10.6 Chapter 10 Summary

Cellular Respiration Overview

Cellular respiration is the catabolic process that extracts energy stored in the chemical bonds of glucose and other organic molecules and converts it into the high-energy molecule ATP (adenosine triphosphate), the primary energy currency of the cell.

• Process and Location: It involves a sequence of oxidation-reduction (redox) reactions. In eukaryotes, the initial step occurs in the cytoplasm, while the remaining, most productive steps take place within the mitochondria.

• Electron Carriers: The energy released from oxidizing glucose is initially captured by electron carrier molecules, primarily NAD⁺ and FAD, which are reduced to NADH and FADH₂. These carriers transport high-energy electrons to the final stage of respiration.

• Four Major Stages:

  1. Glycolysis (Cytoplasm)

  2. Pyruvate Oxidation (Mitochondrial Matrix)

  3. Citric Acid Cycle (Mitochondrial Matrix)

  4. Oxidative Phosphorylation (Inner Mitochondrial Membrane)

Glycolysis is the first step of glucose catabolism. This pathway occurs in the cytoplasm of nearly all living organisms.

• Process: One six-carbon molecule of glucose is broken down into two three-carbon molecules of pyruvate.

• Phases:

  • Energy-Requiring (Investment) Phase: Two ATP molecules are consumed to phosphorylate (activate) the glucose molecule.

  • Energy-Releasing (Recovery) Phase: Four ATP molecules are produced via substrate-level phosphorylation, and two molecules of NADH are generated.

• Net Yield per Glucose: 2 Net ATP (4 produced – 2 consumed), 2 NADH, and 2 Pyruvate.

• Limiting Factor: The continuation of glycolysis depends on the availability of NAD⁺, which must be regenerated from NADH either by aerobic respiration or by fermentation.

Pyruvate Oxidation and the Citric Acid Cycle

In the presence of oxygen, the pyruvate from glycolysis is processed in the mitochondria.

Pyruvate Oxidation (The Transition Reaction)

• Process: Each of the two pyruvate molecules moves into the mitochondrial matrix. Here, a multienzyme complex converts pyruvate into acetyl CoA, a two-carbon compound.

• Products per Pyruvate (or per turn): The conversion is a key oxidation step that releases one molecule of CO₂ and produces one molecule of NADH. Since two pyruvates are made per glucose, the yield is 2 CO₂ and 2 NADH.

The Citric Acid Cycle

• Process: This is a closed loop pathway in the mitochondrial matrix that completes the oxidation of glucose. The 2-carbon acetyl group from Acetyl CoA joins with the 4-carbon molecule oxaloacetate to form the 6-carbon molecule citrate.

• Primary Function: To harvest high-energy electrons (in the form of NADH and FADH₂) by breaking down the acetyl group. The remaining carbon atoms are released as CO₂.

• Products per Glucose (two turns of the cycle):

  • 4 CO₂ (the final carbon atoms from the original glucose are released).

  • 6 NADH and 2 FADH₂ (electron carriers).

  • 2 ATP/GTP (via substrate-level phosphorylation).

Oxidative Phosphorylation

This stage is responsible for generating the majority of the cell’s ATP. It occurs on the inner mitochondrial membrane and consists of two parts: the Electron Transport Chain and ATP synthase.

Electron Transport Chain (ETC)

• Electron Flow: High-energy electrons from NADH and FADH₂ are passed down a series of protein complexes (I-IV) embedded in the inner mitochondrial membrane.

• Proton Pumping: The energy released from the flow of electrons is used to actively pump protons (H$^+$) from the mitochondrial matrix into the intermembrane space, establishing a strong electrochemical gradient (the proton-motive force). * Final Electron Acceptor: Oxygen (O₂) is the final electron acceptor, which combines with electrons and protons to form water (H₂O). Without oxygen, the entire ETC backs up and cellular respiration halts.

ATP Synthase

• ATP Synthesis: The potential energy stored in the proton gradient is utilized by a molecular machine called ATP synthase.

• Mechanism: Protons flow back into the matrix down their concentration gradient through ATP synthase, causing the enzyme to rotate and catalyze the phosphorylation of ADP to produce large amounts of ATP.

• Overall Yield: This process generates the largest ATP yield of all stages in cellular respiration (up to ~34 ATP per glucose, though the actual yield varies).

Metabolism Without Oxygen

When oxygen is unavailable (anaerobic conditions), aerobic respiration stops because the ETC cannot function. To sustain ATP production, cells rely on anaerobic processes to regenerate the NAD$^+$ necessary for glycolysis to continue.

Fermentation

• Fermentation uses an organic molecule as the final electron acceptor to reoxidize NADH to NAD$^+$. It occurs entirely in the cytoplasm.

  • Lactic Acid Fermentation: Occurs in human muscle cells (during intense exercise) and certain bacteria (e.g., in yogurt). Pyruvate is converted to lactate (lactic acid), regenerating NAD$^+$.

  • Alcohol Fermentation: Occurs in yeast and some bacteria. Pyruvate is converted to ethanol and CO₂, regenerating NAD$^+$.

• Energy Output: Fermentation processes only rely on glycolysis and yield a net of 2 ATP per glucose, which is significantly less efficient than aerobic respiration.

Anaerobic Cellular Respiration

• Some prokaryotes use an electron transport chain but utilize an inorganic molecule (like sulfate or nitrate) instead of oxygen as the final electron acceptor. This process is distinct from fermentation and can generate more ATP than glycolysis alone.