生物分子概述 Overview | Biomolecules Overview

AQA生物分子全解析 Biological Molecules Guide

生物分子概述 Overview

聚合反应（Condensation）脱水连接单体成聚合物，水解反应（Hydrolysis）加水分解聚合物回单体。这两个反应是所有生物大分子合成与分解的基础。Condensation reactions join monomers by removing water; hydrolysis breaks polymers by adding water. These two reactions underpin all macromolecule synthesis and breakdown.

碳水化合物 Carbohydrates

单糖如葡萄糖（C6H12O6）是基本单位。关键区分：α-葡萄糖的-OH在碳环下方，形成螺旋淀粉；β-葡萄糖的-OH在上方，形成笔直纤维素链。Glucose is the key monosaccharide. α-glucose has -OH below the ring plane, forming helical starch; β-glucose has -OH above, forming straight cellulose chains.

二糖由两个单糖通过糖苷键连接：麦芽糖=葡萄糖+葡萄糖，蔗糖=葡萄糖+果糖，乳糖=葡萄糖+半乳糖。本尼迪克特试剂加热后由蓝变砖红色即含还原糖。Disaccharides: Maltose (glucose+glucose), Sucrose (glucose+fructose), Lactose (glucose+galactose). Benedict's reagent turns from blue to brick-red when reducing sugars are present.

多糖中，淀粉（直链淀粉α-1,4螺旋+支链淀粉α-1,6分支）是植物储能分子；糖原分支更多，是动物储能分子；纤维素β-1,4糖苷键形成笔直链，氢键交联为微纤维，赋予细胞壁抗拉强度。结构决定功能。Starch (amylose α-1,4 helices + amylopectin α-1,6 branches) stores energy in plants; glycogen has more branches for animal storage; cellulose β-1,4 bonds form straight chains with hydrogen-bonded microfibrils for cell wall tensile strength. Structure dictates function.

脂质 Lipids

甘油三酯：甘油+3脂肪酸通过酯键连接。饱和脂肪酸无双键、排列紧密、常温固态；不饱和脂肪酸含双键、链弯曲、常温液态。主要功能是高密度能量储存、保温和保护。乳浊液测试（乙醇+水，出现白色乳浊）鉴定脂质。Triglycerides: glycerol + 3 fatty acids via ester bonds. Saturated fatty acids lack double bonds (solid at room temp); unsaturated have double bonds causing kinks (liquid). Functions: dense energy storage, insulation, protection. Emulsion test (ethanol + water produces white emulsion) identifies lipids.

磷脂：一个脂肪酸被磷酸基团取代，形成亲水头部+疏水尾部，在水中自发形成双分子层——细胞膜的基础结构。Phospholipids: phosphate replaces one fatty acid, creating hydrophilic head + hydrophobic tails that spontaneously form bilayers in water — the basis of cell membranes.

蛋白质与酶 Proteins and Enzymes

氨基酸共享结构：中心碳连接氨基(-NH2)、羧基(-COOH)、H原子和可变R基团。肽键通过缩合反应形成。四级结构：一级=氨基酸序列，二级=α-螺旋/β-折叠（氢键），三级=三维构象（氢键、离子键、疏水作用、二硫键），四级=多亚基组合（如血红蛋白）。变性破坏三维结构，永久失活。Amino acids share: central C bonded to -NH2, -COOH, H, and variable R group. Peptide bonds form via condensation. Four structure levels: primary (sequence), secondary (α-helix/β-sheet via H-bonds), tertiary (3D conformation), quaternary (multi-subunit). Denaturation disrupts 3D structure irreversibly.

酶遵循诱导契合模型：底物使活性位点构象变化以降低活化能。影响因素：温度、pH、底物浓度、酶浓度。竞争性抑制剂占活性位点，非竞争性抑制剂结合别构位点改变活性位点形状。必考实验：过氧化氢酶对H2O2分解速率的影响（AQA RP1）。Enzymes follow the Induced Fit Model: substrate binding changes active site conformation, lowering activation energy. Factors: temperature, pH, substrate/enzyme concentration. Competitive inhibitors occupy active site; non-competitive inhibitors bind allosteric site. Required Practical 1: catalase effect on H2O2 decomposition.

核酸 Nucleic Acids

核苷酸=戊糖+磷酸+含氮碱基。DNA用脱氧核糖和胸腺嘧啶(T)；RNA用核糖和尿嘧啶(U)。共同碱基：A、G、C。磷酸二酯键连接核苷酸形成5'→3'多核苷酸链。DNA双螺旋：反平行链通过A=T和C≡G互补配对。半保留复制：解旋酶解链，聚合酶合成新链。ATP（腺苷三磷酸）是能量货币——水解为ADP+Pi释放能量，由ATP合酶再磷酸化。Nucleotides = pentose sugar + phosphate + nitrogenous base. DNA uses deoxyribose and thymine; RNA uses ribose and uracil. Shared bases: A, G, C. Phosphodiester bonds link nucleotides 5'→3'. DNA double helix: antiparallel strands with A=T and C≡G base pairing. Semi-conservative replication: helicase unwinds, polymerase synthesizes. ATP is the energy currency — hydrolyzed to ADP+Pi releasing energy, re-phosphorylated by ATP synthase.

水与无机离子 Water and Inorganic Ions

水是极性分子——氧δ-、氢δ+，形成氢键。关键性质：高比热容（缓冲温度）、高汽化热（出汗冷却）、优良溶剂、内聚力（木质部水柱）、冰密度小于水（水面保温层）。Water is polar (O δ-, H δ+) enabling hydrogen bonds. Key properties: high specific heat capacity, high latent heat of vaporization, excellent solvent, cohesion (xylem columns), ice floats (insulating layer).

无机离子功能：Fe2+是血红蛋白血红素核心（氧结合），PO43-是ATP/核酸结构组分，Na+/K+参与神经冲动传导。Inorganic ions: Fe2+ is core of haemoglobin's haem group (O2 binding), PO43- in ATP/nucleic acids, Na+/K+ for nerve impulses.

实验技能与考试技巧 Practical Skills and Exam Tips

核心实验：本尼迪克特测试（还原糖）、碘液测试（淀粉）、乳浊液测试（脂质）、双缩脲试剂（蛋白质）、过氧化氢酶实验（RP1）。每项必须掌握原理、变量控制和反应速率计算。Core practicals: Benedict's (reducing sugars), Iodine (starch), Emulsion (lipids), Biuret (proteins), catalase (RP1). Master principles, variable control, and rate calculations for each.

常见陷阱：混淆α/β-葡萄糖结构功能后果、忘记纤维素β-1,4键形成直链非螺旋、用锁钥模型替代诱导契合模型、混淆两种抑制剂对Vmax/Km的影响、描述结构不联系功能（AQA要求两者同时出现且逻辑关联）。Common pitfalls: confusing α/β-glucose structural-functional outcomes, forgetting cellulose β-1,4 creates straight chains not helices, using Lock and Key instead of Induced Fit, mixing up inhibitor effects on Vmax/Km, describing structure without linking to function.

复习策略 Review Strategy

采用主动回忆法：合上书本默写分子结构、键型和功能。思维导图串联单体→聚合物→功能主线。历年真题是最佳资源，尤其3-5分解释题需反复练习至限时完成。Use Active Recall: close the book and draw structures, bond types, and functions from memory. Mind maps connect monomers→polymers→function. Past papers are the best resource — practice 3-5 mark explanation questions until you can complete them within time limits.

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