Plant Respiration

1. RESPIRATION AND ITS SIGNIFICANCE
Cellular respiration is an enzyme controlled process of biological oxidation of food materials in a living cell, using molecular O₂, producing CO₂ and H₂O, and releasing energy in small steps and storing it in biologically useful forms, generally  ATP.
$\underset{\mathrm{Glucose}}{{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6} + 6{\mathrm{O}}_2\stackrel{\mathrm{Enzymes}}{\to } \underset{\mathrm{Carbondioxide}}{6{\mathrm{CO}}_2}+\underset{\mathrm{Water}}{6{\mathrm{H}}_2\mathrm{O}} + \underset{\left(\mathrm{ATP}\right)}{\mathrm{Energy}}$
Use of energy
Cellular activities like active transport, muscle-contraction, bioluminescence’s, homothermy, locomotion, nerve impulse conduction, cell division, growth, development, seed germination require energy. Main source of energy for these endergonic activities in all living organisms including plants, comes from the oxidation of organic molecules.
The energy released by oxidation of organic molecules is actually transferred to the high energy terminal bonds of ATP, a form that can be readily utilized by the cell to do work. Once ATP is formed, its energy may be utilized at various places in the cell to drive energy- requiring reactions. In these processes, one of the three phosphate groups is removed from the ATP molecule. Thus the role of ATP as an intermediate energy transforming compound between energy releasing and energy consuming reactions.
Significance of respiration
Respiration plays a significant role in the life of plants. The important ones are given below:
1) It releases energy, which is consumed in various metabolic processes necessary for life of plant.
2) Energy produced can be regulated according to requirement of all activities.
3) It converts insoluble foods into soluble form.
4) Intermediate products of cell respiration can be used in different metabolic pathways e.g.,
Acetyl- CoA (in the formation of fatty acid, cutin and isoprenoids) ; α- ketoglutaric acid (in the formation of glutamic acid) ; Oxaloacetic acid (in the formation of aspartic acid, pyrimidines and alkaloids); Succinyl- CoA (synthesis of pyrrole compounds of chlorophyll).
5) It liberates carbon dioxide, which is used in photosynthesis.
6) Krebs cycle is a common pathway of oxidative breakdown of carbohydrates, fatty acids and amino acids.
7) It activates the different meristematic tissues of the plant.
Compensation point
It is that value or point in light intensity and atmospheric CO₂ concentration when rate of photosynthesis is just equivalent to the rate of respiration in photosynthetic organs so that there is no net gaseous exchange. The value is 2.5- 100 ft candles/ 26.91-1076.4 lux in shade plants and 100-400 ft candles/ 1076.4-4305.6 lux in case of sun plants. It is called light compensation point. There is, similarly, a CO₂ compensation point. Its value is 25-100 ppm (25-100 $\mu l.l^{–}$) in C₃ plants and 0-5 ppm (0-5 $\mu l.l^{–}$ ) in C₄ plants. A plant cannot survive for a long at compensation point because the nonphotosynthetic parts and dark respiration will deplete organic reserve of the plant.
CO₂ intake in photosynthesis balanced with CO₂ release in respiration = Compensation point.
Comparison between respiration and combustion
According to Lavoisier cell respiration resembles the combustion (e.g., burning of coal, wood, oil etc.) in the breakdown of complex organic compounds in the presence of oxygen and production of carbon dioxide and energy, but there are certain fundamental differences between the two processes:

Differences between cell respiration and combustion

2. PHASES OF RESPIRATION
There are three phases of respiration:
1) External respiration: It is the exchange of respiratory gases (O₂ and CO₂) between an organism and its environment.
2) Internal or Tissue respiration: Exchange of respiratory gases between tissue and extra cellular environment. Both the exchange of gases occur on the principle of diffusion.
3) Cellular respiration: It is an enzymatically-controlled stepped chemical process in which glucose is  oxidised inside the mitochondria to produce energy-rich ATP  molecules with high-energy bonds.
So, respiration is a biochemical process.

3. RESPIRATORY SUBSTRATE OR FUEL
In respiration many types of high energy compounds are oxidised. These are called respiratory substrate or respiratory fuel and may include carbohydrates, fats and protein.
1) Carbohydrate: Carbohydrates such as glucose, fructose (hexoses), sucrose (disaccharide) or starch, insulin, hemicellulose (polysaccharide) etc; are the main substrates. Glucose are the first energy rich compounds to be oxidised during respiration. Brain cells of mammals utilized only glucose as respiratory substrate. Complex carbohydrates are hydrolysed into hexose sugars before being utilized as respiratory substrates. The energy present in one gram carbohydrate is 4.4 Kcal or 18.4 kJ.
2) Fats: Under certain conditions (mainly when carbohydrate reserves have been exhausted) fats are also oxidised. Fat are used as respiratory substrate after their hydrolysis to fatty acids and glycerol by lipase and their subsequent conversion to hexose sugars. The energy present in one gram of fats is 9.8 Kcal or 41kJ, which is maximum as compared to another substrate.
The respiration using carbohydrate and fat as respiratory substrate, called floating respiration (Blackmann).
3) Protein: In the absence of carbohydrate and fats , protein also serves as respiratory substrate. The energy present in one gram of protein is: 4.8 Kcal or 20 kJ. when protein are used as respiratory substrate respiration is called protoplasmic respiration. 

4. TYPES OF RESPIRATORY ORGANISM
Organism can be grouped into following four classes on the basis of their respiratory habit.
1) Obligate aerobes: These organisms can respire only in the presence of oxygen. Thus oxygen is essential for their survival.
2) Facultative anaerobes: Such organisms usually respire aerobically (i.e., in the presence of oxygen) but under certain condition may also respire anaerobically (e.g., Yeast, parasites of the alimentary canal).
3) Obligate anaerobes: These organisms normally respire anaerobically which is their major ATP- yielding process. Such organisms are in fact killed in the presence of substantial amounts of oxygen (e.g., Clostridium botulinum  and C. tetani).
4) Facultative aerobes: These are primarily anaerobic organisms but under certain condition may also respire aerobically.

5. TYPES OF RESPIRATION
On the basis of the availability of oxygen and the complete or incomplete oxidation of respiratory substrate. The respiration may be either of the following two types: Aerobic respiration and Anaerobic respiration
Aerobic respiration
It uses oxygen and completely oxidises the organic food mainly carbohydrate (Sugars) to carbon dioxide and water. It therefore, releases the entire energy available in glucose.
${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{O}}_2\stackrel{\mathrm{Enzymes}}{\to }6{\mathrm{CO}}_2+6{\mathrm{H}}_2\mathrm{O}+\mathrm{energy}$ (686 Kcal)
It is divided into two phases: Glycolysis, Aerobic oxidation of pyruvic acid.
Glycolysis / EMP pathway
1) Discovery: It was given by Embden, Meyerhof and Parnas in 1930. It is the first stage of breakdown of glucose in the cell.
2) Definition: Glycolysis ( Gr. glykys= sweet, sugar; lysis= breaking) is a stepped process by which one molecule of glucose (6c) breaks into two molecules of pyruvic acid (3c).
3) Site of occurrence: Glycolysis takes place in the cytoplasm and does not use oxygen. Thus, it is an anaerobic pathway. In fact, it occurs in both aerobic and anaerobic respiration.
4) Inter conversions of sugars: Different forms of carbohydrate before entering in glycolysis get converted into simplest form like glucose, glucose 6-phosphate or fructose 6-phosphate. Then these sugars are metabolized into the glycolysis.

5) Special features of glycolysis: The special features of glycolysis can be summarised as follows:
(i) Each molecule of glucose produces 2 molecules of pyruvic acid at the end of the glycolysis.
(ii) The net gain of ATP in this process is two ATP molecules (four ATPs are formed in glycolysis but two of them are used up in the reaction).
(iii) During the conversion of 1, 3-diphosphoglyceraldehyde into 1, 3-diphosphoglyceric acid one molecule of NADH₂ is formed. As each molecule of glucose yields two molecules of 1,3-diphosphoglyceric acid, hence each molecule of glucose forms 2 molecules of NADH₂.
(iv) During aerobic respiration (when oxygen is available) each NADH₂ forms 3 ATP and H₂O through electron transport system of mitochondria. In this process ½ O₂ molecule is utilized for the synthesis of each water molecule.
In this way during aerobic respiration there is additional gain of 6 ATP in glycolysis
$\underset{(\mathrm{net} \mathrm{gain})}{2\mathrm{ATP}}+\underset{\mathrm{addition} \mathrm{gain}}{6\mathrm{ATP}}\to \underset{\mathrm{Total} \mathrm{net} \mathrm{gain}}{8\mathrm{ATP}}$
(v) Reaction of glycolysis do not require oxygen and there is no output of CO₂.
(vi) Formation of 1, 3- diphosphoglyceraldehyde called non enzymatic phosphorylation.
(vii) Overall reaction of glycolysis represented by following reaction:${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\to \underset{\mathrm{Pyruvate}}{2{\mathrm{c}}_3{\mathrm{H}}_4{\mathrm{O}}_3}+4\mathrm{H}$

Total input and output materials in glycolysis

Aerobic oxidation of pyruvic acid
1) Oxidative decarboxylation of pyruvic acid: If sufficient O₂ is available, each 3-carbon pyruvate molecule (CH₃COCOOH) enters the mitochondrial matrix  where its oxidation is completed by aerobic means. It is called gateway step or link reaction between glycolysis and Kreb’s cycle.
Decarboxylation and dehydration:
$\underset{\mathrm{Pyruvic} \mathrm{acid}}{{\mathrm{CH}}_3\mathrm{CO}.\mathrm{COOH}}+\underset{\mathrm{CoA}}{\mathrm{CoA}.\mathrm{SH}}+\mathrm{NAD}\underset{**\mathrm{TPP} **\mathrm{LAA}}{\overset{\mathrm{Pyruvic} \mathrm{dehydrogen} \mathrm{ase} \mathrm{multienzyme} \mathrm{complex}}{\to }}\underset{(\mathrm{acetyl}–\mathrm{s}–\mathrm{CoA})}{{\mathrm{CH}}_3.\mathrm{CO}.\mathrm{S}.\mathrm{CoA}}+\mathrm{NAD}.2\mathrm{H}+{\mathrm{CO}}_2$
**TPP = Thiamine pyrophosphate
**LAA = Lipoic acid amide
Acetyl CoA is a common intermediate of carbohydrate and fat metabolism. Latter this acetyl CoA from both the sources enters Kreb’s cycle. This reaction is not a part of Kreb’s cycle.
2) Kreb’s cycle / TCA cycle / Citric acid cycle
Discovery: This cycle has been named after the German biochemist in England Sir Hans Krebs  who discovered it in 1937. He won Noble Prize for this work in 1953. Krebs cycle is also called the citric acid cycle after one of the participating compounds. It takes place in the mitochondrial matrix.