Respiration in Plants 2 Flashcards
what are the crucial events of aerobic respiration
- 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 of the mitochondria.
how is pyruvate transported into the mitochondrial matrix for kreb cyclce
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 catalysed by pyruvic
dehydrogenase. The reactions catalysed by pyruvic dehydrogenase require
the participation of several coenzymes, including NAD+
and Coenzyme A.
Pyruvic Acid+ CoA+ NAD+——-Mg2+/ Pyruvic dehydrogenase—> Acetyl CoA+ CO2+ NADH+H+
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
give an overview of tricarboxylic acid cycle
The TCA cycle starts with the condensation of acetyl group with oxaloacetic
acid (OAA) and water to yield citric acid (Figure 12.3). 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 oxidised to
OAA allowing the cycle to continue.
what is the energy produced in kreb’s cycle
During the
conversion of succinyl-CoA to succinic acid 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.
Also there are three points in the cycle where
NAD+
is reduced to NADH + H+
and one point
where FAD+
is reduced to FADH2
.
what is electron transport system ?
The metabolic
pathway through which the electron passes from one carrier to another,
is called the electron transport system (ETS) (Figure 12.4) and it is
present in the inner mitochondrial membrane.
trace the pathway of ets
Electrons from NADH produced in the mitochondrial matrix
during citric acid cycle are oxidised 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
oxidised with the transfer of electrons to
cytochrome c via cytochrome bc 1
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.
how is atp produced? from whr is atp released
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 of ATP from ADP and
inorganic phosphate. The number of ATP
molecules synthesised 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. A
what is the role of oxygen
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 utilised for the
production of proton gradient required for phosphorylation, in respiration
it is the energy of oxidation-reduction utilised for the same process. It is
for this reason that the process is called oxidative phosphorylation.
how does ets lead to formation of atp
the energy released during the electron transport system is utilised in synthesising ATP
with the help of ATP synthase (complex V). This
complex consists of two major components, F1
and F0 (Figure 12.5). 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.
what assumptions is the respiratory balance sheet based one
There is a sequential, orderly pathway functioning, with one
substrate forming the next and with glycolysis, TCA cycle and ETS
pathway following one after another.
* The NADH synthesised in glycolysis is transferred into the
mitochondria and undergoes oxidative phosphorylation.
* None of the intermediates in the pathway are utilised to synthesise
any other compound.
* Only glucose is being respired – no other alternative substrates are
entering in the pathway at any of the intermediary stages.
justifyl respiration is amphibolic
Glucose is the favoured substrate for respiration. All carbohydrates are
usually first converted into glucose before they are used for respiration. Fats would need to be broken down into glycerol and fatty acids
first. If fatty acids were to be respired they would first be degraded to
acetyl CoA and enter the pathway. Glycerol would enter the pathway
after being converted to PGAL. The proteins would be degraded by
proteases and the individual amino acids (after deamination) depending
on their structure would enter the pathway at some stage within the Krebs’
cycle or even as pyruvate or acetyl CoA.
Usually respiration is a catabolic process because it is basedon the breakdown of substrates. However, when we see the entry of other substrates, synthesis is also possible. For example, the acetyl CoA could be used to make fatty acids, similarly PGAL to make glycerol. The acids could be used to amino acids as well. So respiration is not only anabolic but also catabolic, hence amphibolic
which complexes act as proton pump
complexes 1,3, and 4 act as proton pump. This is because there is a lot of energy released due to electron tansfer, this energy is then used to pump electrons acrosss the membrane to the intermembrane space.
why does complex 2 not act as proton pump
The electrons released by fadh2 have much less energy than that of e-s released by nadh2. hence the complex 2 cannot pump protons.
how is the gradient formed
a proton gradient is formed across the inner membrane of mitochondira. there is an increase in conc of protons in the intermembrane space and decrease in proton conc in the matrix.
this is because of the pumping of r=protons by complexes 1,2,4. Also oxygen is bound to the Cu (b). This oxygen receives electrons from cyt c oxidase. in addition to these electrons it also picks up protons from the matrix to form water. hence decreasing the proton conc in the matrix.
why are these assumptions not valid
But this kind of assumptions are not really valid in a living system; all
pathways work simultaneously and do not take place one after another;
substrates enter the pathways and are withdrawn from it as and when
necessary; ATP is utilised as and when needed; enzymatic rates are
controlled by multiple means.
