1.4 Flashcards
How do multicellular organism signal between cells?
Multicellular organisms signal between cells using extracellular signalling molecules
Extracellular signalling molecules examples
Steroid hormones
Peptide, hormones
Neurotransmitters
Describe the general process of extracellular signalling
A cell releases a signal molecule that is responded to by another cell (target cell).
The target cell must have receptor molecules (proteins) with a binding site for the specific signal molecule.
When binding occurs, it changes the conformation of the receptor, which brings about the response in the target cell
Describe, hydrophilic signalling
Most signal molecules are hydrophilic, and are therefore unable to cross the plasma membrane directly; instead they bind to cell-surface receptors which generate one or more signals inside the target cell
Describe, hydrophobic signalling
Some small signal molecules diffuse across the plasma membrane and bind to receptors inside a target cell, either in the cytoplasm or in the nucleus.
Many of the small signal molecules are hydrophobic and nearly insoluble in aqueous solutions and therefore are transported in the bloodstream and other extracellular fluids after binding to carrier proteins, from which the associate before entering the cell.
Describe various cell types and the signals they produce
Different cell types produce specific signals that could only be detected and responded to by cells with the specific receptor. Signalling molecules may have different affects on different target cell types, due to differences in the intracellular signalling molecules and pathways that are involved.
Describe Response to signals in multicellular organisms
In a multicellular organism, different cell types may show a tissue-specific response to the same signal
Explain diffusion of hydrophobic signalling molecules
Hydrophobic signalling molecules can diffuse directly through the phospholipid bilayers of membranes, and so bind to intracellular receptors.
What are the receptors of hydrophobic signalling molecules?
The receptors for hydrophobic signalling molecules are transcription factors.
What are transcription factors?
Transcription factors are proteins that when bound to DNA can either simulates or inhibit initiation of transcription
Hydrophobic signalling molecule examples
Steroid hormones
Oestrogen
Testosterone
Where do steroid hormones bind
Steroid hormones bind to specific receptors in the cytosol or the nucleus
Describe the hormone receptor complex
The hormone receptor complex moves to the nucleus where it binds to specific sites on the DNA and affects gene expression
Explain hormone response elements
The hormone-receptor complex binds to specific DNA sequences called hormone response elements. Binding at these sites influences the rate of transcription, with each steroid hormone affecting the gene expression of many different genes.
Describe the binding of hydrophilic signalling molecules
Hydrophilic signalling molecules, bind to transmembrane receptors, and do not enter the cytosol
Hydrophilic signalling molecule examples
Peptide hormones
Neurotransmitters
Describe what happens to transmembrane receptors as a ligand binds
Transmembrane receptors change conformation when the ligand binds to the extracellular face. The signal molecule does not enter the cell. This change in conformation alters the behaviour of the cell.
What do transduced hydrophilic signals often involve?
Transduced hydrophilic signals often involve G-proteins or cascades of phosphorylation by kinase enzymes
What do G-proteins do?
G proteins relay signals from activated receptors (receptors that have found a signalling molecule) to target proteins such as enzymes and ion channels
What do phosphorylation cascades allow for?
Phosphorylation cascade allow more than one intra cellular signalling pathway to be activated
Describe a phosphorylation cascade
Phosphorylation cascade 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
How is the action of hormones on receptors described?
The target cells for signalling molecules have specific receptors at their surface, so the action of the hormones is highly specific
Describe what happens when insulin binds to its receptor?
Binding of the peptide hormone insulin to its receptor results in an intracellular signalling cascade that triggers recruitment of GLUT4 glucose transporter proteins to the cell membrane of fat and muscle cells.
Describe the process of insulin in the control of blood glucose
- Binding of insulin to its receptor in the membranes of fat and muscle cells causes a conformational change in the Receptor.
- The start of phosphorylation cascade inside the cell, which eventually leads to GLUT4 containing vesicles being transported to the cell membrane.
- The GLUT4 glucose transporters now allow glucose to pass across the plasma membrane and enter the cell.
What causes diabetes mellitus
Diabetes mellitus can be caused by failure to produce insulin (take one) or loss of receptor function (take two)
What is generally associated with type two
Obesity
What can trigger the recruitment of GLUT4
Exercise also triggers recruitment of GLUT4, so can improve uptake of glucose to fat and muscle cells in subjects with type two diabetes
What is a resting membrane potential?
Resting membrane potential is a state where there is no net flow of ions across the membrane
(not transmitting signals)
What does the transmission of nerve impulse require?
The transmission of nerve impulse requires changes in the membrane potential of the neurons plasma membrane
What is an action potential?
An action potential is a wave of electrical excitation along a neurons plasma membrane
Describe the ratio of sodium and potassium ions during the resting state
In the resting state, there is a higher concentration of Na+ in the extracellular fluid and higher concentration of K+ in the intracellular fluid
How is a membrane in resting potential described?
Polarised
What does the transmission of nerve impulse require?
The transmission of a nerve impulse requires changes in the membrane potential of the neurons plasma membrane
How do neurotransmitters initiate response?
Neurotransmitters initiate a response by binding to their receptors at a synapse
What type of channels are neurotransmitter receptors?
Ligand gated ion channels
Describe the entire process of triggering a nerve impulse
- Neurotransmitters diffuse across the synapse and bind to a ligand gated ion channels in the postsynaptic membrane.
- This causes a change in conformation, opening the channel and allowing Na+ ions to flow into the nerve cell
- If sufficient ion movement occurs, the voltage change across the membrane reaches critical threshold, and the membrane is depolarised.
- This voltage change causes the neighbouring voltage gated Na+ channel to open, which leads to the depolarisation of the neighbouring voltage gated Na+ channel which also opens and so on
- A positive feedback loop as established, leading to entry of Na+ into the cell, and a wave of depolarisation along the length of the nerve
What is depolarisation?
Depolarisation is a change in the membrane potential to a less negative value inside.
How does depolarisation of the plasma membrane occur?
Depolarisation of the plan for membrane occurs as a result of the entry of positive ions, which triggers the opening of the voltage gated sodium channels, and further depolarisation occurs
What restores the resting membrane potential?
Inactivation of the sodium channels and the opening of potassium channels restores the resting membrane potential
Describe the process of resetting the resting membrane potential
- When the voltage reaches a critically high level, the voltage gated Na+ channels, close and now the voltage gated K+ channels open.
- The K+ ions diffuse out of the neuron, in the opposite direction to the Na+ islands, repolarising the membrane and helping to restore the resting potential
- During this, the Na/K-ATPases continues to work to restore and maintain the resting potential by actively transporting excess sodium ions in and potassium ions out of the cell
- Once the resting potential is reached, the K+ channels closes.
What does depolarisation of a patch of membrane cause
Depolarisation of a patch of membrane causes neighbouring regions of membrane to depolarise and go through the same cycle, as adjacent voltage gated sodium channels are opened
What happens when an action potential reaches the end of a neuron?
When an action potential reaches the end of the neuron, it causes vesicles containing neurotransmitters 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?
Restoration of the resting membrane potential allows 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 reestablished?
Ion concentration, gradients are re-established by the sodium potassium pump, which actively transported excess ions in and out of the cell
What happens to ion concentration, gradients after repolarisation
Following repolarisation 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 retina is the area with an eye that detects light and contains two types of photoreceptor cells (rods and cones)
Describe rod cells
Rods function in dim light, but do not allow colour perception
Describe cone cells
Cones are responsible for colour vision and only function in bright light.
Describe the formation of the light-sensitive molecule in animals
In animals the light sensitive molecule retinal is combined with a membrane protein, opsin to form the photo receptors of the eye
What’s the retinol opsin complex in rod cells called
Rhodopsin
What happens when retinal absorbs a photon of light
Retinal absorbs a photon of light and rhodopsin changes conformation to photoexcited rhodopsin
Describe a special function in rod cells
A cascade of proteins amplifies the signal
Describe rod cells in darkness
When rhodopsin is inactive, the rod cells produce a protein called cyclic GMP (cGMP).
Ligand-gated Na+ channels on the photo-receptors respond to cGMP. When cGMP is bound to the Na+ channels, the channel remain open. In the absence of light, the concentration of cGMP is high. Therefore, the Na+ channels are opened in the absence of light stimulation.
This results in a Na+ leakage into the photo receptors, which changes the voltage of the sales, and depolarises it. An inhibitory neurotransmitter is released.
No nerve impulses generated.
What do rod cells produce when rhodopsin is inactive
A protein called cyclic GMP (cGMP)
Describe the concentration of cGMP in the absence of light
Ligand-gated Na+ channels on the photo receptors respond to cGMP. When cGMP is bound to the Na+ channels, they remain open.
In the absence of light, the concentration of cGMP is high. Therefore, the Na+ channels are opened
in the absence of light stimulation
In the absence of light stimulation what does the opening of the sodium channels cause
This results in a sodium leakage into the photo receptors, which change the voltage of the sales, and depolarises it.
An inhibitory neurotransmitter is released. No nerve impulse is generated.
What happens when rhodopsin is stimulated by light
When stimulated by light retinal absorbs a photon of light and rhodopsin changes conformation to photo-excited rhodopsin
What does photo excited rhodopsin activate
Photo excited rhodopsin activates a G-protein, called transducin, which activates the enzyme phosphodiesterase (PDE).
A single photo excited rhodopsin activate hundreds of molecules of G-proteins. Each activated G-protein activates one molecule of PDE.
Function of phosphodiesterase (PDE)
PDE catalyses the hydrolysis of a molecule called cyclic GMP (cGMP)
Each active PDE molecule breaks down thousands of cGMP molecules per second. The reduction in cGMP concentration as a result of its hydrolysis affects the function of ion channels in the membrane of rod cells.
(This fall in cGMP levels closes ligand-gated Na+ channels in the light-sensitive cells of the retina. Inward leakage of Na+ ions stops)
After the hydrolysis of cGMP and this affect on the function of ion channels, what does this cause
This results in the closure of ion channels in the membrane of the rod cells, which triggers nerve impulses in neurons in the retina
What specialisation allows rod cells to respond to low intensity of light
A very high degree of amplification results in rod cells being able to respond to low intensities of light
Describe the effects of different forms of opsin in cone cells
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
