IB生物 细胞结构 膜运输 有丝分裂 显微镜
细胞层面的内容在 IB 生物里占了很大一块分值，A2.2、B2.1、B2.2 与 D2.1、D2.3 五个子主题都落在细胞这个尺度上，Paper 1B 与 Paper 2A 的数据题也常拿显微照片和实验表格出题。这篇文章按 syllabus 的顺序把细胞结构、膜运输、水势、细胞分裂和与之配套的实验技能串一遍，每个小节都标出对应的子主题编号，方便对照 IBO 官方指南检查自己的漏洞。
Cell-level topics carry a large share of the marks in IB Biology, from A2.2 and B2.1 through B2.2 to D2.1 and D2.3, and the data questions in Paper 1B and Paper 2A often begin with a micrograph or a table of experimental results. This article follows the order of the syllabus, covering cell structure, membrane transport, water potential, cell division and the practical skills that hold them together, with every section labelled by subtopic so you can check your own gaps against the official guide.
A2.2 细胞结构与细胞器分工
细胞学说有三条基本表述：生物体由细胞构成，细胞是生命活动的最小单位，新细胞只能由已经存在的细胞分裂产生。原核细胞没有膜包裹的细胞核，核糖体是 70S，DNA 以环状裸露形式盘绕成拟核，不少种类还带质粒与荚膜。真核细胞把互相打架的生化反应分装到膜性细胞器里，线粒体和叶绿体各有自己的环状 DNA 与 70S 核糖体，这是内共生学说最直接的证据。溶酶体内部的 pH 大约在 5 附近，水解酶在这个酸度下工作，一旦泄漏到细胞质里活性反而下降，分区化让不相容的反应可以同时进行。自由核糖体合成胞内蛋白，附着在内质网上的核糖体合成分泌蛋白与膜蛋白，肽链进入内质网腔折叠，再由高尔基体修饰和分拣。很多学生以为核糖体只待在细胞质里，其实线粒体内部也有核糖体，这一点在 A2.2 的 syllabus 里写得很清楚。常见的错误是把细胞壁当成细胞膜的一部分，植物细胞壁主要由纤维素构成，选择透过性完全由膜负责。
Cell theory rests on three claims: living things are built from cells, the cell is the smallest unit that can carry out the functions of life, and new cells arise only from cells that already exist. Prokaryotes keep their DNA in a nucleoid with 70S ribosomes and no membrane-bound nucleus, while eukaryotes split the same chemistry across compartments, and lysosomal hydrolases tuned to about pH 5 actually lose activity if they leak into the cytosol, which is how incompatible reactions run side by side in one cell. Mitochondria and chloroplasts carry their own circular DNA and 70S ribosomes, the evidence behind the endosymbiotic theory, and ribosomes on the rough endoplasmic reticulum make proteins destined for secretion or for the membrane, with folding in the lumen followed by sorting in the Golgi. Examiners see the cell wall treated as a permeability barrier every year, yet in the syllabus only the membrane controls what enters and leaves.
显微镜放大倍率与分辨率的区别
放大倍率等于像的大小除以实物大小，分辨率指的是能把两个靠近的点分开的最小距离，两者不是同一件事。光学显微镜的分辨极限大约 200 nm，受可见光波长限制，再往上加放大倍率只是把模糊的像放得更大。透射电镜的分辨率能到 0.5 nm 左右，因为它用的电子束波长更短，代价是样品要经过固定、脱水、切片并放在真空里观察，看到的细胞是死的。IB 的实操题常给一张显微照片和一根比例尺，让你算实际长度：先量出像长，换算到同一单位，再除以放大倍率。很多学生把放大倍率乘上比例尺长度，结果数量级差出上百倍。算完记得回看一次单位，微米与毫米之间的换算错一次，这道题的分就基本没了。绘图的规矩也很具体：铅笔单线、不涂阴影、标出放大倍率，标注线不能交叉。
Magnification tells you how many times larger the image is than the specimen, whereas resolution is the smallest gap at which two points still look separate, and only resolution decides how much detail is genuinely visible. A light microscope stalls near 200 nm because of the wavelength of visible light, so pushing the magnification higher merely enlarges a blurred image, while a transmission electron microscope reaches about 0.5 nm with a shorter electron wavelength, at the cost of fixed, dehydrated, sectioned material viewed in a vacuum, which means the cells are dead. Scale-bar questions all follow one route: measure the image, convert to the same unit, divide by the magnification. Students who multiply the scale-bar length by the magnification end up with answers out by a factor of a hundred or more, so check the unit once more after the arithmetic.
B2.1 流动镶嵌模型与被动运输
流动镶嵌模型把膜描述成磷脂双分子层，蛋白质嵌在里面或者横跨整层，胆固醇调节流动性，糖蛋白朝细胞外侧。1972 年 Singer 与 Nicolson 提出这个模型，此后的冷冻蚀刻电镜照片支持了蛋白质分散在脂双层中、而不是铺成一层平膜的判断。简单扩散只让非极性小分子通过，氧气和二氧化碳走这条路，速率与浓度梯度、表面积成正比，与扩散距离成反比。协助扩散靠通道蛋白或载体蛋白，葡萄糖进入红细胞由 GLUT 载体负责，这一过程不消耗 ATP，但载体占满之后速率就饱和。主动运输逆着浓度梯度走，需要 ATP 与泵蛋白，钠钾泵每消耗一个 ATP 把三个钠离子泵出、两个钾离子泵入，是维持静息电位的直接原因。胞吞与胞吐搬运大分子颗粒，膜在这里变形并融合，方向与浓度梯度无关。学生最常把渗透和扩散混成一个概念，渗透特指水跨选择透过膜的移动，溶质并不跟着穿过去。
The fluid mosaic model, published by Singer and Nicolson in 1972, pictures the membrane as a phospholipid bilayer with proteins embedded in it or spanning it, cholesterol tuning fluidity, and glycoproteins facing the outside of the cell; freeze-fracture electron micrographs later showed proteins distributed within the bilayer rather than spread as one flat layer. Small non-polar molecules cross by simple diffusion, where the rate rises with surface area and the concentration gradient and falls as diffusion distance grows. Facilitated diffusion uses channel or carrier proteins such as GLUT in red blood cells, needs no ATP, and saturates once every carrier is occupied. Active transport runs against the gradient on ATP and pump proteins, and the sodium-potassium pump exports three sodium ions and imports two potassium ions for each ATP consumed; endocytosis and exocytosis instead move large particles by deforming and fusing the membrane, independent of any gradient.
渗透与水势 D2.3 的计算
水势衡量单位体积水所含自由能的多少，纯水在标准大气压下定为 0 kPa，加入溶质后水势变小，水因此总是从水势高的一侧移向水势低的一侧。水势等于溶质势加压力势，动物细胞没有细胞壁，放进纯水会一直吸水直到胀破；植物细胞靠细胞壁产生的压力势抵住进一步吸水，达到膨压状态，放进高浓度溶液则发生质壁分离。D2.3 的计算题给出蔗糖溶液的浓度与温度，要求把渗透压换算成水势，注意负号与 kPa 单位。组织块的渗透实验里，把等大小的马铃薯条放进不同浓度溶液，一小时后称质量变化，最接近零变化的那一管说明外部溶液的水势与组织内部相同。常见的错误是用绝对质量而不是百分比变化画图，初始质量不同的小块会把整条趋势拉平。做完实验要写清控制变量：温度、块的大小、浸泡时间与表面吸干的方式都得一致。
Water potential measures the free energy of water per unit volume, fixed at 0 kPa for pure water at standard atmospheric pressure, and because solutes lower it, water always moves from the higher to the lower water potential. It is the sum of solute potential and pressure potential, so an animal cell in pure water swells until it bursts, while a plant cell builds pressure potential against its wall, turns turgid, and plasmolyses in a strong solution. Calculation questions give sucrose concentration and temperature and expect a conversion from osmotic pressure to water potential, keeping the negative sign and the kPa unit. In the classic potato-cylinder practical, equal-sized pieces are weighed before and after an hour in a series of concentrations, so the tube where mass barely changes marks the point where the external solution matches the tissue. Graphing absolute mass rather than percentage change is the mistake that flattens the trend when the cylinders started at different weights.
细胞周期与有丝分裂四个阶段
细胞周期分成间期与分裂期，间期包含 G1、S、G2 三个阶段，DNA 在 S 期复制，这时染色质还是细丝状态，光学显微镜下看不清染色体。有丝分裂通常按前期、中期、后期、末期四步描述：前期染色质凝缩成染色体，核膜开始解体，纺锤体从两极长出；中期染色体排在赤道面上，着丝粒两侧连着纺锤丝；后期姐妹染色单体分开并被拉向两极；末期核膜重建，随后胞质分裂把细胞一分为二。检查点控制周期推进，G1 检查点判断 DNA 是否完好、营养是否充足，过不去就停在 G0 或者走向凋亡。周期蛋白与依赖周期蛋白的激酶在每个阶段起落，这类调控失效时细胞就失去分裂次数的限制。很多学生以为后期最耗时间，其实一个洋葱根尖细胞群里九成以上都停在间期，分裂期各个阶段加起来只占很小一部分。
The cell cycle alternates interphase, which runs through G1, S and G2 with DNA replicated during S while the chromatin is still too fine to resolve under a light microscope, with the division phase itself. Mitosis is described in four steps: chromatin condenses and the nuclear envelope breaks down in prophase as the spindle forms, chromosomes line up on the equator in metaphase with spindle fibres attached at the centromere, sister chromatids are pulled to opposite poles in anaphase, and telophase rebuilds the nuclear envelope before cytokinesis divides the cell. Checkpoints decide whether the cycle continues, the G1 checkpoint can halt a cell in G0 or send it into apoptosis, and cyclins with their kinases rise and fall stage by stage, so when that control fails, cells stop respecting the limit on how many times they may divide.
有丝分裂指数实验与数据题
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，这个指标在医院里用来判断肿瘤的生长速度，也是 IB 数据题常考的量。洋葱根尖压片是标准做法：取 1 cm 左右的根尖，用 1 mol 盐酸在 60 度水浴中解离约五分钟，让细胞壁软化、细胞容易分开，然后染色压片。盐酸处理过久会破坏染色体形态，这一步时间要看着表控制。计数的时候要随机移动视野，从视野边缘按固定方向扫过去，只挑清晰完整的细胞，遇到重叠的细胞团不要硬数。样本量太小是最大的坑，数二三十个细胞算出来的比例波动极大，通常要数到几百个才稳定。阅卷时看到只有百分比而没写原始计数与总细胞数，分析分一般拿不到，写清分母是基本要求。
The mitotic index is the number of cells in the division phase divided by the total number of cells counted, a figure hospitals use to judge how fast a tumour is growing and a favourite in IB data questions. Onion root tips are the standard material, with roughly 1 cm of tip hydrolysed in 1 mol hydrochloric acid at 60 degrees for about five minutes so the walls soften and the cells separate, then stained and squashed, and leaving the acid on too long destroys chromosome structure. Counting has rules of its own, with random fields, a fixed sweep direction, clearly complete cells only, and a sample in the hundreds, since a few dozen cells give a ratio that swings wildly, and a percentage reported without the raw counts rarely earns the analysis mark.
Paper 1B 与 Paper 2A 的答题落点
2025 年 5 月第一次评估的 IB 生物只保留 Paper 1 与 Paper 2，旧版 Paper 3 的选项部分已经取消，IBO 官方指南写得很明确：Paper 1A 是多选题，Paper 1B 是数据题，Paper 2A 是数据题与简答题，Paper 2B 是长答题，外部考试合计占 80%，内部评估占 20%。简答题按点给分，一个得分点对应一句话，写三段废话不如写三条短句。数据题常见的要求是先描述趋势再给出解释，描述用比较级和具体数字，解释要落到机制上，比如温度升高先加快酶促反应，超过最适温度则变性失活。命令词要看清：outline 只要要点，explain 要因果链，compare 必须两边都写，只描一侧拿不到分。内部评估的 3000 字里，研究问题的变量范围与对照设计往往比漂亮的结论更值分。
For first assessment in May 2025 the IB Biology course keeps only Paper 1 and Paper 2, with the old Paper 3 options dropped, as set out in the IBO subject guide: Paper 1A carries multiple-choice questions, Paper 1B data questions, Paper 2A data and short answers, and Paper 2B extended responses, adding up to 80 per cent of the grade with 20 per cent from the internal assessment. Short-answer marks are awarded point by point, so three tight sentences beat a page of prose, and data questions usually ask for a trend plus an explanation, where the description needs comparative figures while the explanation needs a mechanism, such as enzyme activity rising with temperature up to the optimum before denaturation sets in. Command terms decide the shape of an answer, since outline wants the points only, explain wants the causal chain, and compare demands both sides rather than one.
牛津AQA国际GCSE生物?OxfordAQA International GCSE Biology (9201): Onl?进口原版¥399.0📱 去京东购买京东精选
需要一对一辅导？Need One-on-One Tutoring?
IB / A-Level / GCSE 生物与化学全科辅导，历年真题与考纲对照讲解
16621398022 同微信
查看 IB 生物历年真题
或截图后用「扫一扫 → 相册」选择本图