IB生物 细胞结构 膜运输 有丝分裂 检查点
IB 文凭课程的生物从 2025 年首次评估起换了新版大纲，细胞这一块的内容并没有变少，只是换了位置。IBO 在 2023 年 2 月发布的《Biology guide》里，细胞结构放在主题 A 的 A2.2，膜与膜运输放在主题 B 的 B2.1，细胞与细胞核分裂放在主题 D 的 D2.1。旧版大纲把它们归在一个 Topic 1 Cell biology 里，很多学生以为只是编号变了，实际上考题的落点跟着改了。
下面按知识点的顺序，把 IB 生物细胞部分需要写进答案的细节和常见的丢分点逐条过一遍。
Since the first assessment in 2025, IB Biology has been organised into four themes instead of the old numbered topics. Cell structure now sits in A2.2, membranes and transport in B2.1, and cell and nuclear division in D2.1, so the content has not shrunk but the labels and the exam emphasis have shifted.
细胞学说与三个经典例外
细胞学说有三条主张。所有生物都由细胞构成，细胞是生命的最小单位，细胞只能来自已有细胞。施莱登与施旺在 1839 年提出前两条，魏尔啸在 1855 年补上第三条。IBO 大纲要求考生举出三个例外。横纹肌纤维含多个细胞核，伞藻这种巨藻单个细胞可以长到十厘米，无隔菌丝里多个细胞核共用一个连续的细胞质。答这三个例外时要说清它们是单个细胞加上不寻常的结构而不是许多细胞挤在一起。常见的错误是只写名字不写理由，理由分往往整段丢掉。
The three claims of cell theory are that living organisms are made of cells, that the cell is the smallest unit of life and that cells arise only from pre-existing cells; Schleiden and Schwann set out the first two in 1839 and Virchow added the third in 1855. Three exceptions are expected, and each carries a reason: striated muscle fibres hold many nuclei, Acetabularia reaches about ten centimetres as a single cell, and aseptate fungal hyphae share one continuous cytoplasm. An answer that names them without explaining why they still count as cells usually loses the reasoning mark.
表面积与体积比为什么限制细胞大小
细胞靠表面与外界交换物质，需要量却随体积一起增长。边长 1 厘米的立方体表面积是 6 平方厘米，体积是 1 立方厘米，比值 6 比 1。边长变成 2 厘米时比值降到 3 比 1，3 厘米时只剩 2 比 1。交换速率与表面积成正比，内部代谢需求与体积成正比，所以细胞不可能无限长大。吸收面积不够时细胞靠形态补偿。小肠绒毛、肺泡和根毛都在把交换面积摊开。计算题里单位换算最容易出错，1 毫米等于 1000 微米，1 微米等于 1000 纳米。
Exchange happens across the surface of a cell while its needs scale with volume, and the cube model shows the squeeze: a 1 cm cube gives 6:1, a 2 cm cube 3:1 and a 3 cm cube 2:1. Because uptake depends on surface area, absorptive tissues end up folded or branched, which is exactly what villi, alveoli and root hairs are doing. Unit conversion is a steady source of lost marks, so practise moving between millimetres, micrometres and nanometres.
原核细胞与真核细胞的四条分界
原核细胞的直径多在 1 到 5 微米之间，真核细胞多在 10 到 100 微米之间。原核细胞的核糖体是 70S，真核细胞是 80S。原核生物的 DNA 裸露、环状，不与组蛋白结合。真核生物的 DNA 缠在组蛋白上形成核小体，并且装在有膜的细胞核里。有膜的细胞器只出现在真核细胞中，分裂方式也分成二分裂与细胞核分裂。很多学生以为两者只差一层核膜。核对答案时会发现核糖体大小与 DNA 形态同样是判分点。
Size gives the first clue, since prokaryotes measure roughly 1 to 5 micrometres across and eukaryotes 10 to 100; beyond that, prokaryotes carry 70S ribosomes and naked circular DNA, whereas eukaryotes wrap DNA around histones into nucleosomes inside a membrane-bound nucleus. Membrane-bound organelles and nuclear division follow from the same organisation, so a comparison that only mentions the missing nuclear envelope stops at one mark. Ribosome size and DNA form sit in separate rows of the mark scheme, which is why both need a sentence of their own.
光学显微镜与电镜的放大倍数和分辨率
放大倍数的算法是图像尺寸除以实际尺寸，反过来求实际尺寸就用图像尺寸除以倍数。光学显微镜的分辨率大约是 0.2 微米，透射电镜可以到 0.5 纳米。线粒体嵴这类结构只能在电镜图上看到。插图题常用比例尺代替倍数。量出比例尺长度与实际长度就能换算。绘制结构图要用连续单线条、不加阴影，并且标出放大倍数或比例尺。阅卷时常见的是倍数算反，把实际尺寸当成图像尺寸写上去。答这类题的顺序是先量、再换算、最后写单位。
Magnification is image size divided by actual size, and rearranging the same relationship gives the real dimension. A light microscope separates objects about 0.2 micrometres apart, whereas a transmission electron microscope reaches roughly 0.5 nanometres, which is why cristae and ribosomes appear only in electron micrographs. When a question supplies a scale bar instead of a stated figure, measure the bar and convert rather than estimating the factor.
八种主要细胞器与它们的功能
细胞核储存 DNA，并通过核孔与细胞质交换物质，核仁负责组装核糖体。线粒体做有氧呼吸，双层膜的内膜折成嵴，基质里带着自己的环状 DNA 和 70S 核糖体。叶绿体同样双层膜，内部的类囊体堆叠成基粒，光合作用在这里完成。粗面内质网负责蛋白质的运输，滑面内质网合成脂质。高尔基体加工、分选并包装这些产物，溶酶体负责水解废料。核糖体游离在细胞质里，或者贴在粗面内质网上，是蛋白质合成的场所。写功能题时把结构与功能写在同一句里，得分比单背名词稳定。
The nucleus stores DNA and exchanges material through its pores, while the nucleolus assembles ribosomes; mitochondria run aerobic respiration and chloroplasts carry out photosynthesis, both enclosed by a double membrane. The endomembrane system handles the rest, because rough reticulum carries proteins, smooth reticulum makes lipids, the Golgi apparatus modifies and packages them, and lysosomes hydrolyse waste. Free and bound ribosomes are where protein synthesis happens, so link each structure to its job in the same sentence rather than listing names.
流动镶嵌模型与冷冻断裂证据
辛格与尼科尔森在 1972 年提出流动镶嵌模型。磷脂分子一头亲水、一头疏水，在水里自发排成双层，疏水尾部朝内。蛋白质有的嵌在双层内部，有的附着在表面，有的贯穿整个膜。胆固醇插在疏水区，调节膜的流动性和通透性。糖蛋白与糖脂伸向细胞外侧，负责识别、黏着与受体功能。支持这个模型的证据来自冷冻断裂电镜，膜被撕开后蛋白质颗粒在断裂面上分布得不均匀。更早的达夫森与丹尼利模型把蛋白质当成连续的两层，解释不了这些颗粒。很多学生以为磷脂双层的提法来自流动镶嵌模型，其实双层结构在更早的模型里就有了。
Singer and Nicolson proposed the fluid mosaic model in 1972, describing phospholipids that are amphipathic and therefore arrange themselves into a bilayer with the tails facing inward. Proteins sit inside the bilayer, attach to its surface or span it completely, while cholesterol among the tails adjusts fluidity and permeability and glycoproteins on the outer face handle recognition and reception. Freeze-fracture micrographs backed the model because they revealed protein particles scattered unevenly across the split membrane, which the earlier Davson-Danielli sandwich could not explain.
扩散、渗透与水势的方向判断
简单扩散是分子顺着浓度梯度穿过磷脂双层，氧气和二氧化碳都走这条路。渗透是水分子穿过半透膜的过程，判断方向看水势而不是溶质浓度。纯水的水势定为 0 千帕，加入溶质后水势变成负值。水从水势较高的一侧流向水势较低的一侧。水通道蛋白能加快水穿过膜的速度，肾脏和根毛细胞里都比较常见。协助扩散同样顺着梯度，但需要通道蛋白或载体蛋白，葡萄糖和离子走这条路。答渗透题时先写两侧水势的高低，再写水的方向，顺序反了容易被扣分。
Simple diffusion moves molecules such as oxygen and carbon dioxide down a concentration gradient straight through the bilayer. Osmosis is the movement of water across a partially permeable membrane, and its direction follows water potential rather than solute concentration, with pure water set at 0 kPa and any dissolved solute pushing the value below zero. Facilitated diffusion also runs down the gradient, but it needs channel or carrier proteins, and aquaporins speed up water movement in tissues such as kidney and root hair cells.
主动运输与囊泡运输消耗的 ATP
主动运输把物质逆着浓度梯度搬运，必须消耗 ATP。蛋白泵在搬运时自身构象会发生变化。钠钾泵是最常考的模型，每消耗一个 ATP 就把三个钠离子泵出细胞、两个钾离子泵进细胞。神经细胞的静息电位与它直接相关。胞吞与胞吐靠囊泡完成，膜先把物质包起来，再断开或者融合回去。吞入固体叫吞噬，吞入液体叫胞饮，分泌神经递质走胞吐。这一步最容易出错的地方是把方向写反，泵永远把物质从低浓度一侧搬到高浓度一侧。
Active transport moves substances against their concentration gradient and depends on ATP, with the pump protein changing shape as it works. The sodium-potassium pump is the standard example, because one ATP drives three sodium ions out of the cell and two potassium ions in, and that imbalance underlies the resting potential of nerve cells. Endocytosis and exocytosis use vesicles and membrane remodelling, so they also cost ATP and should never be filed under passive transport.
细胞周期里 S 期与分裂期的分工
细胞周期分成间期与分裂期。间期又包含 G1、S、G2 三个阶段。S 期完成 DNA 复制，复制之后每条染色体由两条姐妹染色单体组成，共用一个着丝粒。G1 期细胞生长，并合成复制所需的酶与蛋白质。G2 期继续生长，并且检查复制是否完成。分裂期在整条周期里只占很短的一段。染色体、染色单体、着丝粒这三个词不能混用。复制后一条染色体上有两条染色单体，着丝粒只数一个。
The cell cycle separates interphase from division, and interphase itself runs through G1, S and G2. DNA is replicated during S phase, so each chromosome then consists of two sister chromatids held at a single centromere. Division occupies only a short stretch of the cycle, yet chromosome, chromatid and centromere counts are all judged there, so keep the three terms apart in every answer and remember that one replicated chromosome still counts as one chromosome.
有丝分裂四个阶段的判读要点
前期染色体凝缩变短，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，纺锤丝接到着丝粒上。中期是染色体形态最清楚的一步，数数目也最方便。后期着丝粒分裂，姐妹染色单体被拉向细胞两极，细胞里的染色体数目暂时加倍。末期核膜重新形成，染色体解螺旋。动物细胞靠收缩环完成胞质分裂，植物细胞靠细胞板形成新的细胞壁。识别图题时按染色体形态判断阶段，不要按细胞外形判断。
In prophase the chromosomes condense, the nuclear envelope breaks down and the spindle begins to form. Metaphase lines the chromosomes up on the equator with spindle fibres attached to the centromere, which makes it the clearest stage for counting, while anaphase splits the centromeres and pulls the sister chromatids to opposite poles. Telophase rebuilds the nuclear envelope and separates the two cells, using a cleavage furrow in animals and a cell plate in plants.
检查点失灵与肿瘤形成的关系
细胞周期的推进依赖周期蛋白与周期蛋白依赖激酶。G1 检查点要核对细胞大小、营养供应与 DNA 是否受损。检查点失灵时，带突变的细胞会继续分裂，突变一步步累积，肿瘤往往就是这样形成的。原癌基因与抑癌基因的突变都可能打破这道控制。吸烟、紫外线与某些病毒属于诱变因素，会提高这类突变的概率。D2.1 的另一半是减数分裂。前期 I 的交叉互换与中期 I 的自由组合带来遗传变异，两次分裂把染色体数目减半。比较题里别把减数分裂的同源染色体配对与有丝分裂的中期排列混在一起。
Progression through the cycle depends on cyclins and cyclin-dependent kinases, and the G1 checkpoint checks cell size, nutrient supply and DNA damage. When checkpoints fail, cells carrying mutations keep dividing and collect further damage, and changes in oncogenes or tumour suppressor genes can remove that control on their own, with tobacco smoke, ultraviolet light and some viruses raising the chance of such changes. The other half of D2.1 is meiosis, where crossing over in prophase I and independent assortment in metaphase I generate variation before two divisions halve the chromosome number.
细胞主题的分值结构与数据题读法
IBO 官网给出的分值结构里，2023 年发布、2025 年首次评估的大纲把外部考试定在 80%，内部评估定在 20%。标准级 Paper 1 用时一小时三十分钟、共 55 分，其中 1A 是 30 道选择题，1B 是四道数据题。高级课程 Paper 1 延长到两小时、共 75 分，1A 扩成 40 道选择题。高级课程 Paper 2 用时两小时三十分钟、共 80 分，含数据题、简答题和 Section B 的论述题。内部评估是一项科学探究，按研究设计、数据分析、结论、评价四项评分，每项 6 分。数据题给的是真实实验情境，读图时先看坐标轴单位与对照组的设置。常见的错误是只写结论不写数据支撑，对应的分点就丢掉了。
External assessment carries 80% of the final grade and the internal assessment the remaining 20%. At standard level Paper 1 lasts 1 hour 30 minutes and carries 55 marks, split into 30 multiple-choice questions and four data-based questions, while higher level extends Paper 1 to 2 hours and 75 marks and gives Paper 2 a further 80 marks over 2 hours 30 minutes. The internal assessment is a single scientific investigation marked out of 24 across research design, data analysis, conclusion and evaluation, so practise reading axis units and control settings before drawing conclusions from data.
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