Intro to Glycolysis: Videos & Practice Problems

Glycolysis is a metabolic pathway that occurs in the cytoplasm of cells, where glucose, a six-carbon sugar, is broken down into two molecules of pyruvate, a three-carbon compound. This process is crucial because it is the first step in cellular respiration, providing the necessary substrates for further energy production. During glycolysis, a net gain of 2 ATP molecules and 2 NADH molecules is achieved, which are essential for energy transfer within the cell. The pyruvate produced can then enter the mitochondria to be further oxidized in the Krebs cycle, leading to the production of additional ATP, NADH, and FADH₂. Glycolysis is vital because it provides energy quickly and does not require oxygen, making it an anaerobic process that can occur in both aerobic and anaerobic conditions.

Glycolysis contributes to the production of ATP and NADH through a series of ten enzymatic reactions that convert glucose into pyruvate. During these reactions, two molecules of ATP are consumed in the initial steps, but four molecules of ATP are produced in the later steps, resulting in a net gain of two ATP molecules per glucose molecule. Additionally, two molecules of NAD⁺ are reduced to NADH during glycolysis. The NADH produced is a high-energy electron carrier that can be used in the electron transport chain to generate more ATP through oxidative phosphorylation. Thus, glycolysis not only provides immediate ATP for cellular activities but also supplies NADH for further energy production in aerobic respiration.

Glycolysis consists of two main stages: the energy investment phase and the energy payoff phase. In the energy investment phase, two ATP molecules are used to phosphorylate glucose and its intermediates, which helps destabilize the glucose molecule and prepare it for cleavage. This phase results in the formation of two three-carbon molecules, glyceraldehyde-3-phosphate. In the energy payoff phase, these molecules undergo further reactions that lead to the production of four ATP molecules and two NADH molecules, resulting in a net gain of two ATP and two NADH per glucose molecule. The significance of these stages lies in their ability to convert glucose into pyruvate efficiently, providing energy and reducing power for the cell.

NAD⁺ (nicotinamide adenine dinucleotide) plays a crucial role in glycolysis as an electron carrier. During the glycolytic pathway, specifically in the sixth step, NAD⁺ is reduced to NADH when it accepts electrons from glyceraldehyde-3-phosphate. This reduction is essential because it facilitates the oxidation of glucose, allowing the conversion of glyceraldehyde-3-phosphate into 1,3-bisphosphoglycerate. The NADH produced is a high-energy molecule that can later donate electrons to the electron transport chain in the mitochondria, contributing to the production of additional ATP through oxidative phosphorylation. Thus, NAD⁺ is vital for maintaining the flow of electrons and energy production in cells.

After glycolysis, pyruvate can be utilized in several ways depending on the availability of oxygen. In the presence of oxygen, pyruvate is transported into the mitochondria, where it is converted into Acetyl CoA by the enzyme pyruvate dehydrogenase. Acetyl CoA then enters the Krebs cycle (also known as the citric acid cycle), where it is further oxidized to produce ATP, NADH, and FADH₂. These high-energy molecules are crucial for the electron transport chain and oxidative phosphorylation, leading to the production of a significant amount of ATP. In the absence of oxygen, pyruvate can undergo fermentation to produce lactate in animals or ethanol in yeast, allowing glycolysis to continue by regenerating NAD⁺.