📚 Biological Molecules: Structure and Function | 生物分子：结构与功能
In A-Level Biology, biological molecules are the carbon-based compounds that make up every living organism. They include water, carbohydrates, lipids, proteins and nucleic acids, and they are built largely from a small number of monomer units joined by condensation reactions. Understanding their structure explains how they store energy, carry information, catalyse reactions and form cellular structures.
在 A-Level 生物学中，生物分子是构成所有生命体的含碳化合物。它们包括水、碳水化合物、脂质、蛋白质和核酸，大部分由少量单体通过缩合反应连接而成。理解它们的结构可以解释它们如何储存能量、传递信息、催化反应以及形成细胞结构。
1. Water and Hydrogen Bonding | 水与氢键
Water (H₂O) is a polar molecule because oxygen is more electronegative than hydrogen, creating a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms. This polarity allows water molecules to form hydrogen bonds with each other, which gives water its unusual properties.
水 (H₂O) 是极性分子，因为氧比氢更具电负性，使氧原子带部分负电荷、氢原子带部分正电荷。这种极性使水分子之间能够形成氢键，从而赋予水不同寻常的性质。
Water is an excellent solvent for ions and polar molecules such as glucose and amino acids. Its high specific heat capacity buffers organisms against rapid temperature changes, and its high latent heat of vaporisation makes sweating an effective cooling mechanism.
水是离子以及葡萄糖、氨基酸等极性分子的优良溶剂。其高比热容可以缓冲生物体免受温度快速变化的影响；其高蒸发潜热使出汗成为有效的降温机制。
Cohesion and adhesion are also biologically important. Cohesion allows water to be pulled up xylem vessels in plants, while adhesion helps water climb cell walls. Ice is less dense than liquid water, so it floats and insulates aquatic life.
内聚力和附着力也具有重要生物学意义。内聚力使水能够在植物木质部导管中被向上牵引，而附着力帮助水沿细胞壁上升。冰的密度小于液态水，因此漂浮并隔离保护水生生物。
2. Monosaccharides | 单糖
tose sugars such as ribose (n = 5) and hexose sugars such as glucose and fructose (n = 6).
单糖是最简单的碳水化合物，通式为 (CH₂O)ₙ。常见的例子包括三碳糖如甘油醛 (n = 3)、五碳糖如核糖 (n = 5) 以及六碳糖如葡萄糖和果糖 (n = 6)。
Glucose has the molecular formula C₆H₁₂O₆ and exists as two structural isomers: α-glucose and β-glucose. The only difference is the position of the hydroxyl (−OH) group on carbon 1; in α-glucose it lies below the plane of the ring, while in β-glucose it lies above. This small change has major biological consequences for polymer structure.
葡萄糖的分子式为 C₆H₁₂O₆，以两种结构异构体存在：α-葡萄糖和 β-葡萄糖。唯一区别是 1 号碳上羟基 (−OH) 的位置；在 α-葡萄糖中它位于环平面下方，而在 β-葡萄糖中位于上方。这一微小变化对聚合物结构产生重大生物学影响。
Monosaccharides are reducing sugars and can donate electrons to Benedict's reagent. They serve as an immediate energy source in respiration and as monomers for building larger carbohydrates.
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