Edexcel生物 样方 食物链 能量传递 碳循环
Edexcel 国际 GCSE 生物（4BI1）第 4 章讲生态与环境，考纲编号从 4.1 排到 4.18B，两个 paper 都会考到它。Paper 1B 两小时 110 分，占 61.1%；Paper 2B 一小时十五分钟 70 分，占 38.9%，而且会覆盖带 B 标记的条目。这一章的题目看着像背诵，实际考的是把定义、取样、能量流动和物质循环串起来解释一个真实场景。下面按考纲顺序拆开十二个知识点，每节都给出中国学生常卡住的地方。
Section 4 of the Pearson Edexcel International GCSE Biology specification (4BI1) runs from point 4.1 to 4.18B, and both papers draw on it: Paper 1B lasts two hours and carries 110 marks, while Paper 2B lasts 1 hour 15 minutes and carries 70. On the surface the chapter looks like vocabulary to memorise, but the questions almost always ask you to place those terms inside a real habitat or a polluted river. What follows works through the twelve points in specification order and flags where marks are usually lost.
栖息地 种群 群落 生态系统四个定义（4.1）
考场里最常见的失分不是背不出定义，而是把四个词用混。栖息地（habitat）指一个生物生活的具体地方，比如潮间带的一段石岸。种群（population）指同一时间、同一栖息地里同一个物种的全部个体。群落（community）指这片栖息地里所有物种的种群加在一起。生态系统（ecosystem）再往前一步，把群落和周围的非生物环境算成一个整体。判断标准很直接：题目说的是一个物种就写种群，说的是所有物种就写群落，一旦提到温度、光照、土壤，答案就是生态系统。很多学生以为生态系统必须很大，其实一块腐木、一个水坑也能构成一个生态系统。
Examiners see the same four words mixed up every year, and the confusion is nearly always about scale. A habitat is the physical place an organism lives, a population is every individual of one species there at one time, a community is all the populations of every species sharing that place, and an ecosystem adds the non-living conditions such as temperature, light and soil pH. A rotting log or a puddle counts as an ecosystem just as legitimately as a rainforest, so size is not part of the definition.
样方随机取样与种群数量估算（4.2）
样方（quadrat）通常是一个 0.5 米乘 0.5 米的方框，面积 0.25 平方米。取样时用随机数定坐标，避免专挑花多的地方放框。数完每个样方里的个体数，取平均值，再按总面积放大。举个具体的例子：一块 200 平方米的草坪放 10 个样方，合计数到 40 株蒲公英，平均每个样方 4 株。按 200 除以 0.25 换算，整块草坪相当于 800 个样方的面积。估算总数就是 4 乘 800，等于 3200 株。常见的错误是忘记换算面积，直接把平均数当成总数写上去。
A quadrat is a frame, commonly 0.5 m by 0.5 m, giving an area of 0.25 m squared, and random coordinates keep the sample away from the showiest corner of the field. Ten frames on a 200 m squared lawn holding 40 dandelions give a mean of four per frame, and 200 divided by 0.25 shows the lawn holds 800 frame areas, so the estimate is 3,200 plants. Marks disappear when the mean is reported as the total, or when quadrat size and frame number are changed between two areas that are meant to be compared.
生物多样性怎么量 物种数与覆盖率（4.3B 4.4B）
生物多样性在考纲里是单独一条（4.3B），指一个栖息地里物种的丰富程度。它既看物种数量，也看各物种的个体数是否均衡。测量时可以和样方结合（4.4B），记录每个样方里出现哪些物种。算出物种数以后，再看每种的数量分布，就能比较两块地的差别。覆盖率（percentage cover）是另一种记录方式，用网格估算某种植物占的面积比例。这种做法速度快，但主观成分大，同一个人两次读数都可能不同。想让数据站得住，样方数量要够，取样点要覆盖不同区域。
Biodiversity is a named specification point in its own right (4.3B), covering both how many species are present and how evenly individuals are spread between them. Quadrats can supply that evidence, with each frame recording which species appear and in what numbers, while percentage cover offers a faster but more subjective alternative. A trampled path across a lawn often scores higher than the middle of the lawn, because flattened grasses leave space for small species, so reliability comes from many frames across several zones rather than one careful frame.
非生物因素 温度 光照 湿度 土壤pH（4.5）
非生物因素指环境中所有非生命的条件，考纲点名的有温度、光照强度、湿度和土壤 pH。温度决定酶的活性，也决定变温动物能不能活动。光照强度限制光合作用速率，林冠下的植物往往叶片更大、颜色更深。湿度影响蒸腾，干燥环境里的植物常有卷叶、绒毛或厚角质层。土壤 pH 决定养分能不能被根系吸收，喜酸植物栽在碱性土里会发黄。记录这些数值时要在同一天同一时段测，否则两地的比较没有意义。测量工具也要写清，温度用温度计，光照用光强计，土壤 pH 用 pH 计或指示剂。
Abiotic factors are the non-living conditions of a habitat, and the specification names temperature, light intensity, moisture and soil pH as the ones to know. Each needs a mechanism rather than a label: temperature sets enzyme activity and so the pace of growth, light intensity limits photosynthesis, moisture drives transpiration and shapes leaf adaptations, and soil pH controls which mineral ions stay available. Readings taken at noon cannot be compared with readings taken at dusk, and the instruments belong in the answer too, since a thermometer, a light meter and a pH meter measure different things.
生物因素 竞争 捕食 疾病 食物供应（4.5）
生物因素指其他生物带来的影响，包括竞争、捕食、疾病和食物可得性。竞争发生在资源有限的时候，两种草争水和矿质离子，结果往往是一方数量下降。捕食者把猎物的数量压下去，猎物减少以后捕食者自己也跟着减少。疾病在种群密度高时传播更快，所以密植的作物更容易成片发病。食物不足会降低繁殖成功率，鸟的窝卵数常常随当年的昆虫数量变化。这几类因素合起来，决定一个种群能有多大、分布在哪块地方。写答案时要点明是哪一个因素在起作用，笼统写环境不好拿不到分。
Biotic factors are the effects of other organisms, and the specification lists competition, predation, disease and food availability. The feedback pattern is what earns marks: predators reduce prey numbers, prey shortages later reduce predator numbers, disease spreads fastest where density is high, and a poor food year lowers breeding success. Competition is usually described far too vaguely, so name the resource being contested, whether that is water, mineral ions or light, and state which species declines as a result.
营养级名称 生产者与各级消费者（4.6）
生产者（producer）自己制造有机物，绿色植物和藻类都属于这一类。初级消费者吃生产者，次级消费者吃初级消费者，三级消费者再往上排。分解者（decomposer）包括细菌和真菌，把死体和排泄物里的有机物拆开，让养分回到环境。食物链的箭头表示物质和能量流动的方向，从被吃的一方指向吃的一方。很多学生以为箭头表示谁被谁吃，其实写成草原指向蚱蜢，读作草原被蚱蜢吃。营养级只数到消费者，分解者不出现在食物链的箭头里。阅卷时几乎每场都能看到把分解者摆在第四级的答案。
Trophic levels are named by feeding position, so producers come first, followed by primary, secondary and tertiary consumers, with decomposers working alongside the chain rather than inside it. The arrow convention catches people out, because it points in the direction that substances and energy travel, meaning grass to grasshopper and never the reverse. A recurring error is placing decomposers at the end of a food chain as a fourth trophic level, which ignores that they recycle nutrients from every level at once.
食物链与食物网 移除一个物种的影响（4.7 4.8）
食物链是一条线性路径，食物网是许多条链交织在一起。做食物网的题目，先确定生产者，再顺着箭头找所有可能的路径。移除一个物种的影响取决于它在网里有多少替代食物。只吃一种猎物的捕食者受影响最大，杂食性的物种受影响小得多。物质沿食物链传递，碳和氮从生产者进入消费者体内，组成蛋白质和脂肪。能量沿同一方向流动，但每过一级都要损耗一部分。回答影响类题目时，要点落在数量变化和食物来源变化上。只写生态失衡这类空话，得分通常很低。
A food chain is a single route through a community, while a food web shows how many routes overlap, and questions reward you for tracing them in order instead of spotting one path. Removing a species has different consequences depending on how replaceable its role is: a specialist predator collapses, an omnivore switches food, and a whole web shifts when a producer disappears. Substances travel the same way as energy, with carbon and nitrogen built into proteins and fats at each level, so impact answers should name the population change and the change in food supply rather than announce that the ecosystem is unbalanced.
只有约一成能量传下去 损耗在哪里（4.9）
考纲明确要求解释为什么只有约 10% 的能量传到下一级。能量的去向有几条：一部分在呼吸作用中以热的形式散掉。一部分随粪便和尿液排出体外，还有一部分根本没被吃掉，比如根系、骨骼和外壳。剩下用于生长的那部分，才可能被下一级摄入。一个 10000 千焦的生产者层级，大约只有 1000 千焦进入初级消费者，传到次级消费者只剩约 100 千焦。这解释了为什么食物链一般不超过四到五级。常见的错误是只说能量被浪费，没有指出热量散失和未被同化这两条具体路径。
The specification asks you to explain why roughly one tenth of the energy passes to the next trophic level, not simply to quote the figure. Energy leaves a level as heat from respiration, in undigested material and urine, and in parts that are never eaten, such as roots, bone and shell, so only the fraction built into new growth is available. Starting from 10,000 kJ of producers, about 1,000 kJ enters primary consumers and roughly 100 kJ reaches secondary consumers, which explains why chains rarely run beyond four or five levels.
数量 生物量 能量三种金字塔（4.7）
数量金字塔记录每个营养级的个体数，形状可能倒过来。一棵橡树养活成千上万只蚜虫时，底下那一格就特别窄。生物量金字塔记录每一级的干重，通常逐级减少，因为它反映的是真实物质。能量传递金字塔一定正立，理由就是上一段那 10% 的损耗。画图时每格的宽度要按数值成比例，不能随手画几个方块。单位也要写清，数量用个体数，生物量用克或千克干重，能量用千焦每平方米每年。三类图放在一起比较时，先看清题目问的是哪一个量。
Pyramids of number count individuals at each level, and their shape can invert, as it does when a single oak supports thousands of aphids. Pyramids of biomass measure dry mass and almost always taper upwards, while pyramids of energy transfer must taper because they are built from the ten per cent rule. Bar width has to be drawn to scale and the units stated, using individuals, grams or kilograms of dry mass, and kilojoules per square metre per year respectively.
碳循环 四个过程与方向（4.10）
碳循环考纲只要求描述四个过程，但每一步的方向要写清。光合作用把大气中的二氧化碳固定成葡萄糖，碳由此进入生物体。呼吸作用在植物、动物和分解者体内进行，把有机物分解并释放二氧化碳。分解作用由细菌和真菌完成，死体和排泄物里的碳回到大气或土壤。燃烧化石燃料和木材，把封存了几百万年的碳快速释放出来。答题时把过程名和方向一起写，比单列术语更容易满点。很多学生以为植物白天只进行光合作用，其实呼吸作用全天都在进行。
The carbon cycle is a describe question, and the marks sit in the direction of each step rather than in naming processes alone. Photosynthesis takes carbon dioxide from the atmosphere into glucose, respiration in plants, animals and decomposers returns it, decomposition releases carbon held in dead matter and waste, and combustion of fossil fuels and wood releases carbon that was stored for millions of years. Writing the direction explicitly, atmosphere to organism for photosynthesis and back again for respiration, is what separates a full answer from a list.
氮循环 四类细菌各做什么（4.11B）
氮气占大气约 78%，绝大多数生物却无法直接利用。根瘤菌生活在豆科植物的根瘤里，把氮气转成含氮化合物供植物使用。土壤里的固氮细菌也做同样的事，闪电偶尔也会把氮气变成含氮化合物。分解者把死体和尿素里的蛋白质拆成氨，氨溶于水形成铵盐。硝化细菌在氧气充足的条件下把铵盐变成亚硝酸盐，再变成硝酸盐。植物根系以主动运输吸收硝酸盐，用来合成氨基酸和蛋白质。反硝化细菌在缺氧的积水土壤里把硝酸盐还原成氮气。氮就这样离开土壤回到大气，这也是积水地力下降的原因之一。
Nitrogen makes up about 78 per cent of the atmosphere, yet neither plants nor animals can use the gas directly, which is why the cycle exists. Nitrogen-fixing bacteria in the root nodules of legumes turn the gas into usable nitrogen compounds, decomposers release ammonium compounds from dead matter and urea, and nitrifying bacteria oxidise those compounds into nitrites and then nitrates in well-aerated soil. Plants absorb nitrates by active transport and build amino acids, while denitrifying bacteria in waterlogged, oxygen-poor soil return nitrogen to the air, which is why compacted ground loses fertility.
人类影响 酸性气体 温室气体 富营养化（4.12 至 4.18B）
这一组考纲点从 4.12 排到 4.18B，覆盖空气污染、温室气体、污水、富营养化和森林砍伐。二氧化硫主要来自含硫化石燃料的燃烧，进入大气后形成酸雨。酸雨使土壤和湖水酸化，还会损伤叶片表面，降低光合作用。一氧化碳来自不完全燃烧，它与血红蛋白结合的能力远强于氧气。温室气体考纲列了水蒸气、二氧化碳、一氧化二氮、甲烷和氟氯烃五种。增强的温室效应可能带来全球变暖，进而影响海平面、物种分布和珊瑚礁。污水和肥料里的硝酸盐进入水体后，藻类暴发遮住阳光，水草死亡，分解者大量繁殖并耗尽溶解氧。森林砍伐会加剧淋溶、土壤侵蚀，并干扰蒸腾和碳循环。
The human influence points, 4.12 to 4.18B, are answered worst when students describe the pollutant and skip the biological consequence. Sulfur dioxide from burning fossil fuels forms acid rain that acidifies soils and lakes and damages leaf surfaces, carbon monoxide from incomplete combustion binds haemoglobin far more strongly than oxygen, and the greenhouse gases named in the specification are water vapour, carbon dioxide, nitrous oxide, methane and CFCs. Sewage and leached nitrate follow one sequence: nutrients, algal bloom, light blocked, plants dying, decomposers multiplying, dissolved oxygen exhausted, fish suffocating, while deforestation adds leaching, soil erosion and disruption of evapotranspiration and the carbon cycle.
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