题目列表(包括答案和解析)
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根据短文内容,从短文后的选项中选出能填入空白处的最佳选项。选项中有两处为多余选项。
Active & Passive TransportRecall that all living things are made up of tiny units called cells, and that -cells are surrounded by a covering called a membrane (膜).The membrane allows things to move in and out of the cell, and it also controls what can get through. 1 They must have water, oxygen, sugar, sodium, potassium (钾) , and a variety of other minerals.The body has to have a delivery system to get these materials where they need to go.In addition, every cell produces waste materials that it needs to get rid of. 2
Just like cars, trucks, and other vehicles form a transport system for the pick up and delivery of materials out in the world, the body has a transport system, too. 3
Passive transport is easiest for the cells because they don't need to use any energy to make it happen.Diffusion (扩散) is the simplest and most common form of passive transport.
During the diffusion process, tiny particles (微粒) of the materials that need to be delivered to the cells are spread through either a gas (like oxygen) or a liquid (like water). 4 Osmosis (渗透作用) is a unique form of passive transport that allows water to pass through the cell membrane, but keeps most other materials out.
Active transport is used when the concentration of materials inside the cell is high, and the cell needs to push materials out to areas of low concentration of the materials.Unfortunately, this kind of transport requires the cell to work against the natural direction of diffusion, so the cell uses up energy. 5 They pump sodium out of their cells and pull potassium in.Although they must use energy in this process, doing so allows them to thrive (成长).
A.Areas with high concentrations of the materials slowly diffuse them to areas of low concentration.
B.Cells need many kinds of materials in order to survive.
C.One example of this is the way that nerve cells work.
D.There are two kinds of transport:active transport, and passive transport.
E.Passive transport is much more essential than active transport.
F.The body has to have a method of waste collection and disposal.
G.Then the cell usually needs extra energy to make up for it.
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