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Steps of Respiration


Table of Content

1. Glycolysis - EMP - (Embden, Meyerhof, Pamas) Pathway: 

The glycolysis is common phase for aerobic & anaerobic respirations both.

Glycolysis involves a series of ten biochemical reactions in cytoplasm.

In glycolysis, neither consumption of oxygen nor liberation of CO2 take place.

In glycolysis, 1 glucose, produces 2 mol. (If pyruvic acids (3C)

2NADH2 & 2ATP are generated in glycolysis, which are equal to 8 ATP.

Substrate level phosphorylation forms 4 ATP:

(When the substrate releases energy' for phosphorylation of ADP OR formation of ATP, without ETS then called as substrate level phosphorylation.

Glycolysis is also known as oxidative anabolism or catabolic resynthesis, because it links with anabolism of fats and amino acids. Anintermediate PGAL is used for the synthesis of glycerol later forms fats or lipid. PGA is used for synthesis of Serine, Glycine, Cystine, Alanine forms from pyruvate. 

Biochemical reactions of Glycolysis 

Phosphofructokinase is an allosteric enzyme. The phosphorylation of fructose 6 phosphate is the most important control of glycolysis.

Phosphofructokinase has multiple allosteric modulator. It's activity is inhibited by ATP (-ve modulator) and stimulated by ADP & AMP (+ve modulator). Most of the biochemical reactions catalysed by allosteric enzymes are irreversible type and these are control point of glycolysis.

Further oxidation of pyruvic acid and NADH2 after glycolysis in mitochondria requires oxygen. So the fate of pyruvic acid is decided by presence or absence of oxygen. 

2. Formation of Acetyl-Co-A (Link / Gateway Reaction): 

When respiration is aerobic, then pyruvic acid is oxidised to form

2C compound - Acetyl Co-A. It occurs in presence of O2 and CO2 is released first time during it.

Acetyl Co-A is a connecting link between glycolysis & Krebs-cycle. Decarboxylation and dehydrogenation (Oxidative decarboxylation) take place during formation of acetyl Co-A

Acetyl eo-A is formed in perimitochondrial space by enzyme pyruvate dehydrogenase complex. (Mg++, LA (Lipoic Acid), TPP (Thiamine pyrophosphate), NAD, CoA)

2-PA (Pyruvic acid) + 2Co-A 

Acetyl Co-A is also common intermediate between fat & carbohydrate metabolism. 

3. Kreb Cycle / TCA (Tricarboxylic Add) Cycle / Citric Acid Cycle: 

This cycle was discovered by H.A. Kreb. (Nobel prize).

TCA cycle occurs in mitochondrial matrix or power house of cell

Kreb cycle begins by formation of citric acid [TeA (Tri carboxylic acid)) & O.A.A. is the acceptor molecule of acetyl CoA in Kreb's cycle.

A number of Krebs cycle intermediates are used in synthetic (anabolic) pathways, thus TCA cycle is also called amphibolic pathway or anaplerotic pathway. 

Succinyl Co A is important for synthesis of prophyrin ring compounds like chlorophylls, phytochromes, cytochromes, haemoglobin etc.

a-ketoglutaric acid (SC) Is involved in amino acid formation (Nitrogen-metabolism)

Oxidation occurs at 4 sites in Kreb's cycle.

3NADH2, 1 FADH2 & 1GTP (ATP) produced by each turn of TCA cycle. (= 12 ATP)

All the enzymes of TCA cycle, except malic enzyme, Succinic dehydrogenase (on inner mitochondrial membrane) present in matrix. 

Kreb cycle
Bio chemical reactions in Krebs cycle

4. ETS & Oxidative Phosphorylation: 

(Terminal oxidation of NADH2 & FADH2).

ETS (Respiratory chain) consists of four components


Fe-S Protein,

Co-Q (UQ) &


Cytochrorries are cyto.-b. cyto. - C1 & cyto. - C, cyto-a & eyto a3.

UQ and Cyto. c are mobile e- carriers in mitochondrial ETS. (PQ and PC is mobile in Z­scheme)

Cytochrome a3 is last cytochrome in respiratory chain or electron transport chain (ETC).

O2 is last e- acceptor in oxidative phosphorylation & due to this metabolic water is formed.

During the ETS, NADH2 gives it s 2e- /2H+ to FMN in respiratory chain, thus 3 ATP are generated, while FADH2 give it's 2e-,2H+ to CoQ hence only 2 ATP are formed during the process of oxidative phosphorylation.

Enzyme cytochrome oxidase is responsible for oxidation of cyto. a3 & reduction of O2.

Enzyme cytochrome oxidase has cyto. a & a3 as its components. (Cu present in cyto a and cyto. a3)

Cytosolic or extra mitochondrial or glycolytic 2NADH2 comes at ETS by two type of shuttles (Only in eukaryotes):

(a) Glycerol phosphate shuttle: In brain/muscle cells.

Glycolytic/Cytosolic NADH2 + DHAP

Glycerol 3-Phosphate + NAD

Glycerol 3 Phosphate + FAD +  FADH2 + DHAP

2NADH2 → 4 ATP \ 1 Glucose = 36 ATP

(b) Malate asparate shuttle:

Glycolytic/Cytosolic NADH2 + OAA

 → Malate + NAD

Malate  →  NADH2 + OAA

2NADH2   →  6 ATP     ( 1 Glucose = 38 ATP)

It prokaryotes shuttle mechanism is absent. They always get 38 ATP from aerobic respiration of 1 glucose mol.

Bioenergetic of respiration – (1 mol of glucose)

(1) EMP pathway:

(i) ATP forms at substrate level phosphorylation Þ  4 ATP

(ii) ATP produces via ETS 2NADH2 Þ 6 ATP

(iii) ATP consumed in glycolysis Þ 2 ATP

10 ATP - 2 ATP = 8 ATP

Cross - Expenditure = Net or Total gain

Direct gain = 2 ATP

(2) Link reaction or Gateway reaction:

12NADH2 = 6 ATP (via ET5)

(3) Kreb's cycle:

(i) ATP produced at substrate level phosphrylation = 2 GTP/2ATP

(ii) ATP produced via ETS         6 NADH2 →  18 ATP

2FADH2 → 4 ATP

   24 ATP

Total → 38 ATP

1 Sucrose = 80 ATP

1 Fructose 1, 6-biphosphate = 40 ATP

1 Pyruvic acid = 15 ATP

1 Acetyl CoA or 1 TCA cycle = 12 ATP 

Penrose Phosphate Pathway//HMP (Hexose mono-phosphate) Shunt/Warburg Dickens pathways:

PPP is also called as Warburg - Dickens pathway / HMP shunt

Phosphogluconolactone pathway /Carbohydrate degradation without mitochondria/Cytosolic oxidative decarboxylation/Horecker Racker Pathway

Glycolysis & TCA cycle is the main route of carbohydrate oxidation, but Warburg & Dickens (1935) discovered an alternative route of carbohydrate break down, existing in plants, some animal tissues (Mammary glands, adipose, liver) & microbes 

HMP / PPP occurs when

(i) NADPH2 requirement of cell increases during biosynthetic processes.

(ii) When EMP pathway blocked by iodoacetate, fluorides. arsenates.

(iii) When mitochondria is busy in other pathways.

Most of the intermediates are similar to Calvin cycle but PPP is catabolic and oxidative process.

One ATP is utilised in phosphorylation of glucose, so net gain equals to 35 ATP. (12 NADPH2)

Hexose Monophosphate shunt

  • Significance of HMP Shunt:

1.  An intermediate erytbrose-P (4C) of this pathway is precursor of shikimic acid, which goes to synthesis of aromatic compounds and amino acids.

2. This cycle provides pentose sugars Ribose-P for synthesis of nucleotides, nucleosides, ATP and GTP.

3. A five carbon intermediate Ribulose-5-phosphate may used as CO2 acceptor in green cells.

4. This pathway produces reducing power NADPH2 for the various biosynthetic pathways, other than photosynthesis like fats synthesis, starch synthesis, hormone synthesis and chlorophyll synthesis.

5. Intermediates like PGAL and fructose-6-phosphate of this pathway may link with glycolytic reactions.

b-Oxidation of fatty acids: 

b-oxidation takes place mainly in perimitochondrial space but also in glyoxisome, peroxisome, cytosol.

Liberation of 2C segments from the fatty acid mol. in the form of acetyl Co-A is known as b-oxidation. These acetyl-CoA provides ATP after oxidation in Kreb's cycle.

Acetyl CoA is oxidised in TCA cycle to CO2 & H2O with the release of 12 ATP molecules.

16C palmitic add 

  • Glyoxylate Cycle:

Discovered by Kornberg & Krebs during germination of fatty seeds.

This cycle converts fats into sugars so it is an example of gluconeogenesis in plants.

Glyoxylate cycle occurs in glyoxisome, cytosol, & mitochondria.

Entner - Doudoroff pathway

Entner - Doudoroff path discovered by Entner & Doudoroff. This pathway is also called glycolysis of bacteria.

Certain bacteria such as Pseudomonas sacchorophila, P. fluorescens, P. lindeneri and P. averoginosa lack phosphofructokinase enzyme. They can not degrade glucose by glycolytic process. 

Enter-Doudoroff Pathway

  • Cyanide Resistant Pathway:

Cyanide-resistant respiration seems to be widespread in higher plant tissues. Cyanide prevents flow of electron from Cyt a3 to oxygen, so called ETC inhibitor. In these plant tissues resistance is due to a branch point in the ETS preceeding the highly cyanide-sensitive cytochromes. The tissues lacking this branch point. OF alternate pathway and blockage of cytochromes by cyanide, inhibits the electron flow.


(i) The role of alternative pathway is that it may provide a means for the continued oxidation of NADH and operation of the tricarboxylic acid cycle; even through ATP may not be sufficiently drained off.

(ii) It is significant in respiratory climateric of ripening fruits and leads to the production of hydrogen peroxide and super oxide, which in turn enhances the oxidation and breakdown of membranes.

(iii) Necessary activities in the ripening process because peroxides are necessary for ethylene biosynthesis. 

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