1.4 Communication and signalling

How do multicellular organisms signal between cells

using extracellular signalling molecules

What are examples of extracellular signalling molecules

Steroid hormones, peptide hormones and neurotransmitters

What are receptor molecules of target cells

Proteins with a binding site for a specific signal molecule

What does binding change within a cell

Changes the conformation of the receptor which initiates a response within a cell

If a cell type produces specific signals, how can they be detected and responded to

By cells with the specific receptor

Why might signalling molecules have different effects on different target cell types

due to differences in the intracellular signalling molecules and pathways that are involved

In a multicellular organism how do different cell types respond to the same signal

Show a tissue specific response

How do hydrophobic molecules bind to intra cellular receptors

By diffusing directly through the phospholipid bilayers of membranes

What are transcription factors

proteins that when bound to DNA can either stimulate or inhibit initiation of transcription

What are the receptor for hydrophobic signalling molecules

Transcription factors

What are examples of hydrophobic signalling molecules

Oestrogen and testosterone

Where do steroid hormones bind to specific receptors

In the cytosol or the nucleus

Where does the hormone receptor complex move to

nucleus where it binds to specific sites on DNA and affects gene expression

Where does the hormone receptor complex bind to

Specific DNA sequences called hormone response elements

What is the effect of the hormone receptor complex binding to hormone response elements

Influences the rate of transcription with each steroid hormone affecting the gene expression of many different genes

What do hydrophilic signalling molecules bind to

Transmembrane receptors and do not enter the cytosol

What are examples of hydrophilic extra cellular signalling molecules

peptide hormones and neurotransmitters

When do transmembrane receptors change conformation

when the ligand binds to the extracellular face; the signal molecule does not enter the cell, but the signal is transduced across the plasma membrane

What do transmembrane receptors act as

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

What do transduced hydrophilic signals often involve

G-proteins or cascades of phosphorylation by kinase enzymes

What do G-proteins do

Relay signals from activated receptors to target proteins such as enzymes and ion channels

What does phosphorylation cascades allow

More than one intracellular signalling pathway to be activated

What does phosphorylation cascades involve

A series of events with one kinase activating the next in the sequence and so on

What can phosphorylation cascades result in

The phosphorylation of many proteins as a result of the original signalling event

What does the binding of the peptide hormone insulin to its receptor result in

an intracellular signalling cascade

What does the intracellular cascade from insulin trigger

The recruitment of GLUT4 glucose transporter proteins to the cell membrane of fat and muscle cells

What causes a conformational change

The binding of insulin to its receptor

What does a conformational change trigger

Phosphorylation of the receptor which starts a phosphorylation cascade inside the cell

What does a phosphorylation cascade inside the cell eventually lead to

GLUT4-containing vesicles being transported to the cell membrane

What can diabetes mellitus be caused by in type 1

Failure to produce insulin

What can diabetes mellitus be caused by in type 2

Loss of receptor function

What is type 2 generally associated with

Obesity

What also triggers the recruitment of GLUT4

Exercise - so can improve the uptake of glucose to fat and muscle cells in subjects with type 2.

What is resting membrane potential

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

What does the transmission of a nerve impulse require

changes in the membrane potential of the neuron's plasma membrane

What is an action potential

a wave of electrical excitation along a neuron's plasma membrane

How do neurotransmitters initiate a response

by binding to their receptors at a synapse

What are neurotransmitter receptors in terms of channels

Ligand-gated ion channels

What is depolarisation

a change in the membrane potential to a less negative value inside

What does depolarisation of the plasma membrane as a result of the entry of positive ions trigger

the opening of voltage-gated sodium channels, and further depolarisation occurs

What restores the resting membrane potential

inactivation of sodium channels and opening of potassium channels

What does binding of a neurotransmitter trigger (long answer)

the opening of ligand-gated ion channels at a synapse. lon 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.

What does depolarisation of a patch of membrane cause

neighbouring regions of membrane to depolarise and go through the same cycle, as adjacent voltage-gated sodium channels are opened

What happens when the action potential reaches the end of the neuron

it causes vesicles containing neurotransmitter to fuse with the membrane — this releases neurotransmitter, which stimulates a response in a connecting cell

What does restoration of the resting membrane potential allow

the inactive voltage-gated sodium channels to return to a conformation that allows them to open again in response to depolarisation of the membrane

What are ion concentration gradients re-established by

the sodium-potassium pump, which actively transports excess ions in and out of the cell

What happens with the sodium and potassium ion concentration gradients following repolarisation

They are reduced

What does the sodium-potassium pump do

It restores the sodium and potassium ions back to resting potential levels

What is the retina

The area within the eye that detects light and contains two types of photoreceptor cells: rods and cones

How do rods function

in dim light but do not allow colour perception

What are cones responsible for

Colour vision and only function in bright light

In animals what is the light sensitive molecule retinal combined with

a membrane protein, opsin, to form the photoreceptors of the eye

What is the retinal opsin complex called in rod cells

rhodopsin

How is a nerve impulse initiated in the eye

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

A cascade of proteins amplifies the signal

What does photoexcited rhodopsin activate

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

What does PDE catalyse

The hydrolysis of a molecule called cyclic GMP

what does a single photoexcited rhodopsin activate

Hundreds of molecules of G-protein

What does each activated G-protein activate

One molecule of PDE

What does each active PDE molecule break down

thousands of cGMP molecules per second

What does the reduction in cGMP concentration as a result of its hydrolysis affect

The function of ion channels in the membrane of rod cells

What happens when the function of ion channels in the membrane of rod cells are affected

Results in the closure of ion channels in the membrane of the rod cells

What is triggered by the closure of ion channels in the membrane of the rod cells

Nerve impulses in neurons in the retina

What does a very high degree of amplification result in

rod cells being able to respond to low intensities of light

What happens in cone cells

Different forms of opsin combine with retinal to give different photoreceptor proteins, each with a maximal sensitivity to specific wavelengths: red, green, blue or UV

Suggest a way in which a hormone might have different effects in different tissues

Different tissues will have different responses to receptor binding

Describe the role of rhodopsin

To generate a potential difference

Absorbs light to pump proteins