Mastering Microorganisms for IGCSE Edexcel Biology | IGCSE Edexcel 生物：微生物考点精讲

📚 Mastering Microorganisms for IGCSE Edexcel Biology | IGCSE Edexcel 生物：微生物考点精讲

Microorganisms, though invisible to the naked eye, are a central topic in IGCSE Edexcel Biology. They underpin ecosystems as decomposers, drive nutrient cycles, enable food production, and yet also cause devastating diseases. A clear grasp of their diversity, structure, metabolism and roles is key to exam success. This revision guide highlights the essential concepts, definitions and processes you need to master.

微生物虽然肉眼不可见，却是IGCSE Edexcel生物学的核心主题。它们是生态系统的分解者，驱动养分循环，助力食品生产，同时也能导致严重疾病。清晰掌握其多样性、结构、代谢和功能是考试成功的关键。本复习指南突出你需要掌握的重要概念、定义和过程。

1. Classification and Structure of Microorganisms | 微生物的分类与结构

Microorganisms are classified into several major groups: bacteria (prokaryotes), viruses (acellular infectious agents), fungi (eukaryotes with chitin cell walls), and protoctists (eukaryotic single-celled organisms). Comparing their cellular features is essential for IGCSE Edexcel.

微生物可分为几个主要类群：细菌（原核生物）、病毒（非细胞感染因子）、真菌（具有几丁质细胞壁的真核生物）和原生生物（单细胞真核生物）。比较它们的细胞特征是考试重点。

The table below summarises key structural differences among bacteria, fungi and viruses.

下表总结了细菌、真菌和病毒结构的关键差异。

Feature	Bacteria	Fungi	Viruses
Cell type	Prokaryotic	Eukaryotic	Not cellular
Genetic material	Circular chromosome and plasmids (DNA)	Linear chromosomal DNA in nucleus	DNA or RNA inside a protein coat
Cell wall	Made of peptidoglycan	Made of chitin	No cell wall
Typical size	0.2–10 μm	Yeast ~5 μm; moulds larger	20–300 nm
Living status	Living	Living	Non-living, obligate parasites

Protoctists, such as Amoeba and Paramecium, are unicellular eukaryotes with organelles like a nucleus; some are pathogenic, e.g. Plasmodium causing malaria.

原生生物，如变形虫和草履虫，是单细胞真核生物，拥有细胞核等细胞器；有些是病原体，例如疟原虫引起疟疾。

2. Bacteria: Structure and Reproduction | 细菌：结构与繁殖

Bacteria possess a cell wall made of peptidoglycan, which maintains their shape. Many have a slime capsule for protection, flagella for movement, and circular DNA in the cytoplasm plus small rings of DNA called plasmids. They lack a true nucleus and membrane-bound organelles.

细菌具有由肽聚糖构成的细胞壁，以维持其形状。许多细菌有保护性的荚膜、用于运动的鞭毛，以及细胞质中的环状DNA和称为质粒的小DNA环。它们缺乏真正的细胞核和膜结合的细胞器。

Bacteria reproduce asexually by binary fission. Under warm, moist, nutrient-rich conditions they can divide rapidly, sometimes every 20 minutes. Some can form spores to survive harsh environments.

细菌通过二分裂进行无性繁殖。在温暖、潮湿和营养充足的条件下，它们可以迅速分裂，有时每20分钟一次。某些细菌能形成孢子以在恶劣环境中存活。

Bacteria play dual roles: they are decomposers recycling nutrients, and used in food production, but also cause diseases such as cholera (Vibrio cholerae) and tuberculosis (Mycobacterium tuberculosis).

细菌扮演双重角色：它们是分解者，回收营养，并用于食品生产，但也会引发疾病，如霍乱（霍乱弧菌）和结核病（结核分枝杆菌）。

3. Viruses: Acellular Pathogens | 病毒：非细胞病原体

Viruses are not composed of cells. They consist of a small amount of genetic material (DNA or RNA) enclosed in a protein coat, sometimes with a lipid envelope. They cannot carry out life processes independently and are obligate parasites, meaning they must infect a host cell to reproduce.

病毒不是由细胞构成的。它们由少量遗传物质（DNA或RNA）包裹在蛋白质外壳内，有时带有脂质包膜。它们不能独立进行生命活动，是专性寄生生物，必须感染宿主细胞才能繁殖。

The viral replication cycle: attachment to host cell, injection of nucleic acid, hijacking of host machinery to produce viral components, assembly, and lysis of the cell to release new viruses. Examples include tobacco mosaic virus (TMV), HIV, and influenza virus. IGCSE Edexcel often asks how viruses damage host cells.

病毒复制周期：附着于宿主细胞、注入核酸、劫持宿主机制产生病毒成分、组装以及裂解细胞释放新病毒。例子包括烟草花叶病毒（TMV）、HIV和流感病毒。IGCSE Edexcel常考病毒如何破坏宿主细胞。

Antibiotics are ineffective against viruses because viruses lack metabolic pathways and cell walls. Antiviral drugs and vaccinations are used to control viral infections.

抗生素对病毒无效，因为病毒没有代谢途径和细胞壁。抗病毒药物和接种疫苗用于控制病毒感染。

4. Fungi: Yeasts and Moulds | 真菌：酵母与霉菌

Fungi are heterotrophic eukaryotes with cell walls made of chitin. They include unicellular yeasts and multicellular moulds that form a mycelium made of hyphae. They store carbohydrate as glycogen and reproduce both sexually and asexually.

真菌是异养真核生物，细胞壁由几丁质组成。它们包括单细胞的酵母和形成菌丝体（由菌丝构成）的多细胞霉菌。它们以糖原形式储存碳水化合物，并通过有性和无性方式繁殖。

Yeast (Saccharomyces cerevisiae) is crucial in IGCSE Edexcel as it respires anaerobically to produce ethanol and carbon dioxide, used in baking and brewing. It reproduces rapidly by budding. Saprophytic fungi secrete digestive enzymes onto dead organic matter and absorb the products.

酵母（酿酒酵母）在IGCSE Edexcel中至关重要，因为它进行无氧呼吸产生乙醇和二氧化碳，用于烘焙和酿造。它通过出芽迅速繁殖。腐生真菌向死亡有机物分泌消化酶并吸收产物。

5. Protoctists: Single-Celled Eukaryotes | 原生生物：单细胞真核生物

Protoctists, often called protists, are a diverse group of unicellular eukaryotic organisms. They display animal-like (e.g., Amoeba, Paramecium) or plant-like (e.g., Chlorella) features. Disease-causing protoctists include Plasmodium, which causes malaria and requires a mosquito vector.

原生生物，常被称为原生物，是一群多样的单细胞真核生物。它们表现出类似动物（如变形虫、草履虫）或类似植物（如小球藻）的特征。致病原生生物包括疟原虫，它导致疟疾并需要蚊子作为媒介。

Malaria is a core example in the syllabus: the parasite invades red blood cells, multiplies, and bursts them, causing fever cycles. Control methods include mosquito nets and anti-malarial drugs.

疟疾是教学大纲中的核心例子：寄生虫入侵红细胞，在其中繁殖并使细胞破裂，引起周期性发热。控制方法包括蚊帐和抗疟疾药物。

6. Decomposers and the Carbon Cycle | 分解者与碳循环

Microorganisms such as bacteria and fungi act as decomposers in ecosystems. They break down dead plants, animals and waste materials by secreting enzymes, and then absorb the digested nutrients. This process releases carbon dioxide back into the atmosphere through respiration.

细菌和真菌等微生物在生态系统中充当分解者。它们通过分泌酶分解死亡的植物、动物和废物，然后吸收消化的养分。这一过程通过呼吸作用将二氧化碳释放回大气中。

The carbon cycle involves: photosynthesis removes CO₂ from the air; feeding passes carbon compounds along food chains; respiration by all organisms returns CO₂; decomposition and combustion also release CO₂. Fossil fuels are locked carbon. Students must be able to draw and label the carbon cycle, highlighting microorganisms’ role.

碳循环涉及：光合作用从空气中吸收CO₂；取食将碳化合物沿食物链传递；所有生物的呼吸作用释放CO₂；分解和燃烧也释放CO₂。化石燃料是固定的碳。学生必须能够绘制并标注碳循环，突出微生物的作用。

7. The Nitrogen Cycle | 氮循环

Nitrogen is essential for making proteins and nucleic acids but cannot be used directly from the atmosphere (N₂). Microorganisms drive the nitrogen cycle through four key processes: nitrogen fixation, decomposition (ammonification), nitrification, and denitrification.

氮是制造蛋白质和核酸的必需元素，但不能直接从大气（N₂）中利用。微生物通过四个关键过程驱动氮循环：固氮、分解（氨化）、硝化和反硝化。

Nitrogen fixation: nitrogen-fixing bacteria in soil (e.g., Azotobacter) or in root nodules of legumes (Rhizobium) convert N₂ gas into ammonium ions (NH₄⁺).

固氮作用：土壤中的固氮菌（如固氮菌属）或豆科植物根瘤中的根瘤菌将氮气转化为铵离子（NH₄⁺）。

Ammonification: decomposers break down proteins from dead matter and urea, releasing ammonia and ammonium ions into the soil.

氨化作用：分解者分解来自死亡有机物和尿素中的蛋白质，将氨和铵离子释放到土壤中。

Nitrification: nitrifying bacteria first oxidise ammonium ions to nitrite ions (NO₂⁻) by Nitrosomonas, then nitrite to nitrate ions (NO₃⁻) by Nitrobacter. Plants absorb nitrates for growth.

硝化作用：硝化细菌首先通过亚硝化单胞菌将铵离子氧化为亚硝酸根离子（NO₂⁻），然后通过硝化杆菌将亚硝酸盐氧化为硝酸根离子（NO₃⁻）。植物吸收硝酸盐用于生长。

Denitrification: denitrifying bacteria convert nitrate back into N₂ gas, returning it to the atmosphere under anaerobic conditions. This reduces soil fertility.

反硝化作用：反硝化细菌在厌氧条件下将硝酸盐转化回N₂气体，返回大气。这降低了土壤肥力。

These chemical conversions can be summarised as:

这些化学转化可以总结如下：

N₂ → NH₄⁺ → NO₂⁻ → NO₃⁻ (assimilation by plants) → denitrification → N₂

8. Microorganisms in Food Production | 微生物与食品生产

Microorganisms are exploited to produce many foods. In the production of yogurt, Lactobacillus bacteria are used. Milk is first pasteurised by heating to kill unwanted microbes. The bacteria ferment lactose (milk sugar) into lactic acid, which lowers pH and coagulates milk proteins, forming yogurt. The low pH also inhibits harmful bacteria.

微生物被用来生产许多食品。在酸奶生产中，使用乳酸杆菌。牛奶首先经巴氏消毒加热以杀死不需要的微生物。细菌将乳糖发酵成乳酸，降低pH值并使牛奶蛋白凝固，形成酸奶。较低的pH也抑制有害细菌。

Bread making uses yeast (Saccharomyces cerevisiae). Yeast respires aerobically at first, but when oxygen runs out it switches to anaerobic respiration, producing carbon dioxide and ethanol. The CO₂ bubbles cause the dough to rise. The ethanol evaporates during baking. The equation for anaerobic respiration in yeast is:

面包制作使用酵母（酿酒酵母）。酵母最初进行有氧呼吸，但当氧气耗尽时转为无氧呼吸，产生二氧化碳和乙醇。CO₂气泡使面团膨胀。乙醇在烘焙时蒸发。酵母的无氧呼吸方程式为：

Glucose → Ethanol + Carbon Dioxide (+ energy)

Other food uses include: single-cell protein (mycoprotein) from the fungus Fusarium, cheese ripened by moulds, and soy sauce fermented by Aspergillus. Exams often ask to compare aerobic and anaerobic respiration conditions.

其他食品用途包括：来自真菌镰刀菌的单细胞蛋白（真菌蛋白）、由霉菌成熟的奶酪以及由曲霉发酵的酱油。考试常要求比较有氧和无氧呼吸条件。

9. Microorganisms and Disease | 微生物与疾病

Pathogens are microorganisms that cause infectious diseases. Bacteria cause diseases such as cholera (transmitted via contaminated water) and tuberculosis (spread by airborne droplets). Viruses cause AIDS (HIV, by exchange of body fluids) and influenza. Protoctists cause malaria (Plasmodium, via female Anopheles mosquitoes).

病原体是导致传染病的微生物。细菌引发霍乱（通过受污染的水传播）和结核病（通过空气飞沫传播）。病毒引起艾滋病（HIV，通过体液交换）和流感。原生生物导致疟疾（疟原虫，通过雌性按蚊传播）。

Control of disease can be achieved by hygienic measures, clean water, vaccination, vector control, and quarantine. Testing for specific pathogens is done by culturing samples on sterile nutrient agar.

疾病的控制可以通过卫生措施、清洁饮水、接种疫苗、媒介控制和隔离来实现。特定病原体的检测通过在无菌营养琼脂上培养样本进行。

10. Human Body Defence Against Microorganisms | 人体对微生物的防御

The body has non-specific defences: skin acts as a physical barrier; mucus in the airways traps microbes; cilia waft mucus upward; stomach acid (HCl) kills ingested pathogens; tears contain lysozyme, an enzyme that destroys bacterial cell walls.

人体具有非特异性防御：皮肤作为物理屏障；呼吸道黏液捕捉微生物；纤毛向上摆动黏液；胃酸（HCl）杀灭摄入的病原体；泪液含有溶菌酶，一种破坏细菌细胞壁的酶。

The specific immune response involves lymphocytes. When a pathogen is detected, lymphocytes produce complementary antibodies that bind to antigens on the pathogen, marking it for destruction by phagocytes. Memory lymphocytes remain after infection, providing long-term immunity. This is the basis of vaccination: introducing a harmless form of the pathogen to stimulate antibody and memory cell production.

特异性免疫应答涉及淋巴细胞。当检测到病原体时，淋巴细胞产生互补的抗体，与病原体上的抗原结合，将其标记以供吞噬细胞消灭。记忆淋巴细胞在感染后保留，提供长期免疫力。这是疫苗接种的基础：引入无害形式的病原体，刺激抗体和记忆细胞产生。

11. Antibiotics and Antibiotic Resistance | 抗生素与耐药性

Antibiotics are drugs that kill bacteria or prevent their growth, but they do not affect viruses. Penicillin, for example, inhibits the synthesis of peptidoglycan in bacterial cell walls, causing the cell to burst. They must be used with care to avoid the development of resistant strains.

抗生素是杀死细菌或阻止其生长的药物，但对病毒无效。例如，青霉素抑制细菌细胞壁中肽聚糖的合成，导致细胞破裂。必须谨慎使用以避免产生耐药菌株。

Antibiotic resistance arises by natural selection. Within a bacterial population, a random mutation may give one bacterium resistance. When exposed to an antibiotic, sensitive bacteria die, but resistant ones survive and multiply, passing the resistance gene to offspring, often via plasmids. This leads to the emergence of ‘superbugs’ like MRSA (Methicillin-resistant Staphylococcus aureus). To reduce resistance, doctors avoid prescribing antibiotics for mild infections and patients must complete the full course.

抗生素耐药性通过自然选择产生。在细菌种群中，随机突变可能使某个细菌产生耐药性。当接触抗生素时，敏感细菌死亡，但耐药细菌存活并繁殖，将耐药基因传给后代，通常通过质粒传递。这导致“超级细菌”的出现，例如MRSA（耐甲氧西林金黄色葡萄球菌）。为了减少耐药性，医生避免为轻度感染开抗生素，患者必须完成整个疗程。

12. Culturing Microorganisms: Aseptic Technique | 培养微生物：无菌技术

Exam questions frequently ask about the safe culture of microorganisms. Steps include: sterilise all apparatus (Petri dishes, inoculating loop) by autoclaving or flaming; work near a Bunsen flame to create an updraft; use sterile nutrient agar; transfer a sample using a sterile loop; seal the lid with tape but do not seal completely to allow oxygen. Incubate at a maximum of 25 °C in school labs to prevent the growth of human pathogens, which thrive at 37 °C.

考试经常问到微生物的安全培养。步骤包括：通过高压灭菌或灼烧对所有器具（培养皿、接种环）进行灭菌；在酒精灯火焰附近操作以形成上升气流；使用无菌营养琼脂；用无菌环转移样本；用胶带密封盖子，但不要完全密封以允许氧气。在学校实验室中，最高在25 °C下培养，以防止在37 °C生长的病原体滋生。

After incubation, visible colonies form. Each colony arises from a single microbe. Describe results in terms of colony size, shape, colour and count to compare effectiveness of disinfectants or antibiotic discs (zones of inhibition).

培养后，可见的菌落形成。每个菌落源自一个微生物。根据菌落大小、形状、颜色和计数描述结果，以比较消毒剂或抗生素纸片的效果（抑菌圈）。

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Mastering Microorganisms for IGCSE Edexcel Biology | IGCSE Edexcel 生物：微生物 考点精讲