Respiration: Definition And Meaning. FERMENTATION


 RESPIRATION in living organisms is the series of chemical changes which release energy from food material. It involves a complicated chain of chemical breakdowns, accelerated by enzymes, but it can be regarded, for experimental purposes, as the breakdown of carbohydrates to form carbon dioxide and water, with a corresponding release of energy.  

   The energy so produced is used for such activities as mus- cular contraction, nervous conduction, secretion of enzymes and driving a great many chemical reactions in the living cell. Respiration is one of the most important aspects of the vital chemistry of living matter.

  A distinction is usually made between two forms of, or stages in, respiration called aerobic and anaerobic. Aerobic respiration involves the use of oxygen in the breakdown of carbohydrates or fats which are eventually oxidized completely to carbon dioxide and water. Anaerobic respiration is the breakdown of carbohydrates to release energy without the use of oxygen.

The term "respiration" is also often used loosely in reference to breathing, as in "artificial respiration", pulse and repiration rate" or in connexion with gáseous exchange, "the respiratory surface of a gill", "organs of respiration". For this reason, the "respiration described in this chapter is sometimes called tissue respiration or internal respiration to distinguish it from either the breathing movements (ventilation) or the intake of oxygen and output of carbon dioxide (gaseous exchange).

Aerobic respiration

The following equation summarizes the process of aerobic respiration, i.e. respiration which uses oxygen (the formulae represent the molecular weights of the substances in grams):

CH,O+ 60,-6CO,+ 6H,O +2830 kJ*

From the equation above, it is apparent that for respiration to occur food and oxygen must be taken in and react together. Also, carbon dioxide and water, which are the end-products of the reaction, must constantly be removed.

Methods of demonstrating respiration. A demonstration of respiration in material is one indication that the material is living, and measurements of the rate of respiration in cells, tissues, organs or organisms give some idea of the rate of chemical activity. Consequently, to the biologist, methods of measuring respiration rates are important.

The equation suggests that if an organism is respiring it will (a) use up carbohydrate, (b) take in oxygen, (c) give out carbon dioxide, (d) produce water or water vapour, and (e) release energy. With the exception of (d), if one or more of these changes are taking place the material is likely to be living and respiring.

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(a) Decrease in dry weight (using up carbohydrate) If living material is converting carbohydrate to carbon dioxide and water, which escape into the air, its weight will decrease. However, it is the dry weight which must be measured since any material, living or non-living, may lose weight by the evaporation of water into the atmosphere.

One hundred seeds are soaked in water for 12 hours. Half of them are killed by boiling (controls). The 50 living seeds are placed in one dish with moist cotton wool and the 50 dead seeds in identical conditions. Every day for 5 days, 10 seeds or seedlings are selected from each dish and heated in an oven at 120° C for 12 hours to evaporate all the water. The twoo samples of 10 seeds are then weighed. In this way only the solid matter in the seeds is weighed and, if the seeds are respiring, the solids in the food reserve of the endosperm or cotyledons should be decreasing as the food is used to provide energy.

(b) Uptake of oxygen, The apparatus is arranged After five minutes the tubes will have acquired the temperature of the water in the beaker and the screw clips are closed. If the seeds are respiring they will give out carbon dioxide and take in oxygen so there may be no effective change in the volume of gas in the tube. However, soda lime will absorb all the carbon dioxide produced so that any volunme change may be attributed to the uptake of oxygen. If oxygen is absorbed by the seeds, the level of liquid in the capillary should be seen to rise within 20 minutes or so. Any change in the temperature of the tubes will cause the air in them to expand or contract and produce corresponding movements of the liquid in the capillary, which could be confused with the movements due to oxygen uptake.

The water in the beaker, however, should minimize temperature fluctuations and since these will affect both tubes to the same extent, the change in volume due to oxygen uptake alone can be determined by comparing the levels of liquid in the experi- ment and the control. The control thus allows for changes due to temperature variation and also serves to show that oxygen uptake results from a living process in germinating seeds and is not merely due to physical absorption by the seeds.

If the temperature of the water bath is raised by 10°C and the movement of liquid is timed, it will be seen that a rise in temperature produces an increase in the rate of respiration.

(c) Production of carbon dioxide (i) Germinating seeds

Wet cotton wool is placed in two flasks A and B. Soaked seeds are added to A and an equal number of boiled seeds to B.

Both groups of seeds are soaked for 15 minutes in sodium hypochlorite soluti n to prevent fungal or bacterial growth which might produce carbon dioxide. The flasks are securely corked and left in the same conditions of light and temperature until germination is clearly perceptible in A. The seeds in B should not germinate. The gases in each flask are then tested by removing the cork and tilting the flask over a test-tube of lime water and shaking up the test-tube.

Result. The air from flask A should turn the lime water turn the lime water milky showing carbon dioxide is present. Air from B should have no effect.

Interpretation. The carbon dioxide must have been pro- duced by the germinating seeds. B is a control and proves that it is not the cotton wool or anything other than germinating seeds that give carbon dioxide.

(ii) Animals and plants. This experiment is suitable for giving fairly quick results with small animals but will also work, over a longer period, with plant material.

The animal or plant is placed in the vessel C. If it is a plant, the vessel must be "blacked out" to prevent photosynthesis occurring. If a potted plant is used, the pot must be enclosed in impermeable material so that the respiration of organisms in the soil does not affect the result. A stream of air is drawn slowly through the apparatus by means of a filter pump at E.

In A, soda lime absorbs the carbon dioxide from the incoming air; the ime water in B should stay clear and so prove that carbon dioxide is absent from the air going into vessel C. If carbon dioxide is given out by the organism, the lime water in D will go milky after a time. If the rates of respiration of different animals or plants are to be compared, the time taken for the lime water to go milky should be noted.

(d) Production of water vapour

Since non-living matter may give off water vapour by evaporation, this is not a reliable test of respiration.

(e) Release of energy in germinating seeds 

Heat production is a good indication of energy release. Sufficient seeds to fill two small vacuum flasks are soaked in water for 24 hours and half of them killed by boiling for 10 minutes. Both lots of seeds are soaked for 15 minutes in a solution of sodium hypochlorite (e.g. commercial hypochlorite diluted 1:4) to kill fungal spores on the grains. The seeds are rinsed with tap water; the living seeds are placed in one flask, the dead seeds in the other. Thermometers are inserted and the mouths of the flasksS plugged with cotton wool.

Result. After a few days the temperature in the fiask with living seeds will be considerably higher than in the control.

Interpretation. During the germination of seeds, heat energy is released. The results, however, do not justify the conclusion that the heat is the result of respiration rather than any other chemical process.

Anaerobic respiration

This is the release of energy from food material by a process of chemical breakdown which does not require oxygen. The food, e.g. food, e.g. carbohydrate, is not broken down completely to carbon dioxide and water but to intermediate compounds such as lactic acid or alcohol. The incomplete breakdown of the food means that hass energy is made available during anaerobic respi:atio than is released. during aerobic respiration

Both acesses may be taking place in cells at the same time. Indeea, the first steps in the breakdown of glucose in respiration are anaerobic.

glucose------->lactic acid------>carbon dioxide and water

Luring vigorous activity, the oxygen supply may not be sufficient to oxidize food rapidly enough to meet the energy demands of the body so that the products of the initial, anaerobic, stages accumulate, e.g. lactic acid. These producS are oxidized or converted back to carbohydrate so that even alter vigorous activity has ceased, the uptake of oxygen continues at a high rate. The organism is said to have incurred an Oxygen debt as a result of its excess of anaerobic respiration.

Certain bacteria and fungi derive all or most of their energy from anaerobic respiration and the end products are frequently alcohol and carbon dioxide; the process in this case is called fermentation.

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The term fermentation is not applied exclusively to anaerobic respiration in which alcohol is produced; a variety of organic acids, e.g. citric, butyric, oxalic may be formed by the anaerobic respiration of micro-organisms and such fermenta- tions are exploited commercially to produce these compounds.

The yeasts (unicellular fungi) and bacteria which bring about fermentation are able to employ their enzyme systems to release energy anaerobically from carbohydrates, particularly starch and sugar. Alcoholic fermentation on a commercial scale is usually brought about by yeasts acting on sugar solutions such as the malt sugar prepared from germin- ating barley. The equation below summarizes the reactions:

CH,O2C0,+2C,H,OH+118 kJ

If this equation is compared with the one on it can be seen that far less energy is obtained from a gramme molecule of glucose during anaerobic respiration than is released when the sugar is completely oxidized. Unless the products óf fermentation, in this case ethanol, are removed, they will reach a concentration which will eventually kill the organism producing them.

(f) To show carbon dioxide production during anaerobic respiration (fermentation) in yeast

Some water is boiled to expel all the dissolved oxygen and when cool is used to make up a 5 per cent solution of glucose and a 10 per cent suspension of dried yeast. 5 cm3 of the glucose solution and 1 cm3 of the yeast suspension are placed in a test-tube and covered with a thin layer of liquid paraffin to exclude atmospheric oxygen from the mixture. A delivery tube is fitted as shown in and allowed to dip into clear lime water. After 10-15 minutes, With gentle warming if necessary, there should be signs of fermentation in the yeast gucose mixture and the bubbles of gas escaping through the lime-water turn it milky. The gas is therefore carbon dioxide.

A control can be set up in the same way but using a boiled yeast suspension which will not ferment. The fact that the living yeast produces carbon dioxide despite being deprived of oxygen is evidence to support the contention that anaerobic respiration is taking place.

If the experiment is repeated on a larger scale using 500 cm3 glucose solution and left for several days in a warm place, the alcohol can be distilled off, collecting the fraction that vapor- izes between 70-80° C. The distillate can be identified as alcohol by its taste, odour and the fact that it can be ignited.


The thousands of enzyme-controlled chemical changes which tak place in organisms and the cells of which they are com- posed are often referred to collectively as metabolism. The reactions concerned may be changing one compound into another more useful or more reactive, combining simple stances into more complex ones which can be built into. e tissues, breaking down complex compounds to release a energy or tc make them more easily ransportable. Respir is one manifestation of metabolism.

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