Category: KS3 科学

一、物质是什么？从身边的例子理解”物质”的定义 | What Is Matter? Understanding the Definition Through Everyday Examples

物质是构成我们周围一切事物的基本材料 – 你正在阅读的屏幕、呼吸的空气、喝的水，甚至你自己，都是由物质组成的。在科学中，”物质”被定义为任何具有质量并占据空间的东西。换句话说，如果你可以把它放在天平上称重，它就是物质。KS3阶段的学习从这一定义出发，帮助学生建立对物质世界的基本认知框架。

Matter is the fundamental material that makes up everything around us – the screen you’re reading from, the air you breathe, the water you drink, and even you yourself are all made of matter. In science, “matter” is defined as anything that has mass and takes up space. In other words, if you can put it on a scale and weigh it, it is matter. The KS3 curriculum begins from this definition, helping students build a foundational framework for understanding the material world.

想一想：光和声音是物质吗？答案是否定的 – 光没有质量，不占据空间，所以它不是物质，而是一种能量形式。热也是如此。区分”物质”和”能量”是KS3科学中的第一个重要概念分界点。

Think about it: are light and sound matter? The answer is no – light has no mass and does not take up space, so it is not matter; it is a form of energy. The same applies to heat. Distinguishing between “matter” and “energy” is the first important conceptual boundary in KS3 science.

二、物质的三种状态：固体、液体和气体之间的本质区别 | The Three States of Matter: The Essential Differences Between Solids, Liquids, and Gases

地球上的物质通常以三种状态存在：固体、液体和气体。每一种状态都有其独特的性质，这些性质取决于构成物质的微小粒子如何排列和移动。

Matter on Earth typically exists in three states: solid, liquid, and gas. Each state has its own distinct properties, which depend on how the tiny particles that make up the matter are arranged and how they move.

固体 (Solids)

固体具有固定的形状和固定的体积。无论你把一块石头放在桌子上还是放进杯子里，它的形状都不会改变。这是因为固体中的粒子紧密地排列在一起，形成规则的图案（我们称之为晶格结构），每个粒子只能在固定的位置上振动，无法自由移动。固体粒子之间的作用力非常强，所以固体很难被压缩。

Solids have a fixed shape and a fixed volume. Whether you place a rock on a table or put it inside a cup, its shape does not change. This is because the particles in a solid are tightly packed together in a regular pattern (what we call a lattice structure), and each particle can only vibrate in a fixed position without being able to move freely. The forces between solid particles are very strong, which is why solids are very difficult to compress.

液体 (Liquids)

液体具有固定的体积但没有固定的形状 – 它会呈现容纳它的容器的形状。液体中的粒子仍然紧密地挤在一起，但它们之间的排列是随机的、不规则的。粒子可以相互滑过，这意味着液体可以流动。液体粒子之间的作用力比固体弱，但仍然足够强，使得液体难以被压缩。

Liquids have a fixed volume but no fixed shape – they take the shape of the container that holds them. The particles in a liquid are still closely packed together, but their arrangement is random and irregular. The particles can slide past each other, which means liquids can flow. The forces between liquid particles are weaker than in solids, but still strong enough that liquids are difficult to compress.

气体 (Gases)

气体既没有固定的形状也没有固定的体积 – 它会膨胀以充满任何容器。气体中的粒子彼此远离，以高速向各个方向随机运动。粒子之间几乎没有作用力，所以气体很容易被压缩（想象一下给自行车轮胎打气）。这是三态中最”自由”的状态。

Gases have neither a fixed shape nor a fixed volume – they expand to fill any container. The particles in a gas are far apart from each other, moving randomly at high speeds in all directions. There are almost no forces between the particles, so gases are very easy to compress (think about pumping air into a bicycle tyre). This is the most “free” state of the three.

三、粒子模型的完整解释：看不见的微小粒子如何决定物质的性质 | The Particle Model Explained: How Invisible Tiny Particles Determine the Properties of Matter

粒子模型（也叫”动力学粒子理论”）是KS3科学中最重要的理论框架之一。它提出了一个简单但强大的观点：所有物质都由微小的、不断运动的粒子组成。虽然我们无法用肉眼看到这些粒子，但我们可以通过它们的行为来解释物质的所有宏观性质。

The particle model (also called the “kinetic particle theory”) is one of the most important theoretical frameworks in KS3 science. It proposes a simple but powerful idea: all matter is made up of tiny, constantly moving particles. Although we cannot see these particles with the naked eye, we can explain all the macroscopic properties of matter through their behaviour.

粒子模型的核心假设有四个：(1) 所有物质都由微小的粒子组成；(2) 这些粒子在不断地运动；(3) 粒子之间存在空隙（空间）；(4) 粒子之间存在相互吸引的作用力。这四个假设共同构成了我们理解物质状态变化、扩散、气体压力和密度等现象的基础。

The particle model has four core assumptions: (1) all matter is made of tiny particles; (2) these particles are in constant motion; (3) there are spaces (gaps) between the particles; (4) there are attractive forces between the particles. These four assumptions together form the basis for our understanding of phenomena such as changes of state, diffusion, gas pressure, and density.

一个关键的认识是：同一物质在不同状态下的粒子本身是相同的 – 冰、液态水和蒸汽都由H2O粒子组成。区别仅在于粒子的排列方式和运动速度。这个洞察是理解物态变化的钥匙。

A key insight is that the particles of the same substance are identical in different states – ice, liquid water, and steam are all made up of H2O particles. The only differences lie in how the particles are arranged and how fast they move. This insight is the key to understanding changes of state.

四、物态变化：熔化、凝固、沸腾、蒸发和冷凝的粒子层面解释 | Changes of State: Particle-Level Explanations of Melting, Freezing, Boiling, Evaporation, and Condensation

物态变化是指物质从一种状态转变为另一种状态的过程。在KS3阶段，你需要掌握五种基本的物态变化：熔化（固体→液体）、凝固（液体→固体）、沸腾（液体→气体，发生在整杯液体中）、蒸发（液体→气体，仅发生在液体表面）和冷凝（气体→液体）。还有一种变化叫升华（固体→气体，跳过液体阶段），例如干冰（固体二氧化碳）在室温下直接变成气体。

Changes of state are processes in which matter transitions from one state to another. At KS3 level, you need to master five basic state changes: melting (solid to liquid), freezing (liquid to solid), boiling (liquid to gas, occurring throughout the liquid), evaporation (liquid to gas, occurring only at the surface), and condensation (gas to liquid). There is also sublimation (solid to gas, skipping the liquid stage), for example dry ice (solid carbon dioxide) turning directly into gas at room temperature.

粒子层面的解释

当固体被加热时，粒子获得更多动能并振动得更剧烈。在熔点温度下，粒子获得足够的能量来克服束缚它们的作用力，于是固体熔化成为液体。相反，当液体被冷却时，粒子失去动能，运动减慢，在凝固点重新排列成规则的固体结构。

When a solid is heated, the particles gain more kinetic energy and vibrate more vigorously. At the melting point temperature, the particles gain enough energy to overcome the forces holding them in place, and the solid melts into a liquid. Conversely, when a liquid is cooled, the particles lose kinetic energy, slow down, and at the freezing point rearrange back into a regular solid structure.

沸腾和蒸发的区别经常出现在KS3考试中。沸腾发生在特定的温度（沸点）并且在整杯液体中同时进行 – 你会看到气泡从液体内部升起。蒸发则可以在任何温度下发生，但只在液体表面进行。这是因为表面的一些粒子拥有足够的动能来逃逸到空气中，而液体内部的粒子则被周围的粒子”困住”。

The difference between boiling and evaporation frequently appears in KS3 exams. Boiling occurs at a specific temperature (the boiling point) and happens throughout the liquid simultaneously – you see bubbles rising from within the liquid. Evaporation can occur at any temperature but only takes place at the surface of the liquid. This is because some particles at the surface have enough kinetic energy to escape into the air, while particles inside the liquid are “trapped” by surrounding particles.

五、扩散现象：浓度差驱动下的粒子自发混合过程 | Diffusion: The Spontaneous Mixing of Particles Driven by Concentration Differences

扩散是粒子模型预测的最有力的证据之一。扩散是指粒子从高浓度区域向低浓度区域的净运动 – 不需要任何外部能量输入，完全是一种自发过程。你可以在日常生活中观察到扩散：当有人喷香水时，房间另一端的人几秒钟后就能闻到 – 这是因为香水粒子通过空气扩散到了整个房间。

Diffusion is one of the most powerful pieces of evidence predicted by the particle model. Diffusion is the net movement of particles from an area of high concentration to an area of low concentration – it requires no external energy input and is entirely a spontaneous process. You can observe diffusion in everyday life: when someone sprays perfume, a person at the other end of the room can smell it seconds later – this is because the perfume particles have diffused through the air to fill the entire room.

扩散的速度受多个因素影响。温度越高，扩散越快，因为粒子拥有更多的动能，运动速度更快。粒子的质量（相对分子质量）也是一个关键因素：较轻的粒子比较重的粒子扩散得更快。这就是为什么氨气（NH3，相对分子质量17）比氯化氢气（HCl，相对分子质量36.5）扩散得更快 – 在经典的”氨和氯化氢扩散实验”中，白色氯化铵环会在更靠近氯化氢一端形成。

The rate of diffusion is affected by several factors. Higher temperatures lead to faster diffusion because particles have more kinetic energy and move faster. The mass of the particles (relative molecular mass) is also a key factor: lighter particles diffuse faster than heavier particles. This is why ammonia gas (NH3, relative molecular mass 17) diffuses faster than hydrogen chloride gas (HCl, relative molecular mass 36.5) – in the classic “ammonia and hydrogen chloride diffusion experiment”, the white ammonium chloride ring forms closer to the hydrogen chloride end.

液体的扩散比气体慢得多，因为液体粒子之间的空隙更小，粒子运动受到周围粒子的阻碍。但液体扩散仍然可以观察到 – 在一杯水中滴入一滴食用色素，即使不搅拌，颜色也会慢慢扩散到整杯水中。

Diffusion in liquids is much slower than in gases because the spaces between liquid particles are smaller, and the movement of particles is hindered by surrounding particles. However, liquid diffusion can still be observed – add a drop of food colouring to a glass of water, and even without stirring, the colour will slowly spread throughout the water.

六、气体压力：无数粒子碰撞容器壁产生的宏观效果 | Gas Pressure: The Macroscopic Effect of Countless Particles Colliding with Container Walls

气体压力是粒子模型最精彩的应用之一。气体压力不是一种独立存在的力 – 它是数以亿计的气体粒子不断撞击容器壁面所产生的集体效果。每一次单个撞击都极其微弱，但当每秒有数万亿次撞击发生时，累积的力就产生了我们可测量的压力。

Gas pressure is one of the most elegant applications of the particle model. Gas pressure is not an independent force – it is the collective effect of billions upon billions of gas particles constantly colliding with the walls of their container. Each individual collision is extremely weak, but when trillions of collisions occur every second, the accumulated force produces the pressure we can measure.

理解气体压力的关键是两个变量：温度和体积。当气体被加热时，粒子获得更多动能，运动更快，撞击容器壁面更频繁且力度更大 – 压力增加。当气体被压缩到更小的体积中时，同样数量的粒子被限制在更小的空间里，碰撞频率增加 – 压力也增加。这就是为什么自行车轮胎在炎热的天气里可能爆胎（温度升高→压力增大），也是为什么你可以用手泵将大量空气压缩到一个小轮胎中。

The key to understanding gas pressure lies in two variables: temperature and volume. When a gas is heated, the particles gain more kinetic energy, move faster, and collide with the container walls more frequently and with greater force – pressure increases. When a gas is compressed into a smaller volume, the same number of particles is confined to a smaller space, collision frequency increases – pressure also increases. This is why a bicycle tyre might burst on a hot day (temperature increases → pressure increases), and why you can pump a large amount of air into a small tyre using a hand pump.

七、密度：为什么有些物体能浮在水面上而另一些会沉下去 | Density: Why Some Objects Float on Water While Others Sink

密度是物质的一项基本性质，它描述了单位体积内所含的质量。公式为：密度 = 质量 ÷ 体积（ρ = m/V）。在KS3阶段，密度通常以克每立方厘米（g/cm3）或千克每立方米（kg/m3）为单位来表示。水的密度为1 g/cm3，这是一个重要的参考点：密度小于1 g/cm3的物质会浮在水面上，密度大于1 g/cm3的物质则会沉入水中。

Density is a fundamental property of matter that describes how much mass is contained in a unit of volume. The formula is: density = mass / volume (ρ = m/V). At KS3 level, density is usually expressed in grams per cubic centimetre (g/cm3) or kilograms per cubic metre (kg/m3). The density of water is 1 g/cm3, which is an important reference point: substances with density less than 1 g/cm3 float on water, while substances with density greater than 1 g/cm3 sink in water.

粒子模型为密度提供了直观的解释：如果一个物质中的粒子紧密地堆积在一起（例如大多数金属），那么单位体积内的粒子数量就多，物质密度就大。相反，如果粒子之间的空隙很大（例如大多数气体），那么单位体积内的粒子数量就少，密度就小。这就是为什么一块铁（密度约7.9 g/cm3）比一块同样大小的木头（密度约0.6 g/cm3）要重得多的原因。

The particle model provides an intuitive explanation for density: if particles in a substance are tightly packed together (e.g. most metals), there are many particles per unit volume, and the substance has a high density. Conversely, if there are large spaces between particles (e.g. most gases), there are few particles per unit volume, and the density is low. This is why a block of iron (density about 7.9 g/cm3) is much heavier than a block of wood of the same size (density about 0.6 g/cm3).

值得注意的是，相同物质的密度在不同状态下也会不同。水在0°C时的密度约为1.0 g/cm3，但冰的密度只有约0.92 g/cm3 – 这就是为什么冰能浮在水面上。这似乎反常（大多数物质的固态密度大于液态），但这是因为水分子在固态冰中形成了开放的六边形晶体结构，使得粒子之间的空隙实际上比液态水更大。

It is worth noting that the same substance can have different densities in different states. Water at 0°C has a density of about 1.0 g/cm3, but ice has a density of only about 0.92 g/cm3 – this is why ice floats on water. This seems counterintuitive (most substances are denser as solids than as liquids), but it is because water molecules form an open hexagonal crystal structure in solid ice, making the spaces between particles actually larger than in liquid water.

八、布朗运动：在显微镜下直接”看到”粒子运动的经典实验证据 | Brownian Motion: The Classic Experimental Evidence for “Seeing” Particle Motion Under a Microscope

布朗运动是粒子模型最著名的实验证据之一。1827年，植物学家罗伯特·布朗在显微镜下观察悬浮在水中的花粉粒时，注意到这些微小的颗粒在做一种随机、不规则、永不停止的”舞蹈”运动。起初他以为这是因为花粉粒是”活的”，但后来他用非生物颗粒（如灰尘）重复实验时发现了同样的现象。

Brownian motion is one of the most famous experimental pieces of evidence for the particle model. In 1827, the botanist Robert Brown observed pollen grains suspended in water under a microscope and noticed that these tiny particles performed a random, irregular, never-ending “dance” motion. At first he thought this was because pollen grains were “alive”, but later he repeated the experiment with non-living particles (such as dust) and found the same phenomenon.

直到1905年，阿尔伯特·爱因斯坦才给出了正确的解释：花粉粒之所以随机运动，是因为水分子（我们看不见的微小粒子）在不断运动并不断撞击花粉粒。由于水分子从不同方向撞击花粉粒的力度不均匀，花粉粒就被推向不同方向，产生了看似随机的运动路径。布朗运动提供了”粒子在不停运动”的直接可视证据 – 虽然我们看不见水分子本身，但我们可以看见它们对花粉粒的影响。

It was not until 1905 that Albert Einstein provided the correct explanation: the pollen grains move randomly because water molecules (the tiny particles we cannot see) are constantly in motion and constantly colliding with the pollen grains. Since the water molecules hit the pollen grain with uneven force from different directions, the pollen grain is pushed in different directions, producing a seemingly random motion path. Brownian motion provides direct visible evidence that “particles are in constant motion” – although we cannot see the water molecules themselves, we can see their effect on the pollen grains.

在KS3实验中，通常使用烟灰颗粒悬浮在空气中的演示来观察布朗运动 – 在烟雾室中用强光照射，通过显微镜可以看到烟灰颗粒在空中随机跳动。颗粒越小，布朗运动越明显，因为较小的颗粒受到的不平衡碰撞效应更显著。

In KS3 experiments, Brownian motion is often demonstrated using smoke particles suspended in air – illuminated by a strong light in a smoke cell, the smoke particles can be seen through a microscope bouncing randomly in the air. The smaller the particles, the more obvious the Brownian motion, because the unbalanced collision effect is more pronounced for smaller particles.

九、物质粒子模型的局限性：哪些现象粒子模型无法解释 | Limitations of the Particle Model: What the Model Cannot Explain

虽然粒子模型在解释物态变化、扩散、气体压力和密度方面非常成功，但它也有明显的局限性。作为一个简化模型，它把粒子描绘成微小的、坚硬的球体。这在KS3阶段是可以接受的近似，但在更高级的阶段，学生需要了解粒子本身具有内部结构。

While the particle model is remarkably successful at explaining changes of state, diffusion, gas pressure, and density, it has clear limitations. As a simplified model, it depicts particles as tiny, hard spheres. This is an acceptable approximation at KS3 level, but at more advanced levels, students need to understand that particles themselves have internal structure.

例如，粒子模型无法解释为什么不同物质有不同的熔点和沸点 – 这需要原子结构和化学键的知识。粒子模型也无法解释导电性（为什么金属导电而塑料不导电），因为导电性涉及电子在原子之间的移动，而标准的KS3粒子模型并没有包含”电子”的概念。此外，粒子模型对化学反应的描述也是有限的 – 它告诉我们粒子重新排列，但没有解释化学键的断裂和形成。

For example, the particle model cannot explain why different substances have different melting and boiling points – this requires knowledge of atomic structure and chemical bonding. The particle model also cannot explain electrical conductivity (why metals conduct electricity but plastics do not), because conductivity involves the movement of electrons between atoms, and the standard KS3 particle model does not include the concept of “electrons”. Furthermore, the particle model’s description of chemical reactions is limited – it tells us that particles rearrange, but does not explain the breaking and forming of chemical bonds.

了解模型的局限性本身就是KS3科学课程的一个重要目标 – 它帮助学生理解科学模型是简化现实的工具，而不是现实的完美复制品。在后续的GCSE学习中，粒子模型将被原子结构模型和化学键模型所补充和完善。

Understanding the limitations of models is itself an important objective of the KS3 science curriculum – it helps students understand that scientific models are tools for simplifying reality, not perfect replicas of reality. In subsequent GCSE studies, the particle model will be supplemented and refined by models of atomic structure and chemical bonding.

十、CIE KS3实验技能：密度测量与扩散观察的动手实践 | CIE KS3 Practical Skills: Hands-On Density Measurement and Diffusion Observation

CIE的KS3科学课程特别强调实验技能的培养。”物质”这一单元包含了两个核心实验，学生不仅需要理解实验原理，还需要能够描述实验步骤、识别变量、记录数据并得出结论。

The CIE KS3 Science curriculum places particular emphasis on the development of practical skills. The “Matter” unit includes two core experiments, and students need not only to understand the principles behind them but also to be able to describe experimental procedures, identify variables, record data, and draw conclusions.

实验一：测量规则和不规则固体的密度

对于规则形状的固体（如立方体或长方体），密度的测量相对直接。首先用天平测量物体的质量（单位：克）。然后使用尺子测量物体的长度、宽度和高度，计算出体积（单位：立方厘米）。最后用密度 = 质量÷体积的公式计算结果。在这个实验中，自变量是你所选择的材料，因变量是测量出的密度，控制变量包括使用相同的天平和同一个尺子。

For regularly shaped solids (such as a cube or a rectangular block), measuring density is relatively straightforward. First, measure the mass of the object using a balance (unit: grams). Then use a ruler to measure the length, width, and height of the object, and calculate the volume (unit: cubic centimetres). Finally, use the formula density = mass / volume to calculate the result. In this experiment, the independent variable is the material you choose, the dependent variable is the measured density, and the control variables include using the same balance and the same ruler.

对于不规则形状的固体（如一块石头），不能简单地用尺子测量体积。这时需要使用排水法（也称为阿基米德法）：先用量筒（measuring cylinder）量取一定体积的水并记录读数，然后将不规则物体完全浸入水中，记录新的水位读数。两次读数的差值就是该物体的体积。一个常见的学生错误是忘记在浸入物体之前记录初始水位 – 这会导致无法计算体积差。

For irregularly shaped solids (such as a stone), you cannot simply measure the volume with a ruler. Instead, you need to use the displacement method (also called the Archimedes method): first, measure a certain volume of water in a measuring cylinder and record the reading, then completely submerge the irregular object in the water and record the new water level reading. The difference between the two readings is the volume of the object. A common student mistake is forgetting to record the initial water level before submerging the object – this makes it impossible to calculate the volume difference.

实验二：观察液体中的扩散

扩散实验在KS3阶段通常使用高锰酸钾（potassium permanganate）晶体或食用色素在水中进行。将一小粒高锰酸钾晶体放入装有冷水的烧杯底部，紫色会从晶体周围慢慢扩散到整杯水中。这个实验的关键观察是：扩散在水中是可见的（与在空气中不同），而且可以通过对比冷水和热水中的扩散速度来展示温度对扩散速率的影响。在热水中，扩散明显更快 – 这是粒子获得更多动能、运动速度更快的直接证据。

The diffusion experiment at KS3 level is typically conducted using potassium permanganate crystals or food colouring in water. Place a small crystal of potassium permanganate at the bottom of a beaker of cold water, and the purple colour will slowly spread from around the crystal throughout the water. The key observation in this experiment is that diffusion is visible in water (unlike in air), and you can demonstrate the effect of temperature on diffusion rate by comparing diffusion in cold versus hot water. In hot water, the diffusion is visibly faster – this is direct evidence that particles gain more kinetic energy and move faster.

在写实验报告时，CIE评分标准要求学生明确写出：(1) 安全注意事项（戴护目镜，高锰酸钾会染色皮肤和衣物）；(2) 为什么需要小心地将晶体放入水中而不搅拌（搅拌会引入另一个变量 – 对流）；(3) 如何使实验成为”公平测试”（对比冷水和热水时，使用相同大小的晶体和相同体积的水）。

When writing the lab report, the CIE marking criteria require students to clearly state: (1) safety precautions (wear goggles; potassium permanganate stains skin and clothing); (2) why the crystal needs to be placed gently into the water without stirring (stirring introduces another variable – convection); (3) how to make the experiment a “fair test” (when comparing cold and hot water, use the same size crystal and the same volume of water).

Summary | 总结

物质是我们周围一切事物的基本构成材料，所有物质都由不断运动的微小粒子组成。粒子模型是KS3科学中解释物质行为的核心框架：固体中的粒子紧密排列、只在原位振动；液体中的粒子紧密但可以相互滑动；气体中的粒子彼此远离、高速随机运动。这一模型完美解释了物态变化 – 加热使粒子获得动能，从而克服粒子间作用力，导致熔化或沸腾；冷却使粒子失去动能，导致凝固或冷凝。扩散是粒子自发从高浓度向低浓度运动的过程，气体压力则是无数粒子碰撞容器壁面的集体效果。密度由单位体积内粒子的数量和紧密程度决定。布朗运动提供了粒子在不停运动的直接实验证据 – 我们可以在显微镜下看到花粉粒或烟灰颗粒被不可见的水分子或空气分子撞击而产生的随机运动。虽然粒子模型有局限性（无法解释导电性、化学键等），但它为后续GCSE阶段的原子结构和化学键学习奠定了坚实的概念基础。

Matter is the fundamental building material of everything around us, and all matter is made of tiny particles in constant motion. The particle model is the central framework in KS3 science for explaining the behaviour of matter: particles in solids are tightly packed and only vibrate in fixed positions; particles in liquids are closely packed but can slide past each other; particles in gases are far apart and move randomly at high speeds. This model perfectly explains changes of state – heating gives particles kinetic energy, allowing them to overcome inter-particle forces, causing melting or boiling; cooling removes kinetic energy from particles, causing freezing or condensation. Diffusion is the spontaneous movement of particles from high to low concentration, and gas pressure is the collective effect of countless particles colliding with container walls. Density is determined by how many particles are packed into a unit of volume and how tightly they are arranged. Brownian motion provides direct experimental evidence that particles are in constant motion – we can observe under a microscope how pollen grains or smoke particles are jostled randomly by invisible water or air molecules. Although the particle model has limitations (it cannot explain electrical conductivity, chemical bonding, etc.), it lays a solid conceptual foundation for the study of atomic structure and chemical bonding at GCSE level.

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一、什么是均衡饮食？营养学的基础定义 | What Is a Balanced Diet? The Foundational Definition in Nutrition

均衡饮食是指摄入适当种类和数量的食物，以满足身体生长、修复和日常活动所需的全部营养。对于 KS3 阶段的学生来说，这意味着理解我们每天吃的食物不仅提供能量，还提供了维持生命所必需的各种化学物质。根据 CIE 剑桥初中科学课程大纲，学生需要掌握七大营养素（碳水化合物、蛋白质、脂肪、维生素、矿物质、膳食纤维和水）的基本功能，以及它们在日常饮食中的食物来源。

A balanced diet means consuming the right types and amounts of food to provide all the nutrients the body needs for growth, repair, and daily activities. For KS3 students, this means understanding that the food we eat every day provides not only energy but also a wide range of chemical substances essential for life. According to the CIE Cambridge Lower Secondary Science curriculum, students are expected to master the basic functions of the seven nutrient groups – carbohydrates, proteins, fats, vitamins, minerals, dietary fibre, and water – along with their food sources in everyday meals.

“均衡”这个词本身意味着多样性和适度。没有任何单一食物能提供身体所需的所有营养 – 这就是为什么营养学家反复强调食物多样性的重要性。CIE 考试中常常要求学生解释为什么只吃一种食物（如只吃米饭或只吃水果）是不健康的，这背后考察的就是均衡饮食的核心概念。

The word “balanced” itself implies variety and moderation. No single food can supply all the nutrients the body needs – which is why nutritionists repeatedly emphasise the importance of dietary diversity. CIE examinations frequently ask students to explain why eating only one type of food (such as only rice or only fruit) is unhealthy, which tests the core concept of a balanced diet.

二、七大营养素的分类与核心功能 | The Seven Nutrient Groups: Classification and Core Functions

CIE KS3 科学课程将食物中的营养物质分为七大类，每一类都有其独特且不可替代的功能。理解这些营养素不仅是为了通过考试，更是为了在日常生活中做出明智的食物选择。以下是七大类营养素的系统梳理，包括每种营养素的主要功能、食物来源以及缺乏或过量可能带来的后果。

The CIE KS3 Science curriculum classifies nutrients in food into seven groups, each with its own unique and irreplaceable function. Understanding these nutrients is not only for passing exams but also for making informed food choices in everyday life. Below is a systematic overview of the seven nutrient groups, covering the main function, food sources, and potential consequences of deficiency or excess for each one.

碳水化合物 (Carbohydrates) – 身体的主要能量来源，分为淀粉（复杂碳水化合物，如米饭、面包、土豆）和糖类（简单碳水化合物，如水果中的果糖、蛋糕中的蔗糖）。淀粉在消化过程中被分解为葡萄糖，然后通过血液输送到细胞中进行呼吸作用。

Carbohydrates – The body’s main energy source, divided into starch (complex carbohydrates, e.g. rice, bread, potatoes) and sugars (simple carbohydrates, e.g. fructose in fruit, sucrose in cakes). Starch is broken down into glucose during digestion, which is then transported via the blood to cells for respiration.

蛋白质 (Proteins) – 生长和修复的关键物质。蛋白质由氨基酸组成，是构建肌肉、皮肤、头发、酶和抗体的基本材料。富含蛋白质的食物包括肉类、鱼类、蛋类、豆类和奶制品。

Proteins – Essential for growth and repair. Proteins are made up of amino acids and are the basic building blocks for muscle, skin, hair, enzymes, and antibodies. Protein-rich foods include meat, fish, eggs, beans, and dairy products.

脂肪 (Fats) – 能量的储存形式和保温材料。脂肪提供比碳水化合物更多的能量（每克约 9 千卡，而碳水化合物每克约 4 千卡），同时也是脂溶性维生素（A、D、E、K）的载体。脂肪还构成细胞膜的重要组成部分，并在皮下形成保温层。

Fats – Energy storage and insulation. Fats provide more energy per gram than carbohydrates (approximately 9 kcal per gram, compared to 4 kcal per gram for carbohydrates) and also act as carriers for fat-soluble vitamins (A, D, E, K). Fats also form an essential component of cell membranes and provide an insulating layer under the skin.

维生素 (Vitamins) – 调节身体化学反应的微量有机物。与碳水化合物、蛋白质和脂肪不同，维生素不直接提供能量，但它们是许多酶促反应必不可少的辅因子。例如，维生素 C 对胶原蛋白合成至关重要（缺乏会导致坏血病），维生素 D 促进钙的吸收（缺乏会导致佝偻病）。

Vitamins – Organic micronutrients that regulate chemical reactions in the body. Unlike carbohydrates, proteins, and fats, vitamins do not directly provide energy, but they are essential cofactors for many enzymatic reactions. For example, vitamin C is crucial for collagen synthesis (deficiency causes scurvy), and vitamin D promotes calcium absorption (deficiency causes rickets).

矿物质 (Minerals) – 无机微量元素，对骨骼健康、神经传导和血液功能至关重要。钙构成骨骼和牙齿；铁是血红蛋白的核心成分，负责氧气的运输（缺乏会导致缺铁性贫血）；碘是甲状腺激素的组成部分（缺乏会导致甲状腺肿大）。

Minerals – Inorganic micronutrients crucial for bone health, nerve conduction, and blood function. Calcium builds bones and teeth; iron is the core component of haemoglobin, responsible for oxygen transport (deficiency causes iron-deficiency anaemia); iodine is a component of thyroid hormones (deficiency causes goitre).

膳食纤维 (Dietary Fibre) – 虽然不能被人体消化吸收，但纤维对维持消化系统健康至关重要。它增加粪便体积，促进肠道蠕动，预防便秘。全谷物、水果和蔬菜是膳食纤维的主要来源。

Dietary Fibre – Although it cannot be digested or absorbed by the human body, fibre is essential for maintaining digestive health. It adds bulk to faeces, promotes peristalsis (the movement of food through the gut), and prevents constipation. Whole grains, fruits, and vegetables are the primary sources of dietary fibre.

水 (Water) – 约占人体体重的 60-70%，是所有生化反应的溶剂。水参与体温调节（通过出汗）、营养物质运输和废物排出。每天建议摄入约 1.5-2 升水。

Water – Making up approximately 60-70% of human body weight, water is the solvent for all biochemical reactions. It is involved in temperature regulation (through sweating), nutrient transport, and waste elimination. The recommended daily intake is approximately 1.5-2 litres.

三、碳水化合物与脂肪：身体的双重能量系统 | Carbohydrates and Fats: The Body’s Dual Energy System

碳水化合物和脂肪都是能量的来源，但它们在人体的能量代谢中扮演着不同的角色。理解两者之间的区别和联系，是 KS3 CIE 科学考试中的一个常见考点。

Carbohydrates and fats are both sources of energy, but they play different roles in the body’s energy metabolism. Understanding the differences and connections between the two is a common assessment point in KS3 CIE Science examinations.

碳水化合物是身体的”即时能源”。当我们吃下含淀粉的食物（如面包、米饭、意大利面）时，唾液和胰液中的淀粉酶将淀粉分解为麦芽糖，再进一步分解为葡萄糖。葡萄糖通过小肠绒毛被吸收进入血液，导致血糖水平升高。胰腺随即分泌胰岛素，促使细胞吸收葡萄糖并进行有氧呼吸：

Carbohydrates are the body’s “immediate energy source.” When we eat starchy foods (such as bread, rice, pasta), amylase enzymes in saliva and pancreatic juice break starch down into maltose, then further into glucose. Glucose is absorbed through the villi of the small intestine into the bloodstream, causing blood sugar levels to rise. The pancreas then secretes insulin, prompting cells to absorb glucose and carry out aerobic respiration:

葡萄糖 + 氧气 → 二氧化碳 + 水 + 能量 (ATP)
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP)

这个化学反应释放的能量用于维持体温、肌肉收缩、神经传导以及所有细胞活动。如果摄入的碳水化合物超过了身体的即时需求，多余的葡萄糖会在肝脏和肌肉中转化为糖原储存起来；如果糖原储存也满了，多余的碳水化合物会被转化为脂肪储存。

This chemical reaction releases energy used for maintaining body temperature, muscle contraction, nerve conduction, and all cellular activities. If the carbohydrates consumed exceed the body’s immediate needs, excess glucose is converted into glycogen and stored in the liver and muscles; if glycogen stores are also full, the surplus carbohydrates are converted into fat for long-term storage.

脂肪则是身体的”长期能量储备”。与碳水化合物相比，脂肪的能量密度更高，每克提供约 39 千焦的能量（而碳水化合物只提供约 17 千焦每克）。脂肪组织位于皮下和内脏周围，既是能量储存库，也是保温层和物理缓冲。在长时间运动或饥饿状态下，身体会动员脂肪储备，通过脂肪分解为脂肪酸和甘油来提供能量。

Fats, on the other hand, are the body’s “long-term energy reserve.” Compared to carbohydrates, fats have a higher energy density, providing approximately 39 kJ per gram (while carbohydrates provide only about 17 kJ per gram). Adipose tissue is located under the skin and around internal organs, serving as an energy reservoir as well as an insulating layer and physical cushion. During prolonged exercise or starvation, the body mobilises fat reserves, breaking them down into fatty acids and glycerol to provide energy.

需要注意的是，并非所有脂肪都是”坏”的。不饱和脂肪（如橄榄油、鱼油、坚果中的脂肪）对心血管健康有益，而饱和脂肪（如黄油、肥肉中的脂肪）摄入过多则会增加心脏病的风险。CIE 考试中可能会要求学生区分不同类型的脂肪及其健康影响。

It is important to note that not all fats are “bad.” Unsaturated fats (such as those found in olive oil, fish oil, and nuts) are beneficial for cardiovascular health, while excessive intake of saturated fats (such as those in butter and fatty meat) increases the risk of heart disease. CIE examinations may ask students to distinguish between different types of fats and their health implications.

四、蛋白质：从食物到身体组织的转化 | Protein: From Food to Body Tissue

蛋白质是生命的基础 – 这个说法毫不夸张。人体内的每一个细胞都含有蛋白质，从肌肉纤维到免疫系统的抗体，从消化酶到血液中的血红蛋白。KS3 学生需要理解蛋白质在生长和修复中的核心作用，以及为什么正在发育的青少年对蛋白质的需求尤为迫切。

Protein is the foundation of life – this is not an exaggeration. Every cell in the human body contains protein, from muscle fibres to antibodies in the immune system, from digestive enzymes to haemoglobin in the blood. KS3 students need to understand the central role of protein in growth and repair, and why growing adolescents have a particularly high demand for protein.

蛋白质由氨基酸链组成。人体需要 20 种不同的氨基酸，其中 9 种是”必需氨基酸” – 人体无法自行合成，必须通过食物获取。动物性蛋白质（肉类、鱼类、蛋类、奶制品）通常含有全部必需氨基酸，被称为”完全蛋白质”。植物性蛋白质（豆类、坚果、谷物）往往缺少一种或多种必需氨基酸，但通过将不同植物蛋白搭配食用（如米饭配豆类），仍能获得完整的氨基酸谱。

Proteins are made up of chains of amino acids. The human body requires 20 different amino acids, of which 9 are “essential amino acids” – the body cannot synthesise them and must obtain them from food. Animal proteins (meat, fish, eggs, dairy) usually contain all essential amino acids and are called “complete proteins.” Plant proteins (beans, nuts, grains) often lack one or more essential amino acids, but by combining different plant proteins (such as rice with beans), a complete amino acid profile can still be achieved.

在消化过程中，蛋白质被胃蛋白酶和胰蛋白酶分解为氨基酸，然后通过小肠吸收进入血液。细胞利用这些氨基酸来构建新的蛋白质 – 无论是修复受损的肌肉组织、生成新的皮肤细胞，还是合成激素和酶。这就是为什么运动后摄入蛋白质有助于肌肉恢复，为什么受伤后身体对蛋白质的需求会增加。

During digestion, proteins are broken down by pepsin and trypsin into amino acids, which are then absorbed through the small intestine into the bloodstream. Cells use these amino acids to build new proteins – whether repairing damaged muscle tissue, generating new skin cells, or synthesising hormones and enzymes. This is why consuming protein after exercise aids muscle recovery, and why the body’s demand for protein increases after an injury.

蛋白质缺乏会导致严重的健康问题。夸希奥科病（Kwashiorkor）是一种严重的蛋白质缺乏症，常见于以碳水化合物为主食而蛋白质摄入严重不足的儿童，表现为腹部肿胀、生长迟缓、肌肉萎缩和免疫力下降。这正是 CIE 考试中经常出现的案例分析题型。

Protein deficiency can lead to serious health problems. Kwashiorkor is a severe protein-deficiency disease, commonly seen in children whose diet consists mainly of carbohydrates with severely insufficient protein intake. Symptoms include a swollen abdomen, stunted growth, muscle wasting, and weakened immunity. This is a classic case-study question type that frequently appears in CIE examinations.

五、维生素与矿物质：微量但不可或缺 | Vitamins and Minerals: Micronutrients That Punch Above Their Weight

维生素和矿物质被归类为”微量营养素”，因为人体每天对它们的需求量非常小 – 通常以毫克或微克计算。然而，这些微量的化学物质一旦缺乏，就会引发一系列严重的缺乏症。CIE KS3 考试要求学生掌握至少四种关键的维生素和矿物质的缺乏症及其食物来源。

Vitamins and minerals are classified as “micronutrients” because the body requires them in very small amounts – typically measured in milligrams or micrograms. However, a deficiency in these tiny amounts of chemicals can trigger a range of serious deficiency diseases. CIE KS3 examinations require students to know the deficiency diseases and food sources of at least four key vitamins and minerals.

维生素 C（抗坏血酸） – 缺乏导致坏血病 (Scurvy)，症状包括牙龈出血、伤口愈合缓慢、皮肤出现瘀斑。这是因为维生素 C 是合成胶原蛋白的必要辅因子，而胶原蛋白是血管壁和结缔组织的关键结构蛋白。新鲜水果（尤其是柑橘类水果、猕猴桃、草莓）和绿色蔬菜是维生素 C 的主要来源。

Vitamin C (Ascorbic Acid) – Deficiency causes scurvy, with symptoms including bleeding gums, slow wound healing, and bruising of the skin. This is because vitamin C is an essential cofactor for collagen synthesis, and collagen is the key structural protein in blood vessel walls and connective tissues. Fresh fruits (especially citrus fruits, kiwis, strawberries) and green vegetables are the primary sources of vitamin C.

维生素 D（钙化醇） – 缺乏导致佝偻病 (Rickets)，表现为骨骼软化、弯曲（弓形腿）和生长迟缓。维生素 D 促进肠道对钙和磷的吸收，是骨骼矿化的关键调节因子。人体皮肤在阳光紫外线照射下可以自行合成维生素 D，因此也被称为”阳光维生素”。食物来源包括富含脂肪的鱼类（三文鱼、沙丁鱼）、蛋黄和强化乳制品。

Vitamin D (Calciferol) – Deficiency causes rickets, characterised by soft, bent bones (bow legs) and stunted growth. Vitamin D promotes the absorption of calcium and phosphorus in the intestines and is a key regulator of bone mineralisation. The skin can synthesise vitamin D when exposed to UV light from sunlight, hence its nickname the “sunshine vitamin.” Food sources include oily fish (salmon, sardines), egg yolks, and fortified dairy products.

钙 (Calcium) – 缺乏导致骨骼和牙齿脆弱，增加骨质疏松和骨折的风险。钙不仅用于构建骨骼结构，还在神经信号传递、肌肉收缩和血液凝固中发挥关键作用。奶制品（牛奶、奶酪、酸奶）是钙最丰富的来源，绿叶蔬菜和豆制品也含有一定量的钙。

Calcium – Deficiency leads to weak bones and teeth, increasing the risk of osteoporosis and fractures. Calcium is not only used for building bone structure but also plays a crucial role in nerve signal transmission, muscle contraction, and blood clotting. Dairy products (milk, cheese, yoghurt) are the richest sources of calcium; leafy green vegetables and soy products also contain some calcium.

铁 (Iron) – 缺乏导致缺铁性贫血 (Iron-deficiency Anaemia)，症状包括疲劳、面色苍白、呼吸急促和注意力不集中。铁是血红蛋白的核心成分，每个血红蛋白分子含有四个铁原子，每个铁原子结合一个氧分子。红肉、肝脏、菠菜和强化谷物是铁的主要食物来源。

Iron – Deficiency causes iron-deficiency anaemia, with symptoms including fatigue, pale skin, shortness of breath, and poor concentration. Iron is the core component of haemoglobin; each haemoglobin molecule contains four iron atoms, and each iron atom binds one oxygen molecule. Red meat, liver, spinach, and fortified cereals are the main food sources of iron.

六、膳食纤维与水：消化系统的守护者 | Dietary Fibre and Water: Guardians of the Digestive System

膳食纤维和水虽然不直接提供能量，但它们对维持消化系统健康和全身代谢平衡同样至关重要。在 CIE KS3 课程中，学生需要理解这两种”非常规”营养素如何帮助身体正常运转。

Dietary fibre and water do not directly provide energy, but they are equally crucial for maintaining digestive health and overall metabolic balance. In the CIE KS3 curriculum, students need to understand how these two “non-conventional” nutrients help the body function properly.

膳食纤维是植物细胞壁的主要成分 – 纤维素。人体消化系统缺乏分解纤维素的酶，因此纤维在通过消化道时几乎不被消化。但这恰恰是它发挥作用的方式：纤维吸收水分后膨胀，增加了肠道内容物的体积，刺激肠道壁的肌肉收缩（蠕动），从而推动食物残渣顺利通过大肠。缺乏纤维的饮食会导致便秘、痔疮，长期来看甚至增加结肠癌的风险。全麦面包、燕麦、水果（尤其是带皮食用的苹果和梨）、蔬菜和豆类是膳食纤维的优质来源。

Dietary fibre is the main component of plant cell walls – cellulose. The human digestive system lacks the enzymes to break down cellulose, so fibre passes through the digestive tract almost undigested. But this is precisely how it works: fibre absorbs water and swells, increasing the bulk of intestinal contents and stimulating the muscles in the intestinal wall to contract (peristalsis), thereby pushing food waste smoothly through the large intestine. A diet lacking in fibre can lead to constipation, haemorrhoids, and in the long term, even an increased risk of colon cancer. Wholemeal bread, oats, fruits (especially apples and pears eaten with the skin), vegetables, and beans are excellent sources of dietary fibre.

水是生命最重要的分子。人体大约 60-70% 是水，而水参与了几乎所有的生理过程：它溶解营养物质，使它们能被细胞吸收利用；它作为血液的主要成分，运输氧气、营养物质、激素和废物；它通过出汗和蒸发帮助调节体温；它润滑关节，保护大脑和脊髓。脱水 – 即使只是体重的 1-2% 的水分流失 – 就会导致头痛、疲劳和注意力下降，这对正在上学的 KS3 学生来说尤为重要。

Water is the most important molecule for life. The human body is approximately 60-70% water, and water participates in nearly all physiological processes: it dissolves nutrients so they can be absorbed and used by cells; as the main component of blood, it transports oxygen, nutrients, hormones, and waste; it helps regulate body temperature through sweating and evaporation; it lubricates joints and cushions the brain and spinal cord. Dehydration – even a loss of just 1-2% of body weight in water – can cause headaches, fatigue, and reduced concentration, which is particularly relevant for KS3 students at school.

七、均衡饮食实践：构建健康餐盘 | Practising a Balanced Diet: Building a Healthy Plate

理解了各种营养素的独立功能后，下一步是将这些知识应用到日常饮食选择中。英国公共卫生部门推广的”伊特韦尔指南”（Eatwell Guide）将食物分为五个主要类别，并建议了每类食物在一日饮食中的大致比例。这个模型是 CIE KS3 考试中常见的识图和分析题型。

Having understood the individual functions of each nutrient group, the next step is to apply this knowledge to everyday food choices. The Eatwell Guide, promoted by Public Health England, divides food into five main groups and suggests the approximate proportion each group should contribute to the daily diet. This model is a common diagram-recognition and analysis question type in CIE KS3 examinations.

一个健康的餐盘大致由以下比例构成：水果和蔬菜应占餐盘的 1/3 以上（每天至少 5 份不同颜色的蔬果）；淀粉类碳水化合物（土豆、面包、米饭、意大利面）占餐盘的 1/3 左右，尽量选择全谷物版本；蛋白质来源（肉类、鱼类、蛋类、豆类）占餐盘的不到 1/4；奶制品或替代品保持适量；不饱和脂肪少量使用；高糖、高盐、高饱和脂肪的食物尽量减少。

A healthy plate is roughly composed of the following proportions: fruits and vegetables should make up over a third of the plate (at least five portions of different-coloured fruit and veg daily); starchy carbohydrates (potatoes, bread, rice, pasta) around a third of the plate, preferably wholegrain versions; protein sources (meat, fish, eggs, beans) less than a quarter of the plate; dairy or alternatives in moderate amounts; unsaturated fats used sparingly; and foods high in sugar, salt, and saturated fat kept to a minimum.

对于 KS3 年龄段的学生（11-14 岁），能量需求比儿童时期显著增加 – 男孩每天大约需要 10,000-11,000 千焦，女孩大约需要 9,000-10,000 千焦，具体取决于活动水平。这个年龄段也是骨骼生长的关键时期，因此钙和维生素 D 的充足摄入尤为重要。同时，青少年贫血是常见问题（尤其在女性青少年中），因此铁含量的充足摄入也值得关注。

For KS3-aged students (11-14 years), energy requirements increase significantly compared to childhood – boys need approximately 10,000-11,000 kJ per day and girls approximately 9,000-10,000 kJ per day, depending on activity levels. This age range is also a critical period for bone growth, making adequate calcium and vitamin D intake particularly important. Meanwhile, adolescent anaemia is a common issue (especially among female teenagers), so sufficient iron intake also deserves attention.

八、营养不良的双重面孔：缺乏与过剩 | The Two Faces of Malnutrition: Deficiency and Excess

“营养不良”这个词通常让人联想到饥饿和消瘦，但实际上它包含了两个极端：营养不足（Undernutrition）和营养过剩（Overnutrition）。KS3 CIE 课程要求学生理解营养不良的多种形式及其对健康的影响。

The word “malnutrition” typically conjures images of starvation and emaciation, but in reality it encompasses two extremes: undernutrition and overnutrition. The KS3 CIE curriculum requires students to understand the various forms of malnutrition and their health impacts.

营养不足方面，除了前面讨论过的具体缺乏症（如坏血病、佝偻病、贫血和夸希奥科病），还有因长期能量摄入不足导致的消瘦症（Marasmus） – 全身肌肉和脂肪组织的极度消耗，常见于长期饥荒地区。与之形成对比的是夸希奥科病，后者是蛋白质严重缺乏但总热量摄入可能尚可的情况。

On the undernutrition side, in addition to the specific deficiency diseases discussed earlier (such as scurvy, rickets, anaemia, and kwashiorkor), there is also marasmus – the extreme wasting of muscle and fat tissue throughout the body due to prolonged energy deficiency, commonly seen in areas of chronic famine. This contrasts with kwashiorkor, which is a severe protein deficiency where total calorie intake may still be adequate.

营养过剩则是一个在现代社会更为普遍的问题。当能量摄入持续超过能量消耗时，多余的能量以脂肪形式储存，导致超重和肥胖。肥胖会增加 2 型糖尿病、高血压、冠心病、中风和某些癌症的风险。高饱和脂肪和反式脂肪的摄入与动脉粥样硬化有关 – 胆固醇在动脉壁上沉积形成斑块，使血管变窄变硬，增加心脏病发作的风险。过多摄入精制糖会导致蛀牙和血糖波动。过多的盐（钠）摄入与高血压直接相关。

Overnutrition is a more prevalent problem in modern societies. When energy intake consistently exceeds energy expenditure, the excess energy is stored as fat, leading to overweight and obesity. Obesity increases the risk of type 2 diabetes, hypertension, coronary heart disease, stroke, and certain cancers. High intake of saturated and trans fats is linked to atherosclerosis – cholesterol deposits forming plaques on artery walls, narrowing and hardening the blood vessels and increasing the risk of heart attacks. Excessive intake of refined sugars leads to tooth decay and blood sugar fluctuations. Excessive salt (sodium) intake is directly linked to high blood pressure.

CIE 考试中经常出现的题目类型是：给出一段描述某个孩子日常饮食的文字，要求学生判断可能缺乏或过量的营养素，并分析相关的健康风险。这类题目综合考察了学生对各类营养素功能和缺乏症的理解。

A frequently appearing question type in CIE examinations is: providing a description of a child’s daily diet and asking students to identify which nutrients may be deficient or excessive, and to analyse the associated health risks. These questions comprehensively test students’ understanding of the functions of each nutrient group and their associated deficiency and excess conditions.

九、KS3 CIE 科学实验技能：食物成分检测 | KS3 CIE Science Practical Skills: Food Tests

CIE KS3 科学课程不仅要求理论理解，还要求学生掌握基本的食物检测实验技能。以下四种标准食物检测方法是 KS3 实验考试和笔试中的高频考点。每种检测方法都涉及特定的化学试剂和可观察的颜色变化。

The CIE KS3 Science curriculum requires not only theoretical understanding but also mastery of basic food testing practical skills. The following four standard food tests are high-frequency assessment points in both KS3 practical examinations and written papers. Each test involves specific chemical reagents and observable colour changes.

淀粉检测 (Starch Test) – 碘液试验 (Iodine Test)：在食物样本上滴加几滴碘液（碘溶于碘化钾溶液）。如果食物含有淀粉，碘液会从橙棕色变为蓝黑色。这是因为碘分子嵌入了淀粉螺旋结构中，形成了特征性颜色。需要注意的是，碘液检测的是淀粉（多糖），而不是葡萄糖等单糖。

Starch Test – Iodine Test: Add a few drops of iodine solution (iodine dissolved in potassium iodide solution) to the food sample. If the food contains starch, the iodine solution changes from orange-brown to blue-black. This is because iodine molecules become trapped within the helical structure of starch, producing this characteristic colour. It is important to note that this test detects starch (a polysaccharide), not simple sugars like glucose.

还原糖检测 (Reducing Sugar Test) – 本尼迪克特试验 (Benedict’s Test)：将食物样本与蓝色的本尼迪克特试剂混合，然后在水浴中加热约 5 分钟。如果存在还原糖（如葡萄糖、果糖），溶液的颜色会从蓝色变为绿色、黄色、橙色，最终形成砖红色沉淀，颜色的变化反映了还原糖的浓度。值得注意的是，蔗糖（普通食糖）是非还原糖，需要使用不同的测试方法。

Reducing Sugar Test – Benedict’s Test: Mix the food sample with blue Benedict’s reagent, then heat in a water bath for about five minutes. If reducing sugars (such as glucose or fructose) are present, the solution changes colour from blue to green, yellow, orange, and finally forms a brick-red precipitate, with the colour change reflecting the concentration of reducing sugars. It is worth noting that sucrose (common table sugar) is a non-reducing sugar and requires a different testing method.

蛋白质检测 (Protein Test) – 双缩脲试验 (Biuret Test)：在食物样本中加入几滴双缩脲试剂（氢氧化钠溶液和稀硫酸铜溶液）。如果存在蛋白质，溶液会从蓝色变为紫色或淡紫色。这个反应是基于铜离子与蛋白质中肽键的配位作用。注意双缩脲试剂有腐蚀性，实验时必须佩戴护目镜。

Protein Test – Biuret Test: Add a few drops of Biuret reagent (sodium hydroxide solution and dilute copper sulfate solution) to the food sample. If protein is present, the solution changes from blue to purple or lilac. This reaction is based on the coordination of copper ions with peptide bonds in proteins. Note that Biuret reagent is corrosive, and safety goggles must be worn during the experiment.

脂肪检测 (Fat Test) – 乙醇乳液试验 (Ethanol Emulsion Test)：将食物样本与乙醇混合摇匀，然后将混合物倒入水中。如果存在脂肪，会形成白色乳状浑浊液。这个测试的原理是脂肪溶于乙醇但不溶于水 – 当乙醇-脂肪溶液加入水中时，脂肪以小液滴的形式分散（乳化），散射光线形成乳白色外观。

Fat Test – Ethanol Emulsion Test: Mix the food sample with ethanol and shake, then pour the mixture into water. If fat is present, a white milky emulsion forms. The principle behind this test is that fat dissolves in ethanol but not in water – when the ethanol-fat solution is added to water, the fat disperses as tiny droplets (emulsification), scattering light and producing a cloudy, milky appearance.

十、CIE 考试中常见的均衡饮食应用题 | Common Balanced Diet Application Questions in CIE Exams

掌握了以上知识后，让我们通过几个典型的 CIE KS3 考试题型来巩固理解。CIE 科学考试中的均衡饮食题目通常以应用和分析为主，要求学生将理论知识运用到实际情境中。

Having grasped the above knowledge, let us consolidate our understanding through several typical CIE KS3 examination question types. Balanced diet questions in CIE Science examinations usually focus on application and analysis, requiring students to apply theoretical knowledge to practical scenarios.

典型题型一：饮食日记分析。题干给出一个学生三天的饮食记录，要求分析该学生的饮食中可能缺乏哪些营养素，以及可能导致的健康问题。答题思路：逐一核对七大营养素的食物来源是否出现在饮食记录中 – 如果完全没有奶制品，可能缺乏钙和蛋白质；如果没有水果和蔬菜，可能缺乏维生素 C 和膳食纤维；如果以精制碳水化合物为主，可能缺乏 B 族维生素。

Typical Question Type 1: Diet Diary Analysis. The question provides a three-day food diary for a student and asks students to analyse which nutrients may be lacking in the diet and what health problems may result. Answer approach: check one by one whether food sources for the seven nutrient groups appear in the diary – if there are no dairy products, calcium and protein may be deficient; if there are no fruits and vegetables, vitamin C and dietary fibre may be lacking; if the diet is dominated by refined carbohydrates, B vitamins may be deficient.

典型题型二：数据解读。题干给出一张表格，列出了不同食物的能量含量和营养成分，要求学生为特定人群（如运动员、正在成长的青少年、需要减肥的人）设计合适的饮食方案。这类题目考察的是将营养学知识应用于不同人群特定需求的能力。

Typical Question Type 2: Data Interpretation. The question provides a table listing the energy content and nutritional composition of different foods and asks students to design appropriate meal plans for specific groups of people (such as athletes, growing adolescents, or people needing to lose weight). These questions test the ability to apply nutritional knowledge to the specific needs of different populations.

典型题型三：实验设计。要求描述如何通过实验检测某食物样品中是否含有淀粉、还原糖、蛋白质或脂肪。完整的答案需要包括：试剂名称、实验步骤、安全注意事项（如佩戴护目镜、使用水浴加热而非直接加热）、预期结果和结论。

Typical Question Type 3: Experimental Design. Students are asked to describe how to test a food sample for the presence of starch, reducing sugars, proteins, or fats experimentally. A complete answer needs to include: the name of the reagent, experimental procedure, safety precautions (such as wearing goggles and using a water bath rather than direct heating), expected results, and a conclusion.

典型题型四：比较与评价。给出一系列食品包装上的营养标签，要求学生比较哪款产品”更健康”，并说明理由。答题时需要考虑多种营养素的综合对比 – 不能只看热量，还需要考虑饱和脂肪含量、糖含量、盐含量、纤维含量以及是否含有重要的维生素和矿物质。

Typical Question Type 4: Comparison and Evaluation. A series of nutrition labels from food packaging is provided, and students are asked to compare which product is “healthier” and explain why. When answering, a comprehensive comparison of multiple nutrients is needed – looking at calories alone is insufficient; students should also consider saturated fat content, sugar content, salt content, fibre content, and whether the product contains important vitamins and minerals.

Summary | 总结

均衡饮食是 KS3 CIE 科学课程中营养学单元的核心概念。它涵盖七大营养素 – 碳水化合物、蛋白质、脂肪、维生素、矿物质、膳食纤维和水 – 每一种都有其独特且不可替代的生理功能。理解这些营养素不仅帮助我们通过考试，更重要的是帮助我们在一生中做出明智的饮食选择。从识别食物中的营养成分，到分析饮食日记和营养标签，再到动手进行食物成分检测实验，这个单元将理论知识与实践技能紧密结合，为后续 IGCSE 生物学的深入学习奠定了坚实的基础。

A balanced diet is the core concept of the nutrition unit in the KS3 CIE Science curriculum. It encompasses seven nutrient groups – carbohydrates, proteins, fats, vitamins, minerals, dietary fibre, and water – each with its own unique and irreplaceable physiological function. Understanding these nutrients not only helps us pass examinations but, more importantly, helps us make informed dietary choices throughout our lives. From identifying nutritional components in food, to analysing diet diaries and nutrition labels, to conducting hands-on food testing experiments, this unit tightly integrates theoretical knowledge with practical skills, laying a solid foundation for further study in IGCSE Biology.

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一、什么是呼吸？从细胞层面理解 | What is Respiration? Understanding It at the Cellular Level

呼吸（respiration）是生物体将食物中的化学能转化为可用能量的过程。许多同学一开始会混淆”呼吸”（breathing）和”细胞呼吸”（cellular respiration）这两个概念 – breathing 是指空气进出肺部的物理动作，而 respiration 则是在细胞内发生的化学反应，将葡萄糖与氧气结合，释放出 ATP（三磷酸腺苷），这是细胞能够直接使用的能量货币。在 KS3 科学课程中，理解这一区别是整个呼吸主题的起点。

Respiration is the process by which living organisms convert chemical energy stored in food into usable energy. Many students initially confuse “breathing” with “cellular respiration” – breathing refers to the physical movement of air in and out of the lungs, whereas respiration is a chemical reaction that takes place inside cells, combining glucose with oxygen to release ATP (adenosine triphosphate), the energy currency that cells can use directly. In the KS3 Science curriculum, understanding this distinction is the starting point for the entire respiration topic.

呼吸作用发生的场所是细胞内的线粒体（mitochondria）。线粒体被称为”细胞的发电站”，因为它们负责将葡萄糖分子分解，释放出其中储存的能量。每一个活细胞都需要能量来维持生命活动 – 无论是肌肉收缩、神经信号传递，还是细胞分裂和生长，都离不开呼吸作用产生的 ATP。

Respiration takes place inside the mitochondria, which are organelles found within cells. Mitochondria are often called the “powerhouses of the cell” because they are responsible for breaking down glucose molecules and releasing the energy stored within them. Every living cell requires energy to sustain life processes – whether it is muscle contraction, nerve signal transmission, or cell division and growth, none of these can happen without the ATP produced by respiration.

二、呼吸系统的组成器官及其功能 | Organs of the Respiratory System and Their Functions

人体呼吸系统由一系列专门器官组成，它们协同工作，将空气中的氧气输送到血液中，并将二氧化碳排出体外。呼吸系统的主要器官包括：鼻腔（nasal cavity）、咽部（pharynx）、喉部（larynx）、气管（trachea）、支气管（bronchi）、细支气管（bronchioles）和肺泡（alveoli）。

The human respiratory system consists of a series of specialised organs that work together to deliver oxygen from the air into the bloodstream and expel carbon dioxide from the body. The main organs of the respiratory system include: the nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, and alveoli.

空气首先通过鼻腔或口腔进入体内。鼻腔内部的黏膜和纤毛（cilia）可以过滤空气中的灰尘和微生物，同时温暖和湿润吸入的空气。接着空气经过咽部和喉部 – 喉部还包含声带，使我们能够发声。然后空气进入气管，这是一条由 C 形软骨环支撑的管道，确保气管始终保持打开状态。气管向下分支成两根支气管，分别进入左右两肺。支气管在肺内继续分支，形成越来越细的细支气管，最终通向数百万个微小的气囊 – 肺泡。

Air first enters the body through the nasal cavity or mouth. The mucous membrane and cilia inside the nasal cavity filter dust and microorganisms from the air, while also warming and moistening the inhaled air. The air then passes through the pharynx and larynx – the larynx also contains the vocal cords, enabling us to produce sound. Next, air enters the trachea, a tube supported by C-shaped rings of cartilage that ensure the airway remains open at all times. The trachea branches downwards into two bronchi, each leading into one of the two lungs. Inside the lungs, the bronchi continue to divide into increasingly narrower bronchioles, eventually reaching millions of tiny air sacs called alveoli.

膈肌（diaphragm）和肋间肌（intercostal muscles）是驱动呼吸运动的关键肌肉。膈肌是一层位于胸腔底部的穹顶状肌肉，当它收缩时向下移动，增大胸腔容积；肋间肌位于肋骨之间，收缩时将肋骨向上和向外拉动。这两组肌肉的协调运动产生了吸气和呼气。

The diaphragm and intercostal muscles are the key muscles that drive the breathing movements. The diaphragm is a dome-shaped sheet of muscle located at the base of the chest cavity; when it contracts, it moves downwards, increasing the volume of the chest cavity. The intercostal muscles are located between the ribs and, when they contract, pull the ribs upwards and outwards. The coordinated movement of these two sets of muscles produces inhalation and exhalation.

三、肺泡内的气体交换机制 | The Mechanism of Gas Exchange in the Alveoli

气体交换（gas exchange）是呼吸系统最核心的功能，发生在肺泡与毛细血管之间。肺泡是呼吸树末端的微小气囊，每个肺泡的壁极薄 – 仅有一个细胞的厚度 – 并且被密集的毛细血管网络所包裹。这种结构特点使得氧气和二氧化碳能够通过扩散（diffusion）快速地在空气和血液之间进行交换。

Gas exchange is the most essential function of the respiratory system and occurs between the alveoli and the surrounding capillaries. Alveoli are tiny air sacs at the ends of the respiratory tree; each alveolus has an extremely thin wall – only one cell thick – and is wrapped in a dense network of capillaries. These structural features allow oxygen and carbon dioxide to be exchanged rapidly between the air and the blood through the process of diffusion.

扩散是指粒子从高浓度区域向低浓度区域净移动的过程，不需要消耗额外能量（因此称为被动运输）。在吸气后，肺泡内的氧气浓度高于流经肺泡的血液中的氧气浓度，因此氧气从肺泡扩散进入血液。与此同时，血液中二氧化碳的浓度高于肺泡内空气的二氧化碳浓度，因此二氧化碳从血液扩散进入肺泡，在呼气时被排出体外。

Diffusion is the net movement of particles from an area of high concentration to an area of low concentration, requiring no additional energy (hence it is classified as passive transport). After inhalation, the concentration of oxygen inside the alveoli is higher than the concentration of oxygen in the blood flowing past the alveoli, so oxygen diffuses from the alveoli into the blood. At the same time, the concentration of carbon dioxide in the blood is higher than the concentration of carbon dioxide in the alveolar air, so carbon dioxide diffuses from the blood into the alveoli and is expelled during exhalation.

肺泡具有多种适应性特征来最大化气体交换效率：它们数量庞大（成人约有 3 亿个肺泡），提供了巨大的表面积；壁极薄，缩短了扩散距离；表面湿润，有助于气体溶解；并且被丰富的毛细血管网络紧密包裹，维持了持续的浓度梯度，确保扩散不断进行。

Alveoli possess several adaptive features that maximise the efficiency of gas exchange: they are extremely numerous (an adult has approximately 300 million alveoli), providing an enormous total surface area; their walls are extremely thin, minimising the diffusion distance; their surfaces are moist, which helps gases dissolve; and they are tightly wrapped in a rich network of capillaries, maintaining a steep concentration gradient that ensures continuous diffusion.

四、吸气和呼气的机械过程 | The Mechanical Process of Inhalation and Exhalation

呼吸运动（ventilation）包括吸气（inhalation）和呼气（exhalation）两个阶段，由膈肌和肋间肌的协调收缩与放松来驱动。这是一个物理过程，与气体交换（化学过程）不同，但它为气体交换创造了必要的条件 – 不断更新肺泡内的空气，维持氧气和二氧化碳的浓度梯度。

Ventilation consists of two phases – inhalation and exhalation – driven by the coordinated contraction and relaxation of the diaphragm and intercostal muscles. This is a physical process, distinct from gas exchange (which is chemical), but it creates the necessary conditions for gas exchange by continuously refreshing the air inside the alveoli and maintaining the concentration gradients of oxygen and carbon dioxide.

在吸气过程中，膈肌收缩并向下移动（变平），同时外肋间肌收缩，将肋骨向上和向外拉动。这两个动作共同导致胸腔容积增大，肺内气压因此降至低于大气压的水平。由于空气总是从高压区流向低压区，外部空气被吸入肺部。在进行深呼吸时，颈部和胸部的辅助肌肉也会参与，进一步增加胸腔容积。

During inhalation, the diaphragm contracts and moves downwards (flattening), while the external intercostal muscles contract, pulling the ribs upwards and outwards. Together, these two actions increase the volume of the chest cavity, causing the air pressure inside the lungs to drop below atmospheric pressure. Since air always flows from areas of high pressure to areas of low pressure, external air is drawn into the lungs. During deep breathing, accessory muscles in the neck and chest also become involved, further expanding the chest cavity.

呼气通常是一个被动过程：膈肌和外肋间肌放松，膈肌恢复其穹顶形状向上回弹，肋骨在重力作用下向下回落。胸腔容积减小，肺内气压升高至高于大气压，空气被推出肺部。在用力呼气（如咳嗽或吹气球）时，内肋间肌和腹部肌肉会主动收缩，加速空气排出。

Exhalation is usually a passive process: the diaphragm and external intercostal muscles relax; the diaphragm returns to its dome shape and moves back upwards, and the ribs fall back downwards under gravity. The volume of the chest cavity decreases, raising the air pressure inside the lungs above atmospheric pressure, and air is pushed out of the lungs. During forced exhalation (such as coughing or blowing up a balloon), the internal intercostal muscles and abdominal muscles contract actively to accelerate the expulsion of air.

五、有氧呼吸的化学反应与能量释放 | The Chemical Reaction of Aerobic Respiration and Energy Release

有氧呼吸（aerobic respiration）是在氧气充足的情况下发生的主要呼吸形式。它的总反应方程式可以用以下文字公式和化学方程式表示：

Aerobic respiration is the primary form of respiration that occurs when adequate oxygen is available. Its overall reaction can be represented by the following word equation and chemical equation:

文字公式： 葡萄糖 + 氧气 → 二氧化碳 + 水（+ 能量）

Word equation: glucose + oxygen → carbon dioxide + water (+ energy)

化学方程式： C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ ATP)

Chemical equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ ATP)

有氧呼吸是需氧生物获取能量的最有效方式。每分子葡萄糖通过有氧呼吸可以产生约 36-38 个 ATP 分子。这个过程实际上分为几个阶段 – 糖酵解（发生在细胞质中）、克雷布斯循环和电子传递链（均发生在线粒体内）。在 KS3 阶段，同学们不需要记忆这些中间步骤的细节，但需要知道反应物（葡萄糖和氧气）和产物（二氧化碳、水和能量），并理解这是一个放热反应（exothermic reaction），因为反应过程中向周围环境释放了能量。

Aerobic respiration is the most efficient way for aerobic organisms to obtain energy. Each glucose molecule can yield approximately 36-38 ATP molecules through aerobic respiration. The process is actually divided into several stages – glycolysis (occurring in the cytoplasm), the Krebs cycle, and the electron transport chain (both occurring inside the mitochondria). At the KS3 level, students are not required to memorise the details of these intermediate steps but need to know the reactants (glucose and oxygen) and products (carbon dioxide, water, and energy), and understand that this is an exothermic reaction because energy is released to the surroundings during the process.

六、无氧呼吸：当氧气不足时发生了什么 | Anaerobic Respiration: What Happens When Oxygen Is Insufficient

当剧烈运动导致肌肉细胞无法获得足够的氧气时，细胞会转而进行无氧呼吸（anaerobic respiration）。这种呼吸形式不需要氧气，但效率远低于有氧呼吸 – 每分子葡萄糖仅能产生 2 个 ATP 分子。无氧呼吸在人体内的反应可以概括为：

When intense exercise prevents muscle cells from receiving sufficient oxygen, cells switch to anaerobic respiration. This form of respiration does not require oxygen but is far less efficient than aerobic respiration – it produces only 2 ATP molecules per glucose molecule. The reaction for anaerobic respiration in humans can be summarised as:

文字公式： 葡萄糖 → 乳酸（+ 少量能量）

Word equation: glucose → lactic acid (+ a small amount of energy)

乳酸的积累是导致运动后肌肉酸痛和疲劳的主要原因。当肌肉中的乳酸浓度升高时，肌肉的 pH 值下降，酶活性受到抑制，肌肉收缩能力减弱。这就是为什么进行高强度短跑或举重后，肌肉会感到灼烧感和无力。好消息是，运动停止后，乳酸会被血液运送至肝脏，在那里被转化回葡萄糖或进一步分解 – 这个过程需要额外的氧气，这部分额外需要的氧气被称为”氧债”（oxygen debt）。

The accumulation of lactic acid is the main cause of muscle soreness and fatigue after exercise. When the concentration of lactic acid in muscles rises, the pH of the muscle tissue drops, enzyme activity is inhibited, and the ability of muscles to contract is reduced. This is why muscles feel a burning sensation and weakness after high-intensity sprinting or weightlifting. The good news is that once exercise stops, lactic acid is transported by the blood to the liver, where it is converted back into glucose or broken down further – this process requires additional oxygen, and the extra oxygen required is referred to as the “oxygen debt.”

不同生物的无氧呼吸产物也不同。例如，酵母菌（yeast）在无氧条件下进行发酵，将葡萄糖转化为乙醇和二氧化碳，这一过程被广泛用于酿造啤酒和烘焙面包：

Different organisms produce different end products from anaerobic respiration. For example, yeast carries out fermentation under anaerobic conditions, converting glucose into ethanol and carbon dioxide – a process widely used in brewing beer and baking bread:

酵母发酵： 葡萄糖 → 乙醇 + 二氧化碳（+ 少量能量）

Yeast fermentation: glucose → ethanol + carbon dioxide (+ a small amount of energy)

七、有氧呼吸与无氧呼吸的系统对比 | Systematic Comparison of Aerobic and Anaerobic Respiration

理解有氧呼吸和无氧呼吸之间的差异是 KS3 考试中的高频考点。两者都是将葡萄糖转化为可用能量的过程，但在条件、效率和产物上存在本质区别。以下从六个维度进行系统对比：

Understanding the differences between aerobic and anaerobic respiration is a frequently tested topic in KS3 examinations. Both are processes that convert glucose into usable energy, but they differ fundamentally in their conditions, efficiency, and products. The following is a systematic comparison across six dimensions:

1. 是否需要氧气：有氧呼吸需要氧气；无氧呼吸不需要氧气。

1. Oxygen requirement: Aerobic respiration requires oxygen; anaerobic respiration does not.

2. 发生场所：有氧呼吸大部分在线粒体内完成；无氧呼吸则完全发生在细胞质中。

2. Location: Most of aerobic respiration takes place inside the mitochondria; anaerobic respiration occurs entirely in the cytoplasm.

3. ATP 产量：有氧呼吸每分子葡萄糖产生约 36-38 个 ATP；无氧呼吸每分子葡萄糖仅产生 2 个 ATP。

3. ATP yield: Aerobic respiration produces approximately 36-38 ATP per glucose molecule; anaerobic respiration produces only 2 ATP per glucose molecule.

4. 最终产物：有氧呼吸产生二氧化碳和水（均为无毒产物）；人体无氧呼吸产生乳酸（有毒，会引起肌肉疲劳），酵母无氧呼吸产生乙醇和二氧化碳。

4. End products: Aerobic respiration produces carbon dioxide and water (both non-toxic); anaerobic respiration in humans produces lactic acid (toxic, causes muscle fatigue), and in yeast produces ethanol and carbon dioxide.

5. 反应完全程度：有氧呼吸将葡萄糖完全氧化分解；无氧呼吸仅将葡萄糖部分分解，乳酸分子中仍含有大量未释放的化学能。

5. Completeness of breakdown: Aerobic respiration completely oxidises glucose; anaerobic respiration only partially breaks down glucose, and the lactic acid molecules still contain a significant amount of unreleased chemical energy.

6. 持续时间：有氧呼吸可以持续进行，只要氧气和葡萄糖供应充足；无氧呼吸仅能维持较短时间（通常几十秒到几分钟），因为乳酸的积累最终会抑制肌肉功能。

6. Duration: Aerobic respiration can continue indefinitely as long as oxygen and glucose supplies are adequate; anaerobic respiration can only be sustained for a short period (typically tens of seconds to a few minutes) because the accumulation of lactic acid eventually impairs muscle function.

八、植物中的呼吸作用 | Respiration in Plants

植物和动物一样，每时每刻都在进行呼吸作用，将葡萄糖分解以释放能量来驱动各项生命活动 – 包括主动运输（active transport）、细胞分裂、蛋白质合成和生长。一个常见的误解是认为植物只进行光合作用而不进行呼吸，或者认为植物只在夜间进行呼吸。实际上，植物的呼吸作用是持续不断的，无论白天还是黑夜。

Like animals, plants carry out respiration continuously, breaking down glucose to release energy that drives various life processes – including active transport, cell division, protein synthesis, and growth. A common misconception is that plants only perform photosynthesis and do not respire, or that plants only respire at night. In reality, plant respiration is continuous, occurring both day and night.

然而，植物在白天同时进行光合作用和呼吸作用，情况比动物更为复杂。光合作用吸收二氧化碳并释放氧气，而呼吸作用消耗氧气并释放二氧化碳。在白天，光合作用的速率通常高于呼吸作用速率，因此植物净吸收二氧化碳，净释放氧气。到了夜晚，光合作用因缺少光照而停止，但呼吸作用继续进行，此时植物净吸收氧气，净释放二氧化碳。

However, during the daytime, plants carry out both photosynthesis and respiration simultaneously, making the situation more complex than in animals. Photosynthesis takes in carbon dioxide and releases oxygen, while respiration consumes oxygen and releases carbon dioxide. During the day, the rate of photosynthesis is usually higher than the rate of respiration, so plants have a net uptake of carbon dioxide and a net release of oxygen. At night, photosynthesis ceases due to the absence of light, but respiration continues – at this point, plants have a net uptake of oxygen and a net release of carbon dioxide.

这也是为什么在医院病房中通常不在夜间摆放大量植物的原因 – 它们会与病人竞争有限的氧气。同样地，在卧室中放置过多植物也可能在夜间略微降低室内氧气水平。不过需要指出的是，几盆室内植物对室内氧气水平的影响微乎其微，远低于一个人在房间内呼吸所消耗的氧气量。

This is also why hospital wards typically do not keep large numbers of plants at night – they would compete with patients for the limited oxygen. Similarly, having too many plants in a bedroom may slightly reduce the indoor oxygen level at night. It is worth noting, however, that a few houseplants have a negligible effect on indoor oxygen levels, far less than the oxygen consumed by a single person breathing in the room.

九、运动对呼吸的影响及实验探究 | The Effect of Exercise on Respiration and Experimental Investigation

运动对呼吸速率和深度有显著影响。当开始运动时，肌肉收缩需要更多的 ATP，因此呼吸速率加快以提供更多的氧气并清除产生的二氧化碳。这一变化可以通过简单的实验来测量和记录 – 这是 KS3 科学中的常见实践活动。

Exercise has a significant effect on breathing rate and depth. When exercise begins, muscle contraction requires more ATP, so the breathing rate increases to deliver more oxygen and remove the carbon dioxide produced. This change can be measured and recorded through simple experiments – a common practical activity in KS3 Science.

实验方法：测量静息状态下的呼吸速率（通过计算 30 秒内胸部的起伏次数，然后乘以 2 得到每分钟的呼吸次数）。然后进行一定量的运动（如原地跑步或上下台阶 2 分钟），运动结束后立即再次测量呼吸速率，之后每隔 1 分钟测量一次，直到呼吸速率恢复到静息水平。将数据记录在表格中，并绘制呼吸速率随时间变化的折线图。

Experimental method: Measure the resting breathing rate by counting the number of chest rises in 30 seconds, then multiply by 2 to obtain the breaths per minute. Next, perform a set amount of exercise (such as running on the spot or stepping up and down for 2 minutes), then measure the breathing rate again immediately after exercise, and continue measuring every minute thereafter until the breathing rate returns to the resting level. Record the data in a table and plot a line graph of breathing rate against time.

预期结果：呼吸速率在运动结束时达到峰值，然后随着恢复时间的推移逐渐下降。通常情况下，静息呼吸速率约为 12-16 次/分钟；中等强度运动后可能升高至 30-40 次/分钟；高强度运动后甚至可能超过 50 次/分钟。恢复至静息水平所需的时间取决于个体的健康水平 – 体能越好的人，恢复得越快。

Expected results: Breathing rate peaks at the end of exercise and then gradually declines as recovery time progresses. Typically, resting breathing rate is around 12-16 breaths per minute; after moderate exercise it may rise to 30-40 breaths per minute; after high-intensity exercise it may exceed 50 breaths per minute. The time taken to return to resting level depends on the fitness level of the individual – the fitter a person is, the faster the recovery.

除了呼吸速率，心率也会在运动期间同步增加，以便更快地将氧气输送到肌肉并将二氧化碳运回肺部。这两个系统的协调反应展示了人体内器官系统之间精密的协作关系。

In addition to breathing rate, heart rate also increases in sync during exercise to deliver oxygen to the muscles more quickly and transport carbon dioxide back to the lungs. The coordinated response of these two systems demonstrates the intricate collaboration between organ systems within the human body.

十、呼吸系统的健康与疾病防护 | Respiratory System Health and Disease Prevention

保持呼吸系统健康对于维持正常的呼吸功能至关重要。影响呼吸系统健康的主要因素包括：吸烟（包括二手烟）、空气污染、呼吸道感染以及职业性粉尘暴露。吸烟是导致慢性阻塞性肺疾病（COPD）、肺气肿和肺癌的最主要风险因素 – 香烟烟雾中的焦油会破坏纤毛，使肺部失去自我清洁能力；尼古丁会使气道收缩；一氧化碳会与血红蛋白结合，降低血液的携氧能力。

Maintaining respiratory system health is essential for sustaining normal respiratory function. The main factors affecting respiratory system health include: smoking (including second-hand smoke), air pollution, respiratory infections, and occupational dust exposure. Smoking is the leading risk factor for chronic obstructive pulmonary disease (COPD), emphysema, and lung cancer – tar in cigarette smoke damages the cilia, depriving the lungs of their self-cleaning ability; nicotine constricts the airways; and carbon monoxide binds to haemoglobin, reducing the oxygen-carrying capacity of the blood.

哮喘（asthma）是另一种常见的呼吸系统疾病，表现为气道的慢性炎症和可逆性狭窄。哮喘患者在接触触发因素（如花粉、尘螨、冷空气或运动）时，会出现喘息、胸闷、咳嗽和呼吸困难等症状。哮喘可以通过避免触发因素和使用吸入性药物（如支气管扩张剂）来管理。

Asthma is another common respiratory condition, characterised by chronic inflammation and reversible narrowing of the airways. When exposed to triggers (such as pollen, dust mites, cold air, or exercise), asthma sufferers may experience wheezing, chest tightness, coughing, and shortness of breath. Asthma can be managed by avoiding triggers and using inhaled medications (such as bronchodilators).

预防呼吸系统疾病的有效措施包括：不吸烟并避免二手烟暴露；在空气污染严重时佩戴口罩；定期进行有氧运动以增强肺活量；保持良好的室内通风；以及通过均衡饮食摄取足够的抗氧化剂（如维生素 C 和维生素 E），这些物质有助于保护肺组织免受氧化损伤。

Effective measures for preventing respiratory diseases include: not smoking and avoiding exposure to second-hand smoke; wearing a mask when air pollution is severe; engaging in regular aerobic exercise to improve lung capacity; maintaining good indoor ventilation; and consuming adequate antioxidants (such as vitamin C and vitamin E) through a balanced diet, as these substances help protect lung tissue from oxidative damage.

Summary | 总结

呼吸作用是生命最基本的生化过程之一，它使生物体能够将食物中的化学能转化为可直接利用的 ATP。人类的呼吸系统 – 从鼻腔到肺泡 – 经过精妙的演化，能够高效地进行气体交换，通过扩散作用将氧气输送到血液中，同时将代谢废物二氧化碳排出体外。有氧呼吸是最高效的能量获取方式，而无氧呼吸则是在氧气不足时的应急备用方案，但会产生乳酸作为副产品。植物同样进行呼吸作用，只不过它们在白天还同时进行光合作用，使气体交换的净效应变得更加复杂。运动时呼吸速率和深度的增加是身体满足能量需求升高的正常生理反应，而保护好呼吸系统免受烟雾、污染物和病原体的侵害，则是维持长期健康的关键。

Respiration is one of the most fundamental biochemical processes of life, enabling organisms to convert the chemical energy in food into directly usable ATP. The human respiratory system – from the nasal cavity to the alveoli – has evolved with remarkable precision to carry out gas exchange efficiently, delivering oxygen into the bloodstream by diffusion while removing the metabolic waste product carbon dioxide. Aerobic respiration is the most efficient way to obtain energy, while anaerobic respiration serves as an emergency backup when oxygen is insufficient, albeit producing lactic acid as a by-product. Plants also carry out respiration, but during the daytime they simultaneously perform photosynthesis, making the net effect on gas exchange more complex. The increase in breathing rate and depth during exercise is a normal physiological response to meet the elevated energy demand, and protecting the respiratory system from smoke, pollutants, and pathogens is key to maintaining long-term health.

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