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Lecture 7

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Biological Sciences
Stephen Reid

Mitochondria evolution •Arose from phagocytosis of aerobic prokaryote •Can divide in the cell –Mitochondrial fission •“powerhouse of the cell” –Site of aerobic respirations Mitochondria functions •Site of aerobic respiration –Utilizes oxygen to extract energy from macromolecules and converting it to ATP -Primarily from glucose: ATP production. Mitochondria –ATP production •We use 2 X 10 molecules of ATP a day -ATP: cells energy source: energy is released when ATP is hydrolyzed to ADP •Amount of mitochondria in cells depends on energy needs •many mitochondria in: muscle cells, liver cells, fat cells, plant cells & sperm Mitochondria –Structure •Usually sausage-shaped but can be spherical (early embryos) or elongate, threadlike (fibroblasts) -0.2 -2 um in cross-sectional diameter & 1-4 um in length (similar in size to bacteria) •Size and number of mitochondria reflect energy needs of the cell •Dynamic structures –Change shape, move from place to place in cytoplasm •Mitochondria can fuse with one another, or split in two. –The balance between fusion and fission is likely a major determinant of mitochondrial number, length, and degree of interconnection. •Inner and outer mitochondrial membranes enclose two spaces -The matrix and the Intermembrane space •Outer mitochondrial membrane is outer boundary •Inner mitochondrial membrane subdivided into 2 interconnected domains - Inner boundary membrane - Cristae: where the machinery for ATP is located - Phase-contrast light microscope can be picked up - Bright field stain: - To see inner membrane structure light microscope is not enough: TEM, SEM Outer Mitochondrial Membrane (OMM) •~ 50% protein: very porous •Porin proteins –large channels : Bacteria-like Beta pleated sheets. •Allows very large molecules in Inner Mitochondrial Membrane (IMM) •~ 75% protein •Unusual lipid composition -No cholesterol, rich in cardiolipin: typical of bacterial plasma membranes -like bacterial plasma membranes •Highly impermeable –Requires channels and pumps •Over 100 different proteins –Electron transport chain (ETC) -IMM forms cristae-many thin folds to increase the surface area Mitochondrial Matrix •Gel-like, from high protein concentration •Contains circular DNA molecule(s), ribosomes and tRNA •Mitochondrial DNA encodes for 37 genes •However, mitochondrial function requires 3000 proteins •Mitochondria proteins are completely translated on free ribosomes in cytosol •Proteins must then be imported into the mitochondria Posttranslational uptake of proteins into mitochondria •IMM integral proteins: includes all the proteins of the ETC and the ATP synthase. •Matrix proteins: includes all the enzymes of the Krebs / citric acid cycle. How do mitochondria make ATP? •Start with Glucose (or amino acids, or glycerol and fatty acids) •Series of chemical reactions -Performed by enzymes (proteins) –Acts on substrates (targets) to produce a chemical change in the substrate How do mitochondria make ATP? •Start with Glucose (or amino acids, or glycerol and fatty acids) 1) Glycolysis: cytosol of the cell itself 2) Krebs cycle/Citric Acid/TCA Cycle: mitochondrial matrix 3) Electron-Transport Chain: Inner mitochondrial membrane (cristae) Total Result of Aerobic Respiration: C6H12O6+ 6O2=6C02 +6H2O + 38 ATP 1) Glycolysis •Glucose (6 C) –contains high energy electrons –First hydrolyzed into 2 molecules of pyruvate/pyruvic acid (3 C) –Involves 10 chemical reactions by enzymes Steps 1 –3) Glucose is phosphorylated, rearranged structurally and then phosphorylated again (cost = 2ATP) Steps 4 –5) 6C biphosphate is split into two 3C monophosphates Step 6) 3C aldehyde oxidized to an acid and e-used to reduce NAD+ to NADH and C1 is phosphorylated Step 7) phosphate group from C1 is transferred to ADP forming ATP (gain = 2ATP) Steps 8 –9) rearrangement and dehydration of substrate Step 10) phosphate group transferred to ADP forming ATP (gain = 2ATP) + Glucose + 2NAD + 2 ADP + 2Pi 2 Pyruvate+ 2 ATP + 2 NADH + 2 H + 2 H2O •Pyruvic acid then moves to mitochondrial matrix: through a special pyruvate transporter •Converted to a 2 C molecule called acetyl CoA : (one CO2 and one NADH also generated) 2) Krebs / TCA / citric acid cycle •It is a stepwise cycle where substrate is oxidized and its energy conserved. •Acetyl CoA bound to a 4 C molecule (OAA) to create citrate (6 C) •During the cycle, two carbons are oxidized to CO2, regenerating the four-carbon OAA needed to continue the cycle. •OAA then attached to an incoming Acetyl CoA and cycle continues..... •Contains many enzymes for this conversion •Enzymes break down carbon molecules through oxidation reactions: loss of electrons: electrons are from H atoms in c molecules • Step 1 : NAD+ and FAD become reduced •ie., NAD+ becomes NADH •Molecules within cycle are common compounds generated in other catabolic reactions –Krebs/TCA/citric acid cycle = central metabolic pathway of the cell •Net result: Citrate (6C)  OAA (4C) •By products: - 3 NAD+ are reduced to NADH - 2 CO2molecules are released - 1 FAD is reduced to FADH2 - 1 GTP molecule is produced Next: High energy electrons in NADH and FADH2are used to make ATP. How is ATP made? 1. Hydrogen atoms split into protons (H+) and electrons 2. Electrons are passed along ETC 3. Protons are pumped into Intermembrane space 4. Proton gradient is used to make ATP (positive outside the mit and negative inside the mit) 3) Electron Transport Chain •Consists of 4 IMM trans-membrane complexes –Contains proteins with prosthetic groups •Oxidize NADH and FADH2 and take electrons •Have the ability to accept electrons and donate them to the next protein complex in ``chain`` •Pass electrons from one
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