📚 What Are You Made Of? | 你由什么构成？
Look at your hand. It feels solid, warm and alive. But if you zoom in far beyond what your eyes can see, you will find that your body is a highly organised collection of chemical elements, molecules and reactions. This chapter asks a simple but deep question: what are you made of? The answer connects chemistry to biology and to everyday life, and it explains why a subject that seems to live in laboratories is actually living inside you right now.
看看你的手。它摸起来结实、温暖、有生命。但如果你放大到眼睛看不见的尺度，你会发现身体是化学元素、分子和化学反应的高度有序集合。本章提出一个简单却深刻的问题：你由什么构成？答案将化学与生物学以及日常生活联系在一起，也解释了为什么这门看似只属于实验室的学科，此刻正活在你体内。
1. A Chemical Question | 一个化学问题
Chemistry is the study of matter and how it changes. When you ask "What are you made of?", you are asking a chemistry question. Your body is made of atoms, which join together to form molecules. Those molecules build tissues, organs and entire systems. So chemistry is not just about test tubes in a laboratory; it is about the substance of your own body.
化学是研究物质及其变化方式的科学。当你问"你由什么构成？"时，你其实在问一个化学问题。你的身体由原子组成，原子结合成分子，分子再构建组织、器官和整个系统。因此，化学不仅是实验室里的试管，更是关于你身体本质的科学。
At the simplest level, you are made of chemical elements. An element is a pure substance made of only one type of atom. About 60 elements can be found in the human body, but most are present in tiny amounts. A small number of elements dominate, and that is why the chemistry of life often focuses on just a few key players.
在最简单的层面上，你由化学元素组成。元素是只由一种原子组成的纯物质。人体中大约能找到60种元素，但大多数含量极少。少数几种元素占据主导地位，这就是生命化学常常只关注几个关键角色的原因。
It also helps to see the levels of organisation. Atoms group into molecules, molecules build the structures inside cells, cells form tissues, tissues form organs such as the heart or the liver, and organs work together as organ systems. Chemistry sits at the bottom of that ladder, supporting everything above it.
理解构成的层次也很有帮助。原子组成分子，分子构建细胞内部的结构，细胞形成组织，组织形成心脏、肝脏这样的器官，器官再协同成为器官系统。化学位于这把梯子的最底层，支撑着上面的一切。
2. The Big Four Elements | 四大元素
If you could break your body down into pure elements, four would stand out: oxygen, carbon, hydrogen and nitrogen. Together, these four elements make up about 96% of your body mass. Oxygen is number one, mostly because your body contains so much water. Carbon forms the backbone of nearly all biological molecules. Hydrogen is found in water and organic compounds. Nitrogen is essential for proteins and DNA.
如果你能把身体分解成纯元素，有四种会突出：氧、碳、氢和氮。这四种元素一共约占身体质量的96%。氧排在第一位，主要是因为你体内含有大量水。碳构成几乎所有生物分子的骨架。氢存在于水和有机化合物中。氮对蛋白质和DNA至关重要。
| Element | Symbol | Approx. body mass | Main role |
| Oxygen | O | 65% | Water, respiration |
| Carbon | C | 18% | Backbone of organic molecules |
| Hydrogen | H | 10% | Water, organic compounds |
| Nitrogen | N | 3% | Proteins, DNA |
| Calcium | Ca | 1.5% | Bones, teeth, muscle action |
| Phosphorus | P | 1.0% | DNA, ATP, bones |
The percentages vary slightly from textbook to textbook, so do not worry about memorising exact values. What matters is the order: oxygen is the most abundant element in the human body, followed by carbon, hydrogen and nitrogen.
不同教材给出的百分比会略有差异，所以不必死记精确数值。重要的是顺序：氧是人体中含量最多的元素，其次是碳、氢和氮。
3. Beyond the Big Four | 四大元素之外
The remaining 4% of your mass matters enormously, because small amounts of the right element can control a huge amount of chemistry. Calcium and phosphorus build bones and teeth, potassium and sodium carry nerve signals, sulphur appears in some proteins, and chlorine helps make the hydrochloric acid in your stomach.
剩下的4%质量极其重要，因为少量正确的元素可以控制大量化学反应。钙和磷构建骨骼和牙齿，钾和钠传递神经信号，硫存在于某些蛋白质中，氯则帮助胃部生成盐酸。
Some elements are needed in even smaller quantities. These are called trace elements. Iron is part of haemoglobin, the red pigment in blood that carries oxygen. Iodine is needed to make thyroid hormones, which control your growth and the speed of your metabolism. Zinc and copper help enzymes work. Fluorine hardens tooth enamel.
有些元素的需要量更少，称为微量元素。铁是血红蛋白的组成部分，这种血液中的红色色素负责运输氧。碘用于制造甲状腺激素，控制你的生长和代谢速度。锌和铜帮助酶正常工作。氟则使牙釉质更坚固。
A missing element causes a deficiency disease. Too little iron leads to anaemia, so the person feels tired and short of breath. Too little iodine can cause a swelling of the neck called goitre. Too little calcium over many years weakens bones and can lead to osteoporosis. This is a good example of chemistry and health being the same story.
缺少某种元素会引起缺乏症。缺铁会导致贫血，人会感到疲倦、气短。缺碘可能引起颈部肿大的甲状腺肿。长期缺钙会使骨骼变脆，可能导致骨质疏松。这很好地说明化学与健康本是同一个故事。
4. Atoms: The Building Blocks | 原子：基本构件
Every element is made of one type of atom. An atom has a tiny, dense nucleus containing positively charged protons and neutral neutrons, surrounded by negatively charged electrons arranged in shells. Nearly all the mass of an atom sits in the nucleus, while the electrons occupy most of its volume.
每种元素都由一种原子构成。原子有一个极小、极密的原子核，核内是带正电的质子和不带电的中子，核外是分层排布的带负电的电子。原子几乎全部质量集中在原子核，而电子占据了绝大部分体积。
atomic number = number of protons = number of electrons (in a neutral atom)
The atomic number tells you which element an atom is. Carbon always has 6 protons, oxygen always has 8, and nitrogen always has 7. If the number of protons changes, the element changes. That single fact explains why the periodic table is arranged the way it is.
原子序数决定了原子属于哪种元素。碳总是有6个质子，氧总是8个，氮总是7。如果质子数改变，元素就变了。这一条事实解释了元素周期表为何这样排列。
Atoms of the same element with different numbers of neutrons are called isotopes. Carbon-12 and carbon-14 are both carbon, but carbon-14 is radioactive and is used to date ancient materials and as a tracer in medicine. In your own body, isotopes of carbon, hydrogen and oxygen are constantly being replaced as molecules are built and broken down.
同一种元素中中子数不同的原子称为同位素。碳-12和碳-14都是碳，但碳-14具有放射性，可用于测定古物年代，也可在医学中作为示踪剂。在你体内，随着分子不断合成与分解，碳、氢、氧的同位素也在不断被替换。
5. From Atoms to Molecules | 从原子到分子
Atoms rarely stay alone. They join by forming chemical bonds, and the most common bond in your body is the covalent bond, in which two atoms share a pair of electrons. When atoms join in fixed ratios, they form a compound with a chemical formula. Water is H₂O: two hydrogen atoms bonded to one oxygen atom.
原子很少单独存在。它们通过形成化学键结合，而你体内最常见的键是共价键，即两个原子共用一对电子。当原子按固定比例结合，就形成具有化学式的化合物。水是H₂O：两个氢原子与一个氧原子结合。
The subscript numbers in a formula tell you how many atoms of each element are present. In carbon dioxide, CO₂, there is one carbon atom and two oxygen atoms. In glucose, C₆H₁₂O₆, there are 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms, making 24 atoms in total. Counting atoms in a formula is a skill worth practising.
化学式中的下标数字告诉你每种元素有几个原子。二氧化碳CO₂中有1个碳原子和2个氧原子。葡萄糖C₆H₁₂O₆中有6个碳原子、12个氢原子和6个氧原子，共24个原子。数化学式中的原子是一项值得练习的技能。
It is also worth separating two ideas. O₂ is a molecule of an element, because both atoms are oxygen. H₂O is a molecule of a compound, because it contains atoms of different elements joined together. Your body contains both types: element molecules such as O₂ and N₂, and countless compounds.
还有两个概念值得区分。O₂是单质分子，因为两个原子都是氧。H₂O是化合物分子，因为它由不同元素的原子结合而成。你体内两者都有：O₂、N₂这样的单质分子，以及数不清的化合物。
6. Ions and Electrolytes | 离子与电解质
Atoms can also gain or lose electrons to become ions. Losing electrons gives a positive ion; gaining electrons gives a negative ion. Sodium loses one electron to become Na⁺, chlorine gains one to become Cl⁻, calcium loses two to become Ca²⁺, and potassium becomes K⁺. These charged particles are written with superscript signs.
原子也可以得到或失去电子而成为离子。失去电子形成正离子，得到电子形成负离子。钠失去1个电子成为Na⁺，氯得到1个电子成为Cl⁻，钙失去2个电子成为Ca²⁺，钾成为K⁺。这些带电粒子用上标符号表示。
Oppositely charged ions attract and form ionic compounds. Table salt is sodium chloride, NaCl, a regular lattice of Na⁺ and Cl⁻ ions, not a mixture of two substances. In water, these ions separate and move freely, which is why salt water conducts electricity.
带相反电荷的离子相互吸引，形成离子化合物。食盐是氯化钠NaCl，由Na⁺和Cl⁻离子构成规则的晶格，而不是两种物质的混合物。在水中，这些离子分开并自由移动，所以盐水能导电。
Your body fluids contain dissolved ions, often called electrolytes. The movement of Na⁺ and K⁺ across the membranes of nerve cells produces the electrical signals that let you think, feel and move. Ca²⁺ is needed for muscles to contract and for blood to clot. This is why athletes who sweat heavily need to replace salts as well as water.
你的体液中溶解着离子，通常称为电解质。Na⁺和K⁺穿过神经细胞膜的运动产生电信号，使你能够思考、感觉和运动。Ca²⁺是肌肉收缩和血液凝固所必需的。这就是大量出汗的运动员不仅需要补水，还需要补充盐分的原因。
7. Water: The Molecule That Makes You You | 水：塑造你的分子
Water makes up roughly 60% to 70% of your body mass, and in some organs the figure is even higher. Because oxygen is the heaviest and most abundant element in water, water is the main reason oxygen tops the table of body elements. Water is also where most of the chemistry of life happens.
水约占你身体质量的60%到70%，在某些器官中比例更高。由于氧是水中最重的元素且含量最大，水是氧在人体元素表中位居第一的主要原因。水也是生命化学发生的主要场所。
Water is a polar molecule: the oxygen end carries a slight negative charge and the hydrogen ends carry slight positive charges. This allows water molecules to attract each other and to dissolve ions and many other substances, which is why water is often called the universal solvent. Blood plasma, cytoplasm and digestive juices are all water-based solutions.
水是极性分子：氧端略带负电，氢端略带正电。这使水分子之间相互吸引，并能溶解离子和许多其他物质，因此水常被称为"万能溶剂"。血浆、细胞质和消化液都是以水为基础的溶液。
Water also has a high specific heat capacity, meaning it takes a lot of energy to warm it up or cool it down. That helps keep your body temperature stable. Water has a high latent heat of vaporisation too, so sweating removes a large amount of energy and cools you effectively. Without these properties, warm-blooded life would be impossible.
水还具有较高的比热容，这意味着要让它升温或降温都需要大量能量。这有助于维持体温稳定。水的汽化潜热也很高，所以出汗能带走大量能量，有效地为你降温。如果没有这些性质，温血生命将无法存在。
8. Carbon: The Backbone of Life | 碳：生命的骨架
Carbon is special because each carbon atom can form four covalent bonds. That lets carbon atoms link to each other in long chains, branched chains and rings, and then attach other atoms or groups to the framework. Millions of different carbon compounds are possible, and life uses this variety.
碳之所以特别，是因为每个碳原子能形成4个共价键。这使碳原子可以互相连接成长链、支链和环，并在骨架上再接上其他原子或基团。碳化合物可能有数百万种，生命正是利用这种多样性。
Compounds based on carbon are called organic compounds, and their study is organic chemistry. Simple examples include methane, CH₄, ethanol, C₂H₅OH, and ethanoic acid, CH₃COOH. Many organic molecules carry functional groups such as the hydroxyl group −OH, the carboxyl group −COOH, and the amino group −NH₂, and these groups give the molecule its chemical behaviour.
以碳为基础的化合物称为有机化合物，研究它们的学科是有机化学。简单的例子包括甲烷CH₄、乙醇C₂H₅OH和乙酸CH₃COOH。许多有机分子带有官能团，如羟基−OH、羧基−COOH和氨基−NH₂，这些基团决定了分子的化学行为。
Carbon also moves between living things and the environment in the carbon cycle. Plants take in carbon dioxide from the air and use it to make glucose during photosynthesis; animals eat plants and pass the carbon along; respiration and decay return carbon dioxide to the atmosphere. Every carbon atom in your body has travelled through that cycle many times before reaching you.
碳还通过碳循环在生物与环境之间移动。植物在光合作用中吸收空气中的二氧化碳制造葡萄糖；动物吃植物，把碳传递下去；呼吸作用和分解又把二氧化碳还给大气。你体内每一个碳原子在到达你之前，都已经在这个循环中旅行了许多次。
9. The Molecules of Life | 生命分子
Four families of molecules build your body: carbohydrates, lipids, proteins and nucleic acids. Each is made from a small set of elements arranged in repeating units. Carbohydrates contain carbon, hydrogen and oxygen, and include glucose, starch and cellulose. Lipids, such as fats and oils, are built from glycerol and fatty acids.
构建你身体的有四大类分子：碳水化合物、脂质、蛋白质和核酸。它们都由少数几种元素按重复单元排列而成。碳水化合物含碳、氢、氧，包括葡萄糖、淀粉和纤维素。脂肪和油等脂质由甘油和脂肪酸构成。
Proteins contain carbon, hydrogen, oxygen and nitrogen, and sometimes sulphur. They are long chains of amino acids folded into precise shapes. Enzymes are proteins, and they act as biological catalysts, speeding up the reactions of life without being used up themselves. Nucleic acids such as DNA contain carbon, hydrogen, oxygen, nitrogen and phosphorus, and they store the instructions for building proteins.
蛋白质含碳、氢、氧、氮，有时还含硫。它们是折叠成精确形状的长链氨基酸。酶是蛋白质，充当生物催化剂，加快生命反应的速率而自身不被消耗。DNA等核酸含碳、氢、氧、氮、磷，储存着制造蛋白质的指令。
| Molecule family | Elements | Example | Job in the body |
| Carbohydrate | C, H, O | Glucose, starch | Energy supply |
| Lipid | C, H, O | Fats, oils | Energy store, insulation |
| Protein | C, H, O, N, sometimes S | Enzymes, haemoglobin | Catalysis, structure, transport |
| Nucleic acid | C, H, O, N, P | DNA, RNA | Storing genetic information |
10. Chemical Reactions Inside You | 体内的化学反应
You are not just made of chemicals; you are also a place where chemicals constantly react. Together, all these reactions are called metabolism. Some build larger molecules from smaller ones, and some break larger molecules down. Both kinds happen in your cells every second.
你不只是由化学物质构成，你还是化学物质不断反应的地方。所有这些反应统称为新陈代谢。有些把较小的分子合成较大的分子，有些把较大的分子分解。两种反应每秒钟都在你的细胞中发生。
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
This is aerobic respiration, the reaction that releases the energy your cells need. Glucose reacts with oxygen to produce carbon dioxide and water. The energy released powers muscle contraction, nerve impulses and the maintenance of body temperature. Digestion is the opposite kind of process: enzymes break large food molecules into small ones that can be absorbed into the blood.
这就是有氧呼吸，释放细胞所需能量的反应。葡萄糖与氧反应，生成二氧化碳和水。释放的能量用于肌肉收缩、神经冲动和维持体温。消化则是相反的过程：酶把大的食物分子分解成能被吸收进血液的小分子。
Almost every reaction in your body needs an enzyme, and each enzyme has a specific shape that fits its substrate like a key in a lock. That is why temperature and pH matter so much: too much heat or the wrong acidity changes the shape of the enzyme and it stops working. This idea explains both cooking food and the danger of a very high fever.
你体内几乎每个反应都需要酶，而每种酶都有特定的形状，像钥匙配锁一样与底物结合。这解释了为什么温度和pH如此重要：过热或酸碱度不当会改变酶的形状，使它失去作用。这一原理既能解释烹饪食物，也能解释高烧的危险。
11. How Do We Know? Evidence and Tests | 我们如何知道？证据与检验
Scientists did not guess the composition of the human body; they measured it. Food tests are a simple classroom example. Benedict's solution turns from blue to brick red with reducing sugars, iodine solution turns blue-black with starch, Biuret reagent turns purple with protein, and an ethanol emulsion test turns cloudy white with lipids.
科学家并不是猜测人体的组成，而是测量出来的。食品检验就是课堂上简单的例子。本尼迪克特试剂遇还原糖由蓝色变为砖红色，碘液遇淀粉变蓝黑色，双缩脲试剂遇蛋白质变紫色，乙醇乳化试验遇脂质变浑浊白色。
For elements, chemists use techniques such as flame tests, which give characteristic colours for certain metal ions, and spectroscopy, which identifies elements from the light they absorb or emit. Modern instruments can detect elements present at concentrations of a few parts per million. Medical imaging and radioactive tracers let doctors follow specific elements
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