Cellular Respiration: Glycolysis and Pyruvate Oxidation

Cellular Respiration Overview

• Cellular respiration occurs in four stages:
  • Glycolysis
  • Pyruvate Oxidation
  • Krebs Cycle
  • Electron Transport Chain (ETC)

Glycolysis

• Glycolysis means "splitting of sugar".
• Glucose (a 6-carbon sugar) is split into two 3-carbon molecules, pyruvate.
• During glycolysis:
  • 4 ATP is produced, but 2 are used to start the reaction, resulting in a net gain of 2 ATP.
  • 2 NAD+ are reduced to NADH, which will be oxidized in the electron transport chain, producing more ATP.
  • This process occurs in the cytoplasm and does not require oxygen.

Learning Goals of Glycolysis

1. Understand the main goals of glycolysis.
2. Learn the various steps within the pathway (focus on substrates and products, reduction of NAD+, formation of ATP).
3. Describe pyruvate oxidation (outcomes of pyruvate).
4. Stay confident, you can do it!

Glycolysis Reactions

• Glycolysis consists of 10 reactions:
  • Each step is catalyzed by a different enzyme.
  • Initial reactions (endergonic) use 2 ATP per glucose.
  • Later reactions (exergonic) produce 4 ATP per glucose.

Steps of Glycolysis

• Step 1: Glucose is phosphorylated to form Glucose 6-phosphate. Uses 1 ATP.

• Step 2: Isomerization occurs to form Fructose 6-phosphate (enzyme: Phosphoglucose isomerase).

• Step 3: Fructose 6-phosphate is phosphorylated to produce Fructose 1,6-bisphosphate. Uses 1 ATP.

• Step 4: Cleaving of Fructose 1,6-bisphosphate into DHAP and G3P.

• Step 5: Isomerization of DHAP to G3P.

• Step 6: G3P conversion to 1,3-Bisphosphoglycerate, reducing NAD+ to NADH.

• Step 7: Conversion of 1,3-Bisphosphoglycerate to 3-Phosphoglycerate with substrate-level phosphorylation, producing 1 ATP.

• Steps 8 & 9: H2O is removed in step 9, transforming 2-Phosphoglycerate into Phosphoenolpyruvate (PEP).

• Step 10: PEP donates its Pi to ADP, forming ATP and yielding the final product, pyruvate.

Pyruvate Oxidation

• Part 2 of cellular respiration occurs in the mitochondrial matrix:
  1. 2 pyruvates are transported to the mitochondrial matrix.
  2. A carboxyl group is removed from pyruvate and released as CO2.
  3. The remaining 2-carbon fragment is oxidized.
  4. NAD+ is reduced to NADH.
  5. Coenzyme A attaches to the remaining acetyl group to form Acetyl CoA.

Mitochondrial Inner Structure

• Key components of mitochondria:
  • Cristae: foldings of the inner membrane that increase surface area.
  • Matrix: fluid within the mitochondria where the Krebs cycle takes place.

Summary

• Glycolysis is a critical metabolic pathway that converts glucose into pyruvate, generating ATP and reducing NAD+ in the process. Understanding each step and its associated enzymes is essential for grasping cellular respiration as a whole.