📚 The Building Blocks of Life | 生命的基本构建块
All living organisms are composed of a small set of chemical elements that assemble into an astonishing variety of molecules and structures. In Cambridge A-Level Biology, understanding these building blocks means linking atoms, biological molecules, organelles and cells into one coherent picture of life.
所有生物体都由少数化学元素组成，这些元素组装成种类繁多的分子和结构。在剑桥 A-Level 生物中，理解这些基本构建块意味着将原子、生物分子、细胞器和细胞联系成一幅连贯的生命图景。
This revision article follows that hierarchy step by step: from the atoms that make up biological molecules, through the four great classes of macromolecules and the water that surrounds them, to enzymes, organelles, membranes and finally whole cells and organisms. Each section states the core facts you must recall, the explanations examiners expect, and the pitfalls that lose marks.
本篇复习文章沿着这一层级逐级展开：从构成生物分子的原子，到四大类生物大分子以及环绕它们的水，再到酶、细胞器、细胞膜，最终到完整的细胞与生物体。每一节都给出必须记住的核心事实、考官期待的完整解释，以及最容易丢分的陷阱。
1. From Atoms to Biological Molecules | 从原子到生物分子
The most abundant elements in living organisms are carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur. Carbon is especially important because each carbon atom can form four covalent bonds, allowing the construction of long chains, branched structures and rings.
生物体中最丰富的元素是碳、氢、氧、氮、磷和硫。碳尤其重要，因为每个碳原子可以形成四个共价键，从而能够构建长链、支链结构和环状结构。
Each of these elements has a characteristic biological role, and exam questions often ask you to link an element to the molecule it builds or the process it supports.
这些元素各有其标志性的生物学作用，考试题经常要求你把某种元素与它所构建的分子或它所支持的过程联系起来。
| Element | 元素 | Key biological role | 主要生物学作用 |
| C 碳 | Backbone of all organic molecules; four covalent bonds | 所有有机分子的骨架；可形成四个共价键 |
| H 氢 | In every organic molecule; hydrogen bonds in water and proteins | 存在于所有有机分子中；参与水和蛋白质中的氢键 |
| O 氧 | Terminal electron acceptor in aerobic respiration | 有氧呼吸中电子的最终受体 |
| N 氮 | Amino groups in proteins, bases in nucleotides | 蛋白质中的氨基、核苷酸中的碱基 |
| P 磷 | Phosphate groups in ATP, nucleotides and phospholipids | ATP、核苷酸和磷脂中的磷酸基团 |
| S 硫 | Disulfide bridges that stabilise protein tertiary structure | 稳定蛋白质三级结构的二硫键 |
Small organic subunits called monomers join together by condensation reactions, in which a water molecule is removed. The reverse process, hydrolysis, adds water to split a polymer back into monomers.
称为单体的小有机亚基通过缩合反应连接在一起，反应中脱去一分子水。相反的过程——水解——则加入水将聚合物分解回单体。
monomer + monomer → polymer + H₂O
Condensation is a synthesis reaction that builds up bonds; hydrolysis is a breakdown reaction that occurs during digestion. Both are catalysed by enzymes, and hydrolysis in the gut is what releases absorbable monomers from the food you eat.
缩合是构建化学键的合成反应；水解则是分解反应，发生在消化过程中。两者都由酶催化，而消化道中的水解正是把你吃下的食物释放为可吸收单体的过程。
Trace elements such as iron (in haemoglobin and cytochromes) and magnesium (at the centre of chlorophyll) are needed in tiny amounts but are essential for specific proteins and pigments.
微量元素如铁（存在于血红蛋白和细胞色素中）和镁（位于叶绿素中心）需要量极小，但对特定的蛋白质和色素却是必不可少的。
2. Water: The Medium of Life | 水：生命的介质
Water is a polar molecule because oxygen attracts the shared electrons more strongly than hydrogen. This polarity allows hydrogen bonds to form between water molecules, giving water high cohesion, adhesion and surface tension.
水是一种极性分子，因为氧对共用电子的吸引比氢更强。这种极性使水分子之间形成氢键，赋予水较高的内聚力、附着力和表面张力。
Water has a high specific heat capacity and a high latent heat of vaporisation, so it stabilises temperatures in organisms and cools the body through evaporation of sweat. It is also an excellent solvent for ions and polar molecules, enabling transport and metabolic reactions.
水具有较高的比热容和较高的汽化潜热，因此能够稳定生物体内的温度，并通过汗液蒸发使身体降温。它也是离子和极性分子的优良溶剂，使运输和代谢反应得以进行。
Ice is less dense than liquid water because hydrogen bonds hold water molecules in an open lattice, so ice floats and insulates aquatic habitats.
冰的密度低于液态水，因为氢键使水分子排列成开放的晶格结构，因此冰会浮在水面上，并为其下的水生栖息地提供隔热层。
Cohesion supports the continuous columns of water in xylem vessels and produces surface tension at an air-water interface, which is exploited by insects that walk on ponds. Adhesion between water and the walls of narrow xylem vessels contributes to capillary action and helps water rise against gravity.
内聚力支撑木质部导管中连续的水柱，并在水与空气的交界面产生表面张力，使某些昆虫能够在水面上行走。水与狭窄木质部管壁之间的附着力产生毛细作用，有助于水克服重力上升。
Because a large amount of energy is needed to raise its temperature, water acts as a thermal buffer: cells, blood and aquatic environments change temperature only slowly. The high latent heat of vaporisation means a lot of energy is carried away when water evaporates, which is why sweating and transpiration are effective cooling mechanisms.
由于升高水温需要大量能量，水起到热缓冲作用：细胞、血液和水生环境的温度变化都很缓慢。高汽化潜热意味着水蒸发时会带走大量能量，这正是出汗和蒸腾成为有效降温机制的原因。
Water is also a metabolite: it is a reactant in hydrolysis and photosynthesis, and a product of condensation and aerobic respiration. Its transparency allows light to penetrate aquatic habitats for photosynthesis.
水还是代谢物质：它是水解和光合作用的反应物，也是缩合反应和有氧呼吸的产物。水的透明性使光能穿透水生环境，支持光合作用。
3. Carbohydrates: Energy and Structure | 碳水化合物：能量与结构
Carbohydrates contain carbon, hydrogen and oxygen in the ratio Cₓ(H₂O)ᵧ. They are built from monosaccharides, which are sweet, soluble reducing sugars such as glucose, fructose and galactose, all with the formula C₆H₁₂O₆.
碳水化合物含碳、氢、氧，比例可写作 Cₓ(H₂O)ᵧ。它们由单糖构建而成；单糖是带有甜味、可溶的还原糖，例如葡萄糖、果糖和半乳糖，分子式均为 C₆H₁₂O₆。
Maltose = glucose + glucose; sucrose = glucose + fructose; lactose = glucose + galactose.
两个单糖通过缩合反应形成一个由糖苷键连接的双糖。麦芽糖 = 葡萄糖 + 葡萄糖；蔗糖 = 葡萄糖 + 果糖；乳糖 = 葡萄糖 + 半乳糖。
glucose + glucose → maltose + H₂O
Polysaccharides are large, insoluble polymers formed from many monosaccharides. Starch (amylose and amylopectin) is the storage carbohydrate of plants, glycogen is the storage carbohydrate of animals, and cellulose is the structural carbohydrate of plant cell walls.
多糖是由许多单糖形成的大型不溶性聚合物。淀粉（直链淀粉和支链淀粉）是植物的储能碳水化合物，糖原是动物的储能碳水化合物，而纤维素是植物细胞壁的结构性碳水化合物。
- Amylose | 直链淀粉: unbranched helix of α-glucose with 1,4-glycosidic bonds; compact and ideal for storage. 由 α-葡萄糖通过 1,4-糖苷键构成的无支链螺旋；结构紧凑，适合储存。
- Amylopectin and glycogen | 支链淀粉与糖原: also contain 1,6-glycosidic bonds, giving many free ends for rapid hydrolysis. 还含有 1,6-糖苷键，产生许多游离末端，便于快速水解。
- Cellulose | 纤维素: unbranched β-glucose chains with 1,4-glycosidic bonds; adjacent chains are cross-linked by hydrogen bonds to form microfibrils of high tensile strength. 由 β-葡萄糖通过 1,4-糖苷键构成的无支链长链；相邻链之间以氢键交联，形成抗张强度很高的微纤丝。
Starch and glycogen are good storage molecules because they are insoluble (so they do not affect water potential), compact, and easily hydrolysed to glucose for respiration. Cellulose is a good structural molecule because the hydrogen-bonded microfibrils resist tension, and the chains have alternating orientation so the molecule is straight rather than coiled.
淀粉和糖原是良好的储能分子，因为它们不溶（因此不影响水势）、结构紧凑，并易于水解为葡萄糖用于呼吸。纤维素是良好的结构分子，因为氢键连接的微纤丝能抵抗张力，而且链的走向交替，使分子呈直链而非卷曲。
Benedict's test detects reducing sugars, giving a brick-red precipitate on heating; a non-reducing sugar such as sucrose must first be hydrolysed with dilute hydrochloric acid and then neutralised before testing. Iodine in potassium iodide solution turns blue-black with starch.
本尼迪克特试剂可检测还原糖，加热后产生砖红色沉淀；蔗糖等非还原糖必须先与稀盐酸水解并中和后才能检测。碘-碘化钾溶液遇淀粉变蓝黑色。
4. Lipids: Fats, Oils and Phospholipids | 脂质：脂肪、油与磷脂
Lipids contain carbon, hydrogen and oxygen, but with a much lower proportion of oxygen than carbohydrates. They are insoluble in water but soluble in organic solvents such as ethanol.
脂质含碳、氢、氧，但氧的比例远低于碳水化合物。它们不溶于水，但溶于乙醇等有机溶剂。
A triglyceride forms when one glycerol molecule condenses with three fatty acid molecules, producing three ester bonds and three molecules of water.
一个甘油分子与三个脂肪酸分子缩合，形成三个酯键和三分子水，从而生成甘油三酯。
glycerol + 3 fatty acids → triglyceride + 3H₂O
or more C=C double bonds, kinked chains, lower melting point, liquid at room temperature).
脂肪酸可以是饱和的（没有 C=C 双键，链是直的，熔点较高，室温下为固态），也可以是不饱和的（含一个或多个 C=C 双键，链有折角，熔点较低，室温下为液态）。
- Energy storage | 能量储存: lipids release about twice as much energy per gram as carbohydrates and can be stored without water, so they are light and compact. 每克脂质释放的能量约为碳水化合物的两倍，且储存时不需携带水，因此轻便而紧凑。
- Thermal insulation | 保温: fats beneath the skin reduce heat loss in mammals such as whales and seals. 皮下脂肪减少哺乳动物（如鲸和海豹）的热量散失。
- Protection | 保护: fat surrounds delicate organs such as kidneys and heart. 脂肪包裹肾、心脏等娇嫩器官。
- Waterproofing | 防水: waxy cuticles and oil secretions reduce water loss and keep fur or feathers dry. 蜡质角质层和油脂分泌物减少水分散失，使毛皮或羽毛保持干燥。
Phospholipids have a glycerol backbone with two fatty acid tails and one phosphate group. The phosphate head is hydrophilic while the tails are hydrophobic, making the molecule amphipathic - a property that allows phospholipids to self-assemble into bilayers in water.
磷脂以甘油为骨架，连接两个脂肪酸尾和一个磷酸基团。磷酸头部是亲水的，而尾部是疏水的，因此该分子具有两亲性——这一性质使磷脂能在水中自发组装成双层结构。
Cholesterol is a lipid that fits between phospholipids in animal membranes, reducing fluidity at high temperatures and preventing close packing at low temperatures. It is also the precursor of steroid hormones such as testosterone and oestrogen. The emulsion test, in which ethanol is added and the solution is poured into water, gives a white emulsion if lipid is present.
胆固醇是一种脂质，嵌入动物细胞膜的磷脂之间，高温时降低膜的流动性，低温时防止磷脂紧密堆积。它也是睾酮、雌激素等类固醇激素的前体。乳化实验中加入乙醇后把溶液倒入水中，若存在脂质则出现白色乳浊液。
5. Proteins: The Versatile Polymers | 蛋白质：功能多样的聚合物
Proteins are polymers of amino acids. Each amino acid has a central carbon atom bonded to an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom and a variable R group. The R group determines the properties of that amino acid, such as whether it is polar, charged or hydrophobic.
蛋白质是氨基酸的聚合物。每个氨基酸的中心碳原子连接一个氨基（-NH₂）、一个羧基（-COOH）、一个氢原子和一个可变 R 基团。R 基团决定该氨基酸的性质，例如它是极性、带电还是疏水的。
Amino acids join by condensation between the carboxyl group of one and the amino group of the next, forming a peptide bond and releasing water. Two amino acids form a dipeptide; many form a polypeptide.
氨基酸之间通过一个分子的羧基与下一个分子的氨基缩合，形成肽键并释放水。两个氨基酸形成二肽；许多氨基酸形成多肽。
amino acid + amino acid → dipeptide + H₂O
etermined by DNA. The secondary structure is folding into α-helices and β-pleated sheets, held by hydrogen bonds between C=O and N-H groups in the backbone.
蛋白质的功能取决于其三维形状，这一形状分为四个层次描述。一级结构是氨基酸的序列，由 DNA 决定。二级结构是通过主链上 C=O 与 N-H 之间的氢键折叠成 α-螺旋和 β-折叠。
Tertiary structure is further folding into a precise 3D shape stabilised by hydrogen bonds, ionic bonds, hydrophobic interactions and disulfide bridges between cysteine residues. Quaternary structure arises when two or more polypeptide chains associate, as in haemoglobin, which has four subunits and prosthetic haem groups.
三级结构是进一步折叠成的精确三维形状，由氢键、离子键、疏水相互作用以及半胱氨酸残基之间的二硫键稳定。四级结构则由两条或多条多肽链缔合而成，例如具有四个亚基和血红素辅基的血红蛋白。
| Bond | 键 | Where it acts | 作用位置 |
| Peptide 肽键 | Primary structure, between amino acids | 一级结构，氨基酸之间 |
| Hydrogen 氢键 | Secondary and tertiary structure | 二级和三级结构 |
| Ionic 离子键 | Between charged R groups, tertiary structure | 带电 R 基团之间，三级结构 |
| Disulfide 二硫键 | Strong covalent cross-link, tertiary structure | 强共价交联，三级结构 |
| Hydrophobic interaction 疏水作用 | Pushes non-polar R groups to the interior | 使非极性 R 基团聚集到内部 |
Globular proteins are compact and soluble, with hydrophilic R groups on the outside; examples include enzymes, haemoglobin, antibodies and insulin. Fibrous proteins are long, insoluble and often structural, with repeating sequences that allow many cross-links; examples include collagen in tendons and keratin in hair.
球状蛋白质结构紧凑、可溶，外部带有亲水 R 基团，例如酶、血红蛋白、抗体和胰岛素。纤维状蛋白质长而不溶，多为结构蛋白，重复序列使其能形成大量交联，例如肌腱中的胶原蛋白和毛发中的角蛋白。
Denaturation is the loss of tertiary structure caused by breaking the weak hydrogen and ionic bonds, for example by high temperature or extremes of pH. The primary structure stays intact, but the protein can no longer carry out its function. The Biuret test gives a purple colour when peptide bonds are present.
变性是指氢键和离子键等弱键被破坏而导致三级结构丧失，例如高温或极端 pH 所致。一级结构保持不变，但蛋白质无法再执行其功能。双缩脲试剂遇肽键呈紫色。
6. Nucleic Acids and the Genetic Code | 核酸与遗传密码
Nucleotides are the monomers of nucleic acids. Each nucleotide has a phosphate group, a pentose sugar and a nitrogenous base. In DNA the sugar is deoxyribose and the bases are adenine, thymine, cytosine and guanine; in RNA the sugar is ribose and uracil replaces thymine.
核苷酸是核酸的单体。每个核苷酸含一个磷酸基团、一个戊糖和一个含氮碱基。DNA 中的糖是脱氧核糖，碱基是腺嘌呤、胸腺嘧啶、胞嘧啶和鸟嘌呤；RNA 中的糖是核糖，尿嘧啶取代胸腺嘧啶。
Nucleotides join by condensation between the phosphate of one and the sugar of the next, forming a phosphodiester bond. Two polynucleotide chains run in opposite directions (antiparallel) and are held together by hydrogen bonds between complementary bases: A pairs with T by two hydrogen bonds, and G pairs with C by three hydrogen bonds.
核苷酸之间通过一个核苷酸的磷酸与下一个核苷酸的糖缩合，形成磷酸二酯键。两条多核苷酸链方向相反（反向平行），并通过互补碱基间的氢键相连：A 与 T 以两个氢键配对，G 与 C 以三个氢键配对。
This complementary base pairing explains Chargaff's rule (the amount of A equals T and the amount of G equals C in double-stranded DNA), gives the double helix a constant diameter of about 2 nm, and allows accurate semi-conservative replication.
这种互补碱基配对解释了查加夫法则（双链 DNA 中 A 的量等于 T，G 的量等于 C），使双螺旋直径恒定为约 2 nm，并保证精确的半保留复制。
In replication, DNA helicase breaks the hydrogen bonds to unwind the double helix; each strand acts as a template; free DNA nucleotides are attracted to complementary bases; DNA polymerase catalyses the formation of phosphodiester bonds, joining adjacent nucleotides. Each new molecule contains one original and one new strand - this is semi-conservative replication, demonstrated by the Meselson-Stahl experiment.
在复制过程中，DNA 解旋酶打断氢键使双螺旋解旋；每条链作为模板；游离的 DNA 核苷酸被互补碱基吸引；DNA 聚合酶催化磷酸二酯键的形成，把相邻核苷酸连接起来。每个新分子包含一条原有链和一条新链——这就是半保留复制，由 Meselson-Stahl 实验所证实。
- mRNA | 信使 RNA: carries the complementary copy of a gene from the nucleus to the ribosome. 把基因的互补拷贝从细胞核带到核糖体。
- tRNA | 转运 RNA: clover-leaf shape with an anticodon and an amino acid binding site. 三叶草形，带有反密码子和氨基酸结合位点。
- rRNA | 核糖体 RNA: combines with protein to form ribosomes. 与蛋白质结合形成核糖体。
- ATP | 三磷酸腺苷: a nucleotide derivative (adenine + ribose + three phosphates) that releases energy when the terminal phosphate is hydrolysed. 一种核苷酸衍生物（腺嘌呤 + 核糖 + 三个磷酸），末端磷酸水解时释放能量。
7. Enzymes: Biological Catalysts | 酶：生物催化剂
Enzymes are globular proteins that act as biological catalysts by lowering the activation energy of a reaction. They have a specific active site whose shape is complementary to a particular substrate, so each enzyme typically catalyses only one reaction.
酶是球状蛋白质，通过降低反应的活化能充当生物催化剂。酶具有特定的活性位点，其形状与特定底物互补，因此每种酶通常只催化一种反应。
The lock-and-key model treats the active site as a rigid shape that fits the substrate exactly. The induced-fit model is more accurate: the active site is flexible, and its shape changes slightly as the substrate binds, straining the substrate bonds and lowering the activation energy further.
锁钥模型把活性位点视为与底物完全吻合的刚性形状。诱导契合模型更为准确：活性位点具有柔性，底物结合时其形状略微改变，使底物中的键受到张力，从而进一步降低活化能。
enzyme + substrate ⇌ enzyme-substrate complex → enzyme + product
Rate of reaction increases with temperature up to the optimum because molecules have more kinetic energy and more successful collisions occur. Above the optimum, the weak hydrogen and ionic bonds break, the tertiary structure is lost, and the active site no longer fits the substrate - the enzyme is denatured. Extremes of pH also cause denaturation, while each enzyme has an optimum pH: pepsin about pH 2, salivary amylase about pH 7.
在达到最适温度之前，反应速率随温度上升而增加，因为分子动能更大，成功碰撞更多。超过最适温度后，弱氢键和离子键断裂，三级结构丧失，活性位点不再与底物契合——酶发生变性。极端 pH 同样导致变性，而每种酶都有最适 pH：胃蛋白酶约为 pH 2，唾液淀粉酶约为 pH 7。
Competitive inhibitors bind to the active site and compete with the substrate, so their effect can be reduced by increasing substrate concentration. Non-competitive inhibitors bind elsewhere on the enzyme, changing the shape of the active site; increasing substrate concentration does not overcome the inhibition.
竞争性抑制剂结合于活性位点并与底物竞争，因此增大底物浓度可减弱其作用。非竞争性抑制剂结合在酶的其他部位，改变活性位点的形状；增大底物浓度无法解除抑制。
Enzymes need cofactors and coenzymes. Cofactors are inorganic ions such as Cl⁻ for amylase or Zn²⁺ for carbonic anhydrase; coenzymes are organic molecules such as NAD, FAD and coenzyme A that carry hydrogen atoms or groups between reactions.
酶需要辅助因子与辅酶。辅助因子是无机离子，如淀粉酶所需的 Cl⁻、碳酸酐酶所需的 Zn²⁺；辅酶是有机分子，如 NAD、FAD 和辅酶 A，在反应间传递氢原子或基团。
8. Organelles: Compartmentalising Life | 细胞器：生命的分区化
Eukaryotic cells contain membrane-bound organelles that divide the cell into compartments, allowing incompatible reactions to occur at the same time and increasing the efficiency of metabolism.
真核细胞含有被膜包裹的细胞器，把细胞分隔成不同区域，使不相容的反应可同时进行，并提高代谢效率。
- Nucleus | 细胞核: contains DNA as chromatin, site of transcription; the nuclear envelope is double with pores; the nucleolus makes ribosomes. 以染色质形式储存 DNA，是转录场所；核膜为双层且有核孔；核仁合成核糖体。
- Mitochondrion | 线粒体: double membrane, folded inner membrane (cristae) carrying the electron transport chain; matrix contains enzymes of the Krebs cycle. 双层膜，内膜折叠成嵴并携带电子传递链；基质含克雷布斯循环的酶。
- Chloroplast | 叶绿体: double membrane, thylakoids stacked as grana where light absorption occurs, stroma where the light-independent stage occurs. 双层膜，类囊体堆叠成基粒进行光吸收，基质中进行光独立阶段反应。
- Rough endoplasmic reticulum | 粗糙内质网: ribosomes on the surface synthesise proteins that are folded and transported. 表面附有核糖体，合成并折叠、运输蛋白质。
- Smooth endoplasmic reticulum | 光滑内质网: lipid and steroid synthesis, detoxification. 合成脂质和类固醇，解毒。
- Golgi apparatus | 高尔基体: modifies proteins by adding carbohydrate, packages them into vesicles for secretion. 通过加糖基修饰蛋白质，将其包装成囊泡分泌。
- Lysosome | 溶酶体: contains hydrolytic enzymes to digest worn organelles, foreign material and cells. 含水解酶，消化衰老细胞器、异物和细胞。
- Ribosome | 核糖体: site of translation, made of rRNA and protein, not membrane-bound. 翻译场所，由 rRNA 和蛋白质组成，无膜结构。
| Feature | 特征 | Prokaryote | 原核生物 | Eukaryote | 真核生物 |
| Nucleus 细胞核 | Absent; circular DNA 无；环状 DNA | Present 有 |
| Ribosomes 核糖体 | 70S 型 | 80S 型，多数附着于内质网 |
| Membrane organelles 膜细胞器 | Absent 无 | Present 有 |
| Cell wall 细胞壁 | Peptidoglycan (murein) 肽聚糖 |
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