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Differentiate between Respiration and combustion.





1. Nature of process

Biochemical and stepped.

Physico- chemical and spontaneous.

2. Site of occurrence Inside the cells. Not cellular.
3. Control Biologically controlled Uncontrolled.
4. Energy release Energy released in steps. Energy released simultaneously.
5. Temperature Remains within limits. Rises very high.
6. Light No light is produced. Light may be produced.
7. Enzymes Controlled by enzymes. Not controlled by enzymes.
8. Intermediates Several intermediates are produced. No intermediate is produced.
9. ATP formation Most of the energy is trapped in ATP molecules. No ATP is formed during combustion.



What are respiratory substrates? Name the most common respiratory substrate.

Respiratory substrate. The organic substances, which undergo catabolic reaction in the living cells to release energy during respiration, are known as respiratory substrates.

Glucose is the most common substrates used in cellular respiration. Glycogen and sugars other than glucose are broken down to glucose and fed into glycolysis.


Give the schematic representation of glycolysis.



What are the main steps in aerobic respiration? Where does it take place?

The main steps in aerobic respiration are: -

1. Glycolysis – This takes place in the cytoplasm.

2. The complete oxidation of pyruvate by the stepwise removal of all the hydrogen atoms leaving three molecules of CO2 . This takes place in the mitochondria

3. The passing on of the electrons removed as part of the hydrogen atoms to molecular O2 with simultaneous synthesis of ATP. This also takes place in the mitochondria.


Give the schematic representation of an overall view of Krebs cycle.



Explain ETS.

Electron transport system is a series of enzymes and cytochromes operative in inner mitochondria membrane. The pairs of hydrogen atoms removed from respiratory intermediates by dehydrogenation reactions during glycolysis and Kreb’s cycle do not directly react with oxygen. These are used to reduce pH acceptor molecules such as NAD+ and FAD to NADH2 and FADH2.

These reduced coenzymes releases the protons (H+) in the miitochondrial matrix, while the electrons (e-) are channeled into electron- transport chain. (mitochondria respiratory chain).

ETCs a set of seven electron – carries present in a specific sequence along inner mitochondrial membrane.

These seven electron – carriers function in a specific sequence and are:

Nicotinamide Adenine Dinucleotide (NAD).

Flavin Mononucleotide (FMN).

Co- enzyme Q or Ubiquinone.

Cytochrome – b

Cytochrome – c,

Cytochrome –a and

Cytochrome –a3.

It is also called cytochrome – system (CS), as four out of these seven carriers are cytochromes. Transport of electron – carriers is a down hill journey or a descending stairway. In this transport, the electrons tend to flow from electro- negative to electro- positive system, so there is a decrease in free energy and some energy is released and amount of energy with the electrons goes on decreasing. During electron transfer the electron – donor gets oxidised, while electron – acceptor gets reduced so these transfers involve redox – reaction and are catalysed by enzymes called reeducates. Oxidation and reduction are complimentary. This oxidation- reduction reaction over the ETC is called biological oxidation.

Electron – donor.. e- + electron – acceptor
Here, electron – donor and electron acceptor form redox pair.

During the electron transfer, the energy released at some steps is so high that ATP is formed by the phosphorylation of ADP in the presence of enzyme ATP synthetase present in head of F2 – particles present on the miitochondrial crista. This process of ATP synthesis during oxidation of coenzymes is called oxidative Phosphorylation, so ETC is also called oxidative phosphorylation pathway.

Role of oxygen. From the cytochrome a3, two electrons are received by oxygen atom, which also receives two protons (H+) from the mitochondrial matrix and from water molecule.

So the final acceptor of electrons is oxygen.

So the reaction H2 + ½ O2 H2O (called metabolic water) is made is occur in many steps through ETC, so that most of the energy can be derived into a storage and usable form.


Distinguish between the following:

(a) Aerobic respiration and anaerobic respiration

(b) Glycolysis and fermentation

(c) Glycolysis and citric acid cycle.

(a) Differences between aerobic respiration and anaerobic respiration

Aerobic respiration

Anaerobic respiration

1.It takes place in the presence of oxygen.

1.it takes place in the absence of oxygen.

2.It is completed in cytoplasm and mitochondria of cells.

2.It is completed in the cytoplasm only.

3.In involves the complete oxidation of glucose into CO2 and H2O. 3.It involves the incomplete oxidation of glucose into CO2 and alcohol.

4.One molecule of glucose on complete oxidation releases 2870 kJ of energy.

4.One molecule of glucose on oxidation releases 247 kJ of energy.

(b) Difference between glycolysis and fermentation



1. It takes place in all organisms

1. It occurs in some microorganisms and fungi.

2. Glycolysis can function in the presence or absence of O2. 2. Fermentation takes place only in absence of oxygen.

Differences between Glycolysis and citirc acid cycle.



Citric Acid Cycle (Kreb’s Cycle)

1.Enzymes for glycolysis occurs inside the cytoplasm.

1.Kreb’s cycle operates inside mitochondria because its enzymes are present in matrix.

2.Glycolysis is the first step of respiration in which glucose is completely broken down to the level of pyruvic acid. 2.Kreb’scycle is a cyclic process, which completely breaks an active acetyl group through decarboxylation and dehydr-genation.
3.The process is common in both aerobic and anaerobic modes of respiration. 3.It occurs only in aerobic mode of respiration.
4.Glycolysis yields 2 molecules of pyruvic acid. 4.It degrades pyruvic acid completely into inorganic substances (CO2 + H2O).
5.It forms two molecules of NADH and two molecules of ATP for every molecule of glucose broken down. 5.Kreb’s cycle produces four molecules of NADH2, 2 molecules of NADPH2for every two molecules or acetyl CoA oxidised by it. Two more molecules of NADH2 are liberated during conversion of pyruvic acid to Acetyl CoA.
6. No carbon dioxide is evolved in glycolysis. 6. Carbon dioxide is evolved in kreb’s cycle.



What are the assumptions made during the calculation of net gain of ATP?

It is possible to make calculations of the net gain of ATP for every glucose molecule oxidised. But in reality this can remain only a theoretical exercise. These calculations can be made only on certain assumptions that:

1. There is a sequential, orderly pathway functioning, with one substrate forming the next and with glycolysis, TCA cycle and ETS pathway 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 are utilised to synthesise any other compound.

4. Only glucose is being respired – no other alternative substrates are entering in the pathway at any of the intermediary stages


Discuss "The respiratory pathway is an amphibolic pathway".

Because the respiratory pathway is involved in both anabolism and catabolism, it would hence be better to consider the respiratory pathway as an amphibolic pathway. Fatty acids would be broken down to acetyl CoA before entering the respiratory pathway when it is used as a substrate. But when the organism needs to synthesis fatty acids, acetyl CoA would be withdrawn from the respiratory pathway for it. Hence the respiratory pathway comes into the picture both during breakdown and synthesis of fatty acids. Similarly during breakdown and synthesis of protein too, respiratory intermediates form the link. Breaking down processes within the living organism is catabolism, and synthesis is anabolism.


Define R.Q. What is its value for fats?

Respiratory Quotient (R.Q). The ratio of volume of CO2 evolved to the volume of oxygen consumed per unit time per unit weight. R.Q. indicates the type of respiratory substrate being used and type of respiration being performed R.Q. value is one for aerobic respiration of glucose; more than one for organic acids (e.g. 1.3 for malic acid and 1.14 for succinic acid) less than one for fats (0.7) and proteins (0.85); infinity for anaeobic respiration (as no oxygen is used). It can be measured by respiirometer.


What is Oxidative phophorylation.

The generation of ATP from ADP during electron transport system by utilizing the energy obtained during oxidative reaction is called oxidative phosphorylation.


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

Anaerobic and aerobic respiration are conducted in many steps and energy is released in steps. The advantages of stepwise release of energy are:

1. It facilitates the utilization of the relatively higher proportion of the energy in ATP synthesis.

2. Activities of enzymes for the different steps may be enhanced or inhibited by specific compounds. This provides a mean of controlling the rate of the pathway and the energy output according to the need of the cell.

3. The same pathway may be utilized for forming intermediates used in the synthesis of other biomolecules like amino acids.

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