WJEC GCSE生物 细胞结构 显微观察 扩散渗透
WJEC 的 GCSE Biology 是威尔士考试局的资格，由 Qualifications Wales 监管，2016 年 9 月起首次教学，2018 年夏季首次颁发成绩。Unit 1 叫 Cells, Organ Systems and Ecosystems，笔试 1 小时 45 分钟、80 分、占总成绩的 45%；Unit 2 同样是 1 小时 45 分钟、80 分、45%；Unit 3 是实验与探究评估，占 10%。2026 年 2 月发布的 spec Version 3 给 Unit 3 换了新版，从 2026 年 9 月开始教，2028 年首次颁发。这篇文章只讲 Unit 1 里的 1.1 Cells and movement across membranes，也就是细胞结构、显微镜观察、细胞分化、四级组织层次，以及扩散、渗透和主动运输这三条过膜路径。Higher Tier 覆盖 A* 到 D，Foundation Tier 覆盖 C 到 G，1.1 的内容两个 tier 都要考。
WJEC GCSE Biology is the Welsh board qualification, regulated by Qualifications Wales, first taught from September 2016 with the first awards in summer 2018. Unit 1, Cells, Organ Systems and Ecosystems, is a 1 hour 45 minute paper worth 80 marks and 45% of the qualification; Unit 2 carries the same weight, and Unit 3 covers practical scientific enquiry for the remaining 10%. Version 3 of the specification, published in February 2026, replaces Unit 3 with a new version taught from 2026 and awarded from 2028, while topic 1.1, Cells and movement across membranes, is examined on both the Higher tier (A* to D) and the Foundation tier (C to G).
细胞膜 细胞质 细胞核 线粒体
动物细胞和植物细胞都有四个结构：细胞膜、细胞质、细胞核和线粒体。细胞膜是薄薄一层，管物质进出，它只让一部分分子通过，所以叫选择透过性膜。细胞质是细胞里的液体部分，大部分化学反应在里面发生。细胞核装着 DNA，遗传信息在里面，细胞的一切活动由它指挥。线粒体是需氧呼吸后几个阶段发生的位置，能量在这里被释放出来。很多学生以为线粒体是制造能量的结构，其实它只是把葡萄糖里的能量转成细胞能用的形式。阅卷时常见的一种失分，是把细胞膜和细胞壁的功能混着写，两者的透过性完全不同。
Animal and plant cells share four structures: the cell membrane, the cytoplasm, the nucleus and the mitochondria. The membrane is a thin layer that controls what enters and leaves, and because it lets only some molecules through it is described as selectively permeable; the cytoplasm is the fluid where most of the cell reactions happen, and the nucleus holds the DNA that directs the cell. Mitochondria are the site of the later stages of aerobic respiration, so they release energy rather than create it, and a wall around an animal cell is never correct.
细胞壁 大液泡 叶绿体
植物细胞比动物细胞多出三个结构：细胞壁、大液泡和叶绿体。细胞壁的主要成分是纤维素，给细胞支撑和形状，它本身完全透过，物质进出还是由细胞膜决定。大液泡里装着细胞液，把细胞撑得饱满，植物能挺立靠的就是这种膨压。叶绿体里有叶绿素，光合作用在里面进行，只长在见光的部分，根部细胞就没有叶绿体。常见的错误是给动物细胞也画上细胞壁，或者在洋葱鳞叶内表皮细胞里找叶绿体，那层内表皮是白色的，要观察叶绿体得用池塘植物或菠菜叶。画图时必须标注结构名称，线条要直、彼此不交叉。
Three structures separate a plant cell from an animal cell: the cellulose cell wall, the large permanent vacuole and the chloroplasts. The wall gives the cell shape and support but is fully permeable, so the selectively permeable membrane still decides what crosses; the vacuole fills with cell sap and keeps the cell firm, which is why a well-watered plant stands up. Chloroplasts contain chlorophyll and carry out photosynthesis, so they appear only in the parts of a plant that receive light, and drawing a cell wall around an animal cell is a mark lost every year.
指定实验 光镜观察动物与植物细胞
WJEC 在 1.1 里列了两项指定实验，第一项是用光学显微镜观察动物和植物细胞，并画出带标注的科学图。观察人的口腔上皮细胞，要用牙签在口腔内壁轻刮几下，抹在载玻片的水滴里，加一滴亚甲基蓝染色；洋葱鳞叶内表皮撕一小片，用碘液染色。盖玻片从一侧慢慢放下去，斜着压是为了把气泡赶出去，不是为了让标本更薄。先用低倍物镜找到清晰的物像，再换高倍物镜，换镜之后只调细准焦螺旋。画图不许涂阴影，不许画成卡通，用削尖的铅笔，标注线要平行且不交叉，最后写上放大倍数。
The first specified practical in 1.1 asks you to examine animal and plant cells with a light microscope and produce a labelled scientific drawing. Cheek cells are scraped gently, spread in a drop of water and stained with methylene blue, while a thin layer of onion epidermis is stained with iodine; the cover slip is lowered at an angle from one side so that air bubbles are pushed out instead of trapped under the glass. Focus on low power before switching to the high power objective, and then use only the fine adjustment.
放大倍数与分辨率 单位换算
放大倍数用这样一个式子算：放大倍数等于像的大小除以实物的大小。题目给的单位常常不统一，照片上量出来是毫米，真实细胞只有几十微米，先换算再算。1 毫米等于 1000 微米，一个直径 0.05 毫米的细胞就写成 50 微米。分辨率是两个点还能被区分开的最小距离，光镜的分辨率大约停在 200 纳米，线粒体内部的细节在光镜下看不清，要用电子显微镜。这一步最容易出错的地方是忘记写单位，算出 400 却不知道是 400 倍还是 400 微米。如果题目给放大倍数和像的大小，就反过来求实物大小，除法两侧的关系不要颠倒。
Magnification is found by dividing the size of the image by the actual size of the specimen, and more marks are lost on units than on arithmetic. There are 1000 micrometres in a millimetre, so a cell measuring 0.05 mm across is recorded as 50 micrometres and the conversion must happen before the division; given magnification and image size, the same equation is rearranged to find the real size. Resolution is the smallest distance at which two points can still be told apart, and a light microscope stops at roughly 200 nanometres, which is why an electron microscope is needed for detail inside a mitochondrion.
细胞分化与特化细胞
细胞分化指的是一个细胞改变结构，去承担一项特定任务的过程。动物细胞大多在胚胎早期完成分化，成熟动物体内的细胞分裂主要用于修补和替换；植物细胞不一样，很多植物细胞一辈子都保留分化的能力。分化后的细胞叫特化细胞，结构上的改变都能对应到功能：精子细胞有一条摆动的尾和大量线粒体，神经细胞伸出很长的轴突，纤毛上皮细胞的纤毛把黏液往上扫，红细胞没有细胞核、做成两面凹的圆盘来装更多血红蛋白，根毛细胞拉出细长的突起以增大吸收面积，栅栏组织细胞里塞满叶绿体。学生最常把特化细胞的功能写上，却忘了写结构怎么配合功能，分数往往落在后半句上。
Differentiation is the process by which a cell changes its structure to carry out one particular job. Most animal cells differentiate early in development, and in a mature animal division is largely limited to repair and replacement, whereas many plant cells keep the ability to differentiate throughout life; the tail and packed mitochondria of a sperm cell, or the haemoglobin-filled cytoplasm of a red blood cell, only make sense against the job each one performs. Students often give the function of a specialised cell but forget to link it to the structure, and that second half is where the marks sit.
组织 器官 器官系统 生物体
组织是一群结构和功能相似的细胞，器官由几种不同的组织组成，几件器官配合起来完成一套功能就成了器官系统，器官系统合起来才是完整的生物体。胃可以当例子，里面有上皮组织、肌肉组织、结缔组织和神经组织，它们一起把食物揉碎并混上胃液；胃、小肠、肝脏和胰腺连起来构成消化系统。植物这边，叶片就是一个器官，表皮、栅栏组织、海绵组织、木质部和韧皮部都是它的组织。很多学生以为器官必须能独立完成任务，其实器官只负责某几项功能，完成一整套功能的是器官系统。
A tissue is a group of similar cells with a similar function, an organ is built from several tissues, and organs working together on one job form an organ system, which together with the others makes a whole organism. The stomach contains epithelial, muscular, connective and nervous tissue that between them churn food and mix it with gastric juice, and the stomach, small intestine, liver and pancreas make up the digestive system. A leaf is an organ in the same sense, with epidermis, palisade and spongy tissue plus xylem and phloem inside it.
扩散 浓度梯度与影响因素
扩散是分子从浓度高的地方向浓度低的地方的净移动，沿着浓度梯度走，不需要能量，属于被动过程。能靠扩散过膜的主要是小分子，比如氧气和二氧化碳，分子一直在做无规则运动，扩散的动力就来自这种运动。温度升高，分子动能变大，扩散更快；浓度差越陡，扩散越快；距离越短扩散越快，所以肺泡壁和毛细血管壁都只有一层细胞厚；表面积越大，单位时间通过的量越多，小肠绒毛和根毛细胞都靠这个。常见的错误是把扩散写成需要能量的过程，或者在浓度相等之后还说分子停止运动，平衡只是两个方向的移动速率相等。
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a gradient and without any energy input from the cell. Only small molecules such as oxygen and carbon dioxide cross the membrane this way, and a steeper gradient, a shorter distance, a larger surface area or a higher temperature all speed the process up, which is why the wall of an alveolus is one cell thick. Saying that diffusion needs energy, or that particles stop moving at equilibrium, is wrong on both counts.
Visking 透析袋 膜孔与分子大小
WJEC 的 spec 明确要求用 Visking 透析袋做膜的模型，并按孔径与分子大小解释实验结果。透析袋壁上有很小的孔，孔的大小决定哪些分子能过去：淀粉分子太大，出不去；葡萄糖和碘分子足够小，可以自由穿过。典型做法是把淀粉溶液和葡萄糖溶液装进透析袋，扎紧后放进温水里，过一段时间取袋外的水检验。袋外出现葡萄糖，用本尼迪克试剂加热会得到砖红色沉淀；袋外的水遇碘不显蓝黑色，说明淀粉还留在袋里。很多学生以为这证明细胞膜像筛子，其实它只说明孔径和分子大小的关系，真正的细胞膜还要靠载体蛋白选择物质。回答这类题要先写现象，再写解释，两句话都不能省。
WJEC's specification asks you to use Visking tubing as a model of living material and to explain the results in terms of pore and particle size. The pores are wide enough for glucose and iodine but too narrow for starch, so when a starch and glucose mixture is sealed inside tubing and stood in warm water, glucose appears outside while the starch stays in; iodine on the outside water gives no blue-black colour, and Benedict's reagent on the same water turns brick red after heating. Describing the observation and then explaining it with pore size earns both marks, whereas a description on its own does not.
渗透 马铃薯条的质量变化
渗透是水分子穿过选择透过性膜的净移动，从水浓度高的地方去水浓度低的地方，也就是从溶质少的一边去溶质多的一边，本质仍然是扩散。马铃薯条实验是 Welsh 卷上最常见的形式，把等长的马铃薯条分别放进纯水、稀糖水和浓糖水，称前后质量。泡在纯水里的质量增加，水从外面进入细胞；泡在浓糖水里的质量减少，水从细胞里跑出来；质量几乎不变的那组，外面溶液的水浓度和细胞内的相等。植物细胞泡在浓糖水里，原生质体会缩离细胞壁，这叫质壁分离。常见的错误是把渗透说成糖分子跑进细胞，方向正好反了。
Osmosis is the net movement of water across a selectively permeable membrane, from the side with a higher water concentration to the side with a lower one, and it is still a form of diffusion. In the potato cylinder experiment, strips left in pure water gain mass because water enters the cells, strips in concentrated sugar solution lose mass, and the sample that barely changes must be sitting in a solution with the same water concentration as its cells. Plant cells in strong sugar solution pull their cytoplasm away from the wall, a change called plasmolysis.
根毛细胞 渗透吸水与蒸腾拉流
根毛细胞是表皮细胞向外伸出的细长突起，一个细胞就能把吸收面积扩大很多倍，这是它和普通表皮细胞最大的不同。土壤里的水浓度通常高于根细胞里的水浓度，水靠渗透进入根毛，再沿着细胞间隙和木质部导管往上走。木质部的导管是中空的死细胞，管壁有木质素加固，水在里面形成连续的水柱。叶片气孔蒸发失水产生拉力，把整根水柱往上拽，这就是蒸腾拉流。学生最常把根部吸水和蒸腾写成一件事，其实吸水靠渗透，水往上走才靠蒸腾产生的拉力；矿物离子在土壤里比在根细胞里稀，它们进根毛要走主动运输。
Root hair cells are thin extensions of epidermal cells, and that shape multiplies the surface available for absorption. Water in the soil is usually at a higher water concentration than the cytoplasm inside the root, so it enters by osmosis and then travels up through the xylem, where hollow dead cells strengthened with lignin carry a continuous column of water; evaporation from the stomata pulls that column upwards. Mineral ions are more concentrated inside the root cell than in the surrounding soil, so they must be taken in by active transport instead of osmosis.
主动运输 逆浓度梯度与能量
主动运输把分子从浓度低的一边搬到浓度高的一边，逆着浓度梯度走，所以要用呼吸释放的能量，还要靠膜上的载体蛋白。根毛细胞从稀薄的土壤溶液里吸收硝酸盐和镁离子，靠的就是主动运输；小肠上皮细胞把葡萄糖从肠腔吸进血液，浓度差可能已经很小，这一段同样用主动运输。判断一种物质怎么过膜，看三件事：方向是顺梯度还是逆梯度、要不要能量、有没有载体参与。常见的错误是把主动运输写成物质自己想过去，方向说反了却照样写能量。缺氧时主动运输会慢下来甚至停下，因为能量供不上，这条推理在卷面上常考。
Active transport moves substances from a region of lower concentration to a region of higher concentration, against the gradient, so it needs energy from respiration and a carrier protein in the membrane. Root hair cells take in nitrate and magnesium ions this way from very dilute soil solutions, and glucose is absorbed from the gut into the blood even when the concentration difference is small. Cut off the oxygen supply and active transport slows down or stops, because the energy supply has gone, and that link between energy and transport is a favourite exam question.
表面积与体积比 细胞为什么小
细胞为什么都那么小，答案在表面积和体积的比上。一个边长 1 厘米的正方体，表面积 6 平方厘米，体积 1 立方厘米，比是 6 比 1；边长放大到 3 厘米，表面积 54 平方厘米，体积 27 立方厘米，比降到 2 比 1。体积增大时，需要供应物质的细胞质变多，能用来交换物质的膜面积相对变小，扩散就供不上了。多细胞生物因此长出专门的交换面：小肠的绒毛、肺泡、鳃丝、根毛细胞，都是把面积做大的结构。回答这类题不能只写表面积变大，要写出跟体积的比，以及这个比变小会带来什么后果。
Cells stay small because of the ratio between their surface area and their volume. A cube with 1 cm sides has a surface area of 6 cm squared and a volume of 1 cm cubed, a ratio of 6 to 1, while a cube with 3 cm sides has a ratio of only 2 to 1, so the membrane area available for exchange falls behind the amount of cytoplasm that needs feeding. That is why multicellular organisms build exchange surfaces such as villi, alveoli, gills and root hairs, and why an answer that only says the surface area is larger misses the mark.
酶 活性部位与变性
1.1 的后半段转到酶，WJEC 也在这里设了第二项指定实验，调查影响酶作用的因素。酶是生物催化剂，成分是蛋白质，能降低反应所需的活化能，自己不参与反应、用完还能再用。底物结合的那一小块区域叫活性部位，形状与底物互补，一种酶通常只催化一种反应，这就是锁与钥匙模型。温度升高到一定程度，酶的空间结构被破坏，活性部位变形，酶就变性了；pH 偏离最适值也会让酶变性。常见的错误是写酶被杀死了，酶不是生物，只能写变性。实验里常用淀粉酶加缓冲液控制 pH，每隔一段时间取一滴到点滴板上加碘液，用碘液褪色所需的时间衡量反应快慢。
Topic 1.1 also covers enzymes, and the second specified practical investigates the factors that affect enzyme action. Enzymes are proteins that act as biological catalysts, and the substrate binds to a complementary active site, which is the lock and key model; a change in temperature or pH alters the shape of that site, and the enzyme becomes denatured. In the practical, amylase is mixed with buffer solutions to set different pH values and a drop is tested with iodine every so often, so the time taken for the iodine colour to disappear measures how fast the reaction runs.
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