# **Cellular Respiration** Cellular respiration is a series of complex *oxidation-reduction reactions* by which living cells break down organic molecules (like glucose) to obtain energy in the form of **ATP** (Adenosine Triphosphate). In biological systems, these reactions typically involve the transfer of hydrogen atoms. --- ## **Kinds of Cellular Respiration** Cellular respiration is broadly divided into two types based on the presence or absence of oxygen. The initial stage, **glycolysis**, is common to both. Glycolysis breaks down one glucose molecule into two molecules of *pyruvate*. The fate of pyruvate then depends on oxygen availability. | Feature | **Aerobic Respiration** | **Anaerobic Respiration (Fermentation)** | | :--- | :--- | :--- | | **Oxygen Requirement** | Requires abundant oxygen | Occurs in the absence of oxygen | | **Breakdown of Glucose** | Complete oxidation | Incomplete breakdown | | **Final Products** | Carbon dioxide (CO₂), Water (H₂O) | Lactic acid OR Ethanol and CO₂ | | **Energy Yield (ATP)** | High (36 ATP in eukaryotes, 38 in prokaryotes) | Very low (2 ATP) | | **Location** | Cytoplasm and Mitochondria | Cytoplasm only | --- ## **Anaerobic Respiration (Fermentation)** Anaerobic respiration is the incomplete breakdown of glucose without oxygen, yielding a small amount of ATP. It is also known as **fermentation**. ### **Lactic Acid Fermentation** - **Process:** Glycolysis is followed by the reduction of pyruvate by NADH to form lactic acid. This regenerates NAD⁺, allowing glycolysis to continue. - **Equation:** $$\mathrm{C_3H_4O_3 + NADH + H^+ \rightarrow C_3H_6O_3 + NAD^+}$$ $$\text{Pyruvate} + \text{NADH} \rightarrow \text{Lactic Acid} + \text{NAD}^+$$ - **Occurs in:** Anaerobic bacteria, and vertebrate muscle cells during strenuous exercise when oxygen is scarce. The accumulation of lactic acid causes muscle fatigue. ### **Alcoholic Fermentation** - **Process:** Glycolysis is followed by two steps: 1. Decarboxylation of pyruvate to acetaldehyde (releases CO₂). 2. Reduction of acetaldehyde by NADH to form ethanol. This also regenerates NAD⁺. - **Equation:** $$\mathrm{C_3H_4O_3 \rightarrow C_2H_4O + CO_2}$$ $$\text{Pyruvate} \rightarrow \text{Acetaldehyde} + \text{CO}_2$$ $$\mathrm{C_2H_4O + NADH + H^+ \rightarrow C_2H_5OH + NAD^+}$$ $$\text{Acetaldehyde} + \text{NADH} \rightarrow \text{Ethanol} + \text{NAD}^+$$ - **Occurs in:** Yeast and some plants. --- ## **Mechanism of Aerobic Respiration** Aerobic respiration is a catabolic process that involves the complete oxidative breakdown of glucose into CO₂ and H₂O, releasing a large amount of energy. It occurs in four main phases. ### **1. Glycolysis** The breakdown of a 6-carbon glucose molecule into two 3-carbon pyruvate molecules. - **Location:** Cytoplasm. - **Phases:** 1. ***Preparatory Phase (Energy Investment):*** Two ATP molecules are consumed to energize the glucose molecule, which is then split into two molecules of Glyceraldehyde-3-phosphate (G3P). 2. ***Oxidative Phase (Energy Payoff):*** The two G3P molecules are oxidized to pyruvate. This phase produces 4 ATP and 2 NADH. - **Net Yield of Glycolysis:** - **2 Pyruvate** - **2 ATP** (4 produced - 2 invested) - **2 NADH** ### **2. Oxidation of Pyruvate (Link Reaction)** This reaction links glycolysis (in the cytoplasm) with the Krebs cycle (in the mitochondria). - **Location:** Mitochondrial matrix. - **Process:** Each pyruvate molecule enters the mitochondrion and is converted into a 2-carbon molecule called **Acetyl-CoA**. - A molecule of **CO₂** is released (*decarboxylation*). - A molecule of **NADH** is formed (*oxidation*). - Coenzyme A is attached to the remaining acetyl group. - **Yield per Glucose (2 Pyruvates):** - **2 Acetyl-CoA** - **2 NADH** - **2 CO₂** ### **3. Krebs Cycle (Citric Acid Cycle or TCA Cycle)** A cyclic series of reactions that completes the oxidation of glucose. - **Location:** Mitochondrial matrix. - **Process:** The 2-carbon Acetyl-CoA combines with a 4-carbon molecule (oxaloacetate) to form a 6-carbon molecule (citrate). The cycle then proceeds through a series of reactions, regenerating oxaloacetate at the end. - **Yield per Glucose (2 turns of the cycle):** - **2 ATP** (via substrate-level phosphorylation) - **6 NADH** - **2 FADH₂** - **4 CO₂** ### **4. Electron Transport Chain (ETC) and Oxidative Phosphorylation** The final stage where most of the ATP is produced. - **Location:** Inner mitochondrial membrane. - **Process:** 1. **Electron Transport:** High-energy electrons from **NADH** and **FADH₂** are passed along a series of protein complexes (electron carriers) embedded in the inner mitochondrial membrane. 2. **Proton Pumping:** As electrons move down the chain, energy is released and used to pump protons (H⁺) from the mitochondrial matrix into the intermembrane space, creating a proton gradient. 3. **Final Electron Acceptor:** **Oxygen** is the final acceptor of electrons. It combines with protons to form water (H₂O). - **Components:** Includes NADH-Q reductase (Complex I), Coenzyme Q, Cytochrome reductase (Complex III), Cytochrome c, and Cytochrome oxidase (Complex IV). ### **Chemiosmosis** - The process that couples the energy stored in the proton gradient to ATP synthesis. - Protons flow back into the matrix down their concentration gradient through an enzyme complex called **ATP synthase**. - The flow of protons powers ATP synthase to phosphorylate ADP, forming ATP. This is called **oxidative phosphorylation**. - **Yield:** - 1 NADH → **3 ATP** - 1 FADH₂ → **2 ATP** ### **Substrate-Level Phosphorylation** A direct method of ATP synthesis where an enzyme transfers a phosphate group from a substrate molecule directly to ADP. - **Occurs in:** - **Glycolysis:** 2 net ATP are produced. - **Krebs Cycle:** 2 ATP are produced. - **Total:** 4 ATP per glucose molecule are made via this mechanism in aerobic respiration. ### **Metabolism of Fats and Proteins** Other organic molecules can be used as fuel for cellular respiration. - **Fats:** Break down into glycerol and fatty acids. - *Glycerol* is converted to G3P and enters glycolysis. - *Fatty acids* are converted to Acetyl-CoA and enter the Krebs cycle. - **Proteins:** Broken down into amino acids. - The amino group is removed (*deamination*), forming ammonia (which is converted to urea). - The remaining carbon skeletons enter the respiratory pathway at various points (as pyruvate, Acetyl-CoA, or Krebs cycle intermediates). --- ## **Possible Questions/Answers** - **Q:** What is cellular respiration? **A:** A series of metabolic reactions in cells to convert biochemical energy from nutrients into ATP, and then release waste products. - **Q:** What is the main difference between aerobic and anaerobic respiration? **A:** Aerobic respiration requires oxygen and completely breaks down glucose to produce a large amount of ATP (approx. 36), while anaerobic respiration does not require oxygen and only partially breaks down glucose for a small yield of 2 ATP. - **Q:** What is the final electron acceptor in the electron transport chain? **A:** Molecular oxygen (O₂). - **Q:** What is the difference between substrate-level phosphorylation and oxidative phosphorylation? **A:** *Substrate-level phosphorylation* is the direct enzymatic transfer of a phosphate group from a substrate to ADP. *Oxidative phosphorylation* uses the energy released from the electron transport chain to create a proton gradient, which drives ATP synthesis via the enzyme ATP synthase (chemiosmosis). ---