The Big Picture
Each individual stage of cellular respiration is crucial in inducing the processes of the next. This page will detail each, connecting Glycolysis, Acetyl CoA formation, Krebs Cycle, and Oxidative Phosphorylation, to provide a "big picture" outlook on cellular respiration.
Glycolysis
This cycle is started by a glucose molecule, which has been consumed by the organism, entering the cell. In the cytoplasm, the cell will invest an ATP to cause phosphorylation of glucose. Therefore, a cell needs an energy molecule to even begin glycolysis. This ATP is achieved from previous aerobic or anaerobic respiration. Glycolysis also relies on two pre-existing NAD+, 2ADP, and 2Pi. Using these materials, 2 Pyruvate, 2NADH, 2H+ and 2ATP are produced. Both pyruvates will be sent to Acetyl CoA formation, while the 2NADH and 2H+ are sent to the electron transport chain, part of oxidative phosphorylation. The 2ATP will be used as needed by the cell. They account for 2/38 ATP produced in cellular respiration.
Acetyl CoA
Pyruvate moves into the mitochondria, where the inner mitochondrial membrane will oxidize each pyruvate, thus producing acetyl CoA. Since there are two pyruvate, two acetyl CoA are also produced. Each of these will enter and drive the Krebs cycle.
Krebs Cycle
Once acetyl CoA has been produced from pyruvate, Krebs cycle may begin in the matrix of the mitochondria. Krebs cycle runs once per acetyl CoA, so Krebs will run twice, producing 6NADH, 6H+, 2FADH2, and 2ATP. The 6NADH, 6H+, and 2FADH2 will be sent to the electron transport chain in oxidative phosphorylation. The 2 ATP produced will be used by the cell, together accounting with the 2ATP from glycolysis to have produced 4/38 ATP.
Oxidative Phosphorylation
Here, all materials sent to the electron transport chain will be used. This is ongoing throughout cellular respiration, and is also the final step that will produce 32-34 molecules of ATP. The electron transport chain will fuel the Krebs cycle with NAD+ and FAD before the final run through oxidative phosphorylation. In the electron transport chain, NADH transfers electrons to reductase, making NAD+ that will be stored for used in the Krebs cycle. In the same sense, FADH2 will oxidize to FAD before returning to the Krebs cycle. The electrons from NADH and FADH2 will be transferred and accepted by oxygen, the ultimate electron acceptor in aerobic respiration.
This is the first moment in which oxygen is required, however, since Krebs cycle relies on products of the electron transport chain, Krebs cycle will not be able to occur if oxygen is not present. Anaerobic respiration is mentioned below. As a pair of electrons from NADH or FADH2 is transferred through the membrane of the mitochondria, two to four protons are also transported into the mitochondria. After about three protons have entered, a force has been generated to release ATP from ATP synthase. Another proton will transport each ATP generated from the matrix to the cytoplasm.
Since four protons have the ability to release and transport one ATP, and the oxidation of NADH and FADH2 each make 10 and 6 protons respectively, one NADH will produce three ATP molecules, and one FADH2 will produce two ATP. This accounts for the last 32-34 ATP made in respiration, combining with glycolysis and Krebs cycle to produce 36-38 ATP for the cell.
This is the first moment in which oxygen is required, however, since Krebs cycle relies on products of the electron transport chain, Krebs cycle will not be able to occur if oxygen is not present. Anaerobic respiration is mentioned below. As a pair of electrons from NADH or FADH2 is transferred through the membrane of the mitochondria, two to four protons are also transported into the mitochondria. After about three protons have entered, a force has been generated to release ATP from ATP synthase. Another proton will transport each ATP generated from the matrix to the cytoplasm.
Since four protons have the ability to release and transport one ATP, and the oxidation of NADH and FADH2 each make 10 and 6 protons respectively, one NADH will produce three ATP molecules, and one FADH2 will produce two ATP. This accounts for the last 32-34 ATP made in respiration, combining with glycolysis and Krebs cycle to produce 36-38 ATP for the cell.
36-38 ATP produced
So why is each stage of cellular respiration important?
Each stage is crucial to respiration because without them, glycolysis could not occur to produce pyruvate, pyruvate could not be converted to acetyl CoA, the electron transport chain could not produce FAD and NAD+ to drive Krebs cycle without oxygen, acetyl CoA could not be oxidized in Krebs cycle to make even more NADH, H+ and FADH2, and ATP could not be produced to drive a cell's necessary processes.
Without oxygen, anaerobic respiration could occur, in with alcoholic or lactic acid fermentation. However, fermentation produces only 2ATP per glucose, as glycolysis is the only source of ATP in the cycle. This is inefficient and only ideal for quick bursts of energy in most organisms.
Without oxygen, anaerobic respiration could occur, in with alcoholic or lactic acid fermentation. However, fermentation produces only 2ATP per glucose, as glycolysis is the only source of ATP in the cycle. This is inefficient and only ideal for quick bursts of energy in most organisms.
How does cellular respiration affect the biosphere?
While the breaking down of glucose to ATP is crucial at the cellular or organism level, there is an even more important product to the biosphere.
As you may have noticed on the stage-specific pages of this site, oxygen is a main factor in energy-efficient cellular respiration.Without it, cells must perform anaerobic respiration, either alcoholic or lactic acid fermentation. We have previously mentioned why these are less efficient to the cell and organism, however, if all cells on our planet were forced to perform anaerobic fermentation the biosphere would suffer. Why? Lack of carbon dioxide production, one of the most important products of cellular respiration. While some organisms, such as yeast, will perform alcoholic fermentation in anaerobic conditions, this only outputs two carbon dioxides per glucose rather than aerobic respiration's four. Animals, who must go through lactic acid fermentation, would output no carbon dioxide at all. So why is carbon dioxide important? The answer is shown in the image to the left. Any organism that wishes to respire needs oxygen. After breaking down glucose, this organism will release four carbon dioxide. These carbon dioxide molecules will be used in photosynthesis, kind of like cellular respiration for plants, and after photosynthesis produces glucose, it will release six oxygen molecules. Without photosynthesis having carbon dioxide from cellular respiration to produce oxygen, cellular respiration would have no oxygen to produce the carbon dioxide to drive photosynthesis. This complex cycle drives all life in the biosphere. |