The process of oxidation of organic substances in the living cells resulting in the formation of energy is defined as the respiration. The potential energy stored in the organic compounds such as carbohydrates, proteins, and fats, etc., in the living cells is liberated in the form of kinetic energy (ATP). This kinetic energy is utilized by the protoplasm for its numerous physiological activities such as manufacture of food, movements, growth, and reproduction, etc.
The
ATP stands for Adenosine triphosphate. It is known as energy currency. It
consists of adenine and three inorganic phosphate molecules bound by
high energy phosphate bond. When the bonds between adenine and inorganic
phosphates is broken, enough energy is released to carry out many biological processes.
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Site of respiration
Respiration
occurs in the cells; it is called cellular respiration. The glycolysis
takes places in the cytoplasm of the cells whereas the Krebs’ cycle
in the matrix of mitochondria.
Types of respiration
Based on the requirement of oxygen, the respiration is classified as aerobic respiration and anaerobic respiration.
Aerobic respiration: The complete oxidation of the organic food into CO2, H2O, and energy in presence of oxygen is called aerobic respiration. It is also known as oxy-respiration. Following equation represents the complete oxidation of glucose molecule by aerobic respiration:
Anaerobic respiration: The incomplete oxidation of the organic food into CO2 and ethyl alcohol (C2H5OH) or lactic acid (CH3CHOH.COOH) in absence of oxygen is anaerobic respiration. It can be represented by following equation:
Significance of respiration
- Respiration is essential process for the growth and maintenance of all plant tissues.
- It is important because it produces energy that is essential for the normal functioning of the body.
- It has important role in balancing carbons in the individual cells, plants, ecosystems and the global carbon cycle.
- It provides energy for the biosynthesis of macromolecules like carbohydrates, lipids, proteins, etc., that are required by the cells.
- Anaerobic respiration is helpful in the industrial production of alcohol, antibiotics, vitamins, organic acids, bakery, and dairy products.
Mechanism of respiration
The
chemical events that occur during the respiration are called collectively
called respiratory cycle. The process of aerobic respiration involves glycolysis,
oxidative phosphorylation of pyruvic acid, Krebs'
cycle, and Electron transport system.
A. Glycolysis (EMP-Pathway)
The
process of breaking down one molecule of glucose into two molecules of pyruvic
acids is the glycolysis. It is the first step of cellular respiration occurring
in the cytoplasm. It is common to both the aerobic and anaerobic respiration. It
is also called EMP-pathway as it investigated by German biologists – Embden,
Meyerhof, and Paranas.
Glycolysis
occurs in 10 chemical reactions which are grouped into two phases – energy
spending phase and energy yielding phase.
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| Figure 2 Glycolysis |
Energy spending phase
This phase includes first five reactions of glycolysis, by
which ATP molecules are spent.
In this step, a phosphate group from ATP is added to the glucose molecule, by the action of enzyme heterokinase, to form glucose 6-phosphate.
Step 2: Isomerisation
Glucose 6-phosphate is isomerised into fructose 6-phosphoate by the enzyme phosphohexo isomerase.
Another ATP molecule transfers a phosphate group to fructose 6-phosphate to form fructose 1,6-bisphosphate by the action of enzyme phosphofructokinase.
The enzyme aldolase splits fructose 1,6-bisphosphate into Glyceraldehyde 3-phosphate and dihydroxyacetone phosphate.
Step 5: Isomerisation
The enzyme triosephosphate isomerase converts dihydroxyacetone phosphate into glyceraldehyde 3-phosphate.
Energy yielding phase
This phase includes last five reactions of glycolysis, by
which ATP and NADH molecules are formed.
In this step, firstly, the glyceraldehyde phosphate
dehydrogenase enzyme transfers one hydrogen molecule from glyceraldehyde
3-phosphate to NAD to form NADH + H+. Then, the enzyme again adds a
phosphate group to the oxidized glyceraldehyde 3-phosphate to form
1,3-bisphosphoglyceric acid.
In this step, two molecules of 3-phosphoglyceric acid and ATP are formed by the transferring of a phosphate group from 1,3-bisphosphoglyceric acid to ADP in presence of phosphoglyceric kinase.
Two molecules of 2-phosphoglyceric acid are formed from 3-phosphoglyceric acid by the relocating of phosphate group from its 3rd to 2nd carbon in presence of phosphoglyceromutase.
Step 9: Dehydration
Two molecules of 2-phosphoenol pyruvic acid are formed by the removal of water from 2-phosphoglyceric acid in presence of enolase.
Step 10: Formation of ATP
Two molecules of pyruvic acid and ATP are formed by the transferring of a phosphate group form phosphoenol pyruvic acid to ADP in presence of pyruvate kinase.
The glycolysis can be summarized as:
B. Oxidative decarboxylation of pyruvic acid
The
pyruvic acid, formed in the cytoplasm during glycolysis, enters into the
mitochondria. Then, it reacts with CoA (Co-enzyme-A) in presence of pyruvate
dehydrogenase to produce Acetyl-CoA, CO2, NADH+, and
H+. This release of carbon dioxide from pyruvic acid is called the
oxidative decarboxylation.
C. Krebs’ Cycle or Citric acid cycle or TCA cycle
Citric
acid cycle or TCA (tricarboxylic acid) cycle is named as Krebs' cycle, as it was
investigated by Sir Hans Krebs. It occurs in mitochondria by the necessary
enzymes which are found on mitochondrial cristae. The acetyl-coA, formed after
oxidative decarboxylation, undergoes a series of changes is called Krebs cycle.
As citric
acid is formed in the first step, it is called citric acid cycle or
tricarboxylic acid (TCA) cycle. It involves ten steps as mentioned below:
Step 1: condensation
In presence of citrate synthetase, acetyl-coA reacts with oxaloacetic acid to form citric acid as the first stable product.
Step 2: Dehydration
Citric acid losses one molecule of H2O to change into cis-aconitic acid in presence of aconitase enzyme.
Step 3: Hydration
Cis-aconitic acid reacts with one molecule of water to form iso-citric acid in presence of aconitase enzyme.
Step 4: Dehydrogenation
Iso-citric acid is oxidized into
oxalosuccinic acid in the presence of isocitrate dehydrogenase.
Meanwhile, hydrogen is released and is accepted by NAD to form NADH2.
Step 5: Decarboxylation
Oxalosuccinic acid loses one molecule of CO2 to form α-ketoglutaric acid in presence of decarboxylase enzyme.
Step 6: Dehydrogenation and decarboxylation
α-ketoglutaric acid reacts with CoA and NAD+ to form succinyl CoA, one molecule of CO2, and NADH2 in presence of α-ketoglutaric acid dehydrogenase.
Step 7: Formation ATP/GTP
Succinyl CoA reacts with water molecule to form succinic acid
in presence of enzyme succinyl thiokinase. One molecule of GDP
(Guanosine Diphosphate) is phosphorylated by inorganic phosphate to form GTP
(Guanosine Triphosphate). 
Step 8: Dehydrogenation
Succinic acid is oxidized to form fumaric acid in the presence of succinic dehydrogenase. The coenzyme FAD (Flavin Adenine Dinucleotide) is reduced to FADH2.
Step 9: Hydration
A molecule of water is added to fumaric acid in presence of fumarase to form malic acid.
Step 10: Dehydrogenation
Oxaloacetic acid is regenerated by oxidation of malic acid in
presence of malic dehydrogenase. One molecule of NAD+ is
reduced to NADH and H+. Oxaloacetic acid picks up another molecule
of coenzyme A and repeats the TCA cycle.
Krebs Cycle can be summarised as:
The net reaction of Krebs cycle
Electron Transport System
(ETS), Terminal Oxidation or Oxidative Phosphorylation
The series of reactions in which
energy present in the form of the electrons and protons are carried by NADH and
FADH2 is known as the electron transport. Electron transport is
carried out by electron transport system (ETS) which is made of four
multi-protein complexes – Complex-I, Complex-II, Complex-III, and
Complex-IV. These complexes are localized in the inner mitochondrial membrane.
During electron transport, electrons
are passed from one electron-carrier molecule to another through FAD,
Cytochrome-b, Cytochrome-c, Cytochrome-c, and Cytochrome-a3 until
they are accepted by the oxygen atoms. The reaction of electron transport occur
as follows:
1.
The
hydrogen carriers move to the inner membrane of the mitochondrion, where are
the cristae for increased surface area.
2.
The
hydrogen ions carried to the cristae undergo stepwise oxidation using molecular
oxygen and energy is released in a series of small steps Some of this energy is
used to make ATP from ADP and inorganic phosphate (Pi) in the presence of
enzyme ATPase. This is called oxidative phosphorylation.
3.
During
these reactions, the hydrogen is split into H+ and electrons (e-),
which are accepted by a series of hydrogen or electron carriers ending with
oxygen. This series of carriers make the respiratory chain.
4.
Hydrogen
or electrons at a higher energy level are passed from one carrier to the next,
moving downhill in energy terms, until they reach oxygen, the final acceptor of
electrons. As a result, oxygen is reduced to water.
5.
At
each transfer level, some energy is released as heat and in some of the transfers
this is used for the formation of ATP.
6.
The
final step involves cytochrome oxidase enzyme, which hands over the electrons
to the H+ before being accepted by oxygen to form water.
7.
For
each NADH2 that enters the respiratory chain, 3ATP can be made but
for each FADH2, only 2ATP can be made.
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| Figure Electron transport system |
Factors affecting respiration
The
external and internal factors affect the rate of respiration.
A. External factors
The external factors include temperature,
oxygen, carbon dioxide, water, light, injury, mechanical effects, effects of
some chemical compounds (respiratory inhibitors).
1. Temperature
For respiration, minimum
temperature, maximum temperature, and optimal temperature are 0˚ C, 45˚ C and 30˚ C
respectively. An increase in temperature from 0˚ C to 30˚ C, the rate of respiration tends to increase. At very low temperature, respiration
slows down and may even be stopped due to the denaturation of the respiratory
enzymes.
2. Oxygen
In complete absence of oxygen, the
anaerobic respiration occurs but aerobic respiration stops. When sufficient
amount of O2 is available, the rate of aerobic respiration will be optimal
while the anaerobic respiration will be completely stopped. This is called an extinction
point.
3. Carbon dioxide
The
higher concentration of CO2 in the atmosphere (poorly aerated soil) has
retarding effect on the rate of the respiration.
4. Inorganic salts
When
a plant is transferred from water to salt solution, the rate of respiration
increases due to salt respiration.
5. Water
Respiratory cell needs proper
hydration. However, the rate of respiration decreases with increased amount of
water.
6. Light
Light indirectly affects the rate of
respiration through the synthesis of organic food matter during photosynthesis.
7. Injury
Wounding of plant organs stimulates
respiration in that organ. After the wound is healed, the rate of respiration
becomes normal.
8. Mechanical effects
In
some plants, gentle rubbing or bending of leaf blade (lamina) causes the
increase in the rate of respiration.
9. Effects of some chemical compounds (respiratory inhibitors)
Some compounds like cyanides, azides, 2, 4-dinitrophenol,
CO, fluorides, malonates, and iodoacetate retard the rate of respiration by
inhibiting enzymes of one stage or another of respiratory mechanism.
B. Internal Factors
The internal factors include
protoplasmic factors, concentration of the respiratory substrates, and age of
the plant.
1. Protoplasmic factors
The
amount of protoplasm in the cells and its state of activity influence the rate
respiration. The actively dividing cells like meristematic cells require more
energy so that they have high rate of respiration. Old mature tissues have low
rate of respiration due to lesser amount of not so active protoplasm.
2. Concentration of the respiratory substrate
When other
factors are favourable, the increased concentration of respirable food material
causes increased rate of respiration. Under starvation condition, such as,
etiolated leaves, the rate of respiration is considerably slow.
3. Age of the plant
The rate of respiration decreases with the age of the plant.
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