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Unformatted text preview: Chapter 5 Aerobic Respiration And The Mitochondrion A Brief History of Life on Earth I. Life's first 2 billion years - reducing atmosphere (H 2 , NH 3 , H 2 O); Earth of this period was populated by anaerobes that captured & used energy via O 2-independent metabolism (glycolysis, fermentation) II. ~2.7 billion years ago - Cyanobacteria appeared & were immensely successful; photosynthesis produced O 2 by splitting water; O 2 filled the world (oceans, lake, atmosphere) A. O 2 was toxic to most organisms takes on extra electrons & reacts with a variety of biological molecules B. O 2 in atmosphere must have been powerful agent for natural selection C. Over time, species evolved that were protected from the damaging effects of O 2 D. They also evolved metabolic pathways that used O 2 to extract energy more efficiently from foodstuffs III. These aerobes (the world's first) survived & passed their more efficient metabolism to descendants A. Anaerobes only extract limited amounts of energy from food, excreting lactic acid or ethanol, which they were unable to metabolize further B. The aerobes completely oxidized food to H 2 O & CO 2 & got much more energy eventually gave rise to all oxygen-dependent prokaryotes & eukaryotes living today C. In eukaryotes, the use of O 2 for energy occurs in a specialized organelle, the mitochondrion 1. A massive body of data indicates that they have evolved from an ancient aerobic bacterium that took up residence inside the cytoplasm of an anaerobic host cell Mitochondrial Structure & Function I. Mitochondria known for >100 years; large enough to be visible in light microscope; initially isolated before 1900 by teasing cells apart with fine needle & a number of their properties had been described A. Found to be osmotically active early on they swell in hypotonic media, shrink in hypertonic media B. This suggested that they were surrounded by a semipermeable membrane like that around the cell II. Morphological characteristics & physiological properties of typical mitochondria they possess recognizable morphological characteristics, but also exhibit considerable variability in appearance A. In EM, usually sausage-shaped; can be spherical (early embryos) or elongate, threadlike (as in fibroblasts); ~0.2 -1 m in cross-sectional diameter & 1 - 4 m in length (similar in size to bacteria) 1. At least in yeast & cultured mammalian cells, they are not discrete, individual organelles, but they are instead found as a highly branched, interconnected network or reticulum in...
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