Chloroplasts and mitochondria are bounded by which kind of membrane?

Chloroplasts and mitochondria are bounded by a double membrane. Each organelle is surrounded by two distinct lipid bilayers, which is a key structural feature that plays an important role in their functions.

The outer membrane is smooth and is permeable to small molecules and ions, while the inner membrane is highly specialized and contains proteins that are essential for the organelles’ specific functions, such as ATP production in mitochondria and photosynthesis in chloroplasts. In mitochondria, the inner membrane is folded into structures called cristae, increasing the surface area available for reactions involved in cellular respiration. In chloroplasts, thylakoid membranes are arranged in stacks called grana and are involved in the light-dependent reactions of photosynthesis.

This double-membrane structure is a significant aspect of the endosymbiotic theory, which suggests that chloroplasts and mitochondria originated from ancient prokaryotic cells that were engulfed by a host cell. The double membrane is a remnant of this ancestral structure, supporting the idea that these organelles have a unique evolutionary history.