mitochondrion
/ MY-toh-KON-dree-on /
Almost everything a cell does costs energy, and that energy has to be generated somewhere. The mitochondrion is the cell's power plant — a bean-shaped organelle that takes in fuel from food and oxygen and converts them into a usable energy currency the rest of the cell can spend.
A mitochondrion is wrapped in two membranes. The smooth outer membrane is the boundary; the inner membrane is folded into deep ridges called cristae, which pack a huge amount of working surface into a small space. The fluid filling the innermost space, enclosed by the inner membrane, is the matrix. This layered architecture is not decorative: the folded inner membrane and the matrix hold the machinery that extracts energy, and the heavy folding directly raises how much energy a single mitochondrion can produce.
Mitochondria carry their own small loop of DNA and their own ribosomes, and they make more of themselves by dividing — strong clues that they descend from ancient free-living bacteria taken up by a host cell (the endosymbiotic theory). In humans, mitochondrial DNA is inherited almost entirely from the mother. Cells with high energy demands, such as heart muscle and active neurons, are packed with mitochondria; this field covers their structure, while the chemistry of how they release energy belongs to bioenergetics.
A single human heart-muscle cell can contain several thousand mitochondria, sometimes filling a third of its volume, because the heart never stops working and never gets to rest.
Mitochondrial count tracks a cell's energy demand almost directly.
Calling mitochondria "the powerhouse of the cell" is a useful slogan but oversimplifies: they also help control cell death, calcium balance, and the cell's metabolic decisions.