Cell Respiration, Chapter Notes, Class 11, Biology PDF Download

CELL RESPIRATION

HISTORY

Respiration term was given by Deutrochet.
Malpighie (1679) showed the importance of air at the time of germination of seeds of plants.
Pasteur (1870) discovered anaerobic respiration with the help of fermentation.

INTRODUCTION

Respiration is a catabolic process in which complex organic molecules are oxidized to simpler one with gradual and step wise release of energy.
Such respiration is also called cellular respiration or dark respiration or internal respiration.
It is a catabolic, exothermic and oxidative process.
It is also anabolic
Respiration is represented by following general equation :
C6H12O6 + 6O2 + 6H2O  6CO2 + 12H2O + Energy
Respiration is oxidation of organic food, which release the energy, which is utilized in systhesis of ATP.
Various respiratary intermediates are used up in biosynthesis so amphibolic process.
All living cells participate in the respiration process.

Significance of Respiration :

Energy is necessary for all biological processes.
The energy released during respiration is used for the various metabolic processes.
Various chemical substances are formed in this process which are important for cellular componants.
CO2 released in this process maintains a balance in the atmosphere.
Complex insoluble food materials are converted into simple soluble molecules by this process.
It converts the stored (static potential energy) into more useful (kinetic) form.

TYPE OF RESPIRATION

On the basis of kind of Respiratory Substrate:

Floating respiration :
It is normal respiration, where general respiratory substrate and stored food is oxidized. Starch, fat,
protein

Protoplasmic respiration :
It is respiration of starved cells when protoplasmic constituents are oxidized.
On the basis of availability of Oxygen :

Aerobic respiration

Anaerobic respiration

Aerobic respiration - The complete oxidation of food with the use of oxygen and when entire carbon released, as CO2 is called as aerobic respiration.

Anaerobic respiration - This is incomplete oxidation.
When food is oxidized into alcohol or organic acids without use of oxygen. During it most of energy is lost in form of heat. It occurs in cytoplasm and only 2ATP are produced.
C6H12O6

Enzyme

Anaerobic respiration was first reported by Kostytchev.,  oxidised in to some organic compounds like ethanol, acetic acid, lactic acid.
Ithn em cueslclsle. cells & some bacteria, the energy is produced by breaking of glucose into lactic acid inside

The amount of energy released in anaerobic respiration is much less than aerobic respiration.
pFreorcmeesnst.a 2ti oAnT Pis apreer fporromdeudc ebdy. some fungi & some bacteria (only by microbes) and is an extracellular
C6H12O6 Yeast

Both anaerobic rspiration and fermentation are incomplete oxidations.
Inhibitory effect on respiration (anaerobic respiration) of oxygen is called Pasteur effect.
(Anaerobic  Aerobic)

MECHANISM OF RESPIRATION

Initial steps of aerobic and anaerobic respiration are same i.e. Glucose is converted to pyruvic acid.
Further fate of pyruvic acid is dependent upon presence or absence of O2.
There are 2 major pathways of respiration.
Common Pathway
Pentose Phosphate Pathway (PPP)

An overview of Aerobic Respiration


Common Pathway :

It has 3 main parts.
Glycolysis
Krebs Cycle
Terminal Oxidation

GLYCOLYSIS
Glycolysis was discovered by Embden, Meyerhoff and Parnas and hence it is called as EMP pathway.
Glycolysis is independent of O2, hence it is common in both aerobic and anaerobic conditions.
Glycolysis is completed in cytoplasm.
Glucose is substrate of glycolysis. Most of enzymes requires Mg as cofactor.
The glycolysis is common phase for aerobic & anaerobic respirations both.
Glycolysis involves a series of ten biochemical reactions in cytoplasm.
In glycolysis, neither consumption of oxygen nor liberation of CO2 take place.

In glycolysis, 1 glucose, produces 2 mol. of pyruvic acids (3C) 2NADH2 & 2ATP are generated in glycoysis, which are equal to 8 ATP.
Substrate level phosphorylation forms 2 ATP - [When the substrate releases energy for phosphorylation of ADP OR formation of ATP, without ETS then called as substrate level phosphorylation]
Glycolysis is also known as oxidative anabolism or catabolic resynthesis, because it links with anabolism of fats and amino acids. Its intermediate PGAL is used for the synthesis of glycerol later forms fats or lipid. PGA is used for synthesis of Serine, Glycine, cystine. Alanine synthesized from pyruvate.
Biochemical reactions of Glycolysis are G Glucose GColunctorsoel P- o6Pint reaction


Phosphofructokinase and Hexokinase are allosteric enzymes. The steps catalysed by these enzymes are considerd as control point reactions of glycolysis.
Ist & 3rd and last reaction of glycolysis are considered as irreversible reactions of glycolysis.
Further oxidation of puruvic acid and NADH2 after glycolysis in mitochondria requires oxygen. So the fate of pyruvic acid is decided by presence or absence of Oxygen.


KREBS CYCLE / TRICARBOXYLIC ACID (TCA) CYCLE / CITRIC ACID CYCLE

Krebs cycle was discovered by Sir Hans Kreb in 1937 in pigeon muscles.
It is also called citric acid cycle, citric acid being the first product of krebs cycle.

Krebs cycle occurs in matrix of mitochondria as its enzymes are present in the matrix.
Kreb's cycle is amphibolic or anaplerotic in nature because it is central metabolic pathway playing an important role in both catabolism and anabolism.
Acetyl CoA is substrate entrant of krebs cycle.
During oxidation of 1 molecule of glucose, there occurs two rounds in a kreb's cycle.
4 NADH2, 1 FADH2 and 1 ATP molecules are obtained from one pyruvic acid molecule.

Two pyruvic acid are formed from one glucose. Therefore, two molecules of pyruvic acid yields 8NADH2, 2 FADH2 and 2 ATP molecules.
Before Krebs cycle operates, pyruvic acid is converted into Acetyl CoA.

Formation of Acetyl CoA / Link reaction / Gateway Reaction :

It occurs in perimitochondrial space.
Pyruvic acid is oxidatively decarboxylated to form Acetyl CoA. This requires a multienzyme complex, Pyruvate dehydrogenase complex.
Pyruvate dehydrogenase complex consists of 4 Coenzymes and one cofactors i.e. TPP
(Thiamine pyrophosphate), lipoic acid, coenzyme A, NAD, Mg+2
Acetyl CoA is connecting link between glycolysis and Kreb's cycle.
Kreb cycle begins by formation of citric acid & O.A.A. is the aceptor molecule of Acetyl CoA in Kreb’s cycle. All 6C intermediates have 3, carboxylic groups. So called as tricarboxylic Acid cycle.
A number of Krebs cycle intermediates are used in synthetic (anabolic) pathways, thus TCA cycle is also called amphibolic pathway or anaplerotic pathway.
Succinyl CO-A is important for synthesis of porphyrin ring compounds like Chlorophylls, Phytochromes, Cytochromes, Haemoglobin etc.
-ketoglutaric acid (5c) involves in Amino Acid Glutamate formation (Nitrogen-metabolism) Oxidation occurs at 4 steps in Kreb cycle.

NADH2, 1FADH2 & 1 GTP (ATP) produced by each turn of TCA cycle. (= 12 ATP)

All the enzymes of TCA cycle, except mitochondrial marker enzyme Succinic dehydrogenase (on inner mitochondrial membrane) present in matrix.

Biochemical reactions in Krebs Cycle :

TERMINAL OXIDATION

It is combination of oxygen with electrons and protons released from reduced co-enzymes which produces water (metabolic water).

Terminal oxidation consists of two processes
Electron Transport System (ETS)
Oxidative Phosphorylation
Electron Transport System (ETS) :
NADH2 & FADH2 obtained from glycolysis and Kreb's cycle enter in electron transport chain and form water molecule by oxidation with the help of molecular oxygen.
In this process, energy is released which is used for the synthesis of ATP.
Transfer of electrons from NADH2 or FADH2 occurs through a chain of electron acceptors and donors
arranged in a specific sequence.
Electron transport chain is located in inner mitochondrial membrane in Eukaryotes and in inner side of plasma membrane or in mesosome membrane in prokaryotes.
Cytochromes are cyto.-b, cyto.-C1 & cyto. C, cyto.-a & cyto a3 (cyto a & a3 – Cu containing)
Now compounds of ETS are categories as follows :
Name of complexes Components of ETS Inhibitors

Two electron acceptors coenzyme Q or ubiquinone and cytochrome C can be easily separated from respiratory chain, therefore they are called mobile carriers. CoQ functions for e– transport between complex  and  and cyto.C transports e– between complex  and V.
Hydrogen is transferred from NADH2 to FMN and NAD is obtained back from NADH2. FMN forms FMNH2.

Two protons and electrons are released from FMNH2. Two protons are transported out through membrane and two electrons are taken up by Fe-S protein.
Terminal oxidation of reduced coenzyme FADH2 also occurs at mitochondrial ETS. FADH2 gives its e– & H+ to CoQ and become FAD.
During the ETS, NADH2 gives it’s 2e– / 2H+ to FMN in respiratory chain, thus 3 ATP are generated,while FADH2 give it’s 2e– / 2H+ to CoQ hence only 2 ATP are formed during the process of oxidative

Cyanide inhibits the activity of cytochrome oxidase as it inhibits the oxidation of cyto-a3. The respiration of most of living organisms is cynide sensitive respiration.
In mitochondria, of some plants alternative oxidase system is present, in which ETS continues even in presence of cyanides. This type of respiration is known as cyanide resistance respiration or Alternate electron pathway. Ex. Spinacea, Pisum. Alternate oxidase is inhibited by mCLAM and SHAM

ELECTRON SHUTTLES

2 molecule of NADH2 produced during glycolysis in cytoplasm have to go inside the mitochondria for production of ATP through ETS.
The mitochondrial membrane is impermeable to NADH2.
Hence a special carrier system known as electron shuttle is present in inner mitochondrial membrane which transports electrons from NADH2 to the electron carriers inside the mitochondria.
Shuttle system is only present in eukaryotes, as prokaryotes does not have membrane bounded cell organelle.

Electron shuttles are of two types in eukaryotes :
Malate–Aspartate Shuttle
Glycerol–Phosphate Shuttle
Malate-Aspartate Shuttle :

The electrons and hydrogen of NADH2 are transferred to malate which enter into the matrix of mitochondria.
Malate and NAD react to form NADH2 and Oxaloacetic acid.
Oxaloacetic acid cannot cross mitochondrial membrane. Hence it is ammonified to form aspartic acid which passes from matrix to the cytoplasm.
If this shuttle is effective, then 38 ATPs are formed from one glucose molecule. (2 Glycolytic NADH2 = 6ATP).
It is more efficient and present in heart, liver and kidney cells.

Glycerol Phosphate Shuttle :

It is less efficient and present in skeletal muscles and brain cells or most eukaryotic cells.
In this NADH2 transfers electrons to FAD of mitochondria.
Dihydroxyacetone phosphate (DHAP) and NADH2 react to form Glycerol-Phosphate in cytoplasm.
Glycerol-Phosphate goes to outer surface of inner membrane of mitochondria, where it reacts with FAD to form FADH2 and DHAP.
FADH2 enters the electron system to form 2 ATP. DHAP gets transferred to cytosol.
In the presence of this shuttle, 36 ATP are produced from one glucose molecule. (2 Glycolytic NADH2 = 4 ATP)  Note : When which shuttle will be functional, depends on the tissue and the species.

OXIDATIVE PHOSPHORYLATION AND CHEMIOSMOTIC THEORY

By P.Mitchell
The synthesis of energy rich ATP molecules with the help of energy librated by oxidation of reduced coenzyme produced during respiration is called Oxidative Phosphorylation.
The protons which are expelled out from inner mitochondrial membrane during electron transport, produce proton gradient (pH) and membrane potential in the external environment.
This creates proton motive force. This is utilized in the formation of ATP.
ATP synthesis is explained by chemiosmotic theory.
Oxidative phosphorylation occurs on F1–F0 particles (oxysomes or elementary particles) which are located on inner mitochondrial membrane.
ATPase activity is found in F1 (head) which is protruded towards matrix of the mitochondria. F0 (base) which is embedded in inner membrane has proton channels.
ATPase becomes active only when proton gradient develops.
Passage of 2e– from NADH2 pushes out three pairs of protons to outer chamber of mitochondria causing proton gradient and membrane potential. These collectively create proton motive force (pmf).
pmf cause protons to move back only passing through F0, as inner membrane is impermeable for H+.
Energy is released during transfer of protons to the matrix passing through F1 which is used for ATP
formation. Formation of ATP from ADP is induced by the enzyme ATP synthase (ATPase) present in F1.

Transfer of one pair of proton back into inner chamber through F1 head form one ATP. Thus, 3ATPs are formed by 3 pairs of H+ released during electron transport of 1 molecule of NADH2.
Two pairs of H+ are liberated in respiratory chain by FADH2, thus 2ATPs are formed.

Chemiosmosis during oxidative phosphorylation :

ATP synthesis during oxidative phosphorylation & Photophosphorylation is explained by Chemi-osmotic theory of P.Mitchell 1978.

According to this theory energy liberated during ETS, is used in creation of proton gradient (pH gradient) & membrane potential which constitutes proton motive force (pmf) due to this formation of ATP takes place in F1 particle of oxysome.

Coupling factor :

ATP formation requires H+ transport. These H+ only passes through the proton tunnel or coupling factor or F0 particle in mitochondrial membrane, and bacterial membrane
The process of eldcron transport and oxidative phosphorylation in mitochondria is tightly coupled. Some chemicals like 2,4 Dinitrophenol (2, 4 DNP) and oligomycin acts as uncouplers for this process.

Chemiosmosis during photophosphorylation :

The synthesis of ATP is coupled with electron transport system and creation of proton gradient across the membrane during photophosphorylation and oxidative phosphorlaion. Both are same but the difference is that during oxidative phosphorylation high H+ ion concentration at intermembrane space/ perimitochondrial space and low H+ concentration in mitochondrial matrix. While during
photophosphorylation High H+ conc. inside the thylakoid lumen (due to photolysis of water at thylakoid lumen) and low H+ ion conc. in stroma.

Thylakoid membrane is impermeable for protons so plastoquinone (which acts as both e– and H+ carrier) and Cf0 particle (similar to F0 particle of oxysome in mitochondria) helps in proton transport across the thylakoid membrane.
Due to free energy of electrons, protons or H+ ions transported from stroma side towards lumen of thylakoid by PQ or CF0(PQ + 2H+  PQH2 then PQH2  PQ + 2H+)
So within the chloroplast protones in the stroma decreases in number, while in the lumen there is accumulation of protons. This event creates a proton gradient (PH) across the thylakoid membrane,
which results in development of membrane protential. Both proton gradient and membrane potential constitute proton motive force (Pmf) or electro-chemical gradient.
Due to high proton motive force (Pmf) inside the lumen results in collapse’s of proton gradient and this potential is used in phosphorylation of ADP, results in synthesis of ATP (ADD + ip  ATP) catalysed by enzyme ATP synthase.
The returned protons are accepted by NADP leads to formation of NADPH2 in stroma side.


BIOENERGETICS OF RESPIRATION - (1 MOL. OF GLUCOSE)

This is a theoretical calculation based on some assumptions.
EMP-Pathway -
ATP formed at substrate level phosphorylation  4 ATP
ATP produced via ETS (2NADH2)  4/6 ATP
ATP consumed in glycolysis  2 ATP
10 ATP – 2 ATP = 6/8 ATP
Gross – Expenditure = Net or Total gain
Direct Gain = 2 ATP
Link reaction or Gateway reaction -
2NADH2 = 6 ATP (via ETS)

Kreb’s Cycle -

ATP produced at substrate level phosphorylation  2GTP / 2ATP
ATP produced via ETS

(HMCPy)to-Ssohlunt
It is an alternative pathway for glucose break down which was discovered by Warburg et al in 1935
and Dickens et al in 1938 in animal cells.
It was described in detail by Raecker in 1954.
It is completed in cytoplasm.
This pathway is also known as hexose monophosphate shunt (HMP shunt), Pentose phosphate
shunt, Phosphogluconate shunt, Warburg Dickens pathway, Oxidative pentose pathway
Direct oxidation pathway and cytosolic oxidative decarboxylation.
This pathway starts with 6 molecules of Glucose-6-PO4.12 NADPH2 are obtained and 6 CO2 are
released and 5 molecules of Glucose-6-PO4 are recovered.

Pentose Phosphate pathway can be summarized as :

6 Glucose-6-P + 12 NADP  6 CO2 + 12 NADPH2 + 5 Glucose-6-P
or
Glucose-6-P + 12 NADP  6CO2 + 12 NADPH2
Significance of PPP / HMP shunt :
This pathway produces reducing power NADPH2 for the various biosynthetic pathways, other than photosynthesis like fats synthesis, starch synthesis, hormone synthesis and chlorophyll synthesis.
An intermediate erythrose-P (4C) of this pathway is precursor of shikimic acid, which goes synthesis of aromatic compounds and amino acids.
This cycle provides pentose sugars Ribose-p for synthesis of nucleotides, nucleosides, ATP and GTP.
A five carbon intermediate Ribulose-5-phosphate may used as CO2 acceptor in green cells.
Intermediates like PGAL and fructose-6-phosphate of this pathway may link with glycolytic reactions.

OTHER METHODS OF RESPIRATION

Respiration of Fats :

It occurs during germination of fatty seeds and in plants when carbohydrates reserve declines.
Fats are hydrolysed in presence of enzymes lipase to yield fatty acid and glycerol.

Oxidation of Glycerol :

Glycerol reacts with ATP in presence of glycerol kinase to form glycerol-3-PO4, which is then oxidised in presence of glycerol phosphate dehydrogenase and NAD to form dihydroxy acetone
phosphate (DHAP). DHAP enters into glycolysis.
Glycerol + ATP  Kinase Glycerol-3-PO4
Glycerol-3-PO4 + NAD Dehydrogenase DHAP/PGAL + NADH2

Oxidation of Fatty Acids –oxidation) :

It takes place in mitochondria and glyoxysomes.
It involves sequential removal of 2C in the form of acetyl CoA molecules from the carboxyl end of
the fatty acid.
Each turn of –oxidation generates one FADH2, one NADH2 and one acetyl CoA molecule. So each
turn of –oxidation generates 5 ATP molecules.
aCnodm Hplete oxidation of one mole of acetyl CoA in TCA results in production of 12 ATP molecules, CO2 2O.
Palmitic acid (16 C)
(1 mole)
7 turns of –oxidation 5 ATP per turn
= 35 ATP – 2 ATP (consumed in first turn) = 33 ATP

8 mole of Acetyl CoA
TCA cycle 12 ATP per cycle = 96 ATP
16 mole CO2 + 16 mole H2O Total = 129 ATP

Acetyl COA which is the end product of –oxidation of fatty acids may enter into Kreb's cycle or into glyoxylate cycle in germinating fatty seeds to form sucrose.

Note : Glyoxylate cycle is an example of gluconeogenesis.

Respiration of Proteins :

Proteins are hydrolysed to form amino acids.
Amino acids are deaminated to form organic acids, which are oxidized in glycolysis and Kreb's cycle.

FERMENTATION

It is anaerobic breakdown of carbohydrates and organic compounds into CO2, alcohols and organic acids with the help of microorganisms and their enzymes.

Mechanism of Fermentation :

It occurs in two steps :
Glycolysis
Breakdown of Pyruvic acid

Glycolysis :
Pyruvic acid is formed in glycolysis.
C6H12O6 Complex
Enzyme Pyruvic acid + 2ATP + 2NADH2

Breakdown of Pyruvic acid :

Pyruvic acid in anearobic respiration is broken down in different products.

Types of Fermentation :
Based on different products formed by pyruvic acid, fermentation can be of following types

Alcoholic Fermentation :
Most common type. It occurs in some fungi (yeast) and higher plants during anaerobic conditions.

It is completed in 2 step :

2CH3CO.COOH  Decarboxylase 2CH3CHO + 2CO2

Pyruvic acid Acetaldehyde
2 CH3CHO + 2 NADH2 Alcohol dehydrogenase 2C2H5OH + 2NAD

Acetyldehyde Ethyl Alcohol

Lactic Acid Fermentation :
Lactic acid is formed by the activity of bacteria Lactobacillus lactis in presence of enzyme lactic
dehydrogenase which requires FMN and Zn+2.
2CH3CO.COOH + 2 NADH2 Zn 2
Lactic dehydrogenase


Acetic Acid Fermentation :

It is a fermentation process which requires atmospheric oxygen.
Ethyl alcohol is oxidized into acetic acid by the activity of acetic acid bacteria (Acetobacter aceti).

C6H12O6  yeast 2C2H5OH + 2CO2
2 C2H5OH + O2  Acetobacteraceti CH3COOH + H2O
Acetic acid

Butyric Acid Fermentation :

Pyruvic acid is converted to butyric acid by the activity of anaerobic bacteria. Eg. Bacillus butyricus,Clostridium butyricum.
CO2 is liberated during such fermentation.
Pyruvic acid Acetoacetic acid  Butyric acid.

RESPIRATORY QUOTIENT

The ratio of volume of CO2 released to the volume of O2 absorbed in respiration is called respiratory
quotient (RQ) or respiratory ratio.

Value of RQ measured by Ganong's respirometer.
Value of RQ varies from one substrate to another.
RQ gives the idea of nature of substrate being respired in a particular tissue.
R.Q. value equal to unity (RQ = 1) :
When carbohydrates are the respiratory substrate, as in green leaves, flowers, fruits, germinating seeds of cereals etc.

C6H12O6 + 6O2  6CO2 + 6H2O
RQ = 6 / 6 = 1
RQ value less than unity (RQ < 1) :
Fats as respiratory substrate
When fats are respiratory substrate RQ is less than one because fats are poorer in oxygen and hence require more oxygen for their oxidation. Eg. : germinating seeds of peanuts, mustard, sunflower, castor etc.

2 C51H98O6 + 145O2  102 CO2 + 98H2O
Tripalmitin
RQ = 102/145 = 0.7

Proteins as respiratory substrate –

RQ value for proteins is also less than one. Value of RQ is between 0.8 and 0.9. Eg : germinating
seeds of gram, pea, bean etc.

RQ equal to zero (RQ = 0) or less than 1 :
In some succulent or CAM plants (Eg. – Opuntia, Bryophyllum) carbohydrates are incompletely oxidised to organic acids in dark.
2 C6H12O6 + 3 O2  3 C4H6O5 + 3 H2O + 2 CO2

Malic acid

2/3 = RQ = < 1
CO2 is not evolved.
RQ = 0/3 = 0
RQ value more than unity (RQ > 1) :
When respiratory substrates are organic acids
Succulent plants form organic acids during the night which are completly oxidized during the day.
Organic acids are rich in oxygen and therefore require less O2 during respiration but release more
CO2.
C4H6O5 + 3 O2  4 CO2 + 3 H2O
Malic acid
RQ = 4/3 = 1.33
Hightest R.Q., oxalic acid = 4
RQ value equal to infinity (RQ = ) :
During anaerobic respiration, CO2 is evolved, without intake of oxygen and so value of RQ is infinite.
C6H12O6  2 C2H5OH + 2 CO2
RQ = 2 / 0 = 

FACTORS AFFECTING RESPIRATION

Temperature :

A rise in temperature from 0°C to 40°C increases the rate of respiration.
Optimum temperature for respiration lies between 20° - 40°C.
In most of the plants rate of respiration is affected according to Von't Hoff's Law. Q10 value or temperature
coefficient value (increase in rate of a particular process with every 10°C increase in temp.) for respiration is 2–3.
At low temp. (below 0°C), activity of enzyme is less and hence low respiration. It is for this reason that vegetables and fruits are stored at low temp. (cold storage or refrigerators).
At high temp. (45°C or above), denaturation of enzymes occurs and hence respiratory rate is decreased.

Oxygen :

The presence or absence of O2 determines the kind of respiration and respiratory products.
Oxygen is essential for aerobic respiration, as it is one of the reactants in the process. It is the final
acceptor of electrons and protons in electron transport system of the mitochondria.

The inhibition of anaerobic respiration by O2 concentration is called as Pasteur’s effect.
The minimum amount of oxygen, at which aerobic respiration takes place & anaerobic respiration become extinct is called as extinction point.
Oxygen concn at which both aerobic & anaerobic respiration take place simultaneously is called as transition point.

Carbondioxide :
Increase in CO2 concentration reduces respiration. Consequently inhibits the germination of many seeds and rate of growth falls down.

Water :
Proper hydration of respiring cells is must because decrease in water content, decreases respiratory rate as water is necessary for the activity of enzymes.
Dry seeds having 8–12% of water, have negligible respiratory rate but as the seeds imbibe water the respiration increases.
The rate of respiration of seeds increases with increase of water because water causes hydrolysis and so enzyme activity increases. Oxygen also enters the seed through water.

Light :

Light has no direct effect.
Under suitable light, rate of photosynthesis is optimun which supplies respiratory substrate at a
moderate rate and hence indirectly affects respiration.

Injury :

Injury or wounds causes increase in respiration because healing of wound needs more meristmatic
activity of cells which needs more energy provided by increased rate of respiration.
After some time the rate of respiration returns to normal.

Mineral Salts :

If the plants are transferred from water to adequate salt solution, then rate of respiration increases.
This process is known as salt respiration.

Hormones :

IAA, GA & cytokinin increase the respiration rate.
The rapid increase in rate of respiration during ripening of fruits and senscence of leaves and plant organs is called as “Climacteric respiration”. The rate is decrease after sometime.
It is due to production of ethylene hormone.

Inhibitors :
CN, azides, DNP (Dinitrophenol) CO, rotenone, antimycin, amytal, etc inhibit the respiration.
Heavy metals like lead and zinc inhibit respiration by inactivating respiratory enzymes.