Glycolysis is a metabolic pathway which breaks down glucose into two three-carbon compounds and generates energy.
Glycolysis is the first step in the breakdown of glucose to extract energy for cellular metabolism. Glycolysis is the first of the main metabolic pathways of cellular respiration to produce energy in the form of ATP. Through two distinct phases, the six-carbon ring of glucose is cleaved into two three-carbon sugars of pyruvate through a series of enzymatic reactions. The first phase of glycolysis requires energy, while the second phase completes the conversion to pyruvate and produces ATP and NADH for the cell to use for energy.
Glycolysis can be an aerobic or anaerobic reaction. It occurs in the cytosol of cells. In the presence of oxygen, pyruvate is further oxidized to CO2, and in the absence of oxygen, pyruvate can be fermented to lactate or ethanol. For every glucose molecule that undergoes glycolysis, there is a net production of 2 ATP molecules, 2 NADH molecules.
The enzymes involved in the glycolysis pathway are inhibited by ATP, reducing their activity when the cell has sufficient ATP to meet its energy requirements.
In addition to glucose, many other carbohydrates ultimately enter the pathway to undergo energy-yielding degradation. The most significant are the polysaccharides glycogen and starch.
Glycogen and starch enter the glycolytic pathway through the action of two enzymes: glycogen phosphorylase and amalyse. Glycogen phosphorylase breaks down glycogen into glucose. Starch digestion occurs in reactions catalyzed by enzymes called amylases. The enzymes hydrolyze starch breaking it down into glucose monomers.
Practice Questions
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Key Points
• The net reaction for glycolysis is: Glucose + 2NAD+ + 2Pi + 2ADP → 2 pyruvate + 2ATP + 2NADH + 2H2O
• Glycolysis occurs in the cytosol
• The following compounds inhibit glycolysis: glucagon, high ATP/low ADP
• Glycogen and starch are polysaccharides that feed into glycolysis by enzymes that break them down
Key Terms
starch: found in plants and digested to release individual glucose molecules in the digestive tract is absorbed and transported to tissues
glycogen: the main storage carbohydrate in animals is mobilized by the liver and muscle to release glucose when hormones indicate energy is required
glycogen phosphorylase: enzyme that breaks down glycogen
amalyse: enzyme that breaks down starch into glucose
glucose: a simple monosaccharide (sugar) with a molecular formula of C6H12O6; it is a principal source of energy for cellular metabolism
adenosine triphosphate (ATP): a multifunctional nucleoside triphosphate used in cells as a coenzyme, often called the “molecular unit of energy currency” in intracellular energy transfer
NADH: nicotinamide adenine dinucleotide (NAD) carrying two electrons and bonded with a hydrogen (H) ion; the reduced form of NAD
pyruvate: any salt or ester of pyruvic acid; the end product of glycolysis