What are the key structures of a cell found inside its boundaries?

The key structures of a cell located within its boundaries are the cytoplasm and nucleus. The cytoplasm is a gel-like substance where various organelles are suspended and where many metabolic processes occur. The nucleus is often referred to as the control center of the cell, containing the cell's genetic material and overseeing activities such as growth, metabolism, and reproduction. Together, these structures play crucial roles in maintaining the cell's functions and integrity.

The other choices mention structures that do not fit this definition. For example, cell membranes and plasma membranes refer to the same structure, which is the outer boundary of the cell that separates the interior from the external environment. Chloroplasts and ribosomes are important organelles but are not typically referred to as "key structures" on their own in the context of what exists inside the cell's boundaries. Similarly, the cell wall is found in plant cells and some bacteria but is external to the cell membrane, while the cytoskeleton is a network of fibers maintaining cell shape and facilitating movement but, again, is not primarily considered a boundary structure.