The living organisms require energy for survival. The source of energy is food. It stores energy. The energy rich compounds are Glucose (Carbohydrates), fats and proteins. These substrates are broken down into simpler molecules with the release of energy in a process called Respiration.
It is common to plants and animals. It is the first and last metabolic function. It is a continuous process. It is a catabolic, exothermic, oxidative process that occurs both in plants and animals day and night. The free energy released during respiration is used to synthesise ATP (Adenosine Tri Phosphate) the biological energy currency. ATP releases energy whenever and wherever the cell requires for its various functions. The entire energy stored in the substrates is not converted to ATP. Some energy is released in the form of heat.
Where this respiration occurs?
Mitochondria is the site of respiration. But the first part of Respiration occurs in Cytosol. It prokaryotes entire steps of respiration occur in Cytosol as they do not consist mitochondria.
Eukaryotes → Respiration occurs in Cytosol and Mitochondria.
Prokaryotes → It occurs in Cytosol.
How to define?
The vital, catabolic, exothermic, enzyme mediated, common, continuous biological process where in the complex nutrient substrates are either completely or partially oxidised by taking or not taking Oxygen but releasing energy and CO2 is called respiration.
How many types of Respiration?
* Aerobic respiration * Anaerobic respiration
* Fermentation * Salt respiration
* Nitrate respiration * Photorespiration
* Proto plasmic respiration
AEROBIC RESPIRATION
It occurs in the presence of Oxygen. It is more common. It occurs in 4 steps.
* Glycolysis * Oxidative decarboxylation
* Krebs cycle * Electron transport system
I. GLYCOLYSIS
This term is derived from greek (Glykys = sweet, lysis = splitting). It occurs in Cytosol. All the enzymes required for glycolysis are present in Cytosol. Glucose is partially oxidised and broken into 2 molecules of Pyruvic acid. It neither requires O2 nor liberates CO2. It is common to Aerobic and Anaerobic respiration.
It is also called core respiration or EMP path way (Embden, Meyerhof and Paramas) or Hexose diphosphate pathway.
It can be divided into 2 parts or phases.
A) Preparatory phase. B) Oxidation phase or Energy genation phase.
Preparatory phase
The cell invests 2 ATP and prepares glucose to participate in glycolysis. So it can also be treated as investment phase. The most common carbohydrate that participates is glucose. It is called as substrate. All the reactions in it are enzyme mediated except one. The molecular weight of glucose is 180 (Students should remember it as problem can be given basing on this number).
Glucose + Fructose (Monosaccharides)
Glucose and Fructose are isomers. They enter into glycolysis. Amyloplasts store starch. Sometimes starch is converted to glucose by amylase. Thus the source of glucose is sucrose and starch.
The process of glycolysis occurs as follows
1) Phosphorylation
Glucose (substrate)
Glucose − 6 − Phosphate
This reaction is irreversible
2) Isomerisation
Glucose − 6 − Phosphate
Fructose − 6 − Phosphate
It is a reversible process
3) Phosphorylation
Fructose − 6 − Phosphate
Fructose − 1, 6 − Bisphosphate (Hexose biphosphate).
It is an irreversible reaction.
It is the highly energetic compound of glycolysis. Now it actually undergoes glycolysis.
4) Cleavage
It is the actual first step.
DHAP and G − 3 − P are isomers.
5) Isomerisation
Thus these are 2 molecules of G − 3 − P.
G − 3 − P are converted to 1 − 3 diphosphoglyceraldehyde.
It doesn't require enzyme.
Pay off phase/ Energy genation phase
6) G − 3 − P
1, 3 − Bisphosphoglyceric acid (BPGA)
Respiration is basically an oxidation process.
Glucose undergoes a total of 6 biological oxidations.
If it undergoes complete oxidation.
7) Substrate level phosphorylation (SLP)
1, 3 − B P G A
3 − PGA
8) Isomerisation
It is an intra molecular shift
9) Dehydration
Phospho Enol Pyruvate (PEP)
10) Substrate level phosphorylation (SLP)
It is not a reversible process.
Thus at the end of glycolysis.
4 ATP are produced in SLP. 2 ATP are consumed in it. So net ATP formed in it are 2. The end products are 2 molecules of Pyruvic Acid. 2 NADH2 are also formed.
Summary
* Phosphorylations − 2
* Substrate level phosphorylations − 2
* Isomerisations − 3
* Biological Oxidation − 1
* Dehydration − 1
* Cleavage − 1
* Direct ATP formed − 2
ATP to be formed in ETS through 2 NADH2 = 4
Contribution of glycolysis − 6.
Fate of Pyruvic Acid
Pyruvate carries (−) ve charge. It enters into mitochondrial matrix with the help of Pyruvic translocator (a protein).
II. Oxidative decarboxylation
Pyruvic Acid (Pyruvate) undergoes first decarboxylation and then oxidation which is followed by condensation in the presence of pyruvic dehydrogenase complex.
The enzyme requires co-factors like lipoic acid, TPP (Thymine Pyrophosphate) NAD, Mg2+ and CoA. Acetyl CoA is used in the synthesis of gibberellins and ABA. It is the connecting link between glycolysis and Krebs Cycle.
Contribution of Oxidative Decarboxylation
2 NADH2 produce 6 ATP through ETS. Thus it contributes 6 ATP.
Summary
* Biological oxidation − 1
* Decarboxylations − 2
III. KREBS CYCLE
Glucose undergoes partial oxidation in I and II parts of aerobic respiration. Where as complete oxidation occurs in III part i.e, Krebs Cycle.
A chain or sequence of reactions which occur in a cyclic form in the mitochondrial matrix that lead to the complete oxidation of glucose or its representative i.e., Acetyl CoA is called Krebs Cycle.
It was discovered by German born British Biochemist Sir Hans A. Krebs so it is named after him. The first formed compound in it is Citric acid. So it is also called Citric Acid Cycle or Tricarboxylic Acid Cycle (TCA Cycle) or Organic Acid Cycle or Central Metabolic Pathway. Though it is a part of catabolism, the intermediate compounds of it are useful in the synthesis of Amino acids, proteins, vitamins and hormones. So it is also called Amphibolic Pathway.
Oxygen is not taken but CO2 is released in it.
All enzymes required for it are present in mitochondrial matrix except Succinic dehydrogenase.
Krebs Cycle occurs protein - rich matrix between the cristae as follows
Who greets Acetyl CoA?
Begin with condensation!
1) Acetyl CoA unites with Oxalo Acetic Acid (4C) and undergoes condensation in the presence of H2O and Citric synthetase.
2) Dehydration
3) Hydration
Citric Acid, Cis aconitic acid and Isocitric acid and
Oxalosuccinic acid are 6 - Carbon Compounds or Acids.
4) Biological Oxidation
It is the first of the 4 biological oxidations in Krebs Cycle.
It is 2nd biological oxidation in mitochondrion.
It is the 3rd biological oxidation in aerobic respiration.
5) Decarboxylation
With this we can say that 4 decarboxylations occurred for one glucose
6) Oxidative decarboxylations
α − Ketoglutaric acid is one of the most important intermediate compounds of Krebs Cycle useful in the anabolic side of Krebs Cycle. It becomes amino acid by transamination.
7) Cleavage
Another decarboxylation occurs with this glucose looses all its six carbons. Substrate level phosphorylation also occurs.
Only Substrate level phosphorylation in Krebs Cycle. It is the 3rd substrate level phosphorylation in aerobic respiration.
8) Biological oxidation
It is the 3rd in Krebs Cycle.
This reaction is inhibited by Malonate, a competitive inhibitor.
Succinic dehydrogenase is not present in the matrix. It is present in the inner mitochondrial membrane.
9) Hydration
10) Last biological oxidation
OAA is the regenerated compound in Krebs Cycle.
It is the 4th biological oxidation in Krebs Cycle
5th in mitochondrion and sixth in aerobic respiration.
Summary
* No. of Krebs Cycles for one Glucose → 2.
For One Krebs Cycle
* NADH2 formed = 3
* FADH2 formed = 1
* Direct ATP (SLP) = 1
* H2O used = 3
* CO2 released = 2
* Cleavage = 1
* Oxidative decarboxylation = 1
For two Krebs Cycles
* NADH2 formed = 6
* FADH2 formed = 2
* Direct ATP (SLP) = 2
* H2O used = 6
* CO22 released = 4
* Cleavage = 2
* Oxidative decarboxylations = 2
Contribution of One Krebs Cycle
* 3 NADH2 form 9 ATP in ETS
* 1 FADH2 forms 2 ATP in ETS
* Direct ATP → 1
Total ATP contributed by one Krebs Cycle = 12
Contribution of two Krebs Cycles (Glucose)
* 6 NADH2 form 18 ATP in ETS
* 2 FADH2 form 4 ATP in ETS
* Direct ATP → 2
Total 24 ATP
Contribution of glucose in aerobic respiration
* I Glycolysis = 6 ATP
* II Oxidative decarboxylation = 6 ATP
* III Krebs Cycle = 24 ATP
Total = 36 ATP
Contribution of One Pyruvic acid through ETS & SLP
* II Oxidative decarboxylation = 3 ATP
* III Krebs Cycle = 12 ATP
Total = 15 ATP
Some Simple Questions
1) How many ATP are produced by one Pyruvic acid only through ETS?
2) Is Krebs Cycle, a part of aerobic respiration though Oxygen is not utilised?
3) Do the membranes of mitochondria allow the exchange of gases?
ELECTRON TRANSPORT SYSTEM (ETS)
It is the last part of aerobic respiration. It occurs in the inner membrane which consists a series of complex enzymes (I to V) arranged in a sequence. The reduced co-enzymes (NADH2 and FADH2) undergo oxidation during which the energy that is liberated is used in the process of phosphorylation to synthesise ATP. So it is also called oxidative phosphorylation. Oxygen is taken in this process after complete decarboxylation which occurred in /upto Krebs Cycle. So it is also called Terminal Oxidation.
The structure, arrangement & function of enzyme complexes
Complex - I (NADH - Ubiquinone oxidoreductase = Gateway of Electron Transport) It consists FMN as prosthetic group & many (6 or 7) Iron - Sulphur proteins. It is present in the inner membrane. It passes electrons from mitochondrial NADH2 to Ubiquinone.
Complex - II (Succinate - Ubiquinone oxidoreductase) It consists FAD as prosthetic group, 2 Iron - Sulphur proteins. It passes electrons from FADH2 to Ubiquinone.
Complex - III (Cytochrome C reductase) It consists Cytochrome b560 and cytochrome b565 and Fe - S proteins. It takes electrons from complex- I or complex - II through Ubiquinone.
Complex - IV (Cytochrome 'C' reductase) It consists cytochrome a, cytochrome a1 and 2 Copper containing proteins. It passes electrons from cytochrome 'C' to molecular Oxygen.
Complex - V (ATP Synthase or F0 - F1, ATPase) It has 2 parts
1) F0 - base piece, 2) F1 - head piece.
F0 acts as proton channel. F1 protrudes into the mitochondrial matrix. It is the actual catalytic site for the ATP synthesis.
UBIQUINONE
It is a fat soluble, mobile electron carrier present in the inner membrane. It takes electrons from Complex - I, Complex - II and directly from cytosolic NADH2 and pass them to Complex - III. The name Ubiquinone is for its ubiquitous nature. It is almost equal to Plasto Quinone (PQ) of thylakoid membranes. Ubiquinone forms pool.
CYTOCHROME 'C'
It is a water soluble, mobile electron carrier present in the outer surface of inner membrane. It passes electrons from Complex - III to Complex - IV.
THE PROCESS
A) Mitochondrial NADH2
2 e- from NADH2 are first accepted by FMN, Complex - I pumps 4 H+ from mitochondrial matrix to permitochondrial space. FMN passes the electrons to UQ.
QUINONE CYCLE
UQ shows quinone cycle during which 4 H+ are shifted from matrix to perimitochondrial space for every 2 electrons. Later it passes e- to Complex - III.
Complex - II do not participates
Complex - III passes the e- to cytochrome C. Then it transfers the e- to Complex - IV which finally passes them to molecular Oxygen. Thus Oxygen is the ultimate e- acceptor.
Complex - IV pumps 2 H+ from matrix to perimitochondrial space. Infact for 2 e- actually 4 H+ are pumped from matrix but 2 H+ are consumed in forming water and the rest 2 H+ are pumped into Peri Mitochondrial Space (PMS).
It can be shown as follows
Thus for m - NADH2, 10 H+ are accumulated in PMS. Proton concentration gradient is established between PMS and Matrix. Acc to chemiosmotic hypothesis, H+ pass from PMS to matrix through Complex - V. For every 3 H+ one ATP is formed in F1. One H+ is wasted.
B) FADH2
Complex − I do not participates FAD of Complex − II accepts 2 e− from FADH2.
Complex − II do not pumps any electrons from matrix to PMS. FAD conveys 2 e- to UQ. The rest is same as in mitochondrial NADH2. Thus a total of 6 H+ are accumulated in PMS for which 2 ATP are produced.
C) CYTOSOLIC NADH2
Complex − I and Complex − II do not participate. External NADH dehydrogenase is present in the inner side of inner membrane. It takes 2e− from cytosolic NADH2 and passes them to UQ. Quinone cycle occurs. 4 H+ are shifted from matrix to PMS. The rest is same. Thus a total of 6 H+ are accumulated in PMS which pass through complex − V to produce 2 ATP.
Summary
A) MITOCHONDRIAL NADH2
* For NADH2 → Number of e− transported = 2
* Number of protons accumulated = 10
* Number of quinone cycles = 2
* Number of H+ accumulated due to quinone cycles = 4
* Number of ATP produced = 3
* Number of H+wasted = 1
* Number of O2 taken = 1/2
* Number of H2O formed = 1
FOR 8 NADH2
* Number of e− transported = 8 pairs
* Number of H+ accumulated = 8 × 10 = 80
* Number of quinone cycles = 8 × 2 = 16
* Number of H+ accumulated due to quinone cycles = 8 × 4 = 32
* Number of ATP formed = 8 × 3 = 24
* Number of H+ wasted = 8 × 1 = 8
* Number of O2 taken = 8 × 1/2 = 4
* Number of H2O formed = 8 × 1 = 8
B) FADH2
* Number of e− transported for 1 FADH2 = 1 pair or 2
* Number of protons accumulated = 6
* Number of quinone cycles = 2
* Number of H+ accumulated due to quinone cycles = 4
* Number of ATP produced = 2
* Number of H+ wasted = 0
* Number of O2 taken = 1/2
* Number of H2O formed = 1
For 2 FADH2
* Number of e− transported = 2 × 2 = 4 (or 2 pairs)
* Number of H+ accumulated = 2 × 6 = 12
* Number of quinone cycles = 2 × 2 = 4
* Number of H+ accumulated due to quinone cycles = 2 × 4 = 8
* Number of ATP produced = 2 × 2 = 4
* Number of H+ wasted = 0
* Number of O2 taken = 2 × 1/2 = 1
* Number of H2O formed = 2 × 1 = 2
C) Cytosolic NADH2
For 1 NADH2
* Number of e− transported = 2 (1 pair)
* Number of H+ accumulated = 6
* Number of quinone cycles = 2
* Number of H+ accumulated due to quinone cycles = 2 × 4 = 8
* Number of ATP produced = 2
* Number of H+ wasted = 0
* Number of O2 taken = 1/2
* Number of H2O formed = 1
For 2 Cytosolic NADH2
* Number of e− transported = 2 pairs
* Number of H+ accumulated = 2 × 6 = 12
* Number of quinone cycles = 2 × 2 = 4
* Number of H+ accumulated due to quinone cycles = 2 × 4 = 8
* Number of ATP produced = 2 × 2 = 4
* Number of H+wasted = 0
* Number of O2 taken = 2 × 1/2 = 1
* Number of H2O produced = 2 × 1 = 2
For 1 Glucose
Number of H+ accumulated in PMS = 80 + 12 + 12 = 104
Number of H+ accumulated due to quinone cycles = 32 + 8 + 8 = 48
ANAEROBIC RESPIRATION
Respiration process during which food material is oxidised incompletely and CO2 is released without taking Oxygen is called Anaerobic Respiration.
The organism which undergo anaerobic respiration are called anaerobic organism or anaerobes. Anaerobic organism which can continue aerobic respiration if Oxygen is available are called facultative anaerobes.
e.g.: Yeast.
Anaerobes which die when exposed to Oxygen are called Obligatory anaerobes.
e.g.: Clostridium botulinum.
Anaerobic respiration occurs under following conditions.
1) Organism in the soil 2) Organism in the space
3) Germinating seeds 4) Stored seeds and fruits
5) When Oxygen concentration is less than 9%
Process of Anaerobic Respiration
It occurs in cytoplasm. It is divided into 3 parts.
1) Glycolysis 2) Decarboxylation 3) Dehydrogenation
1) GLYCOLYSIS
Glucose produces
1) 2 Pyruvic Acids
2) 2 NADH2
3) 2 ATP (in substrate level phosphorylations)
2) DECARBOXYLATION
Thus for one Glucose in Anaerobic respiration
ATP produced = 2 CO2 released = 2
Biological Oxidation = 1
DIFFERENCES BETWEEN AEROBIC RESPIRATION AND ANAEROBIC RESPIRATION
FERMENTATION
It is a kind of anaerobic respiration in which certain prokaryotic, eukaryotic micro organisms and rarely higher plants and animal cells also participate. It is extra cellular or intercellular and rarely intra cellular in occurrence. The term fermentation was coined by Louis Pasteur. Buchner discovered Zymase during the fermention in Yeast cells. Fermentation is of different types based on the final acceptor of Hydrogen. It occurs in cytoplasm (or e−) by the organic substance. After glycolysis, the end products (2 Pyruvic acids) show the following reactions in different kinds of fermentation.
1) Alcoholic fermentation
2) Lactic Acid Fermentation
It occurs in Lactic acid bacteria like Lactobacillus aceti. It also occurs in muscles.
Lactic acid, if accumulated in muscles we feel tired.
3) Butyric Acid Fermentation
It occurs in Clostridium butyricum
4) In Yeast cells it occurs as follows
Zymase is extra cellular in action.
Pasteur's effect
When Yeast cells are exposed to Oxygen, the consumption of glucose and formation of alcohol drop. It is called Pasteur effect.
Why it happens?
The reason is that - Fermentation results in 2 ATP per glucose. In aerobic respiration one glucose produces 36 ATP. So Yeast cells consume less glucose for the production of same energy required for its metabolic functions.
RESPIRATORY QUOTIENT (RQ)
It is also called Respiratory Coefficient. The ratio between the number of CO2 molecules released and O2 molecules consumed during the complete oxidation of respiratory substrate is called RQ. It varies with substrate and conditions under which it is undergoing oxidation. RQ is measured by Ganong's Respirometer. RQ for various substrates is as follows
1) Carbohydrates
a) Aerobic respiration C6H12O6 + 6 O2 → 6 CO2 + 6 H2O
b) Anaerobic respiration
Glucose in it releases 2 CO2 without taking O2
2) Proteins
They have more carbons than O2. So they require more O2 and release less CO2 So RQ is less than 1.
3) Fats
They have more Carbons and less Oxygen in their structure. They take more O2 and release less CO2. So their RQ is less than 1.
C57H104O6 + 80 O2 → 57 CO2 + 52 H2O
Triolein
C18H34O2 + 25.5 O2 → 18 CO2 + 17 H2O
2 C51H98 O6 + 145 O2 → 102 CO2 + 98 H2O
ORGANIC ACIDS
They have more Oxygen than Carbons in their structure. So they take less O2 and release more CO2. Hence RQ is more than 1.
C4H6O5 + 3 O2 → 4 CO2 + 3 H2O
(Malic acid)
2 C4H6O6 + 5 O2 → 8 CO2 + 6 H2O
2 C2H2O4 + O2 → 4 CO2 + 2 H2O
How much energy of Glucose in utilised by the cell is aerobic respiration?
Loss or Gain?
1 molecule of glucose produces 36 ATP in aerobic respiration.
1 molecule of Glucose has 686 K. Cal energy.
1 ATP on hydrolysis produces 7.6 K. Cal energy.
So 36 ATP produce 36 × 7.6 = 273.6 K. Cal.
The remaining energy of the Glucose is released in the form of heat. It is 412.4 K. Cal. It is wasted.
In anaerobic respiration
It produces 2 ATP only.
So the cell utilises 2 × 7.6 = 15.2 K .Cal.
The remaining i.e, 686 - 15.2 = 670.8 K. Cal energy is released in the form of heat.
