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Cell Energetics

Cell Energetics. Cell Energetics. Anabolic synthesis of cells from simple precursors (glucose and salts) requires energy Glucose is burned to (either) CO 2 and water (aerobic) or CO 2 and alcohols or acids (anaerobic) to provide this energy. ( Catabolism ). Overall metabolism.

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Cell Energetics

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  1. Cell Energetics

  2. Cell Energetics • Anabolic synthesis of cells from simple precursors (glucose and salts) requires energy • Glucose is burned to (either) CO2 and water (aerobic) or • CO2 and alcohols or acids (anaerobic) to provide this energy. (Catabolism)

  3. Overall metabolism

  4. Cell Energetics (simplified view) Catabolic Processes Metabolic Waste CO2, Alcohol, acids Food & Oxygen ATP - Reducing Power Cell Mass Products Anabolic Processes

  5. ATP to ADP • Metabolites to building blocks and building blocks to polymers are energetically uphill reactions. • The energy for these reactions is supplied by simultaneous hydrolysis of ATP to ADP – an energetically downhill reaction • Net reaction is slightly downhill.

  6. Energy Balances • Precursors to Metabolites (produces net ATP)

  7. Energy Balances • Precursors to Metabolites (produces net ATP)

  8. Energy Balances • Metabolites to building blocks (uses net ATP)

  9. Energy Balances • ATP used for producing BB (from metabolites): • amino acids = • R-nucleotides = • D-nucleotides = • Lipids = • LPS = • Peptidoglycan = • Storage glycogen =

  10. Energy Balances • For producing proteins from BB • Amino acids to proteins (uses net ATP) • 4 ATP’s are used to incorporate each molecule of an amino acid into a protein (5081 umole aa’s/gram cells) = 21,000 umole • 1020 umole ATP per gram of cell for producing m-RNA • 500 umole ATP per gram of cell for protein proofreading • 125 umole ATP per gram of cell for protein modification and assembly • Total = ~22,000 umole/gram

  11. Energy Balances • For producing DNA from BB • Trinucleotides to DNA (uses net ATP) • Unwinding energy (2 ATP per bp) = 100 umole/gram cells • Ligation = 0.6 umole/gram • Proofreading = 35 umole/gram • Supercoiling = 0.5 umole/gram • Total = 136 umole/gram

  12. Energy Balances • For producing RNA from BB • Trinucleotides to RNA (uses net ATP) • Discarded segments = 242 umole/gram • Modifications = 14 umole/gram • Total = 246 umole/gram

  13. Energy Balances • For producing PG from BB • Peptidoglycans • Forming peptide for forming peptide crosslinks = 138 umole/gram • Total = 138 umole/gram

  14. Energy Balances • For producing Lipids from BB • Phospholipids • Forming phophatidylethanolamine = 258 umole/gram • Total = 258 umole/gram

  15. Energy Balances • Grand total for forming metabolites = - 6000 umole ATP per gram of cells • Grand total for forming BB = 19,000 umole ATP/gram cells • Grand total for forming polymers from BB = 23,000 umole ATP/gram cells • Net Total = 36,000 umole ATP/gram cells made • (the actual number appears to be more like 72,000 umole of ATP per gram cells, as if the processes are only ½ efficient)

  16. Question? • Where does the cell get the ATP it needs (72,000 umole/gram dry cells) to synthesize these molecules?

  17. ATP supply • ATP is regenerated from ADP by one of two routes • Substrate level phosphorylation • e.g.Glycolytic reactions… with the formation of (net) 2 ATP per mole of glucose and …

  18. …Oxidative Phosphorylation • Oxidative phosphorylation is complex and somewhat controversial but the points of agreement are: • O/P occurs (only) at the cytoplasmic membranes of bacteria and mitochondrial membranes of eukaryotic cells • NADH is oxidized to NAD+ with the release of an electron. The reaction and electron help pump H+ out of the cell. The free energy of reentry of H+ results in the formation of ATP.

  19. Oxidative Phosphorylation • Oxidative phosphorylation (or electron transport)… with the formation of 20 to 30 moles of ATP per mole of glucose

  20. Formation of NADH • NADH is produced at various points in the TCA cycle

  21. O/P • The excess NADH is oxidized at the membrane resulting in the net production of ATP (38 is probably an overestimate – 25 is more like it in bacterial systems)

  22. Oxidative Phosphorylation • NADH is oxidized to NAD+, ejecting 2 H+ in the process. • Reduction of water with the electron reacts H+ in the cytoplasm • Reentry of H+ into the cell produces ATP in the process

  23. Protomotive Force • The energy to generate ATP from ADP at the membrane is believed to come from the “protomotive force” which drives H+ back into the cell and simultaneously generates ATP

  24. Heterotrophs and Autotrophs • Heterotrophs use glucose (and other carbon containing compounds) produced by other organisms for both anabolism and catabolism. • Autotrophs produce glucose from CO2 and water use for anabolism, using ATP generated by photosynthesis or oxidation of inorganic molecules. • Phototrophs get energy from light (light takes the place of NADH) • Chemolithotrophs get energy from reduced inorganic compounds. (Various compounds take the place of NADH)

  25. ATP generation via photosynthesis

  26. Photosynthesis • The ATP generated by photosynthesis is used to make glucose. • If there is excess glucose synthesis capacity, FDiP is used to make starch or sucrose or G3P to make fats and oils.

  27. Photosynthesis • Light, water and CO2 are required - O2 and glucose (CH2O)6 are produced. • The light produces an electron and O2, the electron is used to produce water and a protomotive gradient. • The protomotive gradient produces ATP

  28. Energy from Other Compounds - chemolithotrophs

  29. Overall Energy Balance • Glucose requirements to supply energy for cell synthesis. • Aerobic ~ 25 umoles of ATP per umole of glucose metabolized. • (Needs 72,000 umoles ATP per gram cells) • 2880 umoles of glucose needed per gram of cells = 0.5 grams metabolized 1.42 + 0.5 = 1.92 grams of glucose per gram of cells total or 0.52 grams of cells/ gram of glucose (observed to be about 0.51 g-cells/g)

  30. Other substrates • Other substrates produce more or less ATP per gram of precursor and so the precursor requirements are not the same as for glucose. Yields are measured experimentally. • ATP yields are high for hydrocarbons (see methane and parafins) • ATP yields are low for organic acids

  31. Yield of cells on various C sources

  32. Anaerobic Metabolism • Glucose requirements to supply energy for cell synthesis. • Anaerobic only ~ 2 umoles of ATP per umole of glucose metabolized. • Needs 72,000 umoles ATP per gram cells** • 36,000 umoles of glucose needed per gram of cells = 6.48 grams metabolized 1.42 + 6.48 = 7.90 grams of glucose per gram of cells total or 0.13 grams of cells/ gram of glucose (observed to be about 0.1 g-cells/g)

  33. Heat Release During Cell Growth • Cells have a higher Gibbs energy (and enthalpy) than glucose (spontaneous formation is unfavorable) • CO2 and H2O have lower Gibbs energy (and enthalpy) than glucose (spontaneous formation is favorable) • Overall reaction is energetically favorable and about 50% of the free energy in the precursors is recovered in the cells.

  34. Cells have a higher heat of combustion per gram than glucose Cell combustion = 5.5 kcal/gm Glucose combustion = 3.75 kcal/gm It takes 2 grams of glucose to produce 1 gram of cells  Heat of reaction is ~ 2.0 (2.3 measured) kcal/gm-cells (exothermic) Heat Release During Cell Growth

  35. Heat Released During Cell Growth

  36. Oxygen Requirements • Glucose catabolism requires 6 moles of O2 per mole of glucose. • C6H12O6 + 6O2 6CO2 + 6H2O • 2.88 mmole of glucose are catabolized per gram of cells = 17.3 mmole of O2 per gram x .032 g / mmole = 0.6 grams O2/gram cells  1.67 grams cells/ g-O2 (1.47 observed) • 20 mmole O2 per gram of cells or 1.5 grams of cells/gram of O2 • More reduced substrates require relatively more O2 per gram of cells (lower yield on O2)

  37. Oxygen Needs for cells

  38. Respiratory Quotient • Stoichiometry suggests that aerobic metabolism results in the production of 1 mole of CO2 per mole of O2 used. The ratio of CO2 produced to O2 used is called the respiratory quotient (RQ). RQ ~ 1.0 • If the cells metabolize anaerobically, they still produce CO2, but use no O2. The RQ goes to infinity. glucose  2 ethanol + 2CO2 • Partial anaerobic growth gives RQ > 1 and is diagnostic of the culture conditions.

  39. Summary • Calculation of cell ATP needs for synthesis and ATP produced by catabolism of substrates gives a value of about 0.5 grams of glucose catabolized per gram of cells produced. This coincidentally gives a yield of 0.5 grams of cells per gram of glucose. • The fact that cell + CO2 formation is energetically downhill from glucose substrate means that there is (an exothermic) enthalpy of reaction of about 2.3 kcal/gram cells produced

  40. Summary 3. The oxygen requirements, as calculated from the glucose metabolized, suggest a requirement of 20 mmole of O2 per gram of cells produced. (And the production of 20 mmole of CO2 per gram of cells) corresponding to a yield of cells on O2 of about 1.5 grams/gram of O2 4. RQ is diagnostic of anaerobic growth

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