7.1 Production of Energy through Cellular Respiration
Cellular respiration is the oxidation process of organic molecules (mainly glucose) in living cells to release energy stored in chemical bonds, which is stored in the form of adenosine triphosphate (ATP).
Energy Requirements in Living Organisms
- Living organisms require energy to carry out metabolic processes such as active transport, cell division, muscle contraction, protein synthesis, and maintaining body temperature.
- Main Substrate for Respiration: Glucose is the primary substrate derived from the digestion of carbohydrates in animals or photosynthesis in plants.
Main Types of Cellular Respiration
- Aerobic Respiration: Breakdown of glucose in the presence of oxygen to produce carbon dioxide, water, and a large amount of energy (36–38 ATP).
- Anaerobic Respiration: Breakdown of glucose in the absence of oxygen (or limited oxygen) to yield smaller amounts of energy.
- Fermentation: Incomplete breakdown of glucose in oxygen-limited conditions or without oxygen via alternative pathways.
7.2 Aerobic Respiration
Aerobic respiration takes place in two main stages: Glycolysis (in the cytoplasm) and Oxidation of Pyruvate / Krebs Cycle (in the mitochondrion).
Stages of Aerobic Respiration
- 1. Glycolysis (Cytoplasm):
- Occurs in the cytoplasm and does not require oxygen.
- One 6-carbon molecule of glucose is broken down through a series of enzymatic reactions into two 3-carbon molecules of pyruvate.
- Net energy yield: 2 ATP and 2 NADH.
- 2. Oxidation of Pyruvate and Krebs Cycle (Mitochondrion Matrix & Inner Membrane):
- Occurs in the presence of oxygen.
- Pyruvate enters the mitochondrion matrix and is oxidized into Acetyl-CoA, releasing $CO_2$.
- Acetyl-CoA enters the Krebs Cycle to yield $CO_2$, ATP, NADH, and $FADH_2$.
- Electrons from NADH and $FADH_2$ are passed through the Electron Transport Chain on the inner mitochondrial membrane (cristae), driving the synthesis of approximately 32 to 34 ATP molecules.
- Oxygen acts as the final electron acceptor, combining with $H^+$ ions to form water ($H_2O$).
Summary Chemical Equation for Aerobic Respiration
$$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + 36\text{--}38\text{ ATP}$$
7.3 Fermentation
Fermentation is the incomplete breakdown of glucose without oxygen or in conditions of limited oxygen supply, yielding much less ATP (2 ATP per glucose molecule) compared to aerobic respiration.
Types of Fermentation
- 1. Alcohol Fermentation:
- Incomplete breakdown of glucose into ethanol, carbon dioxide, and energy.
- Equation: $\text{Glucose} \rightarrow \text{Ethanol} + \text{Carbon Dioxide} + 2\text{ ATP}$
- Occurs in: Yeast (Saccharomyces cerevisiae) and plant cells (e.g., paddy plants growing in flooded waterlogged conditions).
- Applications: Bread making (bubbles of $CO_2$ expand dough) and production of alcoholic beverages (beer, wine).
- 2. Lactic Acid Fermentation:
- Breakdown of glucose into lactic acid and energy without releasing carbon dioxide.
- Equation: $\text{Glucose} \rightarrow \text{Lactic Acid} + 2\text{ ATP}$
- Occurs in:
- Human Muscle Cells: During vigorous physical exercise, oxygen demand exceeds supply (oxygen debt). Muscle cells undergo lactic acid fermentation to produce rapid ATP. Accumulation of lactic acid causes muscle fatigue and cramps.
- Lactobacillus Bacteria: Converts lactose in milk to lactic acid during yogurt and cheese production.
Oxygen Debt in Muscle Cells
- During intense exercise, the rate of oxygen delivery to muscles cannot meet the rapid requirement for aerobic respiration.
- Muscles incur an oxygen debt (deficiency of oxygen) and switch to lactic acid fermentation to generate ATP.
- Recovery Phase: After exercise, rapid and deep breathing supplies excess oxygen to pay off the oxygen debt.
- The extra oxygen oxidizes accumulated lactic acid:
- About $\frac{1}{6}$ of the lactic acid is oxidized to $CO_2$, water, and energy in the liver.
- The remaining $\frac{5}{6}$ is converted back into glucose and stored as glycogen in muscle and liver tissues.
Comparison between Aerobic Respiration and Fermentation
- Similarities: Both break down glucose, release energy stored in ATP, and begin with glycolysis in the cytoplasm.
- Differences:
- Oxygen: Aerobic respiration requires oxygen; Fermentation occurs without or in limited oxygen.
- Completeness: Aerobic respiration completely oxidizes glucose; Fermentation incompletely breaks down glucose.
- Location: Aerobic occurs in cytoplasm and mitochondrion; Fermentation occurs only in cytoplasm.
- Products: Aerobic produces $CO_2$, $H_2O$, and 36–38 ATP; Alcohol fermentation produces ethanol, $CO_2$, and 2 ATP; Lactic acid fermentation produces lactic acid and 2 ATP.