Cellular Energy Production

Homework for chpt 6&7 video 09/23/25

Cells obtain energy through the oxidation of various nutrients, with glucose being a primary example.
This oxidation process releases energy, which is subsequently used to synthesize crucial energy-carrying molecules:
- Adenosine~Triphosphate~(ATP): Often referred to as the "energy currency" of the cell, it directly powers many cellular processes.
- NADH: An electron carrier that plays a vital role in subsequent energy production stages.

Glycolysis is a foundational metabolic pathway that involves a series of steps to oxidize glucose into pyruvate.
This process can be broadly divided into two main phases: an energy investment phase and an energy harvesting phase.

ATP Investment: The initial stage of glycolysis requires an input of energy from ATP.
Phosphorylation of Glucose: Two phosphate groups originating from two separate ATP molecules are transferred to a single glucose molecule.
- This results in the formation of a six~carbon~sugar~diphosphate molecule.
- Concurrently, the two donor ATP molecules are dephosphorylated, yielding two low~energy~adenosine~diphosphate~(ADP) molecules.
Molecular Cleavage: The six~carbon~sugar~diphosphate molecule is then enzymatically split into two separate three~carbon~molecules.

Conversion to Pyruvate: Each of the two three~carbon~molecules formed in the investment phase undergoes a series of reactions, ultimately being converted into pyruvate.
Electron Transfer and NADH Production: During these conversion steps, electrons are liberated and transferred to the coenzyme NAD+ (nicotinamide adenine dinucleotide in its oxidized form).
- This reduction of NAD+ forms NADH (the reduced form), representing stored chemical energy in the form of high-energy electrons.
ATP Production: Simultaneously, ATP is generated directly in these steps through substrate-level phosphorylation.

The pyruvate molecules generated at the end of glycolysis have different fates depending on the availability of oxygen within the cell's environment:
- Under Aerobic Conditions (Presence of Oxygen):
  - Pyruvate is further oxidized. This typically involves its entry into the mitochondria for the citric acid cycle and oxidative phosphorylation, leading to the production of a significantly larger amount of ATP.
- Under Anaerobic Conditions (Absence of Oxygen):
  - Pyruvate undergoes fermentation and is converted into lactic~acid and/or other byproducts. This process, while not producing additional ATP, regenerates NAD+ from NADH, allowing glycolysis to continue and produce a limited amount of ATP.