Mitochondria are organelles within eukaryotic cells that generate significant amounts of energy in the form of adenosine triphosphate (ATP), which fuels various cellular processes. Their internal architecture enables this energy production. Mitochondria are round to oval in shape, ranging from 0.5 to 10 micrometers.
The Outer Boundary
The mitochondrion is enveloped by an outer membrane. This smooth membrane is approximately 60 to 75 angstroms thick and has a protein-to-phospholipid ratio similar to that of the cell’s plasma membrane. It acts as a selective barrier, separating the mitochondrion’s internal environment from the rest of the cytoplasm.
The outer mitochondrial membrane is permeable to small molecules, including ions and sugars, due to specialized integral membrane proteins called porins. These porins form channels that allow small molecules to diffuse freely into the intermembrane space. This permeability allows for the exchange of metabolic anions and ATP/ADP between the cytosol and the mitochondrion.
The Inner Compartments
Beyond the outer membrane, the mitochondrion contains an inner membrane, an intermembrane space, and a central matrix. The inner mitochondrial membrane is highly folded into structures known as cristae. These folds significantly increase the surface area available for chemical reactions, especially those involved in energy production.
The inner membrane is selectively permeable and lacks porins. This membrane houses protein complexes, including those of the electron transport chain and ATP synthase, essential for ATP generation. The intermembrane space is the region between the outer and inner mitochondrial membranes. Due to the outer membrane’s permeability, this space is similar to the cytosol for small molecules, though its protein composition differs.
The mitochondrial matrix is the gel-like substance enclosed by the inner mitochondrial membrane. The matrix contains mitochondrial DNA, ribosomes, and numerous soluble enzymes. These enzymes facilitate reactions such as the citric acid cycle and the beta-oxidation of fatty acids, which are initial steps in ATP production.
Mitochondrial DNA and Ribosomes
Mitochondria possess their own distinct genetic material, mitochondrial DNA (mtDNA). This mtDNA is circular in shape and encodes for a limited number of proteins that are components of the electron transport chain and ribosomal RNAs. mtDNA exhibits maternal inheritance, passed down exclusively from the mother to all offspring, as mitochondria are primarily found in the egg cell.
Within the mitochondrial matrix are mitochondrial ribosomes, or mitoribosomes. These are distinct from cytoplasmic ribosomes. While most mitochondrial proteins are synthesized by cytoplasmic ribosomes and then imported, mitoribosomes translate messenger RNAs (mRNAs) encoded by the mtDNA. Mitoribosomes are attached to the inner mitochondrial membrane.
How Structure Powers the Cell
The arrangement of mitochondrial structures works together to produce ATP. The highly folded cristae of the inner mitochondrial membrane provide an expansive surface area for housing the protein complexes of the electron transport chain. As electrons move through this chain, protons are actively pumped from the mitochondrial matrix into the intermembrane space. This creates a high concentration of protons, establishing an electrochemical gradient across the inner membrane.
The potential energy stored in this proton gradient is harnessed by ATP synthase, embedded in the inner mitochondrial membrane. Protons flow back into the matrix through the ATP synthase complex, driving the synthesis of ATP from adenosine diphosphate (ADP). The mitochondrial matrix handles the initial breakdown of nutrients through cycles like the citric acid cycle, generating electron carriers that feed into the electron transport chain on the inner membrane. This design allows the mitochondrion to generate the cell’s ATP.