Tham khảo tài liệu 'delta's key to the next generation toefl test part 4', ngoại ngữ, toefl - ielts - toeic phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | . Facts and Details Exercise Read the passages and choose the best answer to each question. Questions 1-3 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. 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 arc nucleic acids. 1. Which of the following statements applies to all organic compounds They are composed of carbon and one or more other elements. jT They contain atoms of the seven most abundant elements. T They have stronger chemical bonds than inorganic compounds do. 7 They are produced by linking several simple sugar molecules. 2. Carbohydrates proteins and nucleic acids are types of 3. Why is it important for humans to obtain some amino acids from food T Without
