Respiration in Plants | CBSE Class 11 Biology Notes

This note covers respiration in plants: why living things need energy and how ATP stores it, how plants exchange gases without respiratory organs, why glucose is oxidised in small steps, glycolysis, alcoholic and lactic acid fermentation, the conversion of pyruvate to acetyl CoA, the Krebs' cycle, the electron transport system and oxidative phosphorylation, the respiratory balance sheet, the amphibolic pathway and the respiratory quotient.

What is respiration, and why do living things need it?

All living organisms need energy for their daily life activities, such as absorption, transport, movement, reproduction and even breathing. This energy is obtained by the oxidation of some macromolecules that we call food.

Green plants and cyanobacteria prepare their own food. By photosynthesis they trap light energy and convert it into chemical energy, stored in the bonds of carbohydrates such as glucose, sucrose and starch.

In green plants, only cells containing chloroplasts, most often located in the superficial layers, carry out photosynthesis. The non-green organs, tissues and cells also need food for oxidation, so food has to be translocated to them.

Animals are heterotrophic: they obtain food from plants directly (herbivores) or indirectly (carnivores). Saprophytes such as fungi depend on dead and decaying matter. Ultimately, all the food that is respired comes from photosynthesis.

Definition: Respiration is the breaking of the C-C bonds of complex compounds through oxidation within the cells, leading to the release of a considerable amount of energy.

Cellular respiration is the breakdown of food materials within the cell to release energy, and the trapping of this energy for the synthesis of ATP. In eukaryotes this breakdown takes place in the cytoplasm and in the mitochondria.

What are respiratory substrates?

The compounds oxidised during respiration are called respiratory substrates. Usually carbohydrates are oxidised. In some plants, under certain conditions, proteins, fats and even organic acids can also be used.

Why is ATP called the energy currency of the cell?

The energy in a respiratory substrate is not released free into the cell, or in a single step. It is released in a series of slow, step-wise reactions controlled by enzymes, and trapped as chemical energy in the form of ATP.

This energy cannot be used directly. It is used to synthesise ATP, which is broken down whenever and wherever energy needs to be used. ATP therefore acts as the energy currency of the cell.

The energy in ATP drives the energy-requiring processes of the organism. The carbon skeleton produced during respiration is used as precursors for the biosynthesis of other molecules in the cell.

Do plants breathe?

Plants require O₂ for respiration, and they give out CO₂, so they have systems that ensure O₂ is available. Unlike animals, plants have no specialised organs for gaseous exchange. Instead they have stomata and lenticels, through which gases are exchanged by diffusion.

Why can plants manage without respiratory organs?

1. Each part meets its own needs. Each plant part takes care of its own gas-exchange needs. There is very little transport of gases from one plant part to another.
2. The demand is low. Roots, stems and leaves respire at rates far lower than animals do. Large volumes of gases are exchanged only during photosynthesis, and each leaf is well adapted to its own needs then. In photosynthesising cells O₂ is not a problem, because it is released within the cell.
3. The distances are short. Even in large, bulky plants, gases need not diffuse far. Each living cell is located quite close to the surface of the plant.

What about thick, woody stems and roots?

In stems, the living cells are organised in thin layers inside and beneath the bark. Stems also have openings called lenticels. The cells in the interior are dead and provide only mechanical support.

Thus most cells of a plant have at least a part of their surface in contact with air. This is helped by the loose packing of parenchyma cells in leaves, stems and roots, which provides an interconnected network of air spaces.

Why must glucose be oxidised in small steps?

The complete combustion of glucose produces CO₂ and H₂O as end products. It yields energy, most of which is given out as heat.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

To be useful, the energy must be usable by the cell to synthesise other molecules. So the plant cell catabolises glucose in such a way that not all the liberated energy goes out as heat.

The key is to oxidise glucose not in one step but in several small steps. Some steps are just large enough that the energy released can be coupled to ATP synthesis. How this is done is, essentially, the story of respiration.

What happens where oxygen is not available?

During respiration, oxygen is used, and carbon dioxide, water and energy are released. But some cells live where oxygen may or may not be available.

There are sufficient reasons to believe that the first cells on this planet lived in an atmosphere that lacked oxygen. Several present-day organisms are adapted to anaerobic conditions. Some are facultative anaerobes, while in others the requirement for anaerobic conditions is obligate.

All living organisms retain the enzymatic machinery to partially oxidise glucose without the help of oxygen. This breakdown of glucose to pyruvic acid is called glycolysis.

What is glycolysis, and where does it take place?

The term glycolysis comes from the Greek words glycos, for sugar, and lysis, for splitting. Its scheme was given by Gustav Embden, Otto Meyerhof and J. Parnas, so it is often called the EMP pathway.

In anaerobic organisms, glycolysis is the only process in respiration. It occurs in the cytoplasm and is present in all living organisms.

Glucose undergoes partial oxidation to form two molecules of pyruvic acid. A chain of ten reactions, under the control of different enzymes, produces pyruvate from glucose.

Where does the glucose come from in plants?

In plants, the glucose comes from sucrose, the end product of photosynthesis, or from storage carbohydrates. Sucrose is converted into glucose and fructose by the enzyme invertase, and both readily enter the glycolytic pathway.

Glucose is phosphorylated to glucose-6-phosphate by the enzyme hexokinase. This then isomerises to fructose-6-phosphate. From here on, the steps for glucose and fructose are the same.

A vertical flow chart of blue arrows, with orange side arrows where ATP, NAD⁺ or water enter or leave. Glucose (6C) → glucose-6-phosphate (6C), with ATP → ADP → fructose-6-phosphate (6C) → fructose 1,6-bisphosphate (6C), again with ATP → ADP. This gives triose phosphate (glyceraldehyde-3-phosphate) (3C), joined by a double arrow to triose phosphate (dihydroxy acetone phosphate) (3C). Then 2 × triose bisphosphate (1,3 bisphosphoglyceric acid), with NAD⁺ → NADH+H⁺ → 2 × triose phosphate (3-phosphoglyceric acid), with ADP → ATP → 2 × 2-phosphoglycerate → 2 × phosphoenolpyruvate, with H₂O leaving → 2 × pyruvic acid (3C), with ADP → ATP.

See Fig. 12.1 in your NCERT textbook

Which steps of glycolysis use or make ATP and NADH?

The steps follow Figure 12.1. After fructose 1,6-bisphosphate splits, each glucose gives two three-carbon molecules, so the later molecules are labelled "2 ×".

1. Glucose → glucose-6-phosphate. Hexokinase phosphorylates glucose; one ATP is used.
2. Glucose-6-phosphate → fructose-6-phosphate, by isomerisation.
3. Fructose-6-phosphate → fructose 1,6-bisphosphate. A second ATP is used.
4. Splitting. Fructose 1,6-bisphosphate is split into dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL).
5. PGAL → 1,3-bisphosphoglycerate (BPGA). Two redox equivalents, as two hydrogen atoms, are removed from PGAL and transferred to NAD⁺, forming NADH + H⁺. PGAL is oxidised and, with inorganic phosphate, becomes BPGA.
6. BPGA → 3-phosphoglyceric acid (PGA). This step yields energy, trapped by forming ATP.
7. PGA → 2-phosphoglycerate → phosphoenolpyruvate (PEP). Water leaves in forming PEP.
8. PEP → pyruvic acid. Another ATP is synthesised.

How many ATP are made directly from one glucose?

ATP is used at two steps and made at two steps. Because the ATP-forming steps happen twice for each glucose, the count is as follows.

Note: Pyruvic acid is the key product of glycolysis. The net gain is only two ATP for each glucose degraded to pyruvic acid.

What happens to pyruvic acid, and how does fermentation work?

The fate of pyruvate depends on the cellular need. Cells handle pyruvic acid in three major ways: lactic acid fermentation, alcoholic fermentation and aerobic respiration.

Fermentation takes place under anaerobic conditions in many prokaryotes, unicellular eukaryotes and germinating seeds. For the complete oxidation of glucose to CO₂ and H₂O, organisms use the Krebs' cycle, which requires an O₂ supply.

How do the two kinds of fermentation differ?

In fermentation by yeast, glucose is incompletely oxidised under anaerobic conditions, and pyruvic acid is converted to CO₂ and ethanol. Some bacteria produce lactic acid from pyruvic acid. So do animal muscle cells during exercise, when oxygen is inadequate for cellular respiration.

Glucose → glyceraldehyde 3-phosphate (hollow arrows) → 3-phosphoglyceric acid, with NAD⁺ → NADH+H⁺ → phosphoenol pyruvic acid → a large hollow arrow to pyruvic acid. From pyruvic acid one branch rises to lactic acid and the other falls to ethanol + CO₂; on each branch NADH+H⁺ is oxidised to NAD⁺.

See Fig. 12.2 in your NCERT textbook

How much energy does fermentation release?

Not much energy is released in either kind of fermentation: less than seven per cent of the energy in glucose, and not all of it is trapped as high-energy bonds of ATP. The net gain is two ATP per glucose.

The processes are also hazardous, as either acid or alcohol is produced. Yeasts poison themselves to death when the alcohol concentration reaches about 13 per cent. Naturally fermented beverages therefore hold at most about 13 per cent alcohol; stronger drinks are obtained by distillation.

How does pyruvate enter aerobic respiration?

Definition: Aerobic respiration is the complete oxidation of organic substances in the presence of oxygen, releasing CO₂, water and a large amount of the energy present in the substrate.

Aerobic respiration is the most common type in higher organisms. In eukaryotes its steps take place within the mitochondria and require O₂. So pyruvate, the final product of glycolysis, is transported from the cytoplasm into the mitochondria.

What are the two crucial events of aerobic respiration?

• The complete oxidation of pyruvate by the stepwise removal of all its hydrogen atoms, leaving three molecules of CO₂. This takes place in the matrix of the mitochondria.
• The passing on of the electrons removed as part of the hydrogen atoms to molecular O₂, with simultaneous synthesis of ATP. This takes place on the inner membrane of the mitochondria.

How is pyruvate converted to acetyl CoA?

In the mitochondrial matrix, pyruvate undergoes oxidative decarboxylation by a complex set of reactions catalysed by pyruvate dehydrogenase. These need several coenzymes, including NAD⁺ and coenzyme A.

Pyruvic acid + CoA + NAD⁺ → Acetyl CoA + CO₂ + NADH + H⁺ (pyruvate dehydrogenase, Mg²⁺)

One glucose gives two pyruvic acid molecules, so this step produces two NADH per glucose. The acetyl CoA then enters the tricarboxylic acid cycle, more commonly called the Krebs' cycle after Hans Krebs, who first elucidated it.

What happens in the Krebs' cycle?

The tricarboxylic acid (TCA) cycle, or citric acid cycle, runs in the mitochondrial matrix. One turn proceeds as follows.

1. Condensation. The acetyl group condenses with oxaloacetic acid (OAA) and water to yield citric acid. Citrate synthase catalyses this, and a molecule of CoA is released.
2. Isomerisation. Citrate is isomerised to isocitrate.
3. Two decarboxylations. Two successive decarboxylation steps form α-ketoglutaric acid and then succinyl-CoA.
4. GTP formation. As succinyl-CoA becomes succinic acid, a molecule of GTP is synthesised. This is a substrate level phosphorylation. In a coupled reaction, GTP is converted to GDP as ATP is made from ADP.
5. Regeneration of OAA. In the remaining steps, succinic acid is oxidised, through malic acid, back to OAA, so the cycle continues.

In each turn, NAD⁺ is reduced to NADH + H⁺ at three points, and FAD is reduced to FADH₂ at one point. The cycle needs a continued supply of oxaloacetic acid, its first member, and the regeneration of NAD⁺ and FAD from NADH and FADH₂.

Summary equation (mitochondrial matrix):

Pyruvic acid + 4NAD⁺ + FAD + 2H₂O + ADP + Pᵢ → 3CO₂ + 4NADH + 4H⁺ + FADH₂ + ATP

Pyruvate (3C) takes in CoA and gives off CO₂, with NAD⁺ → NADH+H⁺, forming acetyl coenzyme A (2C). This enters a ring labelled "Citric acid cycle" between oxaloacetic acid (4C) and citric acid (6C). Round the ring: citric acid → α-ketoglutaric acid (5C), releasing CO₂ and forming NADH+H⁺ → succinic acid (4C), releasing CO₂, forming NADH+H⁺ and turning GDP into GTP → malic acid (4C), with FAD⁺ → FADH₂ → oxaloacetic acid, with NAD⁺ → NADH+H⁺.

See Fig. 12.3 in your NCERT textbook

What has been gained so far?

For the two pyruvates from one glucose, these steps have released CO₂ and made eight NADH + H⁺ and two FADH₂, besides just two ATP in the TCA cycle. O₂ has not yet come into the picture, and the large yield of ATP has not yet been made.

How does the electron transport system work?

The next steps release and use the energy stored in NADH + H⁺ and FADH₂. These are oxidised through the electron transport system, and the electrons are passed on to O₂, forming H₂O.

Definition: The electron transport system (ETS) is the metabolic pathway through which the electron passes from one carrier to another. It is present in the inner mitochondrial membrane.

In what order do the electrons travel?

1. NADH produced in the matrix during the citric acid cycle is oxidised by NADH dehydrogenase (complex I).
2. The electrons pass to ubiquinone, located within the inner membrane. Ubiquinone also receives reducing equivalents via FADH₂ (complex II), which is generated during the oxidation of succinate in the citric acid cycle.
3. The reduced ubiquinone (ubiquinol) is oxidised, and its electrons go to cytochrome c via the cytochrome bc₁ complex (complex III).
4. Cytochrome c, a small protein attached to the outer surface of the inner membrane, is a mobile carrier of electrons between complex III and complex IV.
5. Complex IV is the cytochrome c oxidase complex, containing cytochromes a and a₃ and two copper centres. The electrons then reach O₂, which forms water.

Three zones are labelled: inter-membrane space (left), inner mitochondrial membrane (a blue band) and matrix (right). Complex I (NADH dehydrogenase) takes NADH + H⁺ and releases NAD⁺; 2e⁻ pass through FMN and (Fe-S) to UQ, forming UQH₂. Complex III (cytochrome bc₁) passes electrons from Cyt b to Fe-S to Cyt c₁ and on to Cyt c. Complex II (succinate dehydrogenase) turns succinate into fumarate and passes electrons via FAD and (Fe-S) to UQ. Cyt c carries e⁻ to complex IV (cytochrome c oxidase): CuA → Cyt a → Cyt a₃ → CuB, where ½O₂ + 2H⁺ form H₂O. Red arrows move 4H⁺ (complex I), 4H⁺ (complex III) and 2H⁺ (complex IV) into the inter-membrane space. At the bottom, ATP synthase (F₀ in the membrane, F₁ on the matrix side) makes ATP from ADP + Pᵢ as H⁺ returns to the matrix, down a marked electrochemical gradient.

See Fig. 12.4 in your NCERT textbook

What is the role of oxygen?

Aerobic respiration takes place only in the presence of oxygen, but the role of oxygen is limited to the terminal stage. Yet oxygen is vital, because it drives the whole process by removing hydrogen from the system.

Oxygen acts as the final hydrogen acceptor. It is the ultimate acceptor of electrons and is reduced to water.

What is oxidative phosphorylation, and how does ATP synthase make ATP?

As electrons pass from carrier to carrier through complexes I to IV, they are coupled to ATP synthase (complex V), which produces ATP from ADP and inorganic phosphate.

The number of ATP made depends on the electron donor. Oxidation of one NADH gives 3 ATP, while one FADH₂ gives 2 ATP.

How is it different from photophosphorylation?

In photophosphorylation, light energy produces the proton gradient required for phosphorylation. In respiration, the energy of oxidation-reduction is used for the same purpose. For this reason, the process is called oxidative phosphorylation.

The mechanism of this membrane-linked ATP synthesis is explained by the chemiosmotic hypothesis.

What are the parts of ATP synthase?

For each ATP produced, 4H⁺ pass through F₀ from the intermembrane space to the matrix, down the electrochemical proton gradient.

A vertical blue band is the inner mitochondrial membrane, with "Outer side" to its left and "Matrix" to its right. F₀ sits in the membrane and the rounded F₁ head projects into the matrix. An arrow labelled 4H⁺ passes from the outer side through F₀ and F₁. Below F₁, ADP and Pi join, and an arrow rises through F₁ to ATP.

See Fig. 12.5 in your NCERT textbook

How many ATP does one glucose yield? The respiratory balance sheet

The net gain of ATP for each glucose oxidised can be calculated, but in reality this remains only a theoretical exercise. It rests on these assumptions:

1. There is a sequential, orderly pathway, with one substrate forming the next, and with glycolysis, the TCA cycle and the ETS following one after another.
2. The NADH synthesised in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation.
3. None of the intermediates in the pathway is used to synthesise any other compound.
4. Only glucose is being respired; no alternative substrates enter the pathway at any of the intermediary stages.

Why are these assumptions not valid in a living system?

In a living system, all pathways work simultaneously, not one after another. Substrates enter the pathways and are withdrawn as and when necessary. ATP is used as and when needed, and enzymatic rates are controlled by multiple means.

Yet the exercise shows the beauty and efficiency of the living system in extracting and storing energy. On these assumptions, there can be a net gain of 38 ATP during the aerobic respiration of one glucose.

How do the 38 ATP add up?

How does fermentation compare with aerobic respiration?

Why is the respiratory pathway called amphibolic?

Glucose is the favoured substrate for respiration, and all carbohydrates are usually first converted into glucose. Other substrates can also be respired, but they do not enter the respiratory pathway at the first step.

Where do other substrates enter the pathway?

Fats, carbohydrates and proteins sit in boxes along the top. Fats → fatty acids and glycerol; two short arrows run down from this box and stop just above the level of dihydroxy acetone phosphate, and a separate line runs to acetyl CoA. Carbohydrates → simple sugars, e.g. glucose → glucose 6-phosphate → fructose 1,6 bisphosphate → dihydroxy acetone phosphate, linked by a double arrow to glyceraldehyde 3-phosphate → pyruvic acid. Proteins → amino acids, with a line into pyruvic acid. Pyruvic acid → acetyl CoA → a circle labelled Krebs' cycle, giving off H₂O and CO₂.

See Fig. 12.6 in your NCERT textbook

Why is "catabolic" not the full picture?

Because respiration breaks down substrates, it has traditionally been considered a catabolic process, and the respiratory pathway a catabolic pathway.

But the compounds at which substrates enter the pathway are the very compounds withdrawn from it to synthesise those substrates. When fatty acids are respired, they are broken down to acetyl CoA. When the organism needs to synthesise fatty acids, acetyl CoA is withdrawn from the pathway.

So the respiratory pathway is involved in both the breakdown and the synthesis of fatty acids. Similarly, respiratory intermediates form the link in the breakdown and synthesis of protein.

Breaking down is catabolism and synthesis is anabolism. Because the respiratory pathway is involved in both, it is better considered an amphibolic pathway than a catabolic one.

What is the respiratory quotient?

During aerobic respiration, O₂ is consumed and CO₂ is released. Comparing the two volumes tells us about the substrate being respired.

Definition: The respiratory quotient (RQ), or respiratory ratio, is the ratio of the volume of CO₂ evolved to the volume of O₂ consumed in respiration.

RQ = volume of CO₂ evolved ÷ volume of O₂ consumed

The RQ depends upon the type of respiratory substrate used.

What is the RQ of carbohydrates, fats and proteins?

When carbohydrates are completely oxidised, the RQ is 1, because equal amounts of CO₂ and O₂ are evolved and consumed.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy; RQ = 6CO₂ ÷ 6O₂ = 1.0

When fats are used, the RQ is less than 1. For tripalmitin, a fat:

2(C₅₁H₉₈O₆) + 145O₂ → 102CO₂ + 98H₂O + energy; RQ = 102CO₂ ÷ 145O₂ = 0.7

When proteins are the respiratory substrates, the ratio is about 0.9.

Note: In living organisms, respiratory substrates are often more than one. Pure proteins or fats are never used as respiratory substrates.

Glossary

• Respiration — The breaking of the C-C bonds of complex compounds through oxidation within the cells, leading to the release of a considerable amount of energy.
• Respiratory substrate — A compound oxidised during respiration; usually a carbohydrate, though proteins, fats and even organic acids can be used in some plants.
• ATP — The energy currency of the cell: it traps the energy released by oxidation and is broken down whenever and wherever energy is needed.
• Glycolysis — The partial oxidation of glucose to two molecules of pyruvic acid in the cytoplasm, through ten enzyme-controlled reactions; also called the EMP pathway.
• Fermentation — The incomplete oxidation of glucose under anaerobic conditions, in which pyruvic acid is converted either to ethanol and CO₂ or to lactic acid.
• Oxidative decarboxylation — The conversion of pyruvate to acetyl CoA in the mitochondrial matrix by pyruvate dehydrogenase, releasing CO₂ and forming NADH + H⁺.
• Krebs' cycle — The tricarboxylic acid (citric acid) cycle in the mitochondrial matrix, which begins when acetyl CoA condenses with oxaloacetic acid and water to form citric acid.
• Substrate level phosphorylation — Formation of GTP or ATP directly during the reaction of a substrate, as when succinyl-CoA is converted to succinic acid.
• Electron transport system — The pathway in the inner mitochondrial membrane through which electrons pass from one carrier to another, finally reaching O₂ to form water.
• Oxidative phosphorylation — ATP synthesis in which the energy of oxidation-reduction, rather than light, produces the proton gradient needed for phosphorylation.
• ATP synthase — Complex V, made of the F₁ headpiece that synthesises ATP and the F₀ channel through which protons cross the inner membrane.
• Amphibolic pathway — A pathway involved in both catabolism and anabolism, as the respiratory pathway is.
• Respiratory quotient — The ratio of the volume of CO₂ evolved to the volume of O₂ consumed in respiration; also called the respiratory ratio.

Common errors and misconceptions

• Misconception: Plants do not respire because they have no respiratory organs. Correct: Plants require O₂ and give out CO₂; gases are exchanged by diffusion through stomata and lenticels.
• Misconception: The energy released by oxidising food is used directly by the cell. Correct: It is released step by step and used to synthesise ATP, which is broken down wherever energy is needed.
• Misconception: Glycolysis happens in the mitochondria and needs oxygen. Correct: Glycolysis occurs in the cytoplasm, without the help of oxygen, in all living organisms.
• Misconception: Oxygen is used at every stage of aerobic respiration. Correct: Its role is limited to the terminal stage, as the final hydrogen acceptor, but it is vital because it drives the whole process.
• Misconception: Every cell gains exactly 38 ATP from each glucose. Correct: 38 ATP is a theoretical figure based on assumptions that do not hold in a living system.
• Misconception: The respiratory pathway is purely catabolic. Correct: Acetyl CoA is withdrawn to synthesise fatty acids, and respiratory intermediates form the link in protein synthesis, so it is amphibolic.
• Misconception: The RQ is 1 whatever the substrate. Correct: It is 1 for carbohydrates, less than 1 for fats (0.7 for tripalmitin) and about 0.9 for proteins.

Exam-style questions with model answers

Q1. Name the two enzymes that convert pyruvic acid into ethanol and CO₂ in yeast. [1 mark]

1. Pyruvic acid decarboxylase and alcohol dehydrogenase catalyse the conversion of pyruvic acid into CO₂ and ethanol during alcoholic fermentation by yeast.

Q2. Why is ATP called the energy currency of the cell? [2 marks]

1. The energy released by oxidising respiratory substrates cannot be used directly; it is released in enzyme-controlled steps and trapped by synthesising ATP.
2. ATP is broken down whenever and wherever energy is needed, so it is the common form in which the cell stores and spends energy.

Q3. Define RQ. What is its value for fats? [2 marks]

1. The respiratory quotient (RQ) is the ratio of the volume of CO₂ evolved to the volume of O₂ consumed in respiration.
2. For fats it is less than 1; for tripalmitin, RQ = 102CO₂ ÷ 145O₂ = 0.7.

Q4. Differentiate between aerobic respiration and fermentation. [3 marks]

1. Fermentation is only a partial breakdown of glucose; in aerobic respiration glucose is completely degraded to CO₂ and H₂O.
2. Fermentation gives a net gain of only two ATP per glucose; many more ATP are generated under aerobic conditions.
3. In fermentation NADH is oxidised to NAD⁺ rather slowly; in aerobic respiration the reaction is very vigorous.
4. Fermentation occurs under anaerobic conditions; aerobic respiration requires O₂.

Q5. What assumptions are made when calculating the net gain of ATP from one glucose? [4 marks]

1. The pathway is sequential and orderly, with one substrate forming the next, and glycolysis, the TCA cycle and the ETS follow one after another.
2. The NADH made in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation.
3. None of the intermediates is used to synthesise any other compound.
4. Only glucose is respired; no alternative substrates enter at any intermediary stage.
5. Since these do not hold in a living system, the net gain of 38 ATP is only theoretical.

Q6. Give a schematic account of glycolysis, showing where ATP and NADH + H⁺ are used or formed. [5 marks]

1. Glycolysis (the EMP pathway) occurs in the cytoplasm; glucose is partially oxidised to two pyruvic acid molecules through ten enzyme-controlled reactions.
2. Glucose → glucose-6-phosphate by hexokinase; one ATP is used.
3. Glucose-6-phosphate isomerises to fructose-6-phosphate, which becomes fructose 1,6-bisphosphate; a second ATP is used.
4. Fructose 1,6-bisphosphate splits into dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL).
5. PGAL → 1,3-bisphosphoglycerate (BPGA): two hydrogen atoms pass to NAD⁺, forming NADH + H⁺, and inorganic phosphate is added.
6. BPGA → 3-phosphoglyceric acid (PGA), forming ATP.
7. PGA → 2-phosphoglycerate → phosphoenolpyruvate (PEP), with loss of water; PEP → pyruvic acid, forming ATP.
8. For one glucose: 4 ATP formed and 2 used, a net gain of 2 ATP, with 2 NADH + H⁺ formed.

Q7. Describe the Krebs' cycle and write its summary equation. [5 marks]

1. In the mitochondrial matrix, pyruvate undergoes oxidative decarboxylation by pyruvate dehydrogenase to give acetyl CoA, CO₂ and NADH + H⁺.
2. The acetyl group condenses with oxaloacetic acid (OAA) and water to form citric acid, catalysed by citrate synthase; CoA is released.
3. Citrate is isomerised to isocitrate; two successive decarboxylations give α-ketoglutaric acid and then succinyl-CoA.
4. Succinyl-CoA → succinic acid, making GTP (substrate level phosphorylation), which gives ATP from ADP.
5. Further oxidation regenerates OAA. NAD⁺ is reduced to NADH + H⁺ at three points and FAD to FADH₂ at one.
6. Pyruvic acid + 4NAD⁺ + FAD + 2H₂O + ADP + Pᵢ → 3CO₂ + 4NADH + 4H⁺ + FADH₂ + ATP.

Q8. Explain the electron transport system and oxidative phosphorylation. [5 marks]

1. The ETS, in the inner mitochondrial membrane, passes electrons from one carrier to another, releasing the energy stored in NADH + H⁺ and FADH₂.
2. NADH is oxidised by NADH dehydrogenase (complex I), and electrons pass to ubiquinone, which also receives reducing equivalents via FADH₂ (complex II).
3. Ubiquinol passes electrons to cytochrome c via cytochrome bc₁ (complex III). Cytochrome c carries them to complex IV, cytochrome c oxidase (cytochromes a and a₃, two copper centres).
4. O₂, the final hydrogen acceptor, is reduced to water.
5. Electron flow is coupled to ATP synthase (complex V): one NADH gives 3 ATP and one FADH₂ gives 2 ATP.
6. Because the energy of oxidation-reduction, not light, builds the proton gradient, this is oxidative phosphorylation. For each ATP, 4H⁺ pass through F₀ to the matrix, and ATP forms at F₁.

Q9. Discuss the statement "The respiratory pathway is an amphibolic pathway." [5 marks]

1. Respiration breaks down substrates, so the pathway has traditionally been considered catabolic.
2. Other substrates enter at different points: fatty acids as acetyl CoA, glycerol as PGAL, and amino acids, after deamination, as pyruvate, acetyl CoA or a Krebs' cycle intermediate.
3. The same compounds are withdrawn to synthesise these substrates. Fatty acids are broken down to acetyl CoA when respired, and acetyl CoA is withdrawn when fatty acids must be synthesised.
4. Respiratory intermediates likewise link the breakdown and synthesis of protein.
5. Since the pathway serves both catabolism and anabolism, it is better called amphibolic.

Key takeaways

• Respiration breaks the C-C bonds of food by oxidation within cells, and the energy released is trapped as ATP, the energy currency of the cell.
• Plants have no respiratory organs; stomata and lenticels allow gas exchange by diffusion, and almost all living cells have surfaces exposed to air.
• Glucose is oxidised in many small enzyme-controlled steps, so that part of the energy is coupled to ATP synthesis instead of being lost as heat.
• Glycolysis in the cytoplasm splits glucose into two pyruvic acid molecules, forming 4 ATP, using 2, and forming 2 NADH + H⁺.
• Without oxygen, pyruvic acid forms ethanol and CO₂ (yeast) or lactic acid (some bacteria, exercising muscle), releasing less than seven per cent of glucose's energy.
• In the mitochondrial matrix, pyruvate becomes acetyl CoA, and each turn of the Krebs' cycle yields three NADH + H⁺, one FADH₂ and one ATP.
• The ETS in the inner mitochondrial membrane passes electrons to O₂; each NADH gives 3 ATP and each FADH₂ gives 2 ATP by oxidative phosphorylation.
• The theoretical net gain is 38 ATP per glucose, but it rests on assumptions that are not valid in a living system.
• The respiratory pathway is amphibolic, and the RQ is 1 for carbohydrates, 0.7 for tripalmitin and about 0.9 for proteins.

Test yourself

What do the Greek words behind "glycolysis" mean?

Glycos means sugar and lysis means splitting, so glycolysis is the splitting of sugar.

Who gave the scheme of glycolysis?

Gustav Embden, Otto Meyerhof and J. Parnas gave the scheme, which is why it is called the EMP pathway.

Which enzyme converts sucrose into glucose and fructose?

Invertase converts sucrose into glucose and fructose, and both readily enter the glycolytic pathway.

At what alcohol concentration do yeasts poison themselves?

Yeasts poison themselves to death when the concentration of alcohol reaches about 13 per cent.

Which enzyme reduces pyruvic acid to lactic acid in exercising muscle?

Lactate dehydrogenase, when oxygen is inadequate for cellular respiration, using NADH + H⁺ as the reducing agent.

What does complex IV contain?

Complex IV, the cytochrome c oxidase complex, contains cytochromes a and a₃ and two copper centres.

How many protons pass through F₀ for each ATP made, and in which direction?

Four H⁺ pass through F₀ from the intermembrane space to the matrix, down the electrochemical proton gradient.

What is the significance of the step-wise release of energy in respiration?

Oxidising glucose in several small steps lets some steps release just enough energy to be coupled to ATP synthesis, so not all of it is lost as heat.