Intro Cellular Signalling

Question 1

Name the 4 forms of cell signalling.

Answer 1

1. autocrine
2. gap junctions
3. paracrine
4. endocrine

Question 2

What is autocrine signalling?

Answer 2

• a cell releases a ligand that the signalling cell can bind to itself
• in some cases, neighbouring cells of the same type are also influenced by the release ligand

Question 3

When is autocrine signalling used?

Answer 3

• during early development of an organism to ensure that cells develop into the correct tissues & take on proper function
• in embryological development, stimulating a group of neighbouring cells can direct the differentiation of identical cells

Question 4

What is gap junction signalling?

Answer 4

a cell targets a cell connected by the plasma membrane of neighbouring cells
(gap junction in animals and plasmodesmata in plants)
- much more targeted and specific signalling between cells
- no chance for signal to escape and influence a neighbouring cell that it wasn’t aimed at

Question 5

How does gap junction cell signalling work?

Answer 5

Water-filled channels between the plasma membrane of neighbouring cells allow small signalling molecules (called intracellular mediators) to diffuse between the two cells

• small molecules, such as Ca2+, are able to move between cells, but large molecules (proteins) cannot fit through the channels
• specificity of the channel ensures that the cell remains independent

Question 6

What is paracrine cell signalling? Inc example

Answer 6

a cell targets a nearby cell

• signals that act locally between cells that are close together
• e.g. happens in synapses in nerve cells

Question 7

How does paracrine cell signalling work?

Answer 7

paracrine signals move by diffusion through the extracellular matrix

• elicit quick responses that lasts only a short amount of time
• paracrine ligand molecules are normally quickly degraded by enzymes or removed by neighbouring cells
• removing the signals will re-establish the conc gradient for the signal, allowing them to quickly diffuse through the intracellular space if released again

Question 8

What is endocrine cell signalling?

Answer 8

A cell targets a distant cell through the blood stream

- they originate from endocrine cells (many located in endocrine glands, such as thyroid gland, hypothalamus etc)

Question 9

How does endocrine cell signalling work?

Answer 9

Ligands released in endocrine signalling are hormones

• they are able to travel the large distances between endocrine cells and their target cells via blood stream
• types of signals usually produce a slower response, but have a longer lasting effect

Question 10

Name the 2 types of signalling receptors.

Answer 10

1. cell surface receptors

2. intracellular receptors

Question 11

What are cell surface receptors (transmembrane receptors)?

Answer 11

• membrane-anchored / integral proteins that bind to external ligand molecules
• spans the plasma membrane and performs signal transduction, converting an extracellular signal into an intracellular one
• ligands that interact with cell-surface receptors do not have to enter the cell to have an affect (some ligands do tho)
• involved in most of the signalling in multicellular organisms

Question 12

What are the 3 main components of cell surface receptors?

Answer 12

1) extracellular ligand-binding domain
- where the signal comes in
2) trans-membrane domain
- hydrophobic membrane-spanning region
3) intracellular domain
- has to create a new intracellular signal
- inside the cell

Question 13

What are the 3 general categories of cell-surface receptors?

Answer 13

1. ion channel-linked receptors
2. G-protein-coupled receptors
3. enzyme-linked receptors

Question 14

How do ion channel linked receptors work?

Answer 14

> ion channel-linked receptors bind a ligand and open a channel through the membrane that allows specific ions to pass through
to form a channel, it has an extensive membrane-spanning region
in order to interact with the phospholipid fatty acid tails that form the centre of the plasma membrane, many of the a.a. in the membrane-spanning region are hydrophobic
a.a. that line the inside of the channel are hydrophilic to allow passage of water or ions

Question 15

What are G-protein linked receptors?

Answer 15

> g-protein-linked receptors bind a ligand and activate a membrane protein called a G-protein
the activated protein then interacts with either an ion channel or an enzyme in the membrane

Question 16

How do G-protein coupled receptors work?

Answer 16

• cell signalling using g-protein-coupled receptors is a cyclic series of events
• once ligand is bound to the receptor, a conformational shape change activates the G-protein, which releases GDP & picks up GTP
• the subunits of the G-protein then split into the alpha subunit & beta subunit (these are able to activate other proteins)
• The GTP on the active alpha subunit is hydrolysed to GDP and the subunit is deactivated
• the subunits re-associate to form the inactive G-protein
• CYCLE STARTS OVER

Question 17

What are enzyme-linked receptors?

Answer 17

> enzyme-linked receptors are cell-surface receptors with intracellular domains that are associated with an enzyme
the intracellular domain of the receptor itself can be an enzyme or interacts directly with an enzyme
single alpha-helical membrane-spanning region with large extracellular and intracellular domains

Question 18

How do enzyme-linked receptors work? Incl e.g.

Answer 18

• when a ligand binds to the extracellular domain, a signal is transferred through the membrane and activates the enzyme
• this sets off a chain of events within the cell that eventually leads to a response
• e.g. tyrosine kinase receptors that dimerise once ligand is bound & auto-phosphorylate

Question 19

What are intracellular receptors?

Answer 19

> receptors found in the cytoplasm or nuclei of cells and respond to hydrophobic ligand molecules that are able to travel across the plasma membrane
once inside the cell, many of these molecules bind to receptors that act as transcription factors
in some cases, when the ligand binds the receptor, a conformational change exposes a DNA-binding site on the protein
the ligand-receptor complex moves can move into the nucleus and cause its effect

Question 20

What are signalling molecules?

Answer 20

• produced by signalling cells

- ligands act as chemical signals that travel to the target cells to coordinate responses

Question 21

What are the diff types of molecules that serve as ligands?

Answer 21

Incredibly varied range of small proteins to small ions

Question 22

What are water-soluble ligands?

Answer 22

• water-soluble ligands are polar so cannot pass though the plasma membrane w/o aid
• most water-soluble ligands bind to the extracellular domain of cell-surface receptors
• diverse set of ligands, incl small molecules, peptides & proteins

Question 23

What are small hydrophobic ligands?

Answer 23

• can directly diffuse through the plasma membrane

- e.g. steroid hormones such as testosterone or estradiol (and other derivatives of cholesterol)

Question 24

How can nitric oxide (NO) act as a ligand?

Answer 24

• it is able to diffuse across the plasma membrane
• one of its roles is to interact with receptors in smooth muscle & induce relaxation of the tissue
• v. short half-life so can only function over short distances