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Cellular Respiration

Cellular respiration is the metabolic process within cells that harnesses energy from organic molecules, like glucose, to produce ATP. This process is vital for short-term energy storage and is responsible for the universal energy carrier in cells. Cells can obtain energy by oxidizing molecules like glucose, allowing the carbon and hydrogen atoms to combine with oxygen to form carbon dioxide and water. This process, known as aerobic respiration, occurs in four main stages: glycolysis, link reaction, Krebs cycle, and electron transport chain.

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Cellular Respiration

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  1. Cellular Respiration The metabolic processes within cells that makes ATP using energy from organic molecules such as glucose.

  2. The need for respiration

  3. Adenosine triphosphate (ATP) • the short-term energy store of all cells • easily transported and is therefore the universal energy carrier • formed from the nucleotide adenosine monophosphate by the addition of two further phosphate molecules • a metabolically active cell may require up to 2 million ATP molecules every second!!!!!!!!

  4. Adenosine triphosphate (ATP)

  5. Adenosine triphosphate (ATP)

  6. Adenosine triphosphate (ATP)

  7. Cellular Respiration

  8. Why can cells obtain energy from oxidising molecules such as glucose? • The carbon and hydrogen atoms in cells, for example in glucose molecules, are not in their most stable form • The most energetically stable form of carbon is carbon dioxide and the most energetically stable form of hydrogen is water. • A cell can therefore obtain energy from sugar molecules (or amino acids or fatty acids) by allowing the carbon and hydrogen atoms in these molecules to combine with oxygen to form carbon dioxide and water. • This oxidation occurs through a series of steps called aerobic respiration • Overall: C6H12O6 + 6O2  6CO2 + 6H2O

  9. Burning glucose: C6H12O6 CO2 + H2O Respiration: C6H12O6 CO2 + H2O Energy Energy How is this different from burning glucose? one step oxidation multi-step catabolism

  10. Cellular Respiration This can be divided into 4 main stages: • Glycolysis • Link reaction • Krebs cycle • Electron transport chain

  11. Just to confuse you!!! • The stages of respiration have other names • Glycolysis - the Embden-Meyerhof pathway • Krebs cycle -the citric acid cycle -the tricarboxylic acid (TCA) cycle • Electron transport chain -the hydrogen carrier system -the cytochrome system

  12. Where it all happens: Glycolysis Link reaction Krebs cycle Glycolysis – in the cytoplasm Link reaction and Krebs cycle in the matrix of the mitochondria E.T. chain on the inner membrane of the mitochondria Electron transport chain

  13. Krebs cycle Summary of events Substrates Products Stages glycolysis ATP glucose pyruvate carbon dioxide link reaction acetyl CoA carbon dioxide & ATP H Atoms (H+ & e-) water & ATP oxygen electron transport chain

  14. Key terms:

  15. Oxidation and reduction • understanding these two terms is central to understanding respiration • Oxidation is the loss of electrons • Reduction is the gain of electrons OILRIG

  16. Oxidation and reduction F214 NAD [and FAD] act as ‘hydrogen carriers’

  17. Glycolysis 1 • glyco - ‘sugar’ ; lyso - ‘breakdown’ • the breakdown of a hexose sugar • into two molecules of the three-carbon compound pyruvate(pyruvic acid) • occurs in all cells • gives a net gain of 2 ATP molecules • in anaerobic respiration it is the only stage of respiration

  18. Glycolysis 2 • look at the summary of glycolysis on the worksheet • do not try to memorise this sequence of reactions • concentrate on understanding what happens in this stage of respiration and identifying the important points • One molecule of glucose(with 6 C atoms) is broken down into two molecules of pyruvate (with 3 C atoms each) • Two molecules of ATP are used in the very first steps but later four molecules are synthesised by substrate level phosphorylation ( a net gain of two ATP molecules) • hydrogen atoms are removed from glucose molecules when two molecules of NAD+ are reduced to NADH

  19. Summary of glycolysis

  20. Substrate-level phosphorylation Enzyme adenosine P P P ADP substrate product adenosine P P P ATP

  21. hexose 1,6-biphosphate glucose 6-phosphate 2 x glycerate 3-phosphate 2 x pyruvate Energy changes in glycolysis • The stages of glycolysis that involve energy changes are shown below: Potential energy glucose Reaction time

  22. The story so far; 1 ……………………. One molecule of glucose (6c) One molecule of glucose (6c) cytoplasm matrix of mitochondrion

  23. The Link Reaction Link reaction

  24. Link reaction- formation ofAcetyl CoA • This occurs in the fluid filled centre (matrix) of mitochondria • The two pyruvate molecules produced by glycolysis enter the mitochondrial matrix from the cytoplasm • Here each pyruvate is converted to acetyl coenzyme A (usually written as acetyl CoA) • In this reaction carbon dioxide is given off and each pyruvate loses a pair of hydrogen atoms which results in reduced NAD (NADH) • Acetyl CoA is a complex molecule incorporating a coenzyme derived from pantothenic acid (vitamin B5)

  25. Link reaction pyruvate + coenzyme A  acetyl coenzyme A + CO2 2C 1C 3C Coenzyme A [CoA]

  26. Link reaction- formation ofAcetyl CoA

  27. The story so far; 2 ……………………. One molecule of glucose (6c) cytoplasm matrix of mitochondrion

  28. Krebs cycle 1 • important for three main reasons: • enables breakdown of macromolecules - with the release of CO2 • provides reducing power for electron transport system - produces pairs of hydrogen atoms which are ultimately the source of metabolic energy for the cell • acts as an interconversion centre - is a valuable source of intermediate compounds used in making e.g. fatty acids, amino acids

  29. Krebs cycle 2 • This series of reactions also occurs in the matrix of mitochondria • Each molecule of acetyl CoA combines with a 4-C compound (oxalocacetate) to form a 6-C compound (citrate) • This citrate then undergoes four dehydrogenations - the removal of hydrogen • and then two decarboxylations – the removal of carbon dioxide

  30. Krebs cycle - detail

  31. Krebs cycle

  32. Krebs cycle • as a result of these reactions: • oxaloacetate is regenerated and can start the cycle off again • three molecules of NADare reduced to NADH • one molecule of FAD is reduced to FADH2 • two molecules of CO2 are produced • one molecule of ATP is made by substrate level phosphorylation • How many times will the cycle ‘turn’ during the catabolism of one molecule of glucose? 2

  33. Krebs cycle • The overall reaction accomplished by two ‘turns’ of the Krebs cycle is: 2 acetyl CoA + 6NAD + 2FAD + 2ADP + 2Pi 4CO2+ 6NADH + 2FADH2+ 2ATP

  34. The story so far; 3……………………. One molecule of glucose (6c) One molecule of glucose (6c)

  35. Electron transport chain 1 • the means by which the energy, in the form of hydrogen atoms, mostly from the Krebs cycle, is converted to ATP • hydrogen atoms (H+and e-), attached to carriers, are passed along a chain of progressively lower energy levels (the respiratory chain) • the energy released is harnessed to produce ATP through the oxidation of the hydrogen atoms (oxidative phosphorylation)

  36. Electron transport chain 2 • Look back at the equations representing the catabolism of a molecule of glucose so far and you will notice that a number of reduced coenzyme molecules (either NADH or FADH2) have been produced as the glucose molecule has been stripped of its electrons • how many molecules of NADH have been produced? • how many molecules of FADH2 have been produced? 10 NADH; 2 FADH2

  37. Electron transport chain 3 • In the electron transfer chain these reduced coenzymes are re-oxidised. • The enzymes which catalyse this re-oxidation process are located on the folded inner mitochondrial membrane where they are arranged in a series. • What is the likely advantage of this high degree of folding of the inner membrane of the mitochondria to form cristae? increased surface area for the location of the enzymes of the electron transport chain

  38. Electron transport chain 4 • The overall reaction of the electron transport chain: 10NADH + 2FADH2 + 6O2  10NAD + 2FAD + 12H2O • During this oxidative reaction, ATP is produced by oxidative phosphorylation

  39. Electron transport chain 5 • The last cytochrome in the chain is re-oxidised by donating its electrons to oxygen • thus producing water - the other waste product of respiration • Voila!!!! Respiration is complete. Well, almost …….

  40. How exactly is this ATP made? The chemiosmosis theory…….. H  H+ + e-

  41. chemiosmosis

  42. chemiosmosis

  43. chemiosmosis

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