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The first stage of cellular respiration, occurring in the cytoplasm of all living cells.
Every single cell in your body—and almost every living thing on Earth—shares a billion-year-old secret: a way to extract energy from sugar without needing a single breath of oxygen. How did life survive before the atmosphere even had oxygen?
Glycolysis (from the Greek glykys 'sweet' and lysis 'splitting') is the first stage of cellular respiration. It occurs in the cytoplasm of the cell, not the mitochondria. This is a critical distinction: because it happens in the cytosol and does not require oxygen, it is considered an anaerobic process. This pathway is the most ancient metabolic route, likely used by the first prokaryotes billions of years ago before the Earth's atmosphere contained oxygen. The goal is simple: take one 6-carbon molecule of glucose () and break it down into two 3-carbon molecules called pyruvate ().
Quick Check
Why can glycolysis occur in both prokaryotic and eukaryotic cells, regardless of whether oxygen is present?
Answer
Because it occurs in the cytoplasm and does not require oxygen or specialized organelles like mitochondria.
To make money, you often have to spend money. Glycolysis begins with the Energy-Investment Phase. Even though the goal is to produce energy, the cell must first 'prime' the glucose molecule to make it reactive. During this phase, the cell actually consumes 2 molecules of ATP. These ATP molecules donate phosphate groups to the glucose, transforming it into a highly unstable intermediate called fructose-1,6-bisphosphate. This instability is key—it allows the 6-carbon sugar to be easily split into two 3-carbon molecules known as G3P (glyceraldehyde 3-phosphate).
Let's track the carbon atoms to ensure we haven't lost anything. 1. Start with 1 Glucose molecule ( carbons). 2. Add 2 phosphates from 2 ATP ( carbons + phosphates). 3. Split the molecule into 2 G3P molecules ( carbons each). Total carbons: . The carbon count remains balanced.
Now the cell reaps the rewards. In the Energy-Payoff Phase, the two G3P molecules undergo a series of reactions to become pyruvate. This process involves two major energy captures. First, electrons are transferred to the electron carrier , reducing it to NADH. Second, the molecules donate phosphate groups directly to ADP to form ATP through a process called substrate-level phosphorylation. Because there are two G3P molecules moving through this phase, the total production is 4 ATP and 2 NADH.
Quick Check
If the payoff phase produces 4 ATP, why do we say the 'net' yield is only 2 ATP?
Answer
Because 2 ATP were consumed during the investment phase; .
Glycolysis is a 'metabolic fossil.' It is the most widespread metabolic pathway among Earth's organisms. Its location in the cytoplasm suggests it evolved before endosymbiosis (the origin of mitochondria). Furthermore, its ability to function without oxygen () was essential for life on the early Earth, where the atmosphere was primarily composed of volcanic gases. Today, it remains the first step for both aerobic respiration and fermentation, acting as the universal starting point for energy extraction.
Calculate the total energy yield from 5 molecules of glucose entering glycolysis. 1. Total ATP Invested: 2. Total ATP Produced: 3. Total NADH Produced: 4. Total Pyruvate Produced: Net ATP Yield: .
Where in the cell does glycolysis take place?
What is the net gain of ATP from one molecule of glucose during glycolysis?
Glycolysis requires oxygen to produce pyruvate.
Review Tomorrow
In 24 hours, try to sketch the 'Investment vs. Payoff' chart from memory, specifically noting the numbers of ATP and NADH.
Practice Activity
Research what happens to pyruvate after glycolysis if oxygen is NOT present (Hint: Look up lactic acid fermentation).