Republic of Iraq

Ministry of Higher Education

&Scientific Research
University of Anbar
College of Science

Lecture 10
RESPIRATION IN PLANTS
GLYCOLYSIS

Tricarboxylic Acid Cycle

Electron Transport System (ETS) and Oxidative Phosphorylation

For
Dr. Enas Fahd Naji
Department of Biology
College of Sciences Plant Physiology Dr. Enas Fahd Naji
Department of Biology
3rd stage

RESPIRATION IN PLANTS

The breakdown of complex molecules to yield energy takes place in the

cytoplasm and in the mitochondria (also only in eukaryotes). The
breaking of the C-C bonds of complex compounds through oxidation
within the cells, leading to release of considerable amount of energy is
called respiration. The compounds that are oxidised during this process
are known as respiratory substrates. Usually carbohydrates are oxidised to
release energy, but proteins, fats and even organic acids can be used as
respiratory substances in some plants, under certain conditions. During
oxidation within a cell, all the energy contained in respiratory substrates
is not released free into the cell, or in a single step. It is released in a
series of slow step-wise reactions controlled by enzymes, and it is trapped
as chemical energy in the form of ATP. Hence, it is important to
understand that the energy released by oxidation in respiration is not (or
rather cannot be) used directly but is used to synthesize ATP, which is
broken down whenever (and wherever) energy needs to be utilized.
Hence, ATP acts as the energy currency of the cell. This energy trapped
in ATP is utilized in various energy-requiring processes of the organisms,
and the carbon skeleton produced during respiration is used as precursors
for biosynthesis of other molecules in the cell.

Plants, unlike animals, have no specialized organs for gaseous exchange

but they have stomata and lenticels for this purpose.

C6H12O6 + 6O2 6CO2 + 6H2O + Energy

If this energy is to be useful to the cell, it should be able to utilize it to

synthesize other molecules that the cell requires. The strategy that the
plant cell uses is to catabolize the glucose molecule in such a way that not
all the liberated energy goes out as heat. The key is to oxidize glucose not
in one step but in several small steps enabling some steps to be just large
enough such that the energy released can be coupled to ATP synthesis.
During the process of respiration, oxygen is utilized, and carbon dioxide,
water and energy are released as products. The combustion reaction
requires oxygen. But some cells live where oxygen may or may not be
available. Even among present-day living organisms, we know of several
that are adapted to anaerobic conditions. Some of these organisms are

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College of Sciences Plant Physiology Dr. Enas Fahd Naji
Department of Biology
3rd stage

facultative anaerobes, while in others the requirement for anaerobic

condition is obligate. In any case, all living organisms retain the
enzymatic machinery to partially oxidize glucose without the help of
oxygen. This breakdown of glucose to pyruvic acid is called glycolysis.

GLYCOLYSIS

The term glycolysis has originated from the Greek words, glycos for
sugar, and lysis for splitting. The scheme of glycolysis was given by
Gustav Embden, Otto Meyerhof, and J. Parnas, and is often referred to as
the EMP pathway. In anaerobic organisms, it is the only process in
respiration. Glycolysis occurs in the cytoplasm of the cell and is present
in all living organisms. In this process, glucose undergoes partial
oxidation to form two molecules of pyruvic acid. In plants, this glucose is
derived from sucrose, which is the end product of photosynthesis, or from
storage carbohydrates. Sucrose is converted into glucose and fructose by
the enzyme, invertase, and these two monosaccharides readily enter the
glycolytic pathway. Glucose and fructose are phosphorylated to give rise
to glucose-6- phosphate by the activity of the enzyme hexokinase. This
phosphorylated form of glucose then isomerises to produce fructose-6-
phosphate. Subsequent steps of metabolism of glucose and fructose are
same. In glycolysis, a chain of ten reactions, under the control of different
enzymes, takes place to produce pyruvate from glucose. ATP is utilized
at two steps: first in the conversion of glucose into glucose 6-phosphate
and second in the conversion of fructose 6-phosphate to fructose 1, 6-
bisphosphate. The fructose 1, 6-bisphosphate is split into
dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL). We
find that there is one step where NADH + H+ is formed from NAD+ ;
this is when 3-phosphoglyceraldehyde (PGAL) is converted to 1, 3-
bisphosphoglycerate (BPGA). Two redox-equivalents are removed (in the
form of two hydrogen atoms) from PGAL and transferred to a molecule
of NAD+ . PGAL is oxidized and with inorganic phosphate to get
converted into BPGA. The conversion of BPGA to 3-phosphoglyceric
acid (PGA), is also an energy yielding process; this energy is trapped by
the formation of ATP. Another ATP is synthesized during the conversion
of PEP to pyruvic acid. There are three major ways in which different
cells handle pyruvic acid produced by glycolysis. These are lactic acid

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College of Sciences Plant Physiology Dr. Enas Fahd Naji
Department of Biology
3rd stage

fermentation, alcoholic fermentation and aerobic respiration.

Fermentation takes place under anaerobic conditions in many prokaryotes
and unicellular eukaryotes. For the complete oxidation of glucose to CO2
and H2O, however, organisms adopt Krebs’ cycle which is also called as
aerobic respiration. This requires O2 supply.

AEROBIC RESPIRATION

For aerobic respiration to take place within the mitochondria, the final
product of glycolysis, pyruvate is transported from the cytoplasm into the
mitochondria. The crucial events in aerobic respiration are:

• The complete oxidation of pyruvate by the stepwise removal of all the

hydrogen atoms, leaving three molecules of CO2 .

• The passing on of the electrons removed as part of the hydrogen atoms

to molecular O2 with simultaneous synthesis of ATP. What is interesting
to note is that the first process takes place in the matrix of the
mitochondria while the second process is located on the inner membrane

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College of Sciences Plant Physiology Dr. Enas Fahd Naji
Department of Biology
3rd stage

of the mitochondria. Pyruvate, which is formed by the glycolytic

catabolism of carbohydrates in the cytosol, after it enters mitochondrial
matrix undergoes oxidative decarboxylation by a complex set of reactions
catalyzed by pyruvic dehydrogenase. The reactions catalyzed by pyruvic
dehydrogenase require the participation of several coenzymes, including
NAD+ and Coenzyme A.
Mg+2

Pyruvic acid + CoA + NAD+ Acetyl Co A + CO2 + NADH + H+

Pyruvate 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). The acetyl CoA then enters a cyclic
pathway, tricarboxylic acid cycle, more commonly called as Krebs’ cycle
after the scientist Hans Krebs who first elucidated it.

Tricarboxylic Acid Cycle

The TCA cycle starts with the condensation of acetyl group with
oxaloacetic acid (OAA) and water to yield citric acid. The 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, leading to the formation of α-
ketoglutaric acid and then succinyl-CoA. In the remaining steps of
citric acid cycle, succinyl-CoA is oxidized to OAA allowing the cycle to
continue. During the conversion of succinyl-CoA to succinic acid a
molecule of GTP is synthesized. This is a substrate level phosphorylation.
In a coupled reaction GTP is converted to GDP with the simultaneous
synthesis of ATP from ADP. Also 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 acetyl CoA via the TCA
cycle requires the continued replenishment of oxaloacetic acid, the first
member of the cycle. In addition it also requires regeneration of NAD+
and FAD+ from NADH and FADH2 respectively.

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College of Sciences Plant Physiology Dr. Enas Fahd Naji
Department of Biology
3rd stage

Electron Transport System (ETS) and Oxidative Phosphorylation

The following steps in the respiratory process are to release and utilize
the energy stored in NADH+H+ and FADH2. This is accomplished when
they are oxidized 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) and it is present in the inner
mitochondrial membrane. Electrons from NADH produced in the
mitochondrial matrix during citric acid cycle are oxidized by an NADH
dehydrogenase (complex I), and electrons are then transferred to
ubiquinone located within the inner membrane. Ubiquinone also receives
reducing equivalents via FADH2 (complex II) that is generated during
oxidation of succinate in the citric acid cycle. The reduced ubiquinone
(ubiquinol) is then oxidized 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) for the production

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College of Sciences Plant Physiology Dr. Enas Fahd Naji
Department of Biology
3rd stage

of ATP from ADP and inorganic phosphate. The number of ATP

molecules synthesized depends on the nature of the electron donor.
Oxidation of one molecule of NADH gives rise to 3 molecules of ATP,
while that 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. 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. Unlike photophosphorylation where it is the light
energy that is utilized for the production of proton gradient required for
phosphorylation, in respiration it is the energy of oxidation-reduction
utilized for the same process. It is for this reason that the process is called
oxidative phosphorylation.. As mentioned earlier, the energy released
during the electron transport system is utilized in synthesizing ATP
with the help of ATP synthase (complex V). This complex consists of
two major components, F1 and F0. The F1 headpiece is a peripheral
membrane protein complex and contains the site for synthesis of ATP
from ADP and inorganic phosphate. F0 is an integral membrane protein
complex that forms the channel through which protons cross the inner
membrane. The passage of protons through the channel is coupled to the
catalytic site of the F1 component for the production of ATP. For each
ATP produced, 4H+ passes through F0 from the intermembrane space to
the matrix down the electrochemical proton gradient.

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College of Sciences Plant Physiology Dr. Enas Fahd Naji
Department of Biology
3rd stage

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College of Sciences Plant Physiology Dr. Enas Fahd Naji
Department of Biology
3rd stage

References:

1. Plant Physiology, 3rd ed by Lincoln Taiz and Eduardo Zeiger, 2002

2. Plant Physiology, Development and Metabolism, Satish C Bhatla ,

Manju A. Lal, 2018. ISBN 978-981-13-2022-4 ISBN 978-981-13-2023-1
(eBook)

3. PLANT PHYSIOLOGY Vince Ördög , 2011 .

4. Plant Solute Transport Edited by ANTHONY YEO Haywards Heath,

West Sussex, UK, TIM FLOWERS School of Life Sciences University of
Sussex, UK, 2007.

, ‫ كلية العلوم‬,‫ قسم النبات‬,‫ حشمت سليمان احمد الدسوقي‬.‫د‬.‫ أ‬, ‫ اساسيات فسيولوجيا النبات‬.5
.8002 , ‫ جمهورية مصر العربية‬, ‫جامعة المنصورة‬

‫ جامعة‬, ‫ كلية العلوم‬, ‫ قسم العلوم البيولوجية‬, ‫ بسام طه ياسين‬.‫ د‬, ‫ أساسيات فسيولوجيا النبات‬.6
.8002 , ‫قطر‬