All the energy required for ‘life’ processes is obtained by oxidation of some macromolecules that we call ‘food’. In photosynthesis plants trap light energy and convert it into chemical energy that is stored in the bonds of carbohydrates like glucose, sucrose and starch.

Cellular Respiration - the mechanism of breakdown of food materials within the cell to release energy, and the trapping of this energy for synthesis of ATP.

- NAD+ is function as an electron carrier in cellular respiration. (NEET 2018)

Respiration - The breaking of the C-C bonds of complex compounds through oxidation within the cells, leading to release of considerable amount of energy.
Compounds that are oxidised during this process are known as respiratory substrates usually carbohydrates are oxidised to release energy, but proteins, fats and even organic acids can be used as respiratory substances in some plants.

Do plants breathe ?

Plants have no specialised organs for gaseous exchange but they have stomata and lenticels for this purpose.
Reasons why plants can get along without respiratory organs :-

Each plant part takes care of its own gas-exchange needs
Very little transport of gases from one plant part to another.
Plants do not present great demands for gas exchange.
Roots, stems and leaves respire at rates far lower than animals do.
availability of Oxygen is not a problem in these cells since oxygen is released within the cell.
distance that gases must diffuse even in large, bulky plants is not great.
The cells in the interior are dead and provide only mechanical support.
loose packing of parenchyma cells in leaves, stems and roots, which provide an interconnected network of air spaces.

• The complete combustion of glucose, which produces Carbon dioxide and water as end products, yields energy most of which is given out as heat.

This energy is to be useful to the cell, it should be able to utilise it to synthesise other molecules that the cell requires.
Plant cell uses is to catabolise the glucose molecule in such a way that not all the liberated energy goes out as heat and produce ATP.

Glycolysis or EMP pathway

• 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.

- The term glycolysis has originated from the Greek words, glycos for sugar, and lysis for splitting.

• Glycolysis was given by Gustav Embden, Otto Meyerhof, and J. Parnas, and is often referred to as the EMP pathway.

In anaerobic organisms, it is the only process in respiration.
Occurs in the cytoplasm of the cell and is present in all living organisms.
glucose undergoes partial oxidation to form two molecules of pyruvic acid.
In plants glucose is derived from sucrose, which is the end product of photosynthesis,
Sucrose is converted into glucose and fructose by the enzyme, invertase, and these two monosaccharides readily enter the glycolytic pathway.

• Glycolysis Pathways :-

Steps of metabolism of glucose and fructose are same.
A chain of ten Reactions under control of different enzymes.

Glucose and fructose are phosphorylated to give rise to glucose-6phosphate by the activity of the enzyme hexokinase. ( 1 ATP is used) (NEET 2019)
phosphorylated form of glucose then isomerises to produce fructose-6-phosphate.
Conversation of fructose 6-phosphate to fructose 1, 6-bisphosphate. (1 ATP is used)
The fructose 1, 6-bisphosphate is split into dihydroxyacetone phosphate and 3- phosphoglyceraldehyde (PGAL).
3-phosphoglyceraldehyde (PGAL) is converted to 1, 3-bisphosphoglycerate (BPGA).
PGAL is oxidised and with inorganic phosphate to get converted into BPGA.
Conversion of BPGA to 3-phosphoglyceric acid (PGA),[2 molecules] is also an energy yielding process; this energy is trapped by the formation of ATP.

PGA is converted into 2-phosphoglycerate.
2-phosphoglycerate is Converted into phosphoenolpyruvate (PEP) and two water molecule release.
PEP converted into pyruvic acid and 2 molecules of ATP is formed.
Pyruvic acid is then the key product of glycolysis.

• Three major ways in which different cells handle pyruvic acid produced by glycolysis.

1. lactic acid fermentation 2. alcoholic fermentation 3. aerobic respiration.

Fermentation takes place under anaerobic conditions in many prokaryotes and unicellular eukaryotes. For the complete oxidation of glucose to carbon dioxide and water.
Organisms adopt Krebs’ cycle which is also called as aerobic respiration requires oxygen supply.

Fermentation

The incomplete oxidation of glucose is achieved under anaerobic conditions by sets of reactions where pyruvic acid is converted to Carbon dioxide and ethanol.
Enzymes, pyruvic acid decarboxylase and alcohol dehydrogenase catalyse these reactions.
Some bacteria produce lactic acid from pyruvic acid.

In animal cells also, like muscles during exercise, when oxygen is inadequate for cellular respiration pyruvic acid is reduced to lactic acid by lactate dehydrogenase.
The reducing agent is NADH+, H+ which is reoxidised to NAD+ in both the processes.

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.

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

Aerobic Respiration

• Aerobic respiration is the process that leads to a complete oxidation of organic substances in the presence of oxygen, and releases CO2, water and a large amount of energy present in the substrate.

take place within the mitochondria, the final product of glycolysis, pyruvate is transported from the cytoplasm into the mitochondria.

• The crucial events :-

The complete oxidation of pyruvate by the stepwise removal of all the hydrogen atoms, leaving three molecules of Carbon dioxide. (takes place in the matrix of the mitochondria)
The passing on of the electrons removed as part of the hydrogen atoms to molecular oxygen with simultaneous synthesis of ATP. (located on the inner membrane of the mitochondria).
The reactions catalysed by pyruvic dehydrogenase require the participation of several coenzymes, including NAD+ and Coenzyme A.

two molecules of NADH are produced from the metabolism of two molecules of pyruvic acid (produced from one glucose molecule during glycolysis).

Tricarboxylic Acid Cycle / Krebs cycle

Acetyl CoA then enters a cyclic pathway, tricarboxylic acid cycle, more commonly called as Krebs’ cycle after the scientist Hans Krebs who first elucidated it.

The TCA cycle starts with the condensation of acetyl group with oxaloacetic acid (OAA) and water to yield citric acid.
The reaction is catalysed by the enzyme citrate synthase and a molecule of CoA is released.
Citrate is then isomerised to isocitrate.
It is followed by two successive steps of decarboxylation, leading to the formation of α-ketoglutaric acid and then succinyl-CoA.
In the remaining steps of citric acid cycle, succinyl-CoA is oxidised to OAA allowing the cycle to continue.
For detail refer Fig. 14.3

• Summary equation :-

During the conversion of succinyl-CoA to succinic acid a molecule of GTP is synthesised.
This is a substrate level phosphorylation. In a coupled reaction GTP is converted to GDP with the simultaneous synthesis of ATP from ADP.
there are three points in the cycle where NAD+ is reduced to NADH + H+ and one point where FAD+ is reduced to FADH2 (NEET 2017)
TCA cycle requires the continued replenishment of oxaloacetic acid, the first member of the cycle.
glucose has been broken down to release CO2 and eight molecules of NADH + H+ ; two of FADH2 have been synthesised besides just two molecules of ATP in TCA cycle.

Electron Transport System (ETS) and Oxidative Phosphorylation

• The metabolic pathway through which the electron passes from one carrier to another, is called the electron transport system (ETS) and it is present in the inner mitochondrial membrane. (NEET 2018)

steps in the respiratory process are to release and utilise the energy stored in NADH+H+ and FADH2.
Electrons from NADH produced in the mitochondrial matrix during citric acid cycle are oxidised by an NADH dehydrogenase (complex I), and electrons are then transferred to ubiquinone located within the inner membrane.
Ubiquinone also receives reducing equivalents via FADH2 (complex II) that is generated during oxidation of succinate in the citric acid cycle.
The reduced ubiquinone (ubiquinol) is then oxidised with the transfer of electrons to cytochrome c via cytochrome bc1 complex (complex III).
Cytochrome c is a small protein attached to the outer surface of the inner membrane and acts as a mobile carrier for transfer of electrons between complex III and IV.
Complex IV refers to cytochrome c oxidase complex containing cytochromes a and a3, and two copper centres.
When the electrons pass from one carrier to another via complex I to IV in the electron transport chain, they are coupled to ATP synthase (complex V) for the production of ATP from ADP and inorganic phosphate. (NEET 2016)
The number of ATP molecules synthesised depends on the nature of the electron donor.
Oxidation of one molecule of NADH gives rise to 3 molecules of ATP, while that of one molecule of FADH2 produces 2 molecules of ATP.
Although the aerobic process of respiration takes place only in the presence of oxygen, the role of oxygen is limited to the terminal stage of the process.
the presence of oxygen is vital, since it drives the whole process by removing hydrogen from the system.
Oxygen acts as the final hydrogen acceptor.
in respiration it is the energy of oxidation-reduction utilised for the same process. It is for this reason that the process is called oxidative phosphorylation
(complex V). This complex consists of two major components, F1 and F0
The F1 headpiece peripheral membrane protein complex and contains the site for synthesis of ATP from ADP and inorganic phosphate.
F0 is a 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 the F1 component for the production of ATP.
For each ATP produced , 2H+ passes through F0 from the intermembrane space to matrix down the electrochemical proton gradient.

The Respiratory balance sheet

calculations can be made only on certain assumptions that:
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.
The NADH synthesised in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation.
None of the intermediates in the pathway are utilised to synthesise any other compound.
Only glucose is being respired – no other alternative substrates are entering in the pathway at any of the intermediary stages.
There can be a net gain of 38 ATP molecules during aerobic respiration of one molecule of glucose.
Difference between Fermentation and Aerobic Respiration
Fermentation accounts for only a partial breakdown of glucose whereas in aerobic respiration it is completely degraded to CO2 and H2 O.
In fermentation there is a net gain of only two molecules of ATP for each molecule of glucose degraded to pyruvic acid whereas many more molecules of ATP are generated under aerobic conditions.
NADH is oxidised to NAD+ rather slowly in fermentation, however the reaction is very vigorous in case of aerobic respiration. (Aipmt 2008,2010)
There is no release of carbon dioxide in lactate fermentation. (NEET 2014)

Amphibolic pathway

Respiratory pathway is involved in both anabolism and catabolism, it would hence be better to consider the respiratory pathway as an amphibolic pathway rather than as a catabolic one.
Acetyl Co-A biomolecule is common to respiration-medicated breakdown of face carbohydrates and proteins. (NEET 2013,2016)

Respiratory Quotient

The ratio of the volume of carbon dioxide evolved to the volume of oxygen consumed in respiration is called the respiratory quotient (RQ) or respiratory ratio.

Different RQ value are given in Reaction .

RQ of Tripalmitin is 0.7 in NEET 2019.

Respiratory Quotient

The ratio of the volume of carbon dioxide evolved to the volume of oxygen consumed in respiration is called the respiratory quotient (RQ) or respiratory ratio.

Different RQ value are given in Reaction .