AQA生物 细胞结构 细胞膜 有丝分裂 考点
细胞是 AQA A-Level Biology 整门课的地基。Paper 1 中，细胞结构与物质运输几乎年年出现，而且常与酶、免疫、遗传等后续章节串联成大题。很多同学背下了细胞器的名字，却拿不到分，原因不是没记住，而是没有把结构与功能绑定起来答题。本文梳理细胞章节的五大核心板块，中英对照讲解，并给出 AQA 特有的答题套路与高频计算题解法。
Cells form the foundation of the whole AQA A-Level Biology course. In Paper 1, cell structure and transport appear almost every year, and they are often linked to enzymes, immunity and genetics in longer questions. Many students can name the organelles but still lose marks, not because they have not memorised the facts, but because they fail to link structure to function in their answers. This article covers the five core areas of the cells topic with parallel Chinese and English explanations, plus the exam technique and calculation methods that AQA examiners reward.
一、细胞基本结构与细胞器分工
真核细胞的核心逻辑是区室化：不同的反应被分隔在不同的膜结构里，互不干扰。细胞核由双层核膜包裹，核膜上有核孔，mRNA 与核糖体亚基通过核孔进入细胞质，而 DNA 始终留在核内。核仁负责合成 rRNA 并组装核糖体。线粒体是双层膜结构，内膜向内折叠形成皱褶，把表面积做大，为氧化磷酸化提供更多附着位点；基质中含有参与三羧酸循环的酶。粗面内质网表面附着核糖体，负责合成并折叠分泌蛋白与膜蛋白；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，对蛋白进行修饰、分选与包装，最终形成分泌囊泡或溶酶体。
The core logic of a eukaryotic cell is compartmentalisation: different reactions are separated by membrane-bound compartments so that they do not interfere with one another. The nucleus is surrounded by a double membrane containing nuclear pores, through which mRNA and ribosomal subunits pass into the cytoplasm, while DNA remains inside. The nucleolus makes rRNA and assembles ribosomes. Mitochondria have a double membrane, and the inner membrane folds into cristae, increasing the surface area available for oxidative phosphorylation, while the matrix contains the enzymes of the Krebs cycle. Rough endoplasmic reticulum has ribosomes on its surface and synthesises and folds proteins destined for secretion or for membranes; smooth endoplasmic reticulum synthesises lipids and detoxifies substances. The Golgi apparatus receives vesicles from the endoplasmic reticulum, modifies, sorts and packages proteins, and finally produces secretory vesicles or lysosomes.
要拿满分，必须写出结构决定功能的因果链。例如：线粒体内膜褶皱，所以附着 ATP 合成酶的表面积大，所以有氧呼吸产生 ATP 的速率高。溶酶体含有水解酶，且膜为单层，能把酶与细胞质隔离，避免自身被消化。核糖体由 rRNA 与蛋白质组成，没有膜结构，因此在原核细胞中也存在，这是原核与真核对比题的常见得分点。原核细胞没有核膜包裹的细胞核，DNA 呈环状且不与组蛋白结合，没有线粒体和内质网这类膜性细胞器，但仍有 70S 核糖体、细胞壁与质膜，并且可能带有质粒或荚膜。
To reach full marks you must write the causal chain from structure to function. For example: the inner mitochondrial membrane is folded, so a large surface area is available for ATP synthase, so the rate of ATP production in aerobic respiration is high. Lysosomes contain hydrolytic enzymes and are bounded by a single membrane, which keeps those enzymes separate from the cytoplasm and prevents self-digestion. Ribosomes are made of rRNA and protein and have no membrane, so they are also present in prokaryotic cells, which is a common comparison mark. Prokaryotes have no membrane-bound nucleus, their DNA is circular and not associated with histones, and they lack membrane-bound organelles such as mitochondria and endoplasmic reticulum, but they still have 70S ribosomes, a cell wall and a plasma membrane, and they may carry plasmids or a capsule.
二、细胞膜与物质跨膜运输
细胞膜的流动镶嵌模型包含三层信息：磷脂双分子层构成基本骨架，亲水头部朝外、疏水尾部朝内；蛋白质镶嵌或贯穿其中，承担通道、载体与受体功能；胆固醇嵌入疏水区，在高温下限制磷脂移动，在低温下防止膜凝固，从而维持膜的稳定性。糖蛋白与糖脂只分布在膜外侧，参与细胞识别、受体结合与免疫应答。这一条不对称性经常被考到，答案要明确写出只在外表面。
The fluid mosaic model of the cell membrane contains three layers of information. The phospholipid bilayer forms the basic framework, with hydrophilic heads facing outwards and hydrophobic tails facing inwards. Proteins are embedded in or span the membrane, acting as channels, carriers and receptors. Cholesterol is embedded among the hydrophobic tails: at high temperatures it restricts phospholipid movement, and at low temperatures it prevents the membrane from solidifying, so it maintains membrane stability. Glycoproteins and glycolipids occur only on the outer surface and are involved in cell recognition, receptor binding and immune responses. This asymmetry is examined frequently, and the answer must state explicitly that they are found on the outer surface only.
跨膜运输分四类，判断依据是是否耗能、是否用蛋白、是否顺浓度梯度。简单扩散顺浓度梯度，直接穿过磷脂双分子层，不耗能，例如氧气与二氧化碳。协助扩散顺浓度梯度，但必须通过通道蛋白或载体蛋白，不耗能，例如葡萄糖进入红细胞、水通过水通道蛋白。主动运输逆浓度梯度，需要载体蛋白并消耗 ATP，例如钠钾泵把钠离子泵出细胞、把钾离子泵入细胞。协同运输利用钠离子顺浓度梯度回流的能量，把葡萄糖逆浓度梯度带入肠上皮细胞，这条路径是 Paper 3 实验题的常客。渗透作用本质上是水通过部分透性膜的净移动，方向由水势决定，纯水水势为零，溶质越多水势越低，因此水从低浓度溶液流向高浓度溶液。
There are four categories of transport, distinguished by whether energy is used, whether a protein is involved, and whether movement is down or against the concentration gradient. Simple diffusion moves down the gradient directly through the phospholipid bilayer without energy, for example oxygen and carbon dioxide. Facilitated diffusion also moves down the gradient but requires channel or carrier proteins and no energy, for example glucose into red blood cells and water through aquaporins. Active transport moves against the gradient, requires carrier proteins and hydrolyses ATP, for example the sodium-potassium pump moving sodium ions out and potassium ions into the cell. Co-transport uses the energy released as sodium ions flow back down their gradient to bring glucose into epithelial cells of the ileum against its gradient, and this pathway appears regularly in Paper 3 practical questions. Osmosis is essentially the net movement of water across a partially permeable membrane, and its direction is determined by water potential: pure water has a water potential of zero, and the more solute present the lower the water potential, so water moves from a dilute solution to a concentrated one.
三、显微镜技术与细胞分级分离
光学显微镜的分辨率受光波长限制，只能分辨约 0.2 微米，因此看不到细胞器内部细节，只能观察细胞核、叶绿体这类较大结构。电子显微镜用电子束代替光束，分辨率可达 0.0002 微米。透射电镜要求样本极薄，显示内部结构，但只能是二维灰度图像；扫描电镜显示表面三维形貌，图像立体但不能反映内部。放大倍数公式为：放大倍数等于图像长度除以实际长度，计算前务必把单位统一成微米或毫米，这一步失分最多。实际长度等于图像长度除以放大倍数。图像长度等于实际长度乘以放大倍数。
The resolution of a light microscope is limited by the wavelength of light, so it can only distinguish objects about 0.2 micrometres apart. It therefore cannot show internal detail of organelles and can only reveal larger structures such as nuclei and chloroplasts. An electron microscope uses a beam of electrons instead of light and can resolve down to about 0.0002 micrometres. Transmission electron microscopy requires very thin specimens and shows internal structure, but only as a two-dimensional greyscale image, whereas scanning electron microscopy shows three-dimensional surface topography, which is informative about shape but does not reveal what is inside. The magnification formula is: magnification equals image length divided by actual length. Always convert units to the same scale before calculating, because most marks are lost at this step. Actual length equals image length divided by magnification, and image length equals actual length multiplied by magnification.
细胞分级分离是提取细胞器的标准方法，三步走：匀浆、过滤、差速离心。第一步把组织放入冷的等渗缓冲液中匀浆，冷是为了降低酶活性防止自溶，等渗是为了防止细胞器因渗透吸水而涨破或失水皱缩，缓冲液是为了维持 pH 稳定，避免酶变性。第二步过滤除去细胞碎片与完整细胞，只保留滤液。第三步在不同转速下离心：低速先沉淀核与未破碎细胞，中速沉淀线粒体与溶酶体，高速沉淀内质网碎片，最后超速沉淀核糖体与可溶性蛋白。考生必须写出每一步的参数意义，而不只是描述操作顺序。
Cell fractionation is the standard method for isolating organelles, and it has three steps: homogenisation, filtration and differential centrifugation. First, the tissue is homogenised in a cold, isotonic buffer solution. Cold conditions reduce enzyme activity and prevent self-digestion, isotonic conditions prevent organelles from bursting by osmosis or shrinking by water loss, and the buffer maintains a stable pH so that enzymes do not denature. Second, the homogenate is filtered to remove cell debris and intact cells, keeping only the filtrate. Third, the filtrate is centrifuged at increasing speeds: low speed pellets nuclei and unbroken cells, medium speed pellets mitochondria and lysosomes, high speed pellets fragments of endoplasmic reticulum, and ultracentrifugation finally pellets ribosomes and soluble proteins. You must explain the purpose of each condition, not merely describe the order of the steps.
四、细胞周期与有丝分裂
细胞周期分为分裂间期与分裂期。间期包括 G1 期（细胞生长、合成酶与蛋白）、S 期（DNA 半保留复制，每条染色体变成两条姐妹染色单体）与 G2 期（继续生长、合成纺锤体蛋白）。分裂期即有丝分裂，依次为前期、中期、后期、末期。前期：染色质凝缩成染色体，核膜解体，核仁消失，纺锤体开始形成。中期：染色体排列在赤道面上，着丝粒与纺锤体微管连接，是观察染色体数目与形态的最佳时期。后期：着丝粒分裂，姐妹染色单体分开，被纺锤丝拉向两极。末期：染色体解聚，核膜与核仁重新形成，胞质分裂。
The cell cycle is divided into interphase and the mitotic phase. Interphase consists of G1, when the cell grows and synthesises enzymes and proteins; S phase, when DNA is replicated semi-conservatively so that each chromosome becomes two sister chromatids; and G2, when growth continues and spindle proteins are synthesised. The mitotic phase is mitosis, which proceeds through prophase, metaphase, anaphase and telophase. In prophase, chromatin condenses into chromosomes, the nuclear envelope breaks down, the nucleolus disappears and the spindle begins to form. In metaphase, chromosomes line up on the equator and their centromeres attach to spindle microtubules, making this the best stage for counting chromosomes and studying their shape. In anaphase, the centromeres divide, sister chromatids separate and are pulled to opposite poles. In telophase, the chromosomes uncoil, the nuclear envelope and nucleolus re-form, and cytokinesis divides the cytoplasm.
有丝分裂的意义在于产生两个遗传上相同的二倍体子细胞，用于生长、修复与无性繁殖。肿瘤则是细胞周期调控失控的结果，原癌基因突变导致促分裂信号持续激活，抑癌基因突变使检查点失效，细胞在 G1 与 G2 检查点不能停止，于是无限增殖并形成肿块。病毒复制不属于细胞分裂：病毒吸附到宿主细胞表面受体，注入核酸，利用宿主核糖体与酶复制核酸并合成衣壳蛋白，组装后裂解释放。原核细胞用二分裂繁殖，DNA 复制后附着在质膜上，质膜内陷把两个 DNA 分开。这三者的对比是 AQA 简答题的高频考点。
The significance of mitosis is that it produces two genetically identical diploid daughter cells for growth, repair and asexual reproduction. Cancer results from loss of control over the cell cycle: mutations in proto-oncogenes keep division signals permanently active, and mutations in tumour suppressor genes disable checkpoints, so cells fail to stop at the G1 and G2 checkpoints and divide uncontrollably to form a tumour. Viral replication is not cell division. The virus attaches to receptors on the host cell surface, injects its nucleic acid, uses the host ribosomes and enzymes to replicate the nucleic acid and synthesise capsid proteins, assembles new particles and then lyses the cell to release them. Prokaryotes reproduce by binary fission, in which the replicated DNA attaches to the plasma membrane and inward growth of the membrane separates the two DNA molecules. Comparing these three processes is a frequent short-answer question in AQA papers.
五、考试技巧与高频失分点
第一，命令词决定答题深度。Explain 要求写出因果链，描述结构后必须接 therefore 或 so that 引出功能；Suggest 要求把已学原理迁移到新情境，例如给出某细胞器的异常数据让你推断功能受损；Calculate 必须写出公式、代入、带单位的结果，缺一不可。第二，单位与数量级一致，微米与毫米混用是最常见的失分。第三，比较题必须成对写，写出两方各自的差异，只写一方不给分。第四，实验设计题要写对照、单变量、重复次数与统计检验，并说明如何保证可靠性与有效性。
First, the command word determines the depth required. Explain demands a causal chain: after describing a structure you must use therefore or so that to link it to a function. Suggest requires transferring learned principles to an unfamiliar context, for example interpreting abnormal data about an organelle to infer a functional defect. Calculate requires the formula, the substitution and a final answer with units, and omitting any of these loses marks. Second, units and orders of magnitude must be consistent, and mixing micrometres with millimetres is the most common error. Third, comparison questions must be answered in pairs, giving the difference for each side, because a one-sided answer earns nothing. Fourth, practical design questions must state the control, the single independent variable, the number of repeats and an appropriate statistical test, and explain how reliability and validity were ensured.
高频计算题主要有三类。一是放大倍数与实际大小互算，记住两边单位统一。二是细胞周期时长，若 S 期占周期比例为 p，则 S 期时长等于周期总长乘以 p；若给出某时期细胞数与总数之比，可估算该时期时长。三是水势与渗透，注意溶质势为负值，水势等于溶质势加压力势，动物细胞无细胞壁，吸水过多会破裂。心肌细胞与神经元属于不分裂的永久细胞，肝细胞属于可分裂细胞，这类配合题常与有丝分裂速率联合考查。把结构与功能、过程与调控两条线索同时记牢，细胞章节就能从失分点变成稳定得分点。
There are three main types of calculation. The first is converting between magnification and actual size, remembering to make the units match on both sides. The second is cell cycle timing: if S phase occupies a proportion p of the cycle, the duration of S phase equals the total cycle length multiplied by p, and the ratio of cells in a stage to the total number of cells can be used to estimate the duration of that stage. The third is water potential and osmosis: solute potential is negative, water potential equals solute potential plus pressure potential, and animal cells have no cell wall so they burst if too much water enters. Cardiac muscle cells and neurones are permanent cells that no longer divide, whereas liver cells retain the ability to divide, and such combinations are often examined together with mitotic rate. Learn the two strands of structure to function and process to control at the same time, and the cells topic becomes a reliable source of marks rather than a source of lost ones.
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