You have read that nearly all of the energy used by living things comes to them in the bonds of the sugar, glucose. Glycolysis is the first step in the breakdown of glucose to extract energy for cell metabolism. Many living organisms carry out glycolysis as part of their metabolism. Glycolysis takes place in the cytoplasm of most prokaryotic and all eukaryotic cells.
Glycolysis begins with a molecule of glucose (C6H12O6). Various enzymes are used to break glucose down into two molecules of pyruvate (C3H4O3, basically a glucose molecule broken in half). This process releases a small amount of energy.
Glycolysis consists of two distinct phases. In the first part of the glycolysis pathway, energy is used to make adjustments so that the six-carbon sugar molecule can be split evenly into two three-carbon pyruvate molecules. In the second part of glycolysis, ATP and nicotinamide-adenine dinucleotide (NADH) are produced (Figure 67.1).
If the cell cannot catabolize (break down) the pyruvate molecules further, it will harvest only two ATP molecules from one molecule of glucose. For example, mature mammalian red blood cells are only capable of glycolysis, which is their sole source of ATP. If glycolysis is interrupted, these cells would eventually die.
Figure 67.1 Image Description
The image is a diagram of the glycolysis pathway, which is the series of chemical reactions that break down one molecule of glucose into two molecules of pyruvate while generating small amounts of energy for the cell.
At the top is the starting molecule glucose, represented by 6 balls containing the letter C (representing carbon atoms) connected together into a circle. An arrow below the glucose molecule points downward towards the text Fructose diphosphate. A curved arrow intersects this arrow. The arrow is labeled 2 ATP at the beginning and 2 ADP at the end, showing that 2 molecules of ATP are required to provide the energy for this process. Two arrows point downward from the label Fructose diphosphate towards 2 molecules of Glyceraldehyde 3-phosphate (represented as 3 balls containing the letter C attached to one blue ball labeled Pi). Arrows point downward from both of these molecules towards 2 molecules of pyruvate (represented by 3 balls containing the letter C). Two curved arrows intersect each downward arrow. The first curved arrow is labeled NAD+ at the beginning and NADH at the end. The second curved arrow is labeled 2 ADP at the beginning and 2 ATP at the end.
References
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Text adapted from: OpenStax, Concepts of Biology. OpenStax CNX. May 18, 2016. http://cnx.org/contents/b3c1e1d2-839c-42b0-a314-e119a8aafbdd@9.10
