Mitochondrial structure and function
Mitochondria are oval-shaped, rod-shaped, or cylindrical organelles found in virtually all eukaryotic cells, varying in shape and size depending on the cell type. They are just visible with a light microscope but their detailed internal structure requires an electron microscope. Their primary function is to serve as the site of aerobic respiration, a continuous process that produces adenosine triphosphate (ATP), the universal energy currency for cells.
The structure of a mitochondrion is intricately adapted to its function:
Double Membrane (Envelope):
Outer Membrane: This smooth outer boundary contains transport proteins like porin, which create wide aqueous channels allowing easy passage of small, water-soluble molecules like pyruvate, CO2, O2, and NAD from the cytoplasm into the intermembrane space.
Inner Membrane: This membrane is highly folded into structures called cristae, which project into the interior of the mitochondrion. The cristae vastly increase the surface area available for the reactions of aerobic respiration. The inner membrane is also a more selective barrier, controlling the precise entry of ions and molecules into the matrix. It is largely impermeable to hydrogen ions (protons), a crucial feature for establishing an electrochemical gradient.
Intermembrane Space: The narrow region between the inner and outer membranes. This space is essential for the accumulation of hydrogen ions (protons), creating a potential difference that drives ATP synthesis.
Matrix: The fluid-filled interior space enclosed by the inner membrane. The matrix contains an aqueous solution of enzymes necessary for the link reaction and the Krebs cycle.
Mitochondria are central to the overall process of aerobic respiration, which involves four main stages:
Glycolysis: Although glycolysis occurs in the cytoplasm, its product, pyruvate, is subsequently transported into the mitochondrial matrix if oxygen is available.
Link Reaction: Pyruvate is actively transported into the mitochondrial matrix, where it is oxidized to acetate, producing reduced NAD and releasing carbon dioxide. Acetate then combines with coenzyme A to form acetyl coenzyme A.
Krebs Cycle (Citric Acid Cycle): This cycle takes place in the mitochondrial matrix. Acetyl coenzyme A combines with a 4-carbon molecule to form a 6-carbon molecule, which then undergoes a series of oxidation-reduction reactions, generating reduced NAD, reduced FAD, and a small amount of ATP, while releasing carbon dioxide.
Oxidative Phosphorylation: This is the final stage, occurring on the inner mitochondrial membrane (cristae). Energy from electrons carried by reduced NAD and FAD is used to pump protons (H+) from the matrix into the intermembrane space, creating a proton gradient. These protons then flow back into the matrix through ATP synthase (a process called chemiosmosis), driving the synthesis of ATP from ADP and inorganic phosphate. Oxygen acts as the final electron acceptor, combining with protons and electrons to form water.
The complete aerobic respiration of one glucose molecule theoretically yields around 38 molecules of ATP, although in practice, it is closer to 30-32 ATP. Mitochondria also perform other functions, such as the synthesis of lipids.
Mitochondria possess unique features, including their own small, circular DNA molecules and 70S ribosomes, which are smaller than the 80S ribosomes found in the cytoplasm. These characteristics support the endosymbiotic theory, suggesting that mitochondria originated from ancient bacteria that were engulfed by larger cells in a symbiotic relationship.
The number of mitochondria in a cell is directly related to its energy demands. Highly active cells, such as muscle cells (including cardiac muscle), liver cells, nerve cells, and sperm cells, contain thousands of mitochondria to provide the large amounts of ATP required for processes like muscle contraction, active transport, and nerve impulse transmission. For instance, sperm cells have numerous mitochondria to fuel their movement towards an egg, and cells of the proximal convoluted tubule have many mitochondria to supply ATP for active transport.
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