📚 Biology: The Science of Life | 生物学：探索生命的科学
Biology is the scientific study of living organisms and the processes that keep them alive. It ranges from the molecules inside a single cell to the global interactions of whole ecosystems, and it helps us understand how life began, how it evolved, and how it works today.
生物学是探索生命世界的科学，研究对象涵盖从微生物到大型动植物的各个层次。学习生物学，能帮助我们更透彻地理解生命的起源、演化与运作机制。
The subject is usually divided into several branches, such as molecular biology, cell biology, genetics, ecology and evolutionary biology. Each branch has its own perspective, its own core questions, and its own experimental methods, yet all of them connect back to the same central idea: living systems store information, use energy, maintain stability and change over time.
生物学可以划分为多个分支学科，如分子生物学、细胞生物学、遗传学、生态学和进化生物学等，每个领域都有独特的研究视角和核心问题。它们最终都指向同一个核心：生命系统储存信息、利用能量、维持稳定，并在时间中不断变化。
1. What Is Biology? | 什么是生物学
Biology is the branch of science that investigates living things and their interactions with each other and with the non-living environment. A useful way to approach it is through the levels of organisation, because each level has properties that cannot be predicted from the level below it.
生物学是研究生物体及其彼此之间、以及与无机环境之间相互作用的科学分支。理解生物学的一个有效方式是掌握"组织层次"，因为每一个层次都会出现下一层次无法直接预测的新特性。
The traditional sequence runs from molecules to the biosphere. Examiners frequently ask you to place a named example at the correct level, so learn the order and be able to give an example for each step.
传统的层次顺序从分子一直到生物圈。考试中经常要求你把某个具体实例放到正确的层次上，因此要记住顺序，并能给每一层举出例子。
- Molecule 分子 → Organelle 细胞器 → Cell 细胞 → Tissue 组织 → Organ 器官 → Organ system 器官系统 → Organism 个体
- Population 种群 → Community 群落 → Ecosystem 生态系统 → Biome 生物群系 → Biosphere 生物圈
Several unifying themes hold the whole subject together: cell theory, the gene theory of inheritance, homeostasis, the flow of energy through living systems, and evolution by natural selection. When a question looks unfamiliar, ask which of these themes it belongs to.
有几条统一主题把整门学科串联起来：细胞学说、基因遗传理论、稳态、生命系统中的能量流动，以及自然选择驱动的进化。遇到陌生题目时，先判断它属于哪条主题。
2. The Main Branches of Biology | 生物学的主要分支学科
Biology is far too large for one approach, so it is split into overlapping branches. The boundaries are not rigid: a geneticist may use molecular techniques, and an ecologist may use genetic data to track populations.
生物学体量庞大，无法用单一方法研究，因此被划分为彼此交叉的分支。这些边界并不僵硬：遗传学家会使用分子技术，生态学家也会用遗传数据追踪种群。
| Branch 分支 | Focus 研究对象 | Typical question 典型问题 |
|---|---|---|
| Molecular biology 分子生物学 | DNA, RNA, proteins 生物大分子 | How is a gene expressed? 基因如何表达? |
| Cell biology 细胞生物学 | Cell structure and function 细胞结构与功能 | How do organelles divide labour? 细胞器如何分工? |
| Genetics 遗传学 | Inheritance and variation 遗传与变异 | Why do offspring resemble parents? 子代为何像亲代? |
| Ecology 生态学 | Organisms and environment 生物与环境 | What controls population size? 什么控制种群数量? |
| Evolutionary biology 进化生物学 | Change and diversity of life 生命的变化与多样性 | How do new species arise? 新物种如何形成? |
| Physiology 生理学 | How bodies work 生物体如何运作 | How is blood glucose regulated? 血糖如何调节? |
| Microbiology 微生物学 | Bacteria, viruses, fungi 微生物 | How does immunity work? 免疫如何发挥作用? |
| Biotechnology 生物技术 | Using living systems 利用生命系统 | How is human insulin produced? 人胰岛素如何生产? |
Other well-known divisions include biochemistry, botany, zoology, marine biology, bioinformatics and systems biology. Knowing which branch a question comes from is often the fastest route to the right mark scheme.
其他常见分支还包括生物化学、植物学、动物学、海洋生物学、生物信息学和系统生物学。判断题目属于哪个分支，往往是最快找到正确答案的路径。
3. Molecular Biology: The Molecules of Life | 分子生物学：生命的分子基础
Molecular biology examines the molecules that carry out and control life. It focuses on the structure, function and interactions of biological macromolecules, especially the nucleic acids DNA and RNA and the proteins they encode.
分子生物学着眼于生命的基础分子，深入解析 DNA、RNA、蛋白质等生物大分子的结构、功能及相互作用。这类研究使我们更清晰地把握细胞的工作机制和生物体的发育过程。
DNA is a double helix of two antiparallel strands held together by hydrogen bonds between complementary bases. Adenine pairs with thymine through two hydrogen bonds, and guanine pairs with cytosine through three. In RNA, uracil replaces thymine and the sugar is ribose rather than deoxyribose.
DNA 是由两条反向平行的链构成的双螺旋，链间由互补碱基之间的氢键连接。腺嘌呤与胸腺嘧啶以两个氢键配对，鸟嘌呤与胞嘧啶以三个氢键配对。在 RNA 中，尿嘧啶取代胸腺嘧啶，糖为核糖而非脱氧核糖。
The flow of information in a cell is summarised by the central dogma. Transcription copies a gene into messenger RNA, and translation reads that message on a ribosome to build a polypeptide.
细胞内的信息流向可用中心法则概括：转录把基因拷贝成信使 RNA，翻译则在核糖体上读取这一信息，组装出多肽链。
DNA → mRNA → Polypeptide (Transcription → Translation)
Protein shape determines protein function. The primary, secondary, tertiary and quaternary levels of structure all matter, and a single change in the amino acid sequence can alter an active site enough to destroy enzyme activity.
蛋白质的形状决定其功能。一级、二级、三级与四级结构都很关键，氨基酸序列的单一改变就可能改变活性位点，使酶活性完全丧失。
4. Cell Biology: The Basic Unit of Life | 细胞生物学：生命的基本单位
Cell biology focuses on the cell, the basic structural and functional unit of every organism. Studying cell structure, function and how cells are organised into tissues lets us see how life operates and how cells coordinate with one another.
细胞生物学专注生命的基本单位——细胞，它是生物体的结构与功能基础。围绕细胞的结构、功能及组织方式展开探索，能让我们洞悉生命的运转规律和细胞间的协调关系。
Cell theory states that all living organisms are made of cells, that the cell is the smallest unit of life, and that new cells arise from existing cells by division. The two great categories are prokaryotic and eukaryotic cells.
细胞学说指出：所有生物都由细胞构成，细胞是生命的最小单位，新细胞由已有细胞分裂产生。细胞分为两大类别：原核细胞与真核细胞。
| Feature 特征 | Prokaryotic cell 原核细胞 | Eukaryotic cell 真核细胞 |
|---|---|---|
| Nucleus 细胞核 | Absent; circular DNA free in cytoplasm 无，环状 DNA 游离于细胞质 | Present, DNA enclosed by a nuclear envelope 有，DNA 由核膜包围 |
| Size 大小 | Typically 0.5-5 μm | Typically 10-100 μm |
| Organelles 细胞器 | No membrane-bound organelles 无膜性细胞器 | Mitochondria, ER, Golgi, lysosomes etc. 有线粒体、内质网、高尔基体、溶酶体等 |
| Cell wall 细胞壁 | Peptidoglycan (murein) 肽聚糖 | Cellulose in plants, chitin in fungi 植物为纤维素，真菌为几丁质 |
| Ribosomes 核糖体 | 70S | 80S |
The plasma membrane is described by the fluid mosaic model: a phospholipid bilayer with embedded proteins, cholesterol and glycolipids. Substances cross it by diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis.
质膜可用流动镶嵌模型描述：磷脂双分子层中镶嵌着蛋白质、胆固醇和糖脂。物质通过扩散、易化扩散、渗透、主动运输、胞吞和胞吐等方式跨膜。
Cell division matters too. Mitosis produces genetically identical diploid cells for growth and repair, while meiosis halves the chromosome number and generates genetic variation through crossing over and independent assortment.
细胞分裂同样重要。有丝分裂产生遗传上相同的二倍体细胞，用于生长与修复；减数分裂使染色体数目减半，并通过交叉互换和独立分配产生遗传变异。
5. Genetics: Inheritance and Information | 遗传学：遗传与信息传递
Genetics explores genes and the transmission of genetic information. It covers the structure and function of genetic material such as DNA and RNA, and the replication, expression and mutation of that information.
遗传学探究基因和遗传信息的传递，涉及 DNA、RNA 等遗传物质的结构与功能，以及遗传信息的复制、表达和变异。通过遗传学，我们得以揭开性状遗传的规律和变异发生的分子机理。
Genes exist in alternative forms called alleles. An organism's genotype is the combination of alleles it carries; its phenotype is the observable characteristic that results from the genotype interacting with the environment.
基因存在不同的等位形式，称为等位基因。生物体携带的等位基因组合叫基因型；基因型与环境相互作用后表现出的可观察特征叫表现型。
ss between two double heterozygotes (AaBb × AaBb) gives 9 : 3 : 3 : 1 when the genes are unlinked.
两个杂合子杂交（Aa × Aa）的单基因杂交中，子代表现型预期为 3 显性 : 1 隐性。若两个基因不连锁，双杂合子杂交（AaBb × AaBb）则得到 9 : 3 : 3 : 1。
Monohybrid F₂ ratio: 3 : 1 ｜ Dihybrid F₂ ratio: 9 : 3 : 3 : 1
Mutation is the ultimate source of new alleles. Substitution, insertion and deletion of bases can change the amino acid sequence; a frameshift usually has far more serious consequences than a single substitution because every subsequent codon is altered.
突变是新等位基因的最终来源。碱基的替换、插入和缺失都可能改变氨基酸序列；移码突变通常比单个碱基替换后果严重得多，因为其后每一个密码子都被改变。
Beyond Mendel, geneticists study linkage, sex linkage, codominance, multiple alleles and epistasis. Modern techniques such as PCR, gel electrophoresis and DNA sequencing make it possible to read and compare genomes directly.
除孟德尔规律之外，遗传学还研究连锁、伴性遗传、共显性、复等位基因和上位效应。PCR、凝胶电泳和 DNA 测序等现代技术使直接读取和比较基因组成为可能。
6. Ecology: Organisms and Their Environment | 生态学：生物与环境
Ecology studies the interactions between organisms and their environment. It pays close attention to the role of species in an ecosystem, to population dynamics and to niche differentiation.
生态学聚焦生物与环境的互动，重视物种在生态系统中的角色、种群动态和生态位分化。借助生态学，我们不仅能理解生物与环境间的相互影响，还能把握生态系统的稳定性与可持续发展议题。
Energy enters most ecosystems as sunlight and is captured by producers through photosynthesis. It then passes along food chains, but only about 10% of the energy at one trophic level is transferred to the next, which is why food chains rarely exceed four or five levels.
能量以阳光的形式进入大多数生态系统，由生产者通过光合作用固定，再沿食物链传递。但一个营养级的能量大约只有 10% 能传递到下一级，因此食物链通常不超过四到五级。
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Matter, unlike energy, is recycled. The carbon, nitrogen, phosphorus and water cycles move elements between living organisms and the physical environment, and human activity can disrupt them at a global scale.
与能量不同，物质是循环的。碳、氮、磷和水循环把元素在生物体与物理环境之间搬运，而人类活动可能在全局尺度上打乱这些循环。
Population size is controlled by biotic factors such as predation, competition and disease, and by abiotic factors such as temperature, light and mineral availability. A population grows until it reaches the carrying capacity of its habitat, after which it fluctuates around that level.
种群数量受生物因素（捕食、竞争、疾病）和非生物因素（温度、光照、矿物质可得性）共同控制。种群增长直到达到栖息地的环境容纳量，之后便在该水平附近波动。
Each species occupies a niche - the full set of resources it uses and the role it plays. Two species cannot occupy exactly the same niche indefinitely; competition will drive one to local extinction or force resource partitioning.
每个物种都占据一个生态位，即它所利用的全部资源及所扮演的角色。两个物种无法长期占据完全相同的生态位，竞争会使其中一个局部灭绝，或促使资源分配发生分化。
7. Evolutionary Biology: Change Over Time | 进化生物学：时间中的变化
Evolutionary biology asks how species have changed over time and why there is so much diversity on Earth. By tracing evolutionary history and mechanisms, it explains how biodiversity forms and is maintained, and how organisms adapt to changing environments.
进化生物学追问物种的演化历程与多样性，通过追溯进化历史和机制，能解释生物多样性如何形成与维系，以及生物适应环境变化的模式与路径。
Darwin's mechanism of natural selection rests on four observations: individuals vary, some variation is heritable, more offspring are produced than can survive, and survival and reproduction are not random with respect to that variation. The result is a change in allele frequency in the population over generations.
达尔文的自然选择机制建立在四点观察之上：个体存在变异；部分变异可遗传；产生的后代多于能存活的数量；生存与繁殖对于这些变异并非随机。结果是种群的等位基因频率在世代间发生改变。
Evidence comes from the fossil record, comparative anatomy and embryology, biogeography, molecular sequence data, and observed evolution in real time - the rise of antibiotic-resistant bacteria is the classic example.
证据来自化石记录、比较解剖学与胚胎学、生物地理学、分子序列数据，以及实时观察到的进化——抗生素抗性细菌的出现就是经典例子。
The Hardy-Weinberg principle describes the allele frequencies expected in an idealised, non-evolving population. Deviations from it signal that evolution is occurring.
哈迪—温伯格定律描述了理想化、未进化种群中预期的等位基因频率，偏离该定律即表明进化正在发生。
p² + 2pq + q² = 1, and p + q = 1
Speciation usually begins with reproductive isolation, which may be geographic (allopatric) or arise within the same area through polyploidy, timing differences or behavioural barriers (sympatric). Once gene flow stops, the two gene pools diverge independently.
物种形成通常始于生殖隔离，这种隔离可能是地理性的（异地物种形成），也可能在同一区域内通过多倍体、时间差异或行为屏障产生（同地物种形成）。一旦基因流中断，两个基因库便独立分化。
8. Physiology and Homeostasis: Keeping Life in Balance | 生理学与稳态
Physiology studies how the organ systems of organisms work, and homeostasis is its central concept. Homeostasis is the maintenance of a stable internal environment despite external change, and it is almost always achieved by negative feedback.
生理学研究生物体各器官系统如何运作，而稳态是其核心概念。稳态指在外界变化中维持稳定的内环境，几乎总是通过负反馈实现。
A negative feedback loop has a set point, receptors that detect change, a coordination centre, and effectors that counteract the change. Thermoregulation in mammals and blood glucose control by insulin and glucagon are the two most examined examples.
负反馈回路包括设定点、检测变化的受体、协调中枢，以及对抗变化的效应器。哺乳动物的体温调节，以及胰岛素和胰高血糖素对血糖的调节，是最常考的两种实例。
Surface-area-to-volume ratio explains many physiological designs. Small organisms can exchange gases by diffusion alone, while larger organisms need specialised exchange surfaces such as lungs, gills and villi, together with a transport system.
表面积与体积之比可解释许多生理结构的设计。小型生物仅靠扩散即可完成气体交换，而较大的生物需要肺、鳃、小肠绒毛等特化交换表面，并配备运输系统。
Blood pH is kept close to 7.4 by buffer systems. Carbon dioxide reacts with water to form carbonic acid, which dissociates and releases hydrogen ions, so breathing rate directly influences blood pH.
血液 pH 通过缓冲系统维持在 7.4 左右。二氧化碳与水反应生成碳酸，碳酸解离释放氢离子，因此呼吸频率会直接影响血液 pH。
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
Enzymes underpin all of these processes. They lower activation energy, are specific to their substrate because of the shape of the active site, and are affected by temperature, pH, substrate concentration and inhibitors.
酶支撑着上述所有过程。酶降低活化能，因活性位点的形状而对底物具有专一性，并受温度、pH、底物浓度和抑制剂影响。
9. Microbiology and Immunity | 微生物学与免疫
Microbiology deals with organisms too small to see without magnification, mainly bacteria, viruses, fungi and protoctists. Many are harmless or beneficial, but pathogens cause disease by damaging host cells or releasing toxins.
微生物学研究肉眼不可见、需要放大才能观察的生物，主要包括细菌、病毒、真菌和原生生物。许多微生物无害甚至有益，但病原体会通过破坏宿主细胞或释放毒素引发疾病。
Bacteria are prokaryotes that reproduce by binary fission, while viruses are not cells at all - they are nucleic acid surrounded by a protein coat and can only replicate inside a host cell. This difference explains why antibiotics work against bacteria but not against viruses.
细菌是原核生物，以二分裂方式繁殖；病毒根本不是细胞，而是由核酸和蛋白质外壳构成，只能在宿主细胞内复制。这一差异解释了为什么抗生素对细菌有效，而对病毒无效。
Defence has two layers. The innate response includes physical barriers, phagocytosis, inflammation and fever. The adaptive response is slower but specific and provides immunological memory through T lymphocytes and B lymphocytes.
防御分为两层。先天免疫包括物理屏障、吞噬作用、炎症和发热；适应性免疫较慢，但具有特异性，并通过 T 淋巴细胞和 B 淋巴细胞提供免疫记忆。
B cells that are activated differentiate into plasma cells secreting antibodies, which bind to antigens to form agglutinated complexes that are then destroyed by phagocytes. Vaccination exploits memory cells by exposing the immune system to a harmless form of the antigen.
被激活的 B 细胞分化为浆细胞，分泌抗体。抗体与抗原结合形成凝集复合物，随后被吞噬细胞清除。疫苗接种利用记忆细胞原理，让免疫系统接触无害形式的抗原。
Antibiotic resistance is evolution in action. Bacteria with resistance alleles survive treatment and reproduce, so the resistant allele frequency rises rapidly - a direct link between microbiology and natural selection.
抗生素抗性是正在发生的进化。携带抗性等位基因的细菌在治疗中存活并繁殖，使抗性等位基因频率迅速上升——这正是微生物学与自然选择的直接联系。
10. Biotechnology and Applied Biology | 生物技术与应用生物学
Biotechnology uses living organisms
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