📚 IGCSE CIE Biology: Unit Test Revision Guide – Cells and Organisation | IGCSE CIE 生物：单元测试卷复习指南 – 细胞与组织
This revision guide is built around the core Unit Test topics on Cells and Organisation for IGCSE CIE Biology. It consolidates key concepts you must master: from cell structures and specialised cells to diffusion, osmosis, active transport, enzymes, and biological molecules. Each section pairs a concise explanation in English with the same in Chinese, followed by essential revision points and common exam pitfalls. Use it to check your understanding before the unit test.
本复习指南围绕 IGCSE CIE 生物单元测试中细胞与组织的核心主题编写。它整合了必须掌握的关键概念：从细胞结构、特化细胞，到扩散、渗透、主动运输、酶和生物分子。每个小节都配有英文和中文的简洁解释，并列出重要复习要点和常见考试误区。在单元测试前用它来检验你的理解。
1. Cell Structure – Animal and Plant Cells | 细胞结构 – 动物细胞与植物细胞
All living organisms are made of cells. Animal cells contain a nucleus, cytoplasm, cell membrane, mitochondria, and ribosomes. Plant cells have the same basic components but also possess a cellulose cell wall, a large permanent vacuole, and chloroplasts. The nucleus controls cell activities and stores genetic material. The cell membrane controls what enters and leaves the cell. Mitochondria are the site of aerobic respiration. Ribosomes synthesise proteins. Chloroplasts contain chlorophyll and carry out photosynthesis. The vacuole contains cell sap and helps maintain turgor pressure.
所有生物体都由细胞构成。动物细胞包含细胞核、细胞质、细胞膜、线粒体和核糖体。植物细胞具有相同的基本成分，但还拥有纤维素细胞壁、大型永久液泡和叶绿体。细胞核控制细胞活动并储存遗传物质。细胞膜控制物质的进出。线粒体是有氧呼吸的场所。核糖体合成蛋白质。叶绿体含有叶绿素并进行光合作用。液泡含有细胞液，有助于维持膨压。
- Be able to draw and label an animal cell and a plant cell as seen under a light microscope. 能够绘制并标注光学显微镜下观察到的动物细胞和植物细胞。
- Know that the cell wall is fully permeable, while the cell membrane is selectively permeable. 要知道细胞壁是全透性的，而细胞膜是选择透过性的。
- Ribosomes are not surrounded by a membrane. 核糖体没有膜包围。
2. Specialised Cells | 特化细胞
Cells differentiate to carry out specific functions. Examples include red blood cells (no nucleus, biconcave shape, contain haemoglobin to transport oxygen); nerve cells (long axon, dendrites, to transmit electrical impulses); sperm cells (tail for movement, many mitochondria, acrosome with enzymes to penetrate the egg); root hair cells (long extension to increase surface area for water and mineral ion absorption); and xylem vessels (dead, hollow tubes strengthened with lignin, transport water). In multicellular organisms, specialised cells work together to form tissues, organs, and systems.
细胞分化以执行特定功能。例子包括：红细胞（无细胞核、双凹圆盘状、含有血红蛋白以运输氧气）；神经细胞（长轴突、树突、传递电信号）；精子细胞（有尾部用于运动、大量线粒体、顶体含有酶以穿透卵细胞）；根毛细胞（长突起以增大吸收水分和矿物质离子的表面积）；木质部导管（死细胞、中空管状、由木质素加固、运输水分）。在多细胞生物中，特化细胞共同形成组织、器官和系统。
| Cell Type 细胞类型 | Adaptation 适应性结构 | Function 功能 |
|---|---|---|
| Red blood cell 红细胞 | No nucleus, biconcave 无核、双凹形 | Maximise haemoglobin space; large surface area 最大化携氧空间；大表面积 |
| Sperm cell 精子 | Tail, many mitochondria 尾部、大量线粒体 | Swim to egg; energy for movement 游向卵细胞；提供运动能量 |
| Root hair cell 根毛细胞 | Long projection 长突起 | Increases surface area for uptake 增加吸收表面积 |
| Xylem vessel 木质部导管 | Hollow, lignin rings 中空、木质素环 | Continuous water column, support 连续水柱、支撑 |
3. Levels of Organisation | 组织层次
In multicellular organisms, cells of a similar type group together to form a tissue. Several tissues work together to make an organ, and organs are organised into organ systems. For example, in the human circulatory system, blood is a tissue (containing red cells, white cells, platelets, plasma); the heart is an organ made of muscle tissue, nervous tissue, and connective tissue; the circulatory system includes the heart, blood vessels, and blood. In plants, mesophyll tissue is made of palisade and spongy cells for photosynthesis, while the leaf is an organ containing epidermis, mesophyll, and vascular bundles.
在多细胞生物中，相似类型的细胞聚集形成组织。多个组织协同工作构成器官，器官又组成器官系统。例如，在人体循环系统中，血液是一种组织（包含红细胞、白细胞、血小板和血浆）；心脏是由肌肉组织、神经组织和结缔组织构成的器官；循环系统包括心脏、血管和血液。在植物中，叶肉组织由栅栏细胞和海绵细胞组成用于光合作用，而叶片是一个器官，包含表皮、叶肉和维管束。
- Tissue: group of similar cells working together. 组织：一组相似细胞协同工作。
- Organ: group of different tissues working together. 器官：不同组织协同工作。
- Organ system: group of organs working together. 器官系统：一组器官协同工作。
4. Diffusion | 扩散
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is a passive process and does not require energy. Factors affecting diffusion rate include temperature, concentration gradient, surface area, and particle size. In living organisms, diffusion allows oxygen and carbon dioxide to move in and out of cells, and digested food molecules to be absorbed in the intestine.
扩散是粒子从浓度较高的区域向浓度较低的区域发生的净移动，沿着浓度梯度进行。这是一个被动过程，不需要能量。影响扩散速率的因素有温度、浓度梯度、表面积和粒子大小。在生物体中，扩散使氧气和二氧化碳能够进出细胞，并使消化后的食物分子在肠道中被吸收。
Rate ∝ (Surface Area × Concentration Gradient) ÷ Diffusion Distance
速率 ∝ (表面积 × 浓度梯度) ÷ 扩散距离
5. Osmosis | 渗透
Osmosis is the diffusion of water molecules from a region where they are in higher concentration (dilute solution) to a region where they are in lower concentration (concentrated solution) through a partially permeable membrane. It is a special case of diffusion. Plant cells placed in pure water become turgid as water enters by osmosis; placed in concentrated sugar solution, they lose water and become plasmolysed. Animal cells in dilute solutions can swell and burst (lysis), whereas in concentrated solutions they shrink (crenation).
渗透是水分子通过部分渗透膜从水分子浓度较高（稀溶液）的区域向水分子浓度较低（浓溶液）的区域发生的扩散。它是扩散的一种特殊情况。将植物细胞放入纯水中，水通过渗透进入，细胞变得硬挺；放入浓糖溶液中，细胞失水，发生质壁分离。动物细胞在稀溶液中会膨胀甚至破裂（溶血），而在浓溶液中会皱缩。
- Always mention “partially permeable membrane” in osmosis definitions. 在渗透的定义中一定要提到“部分渗透膜”。
- Understand how to calculate percentage change in mass for osmosis experiments. 理解如何计算渗透实验中质量变化的百分比。
6. Active Transport | 主动运输
Active transport is the movement of particles against a concentration gradient, from a region of lower concentration to a region of higher concentration, using energy released by respiration. Carrier proteins in the cell membrane are required. This process is vital for the uptake of mineral ions by root hair cells even when concentrations are higher inside the cell, and for the absorption of glucose in kidney tubules.
主动运输是粒子逆浓度梯度移动，即从浓度较低的区域移动到浓度较高的区域，并利用呼吸作用释放的能量。需要细胞膜上的载体蛋白参与。这一过程对于根毛细胞吸收矿物质离子（即使细胞内浓度更高）以及肾小管对葡萄糖的重吸收至关重要。
| Feature 特征 | Diffusion/Osmosis 扩散/渗透 | Active Transport 主动运输 |
|---|---|---|
| Energy required? 需要能量？ | No 否 | Yes (from respiration) 是（来自呼吸作用） |
| Concentration gradient 浓度梯度 | Down 顺浓度梯度 | Against 逆浓度梯度 |
| Protein involvement 蛋白质参与 | None (except facilitated) 无（协助扩散除外） | Carrier proteins 载体蛋白 |
7. Enzymes – Nature and Role | 酶 – 性质与作用
Enzymes are biological catalysts made of protein. They speed up metabolic reactions by lowering the activation energy. Each enzyme has an active site with a specific shape, complementary to a specific substrate. The lock-and-key model describes this specificity. Factors affecting enzyme activity include temperature, pH, enzyme concentration, and substrate concentration. At extreme pH or high temperatures, the enzyme denatures: the active site changes shape and the substrate can no longer fit. Denaturation is usually irreversible.
酶是由蛋白质构成的生物催化剂。它们通过降低活化能来加速代谢反应。每个酶都有一个活性位点，具有特定的形状，与特定的底物互补。“锁钥模型”描述了这种特异性。影响酶活性的因素包括温度、pH、酶浓度和底物浓度。在极端 pH 或高温下，酶会变性：活性位点形状改变，底物不再能契合。变性通常是不可逆的。
- Optimum temperature for most human enzymes is around 37 °C. 大多数人酶的最适温度约为 37 °C。
- Pepsin works best at pH 2 (stomach), while amylase works at pH 7 (mouth). 胃蛋白酶在 pH 2（胃）时活性最佳，而淀粉酶在 pH 7（口腔）时最佳。
- Always say “enzyme-substrate complex” when describing action. 描述作用时务必提到“酶-底物复合物”。
8. Biological Molecules – Carbohydrates, Proteins, Lipids | 生物分子 – 碳水化合物、蛋白质、脂质
Large biological molecules are made from smaller repeating units. Carbohydrates include starch and glycogen (polysaccharides of glucose) and simple sugars like glucose. Proteins are built from amino acids joined by peptide bonds. Lipids (fats and oils) consist of fatty acids and glycerol. The Benedict’s test detects reducing sugars (blue to brick-red on heating); iodine solution tests for starch (orange-brown to blue-black); Biuret reagent tests for proteins (blue to purple); ethanol emulsion test detects lipids (cloudy white emulsion).
生物大分子由较小的重复单元构成。碳水化合物包括淀粉和糖原（葡萄糖的多糖）以及像葡萄糖这样的简单糖类。蛋白质由通过肽键连接的氨基酸构成。脂质（脂肪和油）由脂肪酸和甘油组成。本尼迪克特试验用于检测还原糖（加热后由蓝色变为砖红色）；碘液检测淀粉（由橙棕色变为蓝黑色）；双缩脲试剂检测蛋白质（蓝色变为紫色）；乙醇乳化试验检测脂质（出现乳白色浑浊）。
| Molecule 分子 | Building Block 构建单元 | Example 例子 |
|---|---|---|
| Carbohydrate 碳水化合物 | Simple sugars (e.g. glucose) 单糖 | Starch, glycogen 淀粉、糖原 |
| Protein 蛋白质 | Amino acids 氨基酸 | Enzymes, haemoglobin 酶、血红蛋白 |
| Lipid 脂质 | Fatty acids + glycerol 脂肪酸 + 甘油 | Triglycerides 甘油三酯 |
9. Magnification and Microscopy | 放大率与显微镜使用
To measure cell sizes, you must be able to use the formula Magnification = Image size ÷ Actual size. Ensure both measurements are in the same units. Convert millimetres (mm) to micrometres (µm) by multiplying by 1000: 1 mm = 1000 µm. When drawing microscope images, use clear, single lines, label with straight ruled lines, and include a scale bar. The IGCSE practical exam often asks you to calculate magnification or draw labelled diagrams from a prepared slide.
要测量细胞大小，你必须会使用公式：放大率 = 图像大小 ÷ 实际大小。确保两个测量值单位一致。将毫米 (mm) 转换为微米 (µm) 要乘以 1000：1 mm = 1000 µm。绘制显微镜图时，使用清晰、单线条，用直尺画标注线，并包含比例尺。IGCSE 实验考试常要求计算放大率或根据现成玻片绘制标注图。
Magnification = Image size (I) ÷ Actual size (A)
放大率 = 图像大小 (I) ÷ 实际大小 (A)
10. Common Errors in Unit Tests | 单元测试常见错误
Many students lose marks by confusing cell wall with cell membrane, or forgetting that plant cells may lack chloroplasts (e.g., root cells). In osmosis, failing to state “net movement of water” or “partially permeable membrane” costs marks. When describing enzyme denaturation, avoid saying the enzyme “dies” – it is denatured, not killed. Finally, in practical data questions, always check units and significant figures.
许多学生因混淆细胞壁和细胞膜而失分，或忘记植物细胞可能没有叶绿体（如根细胞）。在渗透部分，未说明“水的净移动”或“部分渗透膜”会丢分。描述酶变性时，避免说酶“死亡”——它是变性，不是被杀死。最后，在实验数据题中，一定要检查单位和有效数字。
- Cell wall vs membrane: wall is for support and fully permeable. 细胞壁与细胞膜：壁用于支持，全透性。
- All plant cells have a vacuole but not all have chloroplasts. 所有植物细胞都有液泡，但并非都有叶绿体。
- Enzymes: shape of active site is altered, not destroyed, at extreme pH. 酶：极端 pH 下活性位点形状改变，并非被破坏。
- In magnification calculations, show your working. 在放大率计算中，写出解题步骤。
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