Glycolysis- Definition, Equation, Enzymes, 10 Steps, Diagram


Glucose is the most abundant and versatile carbohydrate in nature. It serves as a major source of energy for many living organisms, from bacteria to humans. But how do cells extract energy from glucose? The answer is glycolysis, a series of biochemical reactions that break down glucose into smaller molecules, releasing energy in the process.

Glycolysis is derived from the Greek words glykys (sweet) and lysis (splitting). As the name suggests, glycolysis involves the splitting of a six-carbon glucose molecule into two three-carbon molecules called pyruvate. Along the way, some of the energy stored in glucose is transferred to other molecules, such as ATP (adenosine triphosphate) and NADH (nicotinamide adenine dinucleotide).

ATP is the universal energy currency of cells. It can be used to power various cellular processes, such as muscle contraction, nerve transmission, and biosynthesis. NADH is a coenzyme that acts as an electron carrier. It can donate its electrons to other molecules, such as oxygen, in a process called oxidative phosphorylation. This process generates more ATP and water.

Glycolysis is a central pathway for glucose catabolism because it connects glucose with other metabolic pathways. Depending on the availability of oxygen and the type of organism, pyruvate can undergo different fates after glycolysis. For instance, in aerobic organisms (those that use oxygen), pyruvate can enter the mitochondria and be oxidized further into carbon dioxide and water in a cycle called the citric acid cycle. This cycle produces more NADH and another coenzyme called FADH2 (flavin adenine dinucleotide). These coenzymes can then fuel oxidative phosphorylation and produce more ATP.

In anaerobic organisms (those that do not use oxygen), or under low-oxygen conditions, pyruvate can be reduced into other molecules, such as lactate or ethanol, in a process called fermentation. Fermentation allows glycolysis to continue by regenerating NAD+ (the oxidized form of NADH) from NADH. NAD+ is needed as a coenzyme for one of the steps of glycolysis. However, fermentation does not produce any more ATP or coenzymes.

Glycolysis occurs in the cytosol of cells, which is the fluid part of the cytoplasm. It does not require any membrane-bound organelles or specialized structures. It consists of ten enzyme-catalyzed reactions that can be divided into two phases: an energy-investing phase and an energy-harvesting phase. In the energy-investing phase, two molecules of ATP are used to activate glucose and convert it into fructose-1,6-bisphosphate. In the energy-harvesting phase, fructose-1,6-bisphosphate is cleaved into two molecules of glyceraldehyde-3-phosphate, which are then oxidized and phosphorylated to form two molecules of pyruvate. In this phase, four molecules of ATP and two molecules of NADH are produced.

The net result of glycolysis is:

Glucose + 2NAD+ + 2ADP + 2Pi → 2Pyruvate + 2NADH + 2H+ + 2ATP + 2H2O

where Pi stands for inorganic phosphate.

Glycolysis is an ancient metabolic pathway that evolved long ago and is found in almost all living organisms. It is essential for generating energy from glucose and providing intermediates for other metabolic pathways. It also plays a role in regulating blood glucose levels and responding to hormonal signals.

In this article, we will explore the details of each step of glycolysis, the enzymes involved, the regulation mechanisms, and the clinical implications of glycolysis defects.