OCR GCSE生物 细胞结构 显微镜 酶 有丝分裂
OCR GCSE Biology A（Gateway Science，代码 J247）把细胞层面的内容分在 B1 Cell level systems 与 B2 Scaling up 两个模块，2016 年 9 月起施教，第一次外部考试在 2018 年 6 月。这一块的题型集中在三种：给一张细胞图或显微照片让你指认结构并写出功能，给放大倍数与实际长度让你做单位换算，或者给一组实验数据让你解释变化趋势。八个知识块下面按 spec 的顺序排开，每块都落到具体的结构、公式和数字上。看完拿一张空白纸，把动物细胞、植物细胞、细菌细胞三种图各画一遍，画不出来的部分就是需要回头补的地方。
OCR GCSE Biology A, Gateway Science, carries the code J247 and places cell biology across two modules, B1 Cell level systems and B2 Scaling up; it was first taught from September 2016 with first assessment in June 2018. Questions in this area arrive in three shapes: label a diagram or micrograph and state the function, convert between image size and real size using magnification, or explain a trend in experimental data. The eight blocks below follow the order of the specification and stay with structures, formulas and units; drawing an animal, a plant and a bacterial cell from memory is a quick way to find whatever has not been learned.
亚细胞结构与其功能
动物细胞里被反复考到的结构有五个：细胞核、细胞质、细胞膜、线粒体、核糖体。细胞核里装着染色体，染色体上的 DNA 控制细胞的活动并决定蛋白质合成的顺序。细胞质是化学反应发生的场所，核糖体悬在其中，它就是蛋白质合成的位点。线粒体做有氧呼吸，把葡萄糖和氧气转成 ATP，肌肉细胞和精子里的线粒体数量特别多，因为这两类细胞耗能高。细胞膜把细胞内外隔开，同时控制物质进出，小分子和部分离子能通过而大分子不能，所以它被描述成部分透过性。植物细胞在这五个结构之外还多三样：纤维素构成的细胞壁、含叶绿素的叶绿体、装满细胞液的永久液泡，细胞壁提供支撑，叶绿体做光合作用，液泡把细胞撑挺并储存溶解物。细菌细胞的差别最大，它没有细胞核，只有一条环状 DNA 悬在细胞质里，另外带质粒，也没有线粒体和叶绿体。很多学生容易把细菌的细胞壁当成纤维素，其实它的成分与植物细胞壁不同，这一条常出现在比较题的小题里。
Five structures come up again and again in an animal cell: the nucleus, the cytoplasm, the cell membrane, mitochondria and ribosomes. The nucleus holds the chromosomes, whose DNA controls the cell's activities and the order in which proteins are built; the cytoplasm is where those reactions take place, with ribosomes suspended in it as the site of protein synthesis; mitochondria carry out aerobic respiration, turning glucose and oxygen into ATP, and they are unusually numerous in muscle cells and sperm cells because both use a great deal of energy. The cell membrane separates the inside from the outside and controls what passes through, letting small molecules and some ions across but not large ones, which is why it is described as partially permeable. A plant cell adds a cellulose cell wall for support, chloroplasts containing chlorophyll for photosynthesis, and a permanent vacuole filled with cell sap that keeps the cell firm; a bacterial cell differs most sharply, having no nucleus, only a single loop of DNA plus plasmids, and no mitochondria or chloroplasts, so its cell wall is not cellulose and a comparison question usually turns on exactly that point.
显微镜成像与放大倍数计算
光学显微镜用两组透镜让光穿过薄切片，最大放大倍数约 x1500，分辨率约 200 nm；电子显微镜用电子束代替光，分辨率可到 0.2 nm 左右，穿透式电镜看内部结构，扫描电镜看表面的立体形貌。放大倍数的公式是 放大倍数 = 像的长度 / 实物长度，反过来求实物长度时用 像的长度 / 放大倍数。单位换算是这类题最容易失分的地方：1 mm = 1000 µm，1 µm = 1000 nm，换错一次整道题的数字都跟着错。算例：某细胞在 x1500 下量得像长 30 mm，实物长度 = 30 mm / 1500 = 0.02 mm = 20 µm。反向的题也会考，实物 50 µm 配放大倍数 x400，像长就是 50 x 400 = 20000 µm = 20 mm。切片的质量直接影响读数，盖玻片要一侧先接触水滴再慢慢放下，否则里面留气泡，气泡边缘是粗黑圈，与细胞结构的形状完全不同。OCR 官方 spec 要求学生用显微镜观察并绘制动植物细胞，绘图题的放大倍数要写成 x400 这样的乘法结果，不要只写物镜的倍数。
A light microscope passes light through a thin specimen using two sets of lenses, reaching a maximum magnification near x1500 with a resolution of about 200 nm, while an electron microscope uses a beam of electrons instead and resolves down to roughly 0.2 nm; transmission instruments show internal structure and scanning instruments show three-dimensional surface detail. Magnification is the image length divided by the real length, so real length is the image length divided by magnification. Unit conversion is where most marks are dropped, since 1 mm equals 1000 µm and 1 µm equals 1000 nm, and one slip carries the wrong number through every later step. A cell measuring 30 mm on an image taken at x1500 is therefore 0.02 mm, or 20 µm, in real life, and a 50 µm object at x400 gives an image of 20000 µm, which is 20 mm; a cover slip lowered at an angle from one edge keeps air bubbles out, and their thick black outlines look nothing like a cell structure.
酶与活性位点
酶是蛋白质，起生物催化剂的作用，它降低反应需要的活化能，反应前后自身不改变。酶分子上有活性位点，形状与底物互补，底物结合后被转成产物，这就是锁钥模型的说法。专一性来自形状，一种酶通常只催化一种底物，所以淀粉酶水解淀粉、蛋白酶水解蛋白质。温度影响反应速率：温度升高时分子运动加快，碰撞机会增多，速率上升；超过最适温度后活性位点开始变形，酶变性失活，这一步不可逆。人体内多数酶的最适温度约 37 摄氏度，偏离太远速率就掉下来。pH 同样作用在活性位点上，胃蛋白酶最适 pH 约 2，小肠里的酶最适 pH 约 8，环境过酸或过碱都会让酶变性。底物浓度也是考点：底物少时速率随底物浓度上升，底物足够多以后活性位点全部被占满，曲线走平不再上升。OCR 官方 spec（代码 J247，2016 年 9 月起施教）把酶的作用放在 B1 Cell level systems 里，实验题常用碘液褪色时间或气体产生量来间接测反应速率，读数间隔要均匀，只改一个变量。
Enzymes are proteins that act as biological catalysts, lowering the activation energy a reaction needs while coming out of it unchanged; each enzyme has an active site whose shape is complementary to its substrate, which is the lock and key model. Specificity comes from that shape, so amylase breaks down starch and protease breaks down protein and neither will do the other's job. Temperature changes the rate in two directions: warming speeds the molecules up so collisions become more frequent and the rate rises, but past the optimum the active site distorts, the enzyme is denatured and the change is permanent; most human enzymes work best near 37 degrees Celsius. The same logic applies to pH, since pepsin in the stomach works near pH 2 and enzymes in the small intestine near pH 8, and substrate concentration lifts the rate until every active site is occupied, after which the graph flattens.
细胞分化与特化细胞
分化指细胞在发育过程中改变形状与内部结构，从而承担一项特定功能。动物体内大部分细胞在早期发育阶段完成分化，成熟后还能继续分裂的细胞很少；植物不一样，一生都能靠分生组织产生新细胞。特化细胞的形态与功能一一对应，考题常要求把结构与功能配对。精子有尾用于游动，中段线粒体密集以提供 ATP，头部含酶帮助穿透卵细胞外层。神经细胞的轴突很长，把电信号从一端传到另一端，末端靠突触与下一个细胞相接。肌细胞里充满能收缩的蛋白纤维，常含多个细胞核以便整块纤维协同收缩。根毛细胞向外伸出细长突起，表面面积因此增大，吸水效率提高，它没有叶绿体，因为根部不见光。红细胞没有细胞核，腾出空间装血红蛋白，双凹形状让表面面积更大，运氧效率更高；木质部细胞死亡后留下中空管并含木质素加固，韧皮部靠筛板把糖运向全身。答题时把结构与功能的因果关系写出来，只写结构名称通常只拿一半分。
Differentiation is the process in which a cell changes its shape and internal structure to carry out one particular job; most animal cells finish differentiating early in development, whereas plants keep producing new cells from meristems throughout their lives. Structure and function are always paired in exam questions, so a sperm cell has a tail for swimming, a mid-piece packed with mitochondria to release ATP and enzymes in its head to help it penetrate the outer layer of the egg. A nerve cell has a long axon that carries an electrical impulse from one end to the other and ends in synapses that pass the signal on, while a muscle cell is filled with protein fibres that shorten and often holds several nuclei so a whole block of fibres contracts together. A root hair cell pushes out a long thin extension that raises the surface area for water uptake and holds no chloroplasts because no light reaches the root; a red blood cell has no nucleus, leaving room for haemoglobin, and its biconcave shape increases the surface area available for oxygen transport, with xylem cells dying to leave hollow tubes stiffened by lignin and phloem moving sugars through sieve plates.
干细胞与克隆的用途
干细胞是尚未分化的细胞，能分裂并转成多种细胞类型。胚胎干细胞来自早期胚胎，理论上能转成体内几乎所有细胞类型，医学研究用它替换受损组织，例如用能分泌胰岛素的细胞治疗糖尿病、用神经细胞修复脊髓损伤，这些做法多数还处在试验阶段。成体干细胞存在于骨髓、皮肤等位置，能转成的细胞类型比较有限，骨髓移植治疗白血病正是利用它产生各类血细胞的能力。治疗性克隆把病人的细胞核放进去核卵细胞，得到的胚胎干细胞与病人基因相同，移植时免疫排斥风险小，这一做法在伦理上有争议，考试常要求写出支持与反对各一条理由。植物这边，分生组织里的干细胞能转成植物任何一种细胞，园艺上据此快速繁育出基因相同的植株，切一段枝条就能生根。比较题按来源、可转成的细胞类型、用途、伦理四点逐条写，比笼统作答得分稳。
Stem cells are undifferentiated cells that can divide and become several different cell types. Embryonic stem cells come from an early embryo and can in principle become almost any cell in the body, which is why medical research uses them to replace damaged tissue, including insulin-secreting cells for diabetes and nerve cells for spinal injuries, although most of this work is still at the trial stage. Adult stem cells sit in bone marrow, skin and similar places and can only become a narrower range of cells, and bone marrow transplants for leukaemia rely on exactly that ability to produce blood cells. Therapeutic cloning places a patient's nucleus inside an egg cell whose own nucleus has been removed, so the resulting stem cells match the patient and are less likely to be rejected, a technique that raises ethical objections as well as hopes. In plants, stem cells in meristems can become any plant cell type, which is how gardeners produce genetically identical plants quickly from a single cutting.
细胞周期与有丝分裂
细胞周期分成三个连续阶段：间期、有丝分裂、胞质分裂。间期里细胞生长、复制 DNA 并复制细胞器，染色体此时是细长的染色质，看不出单条的形状。有丝分裂把已经复制的染色体平均分配到两个新核里，过程可以概括为染色体凝缩、排列在细胞中部、姐妹染色单体分离、两个新核形成。人类体细胞有 46 条染色体，配成 23 对，有丝分裂产生的子细胞染色体数与母细胞相同，遗传信息也一致，所以它用于生长、修补受损组织和无性繁殖。配子只有 23 条染色体，是体细胞的一半，受精后才恢复 46 条。洋葱根尖是观察分裂的常用材料，分生区细胞分裂旺盛，一张切片上能看到多个时期。细胞分裂一旦失控，细胞不断分裂堆积就形成肿瘤，这是癌症的细胞学基础。算染色体数目的题要看清问的是分裂前、中期还是末期，姐妹染色单体分开之前，细胞里的 DNA 量是染色体数的两倍，少算这一步答案就偏了。
The cell cycle runs through three continuous stages: interphase, mitosis and cytokinesis. During interphase the cell grows, replicates its DNA and copies its organelles, and the chromosomes exist as long thin chromatin that cannot be counted individually. Mitosis then divides the copied chromosomes evenly between two new nuclei, which can be summarised as the chromosomes condensing, lining up across the middle of the cell, the sister chromatids separating and two new nuclei forming. A human body cell holds 46 chromosomes in 23 pairs, and mitosis produces daughter cells with the same number and the same genetic information, which is why it serves growth, the repair of damaged tissue and asexual reproduction; gametes carry only 23 chromosomes, half the body cell number, and fertilisation restores 46. Onion root tips are standard material for watching the process, since cells in the meristem divide often enough for several stages to appear on one slide, and a cell that divides without control builds up into a tumour, the cellular basis of cancer.
表面面积与体积比
细胞变大时，表面面积按边长的平方增长，体积按立方增长，表面面积与体积的比值随之下降。算例：边长 1 cm 的立方体表面面积 6 cm²、体积 1 cm³，比值 6:1；边长 2 cm 时表面面积 24 cm²、体积 8 cm³，比值降到 3:1；边长 3 cm 时是 54 与 27，比值 2:1。比值下降意味着每单位体积能用来交换物质的表面越来越少，物质进出跟不上内部需求，所以细胞不会长到很大，分裂是维持这个比值的手段。多细胞生物靠交换表面和运输系统解决同一个问题：肺泡数量多、壁只有一层细胞，表面面积大而扩散距离短；小肠绒毛把面积放大并把血流带到很近的位置；鳃丝和根毛细胞是同一条思路。B2 Scaling up 这个模块名指的就是这一层意思，从单个细胞到组织、器官、个体，交换方式必须跟着改造。做这类题把数字代入公式比背结论可靠，写结论时带上具体的比值和单位，说明比值下降导致交换效率不足。
When a cell grows, its surface area rises with the square of the side length while its volume rises with the cube, so the surface area to volume ratio falls as size increases. A cube of 1 cm has a surface area of 6 cm² and a volume of 1 cm³, a ratio of 6:1; at 2 cm the figures are 24 cm² and 8 cm³, giving 3:1; at 3 cm they become 54 and 27, or 2:1. A falling ratio means less surface is available for exchange per unit of volume, so materials cannot get in and out fast enough for the inside, which is why cells stay small and divide instead of growing indefinitely. Multicellular organisms solve the same problem with exchange surfaces and transport systems: alveoli are numerous with walls one cell thick, short diffusion distances and a rich blood supply; villi in the small intestine multiply the surface area and bring blood close to it; gills and root hair cells follow the same pattern. The module name B2 Scaling up points at exactly this step from a single cell to tissues, organs and whole organisms, where the method of exchange has to change with the scale.
必做实验与答题规范
用光学显微镜观察动植物细胞是 OCR 官方 spec 里的 required practical，也是实验题的常见来源。制片步骤要写全：取洋葱鳞片叶内侧的表皮，放在载玻片的水滴里展开，滴一滴碘液染色，盖玻片一侧先接触水滴再慢慢放下，避免气泡。绘图规范比内容更容易丢分：用削尖的铅笔，线条连续不重复涂抹，不画阴影，标出细胞壁、细胞膜、细胞质、细胞核，图的旁边写标题与放大倍数。放大倍数是目镜倍数乘物镜倍数，目镜 x10 配物镜 x40 得 x400，写 x400 而不是 400。结构化题常把观察与解释放在一起，比如问为什么根尖分生区能看到较多正在分裂的细胞，答案要落到细胞周期长度与分裂频率上，而不是重复显微镜的操作步骤。阅卷时能明显看出两类答案的差别，一类只列结构名称，另一类把结构与功能用因果句连起来，后者的得分更高。长度量到毫米先换成微米再代入公式，单位与有效数字是另一处高频扣分点。
Observing plant and animal cells with a light microscope is one of the required practicals in the OCR specification and a frequent source of exam questions. The method needs every step: peel the inner epidermis from an onion scale, spread it in a drop of water on the slide, add a drop of iodine solution, then lower the cover slip from one edge so that no air bubbles are trapped. Drawing rules lose marks more often than the biology does, so use a sharp pencil, keep lines continuous without shading, label the cell wall, cell membrane, cytoplasm and nucleus, and write the title and magnification beside the diagram. Magnification is the eyepiece value multiplied by the objective value, so x10 with x40 gives x400, and the answer should read x400 rather than 400. Structured questions often join observation to explanation, asking why more dividing cells are visible in the meristem of a root tip, where the answer belongs with the length of the cell cycle and the frequency of division rather than with the steps of using the microscope; converting millimetres to micrometres before substituting into a formula catches another cluster of lost marks.
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