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www.GOALias.blogspot.comYeasts are single-celled organisms.They respire anaerobically and duringthis process yield alcohol. They are,therefore, used to make wine and beer.Our muscle cells can also respireanaerobically, but only for a short time,when there is a temporary deficiency ofoxygen. During heavy exercise, fastrunning (Fig. 10.1), cycling, walking formany hours or heavy weight lifting, thedemand for energy is high. But thesupply of oxygen to produce the energyis limited. Then anaerobic respirationtakes places in the muscle cells to fulfilthe demand of energy:Glucose(in muscle)lactic acid + energyin the absence of oxygenFig. 10.1 During exercise, some muscles mayrespire anaerobicallyHave you ever wondered why you getmuscle cramps after heavy exercise? Thecramps occur when muscle cells respireanaerobically. The partial breakdown ofglucose produces lactic acid. Theaccumulation of lactic acid causes musclecramps. We get relief from cramps after ahot water bath or a massage. Can youguess why it is so? Hot water bath ormassage improves circulation of blood.As a result, the supply of oxygen to themuscle cells increases. The increase inthe supply of oxygen results in thecomplete breakdown of lactic acid intocarbon dioxide and water.10.2 BREATHINGActivity 10.1CAUTIONDo this activity under the supervisionof your teacher.Close your nostrils and mouthtightly and look at a watch. What didyou feel after some time? How long wereyou able to keep both of them closed?Note down the time for which you couldhold your breath (Fig. 10.2).So, now you know that you cannotsurvive for long without breathing.You must have seen the movementof the abdomen of dogs, cats and cows,as they breathe. Breathing means takingin air rich in oxygen and giving out airrich in carbon dioxide with the help ofrespiratory organs. The taking in of airrich in oxygen into the body is calledinhalation and giving out of air rich in109RESPIRATIONIN ORGANISMS109