Colored Scanning Electron Micrograph (SEM) of mitochondria and smooth endoplasmic reticulum in an ovarian granulosa-lutein cell. The cell occurs in the corpus luteum of the ovary, which develops after ovulation and secretes the hormone progesterone. Mitochondria (pink) are seen sectioned, containing tubular cristae. Active secretory cells have many mitochondria which play a role in cell respiration. Membranes of smooth endoplasmic reticulum (yellow) are also seen; commonly found in secretory cells ER synthesizes lipids and transports substances around the cell.
The most prominent roles of mitochondria are to produce the energy currency of the cell, ATP, through respiration, and to regulate cellular metabolism. The central set of reactions involved in ATP production are collectively known as the citric acid cycle, or the Krebs Cycle. Other functions of the mitochondria include; regulation of the membrane, apoptosis (programmed cell death), the regulation of cellular metabolism, and steroid synthesis.