AQA ALevel生物 细胞结构 显微镜 有丝分裂
细胞这一章在 AQA A-Level Biology（7402）里属于第 2 主题，specification 的 3.2 节把它拆成细胞结构、细胞分裂、跨膜运输和免疫四块。Paper 1 覆盖第 1 到第 4 主题，时长两小时，满分 91 分，占整个 A-Level 的 35%。这一章名词密度高，图和计算题混在一起，靠背定义拿不到分。下面按考纲顺序把容易混的知识点拆开讲，显微镜的分辨率与放大倍数、分级分离的顺序、有丝分裂的染色体行为、水势与渗透、免疫的细胞分工都单独成节。每节后面配一段英文，方便对照术语。
Cells is topic 2 of AQA A-Level Biology (7402), and section 3.2 of the specification divides the topic into cell structure, cell division, transport across membranes and the immune system. Paper 1 draws on topics 1 to 4, lasts two hours, carries 91 marks and counts for 35% of the A-Level, so this material sits in the paper where diagram and calculation questions mix with definitions. Learning the glossary alone will not carry you through, because most marks come from explaining a structure in terms of what it does.
光学显微镜与电子显微镜的分辨能力
分辨率和放大倍数是两个不同的概念。分辨率指能区分两个点之间的最小距离，放大倍数只描述图像被放大了多少。光学显微镜用可见光成像，波长把它挡在 200 纳米这个量级。透射电子显微镜用电子束，分辨率可以到 0.1 纳米左右，能看到核糖体和膜的三层结构。扫描电子显微镜擅长看表面形貌，给出立体图像，分辨率不及透射电镜。电子显微镜要求样品脱水、固定并处于真空，只能看死细胞。很多学生以为倍数越高越清楚，实际上超过分辨率极限只是把模糊放大。
Resolution and magnification are not the same thing: resolution is the smallest distance at which two points can still be told apart, while magnification only says how much the image has been enlarged. A light microscope is limited by the wavelength of visible light and stalls at around 200 nm, whereas a transmission electron microscope uses a beam of electrons and reaches about 0.1 nm, enough to reveal ribosomes and the layers of a membrane; a scanning electron microscope shows surface topography in three dimensions but resolves less well than TEM. Light microscopes keep cells alive and in colour at low cost, while electron microscopy needs dead, dehydrated specimens in a vacuum. Examiners regularly meet answers that treat higher magnification as a sharper image, and that confusion costs marks.
放大倍数公式与单位换算
公式是放大倍数等于图像尺寸除以实际尺寸，三个量知道两个就能求第三个。题目常给目镜与物镜的倍数，要求先相乘得到总放大倍数。单位换算是另一个失分点，1 毫米等于 1000 微米，1 微米等于 1000 纳米。图像上量到 40 毫米，实际结构是 20 微米，放大倍数就是 2000 倍。求实际尺寸时要把图像尺寸换成同一单位再除，最后按题目要求写成微米或纳米。常见的错误是拿总放大倍数去乘实际尺寸，方向反了。
Magnification equals image size divided by actual size, so any two of the three values fix the third. Questions often give eyepiece and objective powers separately and expect you to multiply them first, then apply the formula in the right direction, so a 40 mm image of a 20 micrometre structure means a magnification of 2000. Unit conversion is where marks disappear, since 1 mm equals 1000 micrometres and 1 micrometre equals 1000 nanometres, and the final answer must be converted back to the unit the question asks for. Reversing the calculation, multiplying an actual size by the magnification, is one of the most common slips on this topic.
细胞分级分离与超速离心顺序
分级分离的第一步是把细胞放在冷、等渗、缓冲的溶液里匀浆，冷液抑制酶活性，等渗防止细胞器吸水涨破，缓冲液维持 pH。匀浆液先过滤，去掉未破碎的细胞和大的组织碎片。随后做差速离心，转速由低到高，最重的细胞核先沉淀。第二档沉淀线粒体、叶绿体和溶酶体，第三档是内质网碎片，最后高速离心才得到核糖体。每次离心后要保留上清液再提高转速，顺序写反就全错。常见的错误是把核糖体写在第一档，核糖体最小最轻，只有最高转速才能沉淀下来。题目问为什么用等渗溶液时，答出防止细胞器渗透破裂即可。
Fractionation starts with homogenisation in a cold, isotonic, buffered solution: the cold slows enzyme activity, the isotonic medium stops organelles swelling and bursting, and the buffer holds pH steady. The homogenate is filtered to remove intact cells and large debris, then spun in steps of increasing speed; nuclei are heaviest and pellet first, mitochondria, chloroplasts and lysosomes follow, endoplasmic reticulum fragments come next, and ribosomes need the highest speed of all. Keeping the supernatant between spins is what makes the sequence work, and placing ribosomes in the first pellet is a standard error because they are the smallest component on the list.
膜性细胞器的结构与功能对应
细胞核由双层膜包被，膜上有核孔，内部是蛋白质结合的线性 DNA 与一个或多个核仁。线粒体的内膜向内折叠成嵴，扩大面积，基质里有呼吸作用的酶。叶绿体有类囊体堆叠成的基粒和基质，光反应在类囊体膜上进行。高尔基体由扁平的囊泡堆叠而成，负责修饰和分选蛋白质，也能形成溶酶体。溶酶体是单层膜囊，内含多种水解酶，用来分解吞入的物质或衰老的细胞器。核糖体没有膜，是蛋白质合成的场所，游离核糖体合成细胞内用的蛋白，附着在内质网上的核糖体合成分泌蛋白。
The nucleus is enclosed by a double membrane pierced by pores, and holds protein-bound linear DNA plus one or more nucleoli. Mitochondria fold their inner membrane into cristae to increase surface area, with the enzymes of aerobic respiration in the matrix, while chloroplasts stack thylakoids into grana where the light-dependent reactions occur. The Golgi apparatus modifies and sorts proteins and gives rise to lysosomes, which are single-membrane sacs of hydrolytic enzymes that digest ingested material or worn-out organelles; ribosomes carry no membrane and build proteins, free ones for use inside the cell and those bound to rough endoplasmic reticulum for export. Smooth endoplasmic reticulum, which lacks ribosomes, handles lipid synthesis.
蛋白质从核糖体到细胞膜的路径
分泌蛋白的合成从附着在粗面内质网上的核糖体开始。多肽链进入内质网腔后折叠，可能加上糖基，形成糖蛋白。内质网出芽形成运输囊泡，把蛋白质送到高尔基体。高尔基体对蛋白质做进一步修饰和分选，再装进囊泡运往细胞膜。囊泡与细胞膜融合，通过胞吐把内容物释放到细胞外。整个过程消耗 ATP，ATP 由线粒体提供。常见的错误是把高尔基体说成合成蛋白质的场所，它只做修饰、分选和包装。题目给放射性标记追踪时，顺序写成核糖体、内质网、高尔基体、囊泡、细胞膜即可。
A secreted protein begins on a ribosome bound to rough endoplasmic reticulum, and the polypeptide folds inside the lumen, often picking up sugar groups to become a glycoprotein. Transport vesicles bud from the reticulum and carry the protein to the Golgi apparatus, which modifies and sorts it before packing it into vesicles that travel to the cell surface. Fusion with the cell-surface membrane releases the contents by exocytosis, and the ATP for all this movement comes from mitochondria; tracking a radioactive label therefore gives the sequence ribosome, reticulum, Golgi, vesicle, membrane.
原核细胞与病毒的结构差别
原核细胞比真核细胞小得多，细胞质里没有膜性细胞器。它的核糖体更小，没有细胞核，只有一条环状 DNA 游离在细胞质中，并且不与蛋白质结合。细胞壁含肽聚糖，AQA specification 的表述是含 murein 的糖蛋白。病毒不是细胞，没有细胞结构，也不算生物。病毒颗粒由遗传物质、衣壳和附着蛋白组成，遗传物质可以是 DNA 或 RNA。病毒不能自己分裂，它把核酸注入宿主细胞，借宿主的核糖体和酶复制自身。抗生素对病毒无效，因为病毒没有细胞壁和核糖体这些药物靶点。
Prokaryotic cells are far smaller than eukaryotic ones and their cytoplasm has no membrane-bound organelles. Their ribosomes are smaller, there is no nucleus, and the DNA is a single circular molecule lying free in the cytoplasm without associated proteins; the wall contains murein, and many species also carry plasmids, a capsule or flagella. Viruses are acellular and non-living, built from genetic material, a capsid and attachment proteins, and they cannot divide on their own: they inject their nucleic acid and use the host machinery to make more virus particles. Antibiotics fail against them because the drug targets, a bacterial wall or a bacterial ribosome, are simply absent.
有丝分裂五个时期的染色体行为
间期是 DNA 复制和细胞生长的阶段，染色体呈细长的染色质丝，光学显微镜下看不清。前期染色质螺旋化变粗，核膜解体，纺锤体开始形成。中期染色体排列在细胞中央的赤道面上，纺锤丝附着在着丝粒上，这个时期的染色体形态最适合计数。后期着丝粒分裂，姐妹染色单体分开，被纺锤丝拉向两极。末期染色体解螺旋，核膜重建，细胞质分裂完成，形成两个遗传物质相同的细胞。很多学生以为间期什么都没发生，实际 DNA 复制就在这一步。
Interphase covers DNA replication and growth, with chromosomes still long thin threads that a light microscope cannot resolve. During prophase the chromatin condenses, the nuclear envelope breaks down and the spindle starts to form; at metaphase the chromosomes line up on the equator with spindle fibres attached to centromeres, which is the stage best suited to counting. Anaphase splits the centromeres so that sister chromatids are pulled to opposite poles, and telophase rebuilds the nuclei before cytokinesis divides the cytoplasm. When micrographs must be placed in order, finding the neat metaphase line first makes the rest fall into place.
细胞周期与有丝分裂指数的计算
细胞周期包含间期和分裂期，间期占的时间最长，通常超过九成。有丝分裂指数等于处于分裂期的细胞数除以观察到的细胞总数，再乘百分之百。根尖分生区里分裂细胞多，指数高，伸长区几乎看不到分裂。取样要有随机性，同一视野里挑着数会偏。计算时只数细胞核形态清楚的细胞，重叠或破损的不算。很多学生以为指数高说明分裂快，实际它反映的是某一时刻处于分裂期的细胞比例。常见的错误是把间期细胞也算进分子，那样指数会接近百分之百。
The cell cycle consists of interphase and mitosis, and interphase takes by far the longer share. A mitotic index is the number of cells in mitosis divided by the total number of cells counted, expressed as a percentage; root tip meristems give high values because many cells divide there, while the elongation zone shows almost none. Sampling has to be random, since choosing fields or cells that look busy biases the result, and only intact nuclei with clear chromosome morphology should be scored. A high index does not mean faster division, it means a larger proportion of cells happened to be in mitosis at that moment, and counting interphase cells in the numerator pushes the value towards one hundred per cent.
根尖压片的制片与镜检操作
这一步对应 AQA 的 Required Practical 2，材料是洋葱或大蒜的根尖。切取根尖末端约 1 厘米，放入 1 摩尔每升的盐酸中水浴加热约 5 分钟，盐酸使细胞分离、细胞壁软化。取出后用清水漂洗，放到载玻片上，加一滴乙酸地衣红染色 5 分钟。盖上盖玻片，用拇指或铅笔橡皮端轻压，把细胞压成单层。镜检先用低倍镜找到分生区，再换高倍镜观察，压得太厚会看不到清晰的染色体。测细胞实际大小用目镜测微尺或公式计算，先量图像再除以放大倍数。
This is Required Practical 2, and onion or garlic root tips are the usual material. Cut about 1 cm of the tip, heat it in 1 mol per dm3 hydrochloric acid for roughly five minutes so that the cells separate and the walls soften, rinse it, then stain with acetic orcein for five minutes on a slide. A coverslip pressed down gently with a thumb or a pencil rubber spreads the tissue into a single layer, because a thick squash hides the chromosomes; locate the meristem at low power first and switch to high power afterwards. Actual cell size comes from measuring the image and dividing by the magnification.
流动镶嵌模型与四种跨膜方式
细胞膜的基本骨架是磷脂双分子层，亲水头部朝外，疏水尾部朝内。蛋白质镶嵌其中，有的贯穿整个双层，有的只在一侧。糖蛋白和糖脂位于膜外侧，参与细胞识别。胆固醇插在磷脂之间，限制它们的运动，让膜保持稳定。简单扩散适合氧气、二氧化碳这类小分子和非极性分子，顺浓度梯度，不耗能。易化扩散靠通道蛋白或载体蛋白，帮助极性和带电的分子顺梯度通过，同样不耗能。主动运输用载体蛋白并水解 ATP，把物质逆浓度梯度搬运。协同运输借一种物质的浓度梯度带动另一种物质，回肠吸收葡萄糖就靠钠离子梯度。
The framework of every membrane is a phospholipid bilayer with hydrophilic heads facing outwards and hydrophobic tails inwards, studded with proteins that may span the bilayer or sit on one side only. Glycoproteins and glycolipids on the outer surface handle cell recognition, and cholesterol wedges between phospholipids to restrict their movement and steady the membrane. Simple diffusion carries small non-polar molecules such as oxygen and carbon dioxide down a gradient at no energy cost, facilitated diffusion uses channel or carrier proteins for polar and charged substances, and active transport spends ATP to move substances against a gradient; co-transport in the ileum then rides the sodium gradient to bring glucose in.
水势与渗透作用的定量判断
水势衡量水分子从一处移动到另一处的趋势，纯水的水势定为零千帕。溶质越多，水势越负，所以 0.5 摩尔每升蔗糖溶液的水势低于纯水。水从水势高的一侧向水势低的一侧移动，这就是渗透。植物细胞放进低水势溶液会失水，原生质层与细胞壁分离，出现质壁分离。放进高水势溶液则吸水，细胞壁限制体积，产生膨压。水势等于溶质势加压力势，压力势在充分膨胀的细胞里为正值。常见的错误是认为水往溶质浓度高的方向走，方向要按水势判断，不是按浓度直接判断。
Water potential measures the tendency of water to move from one place to another, and pure water is defined as zero kilopascals. Solutes make the potential more negative, so a 0.5 mol per dm3 sucrose solution has a lower water potential than pure water, and water moves from the higher potential to the lower one, which is osmosis. A plant cell in a low-potential solution loses water and the protoplast pulls away from the wall, producing plasmolysis, while a cell in a higher-potential solution takes water in and develops turgor pressure; water potential equals solute potential plus pressure potential. In Required Practical 3 a dilution series is used to find the concentration at which tissue mass stops changing, and that solution's solute potential is taken as the tissue's water potential.
吞噬作用与 T 细胞、B 细胞的分工
免疫应答的起点是抗原被识别。吞噬细胞把病原体吞进吞噬泡，溶酶体释放溶菌酶把它分解，然后把抗原片段呈递到细胞表面。辅助 T 细胞识别呈递的抗原后被激活，它再去刺激细胞毒性 T 细胞、B 细胞和吞噬细胞。细胞毒性 T 细胞识别被病毒感染的细胞或异常细胞，把它们杀死。B 细胞的克隆选择指带上对应抗体的 B 细胞与抗原结合后被选中，分裂分化成浆细胞和记忆细胞。浆细胞分泌抗体，抗体与抗原结合形成复合物，通过凝集和吞噬把病原体清除。常见的错误是把辅助 T 细胞说成直接杀死病原体，它的工作是分泌信号调动其他细胞。
An immune response begins when an antigen is recognised. A phagocyte engulfs the pathogen into a phagosome, lysosomes release lysozymes to digest it, and antigen fragments are displayed on the cell surface; this presentation activates helper T cells, which then stimulate cytotoxic T cells, B cells and more phagocytes. Clonal selection picks out the B cell whose surface antibody fits the antigen, and that cell divides into plasma cells secreting antibody and memory cells that keep the record; antibodies form antigen-antibody complexes that clear pathogens by agglutination and phagocytosis. A frequent error is to credit helper T cells with killing pathogens directly, when their job is signalling.
初次应答、再次应答与疫苗、HIV
初次应答要几天才出现抗体高峰，产生的抗体量少，持续时间短。再次应答由记忆细胞介导，抗体上升快、峰值高、维持久。疫苗把减毒或灭活的病原体、抗原片段注入体内，让人在未感染时先建立记忆细胞，这叫主动免疫。被动免疫直接输入抗体，起效快但没有记忆细胞。群体免疫靠足够比例的人有免疫力，切断传播链，保护不能接种的人。HIV 的遗传物质是 RNA，它带有逆转录酶，能把自己的基因整合进辅助 T 细胞的 DNA。辅助 T 细胞数量下降后，免疫系统无法协调应答，机会性感染出现时就进入艾滋病阶段。抗生素作用于细菌的细胞壁或核糖体，对没有这些结构的病毒不起作用。
A primary response takes several days to peak, produces less antibody and fades quickly. Memory cells drive the secondary response, which rises faster, reaches a higher peak and lasts longer; vaccines exploit this by exposing the body to weakened or inactivated pathogens or antigens so that memory cells form before infection, which is active immunity. Passive immunity injects ready-made antibodies, so protection arrives at once but no memory is left behind, and herd immunity works when enough people are immune to break transmission chains and shield those who cannot be vaccinated. HIV carries RNA and reverse transcriptase, integrates its genes into helper T cell DNA, and once helper T cell numbers fall the immune system can no longer coordinate a response, which is why opportunistic infections mark the move to AIDS. Antibiotics target bacterial walls or ribosomes, structures viruses simply do not have.
Paper 1 的题型与常见扣分点
AQA 官网的 specification 写明，Paper 1 覆盖第 1 到第 4 主题，两小时 91 分，其中 76 分是长短答题，15 分是延伸回答。细胞这一块最容易出现在显微镜计算、有丝分裂图像判断和免疫流程排序上。计算题要求写单位，微米和纳米的换算错一步整题就丢分。延伸回答要有逻辑链，从抗原到辅助 T 细胞再到 B 细胞，中间断一环就扣分。阅卷时最常见的情况是答案方向对了但缺少连接词，比如只写抗体结合抗原，没写随后被吞噬。很多学生以为多写几句总能碰到得分点，实际重复同一句不会加分。AQA 在 2026 年 7 月的说明里提到，海外考点从 2028 年起不再提供 7402 的 A-Level，最后一批 2026 年 9 月开课，最后首考在 2028 年夏季。
The AQA specification states that Paper 1 covers topics 1 to 4, runs for two hours and carries 91 marks, of which 76 marks are short and long answer questions and 15 marks form an extended response. Cell content turns up most often in magnification calculations, stage identification from micrographs and sequencing of the immune response; calculations need units, and one slip between micrometres and nanometres can cost the whole question. Description marks require the name of a structure plus what it does, and an extended response needs an unbroken chain from antigen recognition through helper T cells to B cells. From 2028 AQA will no longer offer the 7402 A-Level to UK overseas centres, with the last cohort starting in September 2026 and the last first sitting in summer 2028.
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