Oxidative Phosphorylation ~28-34 (from 10 NADH) (from 2 $FADH_2$) 0 Overall Total (per Glucose) ~30-38 ATP - - 6

Note: ATP yield is variable due to shuttle systems for NADH from glycolysis.

Occurs when: Oxygen is limited or absent.
Purpose: Regenerate $NAD^+$ for glycolysis to continue producing ATP (2 net ATP per glucose).
Stages: Glycolysis followed by fermentation.
Types in Plants:
4.1. Alcoholic Fermentation
- Process: Pyruvate is converted to acetaldehyde, then to ethanol.
- Equation: Pyruvate $\rightarrow$ Acetaldehyde + $CO_2$ $\rightarrow$ Ethanol + $NAD^+$
- Products: Ethanol, $CO_2$, $NAD^+$
- Common in some plant tissues under anoxia (e.g., waterlogged roots, germinating seeds).
4.2. Lactic Acid Fermentation (less common in plants)
- Process: Pyruvate is directly converted to lactate.
- Equation: Pyruvate $\rightarrow$ Lactate + $NAD^+$
- Products: Lactate, $NAD^+$
- Rare in higher plants, but can occur in some tissues under severe stress.
Efficiency: Much less efficient than aerobic respiration (2 ATP vs. ~30-38 ATP).

Temperature:
- Increases with temperature (up to an optimum, usually $20-30^\circ C$).
- High temperatures ($>40^\circ C$) can denature enzymes, decreasing respiration.
Oxygen Concentration:
- Aerobic respiration requires $O_2$. Low $O_2$ leads to anaerobic respiration.
- High $O_2$ can inhibit respiration (Pasteur effect, though complex).
Substrate Availability (Sugars):
- Higher sugar concentration generally leads to higher respiration rates.
- Depletion of sugars (e.g., in senescing leaves) reduces respiration.
Water Content:
- Respiration decreases with reduced water potential (drought stress).
- Enzyme activity and substrate transport are affected.
Light:
- Indirect effect: Light drives photosynthesis, producing sugars for respiration.
- Photorespiration: A separate process (not true respiration) that occurs in light, consuming $O_2$ and releasing $CO_2$ in C3 plants.
Age of Plant/Tissue:
- Meristematic tissues (young, growing) have higher respiration rates.
- Mature, senescing tissues may have altered rates.
Hormones: Hormones like ethylene can increase respiration (e.g., during fruit ripening).

Definition: Ratio of $CO_2$ released to $O_2$ consumed.
Formula: $RQ = \frac{\text{Volume of } CO_2 \text{ released}}{\text{Volume of } O_2 \text{ consumed}}$
Significance: Indicates the type of substrate being respired.
- Carbohydrates: $RQ = 1$ (e.g., glucose: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$)
- Fats: