📚 Science 391: IGCSE Science Mastery | Science 391：IGCSE科学精通
Welcome to Science 391, your comprehensive revision companion for the Edexcel IGCSE Science course. This guide covers the essential concepts, practical skills, and exam strategies you need to succeed, whether you are studying Biology, Chemistry, Physics, or the combined Science route.
欢迎来到 Science 391，这是你备战 Edexcel IGCSE 科学课程的全面复习伙伴。本指南涵盖了考试成功所需的核心理念、实验技能和解题策略，无论你学习的是生物、化学、物理，还是综合科学路径。
1. Biology: Cell Structure and Function | 生物：细胞结构与功能
All living organisms are made of cells. The key organelles to remember are the nucleus (contains DNA), mitochondria (site of respiration), ribosomes (protein synthesis), and, in plant cells, chloroplasts (photosynthesis) and a cellulose cell wall.
所有生物体都由细胞构成。需要记住的关键细胞器包括：细胞核（含 DNA）、线粒体（呼吸作用场所）、核糖体（蛋白质合成），以及植物细胞中的叶绿体（光合作用）和纤维素细胞壁。
The genetic material is stored in the nucleus.
遗传物质储存在细胞核中。
Bacterial cells lack a nucleus but have a circular DNA loop.
细菌细胞没有细胞核，但有一个环状 DNA 分子。
Specialised cells, such as red blood cells and root hair cells, are adapted to their functions.
特化细胞，如红细胞和根毛细胞，具有与其功能相适应的结构。
Magnification = Image size ÷ Actual size
Use this equation to calculate magnification, remembering to convert units (mm → μm → nm).
使用此公式计算放大倍数，注意单位换算（mm → μm → nm）。
2. Biology: Enzymes and Respiration | 生物：酶与呼吸
Enzymes are biological catalysts that speed up reactions. They are specific to substrates due to the shape of their active site. Temperature and pH can denature enzymes.
酶是加快反应速率的生物催化剂。由于其活性中心的形状，酶对底物具有专一性。温度和 pH 值会使酶变性。
Optimum temperature for most human enzymes is 37 °C.
大多数人体酶的最适温度为 37 °C。
Increasing temperature increases collision energy until the optimum, then activity falls sharply.
在达到最适温度前，升温会增加碰撞能量，之后活性急剧下降。
Aerobic respiration releases energy: glucose + oxygen → carbon dioxide + water
有氧呼吸释放能量：葡萄糖 + 氧气 → 二氧化碳 + 水
| Process | Location | Energy yield |
| Aerobic respiration | Mitochondria | High |
| Anaerobic respiration (animals) | Cytoplasm | Low |
3. Chemistry: Atomic Structure and Bonding | 化学：原子结构与化学键
An atom consists of protons, neutrons, and electrons. The atomic number equals the number of protons, while the mass number equals protons plus neutrons. Isotopes are atoms of the same element with different neutron numbers.
原子由质子、中子和电子组成。原子序数等于质子数，质量数等于质子数加中子数。同位素是同一元素中具有不同中子数的原子。
Chemical bonding involves the transfer or sharing of electrons:
化学键涉及电子的转移或共用：
Ionic bonding: metal + non-metal → transfer of electrons
离子键：金属 + 非金属 → 电子转移
Covalent bonding: non-metal + non-metal → sharing of electrons
共价键：非金属 + 非金属 → 电子共用
Metallic bonding: a lattice of positive ions in a sea of delocalised electrons
金属键：正离子晶格沉浸在离域电子的“电子海”中
4. Chemistry: Acids, Bases, and Salts | 化学：酸、碱和盐
Acids release hydrogen ions (H⁺) in water, while bases accept them. The pH scale runs from 0 (strong acid) to 14 (strong alkali), with 7 neutral.
酸在水中释放氢离子（H⁺），碱接受氢离子。pH 范围从 0（强酸）到 14（强碱），7 为中性。
Acid + Base → Salt + Water
For a common salt preparation, use the equation:
对于常见的盐制备，使用以下方程式：
HCl + NaOH → NaCl + H₂O
Insoluble bases can be added to acids until no more reacts.
可向酸中加入不溶性碱，直至不再反应。
Filtration removes excess solid, then the filtrate is crystallised.
过滤除去过量固体，然后结晶滤液。
5. Physics: Forces and Motion | 物理：力与运动
Speed, velocity, and acceleration are fundamental. Velocity includes direction, while speed does not. Acceleration is the rate of change of velocity.
速率、速度和加速度是基础概念。速度包含方向，速率不包含。加速度是速度的变化率。
v = u + at
Newton's Second Law relates force, mass, and acceleration:
牛顿第二定律关联力、质量和加速度：
F = ma
Weight = mass × gravitational field strength (on Earth g ≈ 9.8 N/kg)
重力 = 质量 × 重力场强度（地球表面 g ≈ 9.8 N/kg）
Momentum = mass × velocity
动量 = 质量 × 速度
Stopping distance = thinking distance + braking distance
停车距离 = 反应距离 + 制动距离
6. Physics: Electricity and Magnetism | 物理：电与磁
Electric current is the flow of charge. Voltage (potential difference) pushes charge around the circuit. Resistance opposes the flow.
电流是电荷的流动。电压（电势差）推动电荷在电路中运动。电阻阻碍电荷流动。
V = I × R
In series circuits, current is the same everywhere. In parallel circuits, voltage is the same across each branch.
在串联电路中，电流处处相同。在并联电路中，每个支路两端的电压相同。
Magnetic field lines always point from north to south.
磁感线总是从北极指向南极。
An electromagnet can be made by wrapping a coil around an iron core.
将线圈缠绕在铁芯上即可制成电磁铁。
Transformers use alternating current to change voltage levels.
变压器使用交流电来改变电压水平。
7. Maths Skills for Science | 科学中的数学技能
You must be comfortable with averages, percentages, ratios, and simple algebra. Units are critical—check whether answers are in kg, m, s, or derived units.
你必须熟悉平均值、百分比、比率和简单代数。单位至关重要——检查答案是否以 kg、m、s 或导出单位表示。
| Scientist skill | Example question | Steps |
| Percentage change | Mass increased from 20 g to 24 g | (Change ÷ original) × 100 = 20% |
| Rate | 50 cm³ gas produced in 10 s | 50 ÷ 10 = 5 cm³/s |
| Rearranging equations | Find mass from F and a | m = F ÷ a |
Always show your working, even for multiple-choice questions, to avoid careless errors.
始终写出解题过程，即使是选择题也是如此，以避免粗心错误。
8. Experimental Design and Safety | 实验设计与安全
A well-designed experiment must have independent, dependent, and control variables. The independent variable is the one you change; the dependent variable is the one you measure.
一个设计良好的实验必须包含自变量、因变量和控制变量。自变量是你改变的变量；因变量是你测量的变量。
Use a control group to show that only the independent variable causes the effect.
使用对照组以证明只有自变量导致该效应。
Repeat measurements to calculate a mean and identify anomalies.
重复测量以计算平均值并识别异常值。
Wear safety goggles and handle chemicals with care, especially acids and flammable liquids.
佩戴护目镜并小心处理化学品，尤其是酸和易燃液体。
9. Exam Technique and Common Pitfalls | 考试技巧与常见错误
Read each question carefully. Underline command words such as “describe,” “explain,” “calculate,” and “compare”. They require different levels of detail.
仔细阅读每一道题。在“描述”、“解释”、“计算”和“比较”等指令词下划线标记，它们要求不同层次的细节。
Do not confuse “suggest” with “state”. Suggest requires you to use knowledge in a new context.
不要把“suggest（建议）”与“state（陈述）”混淆。Suggest 要求你运用知识解决新情境。
Include units in numerical answers, and give answers to the correct number of significant figures.
在数字答案中包含单位，并保留正确的有效数字位数。
For dice questions involving a graph, always label axes with quantity and unit.
对于涉及图像的题，务必用物理量和单位标注坐标轴。
10. Revision Strategies and Resources | 复习策略与资源
Use a structured revision plan: break the syllabus into small topics, test yourself regularly, and review mistakes. Active recall and past-paper practice are proven to boost marks.
使用结构化的复习计划：将大纲分解为小主题，定期自测，并回顾错题。主动回忆和真题练习已被证明能显著提高分数。
Create flashcards for definitions and equations.
为定义和方程式制作闪卡。
Draw mind maps to connect topics across Biology, Chemistry, and Physics.
绘制思维导图，将生物、化学和物理的主题联系起来。
Time your practice papers under exam conditions.
在模拟考试条件下计时完成练习卷。
Use the Edexcel specification as your checklist and tick off each point.
使用 Edexcel 考纲作为检查清单，逐项打勾。
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