4
Question
Give reasons for the following:
(i) Testes descend into the scrotum before birth.
(ii) Secondary growth does not occur in monocot stems.
(iii) Nitrogenous fertilizers are not applied in fields where leguminous crops grow.
(iv) Genetic code is 'universal'.
(v) At higher temperatures, green plants start evolving CO2 instead of O2.
(i) Testes descend into the scrotum before birth.
(ii) Secondary growth does not occur in monocot stems.
(iii) Nitrogenous fertilizers are not applied in fields where leguminous crops grow.
(iv) Genetic code is 'universal'.
(v) At higher temperatures, green plants start evolving CO2 instead of O2.
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Solution
(i) Sperm development requires temperature which is a few degrees less than the normal body temperature. Scrotum has almost no fat insulation, so it keeps the testes at a cooler temperature i.e., 2-30 C less temperature than normal body temperature. Also, contraction or relaxation of muscles moves the scrotum close to or far from testes and heat of the body according to the environmental temperatures.
(ii) Secondary growth does not occur in monocot stem because it does not contain the meristematic tissue-cambium, which is responsible for secondary growth in plants.
(iii) Nitrogenous fertilizers are not needed in fields where leguminous crops grow because these plants have root nodules containing nitrogen fixing bacteria. They convert the nitrogen to nitrates which are used by these plants. The nitrates mix with the soil when these plants are ploughed under.
(iv) Genetic code is universal because it consist of only four nitrogenous bases-adenine, guanine, thymine and cytosine. These four bases in different combinations form all the various type of proteins formed by genetic coding.
(v) At higher temperatures CO2 may be released by some plants instead of O2 because of the main enzyme of photosynthesis RuBP-carboxylase or RuBisCO functions as RuBP-oxygenase when the concentration of oxygen is high. It then catalyzes the conversion of RuBP to PGA and phosphoglycolic acid. The later is changed to glycolic acid and then to glycine. In the mitochondria, glycine forms serine and CO2 is released. This process is called photorespiration.
(i) Sperm development requires temperature which is a few degrees less than the normal body temperature. Scrotum has almost no fat insulation, so it keeps the testes at a cooler temperature i.e., 2-30 C less temperature than normal body temperature. Also, contraction or relaxation of muscles moves the scrotum close to or far from testes and heat of the body according to the environmental temperatures.
(ii) Secondary growth does not occur in monocot stem because it does not contain the meristematic tissue-cambium, which is responsible for secondary growth in plants.
(iii) Nitrogenous fertilizers are not needed in fields where leguminous crops grow because these plants have root nodules containing nitrogen fixing bacteria. They convert the nitrogen to nitrates which are used by these plants. The nitrates mix with the soil when these plants are ploughed under.
(iv) Genetic code is universal because it consist of only four nitrogenous bases-adenine, guanine, thymine and cytosine. These four bases in different combinations form all the various type of proteins formed by genetic coding.
(v) At higher temperatures CO2 may be released by some plants instead of O2 because of the main enzyme of photosynthesis RuBP-carboxylase or RuBisCO functions as RuBP-oxygenase when the concentration of oxygen is high. It then catalyzes the conversion of RuBP to PGA and phosphoglycolic acid. The later is changed to glycolic acid and then to glycine. In the mitochondria, glycine forms serine and CO2 is released. This process is called photorespiration.
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