The mitochondrion is often referred to as the powerhouse of the cell. It is a double-membrane organelle that is found in most eukaryotic cells.
The primary function of mitochondria is to generate large quantities of energy in the form of a molecule called adenosine triphosphate (ATP).
ATP is used as a direct source of energy within the cell. Mitochondria are unique in that they have their own genetic material separate from the DNA contained in the nucleus of the cell.
Mitochondrial Structure
Mitochondria are characterized by their unique double-membrane structure. The outer membrane is smooth and encases the entire organelle, while the inner membrane is highly convoluted, creating folds known as cristae.
Outer Membrane
The outer membrane of the mitochondrion contains numerous integral proteins called porins. These proteins form channels that allow molecules 5000 Daltons or smaller to freely cross the membrane. This membrane serves to delineate the mitochondrion from the rest of the cell, creating a unique environment for cellular respiration to occur.
Inner Membrane
The inner membrane of the mitochondrion is highly convoluted, creating folds known as cristae. These folds increase the surface area of the membrane, allowing for a higher number of ATP to be synthesized. The inner membrane also contains proteins that are involved in electron transport and ATP synthesis. Unlike the outer membrane, the inner membrane does not contain porins, and is highly impermeable to most molecules.
Mitochondrial Function
ATP Production
The primary function of the mitochondria is to produce ATP through the process of cellular respiration. This process involves three stages: glycolysis, the citric acid cycle, and the electron transport chain.
Glycolysis
Glycolysis occurs in the cytosol of the cell and does not directly involve the mitochondria. However, the products of glycolysis, pyruvate and NADH, are used in the next stages of cellular respiration that occur in the mitochondria.
Citric Acid Cycle
The citric acid cycle, also known as the Krebs cycle, occurs in the mitochondrial matrix. This cycle breaks down pyruvate into carbon dioxide and produces NADH and FADH2, molecules that carry electrons to the electron transport chain.
Electron Transport Chain
The electron transport chain is located in the inner mitochondrial membrane and is where the majority of ATP is produced. Electrons from NADH and FADH2 are passed along a series of proteins in the membrane. This creates a flow of protons across the membrane, generating a proton gradient. The flow of protons back across the membrane through the protein ATP synthase drives the synthesis of ATP from ADP and inorganic phosphate.
Other Functions of the Mitochondria
In addition to ATP production, mitochondria have several other specialized functions. They play a role in other metabolic tasks such as fatty acid oxidation, amino acid metabolism, and the urea cycle. Mitochondria are also involved in cell signaling, cell differentiation, and cell death.
Mitochondrial DNA
Mitochondria are unique in that they have their own genetic material, separate from the DNA contained in the nucleus of the cell. This mitochondrial DNA is circular and encodes some of the RNA and proteins required for electron transport. This is evidence of the endosymbiotic theory, which suggests that mitochondria were once free-living bacteria that were engulfed by a primitive eukaryotic cell.
Conclusion
Mitochondria are vital organelles that serve many functions in the cell. Their primary role is to generate ATP, the cell’s main source of energy. They also play a role in other metabolic processes and cell functions. Their unique structure and genetic material provide intriguing insights into the evolution of complex life.