Which one of the following was used to...

单项选择题Which one of the following was used to determine the structure of the DNA molecule ( )

A．Transmission(透射) electron microscope
B．Scanning electron microscope
C．Differential centrifugatioin(差速离心)
D．X-ray crystallography

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1.单项选择题Plant guard cells(保卫细胞) have a higher [C1-] than the outside environment. The most likely explanation for this is( ).

A． Water leaves the cell by osmosis.
B． C1- is diffusing through transmembrane channels.
C． C1- diffusion is aided by a mobile carrier.
D．An active transport symport transports C1-/H+.
E．None of the above

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2.单项选择题在下列组织中, 紧密连接特别重要的有( )

A．Stomach(胃) and kidneys
B．Smooth muscle
C．Heart muscle
D．Intestinal epithelial tissue(肠表皮组织)

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3.单项选择题钙泵的作用主要是( )。

A．降低细胞质中Ca2+的浓度；
B．提高细胞质中Ca2+的浓度；
C．降低内质网中Ca2+的浓度；
D．降低线粒体中Ca2+的浓度

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4.单项选择题The hormone glucagons(胰高血糖素) stimulates the breakdown of stored glycogen in liver and muscle cells by the following enzymes. Which is the first enzyme that must be activated ( )

A．Protein kinase A
B．Phosphorylase kinase
C．Glycogen phosphorylase
D．Protein phosphatase
E． Protein phosphatase inhibitor-1

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5.单项选择题Which one of the following is incorrect for the biogenesis of eukaryotic ribosomes( )

A．25-50% of the pre-rRNA is degraded
B． Pre-rRNA binds to ribosomal protein
C． Pre-rRNA is cut to form three classes of rRNA
D．Pre-rRNA is methylated
E．The rRNA gene for the 40S subunit is transcribed separately

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