Cellular Respiration
- Cells must extract energy from organic molecules to provide energy for the processes of life.
- Main source of such chemical energy are carbohydrates, especially glucose C6H12O6.
- Cell respiration is essentially the reverse of photosynthesis. Its general formula is as follows:
- Cellular respiration has 2 general Classifications:
- Aerobic Respiration occurs in the presence of O2.
- Anaerobic Respiration occurs in the absence of O2.
- Aerobic Respiration occurs in 3 steps
- Glycolysis - The splitting of glucose into 2 3-Carbon molecules
- Krebs Cycle (or citric acid cycle)
- Electron Transport
Glycolysis Steps:
- Glucose molecule is split into 2 molecules of PGAL by addition of energy from 2 molecules of ATP (remember ATP ® ADP releases energy)
- Each PGAL molecule is changed by enzymes into 2 molecules of Pyruvic Acid (a 3 carbon molecule). This creates 1 NADH (an energy molecule like NADPH from photosynthesis), and 2 ATP molecules for each pyruvic acid, for a total of 2 NADH, and 4 ATP molecules.
Glycolysis
The Krebs Cycle: - Occurs only in the presence of oxygen.
- Cycle is composed of nine steps, which is summarized as follows:
- Pyruvic acid releases a CO2 and joins a 4C molecule to form a 6C molecule of citric acid
- The citric acid releases 2 CO2 molecules to form 2 molecules of NADH from NAD+
- The resultant 4C molecule creates 1 GTP from GDP (This will be converted to ATP later)
- The resultant 4C molecule creates 1 FADH2 from FAD+ (This will be converted to ATP later)
- Another molecule of NADH is formed from NAD+. The resultant 4C molecule reenters the cycle, and the process begins again.
- Cell must convert FADH2 and NADH to ATP
- 22 ATP molecules are produced through electron transport
- This occurs in a process similar to photosynthesis
- NADH & FADH2 deliver H+ ions (and electrons) to inter-membrane space of mitochondria.
- As H+ concentration gradient increases in inter-membrane space. H+ ions diffuse back into matrix through a special protein called ATP synthetase. This enzyme uses this diffuion energy to generate ATP.
- Electrons are passed through several membrane proteins where they bind with 2 H+ ions and Oxygen to form a molecule of H2O. Thus, in aerobic respiration Oxygen is the final electron acceptor.
Electron transport
Energy Total:Source | Number ATP Produced |
Glycolysis | 2 ATP |
Transport of NADH into Matrix. | -2 ATP |
Krebs Cycle (ATP & GTP) | 2 ATP |
Electron Transport (NADH & FADH2) | 34 ATP |
NET TOTAL | 36 ATP |
Gross ATP yield - Aerobic Respiration
Anaerobic respiration: - Occurs in low Oxygen situations.
- Far less efficient than aerobic respiration - only yields 2 ATP per glucose molecule
- Two types:
- Lactic acid fermentation
- Alcoholic fermentation
- Occurs in human muscle tissue - causes pain under high exertion
- Allows recharging of NADH to NAD+ so glycolysis can continue
- Follows following formula:
Puruvic Acid + NADH Þ NAD+ + Lactic Acid
Lactic Acid Fermentation
Alcoholic fermentation - Is widely used commercially to produce alcoholic beverages
- Allows recharging of NADH to NAD+ so glycolysis can continue
- Follows following formula:
Puruvic Acid + NADH Þ NAD+ + Alcohol + CO2
Alcoholic Fermentation
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