Introduction to Respiration

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, generallyਊTP.

\mathop {{C_6}{H_{12}}{O_6}}\limits_{{\text{glucose}}}#xA0+ 6{O_2}\xrightarrow{{{\text{enzymes}}}}\mathop {6C{O_2}}\limits_{{\text{carbondioxide}}}#xA0+ \mathop {6{H_2}O}\limits_{{\text{Water}}}#xA0+ \mathop {{\text{energy}}}\limits_{{\text{(ATP)}}}

Use of energy : Cellular activities like active transport, muscle-contraction, bioluminescenes, 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) $\alpha$- 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$\mu l.l^{-1}$) in C₃ plants and 0-5 ppm (0-5$\mu l.l^{-1}$) 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 Lavosier 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 :

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.

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. 

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.

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.

Factors affecting rate of respiration

Many external and internal factors affecting the rate of respiration are as follows :

(1) External factors

(i) Temperature : With every 10⁰C rise of temperature from 0^oC to 30^oC the rate of respiration increases 2 to 2.5 times (i.e., temperature coefficient (Q₁₀^o) is = 2 to 2.5), following Vant Hoff’s Law. Maximum rate of respiration takes place at 30^oC, there is an initial rise, soon followed by a decline. Higher the temperature above this limit, more is the initial rise but more is the decline and earlier is the decline in the rate of respiration. Probably this is due to denaturation of enzymes at high temperature.

Below 0^oC the rate of respiration is greatly reduced although in some plants respiration takes place even at-20^oC. Dormant seeds kept at -50^oC survive.

(ii) Supply of oxidisable food : Increase in soluble food content readily available for utilization as respiratory substrate, generally leads to an increase in the rate of respiration upto a certain point when some other factor becomes limiting. 

(iii) Oxygen concentration of the atmosphere : The amount of oxygen in the environment of plants is increased or reduced upto quite low values the rate of respiration is not effected. On decreasing the amount of oxygen to 1.9% in the environment aerobic respiration become negligible (extinction point of aerobic respiration) but anaerobic respiration takes place.

(iv) Oxygen poisoning : The significant fall in respiration rate was observed in many tissues in pure O₂, even at N.T.P. This inhibiting effect was also observed in green peas when they are exposed to pure oxygen exerting a pressure of 5 atm- the respiration rate fall rapidly. The oxygen poisoning effect was reversible, if the exposure to high oxygen pressure was not too prolonged.

(v) Water : With increase in the amount of water the rate of respiration increases. In dry seeds, which have 8-12% of water the rate of respiration is very low but as the seeds imbibe water the respiration increases. As water is necessary for activity of enzymes.

(vi) Light : Respiration takes place in night also which shows that light is not essential for respiration. But light effects the rate of respiration indirectly by increasing the rate of photosynthesis due to which concentration of respiratory substrates is increased. More the respiratory substrate more is the rate of respiration.

(vii) Carbon dioxide (CO₂) :  If the amount of CO₂ in the air is more than the usual rate of respiration is decreased. Germination of seeds is reduced and rate of growth falls down. Heath, (1950) has shown that the stomata are closed at higher cone. of CO₂, due to which oxygen does not penetrate the leaf and rate of respiration is lowered.

(viii) Inorganic salts : The chlorides of alkali cations of Naਊnd K, as also the divalent cations of Li, and Ca and Mg, generally increase the rate of respiration as measured by the amount of CO₂ evolved. Monovalent chlorides of K and Na increases the rate of respiration, while divalent chlorides of Li, Ca and Mg causes less increase in respiration.

(ix) Injury and effects of mechanical stimulation : Wounding or injury almost invariably results in an increase in the rate of respiration.

(x) Effect of various chemical substances : Certain enzymatic inhibitors like cyanides, azides, carbon monoxide, iodoacetate, malonate etc. reduce the rate of respiration even if they are present in very low concentration.

However, various chemical substances such as chloroform, ether, acetone, morphine, etc., brings about an increase in respiratory activity.

(xi) Pollutants : High concentration of gaseous air pollutants like SO₂ , NO_X and O₃ inhibit respiration by damaging cell membrane. These gaseous pollutant causes increase in pH which in turn affects the electron transport system thus inhibiting respiration.

Heavy metal pollutant like lead (Pb) and cadmium (Cd) inhibit respiration by inactivating respiratory enzymes.

(2) Internal Factors

(i) Protoplasm : The meristematic cells (dividing cells of root and shoot apex) have more protoplasm than mature cells. Hence, the meristematic cells have higher rate of respiration than the mature cells. Respiration rate high at growing regions like floral and vegetative buds, germinating seedlings, young leaves, stem and root apices.

(ii) Respiratory substrate : With the increase of in the amount of respiratory substrate, the rate of respiration increases.