📚 Carbohydrates | 碳水化合物
Carbohydrates are biological molecules composed of carbon, hydrogen and oxygen, usually in the ratio Cₙ(H₂O)ₘ. They serve as energy sources, energy stores and structural materials. This article covers monosaccharides, disaccharides, polysaccharides and the main biochemical tests required for Cambridge International A-Level Biology.
碳水化合物是由碳、氢、氧组成的生物分子，通常比例为 Cₙ(H₂O)ₘ。它们是能量来源、能量储存物质和结构材料。本文涵盖剑桥国际 A-Level 生物学要求的单糖、双糖、多糖和主要生化检测方法。
1. Definition and General Formula | 定义与通式
A carbohydrate is an organic compound with the general formula Cₙ(H₂O)ₘ, where n and m may be equal or different. The name ‘carbohydrate’ comes from ‘hydrated carbon’, but the molecule is not literally carbon atoms attached to water molecules.
碳水化合物的通式为 Cₙ(H₂O)ₘ，其中 n 和 m 可以相同或不同。’碳水化合物’这一名称源于’含水的碳’，但分子并非真的是碳原子与水分子相连。
Carbohydrates are classified into three main groups: monosaccharides, disaccharides and polysaccharides. This classification depends on the number of sugar units and whether hydrolysis breaks them into simpler sugars.
碳水化合物主要分为三类：单糖、双糖和多糖。这种分类取决于糖单位的数量以及水解能否将其分解为更简单的糖。
General formula: Cₙ(H₂O)ₘ
2. Monosaccharides: Structure and Examples | 单糖的结构与实例
Monosaccharides are the simplest sugars; they cannot be hydrolysed into smaller carbohydrates. They are classified by carbon number: trioses have 3 carbon atoms, pentoses have 5, and hexoses have 6. Glucose, fructose and galactose are common hexoses with the formula C₆H₁₂O₆.
单糖是最简单的糖，不能水解为更小的碳水化合物。它们按碳原子数分类：丙糖有 3 个碳原子，戊糖有 5 个，己糖有 6 个。葡萄糖、果糖和半乳糖是常见的己糖，分子式为 C₆H₁₂O₆。
Glucose is a hexose sugar with an aldehyde group in its open-chain form; fructose has a ketone group. Ribose and deoxyribose are pentose sugars found in RNA and DNA respectively.
葡萄糖是一种己糖，其开链形式含有醛基；果糖含有酮基。核糖和脱氧核糖分别是 RNA 和 DNA 中的戊糖。
3. α- and β-Glucose Isomers | α-葡萄糖与β-葡萄糖异构体
In aqueous solution, glucose forms a ring structure. The hydroxyl group on carbon 1 can lie below the plane of the ring (α-glucose) or above the plane (β-glucose). These two forms are isomers called anomers.
在水溶液中，葡萄糖形成环状结构。1号碳上的羟基可以位于环平面下方（α-葡萄糖）或上方（β-葡萄糖）。这两种形式是被称为异头物的异构体。
The position of the -OH on carbon 1 has major consequences. Starch and glycogen are built from α-glucose, whereas cellulose is built from β-glucose. This single difference changes the three-dimensional shape and biological properties of the polymer.
1号碳上 -OH 的位置具有重要影响。淀粉和糖原由 α-葡萄糖构成，而纤维素由 β-葡萄糖构成。这一微小差异改变了聚合物的三维形状和生物学性质。
4. Formation of Disaccharides | 双糖的形成
Monosaccharides join by a condensation reaction. A hydroxyl group from one sugar reacts with a hydrogen from another, releasing one water molecule and forming a covalent glycosidic bond.
单糖通过缩合反应连接。一个糖的羟基与另一个糖的氢反应，释放一分子水并形成共价糖苷键。
C₆H₁₂O₆ + C₆H₁₂O₆ → C₁₂H₂₂O₁₁ + H₂O
The bond between carbon 1 of one sugar and carbon 4 of another is a 1,4-glycosidic bond. A 1,6-glycosidic bond can form branches in polysaccharides. Hydrolysis is the reverse process, breaking the bond by adding water.
一个糖的1号碳与另一个糖的4号碳之间的键称为1,4-糖苷键。1,6-糖苷键可在多糖中形成分支。水解是逆过程，通过加水断裂该键。
5. Common Disaccharides | 常见双糖
Maltose is formed from two α-glucose molecules linked by an α-1,4-glycosidic bond; it is a reducing sugar produced during starch digestion. Lactose is galactose joined to glucose by a β-1,4-glycosidic bond.
麦芽糖由两分子 α-葡萄糖通过 α-1,4-糖苷键连接而成；它是淀粉消化过程中产生的一种还原糖。乳糖是半乳糖与葡萄糖通过 β-1,4-糖苷键连接而成。
Sucrose is formed from glucose and fructose. Its glycosidic bond uses the anomeric carbons of both monomers, so sucrose is a non-reducing sugar because neither unit can open to expose a free aldehyde or ketone group.
蔗糖由葡萄糖和果糖形成。其糖苷键同时涉及两个单体的异头碳，因此蔗糖是一种非还原糖，因为没有一个单位能开环暴露游离的醛基或酮基。
6. Reducing Sugars and Benedict’s Test | 还原糖与本尼迪克特试验
A reducing sugar can donate electrons to another substance. All monosaccharides and some disaccharides such as maltose and lactose are reducing sugars because they have a free aldehyde or ketone group in the open-chain form.
还原糖能向其他物质提供电子。所有单糖以及麦芽糖、乳糖等部分双糖都是还原糖，因为它们在开链形式中具有游离的醛基或酮基。
In Benedict’s test, a reducing sugar is heated with Benedict’s reagent, an alkaline solution of copper(II) sulfate. If a reducing sugar is present, blue Cu²⁺ ions are reduced to Cu⁺ and form a brick-red precipitate of copper(I) oxide, Cu₂O. The colour sequence is blue → green → yellow → orange → brick red as concentration increases.
在本尼迪克特试验中，还原糖与本尼迪克特试剂（硫酸铜(II)的碱性溶液）一起加热。如果存在还原糖，蓝色的 Cu²⁺ 离子被还原为 Cu⁺，形成砖红色的氧化亚铜沉淀 Cu₂O。随着浓度升高，颜色变化为蓝色 → 绿色 → 黄色 → 橙色 → 砖红色。
Sucrose gives a negative Benedict’s test until it is hydrolysed, for example by heating with dilute hydrochloric acid, followed by neutralisation with sodium hydrogencarbonate. A positive result after hydrolysis confirms a non-reducing sugar.
蔗糖在本尼迪克特试验中呈阴性，除非先将其水解，例如用稀盐酸加热，然后用碳酸氢钠中和。水解后出现阳性结果即可确认非还原糖。
7. Polysaccharides: Starch | 多糖：淀粉
Starch is a storage polysaccharide in plants. It is made of α-glucose and consists of two polymers: amylose and amylopectin.
淀粉是植物中的储存多糖。它由 α-葡萄糖组成，包含两种聚合物：直链淀粉和支链淀粉。
Amylose is a linear chain with α-1,4-glycosidic bonds; it coils into a helix. Amylopectin is branched, containing α-1,4 bonds along chains and α-1,6 bonds at branch points. The helical and branched structure makes starch compact and insoluble, so it does not affect the water potential of cells.
直链淀粉是通过 α-1,4-糖苷键连接的直链，卷曲成螺旋。支链淀粉具有分支，主链为 α-1,4 键，分支点为 α-1,6 键。螺旋和分支结构使淀粉致密且不溶，因此不会影响细胞的水势。
8. Polysaccharides: Glycogen | 多糖：糖原
Glycogen is the main carbohydrate storage molecule in animals and fungi. It is similar to amylopectin but has more frequent α-1,6 branches, giving it a highly branched, more compact structure.
糖原是动物和真菌中主要的碳水化合物储存分子。它与支链淀粉相似，但具有更多 α-1,6 分支，因此结构高度分支且更致密。
The many free ends allow enzymes to add or remove glucose rapidly, which is important for maintaining blood glucose and supplying energy during exercise. Glycogen is stored mainly in liver and muscle cells as insoluble granules.
大量游离末端使酶能够快速添加或移除葡萄糖，这对于维持血糖水平和运动期间供能非常重要。糖原主要以不溶性颗粒形式储存在肝脏和肌肉细胞中。
9. Polysaccharides: Cellulose | 多糖：纤维素
Cellulose is the main structural component of plant cell walls. It is a linear polymer of β-glucose molecules joined by β-1,4-glycosidic bonds. Each β-glucose is rotated 180° relative to its neighbour.
纤维素是植物细胞壁的主要结构成分。它是由 β-葡萄糖通过 β-1,4-糖苷键连接而成的直链聚合物。每个 β-葡萄糖相对于相邻分子旋转了 180°。
Hydrogen bonds form between hydroxyl groups of parallel cellulose chains, producing strong bundles called microfibrils and macrofibrils. This arrangement gives cellulose very high tensile strength, preventing plant cells from bursting when turgid.
平行纤维素链之间的羟基形成氢键，产生称为微纤维和大纤维的强束。这种排列赋予纤维素很高的抗张强度，防止植物细胞在膨压状态下破裂。
Mammals lack cellulase, so they cannot digest cellulose; some herbivores rely on symbiotic microorganisms. Cellulose remains important in the human diet as dietary fibre.
哺乳动物缺乏纤维素酶，因此不能消化纤维素；一些食草动物依赖共生微生物。纤维素作为膳食纤维在人类饮食中仍然很重要。
10. Polysaccharides: Chitin | 多糖：几丁质
Chitin is a structural polysaccharide found in the exoskeleton of arthropods and in fungal cell walls.
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