U1-KA4 -COMMUNICATION AND SIGNALLING 1) hydrophobic signals and hydrophilic signals

Question 1

Multicellular organisms show division of labour. What does this mean

Answer 1

This means that different cells carry out different functions within defined areas of the body

Question 2

In a multicellular,it is essential that cells communicate with one another.
Why?

Answer 2

Without this communication the integration and coordination of cellular activities would be impossible

Question 3

• How is communication achieve in multicellular organisms

- what does this ensure

Answer 3

• the achieve this communication cells use extra cellular signalling molecules and and complementary receptor proteins.
• this ensures the co-ordinated functioning of the whole organism

Question 4

What are examples of extracellular signalling molecules

Answer 4

• steroid hormones
• peptide hormones
• neurotransmitters

Question 5

The same series of events occur whatever the detail of the signal , target or intended response, what are these events

Answer 5

• the signal molecules released from one cell are specific to the receptor molecules of the target cells - the target cells have a receptors with binding sites for the specific signal molecule
• when binding occurs , it changes the conformation of the RECEPTOR
• this change in conformation changes the behaviour of the target cell in some way and this initiates a response within the cell.

Question 6

Different cell types produce ________ signals that can only be detected and responded to by cells with the ________ ________.

Answer 6

Different cell types produce specific signals that can only be detected and responded to by cells with the specific receptor.

Question 7

tissue specific response

Answer 7

• in a multicellular organism, different cell types may show a tissue specific response to the SAME signal

Question 8

Signalling molecules may have different effects on different target cell types. Why?

Answer 8

Signalling molecules may have different effects on different target cell types due to the differences in the intracellular signalling molecules and pathways that are involved

Question 9

What are the main hormone secreting endocrine glands in the body, and how do they allow multiple signalling events to occur simultaneously.
- give examples

Answer 9

• the main hormone secreting endocrine glands in the human body include : pineal gland , pituitary gland , parathyroid gland , thyroid gland, pancreas , adrenal glands etc
• each gland releases one or more specific signal molecule
• only CERTAIN tissues contain cells with the specific receptors for these molecules
• in this way , multiple signalling events can occur simultaneously
• For example: the pancreas can release insulin , the pituitary gland can release anti diuretic hormone and the thyroid gland can release thyroxine all at the same time , but the different target cells only respond to their intended message.

Question 10

Hormones

• what are they
• how do they bring response

Answer 10

• hormones are extra cellular signalling molecules that are secreted by one tissue (such as an endocrine gland)
• the hormone circulates in the bloodstream until it reaches its receptor protein of the target cell / or until it is broke down
• after binding , conformation of receptor changes causing change in behaviour of target cell to bring a response

Question 11

Hormones are either hydrophobic and hydrophilic , give examples of each

Answer 11

• Hormones are either hydrophobic : such as the steroid hormones oestrogen and testosterone
• or they are hydrophilic , such as peptide hormones

Question 12

What are neurotransmitters

- why is nervous communication so specific and rapid

Answer 12

• The signals that are released into the synaptic gap between a nerve cell and its neighbour are called neurotransmitters
• nervous communication is very specific and rapid due to the intimate association between the signalling cell and its target cell

Question 13

Neurotransmitters are ___________ signalling molecules

Answer 13

Neurotransmitters are hydrophilic signalling molecules

Question 14

Which type of signalling molecules can pass through membranes

Answer 14

• a hydrophobic signalling molecule is able to pass though membranes
• a hydrophilic signalling molecule cannot

Question 15

Where are the location of receptor proteins of hydrophobic and hydrophilic signalling molecules , and why?

Answer 15

• hydrophobic signalling molecules can pass through membranes, therefore their receptors molecules are within the cytoplasm or nucleus of the target cell
• hydrophilic signalling molecules cannot pass through membranes so require integral cell surface receptor proteins

Look at diagrams in page 34 brightred

Question 16

How can hydrophobic signals directly influence transcription of genes

Answer 16

• hydrophobic signalling molecules are lipid soluble : therefore able to diffuse directly through the phospholipid bi layers of membranes and so bind to intracellular receptors.
• the receptors are therefore in the cytosol or within the nucleus of the target cell
• the receptor proteins for steroid hormones are transcription factors
• transcription factors are proteins that when bound to DNA can either stimulate or inhibit initiation of transcription.
• once the hormone signal has bound to the receptor , the proteins change conformation and the hormone-receptor complex is formed. This transcription factor can then bind to specific sites and affect gene expression.
• the hormone-receptor complex binds to specific DNA sequences called hormone response elements (HRES). Binding at these sites influences the rate of transcription , with each steroid hormone affecting the gene expression of many different genes.

Question 17

Summary of hydrophobic signals directly influencing translation

Answer 17

• steroid hormone passes across plasma membrane
• the hormone binds to receptor, changing its conformation activating it
• the hormone-receptor complex binds to specific DNA sequences called hormone response elements (HREs)
• binding at HRE influences the rate of transcription

Question 18

The ________ proteins for steroid hormones are _____________ factors.

Answer 18

The receptor proteins for steroid hormones are transcription factors

Question 19

What are transcription factors

Answer 19

Transcription factors are proteins that when bound to DNA can either stimulate or inhibit transcription.

Question 20

What are sequence named hormone response elements (HRES)

- what does binding at these sites do

Answer 20

HRES are specific DNA sequences at which the hormone - receptor complex binds to
Binding at these sites influences the rate of transcription, with each steroid hormone affecting the gene expression of many different genes

Question 21

hydrophilic signalling molecules examples (recap)

Answer 21

hydrophilic signalling molecules such as neurotransmitters and peptide hormones

Question 22

can hydrophilic signals cross the plasma membrane , how does this effect the way they act

Answer 22

hydrophilic molecules cannot cross the plasma membrane of their target cells.
- they therefore act by binding to cell surface receptors

Question 23

what are the cell surface receptors that hydrophilic molecules bind to

Answer 23

these receptors can be ligand gated ion channels

Question 24

explain signal transduction

Answer 24

• hydrophilic signalling molecules ,such as peptide hormones and neurotransmitters are not lipid soluble , therefore cannot pass the hydrophobic part of the plasma membrane.
• they instead bind to transmembrane receptor molecules and do not enter the cytosol.
• these transmembrane proteins change conformation when the signal ligand binds to the extracellular cell surface; the signal molecule does not enter the cell but the signal is transduced across the plasma membrane
• this means that the behaviour of the cell changes in response to the external binding of the signal molecule . the extracellular ligand binding is converted into intracellular signals

Question 25

describe transducers and what they do / how they do it

Answer 25

• transmembrane receptor proteins act as signal transducers by converting an extracellular ligand binding event to a specific intracellular response through a signal transduction pathway

Question 26

what do transduced hydrophilic signals often involve

Answer 26

transduced hydrophilic signals often involve G proteins or cascades of phosphorylation by kinase enzymes

Question 27

explain what G proteins do

Answer 27

G proteins relay signals from activated receptors (receptors that have bound a signalling molecule) to target proteins such as enzymes and ion channels.
-the enzyme or ion channels will also undergo conformational change and bring about a behavioural change in the cell.

Question 28

explain, in more detail transduction by G proteins (don’t really need to know this very specifically )

Answer 28

• G protein coupled receptors (GPCRs) are linked to a G protein.
• the G protein can act as a switch that is either on or off ( depending on which of the two guanine nucleotides GDP or GTP is attached.
• when a hydrophilic signalling molecule binds to the extracellular side of a G protein GPCR a cascade of events is initiated.
• on binding of a hydrophilic hormone to the receptor , (GTP replaces GDP in the G protein ) the G protein becomes active .
• the active G protein stimulates an enzyme leading to a response in the cell.
• (response is only temporary - the G protein also acts as GTPase and soon hydrolyses the bound GTP into GDP, making the G protein inactive again.

Question 29

explain phosphorylation cascades in transduced hydrophilic signals

Answer 29

• (after; the hydrophilic signal molecule has attached to to the transmembrane receptor , causing a change in conformation in the transmembrane protein-the signal molecule does not enter the cell but the signal is transduced across the plasma membrane )
• phosphorylation cascades allow more than one intracellular signalling pathway to be activated.
• phosphorylation cascades involve a series of events with one kinase activating the next in the sequence and so on.
• phosphorylation cascades can result in the phosphorylation of many proteins as a result of the signalling extent.

Question 30

phosphorylation cascades can result in the phosphorylation of ____ ________ as a result of the __________ ______.

Answer 30

phosphorylation cascades can result in the phosphorylation of many proteins as a result of the signalling extent

Question 31

peptide hormones

• what are they
• give examples of them
• why is the action of these hormones highly specific

Answer 31

• peptide hormones are small hydrophilic proteins
• well known examples include insulin, glucagon, somatotropin (growth hormone) and anti diuretic hormone (ADH)
• each one requires a specific receptor on its target cell surface. since only target cells have the appropriate receptors at their surface , the action of these hormones is highly specific.

Question 32

examples of neurotransmitters

-role/function of them

Answer 32

acetylcholine (ACh) and noradrenaline

• both are hydrophilic peptides
• ACh is the transmitter at the neuromuscular junction connecting motor nerves to muscles. Noradrenaline has a role in the central nervous system and the sympathetic nervous system.