Respiration

CHAPTER: 11(A)

Respiration

Introduction of Respiration:

•      Respiration is a catabolic event involving the breakdown of many organic compounds to CO₂ and water.

•      Apart from the energy that is utilized, many organic compounds appear as intermediates serve as the starting points for other metabolisms which include amino acid metabolism, synthesis of pigments, compounds related to the cellular constitutents.

Ø  Respiration can be defined as follows:

1. Respiration is the oxidative release of energy locked in the chemical bonds of organic molecules designated as foods.
2. Respiration is the process taking place in all living cells in which energy responsible for carrying out different life activities is released by the breakdown of foods.
3. Process of gaseous exchange where by oxygen is usually absorbed from the atmosphere and CO₂ is usually evolved when organic matter is broken down in the cell with consequent release of energy

Types of Respiration:

1. Aerobic Respiartion:

•      Aerobic respiration can be defined as the process of energy transfer that the cells utilize to convert organic molecules into chemical energy. The chemical reactions involved in aerobic respiration are more or less analogous to the chemical reactions proceeding in the oxidation of organic substances, but  less or no ATP will be generated instead energy will be generated in the form of energy and heat. The equation that shows the oxidation of monosaccharide glucose is

                        C₆H₁₂O₆ + 6 O₂ ====> 6 CO₂ + 6 H₂O + light + heat

2. Anaerobic Respiration:

•      When food is oxidised without using molecular oxygen, as found in anaerobic bacteria, yeasts, many parasitic animals like Taenia (tapeworm), Fasciola (liver Fluke), Ascaris, etc., the respiration is called anaerobic respiration. Less energy is produced in anaerobic respiration.

•      In microorganisms the term fermentation is more commonly used in place of anaerobic respiration. Fermentation is defined as the anaerobic breakdown of carbohydrates and other organic compounds into alcohols, organic acids, gases, etc., with the help of microoganisms or their enzymes. In microoganisms the term fermentation is known after the name of product like alcoholic fermentation and lactic acid fermentation, for example, yeasts oxidize glucose to ethyl alcohol and carbon dioxide without utilizing oxygen.

•      In muscles, certain bacteria and parasitic worms, like Taenia and Ascaris, glucose is metabolised to lactic acid without utilizing oxygen and without the formation of carbon dioxide. In cellulose fermentation, cellulose is converted into volatile fatty acid by anaerobic bacteria and protozoa as found in the rumen and reticulum (parts of stomach of ruminant mammals). Mammalian erythrocytes, as they lack mitochondria, carry out anaerobic respiration. Lactic acid thus produced anaerobically as oxidised aerobically by other tissues (e.g., liver and cardiac muscles).

Glucose=====> ethanol + carbon dioxide + energy
Glucose =====> lactic acid + energy

Significance of Respiration:

(1)   It is the energy releasing and supplying process in all living organisms.

(2)   It converts food energy into metabolically usable forms of chemical energy.

(3)   Most of the released energy is properly utilized for the cellular activities (e.g. metabolism, cell division, growth, etc.) Only a small part of energy is lost as heat.

(4)   In other words, respiration conserves energy very efficiently. Out of the total 673 Kcal energy released per molecule of glucose, 456 Kcal is conserved as 38 ATP molecules.

(5)    Various intermediate products of glycolysis and Kreb’s cycle are used for biosynthesis of other complex organic compounds in cell metabolism.

(6)    Carbon dioxide, required for photosynthesis by green plants, is replenished by carbon dioxide released in respiration. Similarly, oxygen required for respiration is replenished by oxygen released in photosynthesis. Thus, photosynthesis and respiration are complementary to each other and together help to maintain the balance of oxygen and carbon dioxide in nature.

(7)   Fermentation, which is a type of anaerobic respiration, is helpful in the industrial production of various useful products such as alcohol, antibiotics, vitamins, organic acids, bakery and dairy products, tanned leather, etc.

Respiratory Quotient (RQ):

•      Respiratory quotient is the ratio between the volume of  CO₂ given out and the volume of O₂ taken in simultaneously, by a given weight of tissue in a given period of time at standard temperature and pressure.

•      It is sometimes useful to be able to deduce which substrate is being used in a person’s metabolism at a specific time. This can be done if the volume of oxygen taken in, and the volume of carbon dioxide given out are measured. From this data the respiratory quotient (RQ) can be calculated:

RQ      = volume of carbon dioxide given off
                       volume of oxygen taken in

•      The value of R.Q theoretically should remain at unity. However the deviations from unity are very common.

•      The deviation in the value of R.Q arise mainly from oxidation and reduction level of substance used in respiration.

•      The RQ values of the following substrates are well documented from previous investigations:
carbohydrate 1.0; protein 0.9; fat 0.7 & oxalic acid 4.0

•      In other words, when highly reduced substances like fats are utilized, the O₂ taken in is much more compared to the CO₂ given out. In this instance the denominator is always more than the numerator. That is why, the R.Q remains less than unity whenever fats are consumed.

•      When carbohydrates are utilized as respiratory substrates the oxygen taken in is usually equal to that of CO₂ given out. Hence, the R.Q remains at unity whenever a carbohydrate molecules serves as a substrate or respiration.

•      Organic acids in contrast to fatty acids are at a high level of oxidation. They contain more oxygen in their molecules than hydrogen, so the requirement of oxygen for the oxidation of an organic acid is far less than that of the fatty acids. Therefore, the volume of O₂ absorbed is less than that of CO₂ given out. With low denominator and high numerator, the R.Q evidently is more than one. The R.Q of oxalic acid , tartaric acid and malic acids are 4, 1.6, and 1.33 respectively.

•      Proteins are rarely employed as respiratory substances and when they do serve than the R.Q fluctuates around 0.9.

Ø  The value of R.Q is formed to deviate from unity , under the following conditions:

1. When O₂ absorbed is utilized for process other than respiration.
2. When CO₂ formed is utilized metabolically by some process instead of being given out.
3. When substance other than carbohydrates such as proteins are consumed.
4. When carbohydrates or other substrates are oxidized partially.

Factors Affecting Respiration:

1. External Factors:

1. Temperature:

•      As with all chemical reactions, the chemical reactions of respiration are sensitive to temperature changes. Since the reactions of respiration are under the control of enzymes, the temperature range in which they may occur is quite narrow. At temperatures approaching 0^o C, the rate of respiration becomes very low. As the temperatures rises, the respiration  rate may again fall due to destruction of enzymes. So, a maximum rate may be attained somewhere between 35^o C and 45^o C.

2. Oxygen:

•      Oxygen is one of the reactants in the normal respiration of higher plants. Its presence or absence determines the kinds of respiration and thee repiratory products.

•      The air contains sufficiently large quantities of oxygen for plant respiration. The amount of oxygen present in the air may be increased or decreased considerably without affecting the rate of respiration.

3. CO₂ :

•      Increase CO₂ concentration in the atmosphere reduces respiration and consequently   inhibits the germination of many seeds.

4. Light:

•      Light can directly or indirectly affects the rate of respiration. Light for example brings about photosynthesis and thus increases the amount of respirable material. Light also affects respiration by increasing the temperature of the respiring organ.

5. Water:

•      Very low water content in dry seeds and stored tubers is responsible for very feeble rate of respiration. In wilted tissues the stored starch converted into sugars which increase the rate of respiration while in well hydrated plants the rate of respiration is not likely to be affected much by slight changes in the water content.

6. Minerals:

•      Many organic ions are known to participate in certain steps of respiration as co-factors of enzyme systems. Absence of these ions decreases the activity of the enzymes concerned and hence , the rate decreases. Apart from the minerals that actually participate in the mechanism of respiration , some affect respiration they are involved in the formation of respiratory enzymes.

•      N & P increases the rate  at high concentration while K increases the rate at low concentration.

7. Moisture:

•      Directly affects the rate.

2. Internal factors:

1. Protoplasmic Factors:

•      Young growing cells exhibit high rate of respiration as compared to mature cells because they possess high protoplasmic factors.

2. Concentration of Respirable materials:

•      Higher availability of respiratory substrates increases the rate of respiration upto a certain limit.

Mechanism of Respiration:

•      The respiration starts with glucose. In aerobic and anaerobic respiration initial reaction are common as a result of which pyruvic acid is formed  by break down of glucose. The process is called Glycolysis or EMP pathway (Embden Meyerhoff’s- Parnas Pathway).

•      This process does not require O₂ although this can be taken place in the presence of an O₂.

•      After this stage the fate of pyruvic acid is different depending upon the absence or presence of oxygen.

•      If  O₂ is present there is complete oxidation of pyruvic acid into water and CO₂ and chemical reaction through which this occurs is Kreb’s Cycle. This cycle occurs in mitochondria.

•      If O₂ is absent, pyruvic acid forms alcohol and CO₂ without the help of any cell organelles. This process is called anaerobic respiration or fermentation.

Ø  The mechanism of respiration can be discussed on the following satges:

1. Glycolysis (EMP)
2. Kreb’s Cycle
3. Electron Transport Chain (ETC)/ Oxidative Phosphorylation