Biology
Chapter 6: Cellular Respiration - Glycolysis
Lecture
Glycolysis Processes
Web Lecture
Cell Respiration
Cells use controlled chemical reactions to free and use the energy stored in glucose molecules. This process is called cellular respiration. When it involves oxygen, the process is called aerobic (air-involving). Under some circumstances, anaerobic respiration or non-oxygen burning respiration may be used by cells to produce energy, but the process is far less efficient that aerobic respiration, so the latter is the dominant process by which cells release energy stored in their carbohydrates to pursue their various functions.
Oxidation-reduction Reactions
Either way, the reactions involved are called oxidation-reduction reactions, or redox reactions (chemists love shorthand). Redox reactions occur when the rearrangement of compounds in the reactants results in a transfer of electrons in the products. Electrons carry with them some amount of energy, so electron transfer always implies an energy transfer between the atoms involved in the reaction.
This sounds really odd: why is the H2 reduced when it is gaining electrons? These terms come from the nineteenth century chemists who first realized that oxygen is a very reactive element. Reactive here means that if you put oxygen near any other element, it tends to undergo a chemical reaction and form a bond with the atoms of the other element. Such oxygen reactions have a special name, combustion, and the result is a new "oxidized" compound, one that has oxygen added to it. In the diagram above, the second carbon already had one bond with oxygen, but now it has two, so it has been "oxidized".
These chemists also realized that substances released by combustion tended not to react with other substances much. The two hydrogens in the original molecule can be pulled off the molecule and will react with other substances. When they have formed an H2 molecule as in this reaction, though, they stop reacting. Their level of reactivity is reduced.
Since the time that these chemists did their work, we've come to realize that there are a whole class of reactions in which electrons are transfered from one atom to another, and that such reactions always involves the transfer of energy as well. In biology, these reactions are important because they involve enzymes to promote or control the rate at which the reaction takes place.
Glycolysis
In aerobic cellular respiration, glucose is converted to water and carbon dioxide. The "net" or overall reaction has the reaction formula:
C6H12O6 + 6O2 + 12H2O --> 6CO2 + 6H2O + ENERGY
The glucose molecule contains electrons which are transferred along with the hydrogen ions to water; energy is given off during the transfer in small amounts which the cell stores in ATP complexes. It is important now that you remember: ADP (adenosine DIphosphate) has 2 phosphate groups attached to the adenosine-ribose complex, and ATP (adenosine TRIphosphate) has 3 phosphate groups.
This is actually a reaction that is made up of many individual steps or smaller reactions. The aerobic respiration process involves four main phases: glycolysis, production of acetyl coenzyme A, the citric acid cycle, and the linked processes of electron transport and chemiosmosis. For this lecture, we'll concentration on glycolysis.
Glycolysis
The first phase of cellular respiration involves taking an existing sugar molecule, glucose, and breaking it down into pyruvate molecules that can be used in the second phase, the Krebs cycle. But we can't just move everything around at once. Each chemical bond has to be broken and reformed one step at a time, in the most efficient way. So we go through many steps. Some merely rearrange atoms within the molecule. Others actually take break the molecule into pieces. Some steps are exothermic, some endothermic. We need to put together a scorecard.
| Step | Starting molecule | Type of reaction | Enzyme | Energy change |
| 1 | Glucose | Rearrange | Hexokinase | Endergonic (energy from ATP used in phosphorylation)b |
| 2 | Glucose-6-phosphate | Rearrange | Phosphoglucoisomerase | Slightly exergonic reaction.b |
| 3 | Fructose-6-phosphate | Rearrange | Phosphofructoskinase | Endergonic (energy from ATP used in phosphorylation)b |
| 4 | Fructose-1,6-biphosphate | Split into dihydroxyacetone phosphate and glyceraldehyde-3-phosphate(PGAL). The dihydroxyacetone phosphate undergoes enzymatic conversion to PGAL.b | Aldolase | Slightly exergonic reaction.b |
| 5 | 2 PGAL | Rearrange | Glyceraldhyde-3-phosphate dehydrogenase | Exergonic. Two NAD+ foster dehydrogenation.b |
| 6 | 2 1,3 biphsophyglycerate | Rearrange | Phosphoglycerokinase | Exergonic. Two ADP undergo phosphorylation to form two ATP.b/td> |
| 7 | 2 3-phosphoglycerate | Rearrange | Phosphoglyceromultase | Slightly exergonic reaction |
| 8 | 2 2-phosphoglycerate | Rearrange | Enolase | Exergonic. A dehydration reaction releases two H2O molecules.b |
| 9 | 2 Phosphopyruvate | Rearrange | Pyruvate kinase | Exergonic. Two ADP undergo phosphorylation to form two ATP.b |
| LAST | 2 Pyruvate | Both molecules move into the Krebs Cycle | ||
No, you do NOT have to memorize the enzyme names for each step! But there are some things to notice about this part of the cellular respiration process.
- Most of the reactions are rearrangement reactions. Atoms change position within a single molecule. Sometimes this requires energy input, especially if it means breaking a doubly-bonded oxygen atom, but often the reaction in slightly exothermic. The molecule is shifting to a somewhat more stable configuration.
- Every step has its own unique enzyme. Later on, we'll see how genetic defects can create deformed enzymes and disrupt entire metabolic sequences. Consider: what would happen if an individual's enolase enzyme were malformed and could not facilitate step 8?
- Enzyme names end in -ase!
- A phosphorylation reaction is one in which a phosphate ion (a phosphorus atom and four oxygens, PO4) attaches to something. In step 1, a phosphate ion attaches to the glucose molecule. This phosphate ion comes from ATP, taking one of the three phosphate ions on that molecule away, and leaving it with only two, so that it becomes ADP.
- The net output of this series of steps are 2 ATP molecules, 2 NADH molecules, and 2 H2O molecules. That's not very much. The energy payoff for this process must come from the next phase.
Looking ahead at the rest of the process
The middle part of the cellular respiration process is a "cycle", the Krebs cycle, named after Hans Adolf Krebs, leader of the team that first identified the processes in the cycle. Krebs was a Jewish doctor barred from practicing medicine in Germany in 1933; he was able to emigrate to England where he worked at Cambridge (during which time he identified the citric acid cycled named for him) and eventually became a professor of biochemistry at the University of Sheffield.
It's important to realize in looking at diagrams of these "cycles" that we really are not talking about some activity that takes place in a nice, clockwise order. The cycle is our way of conceptualizing a series of chemical reactions that take place, one after the other, and leave as their final product a molecule (oxaloacetate) that can start the system of reactions over again. We'll look at it in more detail in the next lecture.
We'll look at the last part of the cellular respiration process, electron transport and chemiosmosis, in detail in the next weblecture -- so for now, just concentrate the idea that when electrons move from one molecule to another, they take on or give off energy. We've seen this already in redox reactions.
- Electron transport is the movement of high energy electrons from one membrane-embedded protein to another. At each step, the electron transfers energy to the protein and the protein uses the energy to pump H+ ions across the membrane and against the concentration gradient into an area of higher H+ concentration.
- This is followed by chemiosmosis, which is the movement of ions across a selectively-permeable membrane (this means that not all ions move across the membrane, but only the chosen few). The idea that ATP is synthesized as the energy harvested from "falling electrons" is used to drive active transport of ions across a membrane was proposed by Peter Mitchell in 1961, and the idea was not well-received at first. Mitchell's theory was finally accepted, and he eventually went on to win the 1978 Nobel prize in chemistry for his work.
The big picture:
>© 2005 - 2024 This course is offered through Scholars Online, a non-profit organization supporting classical Christian education through online courses. Permission to copy course content (lessons and labs) for personal study is granted to students currently or formerly enrolled in the course through Scholars Online. Reproduction for any other purpose, without the express written consent of the author, is prohibited.