（a）Biofuels contain alcohols that are produced by the fermentation of sugars derived from crop waste．This waste contains cellulose and other organic compounds in cell walls． Scientists investigated the production of sugars from crop waste for biofuel production．The scientists discovered that a strain of the fungus Penicillium citrinum，isolated from soil，was a good source of three different extracellular enzymes，M，N and $$\(\mathbf{O}\)$$ ．These enzymes break down polysaccharides in cell walls． The scientists cultured $$\(P\)$$ ．citrinum in a liquid medium containing cell wall material．Samples of the liquid were taken，and the three enzymes were separated from the medium．Each enzyme was placed in a reaction mixture with an appropriate substrate．The activity of each enzyme was determined to give a measurement of the quantity of enzyme produced by $$\(P\)$$ ．citrinum． The results are shown in Table 4．1． Table 4.1 ............................................................................................................................................... . ............................................................................................................................................... . ............................................................................................................................................... . ............................................................................................................................................... . ............................................................................................................................................... . ............................................................................................................................................... . ............................................................................................................................................... . ............................................................................................................................................... . ......................................................................................................................................... ［4］

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