A ligand-gated ion channel responds to a signal by
Answer: A
Binding changes the shape of the channel, which admits ions and changes the membrane potential within milliseconds. Synaptic transmission depends on exactly this speed. A cascade would be far too slow for nerve transmission.
Please log in or register to report this question.
If a nerve impulse crosses a synapse in about 0.5 milliseconds, how many such crossings could occur in one second?
Answer: B
One second divided by 0.5 milliseconds gives 2,000. The figure is theoretical, since refractory periods limit the rate a real neuron can sustain. Even so, chemical synapses are remarkably fast for a diffusion-based process.
Please log in or register to report this question.
State whether the following is true or false.
Signalling through ligand-gated ion channels is faster than signalling through G protein-coupled receptors.
Answer: C
The channel opens directly on binding, whereas a G protein pathway involves several intermediate steps. Speed is traded against the amplification that the longer route provides. Each design suits a different kind of message.
Please log in or register to report this question.
Desensitisation of a receptor after prolonged stimulation serves to
Answer: D
Receptors are phosphorylated, internalised or reduced in number, so the response fades even though the signal persists. This adaptation lets a cell respond to changes rather than to absolute levels. Tolerance to some drugs arises through exactly this mechanism.
Please log in or register to report this question.
Consider the following statements about signal termination.
1. Phosphatases remove phosphate groups added by kinases.
2. Second messengers are degraded by specific enzymes.
3. Signals stop only when the signal molecule is used up.
Answer: A
The first two describe genuine termination mechanisms. The third is wrong, since a cell actively shuts a pathway down rather than waiting passively for the signal to disappear. Termination is as regulated as activation is.
Please log in or register to report this question.
The MAP kinase cascade is a series of
Answer: B
Three kinases acting in sequence amplify the signal and provide several points at which it can be regulated. The pathway relays growth signals from the surface to the nucleus. Its name comes from mitogen-activated protein kinase.
Please log in or register to report this question.
State whether the following is true or false.
Mutations that make a signalling protein permanently active can cause cancer.
Answer: C
A component locked in its on state drives division whether or not a growth signal has arrived. The Ras protein is the classic case, and mutations in it are found in a large fraction of tumours. Roughly a quarter of all human cancers carry a Ras mutation.
Please log in or register to report this question.
The Ras protein is a small G protein that is inactivated by
Answer: D
Mutations that prevent the hydrolysis leave it permanently active, which is why they are found so often in tumours. The timer built into the protein is what normally limits the duration of the signal. Ras switches itself off, which is what a faulty version cannot do.
Please log in or register to report this question.
A signalling molecule that acts on the cell that produced it and on its immediate neighbours is acting in
Answer: A
The categories describe range rather than chemistry, so one molecule may fall into more than one at once. Growth factors released in a tissue commonly do exactly this. The categories overlap because they describe distance travelled.
Please log in or register to report this question.
State whether the following is true or false.
The same signalling molecule can act in more than one mode depending on where it is released.
Answer: B
Correct answer: True
The mode is defined by the distance and the route rather than by the molecule. Adrenaline acts as a hormone when released into the blood by the adrenal medulla and as a neurotransmitter when released into a synapse; many growth factors act autocrine, paracrine or endocrine depending on where they are secreted. The classification describes the delivery, not the chemistry.
Please log in or register to report this question.
