IGCSE Biology Paper-1: Specimen Questions with Answers 123 - 123 of 208

Answer

Explanation

• Members of the family Leguminosae such as beans, gram, groundnut and soyabean etc. on their secondary, tertiary, and sometimes primary roots bear small nodule like swellings. Rhizobium penetrates to the cortex of root through infection thread. Simultaneously cortical cells or root are stimulated to divide more vigorously to form nodules on the root. Neither bacterium nor plant alone can fix nitrogen in such cases. Nitrogen fixation is the outcome of symbiotic relationship between the two. When a section of root nodules is observed the presence of a pigment, leghemoglobin is seen to impart pinkish colour to it. This pigment is closely related to hemoglobin and helpful in creating optimal condition for nitrogen fixation. Like hemoglobin, leghaemo-globin is an oxygen scavenger. Fixation of nitrogen is done with the help of enzyme nitrogenase, which functions under anaerobic conditions. Leghaemo-globin combines with oxygen and protects nitrogenase.
• Symbiotic bacteria have also been found to occur in root nodules of Casuarina, Cycas, Alnus, etc. Leaf nodules develop in some members of family Rubiaceae, the bacteria being Mycobacterium. Some cyanobacteria also have symbiotic association with plants e. g. , lichens; Anthoceros (a liverwort) and Azolla (a water fern).

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