Chapter 9: Cellular Respiration - G. Scott's Bio Page



Chapter 9: Cellular Respiration

• Cellular respiration: C6H12O6 + 6O2 6CO2 + 6H2O + energy

• Flow of energy produces ATP [fig. 9-1]

• How yield energy? REDOX reactions = transfer of 1 or more electrons

▪ Oxidation – partial or complete loss of electrons

▪ Reduction – partial or complete gain of electrons

Xe- + Y X + Ye- C6H12O6 + 6O2 6CO2 + 6H2O + energy

Overview:

• Carbon and Hydrogen valence electrons lose potential energy as they shift towards the electronegative oxygen.

• Hydrogen first passed to the electron acceptor NAD+ before being transferred to oxygen. NAD+ functions as a co-enzyme.

• High energy electrons are transferred from the substrate to NAD+ and are then passed down the electron transport chain to oxygen, powering ATP synthesis.

Process includes 3 metabolic stages:

1. Glycolysis – catabolic pathway that occurs in the cytosol and partially oxidizes glucose (6C) into two pyruvate (3C) molecules.

2. Krebs Cycle – catabolic pathway that occurs in the mitochondrial matrix and completes glucose oxidation by breaking down pyruvate derivative (acetyl-CoA)

Both Glycolysis and Krebs cycle produce:

• Small amounts of ATP by substrate-level phosphorylation

• NADH by transferring electrons from substrate to NAD+ (also produces FADH2 by transferring electrons to FAD)

3. Electron transport chain (ETC) – ATP synthesis

▪ Located at the inner membrane of the mitochondria

▪ Accepts energized electrons from reduced co-enzymes (NADH and FADH2) that are harvested during glycolysis and the Krebs cycle. Oxygen pulls these electrons down the ETC to a lower energy state.

▪ Couples this exergonic slide of electrons to ATP synthesis (oxidative phosphorylation). Produces most of the ATP for the cell (90%)

Oxidative phosphorylation – ATP production that is coupled to exergonic transfer of electrons from food to oxygen

Substrate-level phosphorylation – ATP production by direct enzymatic transfer of phosphate from an intermediate substrate in catabolism of ADP

1. Glycolysis – occurs in the cytosol, “sugar splitting”

• Energy investment phase – cell uses ATP to phosphorylate glycolysis intermediates (2 - 3C molecules)

• Energy yield phase – 2 - 3C intermediates are oxidized to produce 2 pyruvates

2 – 3C Intermediates

(A closer look on page 154 – 155, figure 9.9)

kinase = enzyme involved in phosphate transfer

isomerase = enzyme involved in altering isomer

Step 6 is very exergonic, coupled to endergonic phosphorylation (ΔG = -10.3 Kcal/mol), has more potential to transfer a phosphate than ATP

The presence or absence of oxygen determines the fate of Pyruvate. In presence of oxygen (Aerobic Respiration), pyruvate is transferred from the cytosol, through a transport protein by a carrier protein, into the mitrochondria and converted into Acetyl-CoA. Oxidation of Pyruvate:

2. Krebs Cycle (or citric acid cycle or TCA cycle) – oxidize remaining acetyl fragments of acetyl-CoA to CO2

• Exergonic = energy used to reduce NAD+ and FAD and to phosphorylate ATP

• For every turn of the cycle:

▪ 2 carbons enter in the acetyl fragment of acetyl-CoA

▪ 2 different carbons leave as CO2

▪ coenzymes are reduced: 3 NADH and 1 FADH2 produced

▪ 1 ATP produced through substrate-level phosphorylation

▪ oxaloacetate is regenerated

▪ go through cycle 2 times/glucose molecule (one time for each pyruvate molecule)

▪ closer look on page 157, figure 9.11

3. ETC – made of made of electron carrier molecules embedded in inner mitochondrial membrane

• Electron is passed through a series of protein molecules (mostly cytochromes)

• This chain does not create ATP, but generates a proton gradient across the inner mitochondrial membrane

• Chemiosmosis – an energy coupling mechanism (figure 9.14)

▪ ATP synthase – an enzyme that is in inner membrane, functions in production of ATP by allowing H+ to flow through back into the matrix causing conversion of ADP to ATP.

Review:

|Process |ATP produced by Sub-level |Reduced co-enzyme |ATP produced by oxidative |Total ATP |

| |phosphorylation | |phosphorylation | |

|Glycolysis |Net 2 ATP |2 NADH |4 - 6 ATP |6 - 8 |

|Oxidation of Pyruvate |--- |2 NADH |6 ATP |6 |

|Krebs Cycle |2 ATP |6 NADH |18 ATP |24 |

| | |2 FADH2 |4 ATP | |

| | | |Total estimate = 36 - 38 |

Fermentation: anaerobic (absence of oxygen) catabolism of organic nutrients

• In glycolysis, pyruvate is reduced and NAD+ regenerated as compared to the opposite in aerobic conditions

1. Alcohol Fermentation – bacteria and yeast

2. Lactic Acid Fermentation – occurs in human muscle cells, lactate usually shipped to liver to be converted back to pyruvate when oxygen is available

Classifications of Organisms:

1. Strict (obligate) aerobes – organisms that require oxygen for growth and as the final electron acceptor for aerobic respiration

2. Strict (obligate) anaerobes – microorganisms that only grow in the absence of oxygen, and oxygen is usually toxic to these organisms

3. Facultative anaerobes – organisms capable of growth in either aerobic or anaerobic environments (yeast, bacteria, mammalian muscle cells)

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