Breaking Down Energy: A Guide to Glycolysis

Every time you sprint for a bus, think through a tough problem, or even just breathe, your cells are working hard to keep you powered with cellular energy. The fuel behind all of it? A molecule called ATP. And the first step in making it, no matter if you are a bacteria or a human, is a process called glycolysis.

Glycolysis is where cellular respiration begins. It happens right in the cytosol, the fluid that fills your cells, and it doesn't need oxygen to run. That makes it one of the most fundamental and ancient energy-producing pathways in all of life versions of glycolysis operate in nearly every living organism on Earth.

The process starts with a single glucose molecule, a 6-carbon sugar. Glucose is broken down through a series of molecular reactions into two 3-carbon molecules called pyruvate. During this process 2 NAD+ are reduced to NADH and a net 2 ATP (the molecule of cellular energy) are produced.

Glycolysis runs in two distinct phases. First is the energy investment phase, where the cell spends 2 ATP to perform the first set of chemical reactions. Second is the energy payoff phase, where 4 ATP and 2 NADH are produced. The net gain of cellular energy in this process is 2 ATP per glucose. The 2 NADH are filled up electron carriers that can be used to produce ATP in later stages of respiration in the mitochondria if oxygen is present.

Under aerobic conditions (when oxygen is available), the pyruvate produced by glycolysis gets transported into the mitochondria, where cellular respiration continues through the link reaction, citric acid cycle, and oxidative phosphorylation.

Glycolysis may only produce a small amount of ATP directly, but it's the essential gateway to everything that follows and some prokaryotes rely on glycolysis as their only form of ATP production. Even in the presence of oxygen, glycolysis is needed for cellular respiration to being, and without cellular respiration, life as we know it doesn't run.