The Building Blocks of Life | 生命的构建块

📚 The Building Blocks of Life | 生命的构建块

From the simplest bacterium to the complexity of the human brain, all living organisms are constructed from the same fundamental chemical components. These building blocks – atoms, molecules, macromolecules and cellular structures – form a hierarchy that explains both the unity and the astonishing diversity of life. In Cambridge A‑level Biology, a firm grasp of these molecular foundations is essential, as they connect topics ranging from biochemistry to physiology and genetics.

从最简单的细菌到复杂的人脑，所有生物体都由相同的基本化学成分构建而成。这些构建块——原子、分子、大分子和细胞结构——形成了一个层次体系，既解释了生命的统一性，也揭示了惊人的多样性。在剑桥A‑level生物学中，牢固掌握这些分子基础至关重要，因为它们将生物化学、生理学和遗传学等主题联系在一起。

1. The Chemical Alphabet of Life | 生命的化学字母表

Living matter is built from about 25 chemical elements, but six of them dominate: carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P) and sulfur (S). These CHONPS elements make up over 98% of the mass of most organisms. Trace elements such as iron (Fe), magnesium (Mg), calcium (Ca) and iodine (I) are required in tiny amounts but are still vital for specific functions.

生命物质由大约25种化学元素构成，但其中六种占主导：碳（C）、氢（H）、氧（O）、氮（N）、磷（P）和硫（S）。这些CHONPS元素占大多数生物体质量的98%以上。微量元素如铁（Fe）、镁（Mg）、钙（Ca）和碘（I）虽然需要量极少，但对于特定功能仍然至关重要。

Carbon’s unique ability to form four covalent bonds allows it to create an almost infinite variety of stable, complex skeletons. It can bond with itself to form chains, branched structures and rings, giving rise to the immense diversity of organic molecules.

碳独特地能够形成四个共价键，从而创造出几乎无限种类的稳定复杂骨架。它可以与自己键合形成链、支链和环，从而产生有机分子的巨大多样性。

Element	元素	Biological role	生物学作用
Carbon (C)	碳	Backbone of all organic molecules	所有有机分子的骨架
Hydrogen (H)	氢	Component of water and organic compounds; in H⁺ transport (respiration, photosynthesis)	水和有机化合物的组成；参与H⁺运输（呼吸作用、光合作用）
Oxygen (O)	氧	Required for aerobic respiration; component of water and almost all biomolecules	有氧呼吸所必需；水及几乎所有生物分子的组成
Nitrogen (N)	氮	Key part of amino acids, proteins and nucleic acids	氨基酸、蛋白质和核酸的关键组分
Phosphorus (P)	磷	Found in ATP, nucleic acids and phospholipids	存在于ATP、核酸和磷脂中
Sulfur (S)	硫	Present in some amino acids (cysteine, methionine); forms disulfide bridges in proteins	存在于某些氨基酸（半胱氨酸、蛋氨酸）中；在蛋白质中形成二硫键

2. Water – The Matrix of Life | 水——生命介质

Water is the most abundant molecule in living organisms, typically making up 70–95% of cell mass. Its special properties arise from its polarity and capacity to form hydrogen bonds. Each water molecule (H₂O) can form up to four hydrogen bonds with neighbours, giving it cohesion, adhesion and remarkable thermal properties.

水是生物体中最丰富的分子，通常占细胞质量的70–95%。其特殊性质源于其极性和形成氢键的能力。每个水分子（H₂O）可与相邻分子形成多达四个氢键，使其具有内聚力、附着力以及显著的热学性质。

As a universal solvent, water dissolves ionic compounds and polar molecules such as sugars, amino acids and many proteins. This allows the transport of nutrients and wastes in blood, xylem and phloem, and facilitates metabolic reactions in the cytoplasm.

作为通用溶剂，水溶解离子化合物和极性分子，如糖类、氨基酸和许多蛋白质。这使得营养物质和废物能在血液、木质部和韧皮部中运输，并促进细胞质中的代谢反应。

Water’s high specific heat capacity (4.18 J g⁻¹ °C⁻¹) means it resists rapid temperature changes, providing thermal stability for aquatic and terrestrial organisms. Its high latent heat of vaporisation enables effective cooling through sweating and transpiration.

水的高比热容（4.18 J g⁻¹ °C⁻¹）意味着它能抵抗快速的温度变化，为水生和陆生生物提供热稳定性。其高蒸发潜热使通过出汗和蒸腾作用实现有效降温成为可能。

The fact that ice is less dense than liquid water insulates bodies of water, preventing them from freezing solid and protecting aquatic life. Cohesion and adhesion together create capillary action, which is critical for water movement in plants.

冰的密度小于液态水这一事实隔离了水体，防止其完全冻结，保护了水生生物。内聚力和附着力共同形成毛细管作用，这对植物中水的运动至关重要。

3. Carbohydrates – Fuels and Building Materials | 碳水化合物——燃料与建筑材料

Carbohydrates are organic compounds composed of carbon, hydrogen and oxygen, usually in the ratio (CH₂O)ₙ. They serve as immediate energy sources, short-term energy stores and structural components. The simplest units are monosaccharides, such as glucose (C₆H₁₂O₆), fructose and galactose.

碳水化合物是由碳、氢和氧组成的有机化合物，通常比例为(CH₂O)ₙ。它们作为即时能源、短期能量储存和结构组分。最简单的单位是单糖，如葡萄糖（C₆H₁₂O₆）、果糖和半乳糖。

Two monosaccharides join by a condensation reaction forming a glycosidic bond, producing a disaccharide and a water molecule. Examples include maltose (glucose + glucose), sucrose (glucose + fructose) and lactose (glucose + galactose). Hydrolysis reverses this process using water to break the bond.

两个单糖通过缩合反应连接形成糖苷键，生成一个双糖和一个水分子。例子包括麦芽糖（葡萄糖+葡萄糖）、蔗糖（葡萄糖+果糖）和乳糖（葡萄糖+半乳糖）。水解利用水逆转这一过程，打破该键。

Polysaccharides are long chains of monosaccharide units. Starch (amylose and amylopectin) is the main storage carbohydrate in plants; amylose is a linear chain of α-1,4 linked glucose that coils into a helix, while amylopectin has α-1,4 chains with α-1,6 branches. Glycogen is the animal equivalent, with even more extensive branching. Both are compact, insoluble and easily hydrolysed to release glucose for respiration.

多糖是单糖单元的长链。淀粉（直链淀粉和支链淀粉）是植物中主要的储存碳水化合物；直链淀粉是α-1,4糖苷键连接的直链葡萄糖，卷曲成螺旋，而支链淀粉具有α-1,4链和α-1,6分支。糖原是动物的对应物，分支更为广泛。两者都紧凑、不溶且易于水解，释放葡萄糖用于呼吸作用。

Cellulose, the main structural polysaccharide in plant cell walls, consists of β-1,4 linked glucose chains. The alternating orientation of glucose units allows parallel chains to form hydrogen‑bonded microfibrils, providing immense tensile strength. Humans lack the enzyme cellulase and cannot digest cellulose, but it serves as dietary fibre.

纤维素是植物细胞壁中的主要结构多糖，由β-1,4糖苷键连接的葡萄糖链组成。葡萄糖单元的交替取向使平行链能够形成氢键连接的微纤维，提供巨大的抗拉强度。人类缺乏纤维素酶，无法消化纤维素，但它可作为膳食纤维。

4. Lipids – Diverse Hydrophobic Molecules | 脂类——多样的疏水分子

Lipids are a chemically varied group of molecules united by their hydrophobic nature. They are largely non-polar and insoluble in water but dissolve in organic solvents. The most common types are triglycerides, phospholipids and steroids.

脂类是一类化学性质各异、因疏水性而被归为一类的分子。它们大多非极性，不溶于水但溶于有机溶剂。最常见的类型是甘油三酯、磷脂和类固醇。

A triglyceride forms when a glycerol molecule combines with three fatty acid molecules via ester bonds in condensation reactions. Fatty acids can be saturated (no C=C double bonds, solid at room temperature, e.g. animal fats) or unsaturated (one or more C=C double bonds, liquid at room temperature, e.g. vegetable oils). Triglycerides serve as long-term energy storage, thermal insulation and protection of organs.

一个甘油三酯是由一个甘油分子与三个脂肪酸分子通过缩合反应以酯键结合而成。脂肪酸可以是饱和的（无C=C双键，室温下为固体，如动物脂肪）或不饱和的（含一个或多个C=C双键，室温下为液体，如植物油）。甘油三酯用于长期能量储存、隔热和保护器官。

Phospholipids are similar to triglycerides but one fatty acid is replaced by a phosphate-containing group. This gives the molecule a hydrophilic ‘head’ and two hydrophobic fatty acid ‘tails’, making it amphipathic. In water, phospholipids spontaneously form a bilayer, the foundation of all biological membranes.

磷脂与甘油三酯相似，但一条脂肪酸被含磷酸基团所取代。这使得分子具有一个亲水“头部”和两条疏水脂肪酸“尾部”，呈两亲性。在水中，磷脂自发形成双层，这是所有生物膜的基础。

Steroids, such as cholesterol, have a characteristic four‑ring structure. Cholesterol is an essential component of animal cell membranes, modulating fluidity, and serves as a precursor for steroid hormones like testosterone and oestrogen.

类固醇，如胆固醇，具有特征性的四环结构。胆固醇是动物细胞膜的必要组分，调节流动性，并且是睾酮和雌激素等类固醇激素的前体。

5. Proteins – The Workhorses of the Cell | 蛋白质——细胞的功勋

Proteins are polypeptides made from amino acid monomers. 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 (side chain). There are 20 standard amino acids that differ only in their R groups, providing properties ranging from polar to non‑polar, acidic to basic.

蛋白质是由氨基酸单体构成的多肽。每个氨基酸都有一个中心碳原子，分别与一个氨基（–NH₂）、一个羧基（–COOH）、一个氢原子和一个可变的R基（侧链）相连。共有20种标准氨基酸，仅R基不同，提供从极性到非极性、酸性到碱性的各种性质。

Amino acids link by peptide bonds formed in condensation reactions. The sequence of amino acids in a polypeptide chain is the primary structure. This chain then folds into regular local conformations – the secondary structure – mainly α‑helices and β‑pleated sheets, stabilised by hydrogen bonds between the –C=O and –N–H groups of the backbone.

氨基酸通过缩合反应形成的肽键连接。多肽链中氨基酸的序列是一级结构。随后该链折叠成规则的局部构象——二级结构——主要为α-螺旋和β-折叠片，由主链–C=O和–N–H基团之间的氢键所稳定。

The overall three‑dimensional shape of a single polypeptide, the tertiary structure, is maintained by interactions between R groups: hydrogen bonds, ionic bonds, hydrophobic interactions and disulfide bridges (–S–S–). Some proteins consist of more than one polypeptide subunit; this arrangement is called quaternary structure, as seen in haemoglobin (four subunits).

单条多肽的整体三维形状，即三级结构，由R基团之间的相互作用维持：氢键、离子键、疏水相互作用和二硫键（–S–S–）。有些蛋白质由多个多肽亚基组成，这种排列称为四级结构，如血红蛋白（四个亚基）。

Proteins carry out an astonishing range of functions: enzymes (e.g. DNA polymerase), transport (haemoglobin), structural support (collagen, keratin), movement (actin, myosin), defence (antibodies), signalling (hormones, receptors) and storage (ferritin). The specific shape of a protein determines its function; denaturation by heat or pH permanently alters this shape and renders the protein inactive.

蛋白质执行着惊人的多种功能：酶（如DNA聚合酶）、运输（血红蛋白）、结构支撑（胶原蛋白、角蛋白）、运动（肌动蛋白、肌球蛋白）、防御（抗体）、信号传导（激素、受体）和储存（铁蛋白）。蛋白质的特定形状决定其功能；热或pH引起的变性会永久改变这种形状并使其失活。

6. Nucleic Acids – The Information Molecules | 核酸——信息分子

Nucleic acids, DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), are polymers of nucleotides. Each nucleotide consists of a pentose sugar (deoxyribose in DNA, ribose in RNA), a phosphate group and a nitrogenous base. In DNA the bases are adenine (A), thymine (T), cytosine (C) and guanine (G); in RNA, uracil (U) replaces thymine.

核酸，DNA（脱氧核糖核酸）和RNA（核糖核酸），是核苷酸的聚合物。每个核苷酸由五碳糖（DNA中为脱氧核糖，RNA中为核糖）、一个磷酸基团和一个含氮碱基组成。DNA中的碱基是腺嘌呤（A）、胸腺嘧啶（T）、胞嘧啶（C）和鸟嘌呤（G）；在RNA中，尿嘧啶（U）取代胸腺嘧啶。

DNA molecules typically form a double helix, first described by Watson and Crick. Two antiparallel polynucleotide strands are held together by hydrogen bonds between complementary base pairs: A pairs with T via two hydrogen bonds, and C pairs with G via three hydrogen bonds. This specific pairing enables faithful replication and the storage of genetic information in the sequence of bases.

DNA分子通常形成双螺旋，由沃森和克里克首次描述。两条反向平行的多核苷酸链通过互补碱基对之间的氢键连接：A与T通过两个氢键配对，C与G通过三个氢键配对。这种特异性配对确保了准确复制，并将遗传信息存储在碱基序列中。

RNA is usually single‑stranded and has several forms with distinct roles: messenger RNA (mRNA) carries genetic information from DNA to ribosomes; transfer RNA (tRNA) brings amino acids to the ribosome during translation; and ribosomal RNA (rRNA) forms part of the ribosome structure. RNA’s extra 2′‑OH group makes it more chemically reactive and less stable than DNA.

RNA通常是单链的，并且有几种不同形式，具有不同的角色：信使RNA（mRNA）将遗传信息从DNA携带到核糖体；转运RNA（tRNA）在翻译过程中将氨基酸带到核糖体；核糖体RNA（rRNA）构成核糖体结构的一部分。RNA额有的2′‑OH基团使其化学性质更活泼，稳定性不如DNA。

7. Enzymes – Biological Catalysts | 酶——生物催化剂

Enzymes are globular proteins (and occasionally RNA) that act as biological catalysts, dramatically speeding up biochemical reactions without being permanently changed. They achieve this by lowering the activation energy, providing an alternative reaction pathway.

酶是球状蛋白质（少数为RNA），充当生物催化剂，大幅加速生化反应而自身不发生永久性变化。它们通过降低活化能、提供替代反应途径来实现这一点。

The region of an enzyme where the substrate binds is the active site, a pocket or cleft with a shape and chemical properties complementary to the substrate. The lock‑and‑key model suggests a rigid perfect fit, while the induced‑fit model proposes that the active site changes shape slightly to mould around the substrate, stressing bonds and facilitating the transition state.

酶上与底物结合的区域是活性位点，这是一个形状和化学性质与底物互补的口袋或裂隙。锁钥模型认为是刚性的完美契合，而诱导契合模型则认为活性位点会轻微改变形状以包裹底物，压迫化学键，促进过渡态的形成。

Enzyme activity is influenced by several factors. Temperature: as temperature rises, kinetic energy increases, raising the frequency of collisions and thus the rate, up to an optimum; beyond this, thermal denaturation unfolds the protein, causing a rapid decline. pH: each enzyme has an optimal pH; extreme pH values alter the ionisation of R groups in the active site and can disrupt ionic bonds, denaturing the enzyme.

酶活性受若干因素影响。温度：随着温度升高，动能增加，碰撞频率提高，从而速率上升，直到最适温度；超过此温度，热变性使蛋白质展开，导致速率急剧下降。pH：每种酶都有最适pH；极端的pH值改变活性位点中R基团的电离状态，并可能破坏离子键，使酶变性。

Substrate concentration: at low substrate concentrations, the rate increases almost linearly because many active sites are vacant; at higher concentrations, the rate plateaus as all active sites become saturated, so the maximum velocity (Vₘₐₓ) is reached. Inhibitors reduce enzyme activity: competitive inhibitors resemble the substrate and occupy the active site reversibly, while non‑competitive inhibitors bind elsewhere on the enzyme, altering its shape.

底物浓度：在低底物浓度下，由于许多活性位点空置，速率几乎呈线性增加；在高浓度下，速率趋于平稳，因为所有活性位点都被饱和，达到最大速度（Vₘₐₓ）。抑制剂降低酶活性：竞争性抑制剂与底物相似，可逆地占据活性位点；而非竞争性抑制剂结合在酶的其他部位，改变其形状。

8. The Cell – The Basic Unit of Life | 细胞——生命的基本单位

Cell theory states that all living things are composed of one or more cells, the cell is the basic structural and functional unit of life, and all cells arise from pre‑existing cells. This unifying principle applies to both prokaryotes and eukaryotes.

细胞学说指出，所有生物都由一个或多个细胞组成，细胞是生命的基本结构和功能单位，并且所有细胞都源自已有的细胞。这一统一原则适用于原核生物和真核生物。

Prokaryotic cells (bacteria and archaea) are small (0.5–5 µm), lack a membrane‑bound nucleus, and have a circular chromosome concentrated in a nucleoid region. They contain 70S ribosomes, a plasma membrane, a cell wall made of peptidoglycan, and sometimes a capsule, flagella and plasmids. Organelles such as mitochondria and endoplasmic reticulum are absent.

原核细胞（细菌和古菌）较小（0.5–5 µm），没有膜包被的细胞核，在拟核区域有一条环状染色体。它们含有70S核糖体、质膜、由肽聚糖构成的细胞壁，有时还有荚膜、鞭毛和质粒。不存在线粒体和内质网等细胞器。

Eukaryotic cells (animals, plants, fungi, protists) are larger (10–100 µm) and contain a true nucleus surrounded by a nuclear envelope. They possess numerous membrane‑bound organelles, each performing specialised functions. Key organelles include mitochondria (site of aerobic respiration), chloroplasts (site of photosynthesis, found only in plants and algae), rough and smooth endoplasmic reticulum (protein synthesis and lipid synthesis respectively), Golgi apparatus (modification, packaging and secretion), lysosomes (digestion), and ribosomes (80S, protein synthesis).

真核细胞（动物、植物、真菌、原生生物）较大（10–100 µm），含有由核膜包裹的真正细胞核。它们拥有许多有膜细胞器，各司其职。关键细胞器包括线粒体（有氧呼吸的场所）、叶绿体（光合作用的场所，仅存在于植物和藻类中）、粗面和滑面内质网（分别负责蛋白质和脂质的合成）、高尔基体（修饰、包装和分泌）、溶酶体（消化）以及核糖体（80S，蛋白质合成）。

Plant cells have additional features: a cellulose cell wall, a large permanent vacuole (storage, turgor pressure) and often chloroplasts. Animal cells lack a cell wall but possess centrioles that organise the spindle during cell division.

植物细胞具有额外的特征：纤维素细胞壁、一个大的中央液泡（储存、膨压）以及常有的叶绿体。动物细胞缺乏细胞壁，但拥有中心粒，在细胞分裂时组织纺锤体。

9. Biological Membranes – Boundaries and Gateways | 生物膜——边界与通道

All biological membranes are based on the fluid mosaic model: a phospholipid bilayer provides the basic structure, while various proteins float within or attach to this bilayer, rather like icebergs in a sea. The membrane is fluid – its components can move laterally – and the mosaic refers to the patchwork of proteins, glycoproteins and glycolipids.

所有生物膜都基于流动镶嵌模型：磷脂双分子层提供基本结构，而各种蛋白质漂浮在或附着在这双分子层中，就像大海中的冰山。膜是流动的——其组分可横向移动——而“镶嵌”则是指蛋白质、糖蛋白和糖脂构成的拼凑图案。

Integral proteins span the hydrophobic core, often acting as channels, carriers or pumps for the transport of specific substances. Peripheral proteins are loosely attached to the membrane surface and can function as enzymes or in signalling. Cholesterol, found in animal cell membranes, reduces excessive fluidity at high temperatures and maintains fluidity at low temperatures, thus stabilising the membrane.

整合蛋白横跨疏水核心，常作为通道、载体或泵，运输特定物质。周边蛋白松散地附着在膜表面，可作为酶或参与信号传导。胆固醇存在于动物细胞膜中，在高温下降低过度流动性，在低温下维持流动性，从而稳定膜。

Transport across membranes occurs through passive or active processes. Simple diffusion is the net movement of small, non‑polar molecules (O₂, CO₂) down a concentration gradient through the bilayer. Osmosis is the net movement of water from a region of higher water potential to a region of lower water potential through a partially permeable membrane. Facilitated diffusion uses channel proteins (e.g. aquaporins for water) or carrier proteins (e.g. glucose transporter) to move molecules down their concentration gradient without energy.

跨膜运输通过被动或主动过程发生。简单扩散是小分子非极性物质（O₂， CO₂）顺浓度梯度透过双分子层的净移动。渗透是水通过部分透膜从水势较高的区域向水势较低的区域的净移动。易化扩散利用通道蛋白（如水孔蛋白运输水）或载体蛋白（如葡萄糖转运蛋白）顺浓度梯度移动分子，无需能量。

Active transport requires energy, usually from ATP hydrolysis, to move substances against their concentration gradient. The sodium–potassium pump (Na⁺/K⁺‑ATPase) is a classical example, expelling three Na⁺ ions out of the cell and bringing two K⁺ ions in for each ATP split. Bulk transport includes endocytosis (taking in material) and exocytosis (secreting materials), processes that involve the membrane invaginating or fusing with vesicles.

主动运输需要能量，通常来自ATP水解，以逆浓度梯度移动物质。钠-钾泵（Na⁺/K⁺‑ATP酶）是一个经典例子，每分解一个ATP，将三个Na⁺离子泵出细胞，并将两个K⁺离子泵入细胞内。批量运输包括胞吞（摄入物质）和胞吐（分泌物质），这些过程涉及膜内陷或与囊泡融合。

10. From Simple Molecules to Polymers – The Emergence of Life | 从简单分子到聚合物——生命的起源

Although the origin of life remains a deep scientific question, experimental evidence and theoretical models suggest that the building blocks of life could have arisen on the early Earth. The Miller–Urey experiment (1953) simulated the reducing atmosphere thought to exist on primitive Earth (containing H₂O, CH₄, NH₃ and H₂) and applied electric discharges to mimic lightning. After a week, a range of organic molecules had formed, including several amino acids – demonstrating that key monomers can be synthesised abiotically.

尽管生命的起源仍是一个深刻的科学问题，但实验证据和理论模型表明，生命的构建块可能在早期地球上出现。米勒-尤里实验（1953年）模拟了原始地球被认为存在的还原性大气（含H₂O、CH₄、NH₃和H₂），并施加电火花模拟闪电。一周后，一系列有机分子形成，包括多种氨基酸——证明了关键单体可以在非生物条件下合成。

Further steps would have involved polymerisation of monomers into larger macromolecules, possibly on mineral surfaces (clay) that could concentrate and catalyse the formation of polypeptides and polynucleotides. The assembly of such polymers into membrane‑bound proto‑cells allowed the development of primitive metabolism and replication, setting the stage for the evolution of the first true cells.

进一步的步骤包括单体聚合为更大的大分子，可能发生在矿物表面（粘土）上，该表面可以浓缩并催化多肽和多核苷酸的形成。这些聚合物组装成有膜的原细胞，允许原始代谢和复制的发生，为第一批真正细胞的进化奠定了基础。

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The Building Blocks of Life | 生命的构建块