OCR生物 ALevel 病原体 免疫 疫苗 抗生素
OCR A Level 生物 A 的疾病内容集中在模块 4.1，卷面落点是 H420/02 Biological diversity。这一卷 2 小时 15 分、100 分，占整个 A Level 的 37%，同时抽考模块 1、2、4 与 6，Section A 是 15 分的选择题，Section B 才是结构题与长答题。疾病部分很少单独成题，通常与免疫应答、疫苗或抗生素风险拼在同一道题里。下面按 OCR 官方 spec 的 4.1.1 条目号顺序，把病原体类型、传播途径、植物与动物防御、特异性免疫、抗体与疫苗逐条拆开，每一条后面接上阅卷时真正给分的措辞。
Disease content in OCR A Level Biology A sits in Module 4.1 and reaches the exam through Paper 2, the Biological diversity paper, which runs 2 hours 15 minutes for 100 marks and 37% of the qualification while sampling Modules 1, 2, 4 and 6. Questions on disease rarely stand alone, and they usually share a structured question with immunity, vaccination or antibiotic risk. The sections below follow the 4.1.1 learning outcomes of the OCR specification and keep the examiners' wording alongside each point.
疾病板块在 H420/02 上的位置与分值
疾病这一块属于 OCR 模块 4.1，考试只可能出现在 H420/02 Biological diversity 上。这一卷 2 小时 15 分、100 分，占整个 A Level 的 37%，同时考模块 1、2、4 和 6。官方 examiner report 写明，这一卷对数学技能的要求约占 10%，所有 A Level 生物卷都是这个比例。Section A 是 15 分的选择题，疾病内容在这里多以术语辨析出现，Section B 才是结构题与长答题。选择题只认术语，题干说的是 droplets 而你把传播途径写成 vector，这一分就没了。结构题的分值分布很碎，常见的是 2 分填空加一道 4 分的解释题，解释题要写出机制而不只是结果。长答题按水平给分，写满四行但没有出现 phagosome 与 lysosome 这类关键词，通常只落在中档。
Module 4.1 sits inside Paper 2, the Biological diversity paper, which OCR sets at 2 hours 15 minutes and 100 marks worth 37% of the A Level; it samples Modules 1, 2, 4 and 6, and the examiners' report for the autumn 2020 series records that mathematical skill stays at roughly 10% of the marks across all A Level Biology papers. Section A is a 15-mark multiple choice section where disease content appears as term discrimination, while Section B carries the structured and extended response questions; a term written loosely gains nothing, and an extended response that never names the phagosome or the lysosome tends to settle in the middle band.
四类病原体与各自的代表疾病
OCR 把病原体分成细菌、病毒、原生生物与真菌四类，每一类都要能举出具体疾病。细菌这一栏是肺结核、细菌性脑膜炎与马铃薯环腐病，病毒这一栏是 HIV 与艾滋病、动物流感以及烟草花叶病毒，原生生物对应疟疾与马铃薯和番茄晚疫病，真菌对应香蕉黑条叶斑病、牛的金钱癣与人的脚气。常见错误是把疟疾归到病毒里，疟疾的病原体是疟原虫，属于原生生物，靠按蚊传播。细菌一栏的环腐病要写出马铃薯与番茄这些作物名称，只写 ring rot 拿不到那一分。术语拼写也会影响给分，Tobacco Mosaic Virus 缩写 TMV 在选择题里可以直接用，结构题里第一次出现时最好写全称。分类题很少单独考，通常是先用题干信息判断类别，再解释对应的传播方式。这类题的失分点集中在类别与疾病对不上，四类里写对三类错一类就是一半分。
OCR groups pathogens into bacteria, viruses, protoctista and fungi, and each group has to come with named diseases: tuberculosis, bacterial meningitis and ring rot for bacteria; HIV and AIDS, influenza in animals and Tobacco Mosaic Virus for viruses; malaria and potato or tomato late blight for protoctista; black sigatoka, ringworm in cattle and athlete's foot for fungi. A frequent slip is filing malaria under viruses when its agent is a protoctist carried by the Anopheles mosquito, and naming the crop is safer than writing ring rot on its own. Written answers reward the full name first with the abbreviation afterwards, so Tobacco Mosaic Virus plus TMV reads better than TMV alone.
传播途径题的三类落笔
传播途径题按 OCR 的写法分直接与间接两条线，间接这条线里矢量、孢子与生活条件各自成点。矢量指的是把病原体从一个宿主带到另一个宿主的生物，按蚊传疟疾、蚜虫传植物病毒都属于这一类，答 vector 时要点明中间宿主是谁。孢子这条线多用在真菌上，黑条叶斑病的孢子随风雨落到叶片上，潮湿与温度决定萌发速度。生活条件包括气候与社会因素，人口密集、卫生条件差与医疗资源不足都会抬高传播速率，题干给了数据就用数据说话，只写 living conditions 换不到那一分。动物传染病的直接传播包括接触与飞沫，植物侧的直接传播多通过伤口与嫁接。阅卷时看重的是机制链是否完整，缺掉传播媒介那一环，后面的分析也一起丢分。写这类题最省事的结构是先判定直接还是间接，再指名媒介，再写影响传播速率的条件。
Transmission questions split along direct and indirect lines, and within the indirect line vectors, spores and living conditions are each worth separate marks; a vector is the organism that carries a pathogen from one host to another, so naming the Anopheles mosquito or an aphid matters more than the word itself. Spores belong mainly to fungal examples, where the black sigatoka spores arrive with wind and rain and humidity sets the germination rate; living conditions cover climate alongside social factors such as crowding and poor sanitation, and when the question supplies figures those figures have to be quoted. Direct transmission runs through contact and droplets in animals and through wounds or grafting in plants, so an answer missing the agent in the chain loses the later marks as well.
植物防御 化学物质与结构反应
植物没有抗体，防御靠化学物质与结构反应两条路。结构侧包括蜡质角质层、纤维素细胞壁、气孔关闭与伤口处的愈伤组织，这些都能物理阻断病原体进入。化学侧要写清楚三类物质：具有抗菌作用的酚类与生物碱、能降解病原体细胞壁的酶，以及触发局部细胞死亡的活性氧。愈伤沉积是 OCR 明确点名的机制，葡聚糖在筛板与胞间连丝处堆积，既堵住营养通道又挡住病原体扩散。木质素与木栓质在感染点周围沉积，形成一道加硬的屏障，这道屏障属于局部反应，不改变整株植物的结构。染病部位的组织坏死也是防御，坏死的细胞把病原体困在一小块区域，代价是局部叶片失去光合能力。答题时把化学与结构分开写，两类都答到通常能拿到全分，只写蜡质层属于最浅的一层。
Plants have no antibodies, so defence runs through chemistry and structural responses instead; the structural side covers the waxy cuticle, the cellulose cell wall, stomatal closure and the new tissue laid down at a wound, all of which block entry. On the chemical side three groups are expected, namely antimicrobial phenols and alkaloids, enzymes that break down pathogen cell walls, and the reactive oxygen that triggers localised cell death. Callose deposition is the mechanism OCR names outright, with glucans building up at sieve plates and plasmodesmata to seal the nutrient route and slow the pathogen, and necrosis at the infection site traps the pathogen at the cost of local photosynthesis.
动物非特异防御的六个入口
动物的第一层防御不需要识别具体抗原，OCR 列了皮肤、血凝、伤口修复、炎症、排斥反射与黏膜六个入口。皮肤与黏膜主要靠物理阻挡，黏膜还分泌溶菌酶，呼吸道与消化道里的黏液把病原体粘住后由纤毛或咳嗽排出。血凝反应从血小板开始，血小板释放物质启动级联，最终把纤维蛋白原变成纤维蛋白，纤维蛋白结成网把血小板一起网住形成血块。炎症的四个典型表现是红、肿、热、痛，机制是组胺让局部血管扩张、通透性升高，更多吞噬细胞因此到达感染点。伤口修复要写出胶原沉积与组织重塑两步，这两步在结构题里各占一分。这类题最常见的是把非特异防御与特异性免疫混在一起写，题干问第一层防御时提到抗体，那一分反而拿不到。答满三到四分的组合通常是皮肤、黏膜、血凝与炎症，写四个够用，多写不加分。
Animal defences at this level need no antigen recognition, so OCR lists skin, blood clotting, wound repair, inflammation, expulsive reflexes and mucous membranes. Skin and mucous membranes block entry physically, and the mucus in the airways and gut traps pathogens before cilia or a cough clear them; clotting starts with platelets releasing substances that set off a cascade ending in fibrin, which forms a net trapping platelets into a clot. Inflammation shows as redness, swelling, heat and pain because histamine widens local vessels and raises their permeability, and wound repair adds collagen deposition followed by tissue remodelling; mixing this layer with antibodies when the question asks for the first line of defence is the usual way to drop the mark.
吞噬细胞的步骤与血液涂片
吞噬细胞这条线要按步骤写，OCR 点名的细胞是中性粒细胞与抗原呈递细胞。中性粒细胞先靠趋化作用被吸引到感染点，病原体表面的抗体或补体作为调理素把它标记出来，这一步叫调理作用。细胞膜内陷把病原体包进吞噬体，溶酶体与吞噬体融合形成吞噬溶酶体，水解酶把病原体分解成碎片。分解后的抗原片段被呈递到细胞膜表面，抗原呈递细胞因此得名，T 细胞的活化就接在这一步后面。细胞因子是这条链上的信号分子，白细胞介素负责在细胞之间传递信息，结构题问细胞信号的作用时写白细胞介素最稳。PAG1 要求考生会看血液涂片并画出观察到的细胞，中性粒细胞的多叶核是辨认特征，淋巴细胞则是一个大而圆的核。写这类题最容易漏掉融合这一步，只写吞噬就会丢掉机械细节的分。
Phagocyte answers are marked step by step, and OCR names the neutrophil together with antigen-presenting cells; the neutrophil is drawn in by chemotaxis, and antibodies or complement on the pathogen surface act as opsonins that flag it for engulfment. The membrane then invaginates to form a phagosome, a lysosome fuses with it to make a phagolysosome, and hydrolytic enzymes break the pathogen into fragments that are displayed on the cell surface, which is why the cell is called an antigen-presenting cell. Cytokines carry the signal onward, with interleukins the safest term to quote when a question asks about cell signalling, and PAG1 expects candidates to recognise a neutrophil from its lobed nucleus and a lymphocyte from its single large round nucleus.
B 细胞与 T 细胞的克隆选择
特异性免疫的起点是克隆选择，OCR 要求写出克隆选择与克隆扩张两步的名称。体内预先存在各不相同的 B 细胞与 T 细胞，抗原只与表面受体形状互补的那一个结合，结合就是选择，这一步不涉及新细胞的产生。被选中的细胞随后分裂成一群相同的细胞，这就是克隆扩张，扩张出来的细胞再分化成效应细胞与记忆细胞。T 细胞分三类，辅助 T 细胞释放白细胞介素激活 B 细胞与杀伤 T 细胞，杀伤 T 细胞直接攻击被感染的细胞，调节 T 细胞则抑制免疫反应、防止自体损伤。B 细胞的效应形态是浆细胞，浆细胞专门合成并分泌抗体，抗体进入体液后完成体液免疫。细胞信号这一条在 OCR 的评分方案里反复出现，白细胞介素是唯一必须记住的例子。答题时把选择与扩张分成两句写，漏掉扩张那一步通常扣一分。
Specific immunity begins with clonal selection, and OCR expects both clonal selection and clonal expansion to be named; the body already holds a huge variety of B and T cells, and an antigen binds only the receptor whose shape complements it, which is the selection step rather than the creation of a new cell. The selected cell then divides into a clone of identical cells, and those cells differentiate into effector cells and memory cells. T cells come in three roles, with helper T cells releasing interleukins that activate B cells and killer T cells, killer T cells destroying infected cells directly, and regulator T cells damping the response so self tissue is spared; B cells turn into plasma cells that synthesise and secrete antibodies, and interleukins are the one signalling example worth memorising by name.
初次应答 再次应答与记忆细胞
初次应答与再次应答的差别可以用抗体浓度曲线说明，OCR 要求写出 T 记忆细胞与 B 记忆细胞。初次接触抗原后有一段潜伏期，抗体浓度上升慢、峰值低、维持时间短，因为参与反应的细胞数量有限。记忆细胞在初次应答中形成并留在体内，再次遇到同一抗原时被迅速激活，克隆扩张的起点数量大得多。再次应答的抗体浓度上升更快、峰值更高、维持更久，IgG 的比例也更高，这条曲线在结构题里经常以图的形式给出。疫苗的加强针就是利用这条曲线，第二针之后抗体水平抬升的幅度通常比第一针明显。写这类题常见的错误是把记忆细胞与浆细胞混为一谈，浆细胞分泌抗体但不负责长期记忆，记忆细胞不分泌抗体。曲线题的分值往往落在两个比较点，比速度和比峰值，各用一句写清楚就够。
The gap between the primary and secondary response is easiest to describe with an antibody concentration curve, and OCR expects T memory cells and B memory cells to be named; the first exposure brings a lag phase, then a slow, low and short-lived rise because few cells are involved. Memory cells formed during that first response stay in the body, and on a second encounter with the same antigen they are activated quickly from a much larger starting population. The secondary response therefore rises faster, peaks higher and lasts longer with a greater share of IgG, and booster vaccinations work on exactly this curve because the rise after a second dose is usually steeper than after the first; confusing plasma cells with memory cells is a common error, since plasma cells secrete antibody while memory cells carry the record.
抗体结构与三种作用
抗体是蛋白质，OCR 只要求写出一般结构，不要求背氨基酸序列。一个抗体分子由四条多肽链组成，两条重链与两条轻链通过二硫键连成 Y 形，两条臂顶端的可变区形状各不相同，这就是抗体专一性的来源。恒定区在同类抗体之间差别很小，它决定抗体属于哪一类，也决定抗体与吞噬细胞的结合能力。抗原结合发生在可变区与抗原决定簇之间，形状互补才能结合，这个措辞在评分方案里反复出现。抗体本身不杀死病原体，它的三种作用是调理作用、凝集作用与中和毒素：调理素给病原体做标记让吞噬细胞更容易吞，凝集素把多个病原体连成团限制扩散，抗毒素结合细菌毒素阻止它作用于宿主细胞。写答案时把三种作用各配一句机制说明，只写名称在结构题里只给一分。
Antibodies are proteins, and OCR asks only for the general structure rather than a sequence; the molecule has four polypeptide chains, two heavy and two light, joined by disulphide bonds into a Y shape whose two variable regions differ in shape and give the antibody its specificity. The constant region varies little within a class, which is what assigns the antibody to its class and determines how well it binds phagocytes, while binding itself happens between the variable region and an antigen's epitope and needs complementary shapes. Antibodies do not kill anything directly: opsonins mark a pathogen so phagocytes take it in more readily, agglutinins clump pathogens together and limit their spread, and anti-toxins bind bacterial toxins before they can act on host cells, so each of the three needs a mechanism sentence rather than a bare name.
主动被动 自然人工四类免疫与疫苗
免疫力按获得方式分四类，OCR 要求主动与被动、自然与人工两两组合都能举例。自然主动免疫是感染后自身产生抗体并留下记忆细胞，自然被动免疫是抗体通过胎盘或初乳从母亲传给婴儿。人工主动免疫就是疫苗，注射抗原让身体自己产生抗体与记忆细胞，人工被动免疫是直接注射抗体血清，用于被毒蛇咬伤或暴露后的紧急处理。被动免疫见效快但维持时间短，因为没有记忆细胞，这一对比在结构题里几乎每年都出现。疫苗的原理是引入抗原而不引起疾病，常见做法包括减毒、灭活与只取抗原蛋白，程序会调整的原因包括病原体变异、群体免疫水平变化与全球供应问题。写疫苗题时把群体免疫的门槛说清楚，接种比例够高时病原体找不到足够的易感宿主，传播链就会断。四类免疫的判断常以情境题出现，先问来源是主动还是被动，再问是自然还是人工，两步都对才给分。
Immunity comes in four combinations, and OCR wants active and passive paired with natural and artificial, each with an example. Natural active immunity follows an infection that leaves memory cells behind, while natural passive immunity passes antibodies from mother to infant across the placenta or in colostrum; artificial active immunity is vaccination, where antigen is introduced so the body makes its own antibodies and memory cells, and artificial passive immunity injects antibody serum for emergencies such as a snake bite or post-exposure treatment. Passive immunity acts quickly but fades because no memory cells are made, a contrast that appears almost every series, and questions on vaccination programmes expect the reasons for change, namely pathogen variation, shifts in herd immunity and global supply, with the herd immunity threshold explained by too few susceptible hosts left for the pathogen to keep a transmission chain going.
抗生素风险 药物来源与长答评分
抗生素只对细菌有效，作用靶点包括细胞壁合成、蛋白质合成与核酸复制，对病毒与真菌感染无效。青霉素抑制细胞壁的肽聚糖交联，细菌在低渗环境里吸水胀破，人体细胞没有细胞壁因此不受影响。风险有两层，一层是个体层面的过敏反应与肠道菌群失衡，另一层是群体层面的耐药性，滥用会让敏感菌株被淘汰、耐药菌株大量繁殖。耐药基因还能通过质粒在细菌之间横向转移，这一条常与自然选择的解释题连在一起考。药物来源这一条在 H420 spec 的 4.1.1（m）里单独列出，青霉素来自青霉菌、阿司匹林的有效成分来自柳树皮、抗癌药长春碱来自长春花，这些例子同时说明为什么要维持生物多样性。长答题按水平给分，判分看的是解释链是否完整，从变异来源写到选择压力再写到繁殖结果，缺一环就掉一档。这类题的失分点集中在只写结果不写机制，写耐药就够，但要写明耐药菌株是被选择出来的。
Antibiotics act on bacteria only, targeting cell wall synthesis, protein synthesis or nucleic acid replication, and they do nothing against viruses or fungi; penicillin blocks the cross-linking of peptidoglycan so the bacterium takes in water in a hypotonic medium and bursts, while human cells have no cell wall to lose. The risks sit on two levels, with allergic reactions and gut flora disruption for the individual and resistance for the population, because heavy use kills susceptible strains and lets resistant ones multiply. Resistance genes also move sideways between bacteria on plasmids, which links neatly to natural selection questions: penicillin from Penicillium, the active compound in aspirin from willow bark and the anticancer drug vincristine from the periwinkle all come from living organisms, and extended response marks depend on a complete chain of variation, selection pressure and reproduction rather than a bare statement that resistance appears.
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