biology - 1.4 Flashcards
How do multicellular organisms signal between molecules?
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?
The conformation of the receptor, which initiates a response within the cell.
What do different cell types produce?
Specific signals that can only be detected and responded to by cells with the specific receptor.
Why may 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, what may different cell types show?
A tissue-specific response to the same signal.
What can hydrophobic signalling molecules do?
They can diffuse directly through the phospholipid bilayers of membranes, and so bind to intracellular receptors.
What are the receptors for hydrophobic signalling molecules?
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?
The steroid hormones oestrogen and testosterone.
What do steroid hormones do?
They bind to specific receptors in the cytosol or the nucleus.
What does the hormone-receptor complex do?
It moves to the nucleus where it 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 does binding at hormone response elements do?
It influences the rate of transcription, with each steroid hormone affecting the gene expression of many different genes.
What do hydrophilic signalling molecules do?
They bind to transmembrane receptors and do not enter the cytosol.
What are examples of hydrophilic extracellular signalling molecules?
Peptide hormones and neurotransmitters.
When the ligand binds to the extracellular face, what happens to transmembrane receptors?
They change conformation. 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?
They relay signals from activated receptors to target proteins such as enzymes and ion channels.
What do phosphorylation cascades allow?
More than one intracellular signalling pathway to be activated.
What do 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 binding of the peptide hormone insulin to its receptor result in?
An intracellular signalling cascade that triggers recruitment of GLUT4 glucose transporter proteins to the cell membrane of fat and muscle cells.
What does binding of insulin to its receptor cause?
A conformational change that triggers phosphorylation of the receptor.
What does phosphorylation of the insulin receptor start?
A phosphorylation cascade inside the cell, which eventually leads to GLUT4-containing vesicles being transported to the cell membrane.
What can diabetes mellitus be caused by?
Failure to produce insulin (type 1) or loss of receptor function (type 2).
What is type 2 generally associated with?
Obesity.
What does exercise do?
It triggers recruitment of GLUT4, so can improve 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.
What do neurotransmitters do?
They initiate a response by binding to their receptors at a synapse.
What are neurotransmitter receptors?
Ligand-gated ion channels.
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 is depolarisation?
A change in the membrane potential to a less negative value inside.
What does inactivation of the sodium channels and the opening potassium channels do?
Restores the resting membrane potential.
What does binding of a neurotransmitter trigger?
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, what happens?
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, what happens?
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.
When the action potential reaches the end of the neuron, what does it cause?
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.
How are ion concentration gradients re-established?
By the sodium-potassium pump, which actively transports excess ions in and out of the cell.
Following depolarisation, what happens?
The sodium and potassium 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 and contains two types of photoreceptor cells: rods and cones.
In animals, what form the photoreceptors of the eye?
The light-sensitive molecule retinal is combined with a membrane protein, opsin.
In rod cells, what is the retinal-opsin complex called?
Rhodopsin.
What does retinal do?
It absorbs a photon of light and rhodopsin changes conformation to photoexcited rhodopsin.
What amplifies the signal?
A cascade of proteins.
What does photoexcited rhodopsin activate?
A G-protein, called transducin.
What does the G-protein, transducin, activate?
The enzyme phosphodiesterase (PDE).
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 PDE catalyse?
The hydrolysis of a molecule called cyclic GMP (cGMP).
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 does the hydrolysis of cGMP result in?
The closure of ion channels in the membrane of the rod cells, which triggers 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.
In cone cells, what give different photoreceptor proteins?
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.
Where do rods function?
In dim light but do not allow colour perception.
Where do cones function?
Only in bright light and are responsible for colour vision.
