IB & OCR Biology: Microorganisms Key Points | IB/OCR 生物：微生物考点精讲

📚 IB & OCR Biology: Microorganisms Key Points | IB/OCR 生物：微生物考点精讲

Microorganisms are a fundamental topic across both IB and OCR Biology specifications, carrying significant weight in examinations. From the ultrastructure of bacteria and the intricacies of viral replication to their indispensable roles in ecosystems and biotechnology, a thorough understanding is essential. This revision guide distils the key concepts, common pitfalls, and examiner-favoured details to ensure you are fully prepared.

微生物是 IB 和 OCR 生物课程中的一大核心主题，在考试中占有重要分值。从细菌的超微结构和病毒复制的细节，到它们在生态系统与生物技术中的关键作用，透彻的理解必不可少。本考点精讲浓缩了核心概念、常见易错点以及考官偏爱的细节，确保你备考无忧。

1. Classification of Microorganisms | 微生物的分类

Microorganisms span several domains and kingdoms. Prokaryotes (bacteria and archaea) lack membrane-bound organelles, while eukaryotes include fungi, protoctists, and microscopic algae. Viruses are acellular and are not placed in any domain. Both IB and OCR expect you to distinguish these groups and place them correctly within the three-domain system (Archaea, Bacteria, Eukarya).

微生物涵盖多个域和界。原核生物（细菌和古菌）缺乏膜包围的细胞器，而真核微生物包括真菌、原生生物及显微藻类。病毒无细胞结构，不归入任何域。IB 和 OCR 都要求你区分这些类群，并正确将它们放入三域系统（古菌域、细菌域、真核域）。

2. Prokaryotic Cell Structure (Bacteria) | 原核细胞结构（细菌）

A typical bacterium features a cell wall, cell membrane, cytoplasm, 70S ribosomes, and a circular chromosome in a nucleoid region. Many possess plasmids, a capsule, flagella, and pili. IB HL students must be able to draw and label the ultrastructure including the cell wall, pili, flagella, plasmid, and naked DNA. Crucially, there is no nucleus, no membrane-bound organelles, and no histone proteins associated with DNA (in most bacteria). Compare with eukaryotic cells: 80S ribosomes, linear chromosomes, histones, and compartmentalisation.

一个典型的细菌具有细胞壁、细胞膜、细胞质、70S 核糖体以及位于拟核区的环状染色体。许多细菌含有质粒、荚膜、鞭毛和菌毛。IB 高阶学生必须能画图并标注超微结构，包括细胞壁、菌毛、鞭毛、质粒和裸露 DNA。关键点是无细胞核、无膜包围细胞器，DNA 通常不与组蛋白结合。与真核细胞对比：80S 核糖体、线状染色体、组蛋白及区室化。

3. Gram Staining and Bacterial Cell Walls | 革兰氏染色与细菌细胞壁

Gram-positive bacteria have a thick peptidoglycan layer that retains crystal violet stain, appearing purple. Gram-negative bacteria possess a thin peptidoglycan layer and an outer membrane containing lipopolysaccharide (LPS); they take up the counterstain and appear pink/red. The gram reaction is linked to antibiotic susceptibility: penicillin targets peptidoglycan synthesis and is more effective against gram-positive cells. This concept is frequently tested in OCR practical questions and IB applications.

革兰氏阳性菌具有厚厚的肽聚糖层，能留住结晶紫染料，呈紫色。革兰氏阴性菌的肽聚糖层较薄，且有一层含脂多糖（LPS）的外膜；它们吸附复染剂，呈粉红/红色。革兰氏反应与抗生素敏感性相关：青霉素以肽聚糖合成为靶点，对革兰氏阳性菌更有效。此概念常在 OCR 实验题和 IB 应用题中考查。

4. Viruses: Structure and Classification | 病毒：结构与分类

Viruses are obligate intracellular parasites consisting of a nucleic acid core (DNA or RNA, single- or double-stranded) surrounded by a protein capsid. Some have a lipid envelope derived from the host cell, studded with glycoproteins. They lack ribosomes, cytoplasm, and metabolic machinery. Examples: bacteriophage lambda (DNA, complex symmetry), influenza virus (RNA, enveloped), HIV (RNA, retrovirus). Classification is based on nucleic acid type, capsid symmetry, and presence of envelope.

病毒是专性细胞内寄生物，由核酸核心（DNA 或 RNA，可为单链或双链）和外围蛋白质衣壳构成。部分病毒具有源于宿主细胞的脂质包膜，上面插有糖蛋白。病毒缺乏核糖体、细胞质及代谢机构。举例：λ噬菌体（DNA，复合对称）、流感病毒（RNA，有包膜）、HIV（RNA，逆转录病毒）。分类依据核酸类型、衣壳对称性及有无包膜。

5. Viral Replication: Lytic and Lysogenic Cycles | 病毒复制：裂解与溶原周期

Bacteriophages can replicate via the lytic cycle: attachment, penetration, biosynthesis of viral components, assembly, and lysis of the host cell. In the lysogenic cycle, the viral genome integrates into the host chromosome as a prophage and replicates with the host, potentially switching to the lytic cycle under stress. For HIV, the replicative cycle includes reverse transcription of RNA to DNA, integration into the host genome (provirus), and eventual budding. IB expects detailed knowledge of HIV, while OCR covers both bacterial viruses and HIV.

噬菌体可通过裂解周期复制：吸附、穿入、生物合成病毒组分、装配以及宿主细胞裂解。在溶原周期中，病毒基因组整合到宿主染色体上成为原噬菌体，随宿主一同复制，并在压力下可能转为裂解周期。对于 HIV，复制周期包含 RNA 逆转录为 DNA、整合入宿主基因组（前病毒）及最终出芽。IB 要求深入了解 HIV，OCR 同时涵盖细菌病毒和 HIV。

6. Microbial Nutrition and Metabolism | 微生物的营养与代谢

Microorganisms show remarkable metabolic diversity. Based on carbon and energy sources, they can be chemoheterotrophs (most pathogens, many decomposers), chemoautotrophs (nitrifying bacteria that oxidise ammonia or nitrite), or photoautotrophs (cyanobacteria). Oxygen requirements vary: obligate aerobes must have O₂, obligate anaerobes are killed by O₂, and facultative anaerobes switch between respiration and fermentation. Fermentation regenerates NAD⁺, allowing glycolysis to continue, as seen in yeast producing ethanol and Lactobacillus producing lactic acid.

微生物展现出惊人的代谢多样性。根据碳源和能量来源，它们可以是化能异养型（多数病原菌和分解者）、化能自养型（氧化氨或亚硝酸盐的硝化细菌）或光合自养型（蓝细菌）。对氧的需求各异：专性需氧菌必须有氧气，专性厌氧菌会被氧气杀死，而兼性厌氧菌可在有氧呼吸与发酵之间切换。发酵用于再生 NAD⁺以维持糖酵解，例如酵母产生乙醇、乳酸菌产生乳酸。

7. Microorganisms in Ecosystems: Nitrogen Cycle | 生态系统中的微生物：氮循环

Microorganisms drive all key steps of the nitrogen cycle. Nitrogen fixation: atmospheric N₂ is reduced to NH₃/NH₄⁺ by free-living bacteria (e.g., Azotobacter) and mutualistic Rhizobium in root nodules. Nitrification: chemosynthetic bacteria oxidise ammonium to nitrite (Nitrosomonas) and then to nitrate (Nitrobacter). The overall transformation is often represented as:

NH₄⁺ → NO₂⁻ → NO₃⁻

Denitrification: Pseudomonas converts nitrate back to N₂ under anaerobic conditions. Assimilation and ammonification (decomposition) recycle nitrogen. Both IB and OCR require the linking of specific bacterial genera to each process.

微生物驱动着氮循环的所有关键步骤。固氮作用：大气中的 N₂ 被自由生活的细菌（如固氮菌）和根瘤内的共生根瘤菌还原为 NH₃/NH₄⁺。硝化作用：化能合成细菌将铵态氮氧化为亚硝酸盐（亚硝化单胞菌），再氧化为硝酸盐（硝化杆菌）。总转化常表示为：

NH₄⁺ → NO₂⁻ → NO₃⁻

反硝化作用：假单胞菌在厌氧条件下将硝酸盐还原为 N₂。同化作用和氨化作用（分解）使氮得以循环。IB 和 OCR 都要求将特定细菌属与各个过程对应起来。

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8. Pathogenic Microorganisms and Disease | 病原微生物与疾病

Pathogens include bacteria (Mycobacterium tuberculosis causing TB, Vibrio cholerae causing cholera), viruses (influenza, HIV), fungi (Candida albicans, Trichophyton causing athlete’s foot), and protoctists (Plasmodium causing malaria). They enter via various routes and cause disease by damaging host tissues or producing toxins. Understanding the difference between endotoxins (released from gram-negative cell walls) and exotoxins (secreted) is helpful. Importantly, antibiotics target bacteria, not viruses, a point regularly examined.

病原体包括细菌（结核分枝杆菌引起肺结核，霍乱弧菌引起霍乱）、病毒（流感病毒、HIV）、真菌（白色念珠菌，毛癣菌引起脚气）及原生生物（疟原虫引起疟疾）。它们通过多种途径侵入，通过损伤宿主组织或产生毒素致病。了解内毒素（由革兰氏阴性菌细胞壁释放）与外毒素（分泌型）的区别很有帮助。重要的是，抗生素只针对细菌而非病毒，这是常考要点。

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9. Antibiotics and Antimicrobial Resistance | 抗生素与耐药性

Antibiotics work by disrupting structures or processes unique to prokaryotes. For example, penicillins inhibit transpeptidase, blocking peptidoglycan cross-linking and causing cell lysis. Other targets include the 70S ribosome (tetracycline, chloramphenicol) and DNA gyrase. Resistance arises through mutation or horizontal gene transfer, via mechanisms such as enzymatic breakdown (β-lactamase), alteration of the target site (MRSA alters penicillin-binding protein), or active efflux. Overuse and misuse of antibiotics accelerate selection for resistant strains.

抗生素通过破坏原核生物独有的结构或过程起作用。例如，青霉素抑制转肽酶，阻断肽聚糖交联，引发细胞裂解。其他靶点包括 70S 核糖体（四环素、氯霉素）和 DNA 旋转酶。耐药性通过突变或水平基因转移产生，机制包括酶降解（β-内酰胺酶）、靶点改变（MRSA 改变青霉素结合蛋白）或主动外排。抗生素的过度和不当使用加速了对耐药菌株的选择。

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10. Biotechnology and Microorganisms | 生物技术与微生物

Microorganisms are biotechnology workhorses. The yeast Saccharomyces cerevisiae is used in baking (CO₂ production) and brewing (ethanol fermentation). Lactobacillus species produce lactic acid, turning milk into yogurt and cheese. In genetic engineering, bacteria can be transformed to produce human proteins such as insulin. Bioremediation employs bacteria to degrade pollutants like oil. Large-scale fermenters are controlled for pH, temperature, O₂, and nutrient supply. Exam questions may ask you to interpret data on growth curves or product yield.

微生物是生物技术的主力军。酿酒酵母用于烘焙（产生 CO₂）和酿造（乙醇发酵）。乳酸菌属产生乳酸，将牛奶制成酸奶和奶酪。在基因工程中，细菌可被转化以生产人源蛋白质，如胰岛素。生物修复利用细菌降解石油等污染物。大型发酵罐需对 pH、温度、氧气和营养供给进行控制。考题可能会要求你解读生长曲线或产物产率数据。

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