三七止血机制是（） A．增加血液凝血酶而止血 B．增加血小板数量而止血 C．使局
题型：单项选择题
问题：
三七止血机制是（）
A．增加血液凝血酶而止血
B．增加血小板数量而止血
C．使局部血管收缩而止血
D．增加血小板第三因子促进凝血
E．抑制纤溶酶的活性
题型：单项选择题
It may seem easy to say that plants and animals are different. Any child can tell the difference between a tree and a dog. But those differences are far deeper than just what they look like. The cells (细胞) that plants are made up of are different. In other words, the cell wall of plants has something special.
First of all, it's important to find that not all plant cells have a cell wall. Besides, there are many differences of a cell wall based on the kinds of plant. The following is meant to be what a plant cell wall is like.
The cell wall of a plant is on the outside of the cell membrane (膜). It is usually hard and cannot be bent easily, but the degree of flexibility (韧性) of the cell wall is different. It is based on the special use that the cell has in keeping the plant alive. Cells that make up the trunk of a tree are clearly less flexible than cells on a leaf.
Cell walls act in many ways. They serve to protect the plant. They provide support, as well as help in the movement of chemicals and wastes into and out of the cell. More importantly, they control the water that goes in and out. Plants must have good water controlling systems (系统), allowing them to keep water during dry seasons, and get rid of unnecessary water. Plants can't walk around to find water, so being able to control water is very important.
So there you have it , what a plant cell wall is. Of course, there are many other things that are not talked about here. Knowing more about plant cell wails requires further study and research.
小题1:From the first two paragraphs we can find that _____.
A．all plants have a ceil wall
B．plant cells and animal cells are the same
C．different kinds of plant have different cell walls
D．there are no differences between the cells of a tree and a dog小题2:The underlined word "bent" in the passage is the closest in meaning to _____.
A．变形的
B．防水的
C．太阳灼伤的
D．表皮破损的小题3:Cells in the trunk of a tree are _____ those on a leaf.
A．as flexible as
B．not so flexible as
C．more important than
D．less important than小题4:What is the most important use of a plant cell wall?
A．To protect the plant.
B．To provide support.
C．To move chemicals and wastes.
D．To control the water.
