Test: Respiration & Enzymes - 1 - ACT MCQ

Respiration is a catabolic process, which oxidises reduced substrates, thus, resulting in the energy stored in reduced substrates. The reduced substrates are produced by the process of photosynthesis occurring in green plants and the reduced substrates are oxidised through respiration releasing CO₂, water vapour and energy in the form of ATP.
So, the correct answer is 'It liberates CO₂'

One GTP molecule is produced from one acetyl-CoA molecules. Two GTP molecules are produced from two acetyl-CoA molecules.

Adenosine triphosphate (ATP) is a nucleotide with three phosphates. It is the universal energy currency for all cellular metabolic processes. Endergonic processes are driven by energy input using hydrolysis of ATP. Exergonic processes are coupled to ATP synthesis. The process of ATP synthesis is called phosphorylation because ATP is synthesized by phosphorylation of adenosine diphosphate (ADP) or adenosine monophosphate (AMP). The energy obtained by a cell from catabolic reactions is stored immediately in the form of ATP, as it is the energy currency of the cell.

Alcoholic fermentation is respiration in absence of oxygen. In this, hexose molecule is converted into ethyl alcohol and CO₂, with the activity of enzyme 'zymase' produced by yeast.

During respiration, reduced organic compounds, or stored food molecules, are catabolised through oxidation. Energy is released during this process, which is trapped as cellular ATP.

Respiratory quotient or R.Q is defined as the ratio of carbon dioxide evolved to oxygen taken in for respiratory break down of a substrate. The value of R.Q for oxidation of carbohydrates is unity and less than one for oxidation of fats and proteins. Thus if groundnut, which stores fats is used as respiratory substrate the value of R.Q will be less than one.

All enzymes of the TCA cycle are present in the mitochondrial matrix, except succinate dehydrogenase.
Succinate dehydrogenase is tightly bound to the inner mitochondrial membrane in eukaryotes and to the plasma membrane in prokaryotes.

Glucose-6-phosphate is oxidised to form lactone. NADPH is produced as a by-product of this reaction. NADP⁺ is reduced as glucose-6-phosphate is oxidised.

Free energy available from the complete oxidation of glucose to carbon dioxide and water under standard conditions (pH 7.0 and temperature 298 K) is 686 Kcal or 2840 KJ per moles. But these values are obtained under the standard set of conditions not under physiological conditions in a cell. To describe the energy released under the conditions existing in cells, an expression for the actual free-energy change is essential.

As glycolysis was discovered by three scientists named Embden, Meyerhof and, Parnas, so it also came to be known as the EMP pathway with the initial letters of the discoverers.

According to chemiosmotic theory of Peter Mitchell, the transport of electrons across membranes is coupled to pumping of protons across the membranes, which establishes an electrochemical gradient. The electrochemical gradient of protons has an electrical component due to charge present on protons and a chemical component because of the hydrogen atom present in proton. Both the electrical and chemical gradients are used up to drive ATP synthesis by ATP synthase enzyme complex, when protons flow across the membranes through the ATP synthase enzyme system along the electrochemical gradient.

Complex 2 oxidises succinate to fumarate and reduces ubiquinone. As this reaction releases less energy than the oxidation of NADH, complex 2 does not transport protons across the membrane and does not contribute to the proton gradient.

In glycolysis, two molecules of NADH are formed at the time of oxidation of glyceraldehyde 3-phosphate to 1, 3-diphosphoglycerate which give rise to 6 ATP. This is converted into respective ATP forms during Electron Transport System.

• The two stages of aerobic respiration are Kreb’s cycle and electron transport system.
• The first process takes place in the matric of mitochondria while the second process takes place in the inner membrane of mitochondria in eukaryotes.

• Glycolysis is defined as a sequence of reactions that convert glucose into pyruvate or lactate with the production of ATP (adenosine triphosphate).
• Pyruvic acid is the end product of glycolysis.
• Two molecules of pyruvic acid are generated by the partial oxidation of one glucose molecule.
• The end product of glycolysis is the three-carbon pyruvic acid produced in the cytosol.

The crucial events in aerobic respiration are the complete oxidation of pyruvate and the passing on of the electrons removed as a part of the hydrogen atoms to molecular oxygen with the simultaneous synthesis of ATP.

Fermentation is a metabolic process that converts sugar to acids, gases, or alcohol and is carried out by a wide range of organisms. Yeast and other microorganisms ferment glucose to ethanol and CO₂, rather than to lactate as in muscles of higher vertebrates. Glucose is converted to pyruvate by glycolysis, and the pyruvate is converted to ethanol and CO₂ in a two-step process:
In the first step, pyruvate is decarboxylated in an irreversible reaction catalyzed by pyruvate decarboxylase. Pyruvate decarboxylase requires Mg₂+ and has a tightly bound coenzyme, thiamine pyrophosphate.
In the second step, acetaldehyde is reduced to ethanol through the action of alcohol dehydrogenase, with the reducing power furnished by NADH derived from the dehydrogenation of glyceraldehyde 3-phosphate.
Ethanol and CO2 are thus the end products of ethanol fermentation, and the overall equation is as follows:
Glucose + 2ADP + 2Pi ----> 2 ethanol + 2 CO₂ + 2 ATP + 2 H₂O

• The total number of ATP molecules produced in glycolysis is 4 from one glucose molecule.
• 2 molecules of ATP are utilised in the first half of glycolysis.
• So, there is a net gain of 2 ATP molecules in glycolysis.
• Additionally, 2 NADH molecules are also produced in glycolysis.

The glycolysis results in formation of two molecules of pyruvic acid from each molecule of glucose. The pyruvic acid is oxidatively decarboxylated to acetyl CoA by a complex enzyme pyruvate dehydrogenase. There is release of one molecule of carbon dioxide and production of one molecule of reduced coenzyme NADH2 per molecule of pyruvic acid acted upon by pyruvate dehydrogenase. Thus totally two molecules of acetyl CoA are produced by oxidative decarboxylation of two molecules of pyruvic acid per glucose molecule.

Acetyl CoA is the link between carbohydrate and fat metabolism. In carbohydrate metabolism, during glycolysis, one molecule of hexose is broken down into two molecules of pyruvic acid. The pyruvic acid molecules are oxidatively decarboxylated into acetyl CoA, which is fed into Krebs cycle. For oxidation of fats, the long chains of fatty acids are converted into many acetyl CoA units by sequential removal of two carbon atoms at a time. These units of acetyl CoA are then fed into the usual Krebs cycle.

The pyruvic acid generated in the glycolysis enters into the mitochondrial matrix, undergoes oxidative decarboxylation and forms active acetate in a coordinated series of reactions catalysed by the multienzyme complex. The reaction takes place in the presence of enzyme pyruvate dehydrogenase and several coenzymes including NAD+, coenzyme A, thiamine pyrophosphate and lipoic acid. The reaction occurs in 4 steps involving the conversion of pyruvic acid to acetyl coenzyme A. One molecule of carbon dioxide is released. The coenzyme NAD+ is reduced to NADH+H+ in the reaction. 

Thus, the correct answer is option D.

TCA cycle is also known as Krebs cycle. TCA cycle stands for tricarboxylic acid cycle, which signifies that the first stable product is having three carboxylic groups. The first stable product is citric acid, which is a tricarboxylic acid. It is produced by the condensation of one molecule of acetyl CoA and one molecule of oxaloacetic acid.

Nicotinamide adenine dinucleotide phosphate, abbreviated NADP⁺ or, in older notation, TPN, is a cofactor used in anabolic reactions, such as lipid and nucleic acid synthesis, which require NADPH as a reducing agent. It is the coenzyme - II

Protein synthesis occurs in cellular structures called ribosomes , found out-side the nucleus. The process by which genetic information is transferred from the nucleus to the ribosomes is called transcription. During transcription, a strand of ribonucleic acid (RNA) is synthesized.

Glycolysis means lysis of glucose. It is a sequence of 10 reactions, which occur in the cytoplasm and break down one hexose glucose molecule into two molecules of pyruvic acid, having three carbon atoms in each. Also, there is the net gain of 2 ATP molecules and a synthesis of 2 molecules of reduced coenzyme NADH2. 

The breakdown of the simple sugar i.e. glucose in glycolysis provides the first 6- carbon molecule required for the pentose phosphate pathway. During the first step of glycolysis, glucose is transformed by the addition of a phosphate group, generating glucose-6-phosphate another 6- carbon molecule.

The glucose molecule is first broken down into two molecules of pyruvic acid through glycolysis. The glycolysis results in production of net 2 ATP molecules and 2 molecules of reduced coenzyme NADPH_2​. The 2 molecules of  pyruvic acid are oxidatively decarboxylated by pyruvate dehydrogenase to yield two molecules of acetyl coenzyme and simultaneous production of two molecules of reduced coenzyme NADH_2​​ one per conversion of pyruvic acid to acetyl CoA. The acetyl CoA enters Krebs cycle. Each moleucle of acetyl CoA that enters Krebs cycle generates 3 molecules of NADH_2​​, 1 molecules of FADH_2​​ and 1 molecule of GTP. Thus for two molecules of acetyl CoA from one glucose molecule, the net gain is 6 molecules of NADH_2​ and 2 molecules of FADH2​ and 2 molecules of GTP. Assuming that one molecule of NADH_2​​ is equivalent to 3 molecules of ATP and one molecule of FADH_2​​ is equivalent to 2 molecules of ATP and one molecule of GTP is equivalent to one molecule of ATP. The net gain from entire aerobic breakdown of glucose comes out to be 38 molecules of ATP. 

• For aerobic respiration to take place within the mitochondria, the final product of glycolysis, pyruvate is transported from the cytoplasm into the mitochondria. The crucial events in aerobic respiration are:
• The complete oxidation of pyruvate by the stepwise removal of all the hydrogen atoms, leaving three molecules of CO₂ .
• The passing on of the electrons removed as part of the hydrogen atoms to molecular O₂ with simultaneous synthesis of ATP. What is interesting to note is that the first process takes place in the matrix of the mitochondria while the second process is located on the inner membrane of the mitochondria.