📚 Microorganisms: Key Revision Points for IB and WJEC Biology | 微生物考点精讲（IB与WJEC生物）
Microorganisms are the invisible engines of life on Earth. From the bacteria that cycle nutrients in soil to the viruses that challenge our immune systems, these tiny organisms are central to both IB and WJEC Biology specifications. This article distils the essential concepts you need to master, including classification, structure, growth, ecological roles, disease mechanisms, and industrial applications. Whether you are revising for an IB exam or a WJEC assessment, the principles covered here will help you connect the details of microbial life to the bigger picture of biology.
微生物是地球上生命的无形引擎。从在土壤中循环养分的细菌到挑战我们免疫系统的病毒，这些微小的生物体是 IB 和 WJEC 生物课程的核心内容。本文提炼了你必须掌握的基本概念，包括分类、结构、生长、生态角色、致病机制和工业应用。无论你是在为 IB 考试还是 WJEC 评估做准备，这里涵盖的原理都将帮助你连接微生物生命的细节与生物学的大图景。
1. Classification of Microorganisms | 微生物的分类
Microorganisms are broadly classified into several groups based on cellular structure and genetic makeup. The major groups include bacteria (prokaryotes), archaea (also prokaryotes but genetically distinct), fungi (eukaryotic), protists (a diverse group of mostly unicellular eukaryotes), and viruses, which are acellular and not universally considered living. IB Biology distinguishes between prokaryotic and eukaryotic cells in Topic 1.2, while WJEC Unit 1 also emphasises the key differences between these domains. Understanding this classification is the first step to grasping how different microbes interact with the world.
微生物根据细胞结构和基因组成被分为几大类群。主要类群包括细菌（原核生物）、古菌（同样是原核生物但在遗传上截然不同）、真菌（真核生物）、原生生物（多样化的多为单细胞真核生物），以及病毒，它们是非细胞的，且通常不被认为是活的生物。IB 生物在第 1.2 主题中区分了原核细胞和真核细胞，而 WJEC 第一单元同样强调这些域之间的关键差异。理解这种分类是掌握不同微生物如何与世界互动的第一步。
The three-domain system (Bacteria, Archaea, Eukarya) is a fundamental concept. In both syllabi, students must recognise that archaea are not simply ‘ancient bacteria’ but form a separate domain, often found in extreme environments. For exam purposes, note that pathogenic microbes are drawn from bacteria, viruses, fungi, and protists, while archaea are not typically pathogens.
三域系统（细菌域、古菌域、真核域）是一个基本概念。在两个课程大纲中，学生必须认识到古菌不仅仅是“古老的细菌”，而是形成一个独立的域，常常在极端环境中发现。为了考试，请注意致病微生物来自细菌、病毒、真菌和原生生物，而古菌通常不是病原体。
2. Prokaryotic Cell Structure: Bacteria | 原核细胞结构：细菌
Bacteria are the classic prokaryotic cells you need to know inside out. A typical bacterium possesses a cell wall containing peptidoglycan (in most eubacteria), a cell membrane, cytoplasm, 70S ribosomes, a circular DNA molecule located in a nucleoid region, and often plasmids. Some have flagella, pili, or a capsule. IB Biology expects you to draw and label a diagram of a prokaryotic cell, including the cell wall, plasma membrane, cytoplasm, pili, flagella, ribosomes, and nucleoid. WJEC similarly requires detailed knowledge of these structures and their functions.
细菌是你必须彻底了解的原核细胞的典型代表。一个典型的细菌具有含肽聚糖的细胞壁（在大多数真细菌中）、细胞膜、细胞质、70S 核糖体、位于拟核区的环状 DNA 分子，并常带有质粒。有些细菌有鞭毛、菌毛或荚膜。IB 生物期望你画出并标注原核细胞的示意图，包括细胞壁、质膜、细胞质、菌毛、鞭毛、核糖体和拟核。WJEC 同样要求了解这些结构的详细知识及其功能。
Gram-positive and Gram-negative bacteria differ in cell wall composition, which affects their response to antibiotics. Gram-positive bacteria have a thick peptidoglycan layer and no outer membrane, while Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane containing lipopolysaccharides. This distinction is directly relevant to the mode of action of antibiotics like penicillin, which targets the peptidoglycan cross-linking.
革兰氏阳性菌和革兰氏阴性菌的细胞壁组成不同，这影响了它们对抗生素的反应。革兰氏阳性菌具有厚厚的肽聚糖层且没有外膜，而革兰氏阴性菌具有薄的肽聚糖层和含有脂多糖的外膜。这一区别与抗生素如青霉素的作用方式直接相关，青霉素靶向肽聚糖的交联。
3. Viruses: Acellular Entities | 病毒：非细胞实体
Viruses are not made of cells and do not carry out any metabolic processes outside a host cell. Their structure is minimal: a nucleic acid core (DNA or RNA, single- or double-stranded) surrounded by a protein coat called a capsid, and sometimes a lipid envelope derived from the host cell membrane. Both IB and WJEC specifications require you to understand that viruses lack ribosomes and cannot synthesise proteins independently, which is why they must infect host cells.
病毒不是由细胞构成的，并且在宿主细胞外不进行任何代谢过程。它们的结构极其简单：一个核酸核心（DNA 或 RNA，单链或双链）被一个称为衣壳的蛋白质外壳包围，有时还有一层来自宿主细胞膜的脂质包膜。IB 和 WJEC 的考试大纲都要求你理解病毒缺乏核糖体，无法独立合成蛋白质，因此它们必须感染宿主细胞。
The lytic and lysogenic cycles are standard models for bacteriophage replication. In the lytic cycle, the virus immediately hijacks the host’s machinery to produce new virions, eventually lysing the cell. In the lysogenic cycle, the viral genome integrates into the host chromosome (becoming a prophage) and replicates along with the host DNA until an environmental trigger induces the lytic pathway. This concept often appears in questions about gene transfer and microbial ecology.
裂解循环和溶原循环是噬菌体复制的标准模型。在裂解循环中，病毒立即劫持宿主的机制来产生新的病毒颗粒，最终裂解细胞。在溶原循环中，病毒基因组整合到宿主染色体中（成为前噬菌体），并随着宿主 DNA 一起复制，直到环境触发因素诱导裂解途径。这一概念经常出现在关于基因转移和微生物生态的问题中。
4. Fungi: Eukaryotic Decomposers | 真菌：真核分解者
Fungi are eukaryotic organisms that can be unicellular (yeasts) or multicellular (moulds and mushrooms). Their cells have cell walls made of chitin, not cellulose. They are heterotrophic and obtain nutrients by secreting digestive enzymes onto organic matter and absorbing the breakdown products — a process called saprotrophic nutrition. IB Biology addresses fungi in the context of diversity and nutrient cycling, while WJEC highlights their role in decomposition and food production.
真菌是真核生物，可以是单细胞的（酵母菌）或多细胞的（霉菌和蘑菇）。它们的细胞壁由几丁质构成，而不是纤维素。它们是异养的，通过向有机物分泌消化酶并吸收分解产物来获取营养——这个过程称为腐生营养。IB 生物在多样性和养分循环的背景下涉及真菌，而 WJEC 则突出它们在分解和食品生产中的作用。
Yeasts reproduce asexually by budding and sexually by forming ascospores. Multicellular fungi grow as hyphae, which form a network called mycelium. The distinction between septate and aseptate hyphae is useful for classification. Fungi can also cause diseases (mycoses), such as athlete’s foot or thrush, and produce antibiotics like penicillin. Aspergillus and Penicillium are frequently mentioned in syllabus examples.
酵母菌通过出芽进行无性繁殖，通过形成子囊孢子进行有性繁殖。多细胞真菌以菌丝的形式生长，菌丝形成称为菌丝体网络。有隔菌丝和无隔菌丝的区别有助于分类。真菌也能引起疾病（真菌病），如足癣或鹅口疮，并能产生抗生素如青霉素。曲霉和青霉在课程大纲示例中经常被提及。
5. Growth and Reproduction of Microbes | 微生物的生长与繁殖
Bacteria mainly reproduce asexually by binary fission, where one cell divides into two identical daughter cells. Under favourable conditions, bacterial populations can exhibit exponential growth. Both IB and WJEC exams often present a bacterial growth curve with four phases: lag phase, exponential (log) phase, stationary phase, and death phase. You must be able to explain what limits growth in each phase, such as nutrient depletion and accumulation of toxic waste products.
细菌主要通过二分裂进行无性繁殖，一个细胞分裂成两个相同的子细胞。在有利条件下，细菌种群可呈现指数增长。IB 和 WJEC 考试中经常给出一个包含四个时期的细菌生长曲线：延缓期、指数（对数）期、稳定期和衰亡期。你必须能够解释每个阶段限制生长的因素，例如营养耗尽和有毒代谢废物的积累。
In addition to binary fission, bacteria can exchange genetic material through conjugation (transfer of plasmid DNA via a pilus), transformation (uptake of naked DNA from the environment), and transduction (transfer of DNA via viruses). These processes, particularly conjugation, are central to the spread of antibiotic resistance genes. The reproduction of viruses, as covered earlier, depends entirely on host cell machinery and is not considered autonomous reproduction.
除了二分裂，细菌还可以通过接合（通过菌毛传递质粒 DNA）、转化（从环境摄取裸露 DNA）和转导（通过病毒传递 DNA）交换遗传物质。这些过程，尤其是接合，是抗生素耐药基因传播的核心。如前所述，病毒的繁殖完全依赖宿主细胞的机制，不被视为自主繁殖。
6. Microbial Metabolism and Nutrition | 微生物的代谢与营养
Microorganisms display remarkable metabolic diversity. Depending on their carbon source, bacteria may be autotrophs (fixing CO₂) or heterotrophs (using organic carbon). According to energy source, they can be phototrophs (light) or chemotrophs (chemical compounds). Combining these gives classifications like chemoheterotrophs (most pathogens), photoautotrophs (cyanobacteria), and chemoautotrophs (nitrifying bacteria). This is a key area where IB expects you to apply the terms to unfamiliar examples.
微生物表现出非凡的代谢多样性。根据碳源，细菌可能是自养的（固定 CO₂）或异养的（利用有机碳）。根据能量来源，它们可以是光养生物（光）或化养生物（化合物）。将这些组合起来得到分类，例如化能异养菌（大多数病原体）、光能自养菌（蓝细菌）和化能自养菌（硝化细菌）。这是 IB 期望你能将这些术语应用到不熟悉的例子的关键领域。
Oxygen requirements also define microbial groups: obligate aerobes require oxygen, obligate anaerobes are killed by oxygen, facultative anaerobes can switch between aerobic respiration and fermentation or anaerobic respiration, and microaerophiles need low oxygen concentrations. Fermentation is an important metabolic pathway in many microorganisms, such as yeast producing ethanol and lactic acid bacteria producing lactate. In the WJEC specification, the practical investigation of yeast respiration under different conditions is a common required task.
对氧气的需求同样定义了微生物类群：专性需氧菌需要氧气，专性厌氧菌会被氧气杀死，兼性厌氧菌可在有氧呼吸和发酵或厌氧呼吸之间转换，微需氧菌需要低氧浓度。发酵是许多微生物体内重要的代谢途径，例如酵母产生乙醇，乳酸菌产生乳酸。在 WJEC 考试大纲中，在不同条件下研究酵母呼吸的实践探究是一项常见的指定任务。
7. Microorganisms in Ecosystems | 生态系统中的微生物
Microbes play vital roles in nutrient cycles. In the nitrogen cycle, bacteria are responsible for nitrogen fixation (e.g., Rhizobium in root nodules), nitrification (Nitrosomonas, Nitrobacter), and denitrification. The carbon cycle depends on decomposition by saprotrophic bacteria and fungi, as well as photosynthesis by cyanobacteria and algae. Both IB (Topic 4.3 Ecology) and WJEC (Unit 2) require you to describe these processes and the specific roles of microorganisms.
微生物在养分循环中扮演着至关重要的角色。在氮循环中，细菌负责固氮（如根瘤中的根瘤菌）、硝化作用（亚硝化单胞菌、硝化杆菌）和反硝化作用。碳循环依赖于腐生细菌和真菌的分解，以及蓝细菌和藻类的光合作用。IB（第 4.3 主题生态学）和 WJEC（第二单元）都要求你描述这些过程以及微生物的具体作用。
Mutualistic relationships are also crucial. Mycorrhizal fungi associate with plant roots, enhancing water and mineral absorption. The gut microbiome in animals aids digestion and vitamin synthesis. In ruminants, methanogenic archaea help break down cellulose. These examples are used in exam questions to illustrate symbiosis and energy flow. Note that IB often frames such interactions within the context of communities and ecosystems, while WJEC may link them to agricultural productivity.
互惠关系同样至关重要。菌根真菌与植物根系结合，增强了对水分和矿物质的吸收。动物体内的肠道微生物组有助于消化和维生素合成。在反刍动物体内，产甲烷古菌帮助分解纤维素。这些例子被用于考题以说明共生和能量流动。请注意，IB 常常在群落和生态系统的背景下构建这类相互作用，而 WJEC 则可能将它们与农业生产力联系起来。
8. Pathogens and Disease | 病原体与疾病
Pathogenic microorganisms cause infectious diseases by invading host tissues, producing toxins, or triggering excessive immune responses. Endotoxins are lipopolysaccharide components of Gram-negative bacterial outer membranes, released when the cells die. Exotoxins are soluble proteins secreted by living bacteria, often highly specific and potent. Viruses cause disease by destroying host cells, and protists like Plasmodium (malaria) have complex life cycles involving human and mosquito hosts.
病原微生物通过侵入宿主组织、产生毒素或引发过度的免疫反应来引起传染病。内毒素是革兰氏阴性菌外膜的脂多糖成分，在细胞死亡时释放。外毒素是活细菌分泌的可溶性蛋白质，通常具有高度特异性和很强毒性。病毒通过破坏宿主细胞引起疾病，而像疟原虫（疟疾）这样的原生生物具有涉及人类和蚊子宿主的复杂生命周期。
The transmission of pathogens can be direct (contact, droplet, sexual) or indirect (airborne, vector-borne, foodborne, waterborne). Both syllabi expect you to discuss specific diseases: HIV (virus), TB (bacterium), malaria (protist), and athlete’s foot (fungus) are typical IB examples. WJEC may also include Salmonella, cholera, or measles. For each, you should know the pathogen type, mode of transmission, and key symptoms.
病原体的传播可以是直接的（接触、飞沫、性传播）或间接的（空气传播、媒介传播、食物传播、水传播）。两个课程大纲都期望你讨论具体的疾病：HIV（病毒）、TB（细菌）、疟疾（原生生物）和足癣（真菌）是典型的 IB 例子。WJEC 可能还包括沙门氏菌、霍乱或麻疹。对于每一种，你应该了解病原体类型、传播方式和主要症状。
9. Antibiotics and Resistance | 抗生素与耐药性
Antibiotics are chemicals that kill or inhibit the growth of bacteria. They target processes unique to prokaryotes, such as cell wall synthesis (penicillin), protein synthesis (tetracycline), or nucleic acid synthesis. Antibiotics do not work against viruses because viruses lack the metabolic targets. IB and WJEC both stress the critical importance of understanding why antibiotics are ineffective against viral infections like the common cold or influenza.
抗生素是杀死或抑制细菌生长的化学物质。它们靶向原核生物特有的过程，例如细胞壁合成（青霉素）、蛋白质合成（四环素）或核酸合成。抗生素对病毒无效，因为病毒缺乏这些代谢靶点。IB 和 WJEC 都强调理解为什么抗生素对普通感冒或流感这类病毒感染无效的至关重要性。
Antibiotic resistance arises through genetic mutations or acquisition of resistance genes via plasmids. Methicillin-resistant Staphylococcus aureus (MRSA) is a well-known example. The misuse and overuse of antibiotics in medicine and agriculture accelerate the spread of resistance. Bacteria may become resistant through mechanisms like enzymatic breakdown of the antibiotic (β-lactamase), alteration of the target site, or efflux pumps that expel the drug.
抗生素耐药性通过基因突变或通过质粒获得耐药基因而产生。耐甲氧西林金黄色葡萄球菌（MRSA）是一个著名的例子。在医药和农业中误用和过度使用抗生素加速了耐药性的传播。细菌可能通过如酶解抗生素（β-内酰胺酶）、改变靶点或通过外排泵排出药物等机制变得耐药。
10. Biotechnology and Industrial Uses | 生物技术与工业应用
Microorganisms are harnessed to produce a vast array of useful products. Bacteria and fungi are used in fermentation to make bread, yogurt, cheese, beer, and wine. Penicillium chrysogenum produces penicillin, and genetically modified bacteria (such as E. coli) are used to manufacture human insulin and growth hormone. In the WJEC specification, the role of microorganisms in the food industry, such as the use of Lactobacillus in yogurt production, is examined in practical contexts.
微生物被利用来生产大量有用的产品。细菌和真菌被用于发酵生产面包、酸奶、奶酪、啤酒和葡萄酒。产黄青霉产生青霉素，而转基因细菌（如大肠杆菌）被用于制造人胰岛素和生长激素。在 WJEC 考试大纲中，微生物在食品工业中的作用，例如保加利亚乳杆菌在酸奶生产中的应用，是在实践背景下考察的。
In environmental biotechnology, microbes are used for bioremediation — cleaning up oil spills or degrading pollutants. Genetically engineered microorganisms can express enzymes to break down waste. IB Biology (Topic 3.5 Genetic modification) explores how genes of interest are inserted into bacterial plasmids to produce proteins. This overlaps with the general principle of recombinant DNA technology, which is assessed in both courses.
在环境生物技术中，微生物被用于生物修复——清理石油泄漏或降解污染物。基因工程微生物可以表达酶来分解废物。IB 生物（第 3.5 主题基因改造）探索了如何将目的基因插入细菌质粒以生产蛋白质。这与重组 DNA 技术的一般原理重叠，该原理在两个课程中都会考查。
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