Cellular respiration Study Guide

📖 Core Concepts Cellular respiration: Redox process that oxidizes fuels (glucose, amino acids, fatty acids) using an inorganic electron acceptor to make ATP. Aerobic vs. anaerobic: Aerobic uses O₂ as the final electron acceptor; anaerobic uses other inorganic acceptors (e.g., nitrate, sulfate). Fermentation is anaerobic without an external electron acceptor. ATP yields: Theoretical max = 38 ATP/glucose; realistic ≈ 30 – 32 ATP because of transport costs, proton leak, and revised P/O ratios (NADH ≈ 2.5 ATP, FADH₂ ≈ 1.5 ATP). Key locations: Glycolysis – cytosol (oxygen‑independent) Pyruvate dehydrogenase & TCA – mitochondrial matrix (eukaryotes) or cytosol (prokaryotes) Oxidative phosphorylation – inner mitochondrial membrane (cristae) Chemiosmotic coupling: Electron transport pumps protons, creating a gradient that drives ATP synthase to make ATP. 📌 Must Remember Net ATP from glycolysis = 2 ATP (substrate‑level). Substrate‑level ATP: 2 ATP from glycolysis + 2 ATP (GTP) from TCA per glucose. Reduced carriers per glucose: 10 NADH (2 glycolysis, 2 pyruvate DH, 6 TCA) and 2 FADH₂ (TCA). P/O ratios: NADH ≈ 2.5 ATP, FADH₂ ≈ 1.5 ATP. Overall aerobic reaction: $$\text{C}6\text{H}{12}\text{O}6 + 6\ \text{O}2 \rightarrow 6\ \text{CO}2 + 6\ \text{H}2\text{O} + \text{30–32 ATP}$$ Fermentation ATP yield = 2 ATP/glucose (only glycolysis). Pasteur point: Oxygen level at which cells switch from respiration to fermentation. 🔄 Key Processes Glycolysis (cytosol) Investment: -2 ATP (hexokinase & phosphofructokinase). Payoff: +4 ATP (substrate‑level) + 2 NADH. Net: +2 ATP, 2 pyruvate. Oxidative decarboxylation (mitochondrial matrix) Pyruvate + CoA + NAD⁺ → Acetyl‑CoA + CO₂ + NADH (via pyruvate dehydrogenase complex). Citric Acid Cycle (matrix) per Acetyl‑CoA Products: 3 NADH, 1 FADH₂, 1 GTP (ATP), 2 CO₂. Electron Transport Chain & Oxidative Phosphorylation (inner membrane) NADH → Complex I → Q → Complex III → Cyt c → Complex IV → O₂ → H₂O. FADH₂ enters at Complex II (bypasses Complex I). Proton pumping creates chemiosmotic potential; >3 H⁺/ATP (includes ADP/ATP translocase & Pi carrier). Fermentation (cytosol, no O₂) Lactic acid: Pyruvate + NADH → Lactate + NAD⁺ (muscle). Alcoholic: Pyruvate → Acetaldehyde + CO₂; Acetaldehyde + NADH → Ethanol + NAD⁺ (yeast). 🔍 Key Comparisons Aerobic respiration vs. fermentation Electron acceptor: O₂ (aerobic) vs. none (fermentation). ATP yield: 30 ATP/glucose vs. 2 ATP/glucose. End products: CO₂ + H₂O vs. lactate or ethanol + CO₂. Anaerobic respiration vs. fermentation External acceptor: inorganic (nitrate, sulfate) vs. none. Location of ETC: membrane‑bound chain with alternative terminal acceptor vs. none. NADH vs. FADH₂ P/O ratio NADH ≈ 2.5 ATP per molecule; FADH₂ ≈ 1.5 ATP per molecule (due to fewer pumped protons). ⚠️ Common Misunderstandings “Fermentation is a type of respiration.” – Incorrect; it lacks an external electron acceptor and therefore isn’t respiration. “Each NADH yields 3 ATP.” – Modern data show 2.5 ATP because >3 protons are needed per ATP. “Glycolysis always requires oxygen.” – Wrong; glycolysis occurs anaerobically; oxygen only needed for downstream steps. “All ATP comes from oxidative phosphorylation.” – Only 90 % of total ATP; the rest is substrate‑level (glycolysis + TCA). 🧠 Mental Models / Intuition “Fuel → Reducing power → Proton pump → Gradient → ATP.” Visualize glucose as a battery that first releases electrons (NADH/FADH₂) and then uses those electrons to spin a turbine (ATP synthase). “One‑step‑per‑carrier”: Each NADH = 2.5 ATP, each FADH₂ = 1.5 ATP → quickly estimate yields by counting carriers. “Location matters”: Cytosol steps are oxygen‑independent; membrane‑bound steps need O₂ or an alternative acceptor. 🚩 Exceptions & Edge Cases Transport costs: Import of pyruvate, ADP, Pi consumes part of the proton motive force → lowers net ATP. Proton leak/un-coupling proteins: Dissipate gradient as heat; reduces ATP yield, especially in brown adipose tissue (thermogenin). Anaerobic microbes: May use nitrate or sulfate → yields differ from classic O₂ respiration but still involve an ETC. 📍 When to Use Which If O₂ present → follow aerobic pathway (glycolysis → pyruvate DH → TCA → ETC with O₂). If O₂ absent but NAD⁺ needed → choose fermentation (lactic acid in muscle, alcoholic in yeast). If alternative inorganic acceptor available → anaerobic respiration (use nitrate, sulfate, etc., in the terminal step of the ETC). For quick ATP during high‑intensity work → rely on glycolysis + fermentation despite low yield. 👀 Patterns to Recognize “Two‑ATP investment, four‑ATP payoff” in glycolysis → net +2 ATP. Every acetyl‑CoA → 3 NADH, 1 FADH₂, 1 GTP → multiply by 2 for one glucose. Presence of O₂ ⇒ CO₂ + H₂O as products; absence ⇒ lactate or ethanol + CO₂. Proton‑to‑ATP ratio >3 → expect actual ATP per NADH/FADH₂ to be lower than textbook 3/2. 🗂️ Exam Traps Choosing 38 ATP as the answer – outdated; exam‑ready answer is 30‑32 ATP. Mistaking fermentation for anaerobic respiration – remember fermentation lacks an external electron acceptor. Counting 4 NADH from glycolysis – only 2 NADH are produced (payoff phase). Assuming FADH₂ yields the same ATP as NADH – FADH₂ yields 1.5 ATP, not 2.5. Overlooking transport costs – answers that ignore the cost of moving ADP/ATP across the membrane will overestimate yield. --- All statements are drawn directly from the provided outline.