Edexcel生物 细胞结构 膜运输 有丝分裂
Edexcel 的 A-Level 生物有两套并行考纲：Biology A（Salters-Nuffield，代码 9BN0）和 Biology B（9BI0）。细胞结构、细胞膜运输、细胞分裂这几块在两边都要考，题目形式也接近，很少直接问定义，而是给一张电子显微镜照片、一组实验读数或一段情境材料，让你现场判断。下面按教学顺序把容易混的概念、能算的量、以及答案里必须出现的关键词拆开讲，最后一节单独说计算题和六分题的写法差别。
Edexcel runs two A-Level Biology specifications, Biology A (Salters-Nuffield, 9BN0) and Biology B (9BI0), and both examine cell structure, membrane transport and cell division. The questions seldom ask for a definition on its own. You are handed a micrograph, a set of readings or a short scenario and expected to apply the idea to it. The sections below follow teaching order, flagging the distinctions that mark schemes keep penalising, and the final section deals with calculations and extended writing separately.
放大倍数与分辨率回答的是两个问题
magnification 指像比实物放大了多少倍，写成 x2000 这种形式；resolution 指两个点还能被分辨开的最小距离。两者在评分标准里是分开的分点，写混了就没有分。光学显微镜受可见光波长限制，分辨率大约 200 nm；透射电子显微镜用电子束，波长短得多，分辨率能到 0.1 nm 左右。很多学生以为倍数越高越清楚，在答案里写 higher magnification gives better resolution，这句话直接判错：在光学显微镜上继续放大，只是把一个本来就模糊的像放大得更大。做题前先统一单位，1 mm 等于 1000 µm，1 µm 等于 1000 nm。像长 30 mm、细胞实际长度 15 µm 时，放大倍数 = 30 mm / 0.015 mm = x2000；反过来给倍数 x5000、像长 25 mm，实际长度就是 25 / 5000 = 0.005 mm，即 5 µm。
Magnification and resolution answer different questions, and examiners award them as separate points. Magnification states how many times larger the image is than the object, written in the form x2000. Resolution is the smallest separation at which two points are still distinguishable. A light microscope is limited by the wavelength of visible light to roughly 200 nm, whereas a transmission electron microscope uses electrons of far shorter wavelength and reaches about 0.1 nm. A very common error is to claim that higher magnification improves resolution. Turning the magnification up on a light microscope only enlarges an image that is already blurred by the wavelength limit. Convert before dividing: 1 mm is 1000 um and 1 um is 1000 nm. An image 30 mm long of a cell 15 um across represents a magnification of x2000, and an image of 25 mm at x5000 corresponds to an actual length of 5 um.
细胞分级分离为什么按密度排队
cell fractionation 的目的，是把匀浆里的细胞器按物理性质分开，好单独研究。第一步 homogenisation：细胞放在冷的、等渗的、加过缓冲液的溶液里破碎。冷是为了降低酶活性、减慢自溶；等渗是为了不让细胞器因渗透吸水或失水变形；缓冲液是为了把 pH 稳住，避免蛋白质变性。第二步 filtration，用纱布或滤纸去掉细胞壁碎片、完整的细胞和大的组织块。第三步 ultracentrifugation，转速从低到高逐级加，沉淀按密度和大小先后落下：细胞核最先，接着是线粒体、叶绿体、溶酶体，再往后是内质网碎片、核糖体和细胞质溶胶。常见的错误是把顺序记成按大小排，把核糖体说成最先沉淀，实际最小的颗粒要最高的转速才沉下来。写实验设计题时，离心后要说清取上清还是取沉淀，下一步再加多少转速。
Fractionation separates organelles so that each can be studied on its own. Tissue is first homogenised in a cold, isotonic, buffered solution. Cold reduces enzyme activity and slows autolysis, isotonic conditions stop organelles swelling or shrinking by osmosis, and the buffer holds pH steady so proteins do not denature. The homogenate is then filtered through gauze to remove cell wall debris, intact cells and large tissue fragments. Ultracentrifugation follows, with the speed raised in steps: nuclei sediment first, then mitochondria, chloroplasts and lysosomes, then fragments of endoplasmic reticulum, ribosomes and finally the cytosol. Students often assume the order is set by size alone and put ribosomes in the first pellet, but the smallest particles need the highest speed. When a question asks you to design the procedure, state whether the pellet or the supernatant is kept at each step and what the next speed will be.
原核细胞真正不同的几处结构
原核细胞常被简化成没有细胞器的细胞，这句话只说对一半：它们没有膜结合细胞器，但核糖体、细胞壁、质粒都是实实在在的结构。原核核糖体是 70S，真核细胞质里的核糖体是 80S；线粒体和叶绿体内部的核糖体也是 70S，这一点常被拿来说明内共生起源。细胞壁成分是肽聚糖（murein），不是植物细胞的纤维素。遗传物质是一条环状 DNA，不与组蛋白结合，不形成染色体，另外还有小环状质粒，可以携带抗性基因并在细菌之间转移。分裂方式是二分裂，没有纺锤体，也没有有丝分裂各期。大小通常在 0.5 到 5 µm 之间，所以光学显微镜下能看到形状，看不到内部细节。题目问为什么抗生素能选择性作用于细菌，答案落点就是 70S 核糖体和肽聚糖细胞壁这两处差异。
Prokaryotes are often described as cells without organelles, which is only half true: they lack membrane bound organelles, but ribosomes, a cell wall and plasmids are all present. Prokaryotic ribosomes are 70S while those in eukaryotic cytoplasm are 80S; mitochondria and chloroplasts contain 70S ribosomes, a detail used as evidence for endosymbiosis. The wall is built from peptidoglycan (murein) rather than cellulose. Genetic material sits on a single circular DNA molecule that is not bound to histones and does not form chromosomes, and small circular plasmids may carry resistance genes that pass between cells. Division is by binary fission, with no spindle and none of the stages of mitosis. Cells are typically 0.5 to 5 um across, so a light microscope shows their shape but no internal detail. Questions on antibiotic selectivity expect those two differences: the 70S ribosome and the peptidoglycan wall.
流动镶嵌模型下三种跨膜运输怎么判定
膜的基本骨架是磷脂双分子层，疏水尾部朝内、亲水头部朝外；胆固醇夹在磷脂之间调节流动性；蛋白质有的贯穿、有的只在一侧；糖蛋白和糖脂位于外表面，参与细胞识别。判断运输方式时盯三条：是否需要 ATP、是否有载体蛋白、是否逆浓度梯度。简单扩散不耗能、不要蛋白、顺梯度，只限小分子和非极性分子，比如氧气和二氧化碳。协助扩散顺梯度但要通道蛋白或载体蛋白，葡萄糖进红细胞走的就是这条路。主动运输逆梯度、要载体、要 ATP，钠钾泵是最典型的例子。水走渗透，方向是高水势到低水势，纯水的水势是 0，溶液的水势是负值，溶质越多水势越低。小肠上皮的葡萄糖吸收用协同运输：钠离子顺梯度进入细胞，带着葡萄糖逆梯度一起进，基底面的钠钾泵再把钠排出去，维持梯度。
The membrane is a phospholipid bilayer with hydrophobic tails facing inwards. Cholesterol packs between the phospholipids and modifies fluidity, proteins either span the bilayer or sit on one face, and glycoproteins and glycolipids on the outer surface handle recognition. Three questions decide the transport route: does it need ATP, does it use a carrier or channel, and does it run against the gradient. Simple diffusion costs nothing, uses no protein and follows the gradient, which restricts it to small or non polar molecules such as oxygen and carbon dioxide. Facilitated diffusion also follows the gradient but requires a channel or carrier, as glucose uptake into red blood cells does. Active transport works against the gradient with a carrier and ATP, the sodium potassium pump being the standard example. Water moves by osmosis from higher to lower water potential, pure water being zero and solutions negative. Glucose uptake in the ileum uses co transport, with sodium entering down its gradient and dragging glucose in against its own, while the pump on the basal membrane restores the sodium gradient.
有丝分裂各期能看见什么、数什么
显微镜照片题的第一步是认期。前期染色质凝集成可见的染色体，核膜和核仁解体，纺锤体从两极长出；中期着丝粒排列在细胞赤道面上，此时染色体形态最清楚，核型分析就用这一期；后期着丝粒分裂，姐妹染色单体被拉向两极；末期核膜重建、染色体解聚，胞质分裂随后完成。数染色体只有一个规则：数着丝粒，一个着丝粒就是一条染色体。很多学生以为后期染色体数目不变，实际着丝粒一分裂，染色体数在这一刻翻倍，DNA 含量不变。mitotic index 的计算是视野中处于有丝分裂的细胞数除以细胞总数，通常写成小数或百分数，用来比较不同处理下细胞增殖的快慢，比如根尖细胞在不同浓度溶液里的分裂频率。
Identifying the stage is the first move on any micrograph question. In prophase the chromatin condenses into visible chromosomes, the nuclear envelope and nucleolus break down and the spindle forms from the poles. At metaphase the centromeres lie on the equator, which is when chromosome shape is clearest and when karyotypes are prepared. At anaphase the centromeres divide and sister chromatids are pulled to opposite poles, and at telophase the nuclear envelope re-forms, the chromosomes decondense and cytokinesis finishes the job. Counting chromosomes has one rule: count centromeres, one centromere per chromosome. Many students assume the chromosome number is fixed throughout, but the split of the centromere doubles it at anaphase while DNA content stays the same. The mitotic index is the number of cells in mitosis divided by the total counted, given as a decimal or a percentage, and it is used to compare proliferation rates, for instance in root tips exposed to different concentrations.
检查点失灵之后细胞周期会发生什么
细胞周期有 G1、S、G2、M 四个阶段，中间设了检查点。G1 检查点看细胞大小、营养状况和 DNA 是否损伤，过不了就停在 G1 或者进入休眠的 G0；G2 检查点确认 DNA 复制完整、没有损伤才允许进入分裂；纺锤体检查点确认所有染色体都正确连到纺锤丝上，才放行后期。p53 蛋白在 DNA 损伤时阻止周期推进，要么修复，要么启动凋亡。原癌基因编码促进分裂的信号蛋白，肿瘤抑制基因编码刹车蛋白，两类基因的突变都可能让检查点失效。错得最多的是把原癌基因和肿瘤抑制基因的作用方向记反：前者突变后是功能增强，后者突变后是功能丧失。治疗上，抑制 DNA 复制或者破坏纺锤体形成的药物，作用点都落在分裂活跃的细胞上，这也是副作用集中在骨髓和毛囊的原因。
The cycle runs through G1, S, G2 and M with checkpoints between them. The G1 checkpoint monitors cell size, nutrient status and DNA damage, and a cell that fails it arrests or drops into the resting G0 state. The G2 checkpoint confirms that replication is complete and undamaged before division is allowed, and the spindle checkpoint releases anaphase only once every chromosome is correctly attached. p53 blocks progression when DNA is damaged, allowing repair or triggering apoptosis. Proto oncogenes code for proteins that promote division, tumour suppressor genes code for the braking proteins, and mutation in either class can disable a checkpoint. The confusion that costs marks is direction: a mutated proto oncogene gains function while a mutated tumour suppressor gene loses it. Drugs that block DNA replication or disrupt spindle formation therefore hit dividing cells hardest, which explains why side effects concentrate in bone marrow and hair follicles.
计算题和六分题的写法差别
计算题的分几乎全在过程和单位上。放大倍数用像长除以实长，两个量必须先换成同一单位；细胞数目、比例、百分比变化率要写清分子分母；有效数字按题目给的数据位数保留，写着 0.005 mm 比写 5 µm 更稳妥，因为题干给的是 mm。图表题的顺序是先描述趋势，再引数据点，最后给结论，只写数据不写趋势通常拿不到描述分。六分题看 command word：describe 只写看到的现象，不带机制；explain 要写因果链，每个环节单独成句；suggest 要用题干材料里的信息推出答案，凭课本背的通用结论往往不对。答案按评分点分层写，一个机制一行，别把三个机制挤成一句话。很多学生把 explain 题写成 describe，把现象重复两遍，六分只拿到两分。
Calculations are marked on method and units. Magnification is image size divided by actual size, and both must be converted to the same unit first. Cell counts, ratios and percentage changes need the numerator and denominator stated. Significant figures should follow the data given in the question, so an answer in mm is safer than a converted one in um when the figures were supplied in mm. Graph questions follow a fixed order: describe the trend, quote data points, then state the conclusion, since data without a trend rarely earns the description mark. The command word decides the shape of a six mark answer. Describe means report what is visible and nothing else, explain requires a causal chain written as separate steps, and suggest means reasoning from the material in the question rather than reciting a textbook conclusion. Lay the points out one mechanism per line. Many students answer an explain question in describe mode, repeat the observation twice and collect two marks out of six.
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