What structure surrounds the inner membrane of mitochondria?

The outer membrane surrounds the inner membrane of mitochondria, forming the two-membrane system characteristic of these organelles. The outer membrane is smooth and contains proteins known as porins, which allow the passage of ions and small molecules. In contrast, the inner membrane is highly folded into structures called cristae, which increase the surface area for biochemical reactions, particularly those involved in ATP production during cellular respiration.

The significance of the outer membrane lies in its role in differentiating the mitochondria from the cytosol and creating a distinct environment that is crucial for the mitochondrion's function in energy metabolism. The presence of these two membranes is a key feature of mitochondria, setting them apart from other cellular structures. Other options do not pertain to the structure of mitochondria; for instance, the plasma membrane is associated with the cell itself, the nuclear envelope surrounds the nucleus, and ribosomal membranes are not a recognized structure within cellular biology.