TPO63听力Lecture4原文+MP3+题目+答案下载

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TPO63听力Lecture4原文+MP3+题目+答案下载

TPO63听力Lecture4原文：

Listen to part of a lecture in a biology class.

I think the lens of the eyes is just about the most fascinating part of the body. For one, it’s the only, I repeat, only transparent cellular tissue in the human body. In fact, truly transparent tissue is very uncommon in nature.

There’s some sort of clear creatures that live in oceans and rivers like the jellyfish. Their quasi-transparency allows them to blend in with the water to escape predators. I said quasi because jellyfish are translucent. They are not totally see-through like the lens is. So you have to wonder why the lens is. How is transparent tissue possible? Marlene?

There’s no direct blood supply to that part of the eye.

Good. Exactly. Marlene. The blood and blood vessels would color the lens, the lens would probably end up looking pinkish.

But that’s only part of the explanation. What else?

Well, there was something about crystals and fibers in the chapter we read. It said that the lens is sometimes called a biological crystal, not a true crystal, like a diamond, but it has a very regular arrangement of cells, the cells are exactly aligned and tightly compacted.

Right. It’s made up of layers of cells. But the cells are all of one type, a rigid, elongated cell that’s called a lens fiber. Because it’s all one kind of cell, and because they are aligned so regularly, you don’t have all the angles you get when different types of cells are fitted together, which refract light, bend it as it passes through, and that contributes to its transparency.But there’s something missing in a lens fiber. What doesn’t a lens fiber have?

Well, we said blood vessels, right?

The lens itself, right. Is that it? Anybody? No? This is very important, the lens fibers don’t have organelles. Organelles are parts that function like miniature organs, specialized parts of the cell, like the nucleus that stores the DNA, or mitochondria, which are the powerhouse of a cell.

Okay, now what’s interesting here is that lens fibers, as they’re first forming, they start out as cells with organelles, but as the eye develops, the cells lose all of them, including the nuclei.

。。。。此处省略余下原文！

TPO63听力Lecture4题目：

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