📚 Carbohydrates | 碳水化合物
Carbohydrates are organic molecules made up of carbon (C), hydrogen (H), and oxygen (O) atoms, typically with the empirical formula (CH₂O)ₙ where n is greater than or equal to 3. The term “carbohydrate” reflects the proportion of hydrogen to oxygen as in water (2:1), although some carbohydrates may deviate from this exact ratio. They are one of the four major classes of biomolecules and play critical roles in energy supply, energy storage, and structural support in living organisms. Examples range from simple sugars like glucose to complex polymers such as starch and cellulose.
碳水化合物是由碳（C）、氢（H）和氧（O）原子组成的有机分子，通常实验式为 (CH₂O)ₙ，其中 n ≥ 3。”碳水化合物” 这一名称反映出氢与氧的比例与水相同（2:1），尽管某些碳水化合物的实际比例可能略有偏差。碳水化合物是四大类生物分子之一，在生物体中扮演能量供应、能量储存和结构支撑的关键角色。从简单的糖类如葡萄糖，到复杂的聚合物如淀粉和纤维素，都属于碳水化合物。
1. Introduction to Carbohydrates | 碳水化合物简介
Carbohydrates are often called “saccharides” (from the Greek word for sugar). They are commonly classified into three main groups based on the number of sugar units: monosaccharides (single unit), disaccharides (two units), and polysaccharides (many units). Their properties depend strongly on molecular size and the types of glycosidic bonds linking the monomers.
碳水化合物常被称为 “糖类”（来自希腊语 “糖”）。根据糖单元的数量，它们通常分为三大类：单糖（单个单元）、双糖（两个单元）和多糖（许多单元）。它们的性质强烈依赖于分子大小以及连接单体的糖苷键类型。
The hydrogen-to-oxygen ratio is often 2:1, as in water, giving rise to the general formula Cₓ(H₂O)ᵧ. However, some carbohydrate derivatives contain other elements like nitrogen or sulfur, expanding their biological functions beyond simple fuels.
氢氧比例通常为 2:1，与水相同，由此产生通式 Cₓ(H₂O)ᵧ。然而，某些碳水化合物衍生物含有氮或硫等元素，使其生物学功能扩展至简单的燃料之外。
2. Monosaccharides: The Simplest Sugars | 单糖：最简单的糖
Monosaccharides are the simplest carbohydrate units and cannot be hydrolysed into smaller sugars. They are sweet-tasting, soluble in water, and can exist as either linear chains or ring structures. The most common monosaccharides have 3 to 7 carbon atoms, with hexoses (6 carbons) being particularly important. Examples include glucose, fructose, and galactose, all sharing the molecular formula C₆H₁₂O₆ but differing in structural arrangement (isomers).
单糖是最简单的碳水化合物单位，不能水解为更小的糖。它们具有甜味，可溶于水，能够以直链或环状结构存在。最常见的单糖含有3至7个碳原子，其中六碳糖（己糖）尤为重要。例子包括葡萄糖、果糖和半乳糖，它们的分子式均为 C₆H₁₂O₆，但在结构排列上不同（异构体）。
Because of their small size, monosaccharides can pass through cell membranes and are readily used in cellular respiration. Glucose is the primary respiratory substrate in most organisms. Trioses (3C) such as glyceraldehyde are intermediates in metabolic pathways, and pentoses (5C) like ribose and deoxyribose are vital components of nucleotides.
由于分子小，单糖可以透过细胞膜，并迅速用于细胞呼吸。葡萄糖是大多数生物体最主要的呼吸底物。三碳糖（如甘油醛）是代谢途径的中间产物，而五碳糖如核糖和脱氧核糖则是核苷酸的重要组成部分。
3. Structure of Glucose and its Isomers | 葡萄糖结构及其异构体
Glucose exists in two isomeric ring forms called α-glucose and β-glucose. In the ring structure, carbon atom 1 (the anomeric carbon) can have the hydroxyl group (-OH) positioned either below the plane of the ring (α-glucose) or above the plane (β-glucose). Both forms can interconvert in solution, but the orientation of this single -OH group has profound effects on polymer structure and function.
葡萄糖有两种异构环状形式，称为 α-葡萄糖和 β-葡萄糖。在环状结构中，1号碳原子（异头碳）上的羟基（-OH）可以位于环平面下方（α-葡萄糖）或上方（β-葡萄糖）。两种形式在溶液中可以相互转化，但这个 -OH 基团的方向对聚合物的结构和功能有着深远影响。
In α-glucose, the -OH on C1 is on the opposite side of the ring from the CH₂OH group attached to C5. In β-glucose, the C1 -OH and the C5 CH₂OH are on the same side. This seemingly small difference determines whether the resulting polysaccharide will coil into a helix (starch, glycogen) or form straight, rigid chains (cellulose).
在 α-葡萄糖中，C1 上的 -OH 与连接在 C5 上的 CH₂OH 基团位于环的异侧。在 β-葡萄糖中，C1 的 -OH 与 C5 的 CH₂OH 位于同侧。这个看似微小的差异决定了最终的多糖是会盘绕成螺旋（淀粉、糖原），还是形成笔直刚性的链（纤维素）。
4. Disaccharides: Formation and Examples | 双糖的形成与例子
Disaccharides form when two monosaccharides join through a condensation reaction, removing a water molecule and creating a covalent glycosidic bond. The type of disaccharide depends on the specific monomers and the linkage formed. Maltose is formed from two α-glucose molecules with an α-1,4-glycosidic bond. Sucrose is made of α-glucose and β-fructose, while lactose consists of β-galactose and α-glucose.
双糖由两个单糖通过缩合反应连接形成，脱去一分子水，并生成一个共价糖苷键。双糖的类型取决于具体的单体及形成的键型。麦芽糖由两个 α-葡萄糖分子经 α-1,4-糖苷键形成；蔗糖由 α-葡萄糖和 β-果糖组成；乳糖则由 β-半乳糖和 α-葡萄糖构成。
Maltose and lactose are reducing sugars because they have a free anomeric carbon that can open to expose an aldehyde group, allowing them to reduce Cu²⁺ in Benedict’s test. Sucrose, however, is a non-reducing sugar because both anomeric carbons are locked in the glycosidic linkage, preventing ring opening.
麦芽糖和乳糖是还原糖，因为它们具有游离的异头碳，能够开环暴露出醛基，从而在本尼迪克特试验中还原 Cu²⁺。然而，蔗糖是非还原糖，因为两个异头碳都参与了糖苷键，使得环无法打开。
5. Polysaccharides: Large Biomolecules | 多糖：大分子生物分子
Polysaccharides are macromolecules formed by polymerising many monosaccharide units via glycosidic bonds. Their general formula is (C₆H₁₀O₅)ₙ. Unlike mono- and disaccharides, they are not sweet and are mostly insoluble in water. In living organisms, they serve primarily as energy stores (starch and glycogen) or structural materials (cellulose).
多糖是由许多单糖单元通过糖苷键聚合而成的大分子，通式为 (C₆H₁₀O₅)ₙ。与单糖和双糖不同，多糖没有甜味，且大多不溶于水。在生物体中，它们主要充当能量储备（淀粉和糖原）或结构材料（纤维素）。
The properties of a polysaccharide depend on the monosaccharide isomer used (α or β) and the position of the glycosidic linkages. Linear chains, branched structures, and hydrogen bonding between chains all contribute to their diverse functions.
多糖的性质取决于所用的单糖异构体（α 或 β）以及糖苷键的位置。直链、支链结构以及链间的氢键共同决定了它们多样的功能。
6. Starch: Energy Storage in Plants | 淀粉：植物的能量储存
Starch is a storage polysaccharide found in plant cells, often in the form of starch grains within chloroplasts or amyloplasts. It is a polymer of α-glucose and consists of two components: amylose and amylopectin. Amylose is a linear, unbranched chain of α-glucose units linked by α-1,4-glycosidic bonds, which coils into a helical shape. Amylopectin is branched, with α-1,4-glycosidic chains and occasional α-1,6-glycosidic bonds creating branch points every 20–30 glucose residues.
淀粉是植物细胞中的储存多糖，常以淀粉粒的形式存在于叶绿体或淀粉体内。它是一种 α-葡萄糖的聚合物，由两种组分构成：直链淀粉和支链淀粉。直链淀粉是由 α-1,4-糖苷键连接的不分支直链，盘绕成螺旋状。支链淀粉具有分支，除了 α-1,4-糖苷键之外，每间隔 20–30 个葡萄糖残基就有一个 α-1,6-糖苷键形成分支点。
This combination of linear and branched structures allows starch to be compact and osmotically inactive, ideal for storage without disturbing cell water potential. When iodine–potassium iodide solution is added, amylose’s helical structure traps iodine molecules, producing an intense blue-black colour, which is the basis of the starch test.
这种直链与分支兼具的结构使淀粉紧致且渗透惰性，非常适合储存而不会干扰细胞水势。加入碘-碘化钾溶液时，直链淀粉的螺旋结构容纳碘分子，产生强烈的蓝黑色，这是淀粉检测的基础。
7. Glycogen: Energy Storage in Animals | 糖原：动物的能量储存
Glycogen is the main storage polysaccharide in animal cells, especially abundant in liver and muscle tissues. Like starch, it is a polymer of α-glucose, but glycogen is even more extensively branched. Branches occur every 8–12 glucose units, with both α-1,4- and α-1,6-glycosidic bonds, making it a highly compact molecule that can be rapidly mobilised for energy release.
糖原是动物细胞中的主要储存多糖，尤其在肝脏和肌肉组织中含量丰富。与淀粉相似，它也是 α-葡萄糖的聚合物，但糖原的分支程度更高。每隔 8–12 个葡萄糖单元就出现一个分支，含有 α-1,4- 和 α
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