1000055543

Cellular Respiration Overview

  • Three main steps of cellular respiration:

    • Glycolysis: First step in glucose breakdown.

    • Citric Acid Cycle (Krebs Cycle): Processes the acetyl group from pyruvate.

    • Oxidative Phosphorylation: Produces ATP using the electron transport chain and proton gradient.

Glycolysis

  • Definition: A series of reactions that extract energy from glucose, splitting it into two three-carbon molecules called pyruvates.

  • Importance: It is fundamental to both aerobic and anaerobic cellular respiration.

  • Glucose Role: A six-membered ring molecule typically derived from carbohydrate breakdown, serves as the main energy substrate.

  • Transport into Cells:

    • Glucose enters through specific transporter proteins located in the cell membrane.

    • Two methods of cellular uptake:

      • Secondary Active Transport: Against the glucose concentration gradient.

      • GLUT Proteins: Integral proteins that facilitate glucose transport.

Steps of Glycolysis

  1. Step 1: Hexokinase

    • Reaction: D-Glucose to Glucose-6-Phosphate.

    • Enzyme: Hexokinase (catalyzes conversion).

  2. Step 2: Phosphoglucose Isomerase

    • Reaction: Glucose-6-Phosphate to Fructose-6-Phosphate.

    • Enzyme: Phosphoglucose Isomerase.

  3. Step 3: Phosphofructokinase

    • Reaction: Fructose-6-Phosphate to Fructose-1,6-Bisphosphate.

    • Enzyme: Phosphofructokinase (with Mg2+ as cofactor).

  4. Step 4: Aldolase

    • Reaction: Fructose-1,6-Bisphosphate split into Dihydroxyacetone Phosphate (DHAP) and Glyceraldehyde 3-Phosphate (GAP).

    • Enzyme: Aldolase.

  5. Step 5: Triosephosphate Isomerase

    • Inter-converts DHAP and GAP.

    • Enzyme: Triosephosphate Isomerase.

  6. Step 6: Glyceraldehyde-3-Phosphate Dehydrogenase

    • Reaction: GAP converted to 1,3-Bisphosphoglycerate.

    • Enzyme: GAPDH.

  7. Step 7: Phosphoglycerate Kinase

    • Reaction: Transfers a phosphate from 1,3-Bisphosphoglycerate to ADP forming ATP and 3-Phosphoglycerate.

    • Enzyme: Phosphoglycerate Kinase.

  8. Step 8: Phosphoglycerate Mutase

    • Reaction: Rearrangement of 3-Phosphoglycerate to 2-Phosphoglycerate.

    • Enzyme: Phosphoglycerate Mutase.

  9. Step 9: Enolase

    • Reaction: Removal of water from 2-Phosphoglycerate to form Phosphoenolpyruvate (PEP).

    • Enzyme: Enolase.

  10. Step 10: Pyruvate Kinase

    • Reaction: Transfers a phosphate from PEP to ADP, resulting in Pyruvate and forming ATP.

    • Enzyme: Pyruvate Kinase.

Pyruvate Oxidation

  • Location: Occurs in the mitochondrial matrix post glycolysis.

  • Process: Pyruvate must cross the inner mitochondrial membrane prior to oxidation.

  • Steps of Pyruvate Oxidation:

    1. Removal of carboxyl group from Pyruvate releases CO2.

    2. The remaining 2-carbon compound is oxidized, reducing NAD+ to NADH.

    3. The oxidized 2-carbon acetyl group is combined with CoA to form Acetyl CoA.

Krebs Cycle (Citric Acid Cycle)

  • Initiation: Acetyl-CoA combines with Oxaloacetate to form Citrate.

  • Steps of the Krebs Cycle:

    1. Citrate Synthase: Links Acetyl-CoA with Oxaloacetate.

    2. Aconitase: Converts Citrate to Isocitrate.

    3. Isocitrate Dehydrogenase: Oxidizes Isocitrate to a-Ketoglutarate, producing NADH and releasing CO2.

    4. a-Ketoglutarate Dehydrogenase: Converts a-Ketoglutarate to Succinyl CoA, producing NADH and releasing CO2.

    5. Succinyl-CoA Synthetase: Converts Succinyl CoA to Succinate, producing ATP or GTP.

    6. Succinate Dehydrogenase: Converts Succinate to Fumarate, producing FADH2.

    7. Fumarase: Converts Fumarate to Malate.

    8. Malate Dehydrogenase: Converts Malate back to Oxaloacetate, producing NADH.

Oxidative Phosphorylation

  • Definition: The process resulting in ATP production through the electron transport chain (ETC).

  • Mechanism: Electrons pass through ETC, creating a proton gradient that powers ATP synthesis via ATP synthase.

  • Oxygen's Role: It is the final electron acceptor, combining with electrons and protons to form water, preventing backup in the ETC.

Overall ATP Yield from Glucose

  • Glycolysis produces 2 ATP and 2 NADH.

  • Krebs Cycle produces 2 ATP, 6 NADH, and 2 FADH2 per glucose.

  • Oxidative phosphorylation can yield about 28 ATP from NADH and FADH2.

  • Total ATP yield from one glucose molecule can be approximately 30-32 ATP.

Importance of Cellular Respiration

  • Central for energy production in aerobic organisms.

  • Disruptions can lead to mitochondrial diseases, impacting overall cellular metabolism.