a) Glycolysis. b) Accepting electrons at the end of the electron transport chain. c) The citric acid cycle. d) The oxidation of pyruvate to acetyl CoA. e) The phosphorylation of ADP to form ATP.
The correct answer is b) Accepting electrons at the end of the electron transport chain.
Aerobic cellular respiration depends on the presence of oxygen and involves three stages that occur. The stages of this type of respiration occur in the following order, glycolysis, Kreb’s cycle and then the electron transport chain.
The glycolysis is the splitting of the sugar and formation of pyruvate. In the process, a certain amount of energy is both used and produced. At the end of glycolysis, pyruvate is formed which then enters the second stage of aerobic respiration if oxygen is present.
This second stage is the Kreb’s cycle (tricarboxylic acid cycle) which occurs in the matrix of the mitochondrion. The pyruvate from glycolysis is converted to acetyl groups that combine with oxaloacetate.
Several intermediates are formed and carbon dioxide is released as carbons are stripped from the various sugar molecules. NADH and FADH are formed from the reduction of NAD+ and FAD+, and some ATP is also produced from ADP.
The cofactors and ATP enter the electron transport chain which occurs in the cristae of the mitochondrion. Electrons from NADH and FADH are pulled along a chain of molecules due to the presence of oxygen at the end of the chain.
Oxygen is very electronegative and thus acts directly to pull these electrons along. In the process, the electrons provide energy to establish a proton gradient which allows ATP to be generated in a phosphorylation reaction.
Aerobic cellular respiration
Cell respiration is a catabolic process in which glucose is broken down and in the process energy in the form of ATP is generated and carbon dioxide is released.
Cellular respiration that occurs in the presence of oxygen is called aerobic respiration. The general formula for this type of cell respiration is C6H12O6 (glucose) + 6O2 –> 6CO2 + 6H20.
Oxygen plays a role in the very last stage, the electron transport chain of aerobic cellular respiration. Before this stage can take place though, there are two other stages that have to occur, namely glycolysis and the Kreb’s cycle.
Stage 1: Glycolysis
This is the very first stage of respiration that takes place regardless of the presence of oxygen. In fact, glycolysis also occurs in anaerobic respiration where no oxygen is present.
The process occurs in the cytoplasm and is really the splitting of the sugar molecule. Many enzyme-catalyzed reactions occur and a certain amount of energy is used in the process. This energy is obtained by the oxidation of some molecules of ATP. In the process, ADP and a phosphate group are released.
The phosphate is actually added at some stage to the glucose molecule to make glucose-6-phosphate. This becomes rearranged and modified further to produce fructose and then 1,6-biphosphate.
This molecule is then split to form two molecules of glyceraldehyde 3-phosphate (G3P). At some stage, NAD+ is reduced to produce the energy carrying cofactor NADH.
A glycerate 1,3-biphosphate is formed from the G3P and a phosphate from this molecule is added to ADP in a reduction reaction to form ATP.
Water is also produced during the glycolysis reactions and a pyruvate is eventually formed at the end of glycolysis.
Stage 2: Kreb’s cycle
The Kreb’s cycle is also known as the tricarboxylic acid cycle and occurs in the matrix of the mitochondrion after the pyruvate formed in glycolysis is transported across the mitochondrial membrane.
Pyruvate is oxidized to form acetyl groups which join with oxaloacetate to form the six carbon molecule citrate, and carbon dioxide is released at this time. An NAD+ is also reduced to produce NADH.
During the reactions that occur in the Kreb’s cycle, the ADP is also reduced to ATP and the cofactor FAD+ is reduced to form FADH.
Several intermediate substances are formed during the cycle and more oxaloacetate is formed and recycled so that the process can begin again.
Stage 3: Electron transport chain
The final stage of aerobic respiration is the electron transport chain in which oxidative phosphorylation occurs. This is the stage of cellular respiration in which oxygen is directly involved and plays an important function.
The two cofactors of NADH and FADH that were formed in the Kreb’s cycle now become important for the electron transport chain. This is because these two molecules carry electrons that now enter the chain of molecules.
The electrons move from molecule to molecule along a chain that is found in the inner membrane, the cristae of the mitochondrion.
The electrons are pulled down the chain by the presence of an oxygen molecule at the end. Oxygen is a very electronegative molecule which means that it strongly attracts atoms and thus pulls electrons down the chain from one molecule to the next
Oxygen is, in fact, the final electron acceptor of the chain which accepts hydrogen to form water at the end of the electron transport chain.
Proton gradient and ATP formation
The energy from the electrons is used to pump protons through the membrane into the space between the outer and inner membrane. This establishes a proton gradient and as a result, protons tend to diffuse back across the membrane down their concentration gradient.
The protons though can only diffuse through special membrane proteins that are lined with the enzyme ATP synthase. As the protons move through these proteins they trigger the reaction in which ATP is generated from ADP and an inorganic phosphate.
For every one pair of electrons that the oxygen accepts, three ATP molecules are produced. A great deal of ATP is formed along the chain, much more than is produced during anaerobic respiration where no oxygen is present.
- PH Raven, RF Evert, SE Eichhorn (1987). Biology of Plants. Worth Publishers.
- Editors of Encyclopedia Britannica (2019). Glycolysis. Retrieved from Britannica.com.
- RL Dorit, WF Walker, RD Barnes (1991). Saunders College Publishing.
- Editors of Encyclopedia Britannica (2019). Cellular respiration. Retrieved from Britannica.com.
- Editors of Encyclopedia Britannica (2019). Tricarboxylic acid cycle. Retrieved from Britannica.com.