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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