• activation of the G-protein causes GDP to convert to GTP
• part of the G-protein with the GTP moves along the membrane to and activates the effector molecule

Question 21

Describe the pathway of a protein kinase receptor

Answer 21

• the alpha subunit binds the insulin
• this causes the intracellular portion of the receptor to change shape and be phosphorylated
• Protein kinase phosphorylates the insulin-response-substrate (2nd messenger) which leads to the cellular response

Question 22

What happens to the cell signal once a normal cell has responded?

Answer 22

• when signal binds, GDP id converted to GTP by the addition of a phosphate
• then the Ras (family of protein known as GTP-ases) is able to bind to the GTP
• this causes cell division (Cell response)
• after a brief time, the phosphate in GTP is lost and its converted into GDP. This converts the Ras into its inactive form

Question 23

What happens if there is a mutation in the Ras molecule?

Answer 23

• the GTP do not lose the phosphate group to be turned back into GDP
• this prevents Ras remains in the active form
• therefore, there is constant stimulation of cell division and and isn’t switched off - leading to tumours

Question 24

What is the desired cell response when epinephrine binds to liver cell receptors?

Answer 24

• The epinephrine binds to the liver cell receptors
• G-protein mediated cascade results in the phosphorylation of various enzymes
• the eventual result of this cascade is:
- the inhibition of glycogen synthase and
- release of glucose

• when epinephrine is released the body needs elevated levels of glucose to provide the body with energy to escape from the danger