WJEC GCSE生物 扩散 渗透 循环系统 蒸腾作用
GCSE 生物的运输这一块，WJEC 把它分在三个小节里：1.1 讲物质怎么进出细胞，1.4 讲人的循环系统，1.5 讲植物的蒸腾与运输。三块看着分散，考题却常把它们连着问，同一个扩散原理，既解释肺泡里的气体交换，也解释叶片里的气体进出。WJEC 官方考纲（Version 3，2026 年 2 月）用的说法是 selectively permeable membrane，写渗透定义时照这个词更稳。下面十二节按考纲顺序过一遍定义、图像判断和常见的丢分点。
In the WJEC GCSE Biology specification, transport is split across three areas: movement across membranes in 1.1, the human circulatory system in 1.4, and plant transport with transpiration in 1.5. These topics are rarely tested in isolation, since the same diffusion idea explains gas exchange at the alveoli and inside a leaf. Note the exact wording of the specification, a selectively permeable membrane, because WJEC keeps asking for it. The twelve sections below follow the order of the specification and flag where marks are usually lost.
扩散：浓度梯度决定方向
扩散是粒子从浓度高的地方向浓度低的地方净移动，顺浓度梯度，不需要细胞消耗呼吸释放的能量。细胞膜允许氧气和二氧化碳这样的小分子直接扩散通过，所以肺泡与毛细血管之间、叶片与空气之间的气体交换都靠它。WJEC 考纲用 Visking 透析管做模型：管壁上的孔只让小分子通过，淀粉这类大分子留在管内，葡萄糖可以出来。实验里用碘液检验淀粉、用本尼迪克特试剂检验还原糖，结果就是按分子大小分开的。影响扩散快慢的条件有三个，距离要短、浓度差要大、表面积要大。很多学生以为扩散需要能量，其实它靠的是粒子自身的随机运动。
Diffusion is the net movement of particles down a concentration gradient, and it costs the cell no energy from respiration. Oxygen and carbon dioxide are small enough to cross the membrane this way, which is why gas exchange at the alveoli and inside a leaf both depend on it. WJEC uses Visking tubing as a model: the pores let small molecules such as glucose pass through while starch stays inside, and the iodine and Benedict's tests show exactly that split. How fast it happens depends on a short distance, a steep gradient and a large surface area. Many students assume diffusion needs ATP, yet the driving force is simply random particle motion.
渗透：水从水多的一侧进来
渗透是水分子通过选择性透膜，从水浓度高、溶质浓度低的一侧，移动到水浓度低、溶质浓度高的一侧。WJEC 考纲把渗透直接写成水的扩散，所以定义里要出现水、选择透膜和两个浓度方向这三个要素。动物细胞放进纯水会吸水膨胀甚至破裂，放进浓盐水会失水皱缩。植物细胞外面有细胞壁，吸水后膨压上升，细胞变得坚挺，这叫紧张状态；失水后原生质层与细胞壁分离，叫质壁分离。常见的错误是只写水从低浓度向高浓度移动，没有说明那是溶质浓度。
Osmosis is the diffusion of water across a selectively permeable membrane, from the side with more water and less solute to the side with less water and more solute. Because WJEC defines it as a form of diffusion, a full-mark answer names water, the membrane and both directions of the gradient. An animal cell placed in pure water swells and may burst, while in concentrated salt solution it loses water and shrinks; a plant cell instead becomes turgid as the cell wall resists further expansion, and plasmolysis follows when the protoplast pulls away from the wall. The commonest slip is writing that water moves from low to high concentration without saying solute.
主动运输：逆浓度梯度要耗能
主动运输把物质从浓度低的一侧送到浓度高的一侧，方向与扩散相反，需要载体蛋白和呼吸作用提供的能量。根毛细胞吸收硝酸根、镁离子这类矿质盐，靠的就是主动运输，因为土壤溶液里的离子浓度往往比细胞内低。小肠上皮细胞吸收葡萄糖也有主动运输参与。氧气和二氧化碳不走这条路，它们始终靠扩散。学生最常把主动运输说成速度快一点的扩散，两者的方向完全不同，这才是选择题常设的陷阱。
Active transport moves substances against a concentration gradient, from low to high concentration, and it needs carrier proteins plus energy released by respiration. Root hair cells use it to take up mineral ions such as nitrate and magnesium, because the soil solution is often more dilute than the cell contents, and glucose uptake in the small intestine involves the same mechanism. Oxygen and carbon dioxide never travel this way, since diffusion is enough for them. Describing active transport as faster diffusion misses the real difference, which is direction and energy cost.
表面积与体积比：为什么需要运输系统
细胞靠扩散获得的物质，随着体积增大越来越不够用，因为体积按边长的三次方增长，表面积只按二次方增长。表面积与体积比下降以后，中心的物质来不及扩散到，所以多细胞生物进化出循环系统和专门的运输组织。肺泡、小肠绒毛、根毛都是同一个思路：把交换面积做大，把壁做薄，把距离缩短。人体的毛细血管网密到几乎每个细胞旁边都有一条。常见的错误是把表面积大理解成细胞大，实际正好相反，交换面靠的是无数细小结构堆出来的总面积。
As a cell grows, its volume rises with the cube of its length while its surface area rises only with the square, so the surface area to volume ratio falls. Diffusion then becomes too slow to supply the centre, which is why multicellular organisms need circulatory systems and specialised transport tissues. Alveoli, villi and root hairs all follow one design, a large exchange surface, a thin barrier and a short diffusion distance, and the capillary network is dense enough that almost every cell sits beside one. A large surface area is often read as a large cell, yet the opposite is true, since the total comes from countless small structures.
血液的四种成分
血液由血浆、红细胞、白细胞和血小板组成。血浆是淡黄色的液体，运载葡萄糖、氨基酸、激素、二氧化碳和尿素。哺乳动物的红细胞没有细胞核，呈双凹圆盘状，里面装满血红蛋白，在肺部结合氧，在组织里释放氧。白细胞里的吞噬细胞能伸出伪足把病原体包进细胞内消化。血小板没有完整的细胞结构，负责在伤口处启动凝血。阅卷时经常看到学生把血浆和血清混着写，考纲只要求血浆。
Blood has four main components: plasma, red cells, white cells and platelets. Plasma is the pale yellow liquid that carries glucose, amino acids, hormones, carbon dioxide and urea; a mammalian red cell has no nucleus, is biconcave and is packed with haemoglobin so that it picks up oxygen in the lungs and releases it in the tissues. Phagocytes among the white cells extend pseudopodia and engulf pathogens, while platelets trigger clotting at a wound. Examiners still see plasma and serum used interchangeably, although the specification only asks for plasma.
心脏结构与双循环
心脏由心肌构成，收缩时把血液压向全身。右心房接收上、下腔静脉回流的缺氧血，经三尖瓣进入右心室，再由肺动脉送往肺部。左心房接收肺静脉带回的含氧血，经二尖瓣进入左心室，由主动脉泵向全身。心室与动脉之间有半月瓣，防止血液倒流。左心室壁比右心室厚，因为它要把血推到全身而不只是到肺。心脏自身的肌肉由冠状动脉供血，冠状动脉堵塞就是心梗。很多学生以为动脉里一定是含氧血，肺动脉就是那个例外。
The heart is a muscular pump: the right atrium receives deoxygenated blood from the vena cava, passes it through the tricuspid valve into the right ventricle and out along the pulmonary artery, while the left atrium takes oxygenated blood from the pulmonary vein, sends it through the bicuspid valve into the left ventricle and out through the aorta. Semi-lunar valves at the base of the arteries stop backflow, and the left ventricle wall is thicker because it drives blood around the whole body rather than only to the lungs. The heart muscle itself is supplied by the coronary vessels, whose blockage causes a heart attack. Many students assume every artery carries oxygenated blood, yet the pulmonary artery is the standard exception.
三类血管与毛细血管交换
动脉把血液带离心脏，管壁厚，肌肉和弹性纤维多，管腔窄，能承受高压。静脉把血液送回心脏，管壁薄，管腔宽，内有袋状瓣膜，靠骨骼肌挤压和胸腔负压推动血液。毛细血管只有一层内皮细胞厚，管腔极窄，血流最慢，总横截面积最大。物质就在毛细血管壁上完成交换，氧气和葡萄糖扩散出去，二氧化碳和尿素扩散进来。血管壁薄是扩散距离短的前提，网络密则保证每个细胞都靠近血液。常见的错误是把静脉瓣说成防止血液逆流回心室，它挡的是血液倒流回静脉远端。
Arteries carry blood away from the heart with thick muscular elastic walls, a narrow lumen and the ability to withstand high pressure; veins return blood with thin walls, a wide lumen and pocket valves that rely on skeletal muscle squeezing and the low pressure in the chest. Capillaries are one endothelial cell thick, extremely narrow and slow flowing, and their enormous total cross-sectional area makes them the exchange site, where oxygen and glucose diffuse out and carbon dioxide and urea diffuse in. Thin walls shorten the diffusion path, and dense networks keep every cell close to a supply. Vein valves stop blood falling back towards the capillaries rather than back into the ventricle, which is a common mix-up.
心血管疾病的风险因素与三种治疗
心血管疾病的风险因素包括吸烟、高血压、高胆固醇、肥胖、缺乏运动、糖尿病和家族史，年龄与性别也会影响发病率。吸烟让血管收缩并损伤血管内壁，高血压长期增加动脉壁的负担。他汀类药物抑制肝脏合成胆固醇，降低血液里的胆固醇水平。血管成形术把带气囊的导管送进狭窄的冠状动脉，充气撑开血管，通常再放入支架保持通畅。改变饮食和增加运动属于生活方式干预，风险最小，但对已经严重狭窄的血管作用有限。答题时要按优势与局限两头写，只写好处拿不到全部分。
Risk factors for cardiovascular disease include smoking, high blood pressure, high cholesterol, obesity, inactivity, diabetes and family history, with age and sex also affecting incidence. Smoking narrows vessels and damages the lining, while raised pressure puts long-term strain on artery walls. Statins reduce cholesterol synthesis in the liver and lower the level in the blood, and angioplasty threads a balloon catheter into a narrowed coronary artery, inflates it and usually leaves a stent in place. Diet and exercise carry the fewest risks but cannot open a vessel that is already badly narrowed, so answers need both advantages and limitations to reach the top band.
叶片结构与气孔开合
叶片上表面覆盖角质层，防水并减少水分蒸发。上表皮透明，让光穿过到达栅栏组织。栅栏组织细胞排列紧密，含大量叶绿体，是光合作用的主要场所。海绵组织细胞间隙大，方便气体扩散。叶脉里的木质部运水，韧皮部运蔗糖。气孔多分布在下表皮，由两个保卫细胞控制开合，保卫细胞吸水变膨压时气孔张开，失水时关闭。气孔既让二氧化碳进来、氧气出去，也让水蒸气跑掉，所以它的开合要在光合作用与保水之间权衡。
The leaf surface is covered by a waxy cuticle that limits water loss, and the transparent upper epidermis lets light reach the palisade layer, where tightly packed cells full of chloroplasts carry out most photosynthesis. Spongy mesophyll cells leave large air spaces for gas movement, and the veins contain xylem for water and phloem for sucrose. Stomata sit mainly in the lower epidermis and open when the guard cells take up water and become turgid, closing as they lose it; this lets carbon dioxide in and oxygen out but also lets water vapour escape, so the plant balances photosynthesis against water conservation.
根毛吸水与矿质盐吸收
根毛是根表皮细胞的细长突起，把吸收面积扩大很多倍。土壤溶液比细胞液稀的时候，水通过渗透进入根毛，再沿皮层进入木质部向上运输。矿质盐的处境相反，土壤里的离子浓度常常低于细胞内，所以根毛靠主动运输把它们吸进来，需要呼吸供能。硝酸根用来合成氨基酸和蛋白质，镁是叶绿素的组成成分。植物缺氮时生长缓慢、叶色变淡，缺镁时老叶先褪绿。学生最常把吸水与吸盐写成同一个过程，其实前者靠渗透，后者靠主动运输。
Root hairs are long thin extensions of epidermal cells that multiply the absorbing surface. When the soil solution is more dilute than the cell sap, water enters by osmosis and then moves across the cortex into the xylem; mineral ions face the opposite situation, since they are usually more concentrated inside the root than outside, so uptake depends on active transport and the energy from respiration. Nitrate is built into amino acids and proteins, magnesium sits at the centre of chlorophyll, and a shortage of nitrate stunts growth while a shortage of magnesium yellows the older leaves first. Water uptake and salt uptake are separate processes, and answers that merge them lose marks.
蒸腾作用与影响速率的条件
蒸腾是水从叶片表面蒸发，主要经过气孔。水在叶肉细胞表面蒸发，进入细胞间隙，再扩散出气孔。木质部里的水形成连续的液柱，叶面失水产生的拉力把水从根部往上带，这条路径叫蒸腾流。蒸腾把矿质盐带到地上部分，也帮助叶片降温。光照增强时气孔张开，蒸腾加快；温度升高，水分子动能增大，速率上升；湿度高时叶内外水汽差变小，速率下降；风把叶面湿润的空气吹走，速率上升。测量速率常用蒸腾计，读气泡移动的距离。
Transpiration is the evaporation of water from leaf surfaces, mostly through the stomata. Water evaporates from the mesophyll into the air spaces and diffuses out, and the resulting pull lifts a continuous column of water up the xylem from the roots, a flow known as the transpiration stream that also carries minerals upward and cools the leaf. Light opens the stomata, warmth raises the kinetic energy of water molecules, high humidity reduces the water vapour gradient and wind strips away the humid layer, so the rate responds to all four. A potometer estimates the rate from the movement of an air bubble.
韧皮部运输与营养缺乏
韧皮部把蔗糖从光合作用的叶片运到根、茎、果实和生长点，供呼吸消耗或转成淀粉储存。这个方向与木质部的水流相反，所以同一片叶脉里，水和糖走的是两套管子。筛管细胞成熟后没有细胞核，靠伴胞维持代谢，这些细节超出 WJEC 的要求，考纲只要求写出韧皮部运蔗糖。营养缺乏的表现按元素区分，缺硝酸根生长差、叶色淡，缺钾叶缘发黄，缺磷根系发育不良。农民用含氮、磷、钾的复合肥补这三种元素，用量过多又会污染水体。
Phloem carries sucrose from photosynthesising leaves to roots, stems, fruits and growing points, where it is respired or converted to starch for storage; this runs in the opposite direction to the xylem water, so a single vein contains two separate pipe systems. Sieve tube cells lose their nucleus at maturity and depend on companion cells, a detail beyond what WJEC asks, since the specification only requires phloem transporting sucrose. Deficiency symptoms differ by element: little nitrate gives poor growth and pale leaves, low potassium yellows leaf edges, and low phosphate weakens root growth, which is why NPK fertiliser supplies all three.
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