Mitochondria are organelles within the cytoplasm of nearly all eukaryotic cells, which include animal, plant, and fungal cells. These rod-shaped or oval structures, typically 0.5 to 1.0 micrometers in diameter, have a double-membrane structure that creates specialized compartments. Mitochondria play a foundational role in sustaining cellular life through various biochemical processes.
Generating Cellular Energy
Mitochondria are known for producing adenosine triphosphate (ATP), the cell’s main energy currency. This process, cellular respiration, converts chemical energy from nutrients into ATP. Cells that demand a lot of energy, such as muscle cells, can contain hundreds or even thousands of mitochondria to meet their metabolic needs.
Cellular respiration involves a series of metabolic reactions that break down biological fuels like glucose, amino acids, and fatty acids. It occurs in stages, beginning with glycolysis in the cell’s cytoplasm, which breaks down glucose and yields pyruvate. Pyruvate then moves into the mitochondria.
In the mitochondrial matrix, pyruvate is processed in the Krebs cycle (citric acid cycle), generating high-energy electron carriers like NADH and FADH2. These carriers transfer electrons to the electron transport chain in the inner mitochondrial membrane. This final stage, oxidative phosphorylation, utilizes oxygen as the electron acceptor to produce the majority of the cell’s ATP.
ATP production within mitochondria is highly efficient; approximately 30 molecules of ATP are generated for each glucose molecule oxidized. This contrasts sharply with glycolysis alone, which produces only about 2 ATP molecules per glucose molecule. The ATP produced is transported into the cytosol, fueling various cellular activities and maintaining a high ATP-to-ADP ratio.
Beyond Energy: Metabolic Support
Mitochondria contribute to cellular function beyond ATP production through various metabolic pathways. They break down fats through fatty acid oxidation, which also generates molecules for energy production. Enzymes for this process are located on the outer mitochondrial membrane and within the matrix.
They also metabolize amino acids, breaking them down or synthesizing them to support the cell’s protein needs. Mitochondria also synthesize specialized cofactors. These include heme, a molecule essential for oxygen transport in blood, and iron-sulfur (Fe-S) clusters, which are ancient protein cofactors involved in electron transfer, enzyme catalysis, and gene regulation.
The formation of iron-sulfur clusters is a complex process requiring nearly 30 proteins within the mitochondria and cytosol. These clusters are incorporated into proteins in the mitochondria, cytosol, and nucleus, performing diverse functions. This highlights the extensive reach of mitochondrial metabolic activities, influencing processes far beyond their immediate location.
Orchestrating Cell Survival and Demise
Mitochondria regulate cell survival and programmed cell death (apoptosis). They act as convergence points for signals that promote cell health or trigger self-destruction. This regulation is partly achieved through their influence on calcium signaling and the release of specific proteins.
Calcium ions are important signaling molecules, and mitochondria regulate their cellular levels by taking up and releasing calcium across their inner membrane. An overload of calcium can disrupt their membrane potential, opening permeability transition pores. This can trigger the release of pro-apoptotic proteins like cytochrome c, initiating a cascade that results in cell death.
Mitochondria also contribute to thermogenesis, or heat production, particularly in brown fat cells. This process involves uncoupling oxidative phosphorylation from ATP synthesis, allowing energy to be dissipated as heat rather than stored in ATP. This function is important for maintaining body temperature, especially in response to cold.