Communication within Multicellular Organisms

Question 1

What are the 2 types of signalling systems cells have?

Answer 1

extracellular and intracellular

Question 2

What do signalling systems ensure each cell knows?

Answer 2

its

• position
• role
• involvement in activities (cell growth, division & development)

Question 3

What are receptor molecules of target cells?

Answer 3

proteins with a binding site for a signal molecule.

Question 4

What does binding of receptor molecules do?

Answer 4

Binding changes the conformation of the receptor and this can alter the response of the cell.

Question 5

What do different cell types produce?

Answer 5

Different cell types produce specific signals which can only be detected and responded to by cells with the specific receptor. In a multicellular organism, different cell types may show a tissue-specific response to the same signal.

Question 6

Why can the receptor molecules of hydrophobic signals be found within the nucleus?

Answer 6

Hydrophobic signals can pass through membranes so their receptor molecules can be within the nucleus. They are able to do this because the tails of the phospholipids in the plasma membrane are also hydrophobic and allow the molecules to pass across.

Question 7

What can hydrophobic signals directly influence?

Answer 7

Hydrophobic signals can directly influence transcription of genes

Question 8

Give examples of hydrophobic signalling molecules?

Answer 8

The thyroid hormone thyroxine and steroid hormones e.g. testosterone, oestrogen and progesterone.

Question 9

Describe thyroxine.

Answer 9

Thyroxine is a hydrophobic hormone produced by the thyroid gland. It is involved in regulating the rate of metabolism. Because it is hydrophobic, it can cross the plasma membrane of a cell and interact with proteins inside the cell.

Question 10

Describe what occurs when thyroxine is not present.

Answer 10

Thyroxine receptor binds to DNA
Transcription of Na/K-ATPase gene is inhibited
Less Na/K-ATPase is transcribed
Metabolic rate reduces

Question 11

Describe what occurs when thyroxine is present.

Answer 11

Thyroxine binds to thyroxine receptor protein
Receptor undergoes a conformational change, meaning it can no longer bind to DNA
Gene for Na/K-ATPase is transcribed
Metabolic rate increases

Question 12

What are the receptor proteins for steroid hormone?

Answer 12

transcription factors.

Question 13

What is a transcription factor and what does it do?

Answer 13

A transcription factor is a protein which binds to DNA and controls the rate of transcription. Transcription factors can enhance or block the binding of RNA polymerase to specific genes, thereby controlling whether the gene is transcribed or not.

Question 14

Describe the action of testosterone? (6)

Answer 14

1) Lipid soluble so diffuses into the cells through the plasma membrane
2) Target cells have a specific hormone receptor protein in the cytoplasm or nucleus
3) Testosterone binds the hormone receptor protein to form an activated complex
4) Conformational change makes the hormone receptor complex into an active gene transcription factor (gene regulatory protein)
5) The complex binds to specific DNA sites and stimulates gene transcription
6) Proteins are synthesised that support sperm production and male sexual characteristics

Question 15

Give 2 examples of hydrophilic signalling molecules.

Answer 15

peptide hormones and neurotransmitters.

Question 16

What do hydrophilic signalling molecules require? Why?

Answer 16

Hydrophilic signals require receptor molecules to be at the surface of the cell because they are not capable of passing across the hydrophobic plasma membrane.

Question 17

What are the 3 stages of signal transduction?

Answer 17

Reception
Transduction
Response

Question 18

Describe reception for hydrophilic signalling molecules.

Answer 18

Transmembrane receptors change conformation when the ligand binds outside the cell; the signal molecule does not enter the cell but the signal is transduced across the membrane of the cell.

Question 19

Describe transduction for hydrophilic signalling molecules.

Answer 19

This is the transfer of the molecular signal within the cell. It is a cascade of events, at each stage, the signal is transduced or changed into a different form.

Several molecules are involved in signal transduction pathways:

• Protein kinases
• Cyclic AMP
• Calcium ions.

Question 20

Describe response for hydrophilic signalling molecules.

Answer 20

Signal transduction pathway eventually leads to a response.

Such a response might include:

• Opening/closing of an ion channel in plasma membrane
• Rearrangement of cytoskeleton
• Synthesis/degradation of enzymes or other proteins
• Changes in gene expression.

Question 21

What are three types of signal transduction mechanisms?

Answer 21

Ion-channel linked receptors
Enzyme linked receptors
G-protein Coupled receptors

Question 22

Describe ion-channel linked receptors.

Answer 22

These are found on the surface of muscles and nerves and transduce a signal in the form of a neurotransmitter into an electrical voltage. Binding of the signal ligand causes a conformational change, ‘opening’ the channel.

Question 23

Describe enzyme linked receptors.

Answer 23

An enzyme linked receptor binds an extracellular signal molecule switching on an enzyme activity, usually a kinase, on the other side of the membrane. This kinase activity causes the phosphorylation of other intracellular proteins. These are found in all cells.

Question 24

Describe G-protein coupled receptors.

Answer 24

GPCRs activate the G-protein (G) in response to the signal binding. The G-protein acts on a target protein (T), which maybe an enzyme or an ion-channel protein. This generates the intracellular response.

Question 25

What does a G-protein act as?

Answer 25

a switch that is either on or off, depending on which of the two guanine nucleotides (GDP or GTP) is attached.

Question 26

Describe the switch function of G-proteins

Answer 26

Initially, when GDP is bound, the G-protein is inactive. On binding of a hydrophilic hormone to the receptor, GTP replaces GDP in the G-protein, and the G-protein becomes active.
The active G-protein stimulates an enzyme, leading to a response in the cell.

Question 27

Describe the sequence of events for GPCRs.

Answer 27

1. When a hormone binds to the extracellular protein it changes to the active form. This starts a chain reaction called a signal transduction pathway.
2. The receptor protein activates the G-protein so that it replaces GDP with GTP (guanosine triphosphate).
3. The activated G-protein moves along the membrane and activates the enzyme.
4. The activated enzyme releases hundreds of second messenger molecules.
5. Second messenger molecules are small molecules (usually cyclic AMP). They activate the next step in the reaction.
6. Cyclic AMP phosphorylates and activates kinase enzymes.
7. Kinase enzymes phosphorylate and activate other kinase enzymes and so on until the final enzyme in the chain is activated. This enzyme produces a change in the cell.
8. At each step the number of the activated enzymes increases. This is called signal amplification.