Mitochondria

Mitochondria are double-membraned organelles known as the "powerhouses" of the cell. They generate most of the cell's ATP via aerobic respiration. Mitochondria contain their own small circular DNA and ribosomes, reflecting their evolutionary origin from bacteria (endosymbiosis).

• The inner mitochondrial membrane is highly folded into cristae, which increase surface area for ATP production (electron transport chain and ATP synthase are located there). The matrix (inner compartment) is where the Krebs cycle occurs.
• Mitochondria are inherited almost exclusively from the mother in humans (maternal inheritance). This is because the egg supplies the mitochondria to the embryo. Traits due to mtDNA mutations pass from mothers to all children, but affected fathers do not pass them on.
• Cells that require a lot of energy have many mitochondria. For example, muscle and heart cells are packed with mitochondria to meet energy demands. In contrast, red blood cells have no mitochondria (and thus produce ATP only via glycolysis).

• If a question mentions a "double membrane organelle with its own DNA" or the production of ATP, it's pointing to mitochondria. Common answers: "mitochondrion" or "mitochondria."
• Expect questions about metabolism: e.g., "Where in the cell does the citric acid (Krebs) cycle occur-" -> mitochondrial matrix; "site of oxidative phosphorylation-" -> inner mitochondrial membrane/cristae.
• Inheritance patterns can involve mitochondria: a classic exam scenario is a pedigree where only mothers pass on a condition to all children - indicating a mitochondrial DNA disorder. Recognize that unique maternal inheritance clue means mitochondria.