Mitochondria. Coloured transmission electron micrograph TEM of mitochondria across centre in striated skeletal muscle. Mitochondria have two membranes: an outer surrounding membrane and an inner membrane which forms folds called cristae. Cristae are the sites of aerobic cell respiration, where sugars and fats are oxidised to produce energy. Skeletal muscle is responsible for voluntary muscle movement, the large number mitochondria are indicative of a high energy requirement.