Answer:
Periosteum.
Explanation:
Periosteum is an extremely thin region of non-calcified tissue on the exterior of bones. It covers the entire surface of every bone in the body, with the exception of the joint-forming ends of bones where cartilage forms the bone’s outer layer.
Structure X is only found in plant cells, and structure Y is found in plant and animal cells.
This is extremely vague, but assuming this is regarding concentration gradient in the nervous system, the Sodium-Potassium pump is responsible for transporting sodium and potassium.
The answer is: the "bottleneck effect" .
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<u> The "bottleneck effect" </u><u /> is a type of "genetic drift" resulting from a "reduction in the population" such that the [surviving population] is "no longer genetically representative" [of the original population] .
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