9.(a) through the er translocation channel. The 10.(d)。 nuclear localization sequence is therefore never 11.(b)。 exposed to the cytosol. It will never encounter 12.(b) nuclear import receptors, and the protein will not 13.(d) enter the nucleus 14 15.(a) 答:加在于粗面内质网上合成的蛋白质上的糖 四、简答题(选做4题,每题5分,20分) 基可由两种途径连接:通过天冬氨酸残基的N 原子或通过丝氨酸和苏氨酸残基的O原子。 (1). If only GDP were present, microtubules N连结糖蛋白合成的第步在粗面内质网上 would continue to shrink and eventually 进行,糖链是从磷酸多萜醇转移至新生肽链 sappear, because tubulin dimers with上。这种糖基化在高尔基体中继续被修饰O- GDP have very low affinity for each other 连结的糖基化是在高尔基体中进行的, and will not add stably to microtubules (2) f GTP is present but cannot be hydrolyzed,5答过氧化物酶体中的氧化酶都是利用分子 microtubules will continue to grow until 氧作为氧化剂,催化下面的化学反应 all free tubulin subunits have been used up RH2+O? →R+H2O2 这一反应对细胞内氧的水平有很大的影 响。例如在肝细胞中有20%的氧是由过氧化物 Answer2. Regulated secretion occurs only in酶体消耗的其余的在线粒体中消耗在过氧化 esponse to a signal. The proteins to be secre 物酶体中氧化产生的能量以产热的方式消耗 are stored in special secretory vesicles. Sorting掉而在线粒体中氧化产生的能量贮存在 to the regu lated secretory pathway is contro lled中线粒体与过氧化物酶体对氧的敏感性是不 by selective protein aggregation. Constitutive一样的线粒体氧化所需的最佳氧浓度为296左 secretion appears to occur by default with右增加氧浓度并不提高线粒体的氧化能力 secretory proteins, which do not selectively 过氧化物酶体与线粒体不同,它的氧化率是随 aggregate being included in transport ve 氧张力增强而成正比地提高(图7-44)。因此在 低浓度氧的条件下线粒体利用氧的能力比过 Answer3. The protein is translocated into the氧化物酶体强但在高浓度氧的情况下过氧化 ER. Its ER signal sequence is recognized as soon物酶体的氧化反应占主导地位这种特性使过 as it emerges from the ribosome. The ribosome氧化物酶体具有使细胞免受高浓度氧的毒性 then becomes bound to the er memb.and作用 the growing polypeptide chain is transferred2 9. ( a ) 10. ( d )。 11. ( b )。 12. ( b ) 13. ( d ) 14. ( c ) 15. ( a )。 四、简答题(选做 4 题，每题 5 分，20 分) 1. Answer1: (1). If only GDP were present, microtubules would continue to shrink and eventually disappear, because tubulin dimers with GDP have very low affinity for each other and will not add stably to microtubules. (2). If GTP is present but cannot be hydrolyzed, microtubules will continue to grow until all free tubulin subunits have been used up. 2. Answer2. Regulated secretion occurs only in response to a signal. The proteins to be secreted are stored in special secretory vesicles. Sorting into the regulated secretory pathway is controlled by selective protein aggregation. Constitutive secretion appears to occur by default with secretory proteins, which do not selectively aggregate being included in transport vesicles. 3. Answer 3. The protein is translocated into the ER. Its ER signal sequence is recognized as soon as it emerges from the ribosome. The ribosome then becomes bound to the ER membrane, and the growing polypeptide chain is transferred through the ER translocation channel. The nuclear localization sequence is therefore never exposed to the cytosol. It will never encounter nuclear import receptors, and the protein will not enter the nucleus. 4. 答：加在于粗面内质网上合成的蛋白质上的糖 基可由两种途径连接：通过天冬氨酸残基的 N 原子或通过丝氨酸和苏氨酸残基的 O 原子。 N-连结糖蛋白合成的第一步在粗面内质网上 进行，糖链是从磷酸多萜醇转移至新生肽链 上。这种糖基化在高尔基体中继续被修饰。O- 连结的糖基化是在高尔基体中进行的。 5.答: 过氧化物酶体中的氧化酶都是利用分子 氧作为氧化剂, 催化下面的化学反应: RH2 + O2 ---------→ R + H2O2 这一反应对细胞内氧的水平有很大的影 响。例如在肝细胞中,有 20%的氧是由过氧化物 酶体消耗的,其余的在线粒体中消耗。在过氧化 物酶体中氧化产生的能量以产热的方式消耗 掉, 而在线粒体中氧化产生的能量贮存在 ATP 中。线粒体与过氧化物酶体对氧的敏感性是不 一样的,线粒体氧化所需的最佳氧浓度为 2%左 右,增加氧浓度,并不提高线粒体的氧化能力。 过氧化物酶体与线粒体不同, 它的氧化率是随 氧张力增强而成正比地提高(图 7-44)。因此,在 低浓度氧的条件下,线粒体利用氧的能力比过 氧化物酶体强,但在高浓度氧的情况下,过氧化 物酶体的氧化反应占主导地位,这种特性使过 氧化物酶体具有使细胞免受高浓度氧的毒性 作用