Glycolysis, Cellular Respiration, Aerobic Respiration, Anaerobic Respiration

on Saturday, October 13, 2012

Glycolysis, part of cellular respiration, is a series of reactions that constitute the first phase of most carbohydrate catabolism, catabolism meaning the breaking down of larger molecules into smaller ones. The word glycolysis is derived from two Greek words and means the breakdown of something sweet. Glycolysis breaks down glucose and forms pyruvate with the production of two molecules of ATP. The pyruvate end product of glycolysis can be used in either anaerobic respiration if no oxygen is available or in aerobic respiration via the TCA cycle which yields much more usable energy for the cell.
The following general outline of glycolysis follows the organization of Audesirk & Audesirk.
A glucose molecule is energized by the addition of a high-energy phosphate from ATP, forming glucose-6-phosphate.
A rearrangement of the molecule forms fructose-6-phosphate.
Using the available energy of a second ATP molecule, a second phosphate is added to the fructose.
The fructose-1,6-biphosphate is split into two three-carbon molecules, each having one phosphate group attached. The dihydrooxacetone (DHAP) quickly rearranges to form another G3P molecule, so the net result is two G3P molecules.
In near-simultaneous reactions, each G3P molecule gains an inorganic phosphorous while contributing two electrons and a hydrogen ion to NAD+to form the energized carrier molecules NADH. The resulting molecules have two high-energy phosphates.

Two molecules of low energy ADP are elevated to ATP molecules by phosphates from the biphosphoglycerates.

This recovers the energy invested in the first step of the glycolysis. The remaining phosphorous is relocated to the center position.

The final phosphate is transferred to ADP to form ATP, and this step represents the net yield of 2 ATP for the glycolysis process as a whole.

Cellular Respiration

The term cellular respiration refers to the biochemical pathway by which cells release energy from the chemical bonds of food molecules and provide that energy for the essential processes of life. All living cells must carry out cellular respiration. It can beaerobic respiration in the presence of oxygen or anaerobic respirationProkaryotic cells carry out cellular respiration within the cytoplasm or on the inner surfaces of the cells. More emphasis here will be placed on eukaryotic cells where the mitochondriaare the site of most of the reactions. The energy currency of these cells is ATP, and one way to view the outcome of cellular respiration is as a production process for ATP.

The graphic below can serve as a reminder of some of the processes involved in cellular respiration.

These are active graphics: click anywhere.
Cellular respiration produces CO2 as a metabolic waste. This CO2 binds with water to form carbonic acid, helping to maintain the blood's pH. Since too much CO2would lower the blood's pH too much, the removal of the excess CO2 must be accomplished on an ongoing basis.

Aerobic Respiration

Aerobic respiration, or cell respiration in the presence of oxygen, uses the end product of glycolysis (pyruvate) in the TCA cycle to produce much more energy currency in the form of ATP than can be obtained from any anaerobic pathway. Aerobic respiration is characteristic of eukaryotic cells when they have sufficient oxygen and most of it takes place in the mitochondria.

Anaerobic Respiration

The first step in cellular respiration in all living cells is glycolysis, which can take place without the presence of molecular oxygen. If oxygen is present in the cell, then the cell can subsequently take advantage of aerobic respiration via the TCA cycle to produce much more usable energy in the form of ATP than any anaerobic pathway. Nevertheless, the anaerobic pathways are important and are the sole source of ATP for many anaerobic bacteria. Eukaryotic cells also resort to anaerobic pathways if their oxygen supply is low. For example, when muscle cells are working very hard and exhaust their oxygen supply, they utilize the anaerobic pathway to lactic acid to continue to provide ATP for cell function.
Glycolysis itself yields two ATP molecules, so it is the first step of anaerobic respiration. Pyruvate, the product of glycolysis, can be used in fermentation to produce ethanol and NAD+ or for the production of lactate and NAD+. The production of NAD+ is crucial because glycolysis requires it and would cease when its supply was exhausted, resulting in cell death. A general sketch of the anaerobic steps is shown below. It follows Karp's organization.
Anaerobic respiration (both glycolysis and fermentation) takes place in the fluid portion of the cytoplasm whereas the bulk of the energy yield of aerobic respiration takes place in the mitochondria. Anaerobic respiration leaves a lot of energy in the ethanol or lactate molecules that the cell cannot use and must excrete.

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