Chapter 3.4 Cellular Respiration
Cellular Repsiration (MITOCHONDRIA)
- Cellular respiration in the mitochondria is an exergonic multistep metabolic pathway.
- exergonic means energy producing.
- in cellular respiration organic molecules are oxidized and disassembled by a series of enzymes
- energy is used to synthesize ATP (endergonic process)
- endergonic means using energy
- Oxygen is REQUIRED for maximum ATP production
What Is Glucose?
- glucose is an energy-rich moleulce with many C-C, C-H, C-O bonds.
- energy is within the Carbon bonds
Glucose Oxidation
- glycolysis occurs in cytosol and does not require oxygen.
- intermediate stage
- citric acid cycle
- electron transport system (Stages 2, 3, & 4) occurs in mitochondria and requires oxygen.
Net Chemical Reaction
- C6H12O2 + 6O2 → 6CO2 + 6H2O
- broken bond energy attaches phosphorus to ADP
- Direct it the least common
- substrate level phosphorylation
- indirect is the most common and energy is first release to coenzymes to form ATP
- oxidative phosphorylation
Glycolysis
- glycolysis is a metablic process that occurs in cytosol not requiring oxygen where glucose is the intial substrate and pyruvate is the final product.
- ^^glycolysis produces 2 ATP and uses 4 ATP where the net production of ATP is 2.^^
- during glycolysis 2 NADH is formed.
- negative feedback regulates glycolysis where ATP acts as allosteric inhibitor to turn of PFK
- increase ATP to inhibit PFK
- pyruvate strictly depends on oxygen
- high level of oxygen means pyruvate can enter mitochondria
- low levels of oxygen means pyruvate is converted into lactate
Intermediate Stage
- intermediate stage is where pyruvate releases CO2, NADH+ produces NADH and changes coenzyme A into acetyl CoA.
- Occur in mitochondria, double-membrane organelle (cristae), outer compartment, matrix.
- cristae are inner membrane folds where molecules of electron transport system are embedded
- outer compartment is that space between membranes
- matrix is the innermost space that houses the multienzyme complex & citric acid cycle enzymes
Citric Acid Cycle
- step 1: Acetyl CoA combined with oxaloacetate to form citrate
- steps 2 & 3: Isomer formed by removing water molecule, then reattaching elsewhere
- steps 4 & 5: Transfer of hydrogen to NAD+ to form NADH; CoA attached
- step 6: Removal of CoA and the formation of ATP
- step 7: Dehydrogenase transfers hydrogens to form FADH2
- step 8: water removed
- step 9: Dehydrogenase transfers hydrogen to form NADH
- regulation of citric acid cycles occurs at first stem enzyme citrate synthase.
- high energy demands: Levels of NADH, ATP, and pathway intermediates low, Cycle activity increased
- low energy demands: Levels of substances higher, Cycle activity decreases
- cyclic metabolic pathway involves nine enzymes in the mitochondrial matrix:
- Acetyl CoA is converted to two CO2 molecules, CoA molecule released, and ATP, 3 NADH, and 1 FADH2 formed during one cycle.
Electron Transport System
the electron transport system transfer electrons from NADH & FADH2. The energy is used to make ATP.
e- transport system structures
- in inner membrane (cristae)
- H+ pump
- Proteins that transport H+ from matrix to outer membrane compartment
- Maintains a H+ gradient between outer compartment and mitochondrial matrix
- Electron carriers
- Transport electrons between H+ pumps
e- transport system steps:
- electrons transferred from coenzymes to oxygen
- H+ gradient established
- H+ gradient harnessed to form ATP
in oxidative phosphorylation oxygen is the final e- acceptor, ADP phosphorylation makes ATP, Distinguished from substrate-level phosphorylation:
- substrate-level forms ATP from energy directly released from substrate
- glycolysis & citric acid cycle
ATP Production
- in ATP production the number of molecules generated depends on entry point of electrons into the transport chain. Generates 2 ATP molecules.
- Electrons from NADH pass through 3 H+ pumps and generates 3 ATP molecules
- Electrons from FADH2 enter at second pump.
- ATP in glucose breakdown
stage/total | sustrate-level phosphorylation | oxidative phosphorylation |
---|---|---|
glycolysis | 2 ATP | 2NADH → 6ATP |
intermediate stage | -- | 2NADH → 6ATP |
citric acid cycle | 2 ATP | 6NADH → 18ATP |
2FADH2 → 4ATP | ||
total | 2ATP | 34ATP |
- some ATP used during cellular respiration, so net ATP is 30 (may be slightly less due to enzymes)
Cyanide
- cyanide is a nitrogen triple-bonded with carbon, binds with a specific electron carrier of the electron transport system that inhibits e- transport system and ATP production.
- electrons are unable to reach oxygen
Fate of Pyruvate
pyruvate fate with low oxygen
activity of electron transport chain decreases
- Levels of NADH and FADH2 accumulate, Decreased levels of NAD+ and FAD.
cells become more dependent upon glycolysis
- requires NAD+ to continue
glycolysis eventually shuts down
- due to lack of NAD+
NAD+ must be regenerated for glycolysis to continue
NAD+ Regeneration
- NAD+ regeneration is when a hydrogen transferred from NADH to pyruvate, Pyruvate converted to lactate (lactic acid), Enables glycolysis to continue.
- Only 2 ATP generated versus 30 with sufficient oxygen, Impacts individuals with decreased ability to deliver oxygen to cells (For example, those with respiratory or cardiovascular disease)
Other Oxidized Fuel Molecules
- Fatty acids enzymatically change two carbons at a time to form acetyl CoA (beta oxidation), Acetyl CoA the enters pathway at citric acid cycle, Can only be oxidized aerobically.
- can be changed to sugars
- Amino acids have different pathway if protein is used for fuel, Point of entry depends upon specific type, Amine group is a waste product (Converted to urea, Excreted by kidneys)
- anything ending with -ase is an enzyme
What to Know?
- Glucose goes in, enzyme change the structure, break molecule apart into 2 G3P’s, NAD+ →NADH, 2ADP→ 2ATP, 2 pyruvate
- G3P (glyceraldehyde 3-phosphate)