NEET-Pattern Practice Questions

How This Chapter Is Asked in NEET

Respiration in Plants is asked almost entirely as numbers and locations. Nothing here is derived and nothing is calculated beyond simple multiplication - the marks go to the student who learnt the exact enzyme, the exact number and the exact compartment.

Every question in this chapter is one of four shapes:

1. Name the enzyme, the intermediate or the complex. "The enzyme that converts pyruvic acid to acetyl CoA" - pyruvate dehydrogenase. "The mobile carrier between complex III and complex IV" - cytochrome c.
2. Give the number. ATP, $\mathrm{NADH + H^+}$, carbons, protons. "Net ATP from glycolysis" - 2. "Protons per ATP through $\mathrm{F_0}$" - 4.
3. Say which compartment. "Where glycolysis happens" - the cytoplasm. "Where the TCA cycle happens" - the matrix. "Where the ETS sits" - the inner mitochondrial membrane.
4. Compare two stages. Glycolysis against the Krebs cycle, aerobic against anaerobic, fermentation against aerobic respiration, substrate-level against oxidative phosphorylation.

The traps repeat, and in this chapter they are always the same ones:

• Gross 4 ATP against net 2 ATP in glycolysis. ATP is synthesised at two steps - BPGA to 3-phosphoglyceric acid, and PEP to pyruvic acid - each happening twice, so 4 are made; but ATP is utilised at two steps - glucose to glucose-6-phosphate, and fructose-6-phosphate to fructose 1,6-bisphosphate - so the net gain is 2. The distractor is always 4. Read whether the stem says net or synthesised.
• Glycolysis is in the CYTOPLASM and is universal; the TCA cycle is in the MATRIX and is aerobic. Glycolysis occurs in the cytoplasm of the cell, is present in all living organisms, and in anaerobic organisms it is the only process in respiration. Any option that moves glycolysis into the mitochondrion is wrong.
• The link reaction is neither glycolysis nor the TCA cycle. It is the oxidative decarboxylation of pyruvic acid by pyruvic dehydrogenase in the matrix, needing $\mathrm{NAD^+}$, Coenzyme A and $\mathrm{Mg^{2+}}$, and it releases exactly one $\mathrm{CO_2}$ per pyruvate:

$$\mathrm{Pyruvic\ acid + CoA + NAD^+} \xrightarrow[\mathrm{Mg^{2+}}]{\text{pyruvate dehydrogenase}} \mathrm{Acetyl\ CoA + CO_2 + NADH + H^+}$$

• The TCA cycle produces GTP by SUBSTRATE LEVEL PHOSPHORYLATION, not ATP directly. During the conversion of succinyl-CoA to succinic acid a molecule of GTP is synthesised, and in a coupled reaction GTP is converted to GDP with the simultaneous synthesis of ATP from ADP. The distractor for the label is always oxidative phosphorylation.
• Three $\mathrm{NADH + H^+}$ and one $\mathrm{FADH_2}$ per TURN, but eight $\mathrm{NADH + H^+}$ and two $\mathrm{FADH_2}$ per GLUCOSE. The cycle has three points where $\mathrm{NAD^+}$ is reduced and one point where $\mathrm{FAD^+}$ is reduced, so two turns give 6 and 2; add the 2 from the two link reactions and you have 8 $\mathrm{NADH + H^+}$ and 2 $\mathrm{FADH_2}$. If the stem says in the TCA cycle, the NADH answer is 6; if it says so far or in total, it is 8; and once the 2 from glycolysis are added for the balance sheet, it is 10.
• 3 ATP per NADH against 2 ATP per $\mathrm{FADH_2}$, and the reason. Oxidation of one molecule of NADH gives rise to 3 molecules of ATP; oxidation of one molecule of $\mathrm{FADH_2}$ produces 2 molecules of ATP. $\mathrm{FADH_2}$ yields less because it enters at complex II and bypasses complex I - a shorter run down the chain, so less energy released along the path.
• The five complexes in order, and where cytochrome c sits. Complex I is NADH dehydrogenase; complex II is the $\mathrm{FADH_2}$ entry point from succinate oxidation; complex III is the cytochrome $\mathrm{bc_1}$ complex; complex IV is the cytochrome c oxidase complex, containing cytochromes a and $\mathrm{a_3}$ and two copper centres; complex V is ATP synthase. Cytochrome c is not a complex at all - it is a small protein attached to the outer surface of the inner membrane, acting as a mobile carrier for transfer of electrons between complex III and complex IV. Ubiquinone is the meeting point where complex I and complex II both unload.
• Oxygen acts only at the TERMINAL stage, as the FINAL HYDROGEN ACCEPTOR. The role of oxygen is limited to the terminal stage of the process, yet its presence is vital, since it drives the whole process by removing hydrogen from the system. The trap option says oxygen is therefore unimportant; the correct line keeps both halves of that sentence.
• $\mathrm{F_0}$ is the channel and $\mathrm{F_1}$ is the catalytic headpiece. $\mathrm{F_0}$ is an integral membrane protein complex that forms the channel through which protons cross the inner membrane; the $\mathrm{F_1}$ headpiece is a peripheral membrane protein complex and contains the site for synthesis of ATP from ADP and inorganic phosphate. The giveaway words are integral for $\mathrm{F_0}$ and peripheral for $\mathrm{F_1}$. And for each ATP produced, $\mathrm{4H^+}$ pass through $\mathrm{F_0}$ from the INTERMEMBRANE SPACE to the MATRIX, down the electrochemical proton gradient.
• The chapter 11 contrast. In respiration protons accumulate in the INTERMEMBRANE SPACE; in photosynthesis they accumulate in the THYLAKOID LUMEN. Respiration has $\mathrm{F_0}$ and $\mathrm{F_1}$; photosynthesis has $\mathrm{CF_0}$ and $\mathrm{CF_1}$ - same architecture, different names. And in oxidative phosphorylation it is the energy of OXIDATION-REDUCTION that creates the gradient, while in photophosphorylation it is LIGHT energy - that difference is the definition of oxidative phosphorylation. The trap offers you the matrix and the stroma; those are where the protons return to, not where they gather.
• The 38 ATP figure is theoretical and rests on four assumptions. There is a sequential, orderly pathway functioning; the NADH synthesised in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation; none of the intermediates are utilised to synthesise any other compound; only glucose is being respired. The wording of the conclusion matters too - there CAN BE a net gain of 38 ATP, not "there is". The commonest wrong number is 36, which appears when the glycolytic NADH is not counted at the full rate.
• Fermentation releases LESS THAN SEVEN PER CENT of the energy in glucose, and yeast dies at about THIRTEEN PER CENT alcohol. Not all of even that seven per cent is trapped as high energy bonds of ATP. Yeasts poison themselves to death when the concentration of alcohol reaches about 13 per cent, so naturally fermented beverages cannot exceed about that concentration and stronger drinks are obtained by distillation.
• The four amphibolic entry points. Fats break into glycerol and fatty acids; fatty acids enter as acetyl CoA; glycerol enters as PGAL; amino acids, after deamination, enter at some stage within the Krebs cycle, or even as pyruvate or acetyl CoA. The examiner's trap is to swap glycerol and fatty acid.
• RQ of 1 for carbohydrates, about 0.7 for fats and about 0.9 for proteins. The ratio is the volume of $\mathrm{CO_2}$ evolved over the volume of $\mathrm{O_2}$ consumed, and inverting it is the standard mistake - it turns 0.7 into 1.43.

One habit pays more than any other here. When two options both look plausible, ask which compartment the statement belongs to and whether the number is per pyruvate, per turn or per glucose. The wrong option in this chapter is almost never invented - it is a true fact moved one step sideways: the gross figure given as the net, the per-turn figure given as the per-glucose, the channel component given the catalytic job, the intermembrane space swapped for the matrix.

Before You Start

Give yourself 45 minutes for 45 questions and look nothing up. Mark every question where you were choosing between two options rather than certain - those marked questions, not only the wrong ones, are your revision list.

The facts most likely to decide your score, by topic:

Gas exchange and the logic of respiration. Plants require $\mathrm{O_2}$ for respiration and give out $\mathrm{CO_2}$, but unlike animals they have no specialised organs for gaseous exchange - only stomata and lenticels. Three reasons they manage: each plant part takes care of its own needs, with very little transport of gases between parts; plants do not present great demands, and roots, stems and leaves respire at rates far lower than animals; and the distance gases must diffuse is not great, since each living cell is quite close to the surface. Only during photosynthesis are large volumes of gases exchanged, and then $\mathrm{O_2}$ is released within the cell. In stems the living cells lie in thin layers inside and beneath the bark, with lenticels as openings, and the interior cells are dead and give only mechanical support. Loose packing of parenchyma cells provides an interconnected network of air spaces. The breaking of the C-C bonds of complex compounds through oxidation within cells, leading to release of a considerable amount of energy, is respiration, and the compounds oxidised are the respiratory substrates. Complete combustion of glucose gives $\mathrm{6CO_2 + 6H_2O}$ and energy, most of which is given out as heat; the cell instead oxidises glucose in several small steps so that some steps are just large enough for the released energy to be coupled to ATP synthesis. The first cells lived in an atmosphere lacking oxygen; facultative anaerobes can manage either way, while in obligate ones the anaerobic requirement is absolute; and all living organisms retain the enzymatic machinery to partially oxidise glucose without the help of oxygen.

Glycolysis. From the Greek glycos, sugar, and lysis, splitting. The scheme was given by Gustav Embden, Otto Meyerhof and J. Parnas - the EMP pathway. It occurs in the cytoplasm, is present in all living organisms, and in anaerobic organisms it is the only process in respiration. Glucose undergoes partial oxidation to form two molecules of pyruvic acid. In plants the glucose comes from sucrose, the end product of photosynthesis, or from storage carbohydrates; invertase converts sucrose into glucose and fructose. A chain of ten reactions under the control of different enzymes. Hexokinase phosphorylates glucose to glucose-6-phosphate, which isomerises to fructose-6-phosphate. Fructose 1,6-bisphosphate splits into dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL) - and from there everything happens twice. $\mathrm{NADH + H^+}$ is formed from $\mathrm{NAD^+}$ at one step, PGAL to 1,3-bisphosphoglycerate (BPGA), when two redox-equivalents are removed as two hydrogen atoms. ATP is utilised at two steps - glucose to glucose-6-phosphate, and fructose-6-phosphate to fructose 1,6-bisphosphate. ATP is synthesised at two steps - BPGA to PGA, and PEP to pyruvic acid. Per glucose: 2 ATP used, 4 made, net 2 ATP, plus 2 $\mathrm{NADH + H^+}$ and 2 pyruvic acid. Three fates of pyruvate - lactic acid fermentation, alcoholic fermentation, aerobic respiration.

Fermentation. In fermentation, say by yeast, the incomplete oxidation of glucose is achieved under anaerobic conditions, and pyruvic acid is converted to $\mathrm{CO_2}$ and ethanol. The enzymes pyruvic acid decarboxylase and alcohol dehydrogenase catalyse these reactions. Other organisms like some bacteria produce lactic acid from pyruvic acid, and in animal cells also, like muscles during exercise, when oxygen is inadequate pyruvic acid is reduced to lactic acid by lactate dehydrogenase. The reducing agent is $\mathrm{NADH + H^+}$, which is reoxidised to $\mathrm{NAD^+}$ in both the processes - that is the whole point, because glycolysis needs $\mathrm{NAD^+}$ back at the PGAL step. Only the alcoholic route releases $\mathrm{CO_2}$; lactic acid fermentation releases none. Less than seven per cent of the energy in glucose is released, and not all of it is trapped as high energy bonds of ATP. The processes are hazardous - either acid or alcohol is produced. Net ATP per glucose fermented is 2. Yeasts poison themselves to death at about 13 per cent alcohol, so beverages stronger than that are obtained by distillation. Aerobic respiration leads to a complete oxidation of organic substances in the presence of oxygen, and releases $\mathrm{CO_2}$, water and a large amount of energy present in the substrate; it is most common in higher organisms, and in eukaryotes these steps take place within the mitochondria.

Aerobic respiration and the link reaction. Pyruvate is transported from the cytoplasm into the mitochondria. The crucial events are the complete oxidation of pyruvate by the stepwise removal of all the hydrogen atoms, leaving three molecules of $\mathrm{CO_2}$, and the passing on of the electrons to molecular $\mathrm{O_2}$ with simultaneous synthesis of ATP. The first takes place in the matrix; the second is located on the inner membrane. In the matrix pyruvate undergoes oxidative decarboxylation catalysed by pyruvic dehydrogenase, requiring several coenzymes including $\mathrm{NAD^+}$ and Coenzyme A, with $\mathrm{Mg^{2+}}$ as cofactor. Carbon count - pyruvate 3C, one carbon out as $\mathrm{CO_2}$, acetyl CoA 2C. Two molecules of NADH are produced from the metabolism of two molecules of pyruvic acid, produced from one glucose. The link reaction yields no ATP. Do not confuse pyruvic dehydrogenase, in the matrix, with pyruvic acid decarboxylase, in the cytoplasm of a fermenting yeast.

The TCA cycle. It starts with the condensation of the acetyl group with oxaloacetic acid (OAA) and water to yield citric acid, catalysed by citrate synthase, with a molecule of CoA released. Citrate is then isomerised to isocitrate. Two successive steps of decarboxylation follow, giving $\alpha$-ketoglutaric acid and then succinyl-CoA. In the remaining steps succinyl-CoA is oxidised to OAA, allowing the cycle to continue. Carbon counts - acetyl CoA 2C, OAA 4C, citric acid 6C, $\alpha$-ketoglutaric acid 5C, succinic acid 4C, malic acid 4C. During the conversion of succinyl-CoA to succinic acid a molecule of GTP is synthesised - a substrate level phosphorylation - and in a coupled reaction GTP is converted to GDP with the simultaneous synthesis of ATP from ADP. Three points reduce $\mathrm{NAD^+}$ and one point reduces $\mathrm{FAD^+}$. The cycle requires the continued replenishment of oxaloacetic acid and the regeneration of $\mathrm{NAD^+}$ and $\mathrm{FAD^+}$. The summary equation for this phase, in the matrix:

$$\mathrm{Pyruvic\ acid + 4NAD^+ + FAD^+ + 2H_2O + ADP + P_i} \xrightarrow{\text{mitochondrial matrix}} \mathrm{3CO_2 + 4NADH + 4H^+ + FADH_2 + ATP}$$

That equation is per pyruvic acid and bundles the link reaction in, which is why it shows 4 $\mathrm{NAD^+}$ and 3 $\mathrm{CO_2}$ rather than the 3 and 2 of the cycle alone. By the end of the TCA cycle, eight molecules of $\mathrm{NADH + H^+}$ and two of $\mathrm{FADH_2}$ have been synthesised besides just two molecules of ATP in the TCA cycle.

The electron transport system. The following steps release and utilise the energy stored in $\mathrm{NADH + H^+}$ and $\mathrm{FADH_2}$, by oxidising them through the ETS and passing the electrons on to $\mathrm{O_2}$, resulting in the formation of $\mathrm{H_2O}$. The ETS is present in the inner mitochondrial membrane. Complex I, NADH dehydrogenase, oxidises NADH produced in the matrix and transfers the electrons to ubiquinone located within the inner membrane. Ubiquinone also receives reducing equivalents via $\mathrm{FADH_2}$ (complex II), generated during the oxidation of succinate. Reduced ubiquinone (ubiquinol) is oxidised with transfer of electrons to cytochrome c via the cytochrome $\mathrm{bc_1}$ complex (complex III). Cytochrome c is a small protein attached to the outer surface of the inner membrane and acts as a mobile carrier between complex III and complex IV. Complex IV is the cytochrome c oxidase complex, containing cytochromes a and $\mathrm{a_3}$ and two copper centres. Complex V is ATP synthase. Oxidation of one NADH gives 3 ATP; one $\mathrm{FADH_2}$ gives 2 ATP, because $\mathrm{FADH_2}$ enters at complex II and bypasses complex I. The role of oxygen is limited to the terminal stage, but its presence is vital, since it drives the whole process by removing hydrogen from the system - oxygen acts as the final hydrogen acceptor. Unlike photophosphorylation, where light energy produces the proton gradient, in respiration it is the energy of oxidation-reduction - hence oxidative phosphorylation.

ATP synthase and chemiosmosis. The energy released during the ETS is utilised in synthesising ATP with the help of ATP synthase (complex V), which consists of two major components, $\mathrm{F_1}$ and $\mathrm{F_0}$. The $\mathrm{F_1}$ headpiece is a peripheral membrane protein complex and contains the site for synthesis of ATP from ADP and inorganic phosphate. $\mathrm{F_0}$ 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 $\mathrm{F_1}$. For each ATP produced, $\mathrm{4H^+}$ pass through $\mathrm{F_0}$ from the intermembrane space to the matrix down the electrochemical proton gradient. In respiration protons accumulate in the intermembrane space; in photosynthesis, in the thylakoid lumen. Respiration has $\mathrm{F_0}$ and $\mathrm{F_1}$; photosynthesis has $\mathrm{CF_0}$ and $\mathrm{CF_1}$.

The balance sheet. Four assumptions - a sequential orderly pathway; the glycolytic NADH transferred into the mitochondria and undergoing oxidative phosphorylation; no intermediate used to synthesise any other compound; only glucose respired. They are not really valid, because all pathways work simultaneously and do not take place one after another, substrates enter and are withdrawn as and when necessary, ATP is utilised as and when needed, and enzymatic rates are controlled by multiple means. Yet it is useful to do the exercise, to appreciate the beauty and efficiency of the living system in extraction and storing energy. Per glucose: 4 ATP by substrate-level phosphorylation, 10 $\mathrm{NADH + H^+}$ and 2 $\mathrm{FADH_2}$; at 3 and 2, that is 30 plus 4 from oxidative phosphorylation, plus 4 at the substrate level, giving a possible net gain of 38 ATP. Fermentation against aerobic respiration - only a partial breakdown against complete degradation to $\mathrm{CO_2}$ and $\mathrm{H_2O}$; a net gain of only two ATP against many more; and NADH oxidised to $\mathrm{NAD^+}$ rather slowly against very vigorously.

Amphibolic pathway. Glucose is the favoured substrate, and all carbohydrates are usually first converted into glucose. Fats and proteins can also be respired, but they do not enter the respiratory pathway at the first step. Fats break down to glycerol and fatty acids; fatty acids are degraded to acetyl CoA and enter there; glycerol enters after being converted to PGAL; proteins are degraded by proteases and the amino acids, after deamination, enter at some stage within the Krebs cycle, or even as pyruvate or acetyl CoA. The very compounds that enter when a substrate is respired are withdrawn for the synthesis of that substrate - acetyl CoA in during breakdown of a fatty acid, acetyl CoA out during its synthesis. Catabolism is breaking down, anabolism is synthesis, and because the pathway does both it is better considered amphibolic than catabolic.

Respiratory quotient. The ratio of the volume of $\mathrm{CO_2}$ evolved to the volume of $\mathrm{O_2}$ consumed in respiration is the respiratory quotient (RQ) or respiratory ratio, and it has no unit:

$$\mathrm{RQ} = \frac{\text{volume of }\mathrm{CO_2}\text{ evolved}}{\text{volume of }\mathrm{O_2}\text{ consumed}}$$

RQ depends upon the type of respiratory substrate used. Carbohydrates completely oxidised give 1, because equal amounts of $\mathrm{CO_2}$ and $\mathrm{O_2}$ are evolved and consumed respectively. Fats give less than 1 - for tripalmitin, $\mathrm{2(C_{51}H_{98}O_6) + 145O_2 \rightarrow 102CO_2 + 98H_2O}$, so RQ is 102 over 145, that is 0.7. Proteins give about 0.9. In living organisms respiratory substrates are often more than one, and pure proteins or fats are never used as respiratory substrates.