8. Communication within Multicellular Organisms and Protein Control of Cell Division

how do multicellular organisms signal between cells

extracellular signalling molecules

what are examples of extracellular signalling molecules

steroid hormones

peptide hormones

neurotransmitters

what are receptor molecules of target cells

proteins with a binding site for a specific signal molecule

what does binding to a receptor lead to

changes to the conformation of the receptor which initiates a response within the cell

how do receptors know which signals to detect and respond to

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

what happens if signalling molecules are detected on different target cell types

differences in the intracellular signalling molecules and pathways that are involved

what could happen in a multicellular organism in response to the same signal

different cell types may show a tissue specific response

what are hydrophobic molecules

uncharged, lipid soluble molecules such as steroid hormones

how do hydrophobic signalling molecules bind to intracellular receptors

can diffuse directly through the phospholipid bilayers of membranes

why are hydrophobic signalling molecules able to pass through the phospholipid bilayer

the bilayer is also hydrophobic

what are the receptors for hydrophobic signalling molecules classed as

transcription factors

what are transcription factors

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

what are examples of hydrophobic signalling molecules

oestrogen

testosterone

what and where do steroid hormones bind to

specific receptors in the cytosol or nucleus

where does the hormone - receptor complex move to

nucleus

what happens when the hormone - receptor complex moves to the nucleus

binds to specific sites on DNA and affects gene expression

what does the hormone - receptor complex bind to

specific DNA sequences called hormone response elements (HREs)

what happens when the hormone - receptor complex binds to the HREs

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

what do hydrophilic signalling molecules do

bind to transmembrane receptors and dont enter the cytosol

what are examples of hydrophilic extracellular signalling molecules

peptide hormones

neurotransmitters

how do transmembrane receptors change conformation when a signalling molecule binds to it

when a ligand binds to the extracellular face the signal molecule doesnt enter the cell but the signal is transduced across the plasma membrane

what can transmembrane receptors act as

signal transducers

how do transmembrane receptors act as transducers

converts the extracellular ligand binding event into intracellular signals which alters the behaviour of the cell

what does hydrophilic signal transduction often involve

activating G-proteins

activating 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 are activated receptors

receptors that have bound a signalling molecule

what do phosphorylation cascades allow for

more than 1 intracellular signalling pathway to be activated

what do phosphorylation cascades do

a series of events with 1 kinase activating the next in the sequence and so on

what can phosphoylation cascades result in

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

what is insulin an example of

a peptide hormone

what happens when insulin binds to its receptor

an intracellular signalling cascade that triggers recruitment of GLUT4 glucose transporter proteins to the cell membrane of fat and muscle cells

what is the process of binding of insulin to its receptor

a conformational change that triggers phosphorylation of the receptor

this starts a phosphorylation cascade inside the cell, which eventially leads to GLUT4-containing vesicles being transported to the cell membrane

what can type 1 diabetes mellitus be caused by

failure to produce insulin

what can type 2 diabetes mellitus be caused by

loss of receptor function

what is type 2 diabetes associated with

obesity

what can also trigger recruitment of GLUT4

exercise which can improve uptake of glucose to fat and muscle cells in subjects with type 2

what does resting membrane potential mean

a state where theres no net flow of ions across the membrane

what does transmission of a nerve impulse require

changes in the membrane potential of the neurons plasma membrane

what is an action potential

a wave of electrical excitation along a neurons plasma membrane

how do neurotransmitters initiate a response

binding to their receptors at a synapse

what are neurotransmitter receptors

ligand-gated ion channels

what does depolarisataion mean

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

how does depolarisation of the plasma membrane happen

the result of the entry of positive ions

what does depolarisation of the plasma membrane trigger

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

how is the resting membrane potential restored

inactivation of the sodium channels and the opening of potassium channels

what does the binding of a neurotransmitter trigger

the opening of ligand-gated ion channels at a synapse

ion movement occurs and theres depolarisation of the plasma membrane

what 2 factors are needed to open the voltage-gated sodium channel

if sufficient ion movement occurs

the membrane is depolarised beyond a threshold value

what happens when the 2 factors are met and the voltage-gated sodium channels are opened

sodium ions enter the cell down their electrochemical gradient

leading to a rapid and large change in membrane potential

how long does it take for sodium channels to become inactivated

a short time after opening

what happens after the sodium channels are inactivated

voltage-gated potassium channels open to allow potassium ions to move out the cell to restore the resting membrane potential

what does the 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 does the restoration of the resting membrane potential allow for

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

ion concentration gradients are re-established by the sodium-potassium pump

what does the sodium potassium pump do when ion concetration gradients are re-established

actively transports excess ions in and out the cell

what happens to the sodium and potassium ion concentration gradients following repolarisation

ion concentration gradients are reduced

the sodium-potassium pump restores the sodium and potassium ions back to resting potential levels

what is the retina

the area within the eye that detects light

what are the 2 types of photoreceptor cells

rods

cones

what do rods do

function in dim light but dont allow colour perception

what do cones do

onyl function in bright light and are responsible for colour vision

what are photoreceptors made with in animals

light-sensitive molecule; retinal

membrane protein; opsin

what is the retinal-opsin complex called in rod cells

rhodopsin

what happens when retinal absorbs a photon of light

rhodopsin changes conformation to photoexcited rhodopsin

how is a signal amplified

a cascade of proteins

what does photoexcited rhodopsin activate

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

how many G-proteins are activated by 1 photoexcited rhodopsin

hundreds

how many molecules of PDE are activated by G-proteins

1

what does PDE catalyse

the hydrolysis of cyclic GMP (cGMP)

at what rate does 1 PDE molecule break down cGMP

thousands of cGMP per second

what happens when the cGMP concentration is reduced due to its hydrolysis

affects the function of ion channels in the membrane of rod cells

what happens to the ion channels after the reduction of cGMP concentration

closure of ion channels in the membrane of the rod cells, triggering nerve impulses in neurons in the retina

what allows rod cells to be able to respond to low intensities of light

a very high degree of amplification

how are different photoreceptor proteins made in cone cells

different froms of opsin combine with retinal

each photoreceptor protein has a maximal sensitivity to specific wavelengths of red, green, blue or UV

what does the cytoskeleton do

gives mechanical support and shape to cells

what does the cytoskeleton consist of

different protein structures including microtubules

where type of cell are microtubules found

in all eukaryotic cells

what are microtubules

hollow cylinders composed of the protein tubulin

where are microtubules found

they radiate from the microtubule organising centre (MTOC) or centrosome

what do microtubules do

control the movement of membrane-bound organelles and chromosomes

what does cell division need

remodelling of the cytoskeleton

how does the formation and breakdown of microtubules happen

polymerisation and depolymerisation of tubulin

what do microtubules form

spindle fibres that are active during cell division

what phases are involved with the cell cycle

interphase

mitotic phase

what does interphase involve

growth and DNA synthesis

what sub-phases of interphase are there

G1 - growth phase

S phase - during which DNA is replicated

G2 - further growth phase

what does the mitotic phase involve

mitosis

cytokinesis

what separates the chromosomal material

spindle microtubules

what happens after chromosomal material is separated

cytokinesis

what happens in cytokinesis

the cytoplasm is separated into 2 daughter cells

what phases are involved with mitosis

prophase

metaphase

anaphase

telophase

what happens in prophase

DNA condenses into chromosomes each consisting of two sister chromatids

Nuclear membrane breaks down

spindle microtubules extend from the MTOC by polymerisation and attach to chromosomes via their kinetochores in the centromere region

what happens in metaphase

chromosomes are aligned at the metaphase plate

what happens in metaphase

as spindle microtubules shorten by depolymerisation, sister chromatids are separated, and the chromosomes are pulled to opposite poles

what happens in telophase

the chromosomes decondense and nuclear membranes are formed around them

what is progression through the cell cycle controlled by

checkpoints

what are checkpoints

mechanisms within the cell that assess the condition of the cell during the cell cycle and halt progression to the next phase until certain requirements are met

what do cyclin proteins that accumulate during cell growth do

regulate the cell cycle

what do cyclins combine with and activate

cyclin-dependant kinases (CDKs)

what do active cyclin-CDK complexes do

phosphorylate proteins that regulate progression through the cycle

when does progression occur in the cycle

if sufficient phosphorylation is reached

what checkpoints are there

G1

G2

M

what happens at the G1 checkpoint

retinoblastoma protein (Rb) acts as a tumour suppressor by inhibiting the transcription of genes that code for proteins neede for DNA replication