Brief Summary
This video explains glycolysis, the process by which glucose is broken down into pyruvate. It covers the two phases of glycolysis: the preparatory phase and the oxidative phase. The video details each step of both phases, including the enzymes involved, the energy investment and generation, and the key molecules formed. It also highlights important concepts such as substrate-level phosphorylation and the only oxidation step in glycolysis.
- Glycolysis breaks down glucose into pyruvate.
- It occurs in two phases: preparatory and oxidative.
- The process involves several enzymatic steps, ATP investment, and NADH2 production.
Introduction to Glycolysis
Glycolysis is the enzymatic breakdown of a glucose molecule into two molecules of a three-carbon compound called pyruvic acid. This process occurs in the cytoplasm of the cell.
Two Phases of Glycolysis
Glycolysis consists of two main phases: the preparatory phase and the oxidative phase. The preparatory phase involves the initial steps where glucose is prepared for breakdown, while the oxidative phase involves the energy-yielding reactions.
Preparatory Phase: Steps 1-3
The first step of glycolysis is the phosphorylation of glucose at carbon number six, forming glucose-6-phosphate. This step requires the investment of one ATP molecule and is also known as the priming step, which activates the glucose molecule. Glucose-6-phosphate is then converted into its isomer, fructose-6-phosphate, through isomerization.
Preparatory Phase: Steps 4-5
Fructose-6-phosphate is further phosphorylated at both carbon one and six, resulting in fructose-1,6-bisphosphate, which requires another ATP molecule. Fructose-1,6-bisphosphate is then cleaved by the enzyme aldolase into two three-carbon molecules: glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate. These two molecules are isomers and can interconvert, but only G3P proceeds to the next phase.
Oxidative Phase: Step 6
In the oxidative phase, glyceraldehyde-3-phosphate undergoes oxidation, producing NADH2. During this step, inorganic phosphate is added to G3P, forming 1,3-bisphosphoglyceric acid. This is the only oxidation step in glycolysis and occurs outside the mitochondria.
Oxidative Phase: Steps 7-8
1,3-bisphosphoglyceric acid transfers a phosphate group to ADP, generating ATP and forming 3-phosphoglyceric acid. This is an example of substrate-level phosphorylation. The phosphate group on 3-phosphoglyceric acid then shifts from carbon three to carbon two, resulting in 2-phosphoglyceric acid.
Oxidative Phase: Steps 9-10
2-phosphoglyceric acid undergoes dehydration, catalyzed by enolase, to form phosphoenolpyruvic acid (PEP). This is the only dehydration step in glycolysis, creating a double bond between two carbon atoms. PEP then transfers its phosphate group to ADP, producing another ATP molecule and forming pyruvic acid (pyruvate). This is another instance of substrate-level phosphorylation.
Yield of Glycolysis
In the preparatory phase, two ATP molecules are used. In the oxidative phase, four ATP molecules are produced. Therefore, the net gain is two ATP molecules. Additionally, two NADH2 molecules are formed. The ATP production in glycolysis is an example of substrate-level phosphorylation, where ATP is directly generated from the energy of the substrate.
