Mitochondria - PDF

# **CH 10. MITOCHONDRIA** ## I. **GENERAL DESCRIPTION** - Mitochondria are large enough to be seen by light microscopy, but their ultrastructure is revealed only by electron microscopy. - Present in all eukaryotes but absent in prokaryotes. - In humans, they are transmitted by the mother only, as...

# **CH 10. MITOCHONDRIA** ## I. **GENERAL DESCRIPTION** - Mitochondria are large enough to be seen by light microscopy, but their ultrastructure is revealed only by electron microscopy. - Present in all eukaryotes but absent in prokaryotes. - In humans, they are transmitted by the mother only, as the fertilized egg receives only the nucleus from the spermatozoon. - The mitochondrion is not 100% maternal, as it contains proteins encoded by the nucleus. - Mitochondria exhibit considerable variability in appearance, size, number per cell, and ultrastructure. - They are usually concentrated in metabolically active regions of the cell. - The number of mitochondria per cell is low, the organelles are large, and vice versa. - Mitochondria proliferate by fission of pre-existing mitochondria. - Increase in mitochondrial size requires the import of proteins and lipids produced in other cell compartments, and the replication of its DNA. - Bounded by two membranes, having different surface areas and distinct properties and functions. - Mitochondrial matrix is enclosed in these membranes, and contains nearly all types of organic and inorganic molecules. - Main functions: oxidation of organic compounds and aerobic cell respiration. - Together, they lead to the production of energy in the form of ATP (oxidative phosphorylation) - Although prokaryotes do not have mitochondria, they carry out oxidation of organic compounds and cell respiration by enzymes of the cytoplasm and the plasma membrane. - The ATP synthase in aerobic bacteria is similar to that of mitochondria and is located at the inner face of the plasma membrane. ## **II. STRUCTURE** - Most common shape: ovoid, with 1 to 4 µm in length and 0.2 to 1 µm in diameter. - Other shapes: filamentous, spiral - Bounded by two membranes, which are separated by an intermembrane space. - Intermembrane space contains numerous enzymes, protons, and cytochrome c. ### 1. **MITOCHONDRIAL MEMBRANES** - Inner mitochondrial membrane (IMM) is thicker than the outer one and has greater surface area. - Presents inward folds called cristae. - Outer mitochondrial membrane (OMM) is composed of 50% proteins in weight, whereas the inner one contains more than 75%. - Contains porins or channels. - IMM is devoid of cholesterol and contains cardiolipin. ### 2. **MITOCHONDRIAL MATRIX** - Gel-like consistency, containing high concentrations of proteins and other organic and inorganic molecules. - Contains several circular DNA molecules, ribosomes, tRNAs, and enzymes that are necessary for gene expression and DNA replication. - Matrix contains enzymes that catalyze oxidation of organic compounds, the Krebs cycle enzymes, and β-oxidation enzymes. - Certain enzymes of β-oxidation catabolic pathway are associated with the inner membrane. ### 3. **IMPORT OF PROTEINS** - Mitochondrial proteins are synthesized by free ribosomes and delivered to the mitochondrion post-translationally. - Specific signal (address code) which is named presequence or targeting sequence is located near the N-terminus of the polypeptide chains and contains a number of positively charged “aa” residues. ## **III. MITOCHONDRIAL FUNCTIONS** ### 1. **DEFINITION OF OXIDATION AND ITS GENERAL DESCRIPTION** - Mitochondria extract energy from organic compounds and store it temporarily, in the form of electrochemical energy (a proton gradient across the IMM). - The main function is the oxidation of organic compounds into H2O + CO2 + energy, by means of metabolic oxidation reactions. - Aerobic cell respiration coupled to oxidative phosphorylation. ### 2. **CARBOHYDRATE OXIDATION** - Polysaccharids are first digested into monosaccharides in the digestive tract, liberating an amount of energy. - Monosaccharides are delivered to cells, where glucose oxidation is started in the cytosol and terminated in the mitochondrion. - Glucose oxidation is divided into two series of biochemical reactions: glycolysis and Krebs cycle (also named tricarboxylic acid (TCA) cycle, or citric acid cycle). - **Glycolysis:** takes place in the cytosol. - **Krebs cycle:** takes place in the mitochondrial matrix. ### 3. **FATTY ACIDS OXIDATION** - Mitochondrion is also the site for oxidation of fatty acids, glycerol, and “aa”. - Fatty acids are oxidized in the matrix of mitochondria as well as in the peroxisomes by a cyclic metabolic pathway named β-oxidation. ### 4. **OTHER MITOCHONDRIAL FUNCTIONS** - Mitochondria cooperate with the SER in terms of synthesis of steroid hormones. - Synthesis of certain phospholipids of biomembranes and of lipoprotein particles, in cooperation with the SER. - Synthesis of non-essential amino acids by supplying metabolites of the Krebs cycle that serve as precursors for the synthesis of amino acids in the cytosol. - Storage of diverse substances in form of dense inclusions, which may consist of ions (Ca++, Na+, K+,…) or organic substances such as lipoproteins.