Questions: Glucose CYTOSOL Glycolysis Pyruvate No O2 present: Fermentation Ethanol, lactate, or other products O2 present: Aerobic cellular respiration MITOCHONDR Acetyl CoA CITRIC ACID CYCLE

The text provided outlines the metabolic pathways of glucose under different conditions. Here's a breakdown of the process:

1. Glycolysis in the Cytosol:

   • Glucose is broken down into pyruvate through a series of enzymatic reactions in the cytosol. This process is known as glycolysis and occurs in both the presence and absence of oxygen.

2. No $\mathrm{O}_{2}$ Present (Anaerobic Conditions):

   • Fermentation: In the absence of oxygen, cells undergo fermentation to regenerate NAD+ from NADH, allowing glycolysis to continue. This process results in the production of ethanol, lactate, or other products, depending on the organism. For example, yeast cells produce ethanol, while muscle cells produce lactate.

3. $\mathrm{O}_{2}$ Present (Aerobic Conditions):

   • Aerobic Cellular Respiration: When oxygen is present, pyruvate is transported into the mitochondria and converted into Acetyl CoA. This enters the Citric Acid Cycle (also known as the Krebs Cycle or TCA Cycle), where it is further oxidized to produce ATP, NADH, and FADH2. These high-energy electron carriers then participate in the electron transport chain to produce additional ATP through oxidative phosphorylation.

In summary, the presence or absence of oxygen determines the metabolic pathway that pyruvate will follow after glycolysis, leading to either fermentation or aerobic respiration.