IB生物 细胞结构 显微镜 膜运输 有丝分裂
IB DP 生物的复习几乎都从细胞开始，因为 A2.2 细胞结构、B2.1 膜与跨膜运输、B2.2 细胞器与区室化、D2.1 细胞与核分裂这四块内容横跨主题 A、B、D 三个主题。IBO 发布的 Biology guide（首次评估 2025）把内容重新编成四个主题，细胞相关内容分散在 Unity and diversity、Form and function、Continuity and change 三处，只按某一份旧笔记的章节顺序背，很容易漏掉整整一块。Paper 1 的数据题喜欢给电镜照片或实验数据，Paper 2 的长答题则要求把结构与功能连成一句话。这篇把细胞部分最常考的十二个点按考纲顺序排开，每个点给出定义、易错处和可以落笔的答法。
Cell biology is where most IB DP students begin, and not only because it comes first: A2.2 cell structure, B2.1 membranes and transport, B2.2 organelles and compartmentalisation, and D2.1 cell and nuclear division sit in three different themes of the guide first assessed in 2025. The exam rewards a structural answer rather than a remembered list, since Paper 1 shows you micrographs and data while Paper 2 asks you to connect a structure to the job it does. What follows is the twelve points that come up most often, in syllabus order, each with the definition, the trap and a usable line of answer.
细胞学说的三条结论与一次演绎
细胞学说的三条结论在 A2.2.1 里写得很具体：细胞是所有生物体的基本结构单位，细胞是生命的最小单位，新细胞只能来自已经存在的细胞。IBO 发布的 guide 在这个知识点上挂了一个方法论要求，要考生理解由理论推出预测的演绎过程。如果一切生物都由细胞构成，那么新发现的生物也应当由一个或多个细胞构成。显微镜下的观察一直在验证这条预测，这也是考纲把它列为探究技能的原因。很多学生以为细胞学说只是历史故事，答题时只写 Hooke 和 Leeuwenhoek 的名字。真正拿分的是把观察、理论、预测三者的关系说清楚，而不是背年份。
Cell theory rests on three claims: cells are the structural unit of every organism, the cell is the smallest unit of life, and new cells arise only from cells that already exist. The guide, first assessed in 2025, attaches a nature-of-science point here, asking you to deduce a prediction from the theory itself. If every organism is built from cells, a newly discovered species should also consist of one or more cells, and microscopy keeps confirming that prediction. Naming Hooke and Leeuwenhoek earns little; explaining how observation, theory and prediction support one another is what the marks are for.
放大倍数、分辨力与尺寸换算
放大倍数与分辨力是 A2.2 里最容易混的两个词，前者说图像被放大了多少倍，后者说两个靠得很近的点还能不能分开。放大倍数的算法只有一个：像的大小除以实物大小，写成 magnification = image size / actual size。算之前先把单位统一，1 毫米等于 1000 微米，1 微米等于 1000 纳米，题目给的数据常常一半在毫米一半在微米。光学显微镜的分辨极限大约 200 纳米，再放大也看不清更细的结构，这就是要换电子显微镜的原因。常见的错误是把目镜测微尺的刻度直接当微米用，忘了先用物镜测微尺标定。考纲还要求会画比例尺、会从电镜照片里辨认细胞类型，这两项属于应用技能而不是背诵。
The two words students mix up most often are magnification and resolution: the first says how much larger the image is, the second says whether two points close together can still be told apart. Magnification is simply image size divided by actual size, so convert every length to the same unit before dividing, since 1 mm is 1000 micrometres and 1 micrometre is 1000 nanometres. A light microscope stops resolving at roughly 200 nm, which is why finer detail needs an electron beam, and an eyepiece graticule means nothing until it has been calibrated against a stage micrometer.
原核细胞与真核细胞的结构清单
原核细胞和真核细胞的差别不是一句有无细胞核能概括的，考纲要的是一份结构清单。原核细胞没有膜包被的细胞核，DNA 呈环状集中在拟核区，质粒是额外的小环状 DNA。原核细胞的核糖体是 70S，真核细胞质中的核糖体是 80S，这个数字在选择题里出现频率很高。原核细胞有细胞壁、荚膜、鞭毛和菌毛，却没有线粒体、内质网、高尔基体这些膜性细胞器。真核细胞的核膜是双层膜并带核孔，染色体是 DNA 与组蛋白结合的产物。很多学生把线粒体和叶绿体里的 70S 核糖体当成真核特征写错，这两处恰好是内共生起源的证据。
Prokaryotes and eukaryotes differ by a checklist of structures rather than by one feature. A prokaryote keeps its circular DNA in a nucleoid with no membrane around it, may carry small circular plasmids, and builds 70S ribosomes, while the eukaryotic cytoplasm uses 80S ribosomes and packs DNA with histones inside a double membrane pierced by pores. Prokaryotes also lack membrane-bound organelles altogether, which is why mitochondria and chloroplasts, each holding 70S ribosomes, are read as evidence for endosymbiosis.
真核细胞的细胞器分工
真核细胞内部的分工可以按膜的层数来记，核、线粒体和叶绿体都是双层膜，内质网、高尔基体、溶酶体是单层膜。细胞核里除了染色体还有核仁，核仁是核糖体亚基组装的场所。线粒体的内膜向内折叠成嵴，把有限的体积换成了更大的膜面积，有氧呼吸的电子传递链就铺在这里。粗面内质网表面挂着核糖体，负责合成分泌蛋白和膜蛋白，滑面内质网负责脂质合成与解毒。高尔基体接收内质网送来的囊泡，对蛋白质做修饰、分选和包装，再送往细胞膜或溶酶体。细胞骨架由微管和微丝组成，既撑住形状，也给囊泡运输提供轨道，微绒毛则是吸收细胞扩大表面积的做法。
Inside a eukaryote, membrane count is a useful way to sort organelles: the nucleus, mitochondrion and chloroplast are each wrapped in two membranes, while the endoplasmic reticulum, Golgi apparatus and lysosomes have only one. The nucleolus inside the nucleus is where ribosomal subunits are assembled, and the inner mitochondrial membrane folds into cristae so that a small organelle still offers a large surface for the electron transport chain. Rough endoplasmic reticulum carries ribosomes for secretory and membrane proteins, the Golgi apparatus modifies and sorts what arrives in vesicles, and the cytoskeleton holds the shape while giving vesicles a track to travel on.
植物、动物与真菌细胞的三套版本
同一个真核细胞模板在植物、动物和真菌身上改出了三套版本，差别集中在细胞壁、液泡、质体和中心粒。植物细胞的细胞壁主要成分是纤维素，真菌的细胞壁是几丁质，动物细胞没有细胞壁。成熟植物细胞有一个大的中央液泡，靠溶质积累维持膨压，动物细胞的液泡小而分散。叶绿体和其他质体只出现在植物细胞里，淀粉储存在质体中，动物的糖原存在细胞质里。中心粒和纤毛、鞭毛的组织方式在动物细胞里最典型，高等植物细胞通常没有中心粒。判断细胞类型的顺序很实用：先看有无细胞壁，再看液泡大小，最后看有无叶绿体和中心粒。
The same eukaryotic blueprint comes in three versions, and the differences cluster around the wall, the vacuole, plastids and centrioles. Plant walls are built mainly from cellulose, fungal walls from chitin, and animal cells have no wall at all, while a mature plant cell fills much of its volume with one large sap vacuole that keeps the cell turgid. Chloroplasts and other plastids belong to plants alone, which is why starch is stored there and glycogen in animal cytoplasm, and centrioles with their cilia and flagella are typical of animal cells.
膜结构：从三明治模型到流动镶嵌模型
膜结构的考点沿着两条模型线走，Davson-Danielli 在 1935 年提出的三明治模型，和 Singer-Nicolson 在 1972 年提出的流动镶嵌模型。三明治模型把蛋白质铺在磷脂双层的两侧，电镜下的深色带是它当时成立的主要依据。冷冻蚀刻和荧光抗体标记推翻了它，前者在膜内部看到蛋白质颗粒，后者证明膜蛋白会横向移动。流动镶嵌模型把膜描述成磷脂与蛋白质的二维液体，蛋白质嵌在脂双层里或浮在表面，整体可以流动。胆固醇夹在动物细胞膜的磷脂尾部之间，温度高时限制流动，温度低时阻止紧密堆积。糖蛋白和糖脂的糖链朝向细胞外，参与细胞识别，这一条常与血型和组织相容性一起考。
Membrane structure is taught as a sequence of two models: the Davson-Danielli sandwich of 1935, which laid protein sheets on both faces of a phospholipid bilayer, and the fluid mosaic published by Singer and Nicolson in 1972. Freeze-fracture electron microscopy revealed protein particles inside the bilayer, and fluorescent antibody tagging showed that proteins drift sideways, so the sandwich could not survive. The fluid mosaic treats the membrane as a two-dimensional liquid in which proteins float among phospholipids, with cholesterol tuning fluidity and sugar chains on glycoproteins facing outwards for recognition.
跨膜运输的四种方式
跨膜运输的四种方式按两把尺子分开，要不要膜蛋白，要不要消耗 ATP。简单扩散靠浓度梯度，氧气、二氧化碳和脂溶性小分子直接穿过磷脂双层。协助扩散同样顺浓度梯度，但极性分子和离子必须走通道蛋白或载体蛋白。主动运输逆浓度梯度，载体蛋白既是酶又是泵，水解 ATP 改变构象把离子推过膜。钠钾泵每消耗一个 ATP，把三个钠离子送出细胞、两个钾离子带回细胞内。渗透是水特有的扩散，水既穿过磷脂双层也走水通道蛋白，方向由水势决定而不是由浓度决定。
Four transport routes are separated by two questions: does the route need a membrane protein, and does it spend ATP. Simple diffusion follows a gradient through the bilayer itself and suits oxygen, carbon dioxide and small lipid-soluble molecules, while facilitated diffusion still runs downhill but needs channel or carrier proteins for ions and polar solutes. Active transport climbs the gradient with a carrier that also acts as an enzyme, and the sodium-potassium pump, moving three sodium ions out and two potassium ions in per ATP, is the standard example.
水势与渗透方向的判断
水势的单位是压强，纯水在标准大气压下被定为 0，加溶质后变成负值。水势由溶质势和压力势相加得到，溶质势总是负的或为零，压力势在动物细胞和膨压的植物细胞里为正。水总是从水势高的一侧流向水势低的一侧，这个方向与溶质浓度的高低刚好相反。动物细胞放进低渗溶液会吸水胀破，放进高渗溶液会皱缩，红细胞是常考的模型。植物细胞有细胞壁，吸水后压力势上升顶住水分继续进入，失水过多则发生质壁分离。常见的错误是拿浓度代替水势做题，遇到带压强的题目就会把方向判反。
Water potential is a pressure, defined as zero for pure water at atmospheric pressure, and adding solute drives it below zero. Solute potential is always negative or zero while pressure potential is positive in a turgid plant cell, and water moves from the higher water potential to the lower one, which is the opposite of moving towards the higher solute concentration. Animal cells swell and burst in a hypotonic solution and shrink in a hypertonic one, whereas a plant cell builds turgor against its wall and plasmolyses only when water loss goes too far.
表面积与体积比：细胞为什么不能无限长大
细胞不能无限长大的原因不是膜太薄，而是表面积的增长赶不上体积的增长。边长增加一倍，表面积变成四倍，体积变成八倍，表面积与体积的比值从六降到三。物质进出只能经过膜，代谢需求却随体积走，比值下降意味着供给跟不上消耗。琼脂块实验把这一点变成了可测的数据，含指示剂的琼脂块浸入酸液后褪色，越小的块褪色越快。用褪色时间或褪色深度比较不同边长即可，不必算出绝对速率也能得出结论。小肠上皮的微绒毛、肺泡和肾小管的折叠都是同一个思路，用结构把表面积做大。
A cell cannot keep growing because surface area rises more slowly than volume, not because its membrane is thin. Doubling the side of a cube multiplies area by four and volume by eight, so the surface-to-volume ratio falls from six to three while metabolic demand keeps rising with volume. Agar blocks carrying an indicator make that testable, since smaller blocks decolourise faster in acid, and microvilli, alveoli and folded tubules are all the same answer applied to tissue.
区室化：膜作为隔断的价值
区室化指的是把膜当成隔断，让不同的反应在各自的空间里进行。溶酶体里装着水解酶，这些酶如果漏到细胞质里会消化细胞自身，膜就是那道保险。同一种细胞里可以同时存在 pH 差异，溶酶体偏酸，细胞质接近中性，酶在各自最适条件下工作。线粒体内膜把质子泵到膜间隙，形成电化学梯度，这是区室化直接创造能量条件的例子。膜面积本身也是资源，粗面内质网和高尔基体的堆叠把有限的体积摊成大量反应表面。区室化还有一个附带好处，反应物和酶被浓缩在同一小空间里，碰撞频率比在稀溶液中高。
Compartmentalisation uses membranes as partitions so that incompatible reactions can run in the same cell at the same time. Lysosomes hold hydrolytic enzymes that would digest the cytoplasm if they leaked, and the pH inside a lysosome differs from the near-neutral cytoplasm, letting each set of enzymes work at its own optimum. The inner mitochondrial membrane shows the same principle turned into energy by pumping protons into the intermembrane space, while stacked rough endoplasmic reticulum and Golgi cisternae convert a small volume into a large reaction surface.
细胞周期与有丝分裂的四个阶段
细胞周期从一次分裂结束算到下一次分裂结束，间期占掉大部分时间，分裂期只是最后一段。间期分三个阶段，G1 期细胞生长并合成蛋白质，S 期完成 DNA 复制，G2 期继续生长并检查复制是否准确。有丝分裂按四个阶段记，前期染色体凝缩、核膜解体，中期染色体排列在赤道板上。后期姐妹染色单体分开并被纺锤体拉向两极，末期核膜重建、染色体解螺旋。胞质分裂紧接着发生，动物细胞靠收缩环缢裂，植物细胞靠细胞板形成新的细胞壁。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，通常写成百分比，用来比较组织增殖的快慢。
The cell cycle runs from the end of one division to the end of the next, and interphase takes most of that time: G1 grows the cell and makes protein, S copies the DNA, and G2 keeps growing while checking the copy. Mitosis itself is counted in four stages, with chromosomes condensing and the nuclear envelope breaking down in prophase, lining up on the equator in metaphase, separating towards the poles in anaphase, and re-forming nuclei in telophase. Cytokinesis then splits the cytoplasm, by a contractile ring in animal cells and a cell plate in plants, and the mitotic index compares dividing cells with the total counted.
细胞周期失控与肿瘤
细胞周期有检查点，DNA 复制出错或纺锤体没接好时，周期会在检查点停下。周期蛋白在特定阶段出现又降解，推动细胞从一个阶段进入下一个阶段。致癌因素包括物理的辐射、化学的烟草成分和生物的某些病毒，它们通过改变控制细胞周期的基因起作用。基因改变累积到一定程度，细胞不再响应停止信号，开始不受控地分裂形成肿瘤。良性肿瘤局限在原位，恶性肿瘤的细胞能侵入周围组织并通过血液或淋巴转移到别处。分裂指数在病理切片里被用来判断增殖活跃程度，数值越高通常意味着生长越快。
Checkpoints pause the cycle when DNA is damaged or when chromosomes are not attached properly to the spindle, and cyclins appear and are destroyed at set stages to push the cell forward. Carcinogens such as radiation, tobacco chemicals and some viruses act by altering the genes that control these checkpoints, so a cell that accumulates enough changes stops responding to stop signals and divides without restraint. A benign tumour stays where it started, a malignant one invades and spreads through blood or lymph, and the mitotic index of a biopsy is read as a measure of how actively the tissue is proliferating.
复习这一块最省时的做法是按膜和结构清单走一遍，再把显微镜计算、水势判断、分裂指数三个计算点单独练到不出错。Paper 1 里数据题常常给一张电镜照片或一组琼脂块数据，Paper 2 的长答题则要求把结构与功能连起来写，两卷的拿分方式并不相同。IA 占最终成绩 20%，满分 24 分，报告上限 3000 词，细胞相关的课题很适合做扩散速率或渗透的实验，选课题时先想清楚变量能不能量化。SL 共 150 教学小时、HL 共 240 小时，这一块内容在两者中都要考，HL 多出的深度主要在膜运输的细节与细胞周期的调控上。
The efficient revision route is to walk the membrane and structure checklists once, then drill the three calculation points, magnification, water potential and mitotic index, until they stop going wrong. Paper 1 hands you a micrograph or a set of agar-block data, Paper 2 asks you to link structure to function in extended writing, and the internal assessment, worth 20 percent and marked out of 24 in a report of up to 3000 words, rewards a cell topic only when the variable can actually be measured. Both levels sit the same cell content, with SL at 150 teaching hours and HL at 240, and the extra HL depth lands on transport detail and cycle control.
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