Transmission electron micrograph showing mitochondria in immature red blood cell. Magnification 34,000x Mitochondrion, an organelle in the cytoplasm of cells that functions in energy production. Red blood cell, any of the hemoglobin-containing cells that carry oxygen to the tissues and are responsible for the red color of vertebrate blood. Reticulocytes are immature red blood cells, typically composing about 1% of the red cells in the human body.