Respiration in Plants: Meaning, Types and Factors | Plant Physiology

In this article we will discuss about:- 1. Meaning of Respiration in Plants 2. Types of Respiration 3. Mechanism of Aerobic Respiration 4. Anaerobic Respiration 5. Factors 6. Respiratory Quotient.

Meaning of Respiration in Plants:

The word respiration is derived from the latin word ‘respirare’ (literally means to breathe). It was in the late eighteenth century that the chemistry of respiration was understood. The Dutch plant physiologist Ingen-Housz first established that living plants exchange oxygen and carbon dioxide with the external atmosphere.

In the middle of the nineteenth century, the concept that each and every growing cell of higher plants respires all the time, in the night as well, absorbing oxygen oxidising carbonaceous substances, releasing CO₂ and producing water along with energy was well established. In green plants during photosynthesis, radiant energy is converted into bond energy or chemical energy of the organic materials.

The energy that is present in these compounds cannot be put to use readily, unless it is brought into an utilizable form. Respiration in the presence of oxygen provides a means, by which, energy released from the organic food is trapped in the ATP molecules. The ATP molecules are the energy currency of a cell and can readily meet the energy requirements of a cell.

Thus, cellular respiration is an energy releasing, enzymatically controlled catabolic process which involves a stepwise oxidative breakdown of organic substances inside living cells. But this energy is not released all at once, since it involves a series of reactions controlled by enzymes. The energy is stored in high energy bonds of ATP. For each molecule of oxygen used in respiration, about 100 kcal or 204 kJ of energy is released which is utilized to synthesize 6 molecules of ATP.

Cellular respiration resembles combustion of fuel in the utilization of oxygen, production of carbon dioxide, breakdown of complex organic substances and release of energy. But there are some vital differences between the two processes. During cellular respiration, the energy is released in steps, coupled with the synthesis of ATP. During combustion or burning of the fuel, all the bonds of the fuel molecules are broken simultaneously in an uncontrolled manner, in a single step, so that all energy will be released as heat and sometimes partly as light.

Types of Respiration in Plants:

Respiration is the process of the oxidation of food substances with or without the need of oxygen for liberation of energy. Energy is required by all living organisms for the maintenance of all activities of life.

There are two types of respiration, namely:

(1) Aerobic respiration.

(2) Anaerobic respiration (Anoxic Respiration).

(1) Aerobic Respiration:

If the food substances in living cells are oxidised in presence of oxygen, it is called aerobic respiration (oxic respiration).

Aerobic respiration is the stepwise complete breakdown of the respiratory substrates like glucose into simpler molecules like CO₂ and water with the release of ATP. This process is accompanied by the liberation of heat. It is the common and widely seen type of respiration. It is seen in all eukaryotic organisms.

Aerobic respiration can be represented by the following equation:

C₆H₁₂O₆ + 6O₂ 6CO₂+ 6H₂O + energy (686 kcal)

(2) Anaerobic Respiration (Anoxic Respiration):

It is the biological incomplete oxidation of food molecules without utilization of molecular oxygen resulting in the formation of organic compounds such as ethanol, lactic acid etc. Anaerobic respiration is mostly restricted to microbes. E.g., sulphur bacteria, methanogens.

Fermentation is a type of anaerobic method of deriving energy without using oxygen. This process is commonly seen in prokaryotes (bacteria), eukaryotes (yeasts, roots and seeds of higher plants and muscles of animals).

Mechanism of Aerobic Respiration in Plants:

Aerobic respiration occurs in four stages:

(A) The Glycolytic Pathway

(B) Oxidative decarboxylation of pyruvate

(C) Krebs cycle, and

(D) Electron Transport System (ETS) and Oxidative phosphorylation.

(A) The Glycolytic Pathway:

In glycolysis (Gr., glykys = sweet; lysis = splitting) a molecule of glucose is degraded in a series of enzyme catalysed reactions to yield two molecules of a 3-carbon compound pyruvate. It is a set of reactions that take place in the cytoplasm of prokaryotes and eukaryotes.

Glycolysis is also called Embden-Meyerhof-Parnas or EMP pathway in honour of the three scientists who were responsible for the discovery of the biochemical details of this pathway. It is the first set of reactions common to both aerobic and anaerobic respiration as these reactions can proceed in the absence of molecular oxygen.

The steps in glycolysis are shown in (Fig. 14.3):

i. Phosphorylation:

Glucose is phosphorylated in presence of ATP to form glucose 6-phosphate and ADP. This reaction is catalyzed by the enzyme phosphohexokinase.

Glucose + ATP → Glucose 6 – phosphate + ADP

ii. Isomerisation:

Glucose 6 – phosphate is converted to fructose 6 -phosphate by phosphogluco isomerase. This isomerization involves conversion of an aldose to a ketose.

Glucose -6 – phosphate ⇌ Fructose 6 – phosphate.

iii. Phosphorylation:

Fructose 6 – phosphate is phosphorylated by ATP to form fructose 1, 6 bis­phosphate and ADP. The enzyme catalyzing this step is phosphofructokinase.

Fructose 6 – phosphate + ATP → Fructose 1, 6 – bisphosphate + ADP

These steps constitute the ‘preparatory stage’. This is followed by the ‘pay off stage’.

iv. Cleavage:

The enzyme aldolase splits fructose 1, 6 – bisphosphate (a six carbon molecule) into two three-carbon molecules, glyceraldehyde 3-phosphate (G-3-P) and dihydroxyacetone phosphate (DHAP).

Fructose 1, 6 – bisphosphate ⇌ G-3-P + DHAP

v. Isomerisation:

Glyceraldehyde 3- phosphate is the only molecule that can be used for the rest of glycolysis. However, the dihydroxyacetone phosphate formed in the previous step can rapidly be converted to glyceraldehyde 3-phosphate by the enzyme triosephosphate isomerase. Thus, for each molecule of fructose 1, 6-bisphosphate that is cleaved in step 4, two molecules of glyceraldehyde 3 – phosphate continue down the pathway.

Dihydroxy acetone phosphate ⇌ Glyceraldehyde 3-phosphate

(DHAP) ⇌ (G-3-P)

vi. Formation of 1, 3- bisPGA:

Glyceraldehyde 3-phosphate is dehydrogenated and phosphorylated to get 1, 3-bisphosphoglycerate. The reaction is catalyzed by glyceraldehyde 3-phosphate dehydrogenase. In this reaction inorganic phosphate and NAD⁺ are involved and the hydrogen atoms removed from G-3-P are accepted by NAD⁺ to form NADH.

vii. Formation of 3-PGA:

The newly created high energy phosphate bond of 1, 3 – bisphosphoglycerate is now used to synthesize ATP. Phosphoglycerokinase catalyzes the transfer of the phosphoryl group from 1, 3-bisphosphoglycerate to ADP, generating ATP and 3-phosphoglycerate. This type of phosphorylation i.e., conversion of ADP to ATP is called “substrate level phosphorylation”.

1, 3-bisPGA + ADP → 3-PGA + ATP

viii. Formation of 2- PGA:

3-phosphoglycerate is converted to 2-phosphoglycerate by phosphoglycero mutase. Thus, the reaction is a transfer of phosphate group to a different carbon atom within the same molecule.

3 – PGA ⇌ 2 – PGA

ix. Formation of PEP:

The enzyme enolase catalyzes the dehydration of 2-phosphoglycerate to form phosphoenolpyruvate (PEP). This reaction converts the low energy phosphate ester bond of 2-phosphoglycerate into the high energy phosphate ester bond of PEP. During this conversion, there is a loss of water molecule.

2 PGA ⇌ PEP + H₂O

x. Formation of Pyruvic Acid:

In this last reaction pyruvate kinase catalyzes the physiologically irreversible transfer of the phosphoryl group from PEP to ADP to form ATP and pyruvate.

PEP + ADP → Pyruvate + ATP

Energy Yield in Glycolysis:

Early in glycolysis, one ATP is required for conversion of glucose to glucose 6-phosphate by phosphohexokinase and one more ATP for the conversion of fructose 6 – phosphate to fructose 1, 6 – bisphosphate by phosphofructokinase. However, fructose 1, 6-bisphosphate gives rise to two, three carbon units, each of which generate two ATP in subsequent steps (catalysed by phosphoglycero kinase and pyruvate kinase) giving a net yield of two ATP per glucose molecule.

The overall reaction is:

Glucose + 2 ADP + 2P_i + 2NAD⁺ → 2 Pyruvate + 2 ATP + 2 NADH + 2H₂O

(B) Oxidative Decarboxylation of Pyruvate:

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:

i. The complete oxidation of pyruvate by the stepwise removal of all the hydrogen atoms, leaving three molecules of CO₂.

ii. 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.

Before pyruvate enters Krebs cycle it is decarboxylated, dehydrogenated and a molecule of coenzyme A is added. This results in the formation of acetyl coenzyme A, NADH and release of CO₂.

Pyruvate + NAD⁺ + CoA → Acetyl – CoA + NADH + CO₂↑

This conversion is brought about by pyruvate dehydrogenase complex. This is an oxidative decarboxylation reaction. Acetyl – CoA is then led to the mitochondrial matrix for Krebs cycle. Acetyl – CoA forms the connecting link between glycolysis, and Krebs cycle.

(C) Krebs Cycle or Tricarboxylic Acid Cycle (TCA cycle) or Citric Acid Cycle:

The citric acid cycle is also known as the TCA or tricarboxylic acid cycle or Krebs cycle. Sir Hans Krebs discovered citric acid cycle for which he was awarded Nobel Prize in 1953. During Krebs cycle pyruvate which is the end product of glycolysis is completely oxidised to release CO₂ and H₂O through several steps as explained below (Fig. 14.5).

The citric acid cycle operates in the mitochondria of eukaryotes. Succinate dehydrogenase, the only membrane – bound enzyme in the citric acid cycle, is embedded in the inner mitochondrial membrane in eukaryotes.

i. Citrate is formed from the irreversible condensation of acetyl – CoA and oxaloacetate catalyzed by citrate synthase.

Acetyl – CoA + Oxaloacetate + H₂O → Citrate + CoA

ii. Citrate is converted to isocitrate by an isomerization catalyzed by aconitase. This is actually a two – step reaction during which cis – aconitate is formed as an intermediate.

Citrate → Cisaconitate + H₂O → Isocitrate

iii. Isocitrate is dehydrogenated to a 6 carbon compound oxalosuccinate. During this process hydrogen atoms removed from Iso-citrate are accepted by NAD+ to get a molecule of NADH. The reaction is catalysed by isocitrate dehydrogenase.

Isocitrate + NAD⁺ → Oxalosuccinate + NADH

iv. The oxalosuccinate is decarboxylated to α-ketoglutarate. This involves the removal of CO₂ by the action of an enzyme oxalosuccinate decarboxylase.

v. α-ketoglutarate is dehydrogenated and decarboxylated. It reacts with coenzyme A to form succinyl CoA and by the action of α-ketoglutarate dehydrogenase complex, hydrogen atoms are removed from α-ketoglutarate and transfered to NAD⁺ to get a molecule of NADH and CO₂ is liberated.

α-ketoglutarate + NAD⁺ + Coenzyme A → Succinyl – CoA + NADH + CO₂ ↑

vi. Succinyl-CoA is split into succinate and coenzyme A by succinyl synthetase or succinyl thiokinase. The reaction uses the energy released by cleavage of the succinyl-CoA bond to phosphorylate GDP to GTP (later GTP transfers its phosphate group to ADP producing a molecule of ATP in animals). In plants ADP is directly phosphorylated to ATP. (Formation of ATP/GTP is an example of substrate level phosphorylation).

Succinyl – CoA + H₂O + GDP + P_i → Succinate + GTP + Coenzyme A.

vii. Succinate is dehydrogenated to fumarate (4C) by succinic dehydrogenase.

The hydrogen atoms removed during this are accepted by FAD (flavinadenine dinucleotide) producing a molecule of FADH₂.

Succinate + FAD → Fumaric acid + FADH₂

viii. Fumarate is converted to malate (4C) by fumarase. This is a hydration reaction and requires the addition of a water molecule to fumarate to form malate.

Fumarate + H₂O → Malate

ix. Malate is dehydrogenated to oxaloacetate (4C) by malate dehydrogenase. The dehydrogenation involves removal of hydrogen atoms from malate to form oxaloacetate which can start Krebs cycle again. NAD accepts hydrogen atoms and forms NADH.

Malate + NAD⁺ → Oxaloacetate + NADH

The NADH and FADH₂ produced by the citric acid cycle are passed on to ETS where they are reoxidized and the energy released is used to create proton motive force.

Energy Yield in Krebs Cycle:

Theoretically, each of the three NADH molecules produced per turn of the cycle yields 3 ATP and the single FADH₂ yields 2 ATP by oxidative phosphorylation. One GTP (or ATP) is synthesized directly during the conversion of succinyl – CoA to succinate. Thus the oxidation of a single molecule of pyruvate via the citric acid cycle produces 14 ATP molecules on the entry of NADH and FADH₂ into the terminal oxidation process. These 14 ATP along with the single ATP got by direct synthesis gives 15 ATP/pyruvate molecules via Krebs cycle.

(D) Electron Transport System (ETS) and Oxidative Phosphorylation:

Electron transport chain is a process which oxidises NADH and FADH₂ produced in glycolysis, oxidative decarboxylation of pyruvate and Krebs cycle and traps the energy released as ATP. This is the final step in aerobic respiration. In eukaryotes, this process occurs in the F₀ – F₁ particles or Racker’s particles seated on inner membrane (cristae) of the mitochondria.

In prokaryotes the process occurs in the plasma membrane. Molecular oxygen is absolutely needed for this process. Oxygen combines with the electrons and hydrogen ions liberated from NADH and FADH₂ to form water as a by-product of aerobic respiration. Hence this process is known as terminal oxidation.

NADH and FADH₂ formed during glycolysis and Krebs cycle are oxidized to NAD⁺ and FAD respectively. As oxidation brings about phosphorylation of ADP to ATP this process is called oxidative phosphorylation.

The electron transport system consists of four enzyme complexes:

The enzyme complexes are arranged in a series on the inner membrane of the mitochondria.

i. Complex 1 transfers electrons from NADH to Co-enzyme Q and is called NADH dehydrogenase. This complex pumps protons into the inter-membrane space.

ii. Complex II transfers the electrons derived from succinate oxidation to CoQ. This complex is properly called succinate-Coenzyme Q reductase, though it is often referred by its common name, succinate dehydrogenase. Complex II has FAD and Fe/S.

iii. Complex III is called Coenzyme Q – cytochrome c reductase because it accepts electrons from Coenzyme Q and passes them to cytochrome c. Complex III contains cyt b and cyt c₁. This complex, along with others in the chain (complex I and complex IV), operates as a proton pump, driving protons out of matrix into inter-membrane space.

iv. Complex IV transfers electrons from cytochrome c to oxygen and is called cytochrome c oxidase. It has cyt a and cyt a₃. The complexes I, II and III contain iron/sulphur proteins. Complex IV (cyt a and cyt a₃) contain two copper centres. The cytochrome c is a mobile carrier. Complex IV also pumps protons from the matrix into the inter-membrane space.

Of all the electrons – transferring intermediates involved in aerobic respiration, only cytochrome a₃, at the end of the transport system, is a terminal oxidase, capable of direct transfer of electrons to oxygen. Two electrons, two protons and one atom of oxygen (½ O₂) produce a molecule of water.

½ O₂+ 2 H⁺ + 2e^– → H₂O

Note that as in chloroplast, electron transfer is coupled to ATP synthesis by H⁺ transport. The protons are transferred from matrix to inter-membrane space of mitochondria by three of the four complexes building or creating a proton motive force.

Chemiosmosis in Mitochondria:

The ATPs are synthesized in the stroma region of the chloroplast during photosynthesis whereas the ATPs are synthesized in the matrix region of mitochondria during respiration.

The enzyme complexes namely complex I, complex III and complex IV present in the inner-membrane of the mitochondria act as proton pumps which pump the protons from the matrix of the mitochondria to the inter-mitochondrial space (note: in chloroplasts, cytb₆/ f complex act as proton pump).

The transfer of protons to the inter-membrane space generates a proton motive force across the inner membrane. That is, the proton concentration in the inter-membrane space rises above that in matrix, thus the matrix becomes slightly negative in charge. This internal negativity attracts the positively charged protons and induces them to reenter matrix.

Since membranes are impermeable to ions, the protons that reenter the matrix pass through the ATP synthases, which use the energy of the gradient to catalyse the synthesis of ATP from ADP + P_i. The ATP synthase located in the inner membrane of mitochondria has two parts F₀ and F₁.

F₁ head piece is a peripheral membrane protein complex which 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 F₁ component for the production of ATP. For each flow of 3H⁺ through F₀ from the inter-membrane space to the matrix down the electrochemical gradient, one ATP is produced.

Thus, the vast majority of the energy originally contained in the food material is ultimately used to generate a gradient of protons in the inner membrane of the mitochondria. This gradient is then used to drive the synthesis of ATP. Because the chemical formation of ATP is driven by a diffusion force similar to osmosis across the membrane, the process is referred to as chemiosmosis.

Anaerobic Respiration in Plants:

Anaerobic respiration involves the incomplete breakdown of fuel molecules like glucose into compounds such as ethyl alcohol, lactic acid etc., in the absence of molecular oxygen.

Depending on the end product formed, anaerobic respiration can be classified into the following:

a. Alcoholic Fermentation:

The fermentation process that terminates in the formation of ethyl alcohol is called alcoholic fermentation. In micro-organisms like yeast (Saccharomyces cerevisiae) respiration occurs in absence of molecular oxygen.

Firstly, two molecules of pyruvate are formed by glycolysis which then undergoes decarboxylation to form acetaldehyde in the presence of an enzyme pyruvate decarboxylase. Then acetaldehyde undergoes rapid reduction (using H⁺ of NADH) in the presence of an enzyme alcohol dehydrogenase to form ethyl alcohol.

b. Lactic Acid Fermentation:

The fermentation process that terminates in lactic acid is called lactic acid fermentation. Here also, firstly two molecules of pyruvate are formed by glycolysis. The two molecules of pyruvate are then reduced by the hydrogen atoms given by NADH to form lactate in the presence of an enzyme lactate dehydrogenase. Unlike alcoholic fermentation there is no liberation of CO₂.

Lactic acid fermentation occurs in lactic acid a bacterium which is widely used in dairy industry. It is important commercially because the production of cheese, yogurt, and other dairy products depends on microbial fermentation of lactose, the main sugar found in milk.

It is observed that this type of respiration occurs in mammalian skeletal muscle cells during periods of strenuous exercise. Whenever muscle cells use oxygen faster than it can be supplied by the circulatory system, the cells become temporarily hypoxic. Under these conditions pyruvate is reduced to lactate instead of being oxidised to acetyl-CoA.

Overall Production of ATP during Anaerobic Respiration:

During anaerobic respiration, the breakdown of glucose is described as incomplete or partial since it results in the formation of organic compounds such as ethyl alcohol or lactate as end products. The breakdown of glucose during anaerobic respiration occurs in two phases. They are glycolysis and fermentation.

Glylcolysis converts one molecule of glucose into two molecules of pyruvate. During this process 2 molecules of ATP and 2 molecules of NADH are synthesized.

However, the two molecules of NADH that are formed during glycolysis are used to reduce pyruvate and converted into NAD⁺ during fermentation. Therefore the net energy yield during anaerobic respiration is only two molecules of ATP.

Factors Influencing Respiration:

a. Temperature:

It is noticed that rate of respiration follows Vant Hoff’s law. It states that the respiratory rate is doubled for every rise in temperature by 10°C. Hence, it is also known as law of Q₁₀. The optimum temperature for respiration is 30 to 40°C in different species of plants.

b. Oxygen:

Oxygen is a must for aerobic respiration, though it is not necessary for anaerobic respiration. The minimum oxygen concentration at which aerobic respiration occurs is called extinction point. It is around 3-10% of oxygen. However in facultative anaerobes like yeast and bacteria increase in concentration decreases the rate of glucose breakdown. Oxygen has its role in terminal oxidation of aerobic respiration.

c. Carbon Dioxide:

The atmosphere has a carbon dioxide concentration of 0.03% and it is almost constant. However, any increase in the carbon dioxide concentration has a retarding influence on respiration.

d. Water:

Hydration of the medium is a basic requirement for all metabolic activities. Respiratory enzymes become active only in a hydrated medium. Increased rate of respiration is seen if there is slight desiccation of tissues. But if hydration is either high or low, respiration is decreased.

e. Light:

Light indirectly affects the rate of respiration, as it operates by supplying respiratory material in the form of carbohydrates in plants containing chlorophyll. The fact that respiration proceeds at night as well as in daytime also indicates that light has no major role in respiration. A part of the effect on the rate is caused by the rise in temperature resulting from absorption of the sun’s energy by the cell.

f. Inorganic Salts:

The rate of respiration usually increases when a plant tissue is transferred from water to salt solution. Such an increase over normal rate is called salt respiration or anion respiration. This is due to increase in anion transport.

Respiratory Quotient or RQ of Plants:

The ratio of the volume of CO₂ liberated to the volume of O₂ used during respiration is called respiratory quotient (RQ).

RQ = volume of CO₂ liberated/volume of O₂ used

RQ can be used to find the type of food being oxidized during respiration because different food materials like carbohydrates, fats, proteins and organic acids etc., possess different RQ values. The value of RQ may be unity or one, less than one or more than one depending upon the substrate used.

The RQ values in different respiratory substrates will be as follows:

(1) RQ of Carbohydrates:

If the respiratory substrate is carbohydrate as in germinated seeds like wheat, oat, barley or paddy the value of RQ is always one.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

RQ = volume of CO₂ evolved/volume of O₂ used = 6CO₂/6O₂

(2) RQ of Fats:

If the respiratory substrate is fat as in case of germinated seeds of mustard, castor, RQ of the respiring cells will be less than one because the volume of CO₂ liberated is quite less in comparison to the volume of O₂ consumed. Fats always require more amount of O₂ for their oxidation.

The oxidation of fat can be shown by taking an example of tripalmitin fat. It takes place usually at the time of seed germination.

2 C₅₁H₉₈ + 145 O₂ → 102 CO₂ + 98 H₂O + Energy

RQ = volume of CO₂ evolved/volume of O₂ used = 102CO₂/145O₂ = 0.7

(3) RQ of Organic Acids:

If the respiratory substrate is an organic acid, the RQ of the respiring cells will be more than one because the volume of CO₂ liberated is more than the volume of O₂ consumed. The organic acids already contain more O₂ so they further need only a small amount of O₂. Oxidation of malic acid is an example for this purpose.

C₄H₆O₅ + 3O₂ → 4 CO₂ + 3H₂O + Energy

RQ = volume of CO₂ evolved/volume of O₂ used = 4CO₂/3O₂ = 1.33

(4) RQ of Proteins:

If the respiratory substrate is protein then the RQ value is 0.9.

Significance:

By determining the value of RQ the nature of the respiratory substrate can be understood. If RQ is 1 then it indicates that carbohydrates are utilized for respiration. Cells prefer carbohydrates as the respiratory substrate. When cells utilize fats for respiration, RQ will be around 0.7. When cells use organic acid then RQ is more than 1.