Final answer:
Membranes are structures that separate compartments within cells and have selective channels for molecule transport, with the plasma membrane being a key example in both plant and animal cells.
Step-by-step explanation:
The structures that separate compartments from each other, but have channels or openings for specific molecules are membranes. The plasma membrane in particular plays a crucial role in separating the interior of the cell from the exterior, while allowing selective movement of substances in and out of the cell. Plant cells also have structures known as plasmodesmata, which are channels that pass through the cell walls, connecting the cytoplasm of adjacent plant cells, thus facilitating the transport of materials from cell to cell.
In contrast, the cell walls provide structural support and protection but do not generally have selective channels for molecules. Similarly, capsules and nuclei also lack the function of having selective channels; capsules offer protection to bacterial cells, while nuclei contain genetic material.