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.
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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.
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A signal cascade has four stages, each amplifying tenfold. By what factor is the original signal amplified?
Answer: C
Ten to the fourth power is 10,000, since the stages multiply rather than add. This is why a cascade of only a few steps can produce so enormous a response. Each additional stage multiplies rather than adds to the amplification.
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Compared with an endocrine signal, a signal passing through a gap junction is limited to
Answer: D
Correct answer: cells in direct physical contact
A gap junction is a channel joining the cytoplasms of two adjacent cells, so ions and small molecules pass only between cells that touch. That makes it the shortest-range signalling there is, and the fastest - which is why it synchronises the contraction of cardiac and smooth muscle, where the coupling must be essentially instantaneous.
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Assertion (A): Heart muscle contracts as a coordinated unit rather than cell by cell.
Reason (R): Gap junctions let the signal pass directly between adjacent muscle cells.
Answer: A
Both are true and the reason gives the mechanism: direct cytoplasmic continuity spreads the depolarisation far faster than any released signal could, so the tissue behaves electrically as though it were a single cell rather than millions of separate ones.
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Signalling by direct contact between membrane-bound molecules on two cells is important in
Answer: B
Correct answer: immune recognition and development
Contact-dependent signalling requires the two cells to touch, so it is used where the identity of the particular neighbour matters. A T cell inspecting an antigen-presenting cell and the Notch-Delta interaction that decides which of two adjacent cells adopts which fate are the two classic cases - in both, the signal must be addressed to one specific cell and no other.
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State whether the following is true or false.
All cell signalling requires a molecule to be released into the surroundings.
Answer: C
Correct answer: False
Two important modes involve no release at all. Contact-dependent signalling works through membrane-bound molecules on cells that touch, and gap junction signalling passes small molecules directly between joined cytoplasms without either ever entering the extracellular space. Both are used precisely because they are addressed to particular neighbours rather than broadcast.
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