Download Respiration

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Cellular respiration
Glycolysis:-It is defined as the sequence of reaction converting glucose to pyruvate,
with the production of ATP. Glycolysis occurs in the cytoplasm of virtually all living cells,
both in the absence and presence of O2. Probably it was the first energy releasing process in
organisms when the life evolved.
The oxidative respiration in mitochondria of eukaryotic cells became possible only
after molecular oxygen had accumulated in the earth’s atmosphere as a result of
photosynthesis by cyanobacteria.
Reaction of glycolysis.
1. Glucose is phosphorylated to
glucose 6 phosphate by enzyme hexokinase.
This is a Mg+2 activated
enzyme.
2. Glucose 6 phosphate is isomerised
by enzyme glucose 6 phosphate isomerase
into fructose 6 phosphate.
3.
Fructose 6 phosphate is
phosphorylated
by
enzyme
phosphofructokinase to fructose 1, 6
diphosphate.
4. Fructose 1, 6 diphosphate is
cleaved by enzyme aldolase into two
interconvertible
triosephosphates,
Glyceraldehyde 3 phosphate and Dihydroxy
acetone phosphate.
5.
The enzyme triosephosphate
isomerase catalyses the reversible inter
conversion of glyceraldehyde 3 phosphate
and dihydroxyacetone phosphate, thus two
molecules of glyceraldehyde 3 phosphate are obtained from one molecule of glucose.
6. Glyceraldehyde 3 phosphate is oxidized in presence of enzyme glyceraldehydes 3
phosphate dehydrogenase to 1, 3 biphosphoglycerate, it is simultaneously phosphorylated by
inorganic phosphate. Here 2NAD is changed to 2NADH.
7. 1,3, biphosphoglycerate is dephosphorylated in presence of enzyme
phosphoglycerate kinase to 3 phosphoglycerate and ATP is changed to ADP.
8. Now the phosphate group changes its position from 3 to carbon 2 catalysed by
enzyme phosphoglyceromutase.
9. 2, phosphoglycerate is changed to phosphoenol pyruvate (PEP) by enzyme enolase
containing high energy enol phosphate.
10. Phosphoenol pyruvate is dephosphorylated to pyruvate in presence of enzyme
pyruvate kinase. Here ADP is changed to ATP.
In glycolysis there is a net formation of two ATP molecules and 2 NADH molecules
which in ETC oxidative phosphorylation release 6 molecules of ATP. So total gain of ATP
molecule is glycolysis is 8 i.e. one NADH molecule produces 3 ATP molecules in ETC.
What happens to pyruvaye that is produced in glycolysis. The fate of pyruvate varies
from cell to cell and its environment. In cells having mitochondria and adequate supply of
oxygen, pyruvate is oxidized in TCA cycle. In the absence of oxygen pyruvate is reduced by
NADH to either lactate or alcohol. Such anaerobic routes are called fermentation pathways.
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Oxidative decarboxylation of Pyruvate
Glycolysis occurs in the cytoplasm of the cell but pyruvate is metabolized aerobically
in the mitochondria. The pyruvate is transported to the mitochondria either by simple
diffusion or by the pyruvate – hydroxyl ion antiport system where pyruvate is exchanged for
a hydroxyl ion.
The pyruvate is oxidatively decarboxylated by the multienzyme complex called the
pyruvate dehydrogenase consisting of multiple copies of three enzymes ( E1. E2, E3 ) each
with specific binding site for the substrate and different cofactors. Since this reaction links
glycolysis with TCA it is also termed as link reaction. Pyruvate in this reaction is changed to
acetyle COA with the removal of CO2 and a pair of hydrogen atoms. The hydrogen atoms
released combine with NAD forms NADH.
Tricarboxylic Acid cycle
Discovered by Hans Kreb therefore it is also called Krebs cycle. Krebs cycle is the
most important metabolic pathway for the energy supply to the body. About 65 – 75 % of the
ATP is synthesized in Krebs cycle. This cycle utilizes about two thirds of total oxygen
consumed by the body. The name TCA cycle is used, since at the onset of cycle, tricarboxylic
acids citrate and isocitrate participate
Acetyl CoA now enters the Krebs cycle. Each step is catalysed by a specific enzyme.
The various reactions occurring in the cycle are.
1. Acetyl CoA , condences with oxaloacetate (4–carbon compound )to form citric acid
( citrate) a 6-Carbon compound . CoA is liberated.
2. Citrate is converted into isocitrate by rearrangement of atom groups by the enzyme
aconitase.
3. Isocitrate gives of a pair of H atoms in presence of enzyme isocitrate
dehydrogenase to form oxalosuccinate, and NAD is changed to NADH.
4. Oxalosuccinat in presence of isocitrate dehydrogenase loses a molecule of CO2 and
forms ∞ Ketoglutarate (5 C ). The pair of H atoms passes to NAD forming NADH. (the
enzyme isocitrate dehydrogenase catalyses the reaction )
5. ∞ ketoglutarate is transformed to succinyl CoA (4 C ) . This reaction involves
CoA and NAD, the products being CO2, NADH in addition to succinyl CoA. (With this
reaction all the C atoms that
entered into citric acid cycle
as pyruvic acid are released
as CO2 ).
6. Succinyl CoA is
acted upon by enzyme
(Succinyl thiokinase ) to
form succinate. This reaction
releases sufficient energy to
form ATP (in plants) or GTP
(in animals).
7.
Succinate
undergoes dehydrogenation
to form fumarate (by the
enzyme
succinate
dehydrogenase ). In this
reaction FADH2 (reduced
flavin adenine dinucleotode )
is produced.
8. A molecule of
3
water gets added to fumarate to form malate in presence of enzyme Fumarase.
9. Malate is dehydrogenated (or oxidized) to produce oxaloacetate in presence of
enzyme Malate dehydrogenase. The pair of H atoms passes to NAD forming NADH2.
Oxaloacetate combines with another molecule of acetyl CoA to repeat the cycle.
TCA cycle – the central metabolic pathway
The citric acid cycle is the final common oxidative pathway for carbohydrates fats
and amino acids. This cycle not only supplies energy but also provides many intermediates
required for the synthesis of amino acid, glucose, heame etc. Krebs cycle is the most
important central pathway connecting almost all the individual metabolic pathways.
Energetics of Citric Acid Cycle
During the process of oxidation of acetyl CoA via citric acid cycle 4 reducing
equivalents (3 as NADH and one as FADH2) are produced. Oxidation of one NADH by
electron transport chain coupled with oxidative phosphorylation results in the synthesis of
3ATP, where as FADH2 leads to the formation of 2 ATP.
Total ATP molecules produced during respiration:In Glycolysis
Direct
=
2
2 molecules of NADH
=
6
Total:
=
8
In Link reaction:
Pyruvic acid to Acetyle co-A
2 molecules of NADH
=
6
Citric acid cycle
6NADH
=
18
2FADH2
=
4
Direct
=
2
Total
=
24
Grand total
=
38
Krebs cycle is both catabolic and anabolic in nature. Hence regard as amphibolic.
Pentose phosphate pathway ( PPP )
Glycolysis is a major route of degradation of glucose to pyruvate. Many other routes
of glucose metabolism have also been investigated in plant and animal tissue. One of the
important pathways that operates in a wide variety of organisms including animals, plants and
many organisms called pentose phosphate pathway, or hexose monophsphate shunt or
phosphogluconate pathway given by Warburg-Dickens, so called Warburg-Dickens pathway.
The pathway is divided into Oxidative phase and Non oxidative phase.
1. Oxidative phase :- The various reactions occurring in this phase are
1. Glucose 6 phosphate is oxidized to 6 phosphoglucono 1, 5 lactone, by
removal of
electrons which are accepted by NADP.
2. Hydrolysis of the 6 phosphoglucono 1, 5 lactone results in the formation of
6
phosphogluconate.
3. 6 phosphogluconate is decarboxylated to ribulose 5 phosphate and second
molecule
of NADPH is formed. Enzymes of these reactions are activated by MG2+ .
2. Non oxidation phase :-This pathway operates according to the specific
requirements of the tissue.I tissue active in nucleotide synthesis ribulose 5 phosphate is
isomerised to ribose 5 phosphate. The pathway terminates at this point.
The tissue active in lipid synthesis has a great demand for NADPH. Under this
situation the reactions occurring are as under.
1. Isomerisation of ribulose 5 phosphate occurs by two different enzymes forms
ribose 5 phosphate and xylulose 5 phosphate.
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2. These two 5 carbon sugars condense together by the enzyme transketolase to form
7 carbon sedoheptulose phosphate and three carbon glyceraldehydes 3 phosphate.
3. Now 3 & 7 carbon sugars condense by another enzyme called transaldolase to
form 6 carbon fructose 6 phosphate and 4 carbon erythrose phosphate.
4. Another molecule of 5 carbon of xylulose 5 phosphate condenses with 4 carbon
erythrose phosphate and forms fructose 6 phosphate and 3 carbon glyceraldehyde 3
phosphate.
The end products of the pentose phosphate pathway can enter into the
glycolytic pathway. Thus this pathway is basically a shunt as its branches out from the
glycolytic pathway at the point of glucose 6 phosphate and rejoins at the point of fructose 6
phosphate and glyceraldehyde 3 phosphate. Fructose 6 phosphate may isomerise into glucose
6 phosphate and re-enter in the oxidative phase of cycle.
This pathway does not occur in muscle cells.The pathway occurs in the cytoplasm of
cell.
Glucose-6-phosphate
Glucose 6 phosphate dehydrogenase
NADP
NADPH+ H
6-phospho-glucono-1,5-lactone
6 phospho gluconase
H 2O
6-phosphogluconate
NADP
Phosphogluconate dehydrogenase
Co2
NADPH +H
Robulose-5-phosphate
Isomerase.
Ribose-5-phosphate
(Pathway terminates here when the cell is actively involved in DNA and RNA synthesis).
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Reaction occurring in lipid synthesizing cells
Ribulose-5-phosphate
Isomerase
Ribose-5-phosphate
Xylose-5-phosphate
Condensation.
Transketolase
Pseudo heptulose-7-phosphate
Glyceraldehyde-3-phosphate
Condensation.
Transaldolase
Frutose-6-phosphate
Erythorase-4-phosphate
+
Xylose-5-phosphate
Condensation
Fructose-6-phosphate
Glyceraldehyde-3-phoaphate
Significance of PPP
1. NADPH not NADH is produced in PPP, which is required as reductant in fat
synthesis.
2. This pathway produces several pentoses which are required for nucleic acid
synthesis.
3. The oxidative pentose phosphate pathway is thought to be involved in generating
calvin cycle intermediates before the leaves became fully photoautotrophic.
Fermentation
In the absence of O2 pyruvate is reduced by NADH to either lactate or alcohol. Such
anaerobic routes are called fermentation pathways. The process of conversion of glucose to
alcohol is known as alcoholic fermentation and the process of conversion of glucose to lactate
is called lactate fermentation.
Alcoholic fermentation:- During this process at first glucose is converted into
pyruvate. Pyruvate in the presence of enzyme pyruvate decarboxylase is converted into
acetaldehyde one molecule of CO2 is liberated in this reaction.
Pyruvate decarboxylase
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Pyruvate
Acetaldehyde + CO2
In the 2 reaction acetaldehyde is reduced to ethyl alcohol in presence of enzyme
alcohol dehydrogenase. At this step one molecule of NADH is oxidized to NAD.
Alcoholdehydrogenase
Acetaldehyde
Ethyl alcohol
It can occur in any sugar solution. The fruit juices show alcoholic fermentation when
yeast powder is added or the juice is left as such open in air.
Lactic acid fermentation:- It is carried out by lactic acid bacteria. Pyruvate is
reduced in presence of enzyme pyruvate dehydrogenase in presence of Zn2+ & FMN ( Flavin
mononucleotide ) to lactic acid. One glucose molecule gives two lacticacid molecules CO2 is
not involved in this reation.
In muscle cells,Pyruvate is also changed into lactic acid which is carried to liver through
blood supply, where it is again converted into glucose and enters the blood.
Energy yield in fermentation.
Fermentation yields only about 5% of the energy obtained by aerobic respiration. This
small amount of energy is sufficient to maintain the life of organisms. Such as yeasts, many
bacteria and other anaerobes.
Net gain of ATP in Glycolysis = 8
Loss of ATP in fermentation
=6
Balance ATP
=2
Importance of fermentation:
1. It supplements the energy provided by aerobic respiration during intense muscular
activity.
2. Brewing industry produces beer, wines by fermenting sugar solution with yeasts.
3. Baking industry uses CO2 released by yeast cells in alcoholic fermentation in
raising the dough and making bread spongy.
4. Dairy industry produces yogurt, cheese and butter by fermenting milk sugar
lactose into lactic acid by strepto coccus lacti. Lactic acid coagulates the milk protein casein
and fuses droplets of milk fat.
5. Tea and Tobacco leaves are cured (freed of bitterness and important pleasant
flavour) by fermentation with certain bacteria ( Bacillus megatherium ).
6. Vinegar is produced by fermenting molasses with yeast to ethyl alcohol which is
oxidized to acetic acid by aerobic bacteria Acetobacter aceti.
7. Butyl alcohol and acetone are manufactured from molasses by fermentation with
bacteria Clostridium acetobutylicum
8. Fermentation is used for cleaning hides.
9. Retting of fibers by Pseudomonas.
10. Ensilage a nutrient fodder for cattle is prepared by fermentation with bacteria in
air tight chambers.
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Respiratory quotient (RQ):- During aerobic respiration O2 is consumed and Co2 is released.
The ratio of the volume of the CO2 evolved to the volume of O2 consumed in the respiration
is called respiratory quotient or Repiratory ratio.
Volume of CO2 evolved
RQ =
Volume of O2 consumed.
The RQ depends upon the type of respiratory substrate used during respiration. This is
different for different substrates.
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1.Carbohydrates.
C6H12 O6 + 6O2
6C O2 +6H2O +Energy
6C O2
RQ =
= 1
6O2
So when carbohydrate is the substrate RQ is equall to one. Because equall amount of
O2 and CO2 are consumed and evolved.
2. Fats.
2(C51 H98 O2) + 145 O2
Tripalmatin.
102CO2 + 98H2O +Energy.
102CO2
RQ =
= 0.7
145 O2
So RQ is less than one when fat is the substrate . Fat contains less O2 than
carbohydrates. So requires greater amount of O2 for oxidation.
3. Organic acids
C4H6 O2 +3O2
Mallic acid.
4C O2 +3H2O + Energy
4C O2
RQ
=1.3
3O2
RQ is more than one when the substrate used is organic acid. It is because organic
acids contain more O2 than carbohydrates so relatively less O2 is required for their oxidation.
In anaerobic respiration C O2 is evolved but O2 is not consumed .Therefore RQ in
such cases will be infinite eg.
Zymase
C6H12O6
2C2H5OH + 2C O2+ energy
RQ = ∞
Importance of RQ.
1. Knowledge of RQ helps in determining respiratory substrate.
2. It helps in knowing the type of respiration being performed.
Compensation point:- Photosynthesis and respiration are the process which are exactly
opposite to each other. Photosynthesis builds up carbohydrates with the absorption of O2
and release of CO2. The process of respiration goes on all the times and in all living cells.
Where as photosynthesis proceeds only in sunlight and is confined to the green cells.
Under normal conditions a green plant accumulates organic substances besides
utilizing them in respiration The green cells must therefore built up more of organic food
during the few hours of sunlight than is broken down in respiration. Photosynthesis is
therefore a more rapid process than respiration (approximately10:1) Photosynthesis being a
more rapid process utilizes all the CO2 liberated in respiration , However under a critical low
light intensity (ie. in the morning and evening) both the processes are approximately equal.
The O2 evolved in photosynthesis will be utilized in the respiration and CO2 evolved in
respiration will be utilized in photosynthesis. The light intensity at which the rate of
photosynthesis is just equal to rate of respiration is called compensation point. At the
compensation point there is no increase in the dry matter of plant.