Key Notes: Respiration in Plants - NEET PDF Download

• All living organisms need energy for carrying out daily life activities → the process of breathing is very much connected to the process of release of energy from food   → All the energy required for ‘life’ processes is obtained by oxidation of ‘food’. 

• The process of photosynthesis → Light energy convert into chemical energy that is stored in the bonds of carbohydrates like glucose, sucrose and starch. 
• Animals are heterotrophic, i.e., they obtain food from plants. 
• Saprophytes like fungi are dependent on dead and decaying matter.
• Ultimately all the food that is respired for life processes comes from photosynthesis.
• Photosynthesis takes place within the chloroplasts (in the eukaryotes), whereas the breakdown of complex molecules to yield energy takes place in the cytoplasm and in the mitochondria (also only in eukaryotes).
• The breaking of the C-C bonds of complex compounds through oxidation within the cells, leading to release of considerable amount of energy is called Respiration.

Respiration

• The compounds that are oxidised during this process are known as respiratory substrates. 
• Usually carbohydrates are oxidised to release energy, but proteins, fats and even organic acids can be used as respiratory substances in some plants, under certain conditions. 
• During oxidation within a cell, all the energy contained in respiratory substrates is not released free into the cell, or in a single step. It is released in a series of slow stepwise reactions controlled by enzymes, and it is trapped as chemical energy in the form of ATP. 
• It is important to understand that the energy released by oxidation in respiration is not (or rather cannot be) used directly but is used to synthesise ATP, which is broken down whenever (and wherever) energy needs to be utilised. Hence, ATP acts as the energy currency of the cell.
• This energy trapped in ATP is utilised in various energy-requiring processes.
• Carbon skeleton produced during respiration is used as precursors for biosynthesis of other molecules in the cell. 
• Plants require O₂ for respiration to occur and they also give out CO₂ . Hence, plants have systems in place that ensure the availability of O₂ . Plants, unlike animals, have no specialised organs for gaseous exchange but they have stomata and lenticels for this purpose. 
• Why plant don’t have evolved respiratory system like animals?

• Each plant part takes care of its own gas-exchange needs.
• Very little transport of gases from one plant part to another.
• Plants do not present great demands for gas exchange.
• Roots, stems and leaves respire at rates far lower than animals do. 
• During photosynthesis availability of O₂ is not a problem in these cells since O₂ is released within the cell.
• The distance that gases must diffuse even in large, bulky plants is not great.
• Each living cell in a plant is located quite close to the surface of the plant.

• In stems, the ‘living’ cells are organised in thin layers inside and beneath the bark. They also have openings called Lenticels. The cells in the interior are dead and provide only mechanical support. Thus, most cells of a plant have at least a part of their surface in contact with air. This is also facilitated by the loose packing of parenchyma cells in leaves, stems and roots, which provide an interconnected network of air spaces. 
• The complete combustion of glucose, which produces CO₂ and H₂O as end products, yields energy most of which is given out as heat. 
• The plant cell catabolise the glucose molecule in such a way that not all the liberated energy goes out as heat.
• The key is to oxidise glucose not in one step but in several small steps enabling some steps to be just large enough such that the energy released can be coupled to ATP synthesis. 
• During the process of respiration, oxygen is utilised, and carbon dioxide, water and energy are released as products. 

Anaerobic Organisms

• First cells on this planet lived in an atmosphere that lacked oxygen.
• Even among present-day living organisms, several are adapted to anaerobic conditions.
• Some of these organisms are facultative anaerobes, while in others the requirement for anaerobic condition is obligate anaerobes.

• All living organisms retain the enzymatic machinery to partially oxidise glucose without the help of oxygen. This breakdown of glucose to pyruvic acid is called glycolysis.

Glycolysis

• The term glycolysis → Greek words, glycos for sugar, and lysis for splitting.
• The scheme of glycolysis → given by Embden, Meyerhof, and Parnas. 
• Also known as à EMP pathway.
• In anaerobic organisms, it is the only process in respiration.
• Glycolysis occurs in the cytoplasm of the cell.
• Present in all living organisms.
• In this process, glucose undergoes partial oxidation
  ⇛ Form two molecules of pyruvic acid
  ⇛ Oxygen not consumed
  ⇛ CO₂ not released
• In plants, this glucose is derived from sucrose
  ⇛ which is the end product of photosynthesis, or from storage carbohydrates.
  ⇛ Sucrose is converted into glucose and fructose by the enzyme, invertase,
  ⇛ these two monosaccharides readily enter the glycolytic pathway. 
• Glucose and fructose are phosphorylated to give rise to glucose-6-phosphate by the activity of the enzyme hexokinase.
• This phosphorylated form of glucose then isomerises to produce fructose6- phosphate.
• Subsequent steps of metabolism of glucose and fructose are same.
• In glycolysis, a chain of ten reactions, under the control of different enzymes, takes place to produce pyruvate from glucose.
• Remember the steps
  ⇛ At which utilisation or synthesis of ATP and NADH + H+ take place.
  ⇛ ATPis utilised at two steps:
  1. first in the conversion of glucose into glucose 6-phosphate and
  2. second in the conversion of fructose 6-phosphate to fructose 1, 6-bisphosphate.
• The fructose 1, 6-bisphosphate is SPLIT into dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL).
• There is one step where NADH + H+ is formed from NAD+ ;
  ⇛ When 3-phosphoglyceraldehyde (PGAL) is converted to 1, 3-bisphosphoglycerate (BPGA).
  ⇛ Two redox-equivalents are removed (in the form of two hydrogen atoms) from PGAL and transferred to a molecule of NAD+ .
• PGAL is oxidised and with inorganic phosphate to get converted into BPGA.
• The conversion of BPGA to 3-phosphoglyceric acid (PGA), is also AN energy yielding process; this energy is trapped by the formation of ATP.
• Another ATP is synthesised during the conversion of PEP to pyruvic acid.
• How many ATP molecules are directly synthesised in this pathway from one glucose molecule → Total 4 ATP.
• Pyruvic acid → the key product of glycolysis.
• What is the metabolic fate of pyruvate?
  ⇛This depends on the cellular need.
  ⇛ There are three major ways –
  1. Lactic acid fermentation,
  2. Alcoholic fermentation
  3. Aerobic respiration. 
• Fermentation takes place under anaerobic conditions in many prokaryotes and unicellular eukaryotes. 
• For the complete oxidation of glucose to CO₂ and H₂O → organisms adopt Krebs’ cycle (aerobic respiration); This requires O₂ supply. 

Shuttle Systems

• It is a special electron carrier system which picks up electrons NADH from hydrogen of present in the cytoplasm and transfer them across the inner mitochondrial membrane and give them to electron carrier inside the mitochondria.
• Location: It is present in the inner mitochondrial membrane NADH formed in diffuses cytoplasm during glycolysis into outer chamber of mitochandria though outer membrane, but inner membrane is impermeable to NADH.
• Types of shutle system:

1. Glycerol-Phosphate Shuttle System
2. Malate-Asparate Shuttle System

Glycerol-Phosphate Shuttle System

•  In prokaryotes, shuttle mechanism is absent. They always get 38 ATP from aerobic respiration of 1 glucose mol.

•  Cyanide inhibits the activity of cytochrome oxidase & inhibits the oxidation of cyto-a3.

•  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. Spinacia, Pisum.

Fermentation

• Best example is yeast
• Incomplete oxidation of glucose
• Under anaerobic conditions
• By sets of reactions → Pyruvic acid is converted to CO₂ and ethanol.
• The enzymes required are:
  ⇛ pyruvic acid decarboxylase
  ⇛ alcohol dehydrogenase 
• Some bacteria produce lactic acid from pyruvic acid.
• In animal cells also, like muscles during exercise, when oxygen is inadequate for cellular respiration pyruvic acid is reduced to lactic acid by lactate dehydrogenase.
• The reducing agent is NADH+H+ which is reoxidised to NAD+ in both the processes.
• In both lactic acid and alcohol fermentation not much energy is released
  ⇛ Less than seven per cent of the energy in glucose is released and not all of it is trapped as high energy bonds of ATP.
• Also, the processes are hazardous – either acid or alcohol is produced.
  What is the net ATPs that is synthesised when one molecule of glucose is fermented to alcohol or lactic acid → 2 ATP.
• Yeasts poison themselves to death when the concentration of alcohol reaches about 13 per cent.
• What then would be the maximum concentration of alcohol in beverages that are naturally fermented? → Around 4 to 9% (Not more than 12%)
  ⇛ Beer → 4% to 8%
  ⇛ Wine → 8 to 11%
• How do you think alcoholic beverages of alcohol content greater than this concentration are obtained? → By distillation
  ⇛ Whisky/ Rum/ Vodka / Scotch/Gin à 36% to 60%
• What then is the process by which complete oxidation of glucose and extract the energy stored to synthesise a larger number of ATP molecules needed for cellular metabolism?  Aerobic respiration (Krebs cycle & ETS)
• This type of respiration is most common in higher organisms. 

Aerobic Respiration

• Take place within the mitochondria
• For this à the final product of glycolysis, pyruvate is transported from the cytoplasm into the mitochondria.
• The crucial events in aerobic respiration are:
  ⇛ 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.
• The first process → In the matrix of the mitochondria (Kreb’s cycle).
• The second process → On the inner membrane of mitochondria (ETS).
• Pyruvate 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.
• During this process, two molecules of NADH are produced from the metabolism of two molecules of pyruvic acid (from one glucose).
• The acetyl CoA then enters a cyclic pathway, tricarboxylic acid cycle, more commonly called as krebs’ cycle after the scientist hans krebs who first elucidated it. 

Tricarboxylic Acid Cycle

• The TCA cycle starts with the condensation of acetyl group with oxaloacetic acid (OAA) and water to yield citric acid
  ⇛ Acceptor → OAA
  ⇛ First formed chemical → citric acid (TCA)
  ⇛ Catalysed by the enzyme citrate synthase
  ⇛ 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.
• 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.
• There are three points in the kreb’s cycle where NAD+ is reduced to NADH + H+ and one point where FAD+ is reduced to FADH₂ .
• The continued oxidation of acetyl CoA via the TCA cycle requires the continued replenishment of oxaloacetic acid, the first member of the cycle.
• In addition it also requires regeneration of NAD+ and FAD+ from NADH and FADH2 respectively. 

Electron Transport System (ETS) and Oxidative Phosphorylation

• These steps in the respiratory process are to release and utilise the energy stored in NADH+H+ and FADH₂.
• Through the electron transport system à the electrons are passed on to O₂ resulting in the formation of H₂O.
• The metabolic pathway through which the electron passes from one carrier to another, is called the electron transport system (ETS).
• It is present in the inner mitochondrial membrane.
• 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.
• 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.
• 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.
• 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.
• For the production of proton gradient required for phosphorylation in respiration à The energy of oxidation-reduction utilised so the process is called oxidative phosphorylation.
• 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.
  ⇛ 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, 2H+ passes through F0 from the intermembrane space to the matrix down the electrochemical proton gradient. 

Chemiosmotic Hypothesis

Chemiosmotic Theory / Coupling Theory

• During ETC of respiration CoQ & FMN can releases H+ ions in perimitochondrial space and leads to differenctial H⁺ ion concetration across inner mitochondrial membrane. This differential H+ ion concentration across inner mitochondrial membrane leads to creation of proton gradiant (PH gradient) and Electrical potential (diffrence of charge). Both are collectively known as Proton motive force (PMF).

• PMF do not allow stay of H⁺ ions in Peri mitochondrial space (PMS) so they return towards the matrix through F0 particles selectively.

• The passage of 3H⁺ ions activate ATP synthase and gives rise to 1ATP from ADP & Pi.

• Some physiologist believe that passage of 2H⁺ ions through F0 particle or coupling factor or proton channel leads to synthesis of 1 ATP.

EMP-Pathway

(i) ATP formed at substrate level phosphorylation ⇒ 4.ATP

(ii) ATP produced via ETS (2NADH₂) ⇒ 6 ATP

(iii) ATP consumed in glycolysis ⇒ 2 ATP

10 ATP – 2 ATP = 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 

(i) ATP produced at substrate level phosphorylation = 2 GTP/2ATP

• 1 Sucrose = 80 ATP

•  1 Fructose 1,6–Bisphosphate = 40 ATP

•  1 Pyurvic acid = 15 ATP

•  1 Acetyl Co-A or 1 TCA cycle = 12 ATP

• PPP is also called as Warburg - Dickens pathway/HMP shunt/Phosphogluconolactone pathway/ Carbohydrate degradation without mitochondria/Cytosolic oxidative decarboxylation/Horecker -Racker Pathway

• Glycolysis & TCA cycle is the main route of carbohydrate oxidation, but Warburg & Dickens (1935) discovered an alternative route of carbohydrate break down, existing in plants, some animal tissues (Mammary glands, adipose, liver & microbes).

• HMP/PPP occurs when:

(i) NADPH₂ requirement of cell increases during biosynthetic processes.

(ii) When EMP pathway blocked by iodoacetate, fluorides, arsenates.

(iii) When mitochondria is busy in other pathways.

• Most of the intermediates are similar to Calvin cycle, but PPP is amphibolic and oxidative process.

• One ATP is utilised in phosphorylation of glucose, so net gain equals to 35 ATP. (12 NADPH2) Significance of HMP shunt :-

(1) An intermediate erythrose-P (4C) of this pathway is precursor of shikimic acid, which goes to synthesis of aromatic compounds and amino acids.

(2) This cycle provides pentose sugars Ribose-p for synthesis of nucleotides, nucleosides, ATP and GTP.

(3) A five carbon intermediate Ribulose-5-phosphate may used as CO₂ acceptor in green cells.

(4) This pathway produces reducing power NADPH2 for the various biosynthetic pathways, other than photosynthesis like fats synthesis, starch synthesis, hormone synthesis and chlorophyll synthesis.

(5) Intermediates like PGAL and fructose-6-phosphate of this pathway may link with glycolytic reactions. b-Oxidation of Fatty acids

• b-oxidation takes place mainly in perimitochondrial space but also in glyoxisome, peroxisome, cytosol.

• Liberation of 2C segments from the fatty acid mol. in the form of acetyl Co-A is known as b-oxidation. These acetyl-CoA provides ATP after oxidation in krebs cycle.

• Acetyl CoA is oxidised in TCA cycle to CO₂ & H₂O with the production of 12 ATP molecules.

Glyoxylate Cycle

• Discovered by Kornberg & Krebs,during germination of fatty seeds.

• This cycle converts fats into sugars so it is an example of gluconeogenesis in plants.

• Glyoxylate cycle occurs in glyoxysomes, cytosol, & mitochondria.

The Respiratory Balance Sheet

• There can be a net gain of 36 ATP molecules during aerobic respiration of one molecule of glucose.
• Fermentation accounts for only a partial breakdown of glucose whereas in aerobic respiration it is completely degraded to CO₂ and H₂O.
• In fermentation there is a net gain of only two molecules of ATP for each molecule of glucose degraded to pyruvic acid whereas many more molecules of ATP are generated under aerobic conditions.

• NADH is oxidised to NAD+ rather slowly in fermentation, however the reaction is very vigorous in case of aerobic respiration.

Amphibolic Pathway

• Glucose is the favoured substrate for respiration.
• All carbohydrates are usually first converted into glucose before they are used for respiration.
• Other substrates do not enter the respiratory pathway at the first step.
• 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. 
• Since respiration involves breakdown of substrates à the respiratory process are catabolic process and the respiratory pathway as a catabolic pathway.
• Many compounds that would be withdrawn from the respiratory pathway for the synthesis of the various substrates.
  ⇛ To synthesise fatty acids, acetyl CoA would be withdrawn from the respiratory pathway for it.
  ⇛ During breakdown and synthesis of protein too, respiratory intermediates form the link.
  ⇛ Breaking down processes within the living organism is catabolism, and synthesis is anabolism.
• Because the respiratory pathway is involved in both anabolism and catabolism, it would hence be better to consider the respiratory pathway as an Amphibolic pathway rather than as a catabolic one.

Respiratory Quotient

• The ratio of the volume of CO₂ evolved to the volume of O₂ consumed in respiration is called the respiratory quotient (RQ) or respiratory ratio.
• The respiratory quotient depends upon the type of respiratory substrate used during respiration.
• Carbohydrates → RQ 1
• Fats → RQ is less than 1
• Proteins → RQ is 0.9