• 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

1. Glycolysis - The splitting of glucose into 2 3-Carbon molecules
2. Krebs Cycle (or citric acid cycle)
3. Electron Transport

• 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

1. Pyruvic acid releases a CO₂ and joins a 4C molecule to form a 6C molecule of citric acid
2. The citric acid releases 2 CO₂ molecules to form 2 molecules of NADH from NAD+
3. The resultant 4C molecule creates 1 GTP from GDP (This will be converted to ATP later)
4. The resultant 4C molecule creates 1 FADH₂ from FAD+ (This will be converted to ATP later)
5. 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

1. NADH & FADH₂ deliver H⁺ ions (and electrons) to inter-membrane space of mitochondria.
2. 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.
3. 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

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:

1. Lactic acid fermentation
2. Alcoholic fermentation

Lactic acid fermentation

• Occurs in human muscle tissue - causes pain under high exertion
• Allows recharging of NADH to NAD+ so glycolysis can continue
• Follows following formula:

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:

Alcoholic Fermentation