碳水化合物Carbohydrate

单糖按羰基位置分为醛糖和酮糖，按碳数分为丙糖、戊糖、己糖等，如葡萄糖（己醛糖）、果糖（己酮糖）、核糖（戊醛糖）。五碳以上的单糖在水溶液中主要以五元（呋喃糖）或六元（吡喃糖）环状半缩醛形式存在，并产生 α、β 端基异构体。通式例外如脱氧核糖 C₅H₁₀O₄ 不符合 Cₘ(H₂O)ₙ，而甲醛 CH₂O、乙酸 C₂H₄O₂ 符合通式却不是糖。

Monosaccharides are aldoses or ketoses according to the carbonyl position and trioses, pentoses, hexoses and so on by carbon count: glucose is an aldohexose, fructose a ketohexose and ribose an aldopentose. Those with five or more carbons exist in water mostly as five-membered (furanose) or six-membered (pyranose) cyclic hemiacetals with α and β anomers. Exceptions to the formula abound: deoxyribose, C₅H₁₀O₄, does not fit Cₘ(H₂O)ₙ, while formaldehyde (CH₂O) and acetic acid (C₂H₄O₂) fit it but are not sugars.

19 世纪初人们从元素分析发现糖类中氢氧比为 2:1，以为是“碳的水合物”。1884–1891 年埃米尔·费歇尔用苯肼成脎反应鉴别糖类，并通过逻辑推理确定了葡萄糖等 16 种己醛糖的立体构型，获 1902 年诺贝尔化学奖。1920 年代哈沃斯确立了糖的环状结构并提出哈沃斯投影式。

In the early nineteenth century elemental analysis showed sugars contain hydrogen and oxygen in a 2:1 ratio, so they were thought to be hydrates of carbon. Between 1884 and 1891 Emil Fischer identified sugars by forming osazones with phenylhydrazine and reasoned out the configurations of glucose and the other aldohexoses, receiving the 1902 Nobel Prize in Chemistry. In the 1920s Haworth established the ring structures of sugars and introduced the Haworth projection.

糖类是生物体最主要的能源（每克约 17 kJ），也是结构物质（纤维素、甲壳素）、遗传物质的组成成分（核糖、脱氧核糖），并以糖蛋白、糖脂形式参与细胞识别、免疫和血型决定。中国居民膳食指南建议碳水化合物提供每日总能量的 50–65%。

Carbohydrates are the main energy source of living things (about 17 kJ per gram), structural materials (cellulose, chitin), components of genetic material (ribose, deoxyribose), and, as glycoproteins and glycolipids, take part in cell recognition, immunity and blood-group determination. Chinese dietary guidelines recommend that carbohydrates supply 50–65% of daily energy.