Plant Respiration

Cellular Respiration can be divided into 4 Parts:

1) Glycolysis2) Oxidation of Pyruvate / Transition Reaction3) The Krebs Cycle4) The Electron Transport Chain and Chemiosmotic Phosphorylation

Simple definition: The oxidation of sugars to produce usable energy (ATP), reductant (NADH), and carbon “skeletons” for biosynthesis.

Where do the 4 parts of Cellular Respiration take place?• Glycolysis:

– Cytosol

• Oxidation of Pyruvate:– Mitochondrial Matrix

• The Krebs Cycled:– Mitochondrial Matrix

• Electron Transport Chain and Cheimiosmotic Phosphorylation:– Mitochondrial Cristae

Glycolysis • Glyco-glucose,lysis-

breakdown• Involves member of

enzyme-Controlled reaction

• It takes place –cytoplasm of cell

• It does not require oxygen• Common for both aerobic

and anaerobic reaction

Glycolysis• Glucose phosphorylated by ATP to glucose 6 phosphate• Phosphorylated glucose – no longer recognized –

glucose transport system , therefore – trapped inside the cell

• Enzyme involved is kinase• Glucose 6 phosphate-isomerised-fructose 6 phosphate• Enzyme involved is isomerize• Fructose 6 phosphate-phosphorylated by ATP to

fructose 1,6phosphate • Enzyme involved is kinase• Fructose 1,6 phosphate splits to glycerate 3 phosphate• Glycerate 3 phosphate converts to pyruvate• Glycerate 3 phosphate when converted to pyruvate it

forms 2 NADH2 and ATP

End product of Glycolysis

• 2 molecules of ATP( 4 molecules are produced but 2ATP are used up)

• 2 molecules of NADH

• 2 molecule of pyruvate

• Now this Pyruvate will form Acetyl Co-A

• Cylindrical in shape or rod shape • Width range from 0.5 micro meter to

1.5 micro meter & length from 3 micro meter to 10 micro meter

• Bounded – double membrane • Outer & inner membrane - separated

by inter membrane space • Inner membrane – extensively folded –

form partitions called cristae • Cristae – projected – into – semi fluid

matrix • Circular DNA molecule & 70S

ribosome – present • Endosymbiont theory • Functions of Mitochondria • Involved – cellular respiration • Series of bio chemical reaction – result

in formation of ATP• often known as power station of cell• More than 1000 mitochondria are found

in metabolically active cell

Oxidation of Pyruvate /Transition Reaction• Pyruvate- Matrix of

mitochondria from cytoplasm• Piruvate- Decarboxylated

(Removal of carbon in form of carbon dioxide)

• Piruvate- Dehydrogenated (Removal of hydrogen)

• Hydrogen is transferred to hydrogen acceptor NAD+ to NADH+ H+

• Pyruvate- Acetate• Acetate combines with

coenzyme A to form acetyl coenzyme

End products of linkage step

• 2 molecules of NADH

• 2 molecule of Acetyl Co-A

• Now Acetyl Co-A will enter into TCA/ Kreb cycle

Kreb Cycle:

• Discovered by sir Hans Kerbs-1937

• Citric acid cycle / Tricarboxylic acid cycle (TCA)

• Occurs- In Matrix of mitochondria

• Occurs only in aerobic reaction

• Acetyl co enzyme A (2C) + oxaloacetate(4C) –citrate• Reaction is called condensation• Enzyme involved citrate synthetase• Citrate isomerizes to isocitrate(6C)• Isocitrate undergo Oxidative decarboxylation to give α-ketogluterate(5C)• Carbon dioxide is produced• NAD+, hydrogen acceptors and NADH is formed• Enzyme involved is isocitrate dehydrogenase• X –ketogluterate(5C) undergo oxidative decarboxylation & dehydrogenation gives

succinyl CoA(4C)• CO2 is produced & NADH is formed.• Enzyme used is α- ketogluterate dehydrogenase.• Succinyl CoA(4C) gives succinate (4C)• ATP is formed from ADP+pi• Enzyme used is succinyl CoA Synthetase• Succinate undergoes dehydrogenation and gives Fumerate(4C)• FAD (Flavine adenine dinucleotide) gives hydrogen acceptor and form FADH2• Enzyme used is succinate dehydrogenase• Fumerate undergoes hydrogenation and gives maltate(4C)• Enzyme used is fumerate• Maltate undergo dehydrogenation and gives oxaloacete (4C)• (NAD+)+(H+) gives NADH• Enzyme used is malate dehydrogenase.

• 3 molecules of NADH• 1 molecule of FADH2

• 1 molecule of ATP

• So total:• 6 molecules of NADH

• 2 molecule of FADH2

• 2 molecule of ATP

End product of Kreb/ TCA Cycle

From 1 molecule of Acetyl Co-A

From 2 molecule of Acetyl Co-A

What is the use of NADH & FADH

• Now these NADH & FADH2 will enter into ETC and will give ATP

The Electron Transport Chain• Oxygen is required during the

final stage of reaction.• Oxydative phosphorylation is

a process by which ATP is formed as electron are transferred from NADH & FADH2 to oxygen via series of electron carrier.

• Location- inner membrane of the mitochondria.

• E.T.C involves Chain of electron carrier molecules.

• Electron from NADH & FADH2 are transferred to Oxygen

• Series of reaction---- Redox reaction• Hydrogen atom splits to hydrogen

ions(H+) and electrons.• Transfer of electrons along the chain

releases sufficient energy to make ATP• Hydrogen is passed on to oxygen to form

water.

Mitochondrial electron transport

• NADH & FADH2 – formed during – glycolysis & Krebs cycle are passed to ETC

• ETC – present – inner membrane space and consists of cytochromes

• NADH & FADH2 – oxidized – hydrogen are released • Hydrogen now splits into electrons & protons • Electrons – pass along – electron carrier and transferred to oxygen • Protons H+ are actively pumped from – matrix to the outer

compartment i.e. intermembrane space • A proton gradient – created b/w the outer compartment and the

inner matrix • Protons cannot diffuse through cristae membrane • Protons flow only down the gradient – matrix through ATP synthase

channels this is known as chemiosmosis • Protons flow – ATP synthase channel (F1 channels) they generate

energy to phosphorylate ADP into ATP in the presence of enzyme ATP Synthase

• Later proton combines with oxygen • 2e- + 2H+ + ½ O2 = H2O

Summary of ATP synthesis

Summary of ATP synthesis

Summary of ATP synthesis

Not all carbon atoms entering respiratory pathway ends up as carbon dioxide.

But many important carbon skeletons are made for example: proteins, lipids, DNA, cellulose etc.

Relationship of respiratory intermediates to other plant biosynthetic pathways

Pentose Phosphate Pathway

• Alternative/ Parellel to glycolysis• Also called the hexose monophosphate shunt

(HMP shunt)• Only 5 – 20% respiration occurs this way• But – makes useful intermediates needed for

making DNA, RNA and phenolics• Appears important during plant recovery from

stress

PPP/ HMP Shunt

•There are two distinct phases in the pathway.•First is the oxidative phase, in which NADPH is generated.•Second is the non-oxidative phase in which 5-carbon sugars are synthesized.•For most organisms, the pentose phosphate pathway takes place in the cytosol but in plants, most steps take place in plastids.

ANAEROBIC RESPIRATION • In absence of O2 – glycolysis still occurs • Conversion – pyruvate to acetyl Co A is blocked,

hence kerbs cycle & oxidative phosphorylation also get blocked

• Oxidative phosphorylation is blocked because O2 as final electron acceptor in ETC is not available

• Absence – oxidative phosphorylation – no regeneration of NAD+ & FAD+

• To regenerate NAD+ cells undergo – fermentation • Two types of fermentation which is most common - Lactate fermentation - Alcoholic fermentation

• Alcoholic fermentation – plants & yeast• Lactate fermentation – muscles of animal

during strenuous exercise & & in some bacteria such as Lactobacillus acidophilus

• Both alcoholic & Lactate fermentation don’t produce ATP Molecules but they regenerate NAD+ from NADH in order to keep glycolysis going

Chemistry of Alcoholic Fermentation • Plants – respire – anaerobically only for

short period of time during waterlogged condition

• Ethanol – toxic and plant – unable make use of ethanol

• Yeast – ethanol produced – accumulate – medium – around – cells – concentration rises – level that prevents further fermentation & kills – yeasts

Respiratory Quotient (RQ)• R.Q – ratio of volume of carbon di oxide formed

to the volume of oxygen consumed over a given period of time

• R.Q = CO2 Formed / O2 Consumed • R.Q – used – provide information about what

type of substrate is being oxidized in respiration • R.Q – value – more than 1.0 indicates –

anaerobic respiration • R.Q – value for fats – about 0.7, proteins is 0.9

& carbohydrates is 1.0• Fats and proteins – need more O2 – complete

their oxidation compared with carbohydrates

The phosphate/oxygen ratio, or p/o ratio, refers to the amount of ATP produced from the movement of two electrons through a defined electron transport chain, donated by reduction of an oxygen atom

P/O ratio

The effect was discovered in 1857 by Louis Pasteur, who showed that aerating yeasted broth causes yeast cell growth to increase, while conversely, fermentation rate decreases.

Crabtree EffectThe effect was discovered in 1928 by Herbert Grace Crabtree, who told that addition of glucose in yeast culture decrease the oxygen consumption.

Pasteur Effect

WHY & HOW?

TO FIGHT STRESS

AOX (ALTERNATIVE OXIDASE)

Uncoupling of ETC and ATP synthesis

Respiration in Plants

► Energy coupled

► electron transfer chain is

modified, or other respiratory enzymes

are involved

Uncoupled or non-coupled respiration can

reduce ROS formation following stress

The alternative oxidase (AOX) is mainly responsible for non-coupled respiration in plants

Feature of the AOX

► Thermogenic

► CN insensitive, SHA (Salicylhydroxamic acid) sensitive

► ROS stimulate expression

► Over-expression reduces ROS formation

► Anti-sense AOX increases ROS formation

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