Cell type T secretes cytokine C that acts locally. In a mixed culture, only cells within ~200 µm of T respond. When the medium is continuously stirred, the response disappears even though the total amount of C measured in the dish is unchanged. Which outcome is most likely responsible for the loss of response?
Which outcome is most likely when the medium is stirred?
- Stirring denatures C, preventing any receptor binding at all
- Stirring reduces the local concentration gradient of C near T below activation threshold (correct answer)
- Stirring converts paracrine signaling into endocrine signaling automatically
- Stirring forces C to enter cells without receptors by increasing membrane fluidity
- Stirring blocks secretion of C by removing vesicles from the Golgi apparatus
Explanation: This question assesses understanding of cell communication via signal transduction pathways. Continuous stirring disrupts the local concentration gradient of cytokine C near cell T, reducing it below the activation threshold for nearby cells, even though total C in the dish remains unchanged. Normally, only cells within ~200 µm respond, indicating paracrine signaling reliant on high local concentrations from diffusion gradients. Stirring mixes the medium, preventing accumulation of C near T, so receptors on nearby cells do not reach the binding threshold needed for transduction. A tempting distractor is choice A, suggesting stirring denatures C, but this reflects the misconception that physical agitation denatures proteins, whereas the evidence shows total C is unchanged, pointing to concentration effects. To approach similar questions, consider how environmental factors like mixing affect local signal concentrations in paracrine signaling.