If you are looking for a single organelle, then I would say the mitochondria. They are the site of respiration and are adapted for efficient gas exchange.
mitochondria
the mitochondria
Cellular respiration starts in the cytoplasm of the cell, and then enters the mitochondria.
Cellular respiration is conducted in the mitochondria of the cell.
Mitochondrion is the answer
Cell respiration occurs in the mitochondria.
Mitochondria.
Eukaryotes have organelles that proform various functions for the cell.
Mitochondria .
mitochondria
organelle
mitochondria
Mitochondria.
the mitochondria
You sound like you are describing an organelle.
mitochondria
the mitochondria