By the end of this section, you will be able to:
- Describe the overall result in terms of molecules produced in the breakdown of glucose by glycolysis
- Compare the output of glycolysis in terms of ATP molecules and NADH molecules produced
You have read that nearly all of the energy used by living cells comes to them in the bonds of the sugar, glucose. Glycolysis is the first stage of cellular respiration, and as the name implys (glyco - sugar, lysis - breaking) consists of a series of reactions that break glucose down into simpler molecules releasing energy in the process. Nearly all living organisms carry out glycolysis as part of their metabolism. The process does not use oxygen and is therefore anaerobic. Glycolysis takes place in the cytosol of both prokaryotic and eukaryotic cells and actually a smaller metabolic pathway comprised of ten different reactions catalyzed by as many enzymes.
Glycolysis begins with the six-carbon, ring-shaped structure of a single glucose molecule and ends with two molecules of a three-carbon sugar called pyruvate. Glycolysis consists of two distinct phases. In the first part of the glycolysis pathway, two molecules of ATP are 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, four molecules of ATP and 2 molecules of nicotinamide-adenine dinucleotide (NADH) are produced (Figure 4.13).
If the cell cannot catabolize 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 4.13 In glycolysis, a glucose molecule is converted into two pyruvate molecules. Two ATP are used to get the process going, and four ATP and two NADH are generated in the later steps of the pathway. So, glycolysis yields a net of two ATP, two NADH, and two molecules of pyruvate.
Link to Learning
Gain a better understanding of the breakdown of glucose by glycolysis by visiting this site to see the process in action.
Outcomes of Glycolysis
Glycolysis starts with glucose and ends with two pyruvate molecules, a total of four ATP molecules and two molecules of NADH. Two ATP molecules were used in the first half of the pathway to prepare the six-carbon ring for cleavage, so the cell has a net gain of two ATP molecules and 2 NADH molecules for its use. If the cell cannot catabolize the pyruvate molecules further, it will harvest only two ATP molecules from one molecule of glucose. Mature mammalian red blood cells are not capable of aerobic respiration—the process in which organisms convert energy in the presence of oxygen—and glycolysis is their sole source of ATP. If glycolysis is interrupted, these cells lose their ability to maintain their sodium-potassium pumps, and eventually, they die.
The last step in glycolysis will not occur if the enzyme that catalyzes the formation of pyruvate, is not available in sufficient quantities. In this situation, the entire glycolysis pathway will proceed, but only two ATP molecules will be made in the second half.
Glycolysis is the first pathway used in the breakdown of glucose to extract energy. It occurs in the cytosol of nearly all of the organisms on earth and is anaerobic. Glycolysis consists of two parts: The first part prepares the six-carbon ring of glucose for cleavage into two three-carbon sugars. ATP is invested in the process during this half to energize the separation. The second half of glycolysis extracts ATP and high-energy electrons from hydrogen atoms and attaches them to NAD+. Two ATP molecules are invested in the first half and four ATP molecules are formed by substrate phosphorylation during the second half. This produces a net gain of two ATP and two NADH molecules for the cell.
Which of the following statements about glycolysis is FALSE?
A. It occurs in the cytosol.
B. It is anaerobic.
C. It produces a net total (or profit) of 4 ATP.
D. It produces two molecules of pyruvate
E. It produces 2 NADH.
Nearly all organisms on earth carry out some form of glycolysis. How does that fact support or not support the assertion that glycolysis is one of the oldest metabolic pathways?
If glycolysis evolved relatively late, it likely would not be as universal in organisms as it is. It probably evolved in very primitive organisms and persisted, with the addition of other pathways of carbohydrate metabolism that evolved later.
Red blood cells do not perform aerobic respiration, but they do perform glycolysis. Why do all cells need an energy source, and what would happen if glycolysis were blocked in a red blood cell?
All cells must consume energy to carry out basic functions, such as pumping ions across membranes. A red blood cell would lose its membrane potential if glycolysis were blocked, and it would eventually die.