人的口腔上皮细胞不具有的结构是
A．细胞核
B．线粒体
C．细胞膜
D．细胞壁
答案：D
题目分析：植物细胞的结构有：细胞壁、细胞膜、细胞质、细胞核、液泡、叶绿体和线粒体．动物细胞的结构包括细胞膜、细胞质、细胞核、线粒体．动物细胞和植物细胞相比，植物细胞和动物细胞的都有细胞膜、细胞质、细胞核、线粒体；动物细胞没有的结构是细胞壁、液泡、叶绿体等．口腔上皮细胞属于动物细胞，因此不具有细胞壁．
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hours a day." This is what a teacher usually 1______ when he tells his students how to allot (分配) their time. But is it true? Suppose you have a bucket with a small hole in its bottom. Now you are asked to fill the bucket with the water in a basin. If you are too little a boy to lift the basin full of water, you are sure 2______ a bowl or something alike. You will dip out the water from the basin with the bowl and pour it into the bucket.You may dip from the basin again and again. However, you will soon find that as soon as you pour a bowl of water into the bucket, most of the water 3______ through the hole, so you are 4______ able to fill up the bucket this way. On the contrary, if you are an adult, you will 5______ lift the basin and pour the whole basin of water into the bucket at once. Now, the bucket is full. Though the water in it is also leaking out, you at least have filled up the bucket with water. If you want to 6______ the bucket full, you only have to add a bowl of water now and then. The same is true of English learning. Let us 7______ a simple example. If you learn 10 new words today, you are most likely to remember 9 tomorrow. But if you try to learn 30 words today, though you may forget far more than one word, you may 8______ be able to remember 11 or 12 words tomorrow. Day in and day out, you will be able to remember a large number of words, 9______ you may forget some of them. 10______, the more time you spend at a time, the more you will learn on the whole. Of course, the amount of time you spend each time must be within the limit that you do not exhaust (使筋疲力尽) yourself. |