CHAPTER 7 PT. #1: Energy for Cells - Cellular Respiration

Cellular Respiration Overview

• Definition & Purpose: Cellular respiration is the metabolic process by which cells break down glucose incrementally in a series of enzymatic steps to harvest chemical energy and synthesize adenosine triphosphate ($\text{ATP}$).

• Relationship to Physiological Respiration & Food:

  • Food consumption provides nutrients, notably glucose ($\text{C}_6\text{H}_{12}\text{O}_6$).

  • Breathing provides oxygen ($\text{O}_2$) required by cells.

  • Cells absorb glucose and oxygen to carry out cellular respiration within the cytoplasm and mitochondria.

  • Metabolic byproducts generated by cellular respiration include carbon dioxide ($\text{CO}_2$), which is transported to the lungs and exhaled, and water ($\text{H}_2\text{O}$).

Anatomy of the Mitochondrion

• Role of the Mitochondrion: Mitochondria serve as the primary site for aerobic cellular respiration in eukaryotic cells, housing key enzymes and membrane complexes.

• Structural Components:

  • Double Membrane System:

  • Outer Membrane: A smooth, protective outer boundary layer separating the interior of the organelle from the cytoplasm.

  • Inner Membrane: A highly selective, protein-rich membrane that encloses the innermost compartment and houses the electron transport chain and $\text{ATP}$ synthase complexes.

  • Intermembrane Space: The narrow, fluid-filled space located between the outer membrane and the inner membrane into which hydrogen ions ($\text{H}^+$) are pumped during electron transport.

  • Cristae: Deep invaginations and folds formed by the inner membrane that dramatically increase the surface area available for oxidative phosphorylation.

  • Matrix: The central semi-fluid cavity enclosed by the inner membrane, containing mitochondrial DNA, ribosomes, and the enzymes required for the preparatory reaction and citric acid cycle.

Glucose Processing and Redox Reactions

• Controlled Energy Release: Glucose is degraded gradually in sequential biochemical steps rather than in a single explosive combustion event. This allows cells to capture energy efficiently and store it in $\text{ATP}$ molecules.

• Role of Coenzymes:

  • Hydrogen atoms ($\text{H}^+$) and high-energy electrons ($\text{e}^-$) stripped from glucose intermediates are transferred to specialized coenzymes.

  • $\text{NAD}^+$ (Nicotinamide Adenine Dinucleotide): Accepts electrons and a proton to become reduced to $\text{NADH}$ ($\text{NAD}^+ \rightarrow \text{NADH}$).

  • $\text{FAD}^+$ (Flavin Adenine Dinucleotide): Accepts two electrons and two protons to become reduced to $\text{FADH}_2$ ($\text{FAD}^+ \rightarrow \text{FADH}_2$).

• Redox Principles in Respiration:

  • Cellular respiration is a redox (reduction-oxidation) pathway.

  • Balanced Chemical Equation:     $\text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2 \rightarrow 6\,\text{CO}_2 + 6\,\text{H}_2\text{O} + \text{energy}$

  • Oxidation: Glucose ($\text{C}_6\text{H}_{12}\text{O}_6$) loses hydrogen atoms and electrons, undergoing oxidation to yield carbon dioxide ($\text{CO}_2$).

  • Reduction: Molecular oxygen ($\text{O}_2$) gains hydrogen atoms and electrons, undergoing reduction to yield water ($\text{H}_2\text{O}$).

MEMORIZE THIS FORMULA:

The Four Phases of Glucose Breakdown

• Phase 1: Glycolysis:

  • Location: Cytoplasm (cytosol), outside the mitochondria.

  • Oxygen Requirement: Anaerobic (does not require oxygen).

  • Process: Splits one 6-carbon glucose molecule into two 3-carbon pyruvate molecules.

  • Energy Yield: Net yield of $2\,\text{ATP}$ and $2\,\text{NADH}$.

• Phase 2: Preparatory Reaction (Prep Reaction):

  • Location: Mitochondrial matrix.

  • Process: Converts two pyruvate molecules into two 2-carbon acetyl-CoA units, producing two $\text{CO}_2$ waste molecules and two $\text{NADH}$.

• Phase 3: Citric Acid Cycle (Krebs Cycle):

  • Location: Mitochondrial matrix.

  • Process: A cyclical enzymatic pathway that oxidizes acetyl groups completely, yielding $2\,\text{ATP}$, releasing four $\text{CO}_2$ molecules, and generating high-energy electron carriers ($\text{NADH}$ and $\text{FADH}_2$).

• Phase 4: Electron Transport Chain (ETC):

  • Location: Inner mitochondrial membrane (cristae).

  • Process: High-energy electrons carried by $\text{NADH}$ and $\text{FADH}_2$ pass through membrane-bound carrier complexes. As electrons travel down the chain, energy is harnessed to pump $\text{H}^+$ ions, driving $\text{ATP}$ synthesis. Oxygen acts as the final electron acceptor, combining with $\text{H}^+$ to form $\text{H}_2\text{O}$.

  • Energy Yield: Yields approximately $34\,\text{ATP}$ molecules.

Glycolysis Mechanism and Phases

• Location: Occurs within the cytoplasm of both prokaryotic and eukaryotic cells.

• Oxygen Independence: Can occur under both aerobic and anaerobic conditions.

• Step 1: Energy-Investment Phase:

  • Requires an initial investment of energy from $2\,\text{ATP}$ molecules.

  • $\text{ATP}$ transfers phosphate groups to glucose ($6$-carbon sugar), activating it for cleavage.

  • The phosphorylated sugar is cleaved into two 3-carbon molecules of glyceraldehyde 3-phosphate (G3P).

• Step 2: Energy-Harvesting Phase:

  • Each $\text{G3P}$ molecule is oxidized, transferring electrons and protons to $\text{NAD}^+$ to form $\text{NADH}$.

  • Intermediate molecules generated during this phase include:

  • $\text{BPG}$: 1,3-bisphosphoglycerate

  • $\text{3PG}$: 3-phosphoglycerate

  • Substrate-level phosphorylation transfers phosphate groups directly from substrate intermediates to $\text{ADP}$, creating $\text{ATP}$.

  • High-energy reactions yield four total $\text{ATP}$ molecules and two 3-carbon pyruvate molecules.

• Substrate-Level ATP Synthesis:

  • An enzyme directly transfers a phosphate group from a phosphorylated substrate molecule to $\text{ADP}$, synthesizing $\text{ATP}$.

Glycolysis Balance Sheet and Pyruvate Fate

• Inputs and Outputs Table:

  • Inputs:

  • 1 Glucose molecule ($\text{C}_6\text{H}_{12}\text{O}_6$)

  • $2\,\text{NAD}^+$

  • $2\,\text{ATP}$ (invested)

  • $4\,\text{ADP} + 2\,\text{P}$

  • Outputs:

  • 2 Pyruvate molecules

  • $2\,\text{NADH}$

  • $2\,\text{ADP}$

  • $4\,\text{ATP}$ (gross produced)

  • Net Gain:

  • $2\,\text{ATP}$ net gain per glucose ($4\,\text{ATP}$ produced $- 2\,\text{ATP}$ invested = $2\,\text{ATP}$ net)

  • $2\,\text{NADH}$

  • 2 Pyruvate

• Subsequent Metabolic Fate of Pyruvate:

  • With Oxygen ($\text{O}_2$ Available): Pyruvate is actively transported into the mitochondrial matrix, entering the preparatory reaction to proceed through aerobic cellular respiration.

  • Without Oxygen ($\text{O}_2$ Lacking): Pyruvate remains in the cytoplasm and undergoes reduction via anaerobic fermentation pathways (e.g., lactic acid or alcohol fermentation), regenerating $\text{NAD}^+$ to keep glycolysis functioning.