Cristae Definition
Cristae are sub-compartments of the inner membrane of mitochondria and are essential to mitochondrial function. Mitochondria are often considered the powerhouses of the cell since they are the organelles responsible for the generation of ATP, the energy currency of the cell.
Mitochondria are comprised of an outer and an inner membrane. Each membrane has a distinct form and purpose. The outer membrane controls the organelle’s shape and is essential for the communication of mitochondria with other organelles.
The inner mitochondrial membrane is made up of two sub-compartments:
- the inner boundary membrane, which is adjacent to the outer membrane, and
- the folded cristae whose protrusions and folds penetrate the inner mitochondrial matrix.
The folded cristae membrane contains cylindrical connections to the inner membrane called cristae junctions.
Types of Cristae
Cristae Membrane
The main function of mitochondria is the use of carbohydrates to generate ATP through oxidative phosphorylation. Cristae membranes and cristae junctions are an integral part of this process.
The folding or wrinkling of the cristae on the inner mitochondrial membrane creates a large surface area inside the mitochondria. The number of cristae in the mitochondria reflects the particular cell’s demand for ATP.
For example, heart muscle cells contain up to three times more cristae than other cells due to the greater need for ATP. The cristae membrane is where the electron transport chain, and enzymes of oxidative phosphorylation such as ATP synthase and succinate dehydrogenase are located.
The electron transport chain creates an electrochemical gradient across the inner mitochondrial membrane. This gradient drives the production of ATP from ADP and inorganic phosphate by the mitochondrial F1Fo-ATP synthase reaction. The F0 part of the enzyme is rooted in the cristae, while the F1 extends into the mitochondrial matrix.
The diagram below shows the electron transport chain, the machinery that makes ATP, located in the inner mitochondrial membrane:
Cristae Junctions
Cristae junctions are tubular structures measuring 12-40nm in diameter that demarcate the cristae from the rest of the inner boundary membrane. These junctions allow the selective concentration of enzymes such F1F0-ATP synthase on the cristae.
Many enzymes involved in oxidative phosphorylation are selectively imported to the cristae and can be retained in this region of the mitochondrial membranes due to the presence of junctions. Additionally, the F1F0-ATP synthase is also involved in determining the structure of the cristae itself.
Cristae junctions are also important for inter-mitochondrial communication. Cristae of nearby mitochondria arrange themselves to be parallel to each other and perpendicular to the connections between mitochondria.
This formation facilitates electrochemical coupling allowing the mitochondria to function in synchrony. Thus, cristae are important components of intercellular and intracellular mitochondrial networks as ions and molecules are exchanged across the cristae and in between membranes.
Examples of Cristae Disorders
Unusual inner membrane structures are observed in many human disorders and during programmed cell death. For example, in amyotrophic lateral sclerosis (ALS) disease, Alzheimer’s disease and Parkinson’s disease, cristae morphology is disrupted by inclusions within mitochondria, as well as irregular inner membrane configurations.
Function of Cristae
The cristae greatly increase the surface area of the inner membrane on which the above-mentioned reactions may take place. A widely accepted hypothesis for the function of the cristae is that the high surface area allows an increased capacity for ATP generation.
However, the current model is that active ATP synthase complexes localize preferentially in dimers to the narrow edges of the cristae. Thus, the surface area of mitochondrial membranes allocated to ATP syntheses is actually quite modest.
Mathematical modelling suggested that the optical properties of the cristae in filamentous mitochondria may affect the generation and propagation of light within the tissue.
