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RESPIRATION IN PLANTS
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. CO2 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.
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First of all, glucose and fructose undergo phosphorylation to produce glucose-6-phosphate.
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.
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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:
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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.
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Electrons removed from hydrogen atoms are passed on to molecular O 2. This happens with
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simultaneous synthesis of ATP. This step takes place in the inner membrane of mitochondria. 
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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
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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.
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It is followed by two successive steps of decarboxylation. These steps
decarboxylationlead to the formation of α-ketoglutaric acid and then succinyl-CoA.
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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.
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In a coupled reaction GTP is converted to GDP with the simultaneous synthesis of ATP from 
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of
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+
ADP. Moreover, there are three points in the cycle where NAD is reduced to NADH+H
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and one point where FAD is reduced to FADH2.
+
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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 FADH2 respectively.
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 O2 resulting 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.
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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.
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Ubiquinone also receives reducing equivalents via FADH2 (Complex II). FADH2 is
generated during oxidation of succinate in the citric acid cycle.
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The reduced ubiquinone (ubiquinol) is then oxidised with the transfer of electrons to
cytochrome c via cytochrome bc1 complex (complex III).
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Cytochrome c is a small protein attached to the outer surface of the inner membrane and
acts as a
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mobile carrier for transfer of electrons between complex III and IV.
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Complex IV refers to cytochrome c oxidase complex containing cytochromes a and a3, and
two copper centres.
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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.
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Oxidation of one molecule of NADH gives rise to 3 molecules of ATP, while oxidation of one
molecule of FADH2produces 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.
ELECTRON TRANSPORT PATHWAY
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:
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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.
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NADH is synthesized in glycolysis and is transferred into the mitochondria. The NADH
undergoes oxidative phosphorylation within the mitochondria.
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None of the intermediates in the pathway are utilised to synthesise any other compound. 
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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: