Unit 1: Communication and Signalling

Question 1

Describe the series of events that occur in cell signalling?

Answer 1

1. Extracellular signalling molecule is released from the cell.
2. Extracellular signalling molecule travels to target cell and the signal molecule binds to the receptor on target cell.
3. Binding causes a conformational change in the protein
4. This alters the response of the cell.

Question 2

Describe the structure of receptor molecules of target cells?

Answer 2

Receptor molecules of target cells are
proteins with a binding site for a specific
signal molecule

Question 3

Why might signalling molecules have different effects on different target cells?

Answer 3

this could be due to differences in the intracellular signalling
molecules and pathways that are involved.
In a multicellular organism, different cell
types may show a tissue-specific response to
the same signal

Question 4

How dod hydrophobic signals interact with phospholipid bilayers of membranes?

Answer 4

Hydrophobic signalling molecules can diffuse
directly through the phospholipid bilayers of
membranes, and so bind to intracellular
receptors

Question 5

What is the term used to describe the receptors for hydrophobic signalling molecules?

Answer 5

The receptors for hydrophobic signalling
molecules are transcription factors

Question 6

What are transcription factors?

Answer 6

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

Question 7

What are some examples of hydrophobic signal molecules?

Answer 7

Steroid hormones such as testosterone or oestrogen

Question 8

What do steroid hormones bind to?

Answer 8

Steroid hormones bind to receptors in the cytosol or the nucleus to form a HORMONE-RECEPTOR COMPLEX

Question 9

What do hormone receptor complexes do?

Answer 9

They move to the nucleus and bind to specific sites on DNA and the specific DNA sequences they bind to are called HORMONE RESPONSE ELEMENTS (HRE)

Question 10

How do hydrophilic signalling molecules interact with phospholipid bilayers of membranes?

Answer 10

Hydrophilic signalling molecules bind to receptors on transmembrane proteins and do not enter the cytosol

Question 11

Why do hydrophilic signalling molecules not enter the cytosol?

Answer 11

Because they are not capable of passing through the hydrophobic plasma membrane

Question 12

What do transmembrane receptors do when a ligand binds to the extracellular face?

Answer 12

They change conformation and then transduce the signal across the plasma membrane

Question 13

How do transmembrane receptors transduce signals across the plasma membrane?

Answer 13

Transmembrane receptors act as signal
transducers by converting the extracellular
ligand-binding event into intracellular signals,
which alters the behaviour of the cell

Question 14

What two things do transduced hydrophilic signals often involve?

Answer 14

G proteins
Cascades of phosphorylation by kinase enzymes

Question 15

What do G proteins do?

Answer 15

G-proteins relay signals from activated
receptors to target proteins such
as enzymes and ion channels.

Question 16

What are phosphorylation cascades?

Answer 16

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 original signalling event

Question 17

What do phosphorylation cascades allow to happen?

Answer 17

Phosphorylation cascades allow more than
one intracellular signalling pathway to be
activated

Question 18

What is type 1 diabetes caused by?

Answer 18

Failure to produce insulin.

Question 19

What is type 2 diabetes caused by?

Answer 19

Loss of insulin receptor function

Question 20

Describe the process by which insulin binds to its receptor?

Answer 20

1. Insulin binds to its receptor.
2. Receptor protein undergoes conformational change
   3.Conformational change causes phosphorylation cascade to occur inside cell.
3. This causes vesicles containing GLUT4 to be transported to cell membrane
4. This causes glucose to be able to pass through GLUT4

Question 21

what is resting membrane potential?

Answer 21

Resting membrane potential is a state where
there is no net flow of ions across the
membrane

Question 22

what is an action potential?

Answer 22

An action potential is a wave of electrical
excitation along a neuron’s plasma membrane

Question 23

Describe the steps involved in a nerve transmission?

Answer 23

Binding of a neurotransmitter with its receptor triggers the
opening of ligand-gated ion channels at a synapse.
Ion movement occurs and there is depolarisation of the plasma membrane.

If sufficient ion movement occurs, and the membrane is depolarised beyond a threshold value, the opening of voltage-gated sodium channels is triggered and sodium ions enter the cell down their electrochemical gradient.

This leads to a rapid and large change in the membrane potential. A short time after opening, the sodium channels become inactivated.

Voltage-gated potassium channels then open to allow potassium ions to move out of the cell to restore the resting
membrane potential.

Question 24

What is the function of cone and rod cells in the retina?

Answer 24

Rods function in dim light but do not allow
colour perception. Cones are responsible for
colour vision and only function in bright light.

Question 25

Describe the retinal response to light?

Answer 25

Retinal absorbs a photon of light and rhodopsin changes conformation to photoexcited rhodopsin

Photoexcited rhodopsin activates a G-protein, called transducin, which activates the enzyme phosphodiesterase (PDE)

PDE catalyses the hydrolysis of a molecule called cyclic GMP (cGMP)

This results in the closure of ion channels in the membrane of the rod cells, which triggers nerve impulses in neurons in the retina

A very high degree of amplification results in rod cells being able to respond to low intensities of light