Cellular Respiration: Glycolysis, Pyruvate Oxidation, and the Krebs Cycle

Overview of Cellular Respiration and Metabolism

• Definition: Cellular respiration is the biochemical process by which cells harvest chemical energy stored in organic molecules to power cellular activities.

• Comparison to Photosynthesis:

  • Photosynthesis is an anabolic process carried out by plants and autotrophs to build energy-rich organic molecules, specifically glucose ($C_6H_{12}O_6$), from carbon dioxide ($CO_2$) and water ($H_2O$) while releasing oxygen ($O_2$).

  • Cellular respiration is the functional opposite of photosynthesis. It breaks down glucose ($C_6H_{12}O_6$) in the presence of oxygen ($O_2$) to harvest stored chemical energy, yielding carbon dioxide ($CO_2$) and water ($H_2O$) as chemical byproducts.

• Chemical Equation of Cellular Respiration:

  • Standard Chemical Formula: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$

  • Reactants: Glucose ($C_6H_{12}O_6$) and Oxygen ($6O_2$).

  • Products: Carbon Dioxide ($6CO_2$), Water ($6H_2O$), and released energy in the form of Adenosine Triphosphate ($ATP$).

• Biochemical Classification:

  • Catabolic Pathway (Catabolism): Cellular respiration is a catabolic pathway because it breaks down complex macromolecules (a six-carbon glucose molecule) into simpler inorganic molecules ($CO_2$ and $H_2O$). The enzymatic breakdown of these complex chemical bonds releases stored potential energy for daily cellular operations.

  • Anabolic Pathway (Anabolism): Anabolism is the opposite of catabolism. Anabolic pathways build up complex molecules from simpler precursors (e.g., combining $CO_2$ and $H_2O$ to synthesize glucose during photosynthesis). Anabolic pathways strictly require an input of chemical energy to proceed to completion.

• Major Stages of Cellular Respiration:

  1. Stage 1: Glycolysis

  2. Stage 2: Krebs Cycle (Citric Acid Cycle)

  3. Stage 3: Electron Transport Chain and Chemiosmosis

Structural Foundations of Energy Carriers: ATP, NADH, and FADH2

• Adenosine Triphosphate ($ATP$):

  • Definition: $ATP$ is the organic molecule serving as the universal energy currency across all cell types.

  • Structural Composition:

  • Triphosphate Group: Three phosphate groups containing high-energy chemical bonds between the outer phosphate groups.

  • Ribose Sugar: A five-carbon pentose sugar backbone.

  • Adenine Base: A nitrogenous purine base attached to the ribose sugar (Adenine + Ribose = Adenosine).

  • Mechanism of Energy Release:

  • Energy is harvested from $ATP$ by breaking the high-energy bond situated between the last two (terminal) phosphate groups.

  • This hydrolysis reaction is catalyzed by the enzyme ATPase.

  • Cleavage yields Adenosine Diphosphate ($ADP$), a free inorganic phosphate group, and usable chemical energy.

  • The $ADP$ - $ATP$ Cycle:

  • $ATP$ can be regenerated from $ADP$ and a phosphate group to store energy for subsequent use.

  • Rephosphorylation is catalyzed by the enzyme ATP synthetase.

  • ATP synthetase attaches an inorganic phosphate group back onto $ADP$ to reform $ATP$.

• Electron and Proton Carrier Molecules:

  • Nicotinamide Adenine Dinucleotide ($NAD^+$ / $NADH$):

  • $NAD^+$ functions as an electron acceptor that traps high-energy electrons and hydrogen protons ($H^+$) removed from glucose during catabolism.

  • Reduction Equation: $NAD^+ + 2e^- + H^+ \rightarrow NADH$

  • $NADH$ represents the reduced, energy-stored form; it releases hydrogen and electrons to convert back into oxidized $NAD^+$.

  • Flavin Adenine Dinucleotide ($FAD$ / $FADH_2$):

  • $FAD$ acts as a secondary electron acceptor during metabolic oxidation reactions.

  • Reduction Equation: $FAD + 2e^- + 2H^+ \rightarrow FADH_2$

  • $FADH_2$ represents the reduced energy-stored form; releasing its hydrogen converts it back into oxidized $FAD$.

Cellular Compartmentalization and Mitochondrial Anatomy

• Intracellular Locations of Respiration Stages:

  • Glycolysis: Occurs in the cytoplasm (cytosol) of the cell.

  • Krebs Cycle: Occurs in the mitochondrial matrix.

  • Electron Transport Chain: Occurs on the cristae of the inner mitochondrial membrane.

• Structural Anatomy of the Mitochondrion:

  • Mitochondria are designated as the powerhouse of the cell due to their specialized ability to generate large volumes of $ATP$.

  • Outer Membrane: The smooth, outermost lipid bilayer encapsulating the organelle.

  • Inner Membrane: The continuous internal membrane situated beneath the outer membrane.

  • Cristae: Structural folds formed by invaginations of the inner mitochondrial membrane, providing expanded surface area for electron transport complexes.

  • Matrix: The internal fluid compartment enclosed entirely within the cristae and inner membrane.

Stage 1: Glycolysis

• Key Characteristics:

  • Etymological Meaning: Glycolysis translates to the splitting of sugar (glucose).

  • Location: Cytosol / Cytoplasm.

  • Oxygen Requirement: Anaerobic process; proceeds regardless of whether oxygen ($O_2$) is present or absent.

• Step-by-Step Enzymatic Sequence:

  1. Phosphorylation of Glucose: The initial six-carbon compound, glucose ($6 C$), consumes $1$ molecule of $ATP$. The phosphate group released by ATPase activity binds to glucose, converting it into glucose-6-phosphate ($6 C$).

  2. Isomerization: Glucose-6-phosphate ($6 C$) undergoes structural rearrangement into its chemical isomer, fructose-6-phosphate ($6 C$).

  3. Second Phosphorylation and Cleavage: Fructose-6-phosphate ($6 C$) consumes a second $1$ molecule of $ATP$ to accept another phosphate group. The resulting intermediate splits symmetrically into two three-carbon molecules called glyceraldehyde-3-phosphate ($3 C$), also known as 3-phosphate glyceraldehyde.

  4. Oxidation and Generation of $NADH$: Each of the two glyceraldehyde-3-phosphate ($3 C$) molecules is converted into 1,3-bisphosphoglycerate ($3 C$) (or 1,3-biphosphoglycerate). This step reduces $2$ molecules of $NAD^+$ into $2$ molecules of $NADH$.

  5. First $ATP$ Generation: The two 1,3-bisphosphoglycerate ($3 C$) molecules are converted into two 3-phosphoglycerate ($3 C$) molecules. Each molecule transfers a phosphate group to $ADP$, generating a total of $2$ molecules of $ATP$.

  6. Dehydration Step: The two 3-phosphoglycerate ($3 C$) molecules lose water ($H_2O$) molecules, converting them into two phosphoenolpyruvate ($3 C$) molecules.

  7. Second $ATP$ Generation and Final Product: Each phosphoenolpyruvate ($3 C$) releases its remaining phosphate group to $ADP$, producing another $2$ molecules of $ATP$. The resulting end products of glycolysis are two molecules of pyruvate ($3 C$).

• Summary of Glycolysis Yields:

  • Energy Input: $2$ molecules of $ATP$ consumed.

  • Total Energy Output: $4$ molecules of $ATP$ produced and $2$ molecules of $NADH$ generated.

  • Net Yield per Glucose Molecule: $2$ molecules of $ATP$ (calculated as $4$ produced minus $2$ consumed), $2$ molecules of $NADH$, and $2$ molecules of pyruvate ($3 C$).

Intermediate Stage: Pyruvate Oxidation (Formation of Acetyl-CoA)

• Transition Mechanism:

  • Pyruvate ($3 C$) produced in the cytoplasm during glycolysis must be transformed before entering the mitochondrial matrix for the Krebs cycle.

• Sequence of Steps:

  1. Decarboxylation: Pyruvate ($3 C$) releases one carbon atom in the form of carbon dioxide ($CO_2$), which is expelled into the atmosphere as a waste byproduct.

  2. Oxidation: The remaining two-carbon molecule forms an acetyl group ($2 C$). During this oxidation, $1$ molecule of $NAD^+$ is reduced to $1$ molecule of $NADH$ per pyruvate.

  3. Conjugation: The acetyl group ($2 C$) combines with Coenzyme A ($CoA$) to synthesize acetyl-CoA (acetyl coenzyme A).

• Net Output per Glucose Molecule (for $2$ Pyruvate inputs):

  • $2$ molecules of carbon dioxide ($CO_2$) released.

  • $2$ molecules of $NADH$ produced.

  • $2$ molecules of acetyl-CoA ($2 C$) formed.

Stage 2: The Krebs Cycle (Citric Acid Cycle)

• Overview and Characteristics:

  • Synonyms: Krebs Cycle or Citric Acid Cycle.

  • Location: Mitochondrial matrix.

  • Metabolic Nature: A cyclical series of oxidation reactions.

  • Turn Multiplicity: The cycle turns exactly twice per glucose molecule because glycolysis splits one glucose molecule into two pyruvates, which yield two acetyl-CoA inputs.

• Detailed Eight-Step Enzymatic Sequence:

  1. Step 1 (Formation of Citric Acid): Oxaloacetic acid (oxaloacetate, $4 C$) combines with the two-carbon fragment of acetyl-CoA ($2 C$). Coenzyme A ($CoA$) is released back into the matrix. This reaction forms a six-carbon molecule called citric acid (citrate, $6 C$).

  2. Step 2 (Dehydration/Isomerization): Citric acid ($6 C$) is converted into its structural isomer, isocitric acid (isocitrate, $6 C$).

  3. Step 3 (First Decarboxylation): Isocitric acid ($6 C$) undergoes oxidation and decarboxylation. It releases $1$ molecule of carbon dioxide ($CO_2$) and reduces $1$ molecule of $NAD^+$ to $1$ molecule of $NADH$, forming alpha-ketoglutaric acid (alpha-ketoglutarate, $5 C$).

  4. Step 4 (Second Oxidative Decarboxylation): Alpha-ketoglutaric acid ($5 C$) is oxidized, releasing a second molecule of carbon dioxide ($CO_2$) and reducing another $1$ molecule of $NAD^+$ to $1$ molecule of $NADH$. The remaining four-carbon intermediate binds to Coenzyme A ($CoA$) to form succinyl-CoA ($4 C$).

  5. Step 5 (Substrate-Level Phosphorylation): Succinyl-CoA ($4 C$) releases Coenzyme A ($CoA$), converting into succinic acid (succinate, $4 C$). The released energy converts $1$ molecule of $ADP$ (or GTP in certain cell types) into $1$ molecule of ATP$.\n 6. Step 6 (Oxidation): Succinic acid (4 C$) is oxidized into fumaric acid (fumarate,$4 C$). This reaction reduces$1$molecule of$FAD$to$1$molecule of$FADH_2$.

  6. Step 7 (Hydration): A molecule of water ($H_2O$) is added to fumaric acid ($4 C$), converting it into malic acid (malate, $4 C$).

  7. Step 8 (Dehydrogenation): Malic acid ($4 C$) is oxidized back into oxaloacetic acid ($4 C$). This reaction reduces a third $1$ molecule of $NAD^+$ to $1$ molecule of $NADH$, completing the cycle and regenerating the starting substrate.

• Summary of Krebs Cycle Yields:

  • Per Single Turn (1 Acetyl-CoA):

  • $2$ molecules of $CO_2$

  • $3$ molecules of $NADH$

  • $1$ molecule of $FADH_2$

  • $1$ molecule of $ATP$

  • Per Glucose Molecule (2 Turns / 2 Acetyl-CoA):

  • $4$ molecules of $CO_2$

  • $6$ molecules of $NADH$

  • $2$ molecules of $FADH_2$

  • $2$ molecules of $ATP$

Concept Checks and Interactive Questions

• Expansion of Biological Acronyms:

  • Question: What does the acronym $ATP$ stand for?

  • Answer: Adenosine Triphosphate (Janica).

  • Question: What do the acronyms $NADH$ and $FADH_2$ stand for?

  • Answer: $NADH$ stands for Nicotinamide Adenine Dinucleotide (with hydrogen); $FADH_2$ stands for Flavin Adenine Dinucleotide (with hydrogen).

• Explanation of Cycle Stoichiometry:

  • Question: Why does the Krebs cycle turn twice for every single glucose molecule oxidized?

  • Answer: Glycolysis splits a six-carbon glucose molecule into two three-carbon pyruvate molecules. These two pyruvates are subsequently converted into two acetyl-CoA molecules. Because each turn of the Krebs cycle processes exactly one acetyl-CoA molecule, the cycle must complete two full rotations per original glucose molecule (Samantha).