📚 Year 7 CAIE Biology: Case Study Practical Drills | 案例分析与实战演练
Welcome to this collection of case studies designed for Year 7 CAIE Biology students. Through real-world scenarios, you will practise observation, analysis, and scientific reasoning. Each case study targets key topics such as cells, classification, ecology, and plant biology, helping you to apply textbook knowledge to practical situations. Every case is broken into small, repeatable steps, so you always know what to look at first, what to write down, and how to turn rough observations into a confident scientific conclusion. Let's sharpen your investigation skills with step-by-step workouts.
欢迎来到为 CAIE Year 7 生物学生设计的案例分析合集。通过真实情境的练习，你将锻炼观察、分析和科学推理能力。每项案例都围绕细胞、分类、生态学和植物生物学等关键主题，帮你把课本知识应用到实际问题中。每一个案例都被拆解成可重复的小步骤，因此你始终清楚先看什么、先记什么，以及如何把粗糙的观察转化为有把握的科学结论。让我们通过逐步演练，提升你的探究技能。
1. How to Work Through a Case Study | 如何完成一项案例分析
A case study is a short, realistic story with Biology hidden inside it. Your task is not to memorise the story but to extract the science: what was observed, what it means, and what should be checked next. Every case in this article follows the same four-step routine, which we call ORAC.
案例分析就是一段短小而真实的叙事，生物学知识藏在其中。你的任务不是背下这个故事，而是把科学内容提取出来：观察到了什么、意味着什么、下一步该检查什么。本文的每个案例都遵循同一套四步流程，我们把它称为 ORAC。
- O - Observe: read the scenario and underline every measurable fact, such as magnification, time, temperature or number of organisms.
- R - Record: turn those facts into a table with clear headings and correct units.
- A - Analyse: look for patterns, differences and results that do not fit.
- C - Conclude: answer the question in one sentence, then justify it with evidence from the data.
- O - 观察：阅读情境，划出每一个可测量的事实，例如放大倍数、时间、温度或生物数量。
- R - 记录：把这些事实整理成表格，要求表头清晰、单位正确。
- A - 分析：寻找规律、差异以及不符合规律的结果。
- C - 结论：用一句话回答问题，然后用数据中的证据加以论证。
Notice that a conclusion is never just "because it is green". A strong answer always names the evidence, for example "Organism A is a Euglena because it has chloroplasts and a flagellum but no rigid cell wall".
注意，结论绝不能只是"因为它是绿色的"。优秀的答案一定会点明证据，例如"生物 A 是眼虫，因为它有叶绿体和鞭毛，却没有坚硬的细胞壁"。
2. Case Study 1: The Pond Water Mystery | 案例一：池塘水之谜
Scenario: ith a 400× optical microscope. She saw several types of tiny objects moving in the water and recorded her observations in a table.
情境：一名七年级学生从当地池塘取了水样，将一滴水放在显微镜载玻片上，盖上盖玻片，在 400 倍光学显微镜下观察。她看到了几种微小的物体在水中活动，并将观察结果记录在表格中。
| Organism | Shape and colour | Structures seen | Movement |
| A | Oval, shape, bright green | Many green discs (chloroplasts); one whip-like tail (flagellum); no rigid cell wall visible | Moves quickly, tail first |
| B | Rectangular, bright green | Thick outer wall; chloroplasts; cells joined in short chains | Does not move |
| C | Slipper-shaped, colourless | Tiny hair-like cilia all around the body; no chloroplasts | Spins and darts rapidly |
| D | Tiny round green dots | Thin wall; single green disc inside each cell | Floats passively |
The first observation to make is not about biology but about method: the student used a cover slip, which flattens the water into a single layer so that light can pass through and the organisms stay in focus.
首先要留意的不是生物学本身，而是方法：这名学生使用了盖玻片，它把水压成薄薄一层，使光线可以透过，生物也能保持在焦点上。
3. Reading the Evidence: What Organism A Really Is | 解读证据：生物 A 究竟是什么
Organism A is oval, bright green, has many chloroplasts, has one flagellum, and has no visible rigid cell wall. This combination is unusual because it mixes plant features with animal features.
生物 A 呈椭圆形、亮绿色、含有许多叶绿体、有一根鞭毛，且看不到坚硬的细胞壁。这一组合很不寻常，因为它把植物的特征和动物的特征混合在了一起。
- Plant-like evidence: bright green colour and chloroplasts, which allow photosynthesis.
- Animal-like evidence: a flagellum for movement and no rigid cellulose cell wall.
- 类似植物的证据：亮绿色以及叶绿体，使其能够进行光合作用。
- 类似动物的证据：用于运动的鞭毛，以及没有坚硬的纤维素细胞壁。
A cell that has both sets of features is placed in the kingdom Protoctista. The organism is therefore a Euglena, a single-celled protoctist that photosynthesises in sunlight but can also swim towards brighter light using its flagellum.
同时具备这两类特征的细胞被归入原生生物界（Protoctista）。因此该生物是眼虫（Euglena），一种单细胞原生生物：在阳光下进行光合作用，同时能用鞭毛游向更亮的光照区域。
| Structure | Function |
| Chloroplast | Absorbs light energy for photosynthesis |
| Flagellum | Whips to move the cell through water |
| Cell membrane | Controls what enters and leaves the cell |
| Nucleus | Contains genetic material and controls the cell |
4. Magnification and Actual Size Calculations | 放大倍数与实际大小计算
CAIE questions often ask you to work out the real size of something you have measured on a microscope drawing. You need two pieces of information: the magnification used and the measured image size.
CAIE 的题目经常要求你根据显微镜下测量到的图像，算出物体的真实大小。你需要两条信息：所用的放大倍数，以及测量到的图像大小。
Actual size = Image size ÷ Magnification
Suppose the student' 0 μm. Always convert to micrometres when the answer is very small, because 1 mm = 1000 μm.
假设这名学生画出的生物 A 在 400 倍放大下长 8 mm，则计算为 8 mm ÷ 400 = 0.02 mm，即 20 μm。当答案很小时，一定要换算成微米，因为 1 mm = 1000 μm。
- Measure in millimetres with a ruler, then divide by the magnification written on the diagram.
- Do not forget the unit. A number without a unit scores no mark.
- Check whether the question asks for the magnification instead: Magnification = Image size ÷ Actual size.
- 先用尺子按毫米测量，再除以图上标注的放大倍数。
- 不要漏写单位。只写数字不写单位不得分。
- 注意题目是否反过来求放大倍数：放大倍数 = 图像大小 ÷ 实际大小。
5. Case Study 2: Building a Dichotomous Key | 案例二：构建二歧检索表
Scenario: the same pond water produced eight different organisms. The class was asked to sort them into groups using a dichotomous key, so that anyone could identify a new sample by following the same steps.
情境：同一份池塘水中共发现了八种不同生物。老师要求全班用二歧检索表把它们分组，使任何人只要按照同样的步骤，就能鉴定一份新样品。
A dichotomous key is a series of paired questions. Each pair must contain only two options, and each option must lead either to a name or to the next pair of questions.
二歧检索表由一系列成对的问题组成。每一对只能有两个选项，而每个选项既要能导向一个名称，也要能导向下一对问题。
- 1a. Has chloroplasts (green) → go to 2
- 1b. No chloroplasts (colourless) → go to 3
- 2a. Has a rigid cell wall → green algae
- 2b. No rigid cell wall → Euglena
- 3a. Covered in cilia → Paramecium
- 3b. Not covered in cilia → bacteria
Scientists also give every organism a two-part scientific name in the binomial system: the genus starts with a capital letter and the species starts with a small letter, and the whole name is printed in italics, for example Euglena viridis.
科学家还给每种生物取一个由两部分组成的学名，这就是双名法：属名首字母大写，种名首字母小写，整个学名用斜体印刷，例如Euglena viridis。
| Rank | Example |
| Kingdom | Protoctista |
| Genus | Euglena |
| Species | Euglena viridis |
6. Case Study 3: The Pond Food Web | 案例三：池塘食物网
Scenario: a class surveyed the pond and recorded what each organism ate. They found microscopic green algae, water fleas, small fish, tadpoles, dragonfly larvae and a heron.
情境：某班对这座池塘进行了调查，记录了各种生物的食物。他们发现了微小的绿色藻类、水蚤、小鱼、蝌蚪、蜻蜓幼虫和一只苍鹭。
A food chain always starts with a producer and the arrows always point in the direction that energy travels, that is, towards the animal that eats the previous one.
食物链总是从生产者开始，而箭头始终指向能量流动的方向，也就是指向前一个生物被谁吃掉。
Algae → Water flea → Small fish → Heron
- Producer: algae, which make glucose by photosynthesis.
- Primary consumer: water flea, a herbivore.
- Secondary consumer: small fish, which eats water fleas.
- Top predator: heron, which has no predator in this pond.
- Decomposers: bacteria and fungi that break down dead material and return nutrients to the water.
- 生产者：藻类，通过光合作用制造葡萄糖。
- 初级消费者：水蚤，植食性动物。
- 次级消费者：小鱼，以水蚤为食。
- 顶级捕食者：苍鹭，在这座池塘中没有天敌。
- 分解者：细菌与真菌，分解死亡物质并把养分归还水体。
Only about one tenth of the energy passes from one level to the next, because most of it is lost in movement, heat and undigested waste. This is why the number of organisms usually becomes smaller as you move along a food chain.
相邻两个营养级之间大约只有十分之一的能量能够传递下去，因为大部分能量在运动、散热和未消化的废物中损失了。这就是为什么沿着食物链往后，生物数量通常会越来越少。
7. Case Study 4: The Wilting Tomato Plant | 案例四：萎蔫的番茄植株
Scenario: a tomato plant on a sunny windowsill was watered every morning. On Tuesday, after a hot afternoon with the window open, all its leaves drooped. By the next morning, without any extra water, the leaves were firm again.
情境：窗台上一株番茄每天早上都被浇水。周二，在开窗的炎热午后之后，它的叶片全部下垂。到了第二天早上，并没有额外浇水，叶片却又变得坚挺了。
Water is absorbed by root hair cells, moves up the stem in the xylem, and escapes from the leaves as water vapour through the stomata. This whole process is called transpiration.
水分由根毛细胞吸收，经木质部沿茎向上运输，最后以水蒸气的形式通过气孔从叶片散失。这一整个过程叫做蒸腾作用。
- High temperature: speeds up transpiration, so more water is lost.
- Moving air: removes water vapour from around the leaf, so diffusion is faster.
- Low humidity: creates a steeper concentration gradient, so water leaves faster.
- Bright light: opens the stomata wider.
- 高温：加快蒸腾，水分损失更多。
- 空气流动：带走叶片周围的水蒸气，使扩散更快。
- 湿度低：形成更陡的浓度梯度，水分流失更快。
- 光照强：使气孔张得更开。
On Tuesday afternoon the plant lost water faster than the roots could replace it, so the cells lost their internal pressure and the leaves drooped. Overnight the stomata closed, transpiration slowed down, and the roots had time to refill the xylem, which is why the leaves recovered without extra water.
周二下午，植株失水的速度超过了根部补水的速度，细胞失去内部压力，叶片于是下垂。夜间气孔关闭，蒸腾减慢，根系有时间重新充满木质部，因此叶片在没有额外浇水的情况下也恢复了。
8. Case Study 5: Counting Bubbles with Pondweed | 案例五：用池塘草计气泡
Scenario: a group placed a sprig of pondweed in a beaker of water with dissolved carbon dioxide and counted the oxygen bubbles released per minute while moving a lamp to different distances.
情境：某小组把一小枝池塘草放入含溶解二氧化碳的烧杯中，一边把灯移到不同距离，一边统计每分钟释放的氧气气泡数。
CO₂ + H₂O → C₆H₁₂O₆ + O₂ (light energy and chlorophyll needed)
- Independent variable: the distance of the lamp from the pondweed.
- Dependent variable: the number of bubbles per minute.
- Control variables: temperature, volume of water, mass of pondweed, concentration of carbon dioxide.
- 自变量：灯与池塘草之间的距离。
- 因变量：每分钟的气泡数。
- 控制变量：温度、水的体积、池塘草的质量、二氧化碳浓度。
Results showed that as the lamp moved closer, the bubble count rose quickly at first and then levelled off. The plateau appears because at high light intensity something else, such as carbon dioxide concentration or temperature, becomes the limiting factor.
结果显示，灯越靠近，气泡数起初上升很快，随后趋于平稳。出现平台是因为在强光下，别的因素（如二氧化碳浓度或温度）成了限制因素。
9. Practical Skills: Variables, Controls and Fair Tests | 实验技能：变量、对照与公平实验
Every practical question in CAIE Biology rewards the same vocabulary, so learn it once and reuse it in every case study.
CAIE 生物的每道实验题都奖励同一套术语，所以只要学会一次，就能在所有案例中反复使用。
- A fair test changes only one variable and keeps everything else the same.
- A control is a set-up with no treatment, used for comparison.
- Repeat readings and take a mean to reduce the effect of random error.
- Anomalous results should be identified and left out of the mean, not simply deleted without comment.
- 公平实验只改变一个变量，其余条件全部保持一致。
- 对照是不施加处理的实验组，用于比较。
- 重复读数并求平均值，可以减小随机误差的影响。
- 异常值应被识别并排除在平均值之外，而不是不加说明地直接删掉。
When writing a method, use the future tense and precise measurements: "Place 200 cm³ of water in a 500 cm³ beaker and put it 10 cm from the lamp", not "put some water near a lamp".
书写实验方法时要用将来时和精确的测量数值："在 500 cm³ 烧杯中量取 200 cm³ 水，并将其放置在距灯 10 cm 处"，而不是"把一些水放在灯附近"。
10. Handling Data: Means, Anomalies and Graphs | 数据处理：平均值、异常值与图表
Data questions are worth easy marks, but only if you follow the rules precisely. A mean is calculated by adding the results and dividing by the number of results.
