单选题 1 Most matter exists as compounds---combinations of atoms or oppositely charged ions of two or more different elements held together in fixed proportions by chemical bonds. Compounds are classified as organic or inorganic. Organic compounds contain atoms of the element carbon, usually combined with itself and with atoms of one or more other elements such as hydrogen, oxygen, nitrogen, sulfur, phosphorus, and chlorine. Many materials important to us--food, vitamins, blood, skin, cotton, wool, paper, oil, plastics--are organic compounds.
2 Larger and more complex organic compounds, called polymers, consist of a number of basic structural units linked together by chemical bonds. Important organic polymers include carbohydrates, proteins, and nucleic acids. Carbohydrates, such as the complex starches in rice and potato plants, are composed of a number of simple sugar molecules. Proteins are produced in plant and animal cells by the linking of different numbers and sequences of about twenty different structural units known as amino acids. Most animals, including humans, can manufacture about ten of these amino acids in their cells, but the other ten, called essential amino acids, must be obtained from food in order to prevent protein deficiency. Nucleic acids are composed of hundreds to thousands of four different units called nucleotides linked together in different numbers and sequences. DNA and RNA in plant and animal cells are nucleic acids.

单选题 Which of the following statements applies to all organic compounds?

单选题 Carbohydrates, proteins, and nucleic acids are types of

单选题 Why is it important for humans to obtain some amino acids from food?