BIOLOGY Lecture Notes - Golden Pages 2003

1. THE CELL

Mitochondria

Class="center">Mitochondrial Structure

Cell/35.html">Mitochondria are subcellular Organelles measuring 2 µm in length and 0.5 µm in diameter.

Mitochondria are frequently located within The Cell near structures that require ATP or close to sources of cellular "fuel." Although similar in size to Bacteria, their shape varies. Mitochondria possess two membranes that can be separated from one another. The outer membrane contains monoamine oxidase and Enzymes that activate Fatty acids; it is freely permeable to most soluble low-molecular-weight compounds. The inner membrane has a significantly larger surface area, expanded by folds known as cristae. It is impermeable to Na⁺, Mg²⁺, and Cl^- ions, as well as Most Amino Acids. Distributed across The surface of the inner membrane are discrete groups of Electron Transport Chain enzymes, commonly referred to as respiratory assemblies. Additionally, the inner surface of the inner membrane is associated with structural units containing enzyme molecules involved in ATP synthesis (mitochondrial ATPase). The inner membrane contains several enzymes that facilitate The transport of specific metabolites across the membrane. The inner mitochondrial matrix contains a high concentration of protein, including various Enzymes of the Krebs cycle, fatty acid β-oxidation, and The Urea Cycle, as well as a pool (depot) of ADP, ATP, NAD, NADH, and CoA, which is separated from the cytoplasmic pool by the inner membrane, rendering it impermeable to these compounds. The matrix contains large, electron-dense granules. Ribosomes have been detected in certain mitochondria. Mitochondria contain DNA molecules. Mitochondrial DNA has a circular structure (resembling that of bacteria) that carries the Genetic information for the Synthesis of specific polypeptide chains of Cytochromes and ATPase. Consequently, mitochondria synthesize unusually hydrophobic, Water-insoluble Proteins, thereby avoiding the need to transport them over any distance through the Cytoplasm. The genetic instructions for synthesizing many other polypeptide chains are encoded, as usual, in the nuclear DNA.

Mitochondrial Functions

1. Mitochondria serve as the cellular power plants where ATP—the primary energy source in living organisms—is synthesized. Mitochondria are present in all Eukaryotic Cells, with the exception of mature erythrocytes.

2. Mitochondria carry out The process of cellular Respiration, consuming oxygen and releasing CO₂. In these organelles, Membrane Proteins contain Vitamins and Inorganic Compounds (such as iron, sulfur, and copper) as Cofactors (the non-protein component of the enzyme).

3. Mitochondria mediate The conversion of energy stored in food into Other forms of energy.

Cells acquire the energy they need through the Oxidation of proteins, fats, and CARBOHYDRATES, which are hydrolyzed outside the mitochondrion into amino acids, Monosaccharides, glycerol, and fatty acids. Through sequential reactions, these low-molecular-weight products are converted into an acetic acid derivative—acetyl-CoA. In this form, acetyl groups are oxidized to CO₂ within the Krebs cycle. The enzymes driving this process are localized in the mitochondrial matrix. The Krebs cycle generates hydrogen atoms, with the NAD cofactor acting as the electron acceptor. Subsequently, electrons from this compound enter the transport chain on the inner mitochondrial membrane, where they pass successively from carrier to carrier until they reach molecular O₂, which is reduced to H₂O. This process is termed respiration, and The electron transport chain is known as the Respiratory Chain. Respiration is coupled with phosphorylation. As electrons move along the respiratory chain, an H⁺ concentration gradient is established perpendicular to the membrane surface (Mitchell's chemiosmotic hypothesis, 1966), which is subsequently utilized for ATP synthesis. Thus, Mitochondria are the site of crucial Oxidative Phosphorylation processes.

In addition to respiration coupled with phosphorylation, mitochondria exhibit respiration that is independent of macroergic bond accumulation. This phenomenon is known as non-phosphorylated oxidation. The energy released in the process is dissipated as heat. Several compounds are capable of uncoupling respiration from phosphorylation. Specifically, such uncouplers include dinitrophenol, dicumarol, thyroxine, oligomycin, among others.

Mitochondria are capable of accumulating Ca²⁺ ions using energy released during electron transport; however, the process of Ca²⁺ accumulation in mitochondria is mutually exclusive with oxidative phosphorylation.