An Introduction to Cellular Respiration

Summary

Respiration describes the mechanism by which cells break down food into usable cellular energy
ATP is the key molecule in this process, where it acts as a currency for cellular energy
Respiration consists of 4 steps: glycolysis, pyruvate oxidation, the Kreb’s cycle and the electron transport chain.
Aerobic respiration occurs when oxygen is present and is efficient in producing cellular energy, anaerobic respiration occurs when the cell lacks an oxygen supply, and is not as efficient at generating cellular energy.

Respiration is the process of breaking down organic molecules to harvest chemical energy. For organisms such as humans, it involves taking in food, breaking it down, and turning into useful energy that our cells can use to perform many tasks. This is most efficiently done in the presence of oxygen to make an important molecule called ATP (adenosine triphosphate).

Glucose is the compound most often used to generate cellular energy. During cellular respiration, a glucose molecule is completely oxidised, releasing high energy electrons. The glucose molecule itself is broken down into carbon dioxide and water, and during this process, ATP is produced. ATP is a very important molecule in the process of respiration.

What is ATP?

ATP is the principal molecule for storing and transferring energy in cells, often known as the “molecular currency” of energy transfer. ATP is a small molecule with 3 phosphate groups attached to an adenosine molecule. ATP can be used to store energy for future reactions and can provide an immediate source of energy for reactions needed by the cell. Animals store the energy obtained from the breakdown of food as ATP.

ATP generates energy when it is broken down. When one of the phosphate groups is removed in a process called hydrolysis, ATP is converted to adenosine diphosphate (ADP) and energy is released. Energy is also released when a second phosphate is removed from ADP to form adenosine monophosphate (AMP). The release of this energy can be used to facilitate chemical reactions in the cell.

NAD+/FAD and the importance of redox reactions

NAD+ (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide).are other important molecules in the process of cellular respiration, acting as important electron carriers. Electron carriers are small organic molecules that play key roles in cellular respiration, where they shuttle electrons between molecules.

There are two types of electron carriers that are particularly important in the process of respiration: NAD+ and FAD. When NAD+ and FAD pick up electrons, they also gain one or more hydrogen atoms, switching to a slightly different form (NADH and FADH2), and when they lose electrons, they go back to their original form. The reactions in which NAD+ FAD gain or lose electrons are examples of a class of reactions called redox reactions, which refers to reactions involving electron transfer. Cellular respiration involves many of these reactions.

Summary of the process of respiration

Respiration is a four stage process, consisting of glycolysis, pyruvate oxidation, the Kreb’s cycle and the electron transport chain.

Glycolysis occurs in the cytoplasm, converting a glucose sugar into two 3 carbon molecules called pyruvate. A net of 2 molecules of ATP, 2 molecules of pyruvate, and 2 molecules of NADH are produced during the reaction. When oxygen is present, the pyruvate and NADH enter the mitochondria, and the next stage can begin.

The next stage involves the movement of pyruvate into the mitochondria where it is oxidised and converted into Acetyl-CoA. In this process of pyruvate oxidation, electrons are transferred to NAD+, making NADH, and a carbon is lost, forming carbon dioxide.

The next stage is called the Citric Acid Cycle or the Kreb’s Cycle, which takes place in the mitochondrial matrix. It completes the breakdown of glucose by oxidising acetyl-coA to carbon dioxide. Acetyl-CoA binds to the starting compounds in the cycle, and through series of redox reactions, all carbons, hydrogens and oxygens in pyruvate ultimately end up as carbon dioxide and water. 8 NADH, 2 FADH2, 2 ATP and 6 CO2 are produced for each glucose molecule at this stage of cellular respiration.

The NADH and FADH2 go to the next stage, the electron transport chain. This chain consists of a series of membrane bound carriers in the inner membrane of the mitochondria that pass electrons from one acceptor to another. As the electrons are transferred, the cell is able to capture the energy produced to generate ATP. Oxygen acts as the terminal electron acceptor, and is reduced to form water, a by-product of the respiration reaction.

All of the high energy carriers from previous stages bring their electrons into the electron transport chain. The bulk of energy generated during cellular respiration is produced here, up to 38molecules of ATP.

Aerobic and anaerobic respiration

Cellular respiration refers to both aerobic and anaerobic respiration, but is often synonymous with aerobic respiration. Both aerobic and anaerobic respiration involve chemical reactions which take place in the cell to produce energy, which is needed for active processes. Aerobic respiration requires oxygen to fully oxidise the organic molecule. This releases lots of energy. Anaerobic respiration involves the breakdown of glucose without oxygen. This releases much less energy. Anaerobic respiration is useful in tissues which have a high energy demand such as in muscles, in which there is not enough oxygen to produce all the energy needed by using aerobic respiration alone. Anaerobic respiration takes place in the cell cytoplasm and produces lactic acid.

1. Incomplete oxidation of glucose into pyruvic acid with several intermediate steps is known as

(a) TCA-pathway

(b) glycolysis

(d) Krebs cycle.

Answer and Explanation:

1. (b): Glycolysis is the biochemical change in which one molecule of glucose is converted into 2 molecules of pyruvic acid with the involvement of ten enzymes. It is independent of oxygen and is common to both aerobic and anaerobic condition. It takes place in cytoplasm and all the reactions are reversible.

All the intermediates of glycolysis are not converted into pyruvic acid. Some of them build back the carbohydrates and the phenomenon is called as oxidative anabolism. TCA cycle and Krebs cycle are synonym where the pyruvic acid of glycolysis is utilized to form CO2. HMS is hexose monophosphate shunt or pentose phospate pathway which is an alternative pathway of glycolysis.

2. NADP+ is reduced to NADPH is

(a) HMP

(b) Calvin Cycle

(d) EMP. (1988)

Answer and Explanation:

2. (a): HMP pathway generates NADPH molecule which are used as reductants in biosynthetic process under conditions when NADPH molecules are not generated by photosynthesis. It is, therefore, important in non- photosynthetic tissues such as in differentiating tissues, generating seeds and during periods of darkness. Production of NADPH is not linked to ATP generation in pentose phosphate pathway.

3. End product of glycolysis is

(a) acetyl CoA

(b) pyruvic acid

(d) fructose I-phosphate.

Answer and Explanation:

3. (b): In glycolytic cycle, each molecule of glucose (a hexose sugar) is broken down in step wise biochemical reactions under enzymatic control into two molecules of pyruvic acids. It takes place is cytosol.

4. R.Q- is ratio of

(a) CO2 produced to substrate consumed

(b) CO2 produced to O2 consumed

(d) oxygen consumed to CO2 produced.

Answer:

(b) CO2 produced to O2 consumed

5. EMP can produce a total of

(a) 6 ATP

(b) 8 ATP

(d) 38 ATP.

Answer and Explanation:

5. (b): Glycolysis is also known as EMP pathway after the names of its discoverers. Embden, Meyerhof and Paranas. In glycolysis, 8ATP are produced. 4ATP are formed from substrate level phosphorylation, out of which 2ATP are used up and net gain of 2 AT P. 6ATP are produced from oxidative phosphorylation. Hence, Total ATP produced in glycolysis is 8ATP.

6. Connecting link between glycolysis and Krebs cycle before pyruvate entering Krebs cycle is changed to

(a) oxaloacetate

(b) PEP

(d) acetyl CoA.

Answer and Explanation:

6. (d): End product of glycolysis is pyruvic acid which is converted into acetyl coA before entering into the Krebs cycle, which is aerobic in nature.

7. Terminal cytochrome of respiratory chain which donates electrons to oxygen is

(a) Cyt. b

(b) Cyt. c

(d) Cyt. a3.

Answer and Explanation:

7. (d): Cytochrome a3 helps in transfer of electron to oxygen. The oxygen has great affinity to accept the electrons and in presence of protons a water molecule is formed (figure).

8. Out of 36 ATP molecules produced per glucose molecule during respiration

(a) 2 are produced outside glycolysis and 34 during respiratory chain

(b) 2 are produced outside mitochondria and 34 inside mitochondria

(d) All are formed inside mitochondria.

Answer and Explanation:

8. (b): During respiration, 36 ATP molecules are produced per glucose molecule. 2 molecules of ATP are produced outside mitochondria i.e. during glycolysis and other 34 molecules of ATP are produced inside mitochondria from Krebs cycle.

9. Link between glycolysis, Krebs cycle and P-oxidation of fatty acid or carbohydrate and fat metabolism is

(a) oxaloacetic acid

(b) succinic acid

(d) acetyl CoA.

Answer and Explanation:

9. (d): Krebs cycle is intimately related with fat metabolism. Dihydroxy acetone phosphate produced in glycolysis may be ‘converted into glycerol via glycerol – 3 – phosphate and vice-versa. Glycerol is important constituents of fats. After P-oxidation, fatty acids give rise to active – 2 – C units, the acetyl-CoA which may enter the Krebs cycle. Thus, Acetyl-CoA is a link between glycolysis, Krebs cycle and P- oxidation of fatty acid or carbohydrate and fat metabolism.

10. End products of aerobic respiration are

(a) sugar and oxygen

(b) water and energy

(d) carbon dioxide and energy.

Answer and Explanation:

10. (c): The food substances in living cells are oxidised in presence of oxygen, it is called aerobic respiration. Complete oxidation of food matter (1 .mole of glucose) occurs releasing 686 Kcal of energy. The ends of products formed are CO2 and H2O.

11. At a temperature above 35°C

(a) rate of photosynthesis will decline earlier than that of respiration

(b) rate of respiration will decline earlier than that of photosynthesis

(d) both decline simultaneously.

Answer and Explanation:

11. (a): The plants can perform photosynthesis on a range of temperature, while some cryophytes can do photosynthesis at 35°C. Usually the plants can perform photosynthesis between 10°C – 40°C. The optimum temperature ranges between 25°C – 30°C. At high temperature the enzymes are denatured and hence the photosynthetic rate declines.

12. Oxidative phosphorylation is production of

(a) ATP in photosynthesis

(b) NADPH in photosynthesis

(d) NADH in respiration.

Answer and Explanation:

12. (c): In electron transport system the hydrogen donated by succinate is accepted by FAD which is reduced to FADH2. This hydrogen dissociates into electrons and protons and then passes through a series of carriers involving the phenomenon of oxidation and reduction. During this flow, ATP synthesis occurs at different steps and the phenomenon is called as oxidative phosphorylation.

13. Apparatus to measure rate of respiration and R.Q. is

(a) auxanometer

(b) potometer

(d) manometer.

Answer and Explanation:

13. (c): Respirometer is an instrument used for measuring R.Q and rate of respiration. The apparatus consists of a graduated tube attached at right angles to a bulbous respiratory chamber in its upper end. Desired plant material who’s R.Q is to be determined is placed in the respiratory chamber.

14. R.Q. is

(a) C/N

(b) N/C

(c) CO2/O2

(d) O2/CO2.

Answer:

(c) CO2/O2

15. End product of citric acid cycle/Krebs cycle is

(a) citric acid

(b) lactic acid

(d) CO2 + H2O.

Answer and Explanation:

15. (d): The end product of glycolysis is pyruvic acid whereas acetyl CoA is the connecting link between glycolysis and Krebs cycle. The TCA cycle was first described by Krebs, 1937 as a cyclic process in which acetyl coA is oxidised to C02 and water. Acetyl CoA combines with oxalo acetic acid to form citric acid. After a series of cyclic reactions OAA is recycled back.

16. Out of 38 ATP molecules produced per glucose, 32 ATP molecules are formed from NADH/FADH2 in

(a) respiratory chain

(b) Krebs cycle

(d) EMP.

Answer and Explanation:

16. (a): During respiratory chain, complete degradation of one glucose molecule produced 38 ATP molecules. NAD and FAD is reduced to NADH/FADH2.

17. Life without air would be

(a) reductional

(b) free from oxidative damage

(d) anaerobic.

Answer and Explanation:

17. (d): Anaerobic respiration (absence of oxygen) takes place in anaerobic bacteria and in plant seeds. Anaerobic respiration occurs in the organism which can live without oxygen. In this respiration, only glycolysis takes place due to the absence of oxygen.

18. The First phase in the breakdown of glucose, in animal cell, is

(a) fermentation

(b) Krebs cycle

(d) E.T.S.

Answer:

19. When yeast ferments glucose, the products obtained are

(a) ethanol and CO2

(b) methanol and CO2

(d) water and CO2.

Answer:

(a) ethanol and CO2

20. The ultimate respiratory substrate, yielding maximum number of ATP molecules, is

(a) glycogen

(b) ketogenic amino acid

(d) amylose.

Answer and Explanation:

20. (c): Glucose is the chief respiratory substrate which fields maximum number of ATP molecules. Glucose is the most common substate in glycolysis. Any other carbohydrate is first converted into glucose. During glycolysis it changes to pyruvic acid and net gain is of 2 ATP and 2 NADH2molecules. And later on during Krebs cycle 30 molecules of ATP are produced. So a total of 38 ATP molecules are produced from 1 mol of glucose during aerobic respiration.

21. Poisons like cyanide inhibit Na+ efflux and K+ influx during cellular transport. This inhibitory effect is reversed by an injection of ATP. This demonstrates that

(a) ATP is the carrier protein in the transport system

(b) energy for Na+-K+ exchange pump comes from ATP

(d) Na+-K+ exchange pump operates in the cell.

Answer and Explanation:

21. (b): Active transport is uphill movement of materials across the membrane where the solute particles move against their chemical concentration or electrochemical gradient. Hence the transport requires energy in the form of ATP. Metabolic inhibitors like cyanide inhibit absorption of solutes by lowering the rate of respiration. Consequently less ATP are formed. However, by adding ATP, active transport is facilitated.

It occurs in plants as in climacteric fruits and under cold stress. ATP synthesis does not occur. Reducing power present in reduced coenzymes is oxidised to producc heat energy. Therefore, the heat liberation pathway of terminal oxidation is cyanide resistant.

In normal aerobic respiration, the effect of cyanide poisoning can be minimised by immediate supply of ATP.

22. When one molecule of ATP is disintegrated, what amount of energy is liberated?

(a) 8 kcal

(b) 38 kcal

(d) 4.5 kcal.

Answer and Explanation:

22. (c): ATP is adenosine triphosphate. It was discovered by Lohmann in 1929. It consists of a purine, adenine, a pentose sugar (ribose) and a row of three phosphates out of which the last two are attached by high energy bonds. The last phosphate bond yields an energy equivalent of 7 kcal.

However the latest concept holds that an energy equivalent of 8.15 kcal per mole is released.

23. At the end of glycolysis, six carbon compounds ultimately changes into

(a) ethyl alcohol

(b) acetyl Co-A

(d) ATP.

Answer and Explanation:

23. (c): Glycolysis or EMP pathway is the breakdown of glucose to two molecules of pyruvic acid through a series of enzyme mediated reaction releasing energy. Pyruvic acid is a 3-carbon compound. In glycolysis net gain of 2ATP and 2 NADH2 molecules occurs. It can be represented in equation form as –

C6H12O6 + 2ADP + 2Pi + 2NAD

2CH3COCOOH + 2 ATP + 2 NADH2

24. Which of the following products are obtained by anaerobic respiration from yeast?

(a) beer and wine

(b) alcohols

(d) all of these.

Answer and Explanation:

24. (d): In the absence of O2, fermentation or anaerobic respiration occurs. The cells of yeast contain zymase complex enzyme that are capable of fermentation. It is completed in cytoplasm. In this process pyruvic acid forms ethyl alcohol and CO2.

Brewing is the name given to the combined process of preparing beverages from infusions of grains that have undergone sprouting (malting) and the fermenting of the sugary solution by yeast, whereby a portion of the carbohydrate is changed to alcohol and carbondioxide various types of beer, whisky and wine are produced. Wine is the product made by normal fermentation of the juice of ripe grapes (Vitis vinifero) using a pure culture of yeast.

25. The end products of fermentation are

(a) O2 and C2H5OH

(b) CO2 and acetaldehyde

(d) CO2 and C2H5OH.

Answer and Explanation:

25. (d): Fermentation or anaerobic respiration occurs in the absence of 02. It involves breakdown of organic substance particularly carbohydrates under anaerobic conditions to form ethyl alcohol and carbon dioxide. It can be represented in equation form as

C6H12O6 2CH3 CH2OH + 2CO2

26. In Krebs’ cycle, the FAD precipitates as electron acceptor during the conversion of

(a) fumaric acid to malic acid

(b) succinic acid to fumaric acid

(d) a-ketoglutarate to succinyl CoA.

Answer:

(b) succinic acid to fumaric acid

27. Which of the following is the key intermediate compound linking glycolysis to the Krebs’ cycle?

(a) malic acid

(b) acetyl CoA

(d) ATP.

Answer and Explanation:

27. (b): During glycolysis pyruvic acid is produced from glucose and is oxidatively decarboxylated to form acetyl CoA. This formation of acetyl CoA from pyruvic acid needs a multienzyme complex and 5 essential cofactors, i.e. lipoic acid, CoA, Mg2+, NAD and TPP (thiamine pyrophosphate).

It results in production of 2 molecules of CO2 and 2 molecules of NADH2. This acetyl CoA enters mitochondria and is completely oxidised during Kreb’s cycle. Thus acetyl CoA acts as the linker of glycolysis and Kreb’s cycle.

28. Net gain of ATP molecules, during aerobic respiration, is

(a) 40 molecules

(b) 48 molecules

(d) 38 molecules.

Answer:

29. Organisms which obtain energy by the oxidation of reduced inorganic compounds are called

(a) photoautotrophs

(b) chemoautotrophs

(d) coproheterotrophs.

Answer and Explanation:

29. (b): Chemoautotrophs are organisms that are capable of manufacturing their organic food utilizing chemical energy released in oxidation of some inorganic substances. The process of manufacture of food in such organisms is called chemosynthesis. It includes some acrobic bacteria. Photoautotrophs obtain energy for their synthesis of food from light.

Fungi living on dead or decaying plant or animal remains and also growing on dung of herbivores are saprophytes.

30. How many ATP molecules are produced by aerobic oxidation of one molecule of glucose?

(a) 2

(b) 4

(d) 34.

Answer and Explanation:

30. (c): Refer answer 29

31. In which one of the following do the two names refer to one and the same thing?

(a) Krebs cycle and Calvin cycle

(b) tricarboxylic acid cycle and citric acid cycle

(d) tricarboxylic acid cycle and urea cycle

Answer and Explanation:

31. (b): The reactions of Krebs cycle were worked out by Sir Hans Kreb, hence the name Krebs cycle. It involves many 3-C compounds such as citric acid, cis-aconitic acid and iso-citric acid etc. so it is called TCA cycle tricarboxylic acid cycle. It involves formation of citric acid as its first product so it is called citric acid cycle. It involves production of 24 ATP molecules.

32. In alcohol fermentation

(a) triose phosphate is the electron donor while acetaldehyde is the electron accept

(b) triose phosphate is the electron donor while pyruvic acid is the electron acceptor

(d) oxygen is the electron acceptor

Answer:

(a) triose phosphate is the electron donor while acetaldehyde is the electron accept

33. In glycolysis, during oxidation electrons are removed by

(a) ATP

(b) glyceraldehyde-3-phosphate

(d) molecular oxygen.

Answer and Explanation:

33. (c): During glycolysis NAD (Nicotinamide adenine dinucleotide) removes electrons from 1, 3- diphosphoglyceric acid using diphosphoglycrealdehyde dehydrogenase. NAD changes to NADH2 and this is either utilized as such in anaerobic respiration or in the presence of oxygen.

34. During which stage in the complete oxidation of glucose are the greatest number of ATP molecules formed from ADP?

(a) glycolysis

(b) Kreb’s cycle

(d) electron transport chain.

Answer and Explanation:

34. (d): The last step of aerobic respiration is the oxidation of reduced coenzymes, i.e., NADH2 and FADH2 by molecular oxygen through FAD, ubiquinone, cyt. f, cyt. c, Cyt c,, Cyt. a and cyt. ay By oxidation of 1 molecule of NADH,, 3ATP molecules are produced and by oxidation of 1 molecule of FADH2 2 ATP molecules are produced.

In glycolysis 2 ATP molecules are produced from ADP. Further 2NADH2produced, give 2×3=6 ATP, on oxidative phosphorylation. Similarly in Kreb’s cycle 2 ATP molecules are produced. So the greatest numbers of ATP molecules are produced in the electron transport chain.

35. How many ATP molecules could maximally be generated from one molecule of glucose, if the complete oxidation of one mole of glucose to C02 and H20 yields 686 kcal and the useful chemical energy available in the high energy phosphate bond of one mole of ATP is 12 kcal?

(a) 1

(b) 2

(d) 57.

Answer and Explanation:

35. (d): One mole of ATP liberates 12 kcal of energy. So 686 kcal will be liberated by 686/12 = 57.1 ATP molecules.

36. All enzymes of TCA cycle are located in the mitochondrial matrix except one which is located in inner mitochondrial membranes in eukaryotes and in cytosol in prokaryotes. This enzyme is

(a) isocitrate dehydrogenase

(b) malate dehydrogenase

(d) lactate dehydrogenase.

Answer and Explanation:

36. (c): Mitochondrion is the organelle which bears various enzymes participating in Krebs cycle. Each mitochondrion is covered by double membrane. The inner membrane is selectively permeable and forms foldings called cristae. The inner membrane bears oxysomes, enzymes of fatty acids, succinate dehydrogenase (of Krebs cycle) and electron transport system. All other enzymes of Krebs cycle are present in the mitochondrial matrix.

37. The overall goal of glycolysis, Krebs cycle and the electron transport system is the formation of

(a) ATP in one large oxidation reaction

(b) sugars

(d) ATP in small stepwise units.

Answer and Explanation:

37. (d): Respiration is an energy liberating enzymatically controlled multistep catabolic process of step wise breakdown of organic substances (hexose sugar) inside the living cells. Aerobic respiration includes the 3 major process, glycolysis, Krebs cycle and electrons transport chain. The substrate is completely broken down to form CO2 and water. A large amount of energy is released stepwise in the form of ATP.