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Plant Respiration
Exchange of Gases in Plants:
Plants do not have great demands for gaseous exchange. The rate of respiration in
plants is much lower than in animals. Large amounts of gases are exchanged only
during photosynthesis, and leaves are well equipped for that. The distance travelled by
gases in plants is not much and hence diffusion is enough to meet the need. Hence,
plants do not have specialized organs for exchange of gases. Lenticels and stomata
serve as the openings through which exchange of gases takes place in plants.
Respiration:
The complete combustion of glucose yields energy during respiration. Most of the
energy produced during respiration is given out as heat. CO 2 and H2O are the end
products of respiration.
The energy produced during respiration is also used for synthesizing other molecules.
To ensure the adequate supply of energy for synthesis of different molecules; plants
catabolise the glucose molecule in such a way that not all the liberated energy goes out
as heat. Glucose is oxidized in several small steps. Some steps are large enough to
ensure that the released energy can be coupled with ATP synthesis.
Steps of Respiration:
Respiration happens in two main steps in all living beings, viz. glycolysis and processing
of pyruvate. Glycolysis involves breaking down glucose into pyruvate. This is common
in all living beings. Further processing of pyruvate depends on the aerobic or anaerobic
nature of an organism. In anaerobic respiration, pyruvate is further processed to
produce either lactic acid or ethyl alcohol. There is incomplete oxidation of glucose in
anaerobic respiration. In aerobic respiration, pyruvate is further processed to produce
carbon dioxide and water; alongwith energy. There is complete oxidation of glucose in
case of aerobic respiration.
GLYCOLYSIS
The scheme of glycolysis was given by Gustav Embden, Otto Meyerhof and J Parnas.
Due to this, it is also called the EMP Pathway. Glycolysis takes place in the cytoplasm.
Glucose undergoes partial oxidation in glycolysis; to form two molecules of pyruvic acid.
Four molecules of pyruvic acid are formed after partial oxidation of one molecule of
glucose during this process.
First of all, glucose and fructose undergo phosphorylation to produce glucose-6phosphate. The enzyme hexokinase facilitates this process. Two molecules of
ATP are utilised during phosphorylation of one molecule of glucose. Two
molecules of fructose-6-phosphate are formed at the end of this step.
Fructose-6-phosphate is then converted into PGAL (Phosphoglyceraldehyde).
Each molecule of PGAL then undergoes various steps to finally produce Pyruvic
Acid. Four molecules of ATP are produced during this conversion. Since two
molecules of ATP were utilised during phosphorylation of glucose, hence net
production of ATP at the end of glycolysis is two for each molecule of glucose.
Fate of Pyruvic Acid: Pyurvic acid further undergoes subsequent processes which are
different for anaerobic and aerobic conditions.
FERMENTATION:Endogenous electron acceptors are used for oxidation of organic compounds during
fermentation. This is in contrast to aerobic respiration in which exogenous electron
acceptors are used. Anaerobic does not necessarily mean absence of oxygen, rather it
can also take place even in the presence of oxygen.
Sugar is the most common substrate of fermentation. Ethanol, lactic acid and hydrogen
are the common fermentation products. However, other compounds can also be
produced by fermentation, e.g. butyric acid and acetone. Apart from taking place in
yeast and many other anaerobes, fermentation also takes place in mammalian muscles.
In our muscle cells, fermentation takes place during intense exercise; to meet out the
excess demand of oxygen.
AEROBIC RESPIRATION
Aerobic respiration takes place within the mitochondria. Following are the main steps in
aerobic respiration:
Stepwise removal of all the hydrogen atoms leads to complete oxidation of
pyruvate. This leaves three molecules of CO2. This step takes place in the matrix
of mitochondria.
Electrons removed from hydrogen atoms are passed on to molecular O 2. This
happens with simultaneous synthesis of ATP. This step takes place in the inner
membrane of mitochondria.
Pyruvate enters the mitochondira matrix and undergoes oxidative
decarboxylation. This involves a complex set of reactions which are catalysed by
pyruvic dehydrogenase.
During this process, two molecules of NADH are produced from the metabolism of two
molecules of pyruvic acid (produced from one glucose molecule during glycolysis).
After this, acetyl CoA enters a cyclic pathway. This pathway is called tricarboxylic acid
cycle or Citric Acid Cycle or Krebs’ Cycle. This was first explained by Hans Krebs.
Kreb's Cycle
The TCA cycle starts with the condensation of acetyl group with oxaloacetic acid
(OAA) and water to yield citric acid. This 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. These steps of
decarboxylation lead to the formation of α-ketoglutaric acid and then succinylCoA.
After that, succinyl-CoA is oxidised to OAA allowing the cycle to continue. During
this step, 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. Moreover, there are three points in the cycle where NAD + is
reduced to NADH+H+ and one point where FAD+ is reduced to FADH2.
The continued oxidation of acetic acid via the TCA cycle requires the continued
replenishment of oxaloacetic acid. It also requires regeneration of NAD + and
FAD+ from NADH and FADH2respectively.
Electron Transport System (ETS) and Oxidative Phosphorylation
The next steps are to release and utilize the energy stored in NADH+H + and FADH2.
This is accomplished when they are oxidised through the electron transport system and
the electrons are passed on to O2resulting in the formation of H2O.
The metabolic pathway through which the electron passes from one carrier to another,
is called the electron transport system (ETS). This pathway is present in the inner
mitochondrial membrane.
Electrons from NADH (produced in the mitochondria matrix) are oxidized by an
NADH dehydrogenase (Complex I). After that, electrons are transferred to
ubiquinone which is located within the inner membrane.
Ubiquinone also receives reducing equivalents via FADH2 (Complex II). FADH2 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). This
coupling is necessary for the production of ATP from ADP and inorganic
phosphate. The nature of the electron donor decides the number of ATP
molecules synthesized.
Oxidation of one molecule of NADH gives rise to 3 molecules of ATP, while oxidation 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. But since oxygen
drives the whole process by removing hydrogen from the system, the presence of
oxygen is vital.
Yet, 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.
During photophosphorylation, light energy is utilised for the production of proton
gradient. But in respiration, the energy of oxidation-reduction is utilised for the
production of proton gradient. Hence, this process is called oxidative phosphorylation.
The energy released during the electron transport system is utilised in synthesizing ATP
with the help of ATP synthase (Complex V). This complex is composed of two major
components, viz. F1 and F0. The F1 headpiece is a peripheral membrane protein
complex. It contains the site for synthesis of ATP. F0 is an integral membrane protein
complex which forms the channel through which protons cross the inner membrane.
The passage of protons through the channel is accompanied by catalytic site of the F1
component for the production of ATP. For each ATP produced, 2H +passed through F0
down the electrochemical proton gradient.
The Respiratory Balance Sheet
The respiratory balance sheet gives theoretical value about net gain of ATP for every
glucose molecule oxidized. The calculations for respiratory balance sheet are based on
some assumptions which are as follows:
There is a sequential and orderly pathway in which one substrate makes the next
substrate. Glycolysis, TCA cycle and ETS pathway follow one after another.
NADH is synthesized in glycolysis and is transferred into the mitochondria. The
NADH undergoes oxidative phosphorylation within the mitochondria.
None of the intermediates in the pathway are utilised to synthesise any other
compound.
Glucose is the only substrate undergoing respiration. No other alternative
substrates are entering in the pathway at any stage.
But these assumptions may not be valid in a living system because all pathways work
simultaneously. There can be a net gain of 36 ATP molecules during aerobic respiration
of one molecule of glucose.
Amphibolic Pathway
Glucose is the most favoured substrate for respiration. Other substrates can also be
respired but they do not enter the respiratory pathway at the first step. Respiratory
process involves both catabolism and anabolism; because breakdown and synthesis of
substrates are involved. Hence, respiratory pathway is considered as an amphibolic
pathway rather than a catabolic one.
Respiratory Quotient
The ratio of the volume of CO2 evolved to the volume of O2 consumed during respiration
is called the respiratory quotient (RQ) or respiratory ratio. The RQ for carbohydrates is
1. The RQ for fat and protein is less than 1.
Reaction for respiration of fat:
Question – 1- Differentiate between
(a) Respiration and Combustion
Answer: Respiration is a type of combustion. But while combustion is an uncontrolled
process, respiration is controlled with high precision. Respiration takes place inside the
cells of living beings, while combustion can take place anywhere.
(b) Glycolysis and Krebs’ cycle
Answer: Breakdown of glucose into pyruvic acid is called glycolysis, while further
processing of pyruvic acid through aerobic route is called Krebs’ cycle. Glycolysis
happens in all living beings, while Krebs’ cycle happens in aerobes only. Glycolysis
happens in cytoplasm, while Krebs’ cycle happens in mitochondria.
(c) Aerobic respiration and Fermentation
Answer: Anaerobic respiration is also called fermentation. Ethanol and lactic acid are
the major products of fermentation.
Question – 2 - What are respiratory substrates? Name the most common respiratory
substrate.
Answer: A compound which is oxidized during respiration is called respiratory substrate.
Glucose is the most common respiratory substrate.
Question – 3 - Give the schematic representation of glycolysis?
Answer: Refer to the chapter notes
Question – 4 - What are the main steps in aerobic respiration? Where does it take
place?
Answer: Aerobic respiration takes place within the mitochondria. Following are the main
steps in aerobic respiration:
Stepwise removal of all the hydrogen atoms leads to complete oxidation of
pyruvate. This leaves three molecules of CO2. This step takes place in the matrix
of mitochondria.
Electrons removed from hydrogen atoms are passed on to molecular O2. This
happens with simultaneous synthesis of ATP. This step takes place in the inner
membrane of mitochondria.
Pyruvate enters the mitochondira matrix and undergoes oxidative decarboxylation. This
involves a complex set of reactions which are catalysed by pyruvic dehydrogenase.
Question – 5- Give the schematic representation of an overall view of Krebs’ cycle.
Answer: Refer to chapter notes
Question – 6 - Explain ETS.
Answer: The metabolic pathway through which the electron passes from one carrier to
another, is called the electron transport system (ETS). This pathway is present in the
inner mitochondrial membrane.
Electrons from NADH (produced in the mitochondria matrix) are oxidized by an
NADH dehydrogenase (Complex I). After that, electrons are transferred to
ubiquinone which is located within the inner membrane.
Ubiquinone also receives reducing equivalents via FADH2 (Complex II). FADH2
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). This
coupling is necessary for the production of ATP from ADP and inorganic
phosphate. The nature of the electron donor decides the number of ATP
molecules synthesized.
Question – 7 - Distinguish between the following:
(a) Aerobic respiration and Anaerobic respiration
Answer: Aerobic respiration needs oxygen, while anaerobic respiration does not need
oxygen. There is complete oxidation of glucose in aerobic respiration, while it is
incomplete in anaerobic respiration. Lactic acid and ethanol are the main products of
anaerobic respiration, while carbon dioxide is the end product of aerobic respiration.
(b) Glycolysis and Fermentation
Answer: Breakdown of glucose into pyruvic acid is called glycolysis, while further
processing of pyruvic acid in anaerobes is called fermentation.
(c) Glycolysis and Citric acid Cycle
Answer: Breakdown of glucose into pyruvic acid is called glycolysis, while further
processing of pyruvic acid through aerobic route is called Citric acid cycle. Glycolysis
happens in all living beings, while Citric acid cycle happens in aerobes only. Glycolysis
happens in cytoplasm, while Citric acid cycle happens in mitochondria.
Question – 8 - What are the assumptions made during the calculation of net gain of
ATP?
Answer: The calculations for respiratory balance sheet are based on some assumptions
which are as follows:
There is a sequential and orderly pathway in which one substrate makes the next
substrate. Glycolysis, TCA cycle and ETS pathway follow one after another.
NADH is synthesized in glycolysis and is transferred into the mitochondria. The
NADH undergoes oxidative phosphorylation within the mitochondria.
None of the intermediates in the pathway are utilised to synthesise any other
compound.
Glucose is the only substrate undergoing respiration. No other alternative
substrates are entering in the pathway at any stage.
Question – 9 - Discuss “The respiratory pathway is an amphibolic pathway.”
Answer: Respiratory process involves both catabolism and anabolism; because
breakdown and synthesis of substrates are involved. Hence, respiratory pathway is
considered as an amphibolic pathway rather than a catabolic one.
Question – 10 - Define RQ. What is its value for fats?
Answer: The ratio of the volume of CO2 evolved to the volume of O2 consumed during
respiration is called the respiratory quotient (RQ) or respiratory ratio. The RQ for
carbohydrates is 1. The RQ for fat and protein is less than 1.
Question – 11 - What is oxidative phosphorylation?
Answer: During photophosphorylation, light energy is utilised for the production of
proton gradient. But in respiration, the energy of oxidation-reduction is utilised for the
production of proton gradient. Hence, this process is called oxidative phosphorylation.
Question – 12 - What is the significance of step-wise release of energy in respiration?
Answer: The energy produced during respiration is also used for synthesizing other
molecules. To ensure the adequate supply of energy for synthesis of different
molecules; plants catabolise the glucose molecule in such a way that not all the
liberated energy goes out as heat. Glucose is oxidized in several small steps. Some
steps are large enough to ensure that the released energy can be coupled with ATP
synthesis.