Which organelles in eukaryotic cells are separated from the cytoplasm by a membrane?
 

The nucleus is separated from the cytoplasm by a double membrane called the nuclear envelope, which has nuclear pores that regulate the movement of molecules between the nucleus and the cytoplasm.

The mitochondria are also separated from the cytoplasm by a double membrane, with the outer membrane being smooth and the inner membrane is folded into cristae. This separation creates two compartments within the mitochondria: the intermembrane space and the mitochondrial matrix.

Ribosomes are not separated from the cytoplasm by a membrane, as they are either free-floating in the cytoplasm or attached to the endoplasmic reticulum. The endoplasmic reticulum itself is separated from the cytoplasm by a membrane.

Therefore, the answer is the nucleus and mitochondria.