| Cell biology | |
|---|---|
| mitochondrion | |
 Components of a typical mitochondrion
4Mitochondrial DNA |
Acrista (/ˈkrɪstə/;pl.:cristae) is a fold in theinner membrane of amitochondrion. The name is from the Latin forcrest orplume, and it gives the inner membrane its characteristic wrinkled shape, providing a large amount ofsurface area forchemical reactions to occur on. This aidsaerobic cellular respiration, because the mitochondrion requiresoxygen. Cristae are studded withproteins, includingATP synthase and a variety ofcytochromes.
With the discovery of the dual-membrane nature of mitochondria, the pioneers of mitochondrialultrastructural research proposed different models for the organization of the mitochondrial inner membrane.[1] Three models proposed were:
More recent research (2019) finds rows ofATP synthase dimers (formerly known as "elementary particles" or "oxysomes") forming at the cristae. These membrane-curving dimers have a bent shape, and may be the first step to cristae formation.[4] They are situated at the base of the crista. A mitochondrial contact site cristae organizing system (MICOS) protein complex occupies the crista junction. Proteins likeOPA1 are involved in cristae remodeling.[5]
Crista are traditionally sorted by shapes into lamellar, tubular, and vesicular cristae.[6] They appear in different cell types. It is debated whether these shapes arise by different pathways.[7]
NADH is oxidized intoNAD+, H+ions, andelectrons by anenzyme.FADH2 is also oxidized into H+ ions, electrons, andFAD. As thoseelectrons travel farther through theelectron transport chain in the inner membrane, energy is gradually released and used to pump the hydrogen ions from the splitting of NADH and FADH2 into the space between the inner membrane and the outer membrane (called theintermembrane space), creating anelectrochemical gradient.
Thiselectrochemical gradient creates potential energy (seepotential energy § chemical potential energy) across the inner mitochondrial membrane known as theproton-motive force. As a result,chemiosmosis occurs, and the enzymeATP synthase producesATP fromADP and aphosphate group. This harnesses thepotential energy from the concentration gradient formed by the amount of H+ ions. H+ ions passively pass into the mitochondrialmatrix by the ATP synthase, and later help to re-form H2O (water).
Theelectron transport chain requires a varying supply of electrons in order to properly function and generate ATP. However, the electrons that have entered the electron transport chain would eventually pile up like cars traveling down a blocked one-way street. Those electrons are finally accepted byoxygen (O2). As a result, they form two molecules ofwater (H2O). By accepting the electrons, oxygen allows the electron transport chain to continue functioning. The chain is organized in the cristae lumen membrane, i.e. the membrane inside the junction.[5]
The electrons from each NADH molecule can form a total of 3 ATP's from ADPs and phosphate groups through the electron transport chain, while each FADH2 molecule can produce a total of 2 ATPs.
As a result, 10 NADH molecules (fromglycolysis and theKrebs cycle), along with 2 FADH2 molecules, can form a total of 34 ATPs duringaerobic respiration (from a single electron transport chain). This means that combined with the Krebs Cycle andglycolysis, the efficiency for the electron transport chain is about 65%, as compared to only 3.5% efficiency for glycolysis alone.
The cristae greatly increase the surface area of theinner 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 activeATP 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.[8]