WJEC生物 血液循环 心脏 血管 蒸腾 木质部
运输系统在 WJEC GCSE Biology 里横跨两处内容：物质怎样穿过细胞膜，以及动物和植物体内把材料送到全身的管道网络。这一块的题目很少单独考定义，多是把心脏结构图、血管特征和小麦蒸腾速率的数据放进同一道结构化题，要求你解释原因。WJEC 官方 spec（代码 3400，2016 年 9 月起施教）把它放在 Unit 1，与呼吸、消化、光合作用共用一张 45% 权重的卷子。以下按 spec 的顺序拆成九个知识块，每块都标出学生最容易丢分的具体位置。看完之后建议对着空白的心脏图自己标一遍，标不出来的部分就是下一轮复习的起点。
Transport systems sit in two places in WJEC GCSE Biology Unit 1: how substances cross a cell membrane, and the pipework that moves materials around animals and plants. Questions rarely ask for a bare definition; they put a heart diagram, vessel features and transpiration data into one structured question and ask for the explanation behind it. WJEC awards 45 per cent of the qualification through that single paper, which also carries respiration, digestion and photosynthesis, so revision has to be selective and precise. What follows keeps the order of the official specification and marks out where marks usually disappear.
细胞膜上的三条通道: 扩散 渗透 主动运输
物质进出细胞只有三条路：扩散、渗透和主动运输。扩散是粒子从高浓度一侧向低浓度一侧的净移动，氧气与二氧化碳走这条通道，整个过程不需要细胞提供能量。渗透是水分子穿过选择性渗透膜的扩散，方向只由水浓度决定，水多的一侧流向水少的一侧，与溶质是什么无关。主动运输方向相反，把物质逆着浓度梯度搬进细胞，根毛细胞吸收土壤中的矿物离子就是靠它，代价是消耗呼吸作用释放的能量。很多学生以为扩散和渗透是两套互不相干的机制，其实渗透只是把扩散用在水的身上，判断方向时盯住水分子浓度就够。Visking tubing 的实验题问的是膜的孔径与粒子大小：小分子穿膜而过，淀粉一类的大分子被留在管内，外面液体用本尼迪克试剂和碘液检测就能看出发生了什么。答题时把过程名写完整，只写"物质移动"或"穿了过去"拿不到过程分。
Substances enter and leave a cell by three routes: diffusion, osmosis and active transport. Diffusion is the net movement of particles down a concentration gradient, which is how oxygen and carbon dioxide travel, and it costs the cell no energy; osmosis is the diffusion of water through a selectively permeable membrane, with direction set by water concentration rather than by the solute involved. Active transport runs against the gradient, as root hair cells do when they take in mineral ions from the soil, and the work is paid for by respiration, which is why cold or waterlogged roots absorb ions far more slowly. In Visking tubing questions, examiners want pore size compared with particle size: small molecules cross the membrane while starch is held inside, and food tests on the outside liquid show which of the two occurred. A vague phrase such as "it went through" earns no mark at all.
双循环: 肺循环与体循环
人类的循环系统是双循环，一条去肺，一条去全身。右心室把缺氧血经肺动脉送到肺部，气体交换完成后含氧血经肺静脉回到左心房，这一段叫肺循环。左心室把含氧血经主动脉推向全身，组织处交换之后缺氧血经腔静脉回到右心房，这一段叫体循环。血液每绕全身一圈要经过心脏两次，这样肺里管低压、全身管高压，两套压力分开处理，血液流动更稳。结构化题常问双循环对体温恒定的动物有什么好处，答点落在两处：含氧血与缺氧血不混合，全身组织能得到较高的血压。血液在心脏里不会倒流，靠的是瓣膜随压力变化开合，房室瓣在心室收缩时被顶住，半月瓣在心室舒张时关拢。把血液走行顺序写成一句话再去背，比死记四个心腔的名字有效得多。
The human circulatory system is double, with one loop through the lungs and one through the rest of the body. The right ventricle sends deoxygenated blood to the lungs along the pulmonary artery, and oxygenated blood returns to the left atrium through the pulmonary vein; the left ventricle then drives oxygenated blood out through the aorta, and deoxygenated blood comes back to the right atrium in the vena cava. Blood therefore passes through the heart twice per circuit, which keeps the low pressure of the lungs separate from the high pressure needed in the body. Structured questions ask why this matters for a warm-blooded animal, and the answers are that oxygenated and deoxygenated blood never mix, and that body tissues receive blood at a workable pressure. Backflow is prevented by valves opening and closing as pressure changes.
心脏结构图上的标注点: 心房 心室 瓣膜 冠状动脉
心脏是肌肉做的泵，四个腔的壁厚差别正是考图题的常客。右心房接收腔静脉的缺氧血，右心室把它送进肺动脉；左心房接收肺静脉的含氧血，左心室把它压进主动脉。左心室的肌壁明显比右心室厚，因为它要把血推到全身而不是只推到肺。三尖瓣守在右心房与右心室之间，二尖瓣守在左侧，两个半月瓣分别位于主动脉与肺动脉的出口。图上还常要求标出冠状动脉，它是给心肌本身供血的血管，被脂肪沉积堵住就是冠心病。常见的错误是把肺动脉和肺静脉写反，这里记住一条：肺动脉里流的是缺氧血，命名只按方向不按含氧量。阅卷时能看到的另一处丢分是瓣膜名称与位置对不上，答题前先在图上画箭头把血流方向定下来，再写名字。
The heart is a muscular pump, and the wall thickness of its four chambers is a favourite target of diagram questions. The right atrium collects deoxygenated blood from the vena cava and the right ventricle pushes it into the pulmonary artery; the left atrium receives oxygenated blood from the pulmonary vein and the left ventricle drives it into the aorta. The wall of the left ventricle is visibly thicker because the blood it ejects has to reach the whole body, not just the lungs. The tricuspid valve guards the right opening, the bicuspid valve the left, and the two semilunar valves sit at the exit of the aorta and the pulmonary artery. Diagrams also ask for the coronary vessels, which supply the heart muscle itself; blockage by fatty deposits is what causes coronary heart disease. Pulmonary artery and pulmonary vein are swapped more often than any other pair, so label by direction of travel rather than by oxygen content.
三类血管的结构对比: 动脉 静脉 毛细血管
血管的差别可以用三层结构讲清：外层是坚韧的结缔组织，中层是肌肉与弹性纤维，内层是内皮，中间的空腔叫管腔。动脉的肌肉层厚、管腔窄，能承受心室泵出的高压，管壁有弹性可以回弹，把血液推成持续流动。静脉的肌肉层薄、管腔宽，压力低，所以需要静脉瓣防止血液倒流，腿部肌肉收缩也是把血挤回心脏的动力。毛细血管只有一层内皮细胞厚，管腔极窄，血流放慢，管壁薄正好缩短扩散距离，在组织里连成密网，保证每个细胞附近都有一条毛细血管。题里问结构怎样配合功能，答法是把特征和后果连起来写：管壁薄所以扩散路径短，管腔窄所以流速慢，时间够长，交换才充分。很多学生把毛细血管的"薄"写成优点就停手，没有写出它换来的是更快的扩散，这类答案只能拿到一半分。
The three vessel types can be told apart by their layers: a tough outer coat, a middle layer of muscle and elastic fibres, and an inner endothelium, with the lumen running through the centre. Arteries carry a thick muscle layer and a narrow lumen to withstand the high pressure created by the ventricles, and their elastic walls recoil to keep blood moving between beats. Veins have thin walls and a wide lumen, and because the pressure inside them is low they rely on valves and on the squeezing action of skeletal muscle to return blood to the heart. Capillaries are one endothelium cell thick with very narrow lumens, which slows the flow and shortens the diffusion distance, and their dense networks place a capillary close to almost every cell. Comparison questions want the feature and its consequence linked in the same sentence; thin walls matter only because they shorten the path for diffusion.
血液的四种成分: 红细胞 白细胞 血小板 血浆
血液由红细胞、白细胞、血小板和血浆组成，四者各有分工。红细胞含血红蛋白，在肺部与氧结合、在组织处把氧放出，成熟的红细胞没有细胞核，呈双凹圆盘状，表面积大而扩散距离短。血浆是液体部分，运送二氧化碳、可溶性食物、尿素和激素，也把呼吸产生的热量分配到全身。血小板负责凝血，血管破损时聚集封住伤口。白细胞参与防御，能吞噬病原体。WJEC 官方 spec 在这一点上写得很清楚：Unit 1 只要求知道四者的功能，不需要凝血机制和免疫反应的细节，所以答题时不要花篇幅描述凝血因子级联。学生最常把红细胞说成运送二氧化碳的载体，事实上大部分二氧化碳以溶解形式由血浆带走，只有少量与血红蛋白结合。写功能时把载体写清楚，是这类题拿满分的关键动作。
Blood contains red cells, white cells, platelets and plasma, and each part has its own job. Red cells are packed with haemoglobin, which picks up oxygen in the lungs and releases it in the tissues; they lose their nucleus as they mature and sit as biconcave discs, which raises surface area and shortens diffusion distance. Plasma is the liquid fraction and carries carbon dioxide, soluble food, urea and hormones, and it also spreads respiratory heat around the body. Platelets gather at a damaged vessel wall and seal the break, while white cells defend the body by engulfing pathogens. The official specification limits Unit 1 to the functions of the four parts and deliberately excludes the clotting cascade and the detail of immunity, so extra description earns nothing. A frequent slip is calling red cells the main carrier of carbon dioxide; most of it travels dissolved in plasma.
心血管疾病: 风险因素与三种治疗的评价
心血管疾病的风险因素可以分成两类：无法改变的和可以改变的。年龄、性别和家族史属于前者，吸烟、高脂饮食、缺乏运动、肥胖与高血压属于后者，其中吸烟会让血管收缩并损伤内皮，高血压加快动脉壁的磨损。WJEC 官方 spec 要求评价三种处理方式的优缺点，所以答案必须两面都写。他汀类药物降低血液胆固醇，长期服用能减少发病风险，缺点是需持续服药并可能带来肌肉酸痛等副作用。血管成形术用球囊把狭窄的冠状动脉撑开，常配合支架，见效快，缺点是过程有风险、狭窄可能再次发生。改变饮食与运动量的生活方式干预没有侵入性，成本低，但需要长期坚持，单独使用的效果比配合药物慢。结构化题给出一位病人的情况，要求你选一种方式并说明理由，答题时把病人的年龄、症状与生活方式都写进理由里，才是评价而不是背书。
Risk factors for cardiovascular disease split into those that cannot be changed, such as age, sex and family history, and those that can, including smoking, a high-fat diet, inactivity, obesity and high blood pressure. Smoking narrows vessels and damages the endothelium, while raised pressure wears the artery wall down faster. The official specification asks for advantages and disadvantages of three treatments, so both sides have to appear in the answer. Statins lower blood cholesterol and cut the risk over years, at the cost of continuous medication and possible muscle side effects. Angioplasty, usually with a stent, reopens a narrowed coronary artery quickly but carries procedural risk and the artery may narrow again. Diet and exercise changes are non-invasive and cheap, yet they depend on long-term adherence, which is why the strongest answers weigh them against a named patient rather than listing them.
根毛与木质部: 蒸腾拉力把水送到叶片
水从土壤进入植物，起点是根毛细胞。根毛细胞向外伸出细长突起，表面积大，细胞壁薄，没有角质层，水按渗透进入，矿物离子靠主动运输吸收，所以这类细胞里线粒体数量多。水随后穿过皮层进入木质部。木质部由死细胞首尾相接形成连续的管子，细胞里没有细胞质，管壁含木质素，坚硬不透水，功能是把水和溶解其中的矿物盐从根部向上输送。叶片海绵组织里的水蒸发成水蒸气，经气孔扩散出去，这个过程叫蒸腾。蒸腾把水柱向上拉，因为水分子之间有内聚力，前面蒸发掉一个水分子，后面整列水就被牵着上移。影响蒸腾速率的外界条件有温度、湿度、风速和光照强度：温度高、湿度低、有风、光照强时蒸腾加快；夜里气孔关闭，蒸腾明显减慢。答速率变化题时，把条件先连到气孔或蒸发，再连到速率，中间少一环就扣分。
Water enters a plant at the root hair cells, whose long projections give a large surface area, and whose thin walls and missing cuticle make osmosis easy; mineral ions follow by active transport, which is why these cells hold plenty of mitochondria. Water then crosses the cortex into the xylem. Xylem is built from dead cells joined end to end into continuous tubes with no cytoplasm, and its walls are reinforced with lignin, which is strong and impermeable, so the tissue can carry water and dissolved mineral salts upward from the root. In the leaf, water evaporates from the spongy mesophyll and diffuses out through the stomata, and that loss is transpiration. Cohesion between water molecules means the column is pulled up as the top of it evaporates. Temperature, humidity, wind speed and light intensity all change the rate, with warm, dry, windy, bright conditions speeding it up and closed stomata at night slowing it down.
韧皮部与有机养料运输: 源与库
有机养料的运输走韧皮部，方向与木质部不同。韧皮部由活细胞组成，细胞之间有筛板，细胞质穿过筛孔彼此相连，运输的物质是蔗糖和氨基酸，向上和向下都能进行，这个过程叫转流。运输的起点叫源，终点叫库：夏天叶片制造有机物，叶片是源、根与果实是库；春天块根里储存的养料要供给新芽，根变成源、芽变成库。因为方向由源和库的位置决定，同一器官在不同季节可以扮演不同角色。喷洒在叶片上的内吸性农药也靠转流移动到植物其他部位，这一点常出现在应用型题里。比较木质部与韧皮部时，按"细胞死活、运输物质、运输方向"三栏写，就能把两栏都拿满。把两者混成一套管道是这题最常见的失分方式。
Organic materials travel in the phloem, and the direction of travel separates it from the xylem. Phloem is made of living cells joined end to end, with cytoplasm passing from one cell to the next through the holes in the sieve plates; it carries sucrose and amino acids, and it can move them either upward or downward, a process called translocation. Movement runs from a source, where the material is made, to a sink, where it is used or stored, so leaves act as sources in summer while roots and fruits act as sinks, and in spring the storage tissue in a root becomes the source for new shoots. Because the route depends on where sources and sinks are, one organ can play either role at different times of year. Systemic pesticides applied to leaves also travel by translocation, a point that turns up in applied questions.
数据题: 蒸腾速率的测量与变量控制
Unit 1 的题目里有一类专门给出数据，要求你算速率或评价实验。测蒸腾速率常用气泡计，把带叶的枝条插入装满水的装置，用秒表记录气泡在刻度管里移动的距离，速率按距离除以时间算，单位写成毫米每分钟。设计实验时，要控制的变量包括枝条的种类与大小、光照强度、温度和空气流动，只改变其中一个，才能把速率变化归因于它。这一步最容易出错的地方是没说明重复测量与取平均值，单次读数在简答题里通常不给满分。给出一张蒸腾速率对光照强度的曲线，要求描述趋势并解释，描述部分写清起点与上升段，解释部分回到气孔开度与蒸发，两部分都要写到。数据题还有一类是给心脏超声或血液流动速度，让你比较动脉与毛细血管的流速，答法是把管腔截面积与流速连起来讲。做完这类题后回看评分方案，逐点对分，比再刷一套新卷更有用。
A whole family of Unit 1 questions hands over data and asks for a rate or an evaluation. Transpiration rate is usually measured with a potometer, where a leafy shoot sits in a water-filled apparatus and a stopwatch times a bubble moving along a scale; the rate is the distance divided by the time, reported in millimetres per minute. Variables that must be held constant include the species and size of the shoot, light intensity, temperature and air movement, so that only one factor changes and the shift in rate can be attributed to it. Repeats and a mean are expected, and a single reading rarely earns full marks. When a graph of rate against light intensity appears, describe the trend from its starting point through the rise, then explain it through stomatal opening and evaporation, and give both halves. After marking, work through the scheme point by point rather than starting a fresh paper.
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