The complex enzymatic machinery that assembles cellulose chains spans the plasma membrane, with one part on the cytoplasmic side positioned to bind the substrate, UDP-glucose, and elongate the chains, and another part extending to the outside, responsible for exporting the cellulose molecules to the extracellular space.

Freeze-fracture electron microscopy shows a cellulose synthesis complex, or rosette, composed of six large particles arranged in a regular hexagon with a diameter of about 30 nm. Several proteins, including the catalytic subunit of cellulose synthase, make up this structure. Each of the six particles of the rosette most likely contains three cellulose synthase molecules, each synthesizing a single cellulose chain.

In one working model of cellulose synthesis, cellulose chains are initiated by the transfer of a glucose residue from UDP-glucose to a “primer” glucose already bound to cellulose synthase on the cytoplasmic side of the plasma membrane, to form a disaccharide. As addition of further glucose residues lengthens the chain, it is extruded through a channel formed by the transmembrane helices of cellulose synthase and, on the outer surface of the plasma membrane, joins growing chains from neighboring cellulose synthase molecules to form a cellulose microfibril.

Polymers of more than 6 to 8 glucose units are insoluble in water, promoting microfibril crystallization. There is no definite length for a cellulose polymer; synthesis is highly processive, and some polymers are as long as 15,000 glucose units.

正确

错误

让游客记住接待社的名称、旅游车车牌号或标志、下榻饭店的名称和电话

做好每天行程预报工作，包括游览景点及用餐地点

讲清旅游路线、所需时间，强调集合时间和地点

地陪、全陪和领队要分工协作、密切配合

正确

错误

“T+探伤人员代号”；不做探伤标识

“T+探伤人员代号”；自做探伤标识

“T+探伤人员代号”；做探伤标识

“T+探伤人员代号”；“T+探伤人员代号”

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错误

The orientation of the newly deposited extracellular cellulose microfibrils is determined by the orientation of the underlying intracellular microtubules.

The large cellulose synthase enzyme complexes are integral membrane proteins that continuously synthesize cellulose microfibrils on the outer face of the plasma membrane.

The distal ends of the stiff microfibrils become integrated into the texture of the cell wall, and their elongation at the other end pushes the synthase complex along in the plane of the plasma membrane. The microtubules serve as tracks that help guide the movement of the enzyme complexes.

The cortical array of microtubules attached to the plasma membrane by transmembrane proteins helps determine the direction in which the microfibrils are laid down. In this curiously indirect way, the cytoskeleton controls the shape of the plant cell and the modeling of the plant tissues.