Listening audioscript

Speaking Review Exercise (Skills 16 through 18) 
Page 313
[ mp3 137-138]
Listen to part of a lecture in a biology class. 
(Professor)
Now let’s talk about the structure of proteins, and let me start with the 
primary structure and then we’ll discuss the secondary structure. Now, as 
you know, proteins are made up of strings of amino acids hooked 
together. Think of it as a head and a tail; the heads hook together in a 
chain one after the other. The heads are all the same, but it’s the tails— 
there are 20 different ones—it’s the tails that identify the amino acid. 
These 20 different amino acids can hook together in chains in practically 
an unlimited number of different combinations. This sequence, uh…the 
order of which amino acid comes after another one, this sequence of the 
chain is called the primary structure of the protein. 
Again, as you know, each amino acid has different properties. Some 
dissolve easily in water, and others are more like oils and don’t dissolve 
in water.
Now then, secondary structures arise when long strings of amino acids 
interact with other parts of the string to fold in various ways, forming 
three-dimensional structures. That is they um have height, width, and 
depth. For example, parts of the protein composed of amino acids that 
don’t dissolve in water will be tucked away inside the protein. In contrast, 
the amino acids from the chain, which dissolve readily in water, will be on 
the surface of the protein. The cells in our bodies are mostly water, after 
all, and that’s where the proteins are floating. Now other kinds of bonds 
can form between amino acids on different parts of the chain that forms 
the protein. Some of these bonds are weaker, and some are much 
stronger and give the protein a more secure structure. These different 
properties and bonds allow the protein to form in a very specific three-
dimensional shape that is essential to its proper function.