Unit 1 : Cells and Proteins Key Area 4 Flashcards
What are multicellular organisms made of?
Billions of cells that must communicate with each other.
Why must multicellular organisms cells communicate with eachother?
ensure a co-ordinated response to stimuli
what do extracellular signalling molecules do?
allows cells to communicate and these signals are often produced by either endocrine or nervous system.
Name 3 examples of extracellular signalling molecules
Steroid hormones, peptide hormones and neurotransmitters.
Target cells have ____________ receptors
Complementary
What happens if a cell doesn’t have receptors for that specific signal molecule?
It will not bind and the cells will not respond
These complementary receptors can be described as?
Specific to signalling molecules
What causes the response by the cell?
the conformational change of the receptor
Note steps involved in cell to cell communication
- receptor molecules of target cells are proteins with a binding site for a specific signal molecule.
- binding changes the conformation of the receptor.
- initiates a response within the cell through the intracellular pathways
Different tissues can react differently to what?
The same signalling molecules
What’s this tissue-specific response caused by ?
Intracellular signalling molecules and pathways
What’s an example of different tissues reacting differently to the same signalling molecules?
acetylcholine causes opposite effects in skeletal and heart muscle because these cell types produce different kinds f acetylcholine receptors that trigger different intracellular signalling pathways.
How do hydrophobic signalling molecules travel through the phospholipid bilayer?
Diffuse through phospholipid bilayer and bind to receptors in the intracellular part of the cell.
What is the name for the receptors for hydrophobic signalling molecules?
transcription factors
name two hydrophobic signalling molecules include steroid hormones;
oestrogen and testosterone
What does steroid hormones bind to?
Specific receptors in the cytosol or the nucleus
What’s a transcription factor
Proteins which can affect gene expression by binding to DNA and stimulating/inhibiting transcription of genes
Describe the stages of steroid hormones influencing transcription
- hydrophobic signalling molecules can pass through the membrane directly.
- hydrophobic signalling molecule binds to a receptor to create the hormone-receptor complex (HRC)
- HRC moves to the nucleus where it binds to specific sections of DNA called hormone response elements (HRE’s)
- HRC is a transcription factor + so it influences the transcription/expression lots of genes in the DNA - it can stimulate/ inhibit it.
Hydrophilic signalling molecules _________ pass through the plasma membrane
cannot
What do the hydrophilic signalling molecules bind to?
Transmembrane receptors on the extracellular surface of target cells
examples of hydrophilic signalling molecules (3)
peptide hormones, insulin and neurotransmitters
Does the signal enter the cell?
no - but the transmembrane proteins, to which it binds, changes conformation on binding and the signal is transduced (converted into intracellular signals)
What do transduced hydrophilic signals often involve?
Cascades activated by G-proteins or phosphorylation by kinase enzymes
What do G-proteins often do?
relay signals from activated receptors to target proteins such as enzymes or ion channels.
What does phosphorylation cascades cause?
the phosphorylation of multiple proteins and allow more than one intracellular signalling pathway to be activated.
Whats insulin?
A peptide hormone produced by the pancreas when blood glucose levels are high
Whats the phosphorylation cascade started by?
the binding of the insulin to receptors causes the recruitment of GLUT4 glucose transporters into the plasma membrane of fat and muscle cells.
Whats GLUT4?
a transporter protein - glucose moves by facilitated diffusion down it concentration gradient
Describe steps involved in the recruitment of GLUT4 transporters?
- insulin binds to etracellular receptor, causing conformational change + phosphorylation at the receptor.
- triggers an intracellular signalling phosphorylation cascade.
- vesicles move GLUT4 transporters to membrane
(recruitment) - glucose moves into the cell through the GLUT4 transporters, down its concentration gradient. this is an example of facilitated diffusion.
Cause and treatment of type 1 diabetes?
Failure to produce insulin, genetic - insulin injections
Cause and treatment of type 2 diabetes?
loss of receptor function, lifestyle choices (ie. obesity) - lifestyle changes (healthier diet and more exersize) - triggers recruitment of GLUT4
Whats the membrane potential of a cell?
is the difference in electrical charge across the membrane and is also known as the electrical potential difference or voltage.
How is the membrane potential of a cell maintained?
by the sodium-potassium pump which also contributes to the electrochemical gradient
describe the meaning of the term ‘resting membrane potential’?
Where there is no net flow of ions across the membrane and so the membrane potential is called the resting membrane potential. This is usually around -70mv (the cell is more -ve inside than outisde)
What does the transmission of a nerve impulse require?
changes in the membrane potential of the neurons plasma membrane
Neurotrasmitters cross the synaptic cleft and bind to what?
Specific receptors on the post-synaptic neuron - these receptors are ligand-gated sodium ion channels
what happens when the neurotransmitter binds?
it changes the conformation of the ion channel and allows sodium ions to move into the cell. if there is enough in movement an action potential is generated.
what is an action potential?
a wave of electrical excitation along a neurons plasma membrane
What’s depolarisation?
when the cell becomes more positive due to the influx of sodium ions as a result of neurotransmitter binding at the synapse.
What Neurotransmitters are involved in depolarisation? what do they do?
Ligand-gated sodium ion channels - open to allow the sodium ions to diffuse into the cell down their electrochemical gradient. This causes the initial depolarisation of the neuron plasma membrane.
What happens once a threshold depolarisation is reached?
voltage gated sodium channels open causing rapid further depolarisation as more sodium ions enter the cell.
The triggering of one voltage gated sodium channel depolarises the membrane, so triggering the adjacent voltage-gated sodium channel and so on. this passes the signal along the neuron, depolarising the next section of the membrane.
what is repolarisation?
after the wave of depolarisation passes along the neuron, there has to be a process to re-establish the resting potential, ready for the next impulse.
what happens when the volatge reaches a critically high level?
the Na+ channels close again and the volatge gained K+ channels open.
What does the positive potassium ions do during repolarisation?
They diffuse out the neuron cell down their electrochemical gradient - making the inside more negative. This can go too far and the cell can become more negative than the resting membrane potential - hyperpolarisation.
what happens following repolarisation?
The Na+ and K+ ion concentration gradients are reduced. The sodium-potassium pump restores the Na+ and K+ ions back to resting potential levels.
What happens to K+ channels once resting potential is reached?
They close
What does restoration of the resting membrane potential allow?
Allows the inactive voltage-gated Sodium channels to open again in response to depolarisation of the membrane.
When light enter the eye, what part of the eye does it hit?
The retina
What is the retina made up of?
Two photoreceptor cells - called rods and cones.
What is the function of rods?
Function in dim light but don’t allow colour perception.
What is the function of cones?
Responsible for colour vision but only function in bright light.
What are rod cells particularly sensitive to?
Light intensity - nocturnal animals have a greater proportion of rod cells = better night vision.
What are cone cells particularly sensitive to?
specific colours (wavelengths) of light : green, red, blue and (in some animals) UV. - cone cells allow animals to have colour vision.
How does light lead to vision in rod cells?
Light sensitive molecules called retinal combine with a protein in the membrane of rod cells called opsin. This forms the molecule rhodopsin which can absorb photons of light, causing a conformational change in the protein to become photoexcited rhodopsin.
How do cone cells see colour?
Different forms of opsin combine with retinal to give different photoreceptor proteins, each with a maximal sensitivity to specific wavelengths: green, red, blue or UV
Note the steps involved in the generation of a nerve impulse within the retina?
- a photon is absorbed by retinal molecule.
- photoexcited rhodopsin activates 100s of g-proteins called transducin.
- each transducin activates one molecule of the enzyme phosphdiesterase (PDE)
- each PDE catalyses the hydrolysis of 100s of cGMP into GMP per second.
- the reduced concentration of cGMP causes the Na+ ion channel to close.
- the lack of postive Na+ ions hyperpolarises the cell and triggers the nerve impulse in the rod cell.
