In humans, most carbohydrates (glycogen, starch, sucrose) get chopped up to form glucose. Once inside a cell, enzymes in the cytoplasm start to break down glucose and extract energy to form ATP. More ATP production happens in the mitochondria, but mitochondria can’t use glucose directly. Glycolysis must happen before mitochondria can take advantage of the energy in glucose.

Why this is important…Glycolysis is the first stage of the set of reactions referred to as cellular respiration. These are crucial for converting carbohydrates into energy that cells can use to do work.

Foundations

Almost all cells can extract energy from glucose using the process of glycolysis. Animals, plants, fungi and most bacteria have the enzymes in their cytoplasm that allow them to break down glucose this way.

During this process glucose (a 6-carbon molecule) is basically split in half, resulting in two 3-carbon molecules. This extracts enough energy to produce 2 ATP molecules.

Key terms

Glycolysis – series of chemical reactions that extracts energy from glucose by partially breaking it down (Greek: glyco = sugar or sweet, lysis = break)

Details

Glycolysis refers to a collection of 10 chemical reactions, each requiring a different enzyme (Figure 8-6). We often simplify chemical reactions to emphasize the main point, but some of the complexity is important to understand. For one thing, glycolysis is not a one-step process. If glucose were to be broken down all at once it would release too much energy – enough to cook us from the inside!

Stop & Think #6: Can you think of another process that takes a lot of steps to accomplish a small payoff? Use that to make an analogy for glycolysis.

To control the breakdown of glucose and extract energy in useful bits, the process is spread over several steps. The first three reactions add energy to glucose from 2 ATP. The last three steps extract energy from glucose (or what remains of it) and transfer it to 4 ATP. Overall, the cell gains 2 ATP from this process, often referred to as the net yield. Two NADH are also produced by extracting electrons from glucose and reducing 2 NAD⁺ molecules.

VIDEO 8c: Glycolysis

After watching this video, you should be able to:

describe how energy is extracted from glucose
describe where energy is stored after glycolysis
describe what happens to ATP and pyruvate next

Since glucose is only partially broken down, only a small amount of energy is extracted. After glycolysis, the cell has a small amount of energy in the form of ATP that it can use to do work. It also produced some heat as a byproduct. But there are also 2 organic molecules – pyruvates – which still store energy.

Glycolysis diagram. Shows ten steps to break down glucose into pyruvate, giving off energy into ATP and NADH along the way.

FIGURE 8-6. Glycolysis. Glucose is broken down over a series of 10 enzyme-controlled reactions. The first 3 reactions require energy input. The last 7 reactions release energy, which is captured in ATP or NADH. Notice that there are 2 of every molecule on the bottom row. (“Glycolysis” by J. Buegge is licensed under CC BY-SA 4.0)

What happens to pyruvate depends on the conditions within the cell (Figure 8-7). If the cell has enough oxygen (O₂) then it can pass the pyruvates to mitochondria, which have the right enzymes to extract more energy. If the cell does not have oxygen, then pyruvates are converted in fermentation reactions, which don’t extract any more energy.

Diagram depicting what happens after glycolysis with and without oxygen. With oxygen, pyruvate is sent to mitochondria. Without, it is sent to fermentation.

FIGURE 8-7. What happens after glycolysis? Summary of glycolysis is shown in green. The pyruvates are either transferred into mitochondria to perform more reactions and produce more ATP, or undergo fermentation and produce no more ATP. (“What happens after glycolysis?” by J. Buegge is licensed under CC BY-SA 4.0)

Learning objectives

Describe how glycolysis extracts energy from glucose.
Describe the reactants and products of glycolysis, including energy-carrying molecules.
Describe what further chemical reactions can be used to extract energy after glycolysis.

Stop & Think Questions Explained

6. Any example that is familiar to you will work for this. If you are stuck, here are a couple ideas. Making a meal from scratch takes a lot of small steps before the final payoff, and it doesn’t do much good if it is only half done. Taking a walk or jog very literally takes many steps, which can add up to a long journey taken.