IGCSE WJEC Biology: Microorganisms Key Points | IGCSE WJEC 生物：微生物考点精讲

📚 IGCSE WJEC Biology: Microorganisms Key Points | IGCSE WJEC 生物：微生物考点精讲

Microorganisms, or microbes, are tiny living organisms that can only be seen under a microscope. They include bacteria, viruses, fungi and protists, and they play essential roles both in nature and in human life. Some are beneficial, like those used in food production and decomposition, while others cause diseases. In the WJEC IGCSE Biology specification, understanding the structure, life processes and interactions of microorganisms is fundamental to topics such as health, disease, immunity, biotechnology and ecosystems.

微生物是一类非常微小的生物，通常只能在显微镜下观察到。它们包括细菌、病毒、真菌和原生生物，在自然界和人类生活中发挥着重要作用。有些微生物是有益的，例如用于食品生产和物质分解的微生物，而有些则会导致疾病。在 WJEC IGCSE 生物学课程中，理解微生物的结构、生命活动以及与环境和其他生物的相互作用，是学习健康、疾病、免疫、生物技术和生态系统等主题的基础。

1. Types of Microorganisms | 微生物的类型

Microorganisms can be grouped into several major categories based on their cell structure and characteristics. The four main groups studied at IGCSE level are bacteria, viruses, fungi and protists. Bacteria are single-celled organisms without a nucleus; viruses are non-living particles that require a host cell to reproduce; fungi include both single-celled (yeast) and multicellular (moulds, mushrooms) forms; and protists are a diverse group of eukaryotic single-celled organisms, including amoeba and Plasmodium.

根据细胞结构和特征，微生物可以分为几个主要类别。IGCSE 阶段需要掌握的四大类微生物是：细菌、病毒、真菌和原生生物。细菌是没有细胞核的单细胞生物；病毒是没有细胞结构的颗粒，必须借助宿主细胞才能繁殖；真菌包括单细胞（如酵母菌）和多细胞（如霉菌、蘑菇）两种形式；原生生物则是一类多样的真核单细胞生物，包括变形虫和疟原虫等。

A common feature of bacteria, fungi and protists is that they are living organisms, carrying out life processes such as nutrition, respiration and reproduction. Viruses, on the other hand, do not show all the characteristics of life and are often described as ‘obligate parasites’. All groups can be found in a wide variety of habitats, from soil and water to inside other organisms.

细菌、真菌和原生生物的共同特征是它们都是生物体，能够进行营养、呼吸和繁殖等生命活动。而病毒并不表现全部的生命特征，常被称为“专性寄生物”。所有这些微生物都能在各种环境中生存，从土壤、水体到其他生物体内。

2. Structure of Bacteria | 细菌的结构

Bacterial cells are prokaryotic, meaning they lack a true nucleus and membrane-bound organelles. The genetic material is a single circular chromosome of DNA, sometimes accompanied by small rings of DNA called plasmids. The cell is surrounded by a cell membrane and a rigid cell wall made of peptidoglycan. Some bacteria have a slimy capsule for protection, and many possess flagella for movement.

细菌细胞是原核细胞，没有真正的细胞核和膜包裹的细胞器。遗传物质是一条环状的 DNA 染色体，有时还有称为质粒的小环状 DNA。细胞被细胞膜和由肽聚糖构成的坚硬细胞壁所包围。部分细菌具有保护性的黏滑荚膜，许多细菌借助鞭毛运动。

Unlike plant and animal cells, bacteria do not contain mitochondria or chloroplasts. Respiration takes place in the cytoplasm using enzymes associated with the cell membrane. Some bacteria are capable of photosynthesis using specialised pigments, but they do not have true chloroplasts. Ribosomes in bacteria are smaller (70S) than those in eukaryotic cells (80S), a detail often used in the context of antibiotic action.

与动植物细胞不同，细菌没有线粒体或叶绿体。呼吸作用在细胞质中进行，利用与细胞膜结合相关的酶。一些细菌能够利用特殊色素进行光合作用，但并不具备真正的叶绿体。细菌的核糖体较小（70S），而真核细胞核糖体较大（80S），这一点常与抗生素的作用机制相关。

3. Viruses: Obligate Parasites | 病毒：专性寄生生物

Viruses are much smaller than bacteria and have a very simple structure. They consist of a core of genetic material (either DNA or RNA) surrounded by a protective protein coat called a capsid. Some viruses also have an outer lipid envelope. Because they lack cellular machinery, viruses cannot carry out metabolism or reproduce on their own.

病毒比细菌小得多，结构也非常简单。它们由遗传物质核心（DNA 或 RNA）和包裹在外的蛋白质外壳（称为衣壳）组成。有些病毒还具有外层脂质包膜。由于没有细胞结构，病毒不能独立进行代谢或繁殖。

To replicate, a virus must attach to a specific host cell and inject its genetic material. The host cell’s machinery is then used to produce new viral components, which assemble into complete virus particles. The host cell is often destroyed when the new viruses are released, causing disease symptoms. This obligate intracellular parasitism is a key characteristic used to distinguish viruses from living organisms.

病毒要繁殖，必须先附着在特定的宿主细胞上，并将其遗传物质注入细胞内。宿主细胞的代谢系统被用来合成新的病毒组分，这些组分再装配成完整的病毒颗粒。新病毒释放时通常会破坏宿主细胞，从而引发疾病症状。这种专性胞内寄生是区分病毒与生物体的关键特征。

4. Fungi and Their Features | 真菌及其特征

Fungi are eukaryotic organisms with cell walls made of chitin, not cellulose. They include unicellular yeasts and multicellular moulds and mushrooms. Fungi do not photosynthesise; instead, they obtain nutrients by secreting digestive enzymes onto their food source and then absorbing the broken-down products. This mode of nutrition is called saprotrophic or saprophytic nutrition.

真菌是真核生物，细胞壁由几丁质构成，而不是纤维素。真菌包括单细胞的酵母菌以及多细胞的霉菌和蘑菇。真菌不进行光合作用；它们通过向食物源分泌消化酶，然后将分解后的产物吸收进体内来获取营养。这种营养方式称为腐生营养。

The basic body structure of a multicellular fungus is composed of thread-like hyphae, which together form a network called a mycelium. Hyphae provide a large surface area for absorption of nutrients. Yeast, a single-celled fungus, reproduces asexually by budding and is widely used in baking and brewing. Moulds such as Rhizopus are common examples of saprotrophic fungi that break down organic matter.

多细胞真菌的基本结构是由菌丝组成的，菌丝交织在一起形成菌丝体。菌丝提供了很大的表面积，有利于营养物质的吸收。酵母菌是一种单细胞真菌，通过出芽进行无性繁殖，被广泛用于烘焙和酿酒。毛霉等霉菌则是常见的腐生真菌，能分解有机物。

5. Nutrition in Microorganisms | 微生物的营养方式

Microorganisms show a variety of nutritional strategies. Saprotrophs (e.g. most fungi and many bacteria) feed on dead organic matter, playing a crucial role in decomposition. Parasites (e.g. Mycobacterium tuberculosis, viruses) live on or in another living organism and harm it. Some bacteria are autotrophic: photoautotrophs use light energy (e.g. cyanobacteria) and chemoautotrophs use energy from chemical reactions (e.g. nitrifying bacteria).

微生物有多种多样的营养方式。腐生生物（如大多数真菌和许多细菌）以死的有机物为食，在物质分解中起着重要作用。寄生生物（如结核分枝杆菌、病毒）生活在其他生物体内或体表，对宿主造成伤害。一些细菌是自养型：光能自养菌利用光能（如蓝细菌），化能自养菌利用化学反应释放的能量（如硝化细菌）。

This variety allows microorganisms to occupy almost every ecological niche on Earth. Saprotrophs recycle nutrients such as carbon and nitrogen, making them available to plants. Parasitic microorganisms are responsible for many infectious diseases. Understanding their nutritional needs is also important in culture techniques and controlling their growth.

这种多样性使微生物能够占据地球上几乎所有的生态位。腐生生物循环碳、氮等营养物质，使其能重新被植物利用。寄生微生物是许多传染病的元凶。了解微生物的营养需求对于培养和控制其生长同样重要。

6. Microorganisms and Disease | 微生物与疾病

Pathogenic microorganisms cause infectious diseases when they enter the body and interfere with normal functions. Bacteria can produce toxins that damage cells and tissues (e.g. Salmonella causes food poisoning, Vibrio cholerae causes cholera). Viruses hijack host cells to reproduce, leading to cell death and tissue damage (e.g. influenza virus, HIV, measles virus). Protist pathogens include Plasmodium, which causes malaria.

病原微生物进入人体后会干扰正常的生理功能，从而引起传染病。细菌可产生毒素，损伤细胞和组织（例如沙门氏菌导致食物中毒，霍乱弧菌引起霍乱）。病毒劫持宿主细胞进行繁殖，导致细胞死亡和组织损伤（如流感病毒、HIV、麻疹病毒）。原生生物病原体包括引起疟疾的疟原虫。

The transmission of pathogens can occur through various routes, including direct contact, airborne droplets, contaminated food and water, and vectors such as mosquitoes. Understanding these routes is essential for developing prevention strategies, such as hygiene, clean water supplies, safe food handling and vector control.

病原体的传播途径多种多样，包括直接接触、空气飞沫、受污染的食物和水，以及蚊子等媒介生物。了解这些传播途径对于制定预防策略至关重要，例如注意卫生、提供清洁饮用水、安全处理食品以及控制病媒。

7. Human Defence Against Pathogens | 人体对抗病原体的防御

The human body has several lines of defence against pathogens. The first line of defence consists of physical barriers such as the skin, which acts as a tough outer layer, and mucous membranes that trap microbes. Chemical barriers include enzymes in tears (lysozyme) that destroy bacterial cell walls, and stomach acid (hydrochloric acid) that kills ingested pathogens.

人体对抗病原体有数道防线。第一道防线由物理屏障构成，例如作为坚固外层的皮肤，以及能捕获微生物的黏膜。化学屏障包括泪液中的溶菌酶，可破坏细菌细胞壁；还有胃酸（盐酸），能杀灭随食物进入的病原体。

If pathogens breach these barriers, the second line of defence – the non-specific immune response – is activated. Phagocytes, a type of white blood cell, engulf and digest foreign pathogens in a process called phagocytosis. Inflammation and fever also help to limit the spread of infection. The specific immune response involves lymphocytes producing antibodies that are complementary in shape to specific antigens on the pathogen. This leads to the destruction of the pathogen and the formation of memory cells that provide long-term immunity.

如果病原体突破了这些屏障，就会启动第二道防线——非特异性免疫反应。吞噬细胞是一种白细胞，通过吞噬作用吞入并消化外来病原体。炎症和发热也有助于限制感染的扩散。特异性免疫反应涉及淋巴细胞产生抗体，其形状与病原体上特定的抗原互补。这导致病原体被消灭，并形成记忆细胞，提供长期免疫。

8. Antibiotics and Antibiotic Resistance | 抗生素与抗生素耐药性

Antibiotics are chemical substances that kill bacteria or inhibit their growth without harming human cells. They work by targeting structures or processes unique to bacteria, such as the synthesis of peptidoglycan cell walls (e.g. penicillin) or bacterial ribosomes. Antibiotics are ineffective against viruses because viruses lack these bacterial targets and require host cells for replication.

抗生素是能够杀死细菌或抑制其生长而不损害人体细胞的化学物质。它们通过针对细菌特有的结构或过程起作用，例如抑制肽聚糖细胞壁的合成（如青霉素）或干扰细菌核糖体。抗生素对病毒无效，因为病毒没有这些细菌特有的靶点，并且需要借助宿主细胞才能复制。

Misuse and overuse of antibiotics have led to the evolution of antibiotic-resistant bacteria. Random mutations or plasmid transfer can give certain bacteria the ability to survive antibiotic treatment. These resistant bacteria reproduce, passing on the resistance genes. MRSA (methicillin-resistant Staphylococcus aureus) is a well-known example. Reducing unnecessary antibiotic prescriptions and completing full courses of treatment are crucial to slowing the spread of resistance.

抗生素的滥用和过度使用导致了耐药细菌的进化。随机突变或质粒转移可使某些细菌获得在抗生素治疗下存活的能力。这些耐药细菌繁殖，将耐药基因传递给后代。耐甲氧西林金黄色葡萄球菌（MRSA）就是一个众所周知的例子。减少不必要的抗生素处方，并完整完成疗程，对于减缓耐药性的传播至关重要。

9. Vaccination and Immunity | 疫苗接种与免疫

Vaccination is a method of inducing active immunity without causing illness. A vaccine typically contains a weakened or inactivated form of a pathogen, or its antigens. When introduced into the body, it stimulates the specific immune response, leading to the production of memory lymphocytes and antibodies. As a result, if the individual is later exposed to the actual pathogen, the secondary immune response is rapid and strong, preventing disease.

疫苗接种是一种在不引起疾病的情况下诱导主动免疫的方法。疫苗通常含有减毒或灭活的病原体，或其抗原成分。注入人体后，疫苗会刺激特异性免疫应答，产生记忆淋巴细胞和抗体。这样一来，如果此人日后接触到真正的病原体，就会产生迅速而强烈的二次免疫应答，从而预防疾病。

Herd immunity occurs when a large proportion of a population becomes immune, either through vaccination or prior infection, thereby reducing the spread of the disease and protecting those who cannot be vaccinated, such as newborns or people with compromised immune systems. The success of vaccination programmes has led to the global eradication of smallpox and a dramatic reduction in diseases like polio and measles.

当人群中大部分人通过接种疫苗或既往感染获得免疫力时，就能形成群体免疫，从而减少疾病的传播，保护那些无法接种疫苗的人，如新生儿或免疫系统受损者。疫苗接种计划的成功已经使天花在全球根除，并大大降低了脊髓灰质炎和麻疹等疾病的发病率。

10. Microorganisms in Food Production | 微生物在食品生产中的应用

Microorganisms are invaluable in the food industry. Yeast (Saccharomyces cerevisiae) is used in bread-making and alcohol production. In bread, yeast undergoes aerobic respiration, producing carbon dioxide that causes the dough to rise. In brewing and winemaking, yeast ferments sugars anaerobically, producing ethanol and carbon dioxide. The carbon dioxide provides the fizz in beer and champagne.

微生物在食品工业中价值巨大。酵母菌（Saccharomyces cerevisiae）被用于制作面包和酒精。在面包制作中，酵母进行有氧呼吸，产生二氧化碳使面团膨胀。在酿造啤酒和葡萄酒时，酵母在无氧条件下发酵糖类，生成乙醇和二氧化碳。二氧化碳赋予了啤酒和香槟独特的气泡。

Bacteria also have essential roles: Lactobacillus species are used to make yoghurt and cheese. In yoghurt production, bacteria ferment lactose (milk sugar) into lactic acid. The lactic acid lowers the pH, causing milk proteins to coagulate and giving yoghurt its thick texture and tangy taste. Specific moulds, such as Penicillium species, are used in the production of certain cheeses (e.g. blue cheese) to add distinctive flavours and textures.

细菌同样起着重要作用：乳酸杆菌被用于制作酸奶和奶酪。在酸奶生产中，细菌将乳糖发酵成乳酸。乳酸降低 pH 值，使牛奶蛋白凝固，赋予酸奶浓稠的质地和独特的酸味。特定的霉菌，如青霉菌，被用于某些奶酪（如蓝纹奶酪）的生产，以增添独特的风味和质地。

11. Role of Microorganisms in Ecosystems | 微生物在生态系统中的作用

Microorganisms are crucial to the cycling of nutrients, especially carbon and nitrogen. Saprophytic bacteria and fungi decompose dead plants and animals, releasing carbon dioxide back into the atmosphere and returning mineral ions (such as nitrates and phosphates) to the soil. Without decomposition, nutrients would remain locked in dead matter and ecosystems would collapse.

微生物对营养物质的循环，尤其是碳循环和氮循环，至关重要。腐生细菌和真菌分解死去的动植物，将二氧化碳释放回大气，并将矿物离子（如硝酸盐和磷酸盐）归还土壤。如果没有分解作用，营养物质将被封锁在死去的生物体中，生态系统将崩溃。

In the nitrogen cycle, different groups of bacteria carry out key transformations. Nitrogen-fixing bacteria (e.g. Rhizobium in legume root nodules) convert atmospheric nitrogen gas into ammonia. Nitrifying bacteria in the soil oxidise ammonia to nitrites and then to nitrates, which plants can absorb. Denitrifying bacteria convert nitrates back to nitrogen gas, returning it to the atmosphere. These microbial processes maintain the balance of nitrogen in the environment.

在氮循环中，不同类群的细菌执行着关键的转化过程。固氮细菌（如豆科植物根瘤中的根瘤菌）将大气中的氮气转化为氨。土壤中的硝化细菌将氨氧化成亚硝酸盐，再氧化成硝酸盐，后者可被植物吸收。反硝化细菌将硝酸盐转化回氮气，使之回归大气。这些微生物活动维持着环境中氮的平衡。

12. Practical Investigations with Microorganisms | 微生物相关实验研究

Studying microorganisms in the laboratory requires aseptic technique to prevent contamination by unwanted microbes and to ensure safety. Key practices include sterilising equipment and culture media using an autoclave or pressure cooker, flaming the necks of containers and inoculating loops, and working near a Bunsen burner to create an updraft. Petri dishes containing cultured microorganisms should be sealed with adhesive tape (not completely airtight) and incubated at a maximum temperature of 25°C in schools to avoid growing potential pathogens that thrive at human body temperature (37°C).

在实验室研究微生物需要采用无菌操作技术，以防止杂菌污染并确保安全。关键操作包括：使用高压蒸汽灭菌器对设备和培养基进行灭菌；在火焰上灼烧容器口和接种环；在本生灯附近操作以形成上升气流。培养微生物的培养皿应用胶带密封（不可完全气密），学校的培养温度最高不超过 25°C，以避免培养出可能在人体温度（37°C）下生长的潜在致病菌。

Common investigations include measuring the effect of different antiseptics or antibiotics on bacterial growth by placing impregnated paper discs on an agar plate seeded with bacteria and observing zones of inhibition. Another classic experiment uses yeast and sugar solutions with a delivery tube to collect carbon dioxide and demonstrate fermentation, illustrating the metabolic processes of microorganisms under controlled conditions.

常见的实验包括将浸润不同消毒剂或抗生素的纸片放在涂布了细菌的琼脂平板上，通过观察抑菌圈的大小来比较其效果。另一个经典实验是用酵母和糖溶液，连接导管收集二氧化碳，以演示发酵过程，从而在受控条件下展示微生物的代谢活动。

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IGCSE WJEC Biology: Microorganisms Key Points | IGCSE WJEC 生物：微生物 考点精讲