 |
The University of the West Indies Department of Biological and Chemical Sciences |
BL 05B - Preliminary Biology II |
|
The University of the West Indies Department of Biological and Chemical Sciences |
BL 05B - Preliminary Biology II |
CELLULAR RESPIRATION
Energy is released by the metabolic breakdown of organic compounds, mainly carbohydrates. Oxygen is normally required for this process while CO2 and H2O are released. Respiration occurs in all organisms and in all living cells, for the breakdown of organic compounds is the only way a cell can obtain enough usable energy. This applies to animal and plant cells, bacteria and fungi.
In living organisms the main substrate for respiration is glucose. This may come from starch (plants) or glycogen (animals). The storage polysaccharides are first hydrolysed to monosaccharides before they enter the respiratory pathway. Fats and proteins can also be used but protein is used only in extreme cases, e.g. starvation.
The group of reactions, collectively called
glycolysis coverts glucose, glucose-1-phosphate or fructose to pyruvate occurs in the cytoplasm of cells. These reactions do not require the presence of oxygen.Figure: Glycolysis
Outline of glycolysis:
1. Conversion of one 6-carbon molecule into 2 molecules of a 3-carbon compound
and oxidation (removal of 2 H atoms from the sugar) occurs. The oxidation produces reduced NAD (NADH/H+) which can be oxidised to NAD+ in mitochondria to produce ATP. Enzymes that remove hydrogen from organic compounds (dehydrogenation) are referred to as dehydrogenases.NAD+ + 2H+ + 2e
G º NADH/H+2. Production of ATP
. There is the net production of 2 ATP molecules per hexose molecule. In photosynthetic cells, PGA and TP produced in the Calvin cycle can be transported into the cytosol from the chloroplasts and be put into glycolysis.3. Production of pyruvate for later reactions.
Outline of the Krebs cycle
What happens to the pyruvate depends on whether oxygen is available or not. If oxygen is present, the pyruvate enters a mitochondrion where it combines with coenzyme A to form
acetyl coenzyme A. In this reaction, CO2 is given off (decarboxylation) and the pyruvate loses 2 H-atoms (dehydrogenation, oxidation). These H-atoms are transferred to NAD+.Pyruvate + Coenzyme A + NAD+
º Acetyl coenzyme A + CO2 + NADH/H+Acetyl coenzyme A is a very important link between glycolysis and the next series of reactions. Acetyl CoA has 2 carbon atoms and this now reacts with the 4-carbon compound oxaloacetic acid (oxaloacetate) to form citric acid (citrate), a 6-carbon compound.
Oxaloacetate + Acetyl CoA
º citrate + CoAThere now follows a series of reactions in which citrate is gradually converted back to oxaloacetate in stepwise fashion. This cyclic series of reactions is called the
Krebs cycle, the citric acid cycle or the Tricarboxylic acid (TCA) cycle.Two of the steps in TCA cycle involve decarboxylation and four of the steps involve dehydrogenation (oxidation). The hydrogen atoms removed in glycolysis and the Krebs cycle are passed along a chain of
hydrogen and electron carriers (the electron transport system, ETS) and are oxidised to water by combining with oxygen. As the pairs of hydrogen atoms and electrons are passed along the carrier chain from one intermediate to the other, at three points a small amount of energy is liberated and incorporated into a molecule of ATP.Figure: The Krebs Cycle
A number of points are shown in the figure:
1. Oxidative decarboxylation: removal of carbon from a compound as carbon dioxide. There are three decarboxylation reactions in the conversion of pyruvate to oxaloacetate.
2. Fate of hydrogen atoms following dehydrogenation: at three stages in the cycle, 2 H-atoms are removed and transferred to hydrogen acceptor NAD+ and at one stage a pair of H-atoms is transferred to FAD+. When NAD+ is the acceptor, 3 molecules of ATP are synthesized for each pair of H-atoms; when FAD+ is the acceptor, 2 molecules of ATP are formed.
3. Oxidative phosphorylation: this is the oxidation of carriers in the ETS which liberates energy so that ADP is phosphorylated to form ATP.
ADP + Pi
º ATP4. Substrate-level phosphorylation
in the conversion of succinyl coA to succinate, enough energy is liberated to make one molecule of GTP (=ATP).
On the Web:
Last modified: August 27, 2004