Cellular Respiration PDF

# Energy Transformation: Cellular Respiration ## Cellular Respiration * **Mitochondrion** is the site for cellular respiration. ### Cellular Respiration: The Process * Cellular respiration is the set of catabolic pathways that break down the nutrients we consume into usable forms of chemical ene...

# Energy Transformation: Cellular Respiration ## Cellular Respiration * **Mitochondrion** is the site for cellular respiration. ### Cellular Respiration: The Process * Cellular respiration is the set of catabolic pathways that break down the nutrients we consume into usable forms of chemical energy (*ATP*). * Cellular respiration includes both **aerobic respiration** and **anaerobic respiration**. **Aerobic Respiration** * **C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + Energy (*ATP* + heat)** **The Stages of Cellular Respiration: A Preview** Harvesting of energy from glucose has three stages: 1. **Glycolysis** breaks down glucose into two molecules of pyruvate. 2. **Pyruvate oxidation** and the **citric acid cycle** complete the breakdown of glucose to CO₂. 3. During **oxidative phosphorylation**, the **electron transfer chain** and **chemiosmosis** facilitate synthesis of most of the cell's *ATP*. ## Glycolysis ### Glycolysis Breaks Down Glucose to Pyruvate * **"Glyco-"** means "sugar," and **"-lysis"** means "to break." * Requires many steps, all of which occur in the **cell's cytosol**. **Two Stages:** * **Energy Investment** * **Energy Harvest** **Overall, the net gain is two NADHs and two ATPs per molecule of glucose.** ## Pyruvate Oxidation ### Pyruvate Is Oxidized to Acetyl CoA (Transition) * Pyruvate moves into the mitochondrial matrix, where a preliminary "transition step" further oxidizes each pyruvate molecule. * A molecule of CO₂ is removed, and *NAD*⁺ is reduced to *NADH*. * The remaining two-carbon molecule, called an acetyl group, is transferred to a molecule called coenzyme *A* to form acetyl coenzyme *A* (abbreviated acetyl CoA). * Acetyl CoA is the compound that enters the Krebs cycle. ## Citric Acid Cycle * The citric acid cycle, also called the **Krebs cycle**, oxidizes organic fuel derived from pyruvate, generating 1 *ATP*, 3 *NADH*, and 1 *FADH₂* per turn. * Another **2 CO₂** are produced as a waste product (for a total of 3 CO₂ including one from pyruvate oxidation). * Because 2 pyruvate are produced per glucose, the cycle runs twice per glucose molecule consumed. ## Oxidative Phosphorylation * Electrons drop in free energy as they are transferred down the chain, finally passing to O₂ to form H₂O. * The electron transport chain breaks the large free-energy drop from glucose to O₂ into smaller steps, releasing energy in manageable amounts. * No *ATP* is produced directly by the chain. ### Chemiosmosis: The Energy Coupling Mechanism * The energy released as electrons are passed down the electron transport chain is used to pump *H*⁺ from the mitochondrial matrix to the intermembrane space. * *H*⁺ then moves down its concentration gradient back across the membrane, passing through the protein complex *ATP synthase*. * *H*⁺ moves into binding sites on the rotor of *ATP synthase*, causing it to spin in a way that catalyzes phosphorylation of *ADP* to *ATP*. * This is an example of **chemiosmosis,** the use of energy in a *H*⁺ gradient to drive cellular work. * Certain electron carriers in the electron transport chain accept and release *H*⁺ along with the electrons. * In this way, the energy stored in a *H*⁺ gradient across a membrane couples the redox reactions of the electron transport chain to *ATP* synthesis. * The *H*⁺ gradient is referred to as a **proton-motive force**, emphasizing its capacity to do work. ## Cellular Respiration: Energy Flow * During cellular respiration, most energy flows in this sequence: * **glucose --> NADH --> electron transport chain --> proton-motive force --> ATP** * About 34% of the energy in a glucose molecule is transferred to *ATP* during cellular respiration, making about 32 *ATP*. * The rest of the energy is lost as heat. ## Anaerobic Respiration * **Fermentation** is an extension of glycolysis that oxidizes *NADH* by transferring electrons to pyruvate or its derivatives. * Two common types are **alcohol fermentation** and **lactic acid fermentation**. ### Alcohol Fermentation * In alcohol fermentation, pyruvate is converted to ethanol in two steps: * The first step releases CO₂ from pyruvate. * The second step produces *NAD*⁺ and ethanol. * Alcohol fermentation by yeast is used in brewing, winemaking, and baking. ### Lactic Acid Fermentation * In lactic acid fermentation, pyruvate is reduced by *NADH*, forming *NAD*⁺ and lactate as end products, with no release of CO₂. * Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt. * Human muscle cells use lactic acid fermentation to generate *ATP* during strenuous exercise when O₂ is scarce.