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RESPIRATION IN PLANTS: All living organisms require energy to carry out daily life activities. This energy is acquired from the macromolecules (carbohydrates, fats, proteins, and nucleic acids) by oxidizing them.
Plants can create their own food through photosynthesis — a process in which light energy is trapped and converted into chemical energy that is stored in the bonds of carbohydrates such as glucose and starch. The process of photosynthesis occur only in the cells that contain chloroplast (green). Therefore, food needs to be transported from the cell (green) that make it to cells that do not contain chloroplast (can’t make food). |
Animals are heterotrophic. Heterotrophic means that animals obtain food either directly from plants (herbivores) or indirectly from other animals (carnivores).
In the following sections, we will discuss about the cellular respiration — the process of breaking down of C-C bond in food materials through oxidation within the cell to release energy and trapping this energy to synthesize ATP. The compounds such as carbohydrates, proteins, and fats that are oxidized during the cellular respiration are called respiratory substrates. During the oxidation of respiratory substrates in a cell, all the energy is not released free in a single step. Instead, the energy is released in a series of step-wise reaction and it is trapped as chemical energy in the form of ATP, which is broken down whenever energy needs to be utilized.
In the following sections, we will discuss about the cellular respiration — the process of breaking down of C-C bond in food materials through oxidation within the cell to release energy and trapping this energy to synthesize ATP. The compounds such as carbohydrates, proteins, and fats that are oxidized during the cellular respiration are called respiratory substrates. During the oxidation of respiratory substrates in a cell, all the energy is not released free in a single step. Instead, the energy is released in a series of step-wise reaction and it is trapped as chemical energy in the form of ATP, which is broken down whenever energy needs to be utilized.
Glycolysis:
Glycolysis occurs in the cytoplasm of the cell and does not involve the use of oxygen. It is important to note that in anaerobic respiration, glycolysis is the only process of cellular respiration. In aerobic respiration, glycolysis is the first of four processes that occur for the complete oxidation of a glucose molecule.
In glycolysis, the glucose undergoes partial oxidation to form two molecules of pyruvic acid. In plants, glucose is obtained from the break down of sucrose (the end product of photosynthesis) or from storage carbohydrates (starch and glycogen). Animal obtain food and break down the carbohydrate to get glucose and fructose. The steps involved in glycolysis is shown in figure 1 at right. Note that four total molecules of ATP are produced from a molecule of glucose during glycolysis of which two are used during the glycolysis. Therefore, the net ATP gain from a molecule of glucose in glycolysis is two molecules of ATP. Two molecules of NADH is also produced during glycolysis. NADH and FADH2 are coenzymes that are oxidized in oxidative phosphorylation (after Krebs’ cycle) to form ATP. One NADH produces three ATP and one FADH2 creates two ATP molecules. |
After the pyruvic acid is produced, it enters Krebs' cycle (aerobic respiration) if oxygen is present.
In absence of oxygen, it enters alcoholic or lactic acid fermentation (anaerobic respirations) as shown in Figure 2a and 2b below.
In absence of oxygen, it enters alcoholic or lactic acid fermentation (anaerobic respirations) as shown in Figure 2a and 2b below.
Fermentation:
In the absense of oxygen, pyruvic acid will undergo fermentation reaction. In lactic acid or alcoholic fermentation, the glucose undergoes incomplete oxidation extracting less than seven percent of energy stored in glucose. In alcoholic fermentation, pyruvic acid is converted into CO2 and ethanol (alcohol). In lactic acid fermentation, pyruvic acid is converted into lactic acid. Steps involved in fermentation are shown in figure 3 below.
Lactic acid fermentation occurs in our muscle cells also. During exercise, when muscles don't get enough oxygen supply, pyruvic acid is converted to lactic acid by lactate dehydrogenase.
In lactic acid or alcoholic fermentation reaction, no ATP is produced. As we described earlier, four total molecules of ATP are produced from a molecule of glucose during glycolysis of which two are used during the glycolysis reaction. Therefore, the net ATP gain from glycolysis and fermentation reaction is only two molecule of ATP.
Lactic acid fermentation occurs in our muscle cells also. During exercise, when muscles don't get enough oxygen supply, pyruvic acid is converted to lactic acid by lactate dehydrogenase.
In lactic acid or alcoholic fermentation reaction, no ATP is produced. As we described earlier, four total molecules of ATP are produced from a molecule of glucose during glycolysis of which two are used during the glycolysis reaction. Therefore, the net ATP gain from glycolysis and fermentation reaction is only two molecule of ATP.
Aerobic Respiration:
In the presence of oxygen, the pyruvic acid produced from glycolysis undergoes aerobic respiration. Aerobic respiration is the process that leads to a complete oxidation of organic substances (eg. pyruvic acid) in the presence of oxygen, and releases CO2, water and a large amount of energy present in the substrate.
Aerobic respiration involves three steps : (i) pyruvate oxidation, (ii) citric acid cycle (also called Krebs’ cycle), and (iii) oxidative phosphorylation (shown in figure 4 below). In eukaryotes, all three of these processes occur within the mitochondria (pyruvate oxidation and citric acid cycle occurs in mitochondrial matrix whereas oxidative phosphorylation occur in inner mitochondrial membrane).
Aerobic respiration involves three steps : (i) pyruvate oxidation, (ii) citric acid cycle (also called Krebs’ cycle), and (iii) oxidative phosphorylation (shown in figure 4 below). In eukaryotes, all three of these processes occur within the mitochondria (pyruvate oxidation and citric acid cycle occurs in mitochondrial matrix whereas oxidative phosphorylation occur in inner mitochondrial membrane).
(i) Pyruvate oxidation:
This is a process that occurs in mitochodrial matrix and links glycolysis and citric acid cycle. In aerobic respiration, pyruvic acid is transported from cytoplasm to mitochondrial matrix, where pyruvate oxidation takes place. This transport costs 1 ATP per pyruvic acid.This transport costs 1 ATP per pyruvic acid.
During the reaction, two molecules of NADH, two molecules of acetyl CoA, and two molecules of CO2 are produced from a molecule of glucose (remember that one molecule of glucose gave rise to two molecules of pyruvate). This acetyl CoA will enter a cyclic pathway called tricarboxylic acid cycle also called citric acid cycle and Kreb's cycle.
It is also important to note in this pyruvate oxidation reaction that one molecule of NADH is produced from a molecule of pyruvic acid as shown in the reaction above. Since two molecules of pyruvic acids are created from a molecule of glucose during glycolysis, a total of two molecules of NADH are produced from pyruvate oxidation of a glucose molecule. No ATP is produced directly from pyruvic acid oxidation as can be observed from the reaction above.
It is also important to note in this pyruvate oxidation reaction that one molecule of NADH is produced from a molecule of pyruvic acid as shown in the reaction above. Since two molecules of pyruvic acids are created from a molecule of glucose during glycolysis, a total of two molecules of NADH are produced from pyruvate oxidation of a glucose molecule. No ATP is produced directly from pyruvic acid oxidation as can be observed from the reaction above.
(ii) Citric acid cycle or Tricarboxylic acid cycle (Also called Kreb's cycle):
The two molecules of acetyl CoA produced from pyruvate oxidation will enter the citric acid cycle. In citric acid cycle (also occur in mitrochonrial matrix) a series of reactions will take place creating an end product called oxaloacetic acid that again enters the reaction creating a repeating cyclic reaction as shown in Fiugure 5 below.
A citric acid cycle completely oxidizes the acetyl CoA molecules producing CO2 molecules. The free energy released from the reactions are captured by ADP, NAD+, and FAD creating ATP, NADH, and FADH2 respectively as seen in the figure 5 above.
Note that in a citric acid cycle, a molecule of pyruvate is oxidized creating 1 molecule of ATP, 3 molecules of NADH, and 1 molecules of FADH2. Since two acetyl CoA are produced from a molecule of glucose, this cycle will occur twice producing a total of 2 molecule of ATP, 6 molecule of NADH, and 2 molecules of FADH2.
Note that in a citric acid cycle, a molecule of pyruvate is oxidized creating 1 molecule of ATP, 3 molecules of NADH, and 1 molecules of FADH2. Since two acetyl CoA are produced from a molecule of glucose, this cycle will occur twice producing a total of 2 molecule of ATP, 6 molecule of NADH, and 2 molecules of FADH2.
(iii) Oxidative phosphorylation (copied from textbook) :
The following steps in the respiratory process are to release and utilise the energy stored in NADH+H+ and FADH2. This is accomplished when they are oxidised through the electron transport system and the electrons are passed on to O2 resulting in the formation of H2O. The metabolic pathway through which the electron passes from one carrier to another, is called the electron transport system (ETS) (Figure 6) and it is present in the inner mitochondrial membrane. Electrons from NADH produced in the mitochondrial matrix during citric acid cycle are oxidised by an NADH dehydrogenase (complex I), and electrons are then transferred to ubiquinone located within the inner membrane. Ubiquinone also receives reducing equivalents via FADH2 (complex II) that is generated during oxidation of succinate in the citric acid cycle. The reduced ubiquinone (ubiquinol) is then oxidised with the transfer of electrons to cytochrome c via cytochrome bc1 complex (complex III). Cytochrome c is a small protein attached to the outer surface of the inner membrane and acts as a mobile carrier for transfer of electrons between complex III and IV. Complex IV refers to cytochrome c oxidase complex containing cytochromes a and a3, and two copper centres.
When the electrons pass from one carrier to another via complex I to IV in the electron transport chain, they are coupled to ATP synthase (complex V) for the production of ATP from ADP and inorganic phosphate. The number of ATP molecules synthesized depends on the nature of the electron donor. Oxidation of one molecule of NADH gives rise to 3 molecules of ATP, while that of one molecule of FADH2 produces 2 molecules of ATP. Although the aerobic process of respiration takes place only in the presence of oxygen, the role of oxygen is limited to the terminal stage of the process. Yet, the presence of oxygen is vital, since it |
drives the whole process by removing hydrogen from the system. Oxygen acts as the final hydrogen acceptor. Unlike photophosphorylation where it is the light energy that is utilised for the production of proton gradient required for phosphorylation, in respiration it is the energy of oxidation-reduction utilised for the same process. It is for this reason that the process is called oxidative phosphorylation.
You have already studied about the mechanism of membrane-linked ATP synthesis as explained by chemiosmotic hypothesis in the earlier chapter. As mentioned earlier, the energy released during the electron transport system is utilised in synthesising ATP with the help of ATP synthase (complex V). This complex consists of two major components, F1 and Fo (Figure 7). The F1 headpiece is a peripheral membrane protein complex and contains the site for synthesis of ATP from ADP and inorganic phosphate. F is an integral membrane protein complex that forms the channel through which protons cross the inner membrane. The passage of protons through the channel is coupled to the catalytic site of the F1 component for the production of ATP. For each ATP produced, 2H+ passes through Fo from the intermembrane space to the matrix down the electrochemical proton gradient.
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The Net ATP Gain From Cellular Respiration of a Glucose Molecule:
So far we know of the amount of ATP, NADH and FADH2 molecule made from glycolysis, pyruvate oxidation, and Krebs' cycle.
We also learned that the oxidation of one molecule of NADH produces 3 ATP's and one molecule of FADH2 molecule produces 2 ATP's.
Shown below is the overall breakdown:
Glycolysis (4 ATP's - 2 ATP's used in glycolysis, 2 NADH)-----------> 6 ATP (2+2*3-2 ATP used to transport ATP to mitochondria)
Pyruvate oxidation (2 NADH) -------------------------------------------------> 6 ATP (2*3)
Citric acid cycle (6 NADH , 2 FADH2, 2 ATP) ----------------------------> 24 ATP (6*3+2*2+2)
TOTAL 36 Net ATP
Therefore, fermentation gives rise to only ATP's while aerobic respiration gives rise to 36 ATP's.
Comparison of fermentation and aerobic respiration is given below:
We also learned that the oxidation of one molecule of NADH produces 3 ATP's and one molecule of FADH2 molecule produces 2 ATP's.
Shown below is the overall breakdown:
Glycolysis (4 ATP's - 2 ATP's used in glycolysis, 2 NADH)-----------> 6 ATP (2+2*3-2 ATP used to transport ATP to mitochondria)
Pyruvate oxidation (2 NADH) -------------------------------------------------> 6 ATP (2*3)
Citric acid cycle (6 NADH , 2 FADH2, 2 ATP) ----------------------------> 24 ATP (6*3+2*2+2)
TOTAL 36 Net ATP
Therefore, fermentation gives rise to only ATP's while aerobic respiration gives rise to 36 ATP's.
Comparison of fermentation and aerobic respiration is given below:
Amphibolic Pathway:
All carbohydrates are broken first broken down to glucose before they are used for cellular respiration. Even though glucose is the favored substrate for cellular respiration, it is not the only substrate that enter cellular respiration. Other substrates like fatty acid and glycerol (from breaking down of fats) and amino acids (from breading down of proteins) enter the respiratory pathway at different stages in the respiratory pathway (Figure 8). Fatty acid is degraded to acetyl CoA and glycerol to PGAL during the cellular respiration.
Respiration is an amphibolic pathway : a pathway that involves both catabolism and anabolism. Respiration involves catabolic pathway because substrates such as fatty acids and glycerol are broken down to CoA and PGAL respectively to derive energy. But respiration also involves anabolic pathways because these substrate such as fatty acid and glycerol can be synthesized using the acetyl CoA and PGAL withdrawn from the reparatory pathway. Hence, respiratory pathway is an amphibolic pathway.
Respiratory Quotient:
You know that during aerobic respiration, oxygen(O2) is consumed and carbon dioxide (CO2) is released. The ratio of the volume of CO2 evolved to the volume of O2 consumed in respiration is called the respiratory quotient (RQ) or respiratory ratio.
The respiratory quotient depends upon the type of respiratory substrate used during respiration.
For carbohydrates, the RQ is 1 because the number of CO2 produced is equal to the amount of O2 used as shown below.
For carbohydrates, the RQ is 1 because the number of CO2 produced is equal to the amount of O2 used as shown below.
For fats, the RQ is less than 1 because the number of CO2 produced is less to the amount of O2 used as shown below.
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