Describe the structure of mitochondria.

• A. Inner membrane with chlorophyll pigments.
• B. Single membrane with no folds and a fluid inside.
• C. Outer membrane only with ribosomes on the surface.
• D. Double membrane with inner folds called cristae and a mitochondrial matrix inside.

Answer: (D)

Energy production hinges on a specialized organelle with membranes. Mitochondria have a double membrane—an outer membrane that encloses the organelle and an inner membrane that folds inward to form cristae. Those cristae increase the surface area available for the electron transport chain and ATP synthase, boosting how much ATP the cell can make. The space inside the inner membrane is the mitochondrial matrix, which hosts enzymes of the Krebs cycle and contains mitochondrial DNA and ribosomes. The gap between the membranes is the intermembrane space, important for building the proton gradient that powers ATP synthesis. Descriptions mentioning a single membrane or chlorophyll pigments point to other structures (like chloroplasts), so the double membrane with inner folds and a matrix is the correct description of mitochondria.