RESPIRATION
IN PLANTS
RESPIRATION
Respiration can be defined as an energy-releasing
enzymatically controlled catabolic process which involves a step-wise oxidative
breakdown of food substances inside the living cells.
Respiration refers to the breaking of the carbon -carbon
bonds of the complex compounds by oxidation within the cells which leads to
release of energy. This energy is utilised in the synthesis of ATP. Compounds
that are oxidised during this process are known as respiratory substrates.
Plants need oxygen to breathe, yet they emit carbon dioxide.
Stomata in the leaves and lenticels in the stems allow for gas exchange. There
is a very little gas transmission from one section to another. There are no
high demands for breathing as found in mammals. Plants have living cells that
are near to the surface.
(1) In the presence of oxygen, during respiration carbon
dioxide and water is released as by-products along with a high amount of
energy.
(2) In absence of oxygen, all living things have the enzymatic
equipment to partly oxidise glucose and very little amount of energy is
produced.
Glycolysis
The word glycolysis stands for the splitting of sugar.
It is also called EMP pathway because it was discovered by
three German scientists - Gustav Embden, Otto Meyerhof and J Pamas in 1930.
Glycolysis is the process of partial oxidation of glucose or similar hexose
sugar into two molecules of pyruvic acid through a series of 10 enzyme mediated
reactions releasing some energy (as ATP) and reducing power (as NADH₂). It
occurs in cytosol or cytoplasm. Glycolysis is common in the both aerobic and
anaerobic modes of respiration. So, this is called universal pathway of
respiration.
It is the first stage of glucose breakdown in aerobic respiration
and the only step in glucose breakdown in anaerobic respiration. Glycolysis has
two phases, preparatory and payoff. In the preparatory phase, glucose is broken
down to glyceraldehyde 3-phosphate. In payoff phase, the latter is changed into
pyruvate producing NADH and ATP.
Steps of Glycolysis
(1) Glucose is derived from sucrose.
(2) Sucrose is converted into glucose and fructose by
invertase (enzyme).
(3) These two monosaccharides enter to glycolysis pathway.
(4) Hexokinase phosphorylates glucose and fructose to
produce glucose-6-phosphate.
(5) This phosphorylated glucose isomerises to produce
fructose-6-phosphate.
(6) Fructose-6-Phosphate is converted into
Fructose1,6-Bisphosphate.
(7) Aldolase converts Fructose-1,6-Bisphosphate to GLAP
(Glyceraldehyde-3-Phosphate) or DHAP (Dihydroxy acetone phosphate).
(8) Which is further converted into 1,3 Bisphosphoglycerate
by GLAP Hydrogenase.
(9) 1, 3 Bisphosphoglycerate is converted into
3-Phosphoglycerate by Phosphoglycerate Kinase.
(10) Phosphoglycerate Mutase converts 3- Phosphoglycerate to
2-Phosphoglycerate, which is further converted by Enolase into phosphenol
Pyruvate.
(11) Phosphenol pyruvate is converted to Pyruvate by
Pyruvate kinase.
(12) ATP is utilised in two steps
I. Conversion of glucose to glucose-6- phosphate.
II. Conversion of fructose-6-phosphate
fructose-1.6-bisphosphate.
IMPORTANT
There are three fates of pyruvic add which is produced
during glycolysis These are lactic acid fermentation alcoholic fermentation and
aerobic respiration. Pyruvic acid is the key product of glycolysis.
Fermentation
Lactic acid is produced in muscles during heavy
exercise. It is produced in large quantities in skeletal muscles in humans. The
conversion of pyruvate into lactic acid in lack of oxygen during exercise is
called lactic acid fermentation. This lactic acid is produced as a result of
incomplete oxidation due to inadequate oxygen supply to the muscles. Incomplete
oxidation of glucose occurs when anaerobic conditions are present. Pyruvate is
broken down to produce ethanol and carbon dioxide in alcoholic fermentation.
The amount of energy generated is really very low. If manufactured in
uncontrolled quantities, the process is dangerous.
Important
Fermentation takes place in the absence of oxygen in
bacteria (Rhizopus). some fungi and yeast, where Ethyl alcohol is produced.
Whereas, LAB (Lactic Acid Bacteria -Lactobacillus) and muscles of humans do
anaerobic respiration but form lactic acid as an end product.
AEROBIC RESPIRATION
Having a glass of glucose in between playing, during summer
gives instant energy to our body. Ever wondered why is it so? Why don't we
instead have a sandwich? Because glucose is a monosaccharide which requires no
digestion. When any person uses glucose, it directly absorb in our body and
readily oxidises to give us energy in our cells.
It is an enzymatically controlled release of energy in a
stepwise catabolic process of complete oxidation of organic food into CO
2 and H2O with 02 acting as
a terminal oxidant.
The common pathway of aerobic respiration consists of three
steps - glycolysis (common for both anaerobic and aerobic respiration), Krebs'
cycle or TCA cycle, and Electron transport system and oxidative
Phosphorylation.
Complete oxidation of organic substances or glucose
molecules releases carbon dioxide, water and large amounts of energy.
Steps in Aerobic Respiration
Final product of glycolysis i.e. pyruvate is transported
from cytoplasm to mitochondria.
Its two crucial steps are:
(1) Complete oxidation of pyruvate, leaving three molecules
of carbon dioxide [occurs in matrix of mitochondria].
(2) Passing on of electrons to molecular oxygen, with
production of ATP [occurs in inner membrane of mitochondria].
Oxidation of Pyruvate to Acetyl CoA:
Important
This reaction Is also known as a link reaction as it inks
the glycolysis and TCA cycle.
Tricarboxylic Acid Cycle (TCA Cycle)
Acetyl-CoA enters a cyclic pathway i.e. Tricarboxylic Acid
Cycle also known as Krebs' cycle as it was given by scientist Hans Krebs.
It occurs inside the matrix of mitochondria. The cycle is
also known as Citric Acid Cycle (CAC) after the name of the first stable
product of the cycle which is citric acid. TCA cycle is stepwise oxidative and
cyclic degradation of activated acetate derived from pyruvate.
Acetyl group of acetyl-CoA condenses with oxaloacetate and
water to produce citric acid. Above reaction is catalysed by enzyme citrate
synthase and a molecule of CoA is released.
Important
→ Cycle requires continuous replenishment of Oxaloacetic
acid, i.e. the first member of the cycle Requires regeneration of NAD and FAD+
from NADH and FADH₂.
Electron
Transport System [ETS] and Oxidative Phosphorylation
These respiratory processes help to release and utilise
energy stored in NADH + H+ and FADH2 molecules. The process is
completed when these are oxidised through ETS and the electrons are transferred
to 02 to form Н₂O.
It refers to the metabolic mechanism through which an electron is transferred from one carrier to another. it is present in the mitochondrial membrane.
Electrons from NADH (produced in the mitochondrial matrix
during the TCA cycle) are oxidised by NADH Dehydrogenase (Complex II).
Electrons are transferred to ubiquinone (inner mitochondrial membrane) and it
also receives reducing equivalents via FADH2 (Complex II) generated
in TCA cycle. Reduced ubiquinone is oxidised with transfer of electrons to
cytochrome с via cytochrome b and bc1 complex (Complex III) Cytochrome c is a
small protein acting as a mobile carrier for transfer of electrons between
complex III and complex IV, its found attached to the outer surface of inner
mitochondrial membrane. (Complex IV) is a cytochrome c oxidase complex
containing cytochromes a and a3 and two copper centres. Electrons
pass from one carrier to another via Complex I to IV in ETC, they are coupled
with ATP. Synthase (Complex V) thus producing ATP from ADP and inorganic
phosphates.
Number of ATP molecules produced depends on the nature of
the electron donor.
(1) One molecule of NADH gives 3 molecules of ATP.
(2) One molecule of FADH2 produces 2 molecules of
ATP.
Although the aerobic respiration takes place only in the
presence of oxygen but the role of oxygen is limited, i.e. used in the terminal
stage, but presence is vital It drives the whole process by removing hydrogen
from the system and acts as the final acceptor of hydrogen. Since energy of
oxidation and reduction is utilised for producing a proton gradient which is essential
for phosphorylation thus this process is termed as oxidative phosphorylation.
The Respiratory Balance Sheet
Net gain of ATP can be calculated for every molecule of
glucose broken down but it's just theoretical. The calculation of ATP synthesis
from the complete oxidation of one molecule of glucose is based on assumption.
Following assumptions are made while making a respiratory
balance sheet, which are enlisted below.
(1) 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.
(2) The NADH synthesised in glycolysis is transferred into
the mitochondria and undergoes oxidative phosphorylation.
(3) None of the intermediates in the pathway are utilised to
synthesise any other compound.
(4) Only glucose is being respired - no other alternative
substrates are entering in the pathway at any of the intermediary stages.
But this kind of assumption does not work in living
organisms, since substrates enter and leave the pathway as when needed.
Approximately; breakdown of one molecule of glucose yields 36 ATP.
|
ANAEROBIC RESPIRATION /
FERMENTATION |
AEROBIC RESPIRATION |
|
Takes place in absence of oxygen. |
Takes place in presence of oxygen. |
|
Occurs in cytoplasm. |
Occurs in cytoplasm followed by mitochondria. |
|
End products are ethanol and CO2. |
End products are water and CO2. |
|
Incomplete oxidation of substrate takes place. |
Complete oxidation of substrate takes place. |
|
Energy produced is low i.e., only 2 ATP produced from each molecule
of glucose. |
High amount of energy produced, i.e. 36 ATP by one molecule of
glucose. |
|
Example: Yeast |
Example: Higher plants and animals. |
Amphibolic
Pathway
Glucose is used as a substrate for respiratory pathway and
energy synthesis, other molecules such as fats and proteins are also used but
they cannot enter the respiratory pathway in the first step. All carbohydrates
are converted into glucose, it can enter the very first step of the respiratory
pathway.
Fats can enter the respiratory pathway, it needs to be
broken into glycerol and fatty acids which are converted into acetyl-CoA and
PGAL, respectively which then enters the respiratory pathway.
Proteins enter the pathway. first degraded into amino acids
by proteases and amino acids enter the pathway after deamination, it then enters
the respiratory pathway at some stage in Kerb’s cycle, or as pyruvate or
acetyl-CoA Respiratory pathway is termed as amphibolic pathway since substrates
like carbohydrates, fats, and proteins the enter the respiratory pathway for
synthesis, it is also withdrawn when there is a need for abovementioned
substrates. Thus, respiratory pathway is involved in both anabolism as well as
catabolism, therefore it is known as an amphibolic pathway.
Respiratory
Quotient
As we know, during aerobic respiration two things happen- O2 is consumed and CO2 Is released. Thus, respiratory quotient or respiratory ratio is given by:
Respiratory quotient or respiratory ratio for different
substrates are as follows:
(1) For carbohydrates, RQ = 1, they are completely oxidised
and equal amounts of oxygen and carbon dioxide are produced.
For example:
(2) For proteins, RQ = 0.9 approximately.
(3) For fats and proteins, RQ is less than 1.
