Topic Overview 主题概览
IB Biology Topic 2: Molecular Biology 是IB生物课程最核心的章节之一，涵盖从分子代谢到光合作用的关键概念，占SL考试约20%的权重。本文系统梳理考点、高频易错和答题技巧。
IB Biology Topic 2: Molecular Biology is one of the most fundamental chapters, covering concepts from molecular metabolism to photosynthesis, accounting for approximately 20% of the SL exam weighting.
2.1 Molecules to Metabolism 分子与代谢
碳是有机分子的骨架，四大类生物大分子（碳水化合物、脂质、蛋白质、核酸）的结构与功能是基础。Anabolism builds complex molecules from simpler ones (protein synthesis), while catabolism breaks down complex molecules to release energy (cellular respiration). 考试常考尿素结构识别及合成与分解代谢的区别。Urea is an organic compound synthesized artificially by Wohler, disproving vitalism.
2.2 Water 水的性质
水的极性、氢键及其对生物体的重要性是必考内容。Water's cohesive, adhesive, and thermal properties arise from hydrogen bonding. 考试常见解释水作为优良溶剂（血液运输、细胞代谢）及其高比热容维持体温的机制。Hydrophilic substances dissolve due to polarity; hydrophobic substances (lipids) are non-polar and insoluble.
2.3 Carbohydrates and Lipids 碳水化合物与脂质
Condensation reactions link monosaccharides via glycosidic bonds, forming maltose, sucrose, and lactose. 多糖重点掌握淀粉（amylose vs amylopectin）、糖原和纤维素的结构差异及功能适应。Triglycerides are formed by esterification between glycerol and three fatty acids. 区分饱和与不饱和脂肪酸（cis vs trans），理解cis-unsaturated fatty acids保持液态而saturated fatty acids形成固态的原因。
2.4 Proteins 蛋白质
Amino acids are linked by peptide bonds formed through condensation. 必须熟记蛋白质四级结构：Primary (sequence), Secondary (alpha-helix and beta-pleated sheet via hydrogen bonds), Tertiary (3D folding by hydrophobic interactions, disulfide bridges, ionic bonds), and Quaternary (multiple polypeptides like hemoglobin). Proteome定义：一个细胞或生物体在特定条件下表达的全部蛋白质。Denaturation permanently alters protein conformation causing loss of function.
2.5 Enzymes 酶
Enzymes lower activation energy by binding substrates at the active site. IB采用Induced-Fit模型：active site conformation changes slightly when substrate binds, straining bonds and facilitating catalysis. Temperature increases collision frequency up to the optimum, beyond which denaturation occurs. pH affects active site charge. 竞争性抑制剂结合活性位点可被高浓度底物克服；非竞争性抑制剂结合别构部位，Vmax降低但Km不变。IB实验题常考设计实验探究温度/pH/底物浓度对酶活性的影响。
2.6 Structure of DNA and RNA
Nucleotides consist of pentose sugar, phosphate group, and nitrogenous base. DNA含脱氧核糖和A/T/C/G碱基，RNA含核糖和A/U/C/G。双螺旋由反平行链通过A-T间两个氢键、C-G间三个氢键配对。Crick和Watson基于Franklin的X射线衍射数据提出双螺旋模型。DNA is a stable double-stranded hereditary molecule; RNA is single-stranded and functions in protein synthesis (mRNA, tRNA, rRNA).
2.7 DNA Replication, Transcription and Translation
DNA复制（半保守性）：Helicase unwinds the double helix; DNA polymerase III adds nucleotides in 5' to 3' direction; leading strand is continuous, lagging strand forms Okazaki fragments joined by DNA ligase. Meselson和Stahl实验使用N-15同位素证明了半保守复制。转录：RNA polymerase binds to the promoter; antisense strand serves as template; mRNA synthesized in 5' to 3' direction. 真核细胞中introns被剪接去除，exons拼接形成成熟mRNA。翻译：mRNA codons are read by ribosomes; tRNA anticodons carry specific amino acids. 掌握从DNA到氨基酸序列的转换——使用遗传密码表。
2.8 Cell Respiration 细胞呼吸
Glycolysis (cytoplasm): glucose to 2 pyruvate, net 2 ATP, 2 NADH. Anaerobic respiration: pyruvate to lactate (animals) or ethanol plus CO2 (yeast/plants), regenerating NAD+. 需氧呼吸：Link reaction中pyruvate转化为acetyl-CoA。Krebs cycle每轮产出2CO2, 3NADH, 1FADH2, 1ATP。Electron transport chain中NADH和FADH2供电子，质子梯度驱动ATP synthase完成氧化磷酸化，O2是最终电子受体。总和约32-36 ATP每葡萄糖分子。
2.9 Photosynthesis 光合作用
Chlorophyll absorbs red and blue light most effectively; green light is reflected. 光反应（类囊体膜）：Light energy splits water (photolysis) producing O2, H+, electrons; ATP synthesized by photophosphorylation; NADP+ reduced to NADPH. 卡尔文循环（基质）：CO2 fixed by Rubisco; G3P produced using ATP and NADPH; RuBP regenerated. 光合速率受光照强度、CO2浓度和温度制约。
Exam Strategy 备考策略
Paper 1选择题注意否定词。Paper 2数据题要求精确描述趋势并引用数值，拓展题常考DNA复制、蛋白质合成或细胞呼吸全流程配图。Command terms关键："Describe"陈述事实，"Explain"给出机制，"Compare"指出异同各至少两点，"Discuss"呈现多方观点并评估。混淆缩合与水解反应、认为酶被消耗、搞错DNA聚合酶方向、将卡尔文循环简单称作"暗反应"忽略其对光反应产物的依赖，这些都是典型失分点。
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