iBT TOEFL Topic Vocabulary Breakthrough Unit 061

iBT TOEFL Topic Vocabulary Breakthrough Unit 061
Author: Li Xiaolai

61. Among the most important classes of biomolecules are nucleic acids, proteins, carbohydrates, and lipids. & (1) Nucleic acids are responsible for storing and transferring genetic information. They are enormous molecules made up of long strands of subunits, called bases, that are arranged in a precise sequence. These are "read" by other components of the cell and used as a guide in making proteins. & (2) Proteins are large molecules built up of small subunits called amino acids. The proteins of greatest interest to biochemists are the enzymes, which serve as catalysts, of chemical reactions. & (3) Carbohydrates are the basic fuel molecules of the cell. they contain carbon, hydrogen, and oxygen in approximately equal amounts. Green plants and some bacteria use a process known as photosynthesis to make simple carbohydrates (sugars) from carbon dioxide, water, and sunlight. Animals, however, obtain their carbohydrates from foods. Once a cell possesses carbohydrates, it may break them down to yield chemical energy or use them as raw material to produce other biomolecules. & (4) Lipids are fatty substances that play a variety of roles in the cell. Some are held in storage for use as high-energy fuel; others serve as essential components of the cell membrane.

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