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

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

Microorganisms, or microbes, are tiny living organisms that are invisible to the naked eye and include bacteria, viruses, fungi, and protists. They can be both harmful pathogens and beneficial allies in ecosystems and industry. This guide will cover the key IGCSE Biology topics about microorganisms, from their structure and diseases to their roles in nutrient cycles and food production.

微生物（或称微观生物）是肉眼看不见的微小生物，包括细菌、病毒、真菌和原生生物。它们既可能是有害的病原体，也可以是生态系统和工业中的有益伙伴。本指南将涵盖 IGCSE 生物学中关于微生物的关键考点，从它们的结构和疾病，到它们在营养循环和食品生产中的作用。

1. The Main Groups of Microorganisms | 微生物的主要类群

Microorganisms are classified into several major groups based on their cell structure and function. The four main groups studied in IGCSE are bacteria, viruses, fungi, and protoctists (protists).

根据细胞结构和功能，微生物分为几个主要类群。IGCSE 中学习的四个主要类群是细菌、病毒、真菌和原生生物（原生动物）。

Bacteria are single-celled prokaryotes that lack a true nucleus. They have a cell wall, cell membrane, cytoplasm, and a circular chromosome of DNA, sometimes with plasmids. Some bacteria have flagella for movement.

细菌是单细胞原核生物，缺乏真正的细胞核。它们有细胞壁、细胞膜、细胞质和环状 DNA 染色体，有时还有质粒。一些细菌有用于运动的鞭毛。

Viruses are not considered living by many scientists because they cannot carry out life processes independently. They consist of genetic material (DNA or RNA) enclosed in a protein coat, and they replicate only inside host cells.

病毒不被许多科学家视为生物，因为它们不能独立进行生命活动。它们由遗传物质（DNA 或 RNA）包裹在蛋白质外壳中构成，并且只能在宿主细胞内复制。

Fungi, such as yeast and moulds, are eukaryotic organisms with a nucleus and a cell wall made of chitin. They feed by secreting enzymes onto food and absorbing the digested products (saprotrophic nutrition).

真菌，如酵母和霉菌，是真核生物，有细胞核和由几丁质构成的细胞壁。它们通过将酶分泌到食物上并吸收消化产物（腐生营养）来取食。

Protoctists are a diverse group of mainly unicellular eukaryotic organisms. Examples include Plasmodium (the malaria parasite) which is a protoctist that causes disease.

原生生物是一类多样化的主要单细胞真核生物。例子包括引起疟疾的疟原虫 (Plasmodium)，它是一种致病原生生物。

2. Bacterial Cell Structure and Binary Fission | 细菌细胞结构和二分裂

Bacteria are prokaryotic cells that are much smaller than eukaryotic cells. They have a cell wall made of peptidoglycan, a cell membrane, cytoplasm, ribosomes, and a single circular chromosome located in a region called the nucleoid.

细菌是比真核细胞小得多的原核细胞。它们有由肽聚糖构成的细胞壁、细胞膜、细胞质、核糖体和位于称为类核区域的单条环状染色体。

Some bacteria possess additional structures: a slime capsule for protection, plasmids (small rings of DNA) that can carry antibiotic resistance genes, and flagella for propulsion.

一些细菌具有额外的结构：用于保护的荚膜，质粒（DNA 小环）可携带抗生素抗性基因，以及用于推进的鞭毛。

Bacteria reproduce asexually by binary fission, where the DNA replicates and the cell divides into two genetically identical daughter cells. Under ideal conditions, bacteria can divide every 20 minutes, leading to rapid population growth.

细菌通过二分裂进行无性繁殖，DNA 复制后，细胞分裂为两个遗传上相同的子细胞。在理想条件下，细菌每 20 分钟可分裂一次，导致种群快速增长。

3. Virus Structure and Life Cycle | 病毒的结构和生命周期

Viruses are acellular and consist of a core of genetic material (DNA or RNA) surrounded by a capsid, a protein coat. Some viruses have an additional lipid envelope derived from the host cell membrane.

病毒是非细胞的，由遗传物质核心（DNA 或 RNA）以及包围的衣壳（蛋白质外壳）组成。一些病毒还具有源自宿主细胞膜的额外脂质包膜。

The virus life cycle begins when it attaches to a specific host cell. It injects its nucleic acid into the cell, hijacking the cell’s machinery to produce viral components. New viruses are assembled and burst out, often destroying the host cell.

病毒的生命周期始于附着到特定宿主细胞。它将核酸注入细胞，劫持细胞的机器来生产病毒组分。新病毒组装并爆发出来，常导致宿主细胞被破坏。

Because viruses rely on host cells, they are not affected by antibiotics. Antiviral drugs target stages of the viral life cycle.

由于病毒依赖宿主细胞，它们不受抗生素影响。抗病毒药物以病毒生命周期的各个阶段为目标。

4. Fungal and Protoctist Diseases | 真菌和原生生物疾病

Some fungi can cause diseases in plants and animals. For example, athlete’s foot in humans is caused by a fungus that feeds on skin. In plants, rose black spot is a fungal disease that causes purple-black spots on leaves, reducing photosynthesis.

一些真菌可在动植物中引起疾病。例如，人类的脚癣是由一种以皮肤为食的真菌引起的。在植物中，玫瑰黑斑病是一种真菌病害，导致叶片上出现紫黑色斑点，降低光合作用。

Protoctist pathogens include Plasmodium, which causes malaria. The parasite is transmitted by female Anopheles mosquitoes and infects red blood cells and the liver.

原生生物病原体包括引起疟疾的疟原虫。该寄生虫由雌性按蚊传播，感染红细胞和肝脏。

5. How Pathogens Spread | 病原体如何传播

Infectious diseases can spread through direct contact, airborne droplets, contaminated food and water, vector organisms, and body fluids. The transmission method depends on the pathogen.

传染病可以通过直接接触、空气飞沫、被污染的食物和水、媒介生物以及体液传播。传播方式取决于病原体。

For example, HIV spreads through blood and sexual contact; influenza spreads via airborne droplets from coughs and sneezes; cholera is transmitted through water contaminated with faeces.

例如，HIV 通过血液和性接触传播；流感通过咳嗽和打喷嚏产生的空气飞沫传播；霍乱通过被粪便污染的水传播。

Measures to prevent transmission include good hygiene, clean water supplies, proper sanitation, safe food handling, vaccination, and isolating infected individuals.

预防传播的措施包括良好卫生、清洁供水、适当卫生设施、安全食品处理、疫苗接种和隔离感染者。

6. Physical and Chemical Barriers | 物理和化学屏障

The human body has primary defences that prevent the entry of pathogens. The skin acts as a physical barrier, and sebum produced by skin glands is slightly acidic to inhibit bacterial growth.

人体具有阻止病原体进入的一级防御。皮肤充当物理屏障，皮肤腺体产生的皮脂呈弱酸性，可抑制细菌生长。

Mucus in the respiratory and digestive tracts traps pathogens, and cilia sweep mucus towards the throat to be swallowed. Stomach acid kills most ingested bacteria.

呼吸道和消化道中的黏液捕获病原体，纤毛将黏液扫向喉咙以便吞咽。胃酸杀死大部分摄入的细菌。

Tears contain lysozyme, an enzyme that destroys bacterial cell walls. These nonspecific barriers are the first line of defence.

眼泪含有溶菌酶，一种破坏细菌细胞壁的酶。这些非特异性屏障是第一道防线。

7. The Non-Specific Immune Response | 非特异性免疫反应

When pathogens breach barriers, white blood cells respond. Phagocytes (e.g., neutrophils and macrophages) engulf and digest foreign cells in a process called phagocytosis. They are attracted to infection sites by chemical signals.

当病原体突破屏障时，白细胞做出反应。吞噬细胞（如中性粒细胞和巨噬细胞）通过称为吞噬作用的过程吞噬并消化外来细胞。它们被化学信号吸引到感染部位。

The phagocyte surrounds the pathogen with its cell membrane, forming a phagosome, which fuses with lysosomes containing digestive enzymes to destroy the microbe.

吞噬细胞用细胞膜包围病原体，形成吞噬体，吞噬体与含有消化酶的溶酶体融合以摧毁微生物。

8. Lymphocytes and Antibody Production | 淋巴细胞和抗体产生

The specific immune response involves lymphocytes, which recognise antigens on the surface of pathogens. B-lymphocytes produce specific antibodies that bind to antigens, neutralising the pathogen or marking it for destruction.

特异性免疫反应涉及淋巴细胞，它们识别病原体表面的抗原。B 淋巴细胞产生特异性抗体，这些抗体与抗原结合，中和病原体或标记其以供摧毁。

Memory cells remain after an infection, enabling a faster and stronger response upon re-exposure. This is the basis of vaccination, where a harmless form of the antigen stimulates antibody production and memory cell formation without causing disease.

记忆细胞在感染后留存，使得再次暴露时能产生更快更强的反应。这就是疫苗接种的基础，即使用无害形式的抗原刺激抗体产生和记忆细胞形成而不引起疾病。

Herd immunity occurs when a high percentage of the population is vaccinated, reducing the spread of the pathogen to vulnerable individuals.

当高比例人群接种疫苗时，产生群体免疫，减少病原体向易感个体的传播。

9. Treating Bacterial Infections | 治疗细菌感染

Antibiotics are medicines that kill bacteria or stop their reproduction. They work by targeting bacterial structures, such as the cell wall (e.g., penicillin) or protein synthesis, without harming human cells.

抗生素是杀死细菌或阻止其繁殖的药物。它们通过靶向细菌结构起作用，如细胞壁（如青霉素）或蛋白质合成，而不伤害人体细胞。

Antibiotics are ineffective against viruses because viruses lack the target structures. Misuse and overuse of antibiotics have led to the evolution of antibiotic-resistant bacteria, such as MRSA.

抗生素对病毒无效，因为病毒缺乏目标结构。抗生素的误用和过度使用导致了耐药细菌的进化，如 MRSA。

To reduce resistance, patients must complete the full course of antibiotics, antibiotics should not be used for viral infections, and doctors should prescribe them only when necessary.

为了减少耐药性，患者必须完成整个抗生素疗程，抗生素不应用于病毒感染，医生应仅在必要时开处方。

10. Yeast and Bacteria in Making Food | 酵母和细菌在制作食品中的作用

Yeast (a fungus) is used in bread making and brewing. In anaerobic respiration, yeast converts glucose into ethanol and carbon dioxide:

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

The CO₂ causes bread to rise.

酵母（一种真菌）用于面包制作和酿造。在无氧呼吸中，酵母将葡萄糖转化为乙醇和二氧化碳：

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

CO₂ 使面包膨胀。

Bacteria are used in yogurt production. Lactobacillus bacteria ferment lactose in milk into lactic acid, which lowers the pH and causes milk proteins to coagulate, forming yogurt.

细菌用于酸奶生产。乳酸杆菌将牛奶中的乳糖发酵为乳酸，降低 pH 值，使牛奶蛋白凝固，形成酸奶。

Fermentation is carried out in controlled vessels with maintained temperature and pH to optimise microbial growth and product yield.

发酵在受控容器中进行，维持温度和 pH 值以优化微生物生长和产品产量。

11. Decomposers and the Nitrogen Cycle | 分解者与氮循环

Decomposer microorganisms (bacteria and fungi) break down dead organic matter and waste, releasing nutrients such as ammonium ions back into the soil. This process is known as decay or decomposition.

分解者微生物（细菌和真菌）分解死亡的有机物和废物，将营养物质如铵离子释放回土壤。这个过程称为腐烂或分解。

The nitrogen cycle relies on several types of bacteria:

氮循环依赖几种类型的细菌：

Nitrogen-fixing bacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃) in root nodules of legumes or freely in soil.

固氮菌将大气中的氮（N₂）转化为氨（NH₃），存在于豆科植物根瘤中或土壤中自由生活。
Nitrifying bacteria oxidise ammonium (NH₄⁺) to nitrites (NO₂⁻) then to nitrates (NO₃⁻), which plants can absorb.

硝化细菌将铵（NH₄⁺）氧化为亚硝酸盐（NO₂⁻），然后氧化为硝酸盐（NO₃⁻），植物可吸收。
Denitrifying bacteria convert nitrates back to nitrogen gas, completing the cycle.

反硝化细菌将硝酸盐转化为氮气，完成循环。

12. Investigating Microorganisms Safely | 安全研究微生物

In the lab, microorganisms are cultured on agar plates containing nutrients. Aseptic technique is essential to prevent contamination by unwanted microbes and to ensure safety.

在实验室中，微生物在含有营养物的琼脂平板上培养。无菌技术对于防止不需要的微生物污染和确保安全至关重要。

Steps include sterilising equipment and media by autoclaving, working near a Bunsen burner to create an updraft, flaming the neck of bottles, and sealing petri dishes with tape but not fully (to allow oxygen for aerobic bacteria while preventing entry of contaminants).

步骤包括通过高压灭菌消毒设备和培养基，在本生灯附近工作以产生上升气流，灼烧瓶口，并用胶带密封培养皿但不完全封闭（为好氧细菌提供氧气同时防止污染物进入）。

In schools, incubation is usually done at 25°C, not 37°C, to discourage the growth of human pathogens. Results are observed as colonies, and zones of inhibition can be measured when testing antibiotics.

在学校，培养通常在 25°C 下进行，而不是 37°C，以抑制人类病原体的生长。结果以菌落形式观察，测试抗生素时可以测量抑菌圈。

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