WJEC生物 细胞 生化 遗传 生态 免疫 备考
WJEC A-Level Biology covers four units spanning molecular biology to ecosystems, assessed through AS (Units 1-2) and A2 (Units 3-4) exams plus a practical endorsement. 本备考攻略为你梳理全部四个单元的核心考点与高效复习方法，目标冲刺A*。
Unit 1: Basic Biochemistry and Cell Organisation 基础生化与细胞组织
Unit 1 is the foundation layer -- you will study the chemical building blocks of life and how they assemble into functional cells. 本单元涵盖生物大分子的结构与功能、细胞膜转运机制、酶动力学以及核酸作为遗传信息载体。This unit carries the most marks in the AS exam (around 40% of the total AS grade), so invest serious time in mastering every sub-topic thoroughly.
Biomolecules 生物大分子: Master the four groups -- carbohydrates (monosaccharides, disaccharides like sucrose and lactose, polysaccharides starch/glycogen/cellulose), lipids (triglycerides, phospholipids, cholesterol), proteins (primary to quaternary structure, fibrous vs globular), and nucleic acids (DNA vs RNA structure, semi-conservative replication). WJEC特别喜欢考 chemical bond types in each group -- 糖苷键、酯键、肽键和二硫键的区别必须记牢。Practical skill: the Benedict's test for reducing/non-reducing sugars appears in almost every exam series.
Cell Membranes and Transport 细胞膜与物质转运: The fluid mosaic model -- phospholipid bilayer, intrinsic vs extrinsic proteins, glycoproteins, cholesterol. Transport mechanisms: diffusion, facilitated diffusion (channel vs carrier proteins), osmosis (water potential calculations using ψ = ψs + ψp), active transport, endocytosis/exocytosis. WJEC expects you to explain how CFTR protein malfunction causes cystic fibrosis as a classic example of membrane protein pathology. 必考实验：甜菜根或红甘蓝的花青素渗漏实验，用来测定温度或溶剂对膜通透性的影响。
Enzymes 酶: Lock-and-key vs induced fit models. Factors affecting rate: temperature (Q10 coefficient), pH, enzyme concentration, substrate concentration (Michaelis-Menten curve). Competitive vs non-competitive inhibition. Immobilised enzymes in industrial applications (lactose-free milk production using lactase). 重点计算：用初始速率法求Km和Vmax，WJEC经常在图解题中出现。
Nucleic Acids 核酸: DNA replication -- helicase, DNA polymerase, leading/lagging strands, Okazaki fragments. Meselson-Stahl experiment evidence for semi-conservative replication. Comparison of DNA and RNA. Genetic code features: triplet, degenerate, non-overlapping, universal. Transcription and translation mechanisms. 易错点：DNA聚合酶只能从5'到3'方向合成，因此滞后链需要引物酶不断合成新引物。
Unit 2: Biodiversity and Physiology 生物多样性与生理学
Unit 2 shifts from molecules to whole organisms -- classification, gas exchange, transport systems, and adaptations. 本单元把微观生化知识应用到宏观生物体中，是AS考试中占分最大的部分。
Classification and Biodiversity 分类与生物多样性: The five-kingdom system vs three-domain system (Archaea, Bacteria, Eukarya). Species concepts: morphological, biological (reproductive isolation), phylogenetic. Phylogenetic trees and cladistics. Biodiversity indices: Simpson's Diversity Index (D = 1 - Σ(n/N)²), species richness vs evenness. 考试技巧：Simpson指数计算题几乎每卷必出，必须熟悉公式并能解释高D值代表什么。
Gas Exchange 气体交换: Across the taxonomic spectrum -- unicellular (diffusion across cell surface), insects (tracheal system with spiracles and tracheoles), fish (countercurrent flow in gill lamellae -- DRAW and label this), amphibians (skin + simple lungs), mammals (alveoli with large surface area, thin epithelium, dense capillary network). Fick's Law: rate = (surface area × concentration gradient) / diffusion distance. 反流交换机制是WJEC的高频考点，必须能用氧浓度数据证明其效率优于并流。
Transport Systems 运输系统: Mammalian circulatory system -- double circulation, cardiac cycle (atrial systole, ventricular systole, diastole), SAN/AVN conduction, ECG interpretation. Blood: plasma, erythrocytes, leukocytes, platelets. Oxygen dissociation curves: fetal vs adult haemoglobin, Bohr effect (shift right with increased CO₂). Transport in plants: xylem (cohesion-tension theory, transpiration stream) and phloem (mass flow hypothesis, source-sink relationships). 核心实验：用气泡蒸腾计测定环境因素对蒸腾速率的影响。
Adaptations for Nutrition 营养适应: Autotrophic (photoautotrophs, chemoautotrophs) vs heterotrophic nutrition. Human digestive system: mechanical and chemical digestion, role of enzymes (amylase, pepsin, lipase), absorption in the ileum (villi and microvilli). Saprophytic nutrition in fungi (extracellular digestion). Parasitic nutrition adaptations (e.g., tapeworm -- reduced gut, large surface area). 必记：小肠绒毛上皮细胞的钠钾泵如何驱动葡萄糖的共转运。
Unit 3: Energy, Homeostasis and the Environment 能量、稳态与环境
Unit 3 combines biochemistry (respiration and photosynthesis) with whole-organism physiology (homeostasis, nervous system, kidney) and ecology. 这是A2三个单元中内容最广的，需要平衡理解与记忆。
Respiration 呼吸作用: Glycolysis (cytoplasm, 2ATP net), Link reaction (mitochondrial matrix, pyruvate -> acetyl CoA), Krebs cycle (matrix, produces NADH/FADH₂), Oxidative phosphorylation (cristae, chemiosmosis, ATP synthase). Anaerobic respiration: lactate fermentation (animals) vs ethanol fermentation (yeast/plants). Respiratory quotients (RQ = CO₂ produced / O₂ consumed). 高频考点：线粒体嵴上的电子传递链，要求会描述质子梯度如何驱动ATP合酶（Peter Mitchell's chemiosmotic theory）。Key numbers to memorise: glycolysis yields 2 ATP net + 2 NADH; link reaction yields 2 NADH; Krebs cycle yields 2 ATP + 6 NADH + 2 FADH₂ (per glucose); oxidative phosphorylation yields ~34 ATP. Total ATP per glucose: ~38 in prokaryotes, ~36 in eukaryotes (2 ATP cost for NADH shuttle into mitochondrion).
Photosynthesis 光合作用: Light-dependent reactions (thylakoid membrane): photoionisation of chlorophyll, photolysis of water (2H₂O -> 4H⁺ + 4e⁻ + O₂), cyclic vs non-cyclic photophosphorylation. Light-independent reactions (Calvin cycle, stroma): carbon fixation (RuBisCO), reduction (GP -> TP), regeneration of RuBP. Limiting factors: light intensity, CO₂ concentration, temperature. 实验重点：用氧化还原指示剂DCPIP测定光反应速率（提取叶绿体、Hill reaction）。
Homeostasis and the Kidney 稳态与肾脏: Negative feedback principles. Thermoregulation: hypothalamus, vasodilation/constriction, sweating, shivering. Blood glucose regulation: insulin (beta cells) and glucagon (alpha cells), type 1 vs type 2 diabetes. Kidney structure: nephron (Bowman's capsule, PCT, loop of Henle, DCT, collecting duct). Ultrafiltration, selective reabsorption (PCT -- Na⁺/K⁺ pump, co-transport of glucose/amino acids), osmoregulation (ADH, aquaporins, countercurrent multiplier in loop of Henle). 关键图表：必须能绘制并解释肾元各段的渗透压变化。
Nervous System 神经系统: Structure of a myelinated motor neurone. Resting potential (-70mV, Na⁺/K⁺ pump, K⁺ leak channels). Action potential: depolarisation (Na⁺ voltage-gated channels open, +40mV), repolarisation (K⁺ channels open), hyperpolarisation, refractory period. Saltatory conduction in myelinated axons (nodes of Ranvier). Synaptic transmission: Ca²⁺ influx, vesicle exocytosis, neurotransmitter (acetylcholine) diffusion, postsynaptic receptors, EPSP/IPSP summation. 易混淆点：动作电位的"全或无"特性与突触电位的分级电位之间的区别。
Ecosystems and Nutrient Cycles 生态系统与物质循环: Energy flow: producer -> primary consumer -> secondary consumer, only ~10% transfer between trophic levels. Pyramids of number, biomass, energy. Carbon cycle: photosynthesis, respiration, combustion, decomposition, fossilisation. Nitrogen cycle: nitrogen fixation (Rhizobium, Azotobacter), nitrification (Nitrosomonas, Nitrobacter), denitrification, ammonification. Eutrophication cascade -- excess nitrates -> algal bloom -> light blocked -> aquatic plants die -> decomposers use up oxygen -> fish die. 计算题：能量传递效率与净初级生产力的数值计算。NPP = GPP - R, where R is respiratory losses; typical values for a temperate grassland are GPP ~20,000 kJ m⁻² yr⁻¹ and NPP ~8,000 kJ m⁻² yr⁻¹, meaning 60% of fixed energy is used in respiration.
Unit 4: Variation, Inheritance and Options 变异、遗传与选修
Unit 4 covers classical and molecular genetics, plus one optional topic (choose wisely based on your teacher's strengths). 本单元包含A-Level最难的遗传学计算和进化论论述题。
Inheritance 遗传: Monohybrid and dihybrid crosses (Punnett squares). Codominance and multiple alleles (ABO blood groups). Sex linkage (haemophilia, colour blindness). Autosomal linkage and epistasis (recessive and dominant). Chi-squared test for goodness-of-fit in genetic ratios. 计算重点：WJEC的二倍体杂交题经常要求先推导亲本基因型，再预测F1和F2比例，而不是直接给基因型。A classic exam question pattern: a pedigree chart showing a recessive condition, you must deduce genotypes for every numbered individual, then calculate the probability that individual X's child inherits the condition -- these multi-step pedigree problems are worth 6-8 marks each.
Variation and Evolution 变异与进化: Sources of variation: mutation (gene and chromosome), meiosis (crossing over, independent assortment), random fertilisation. Continuous vs discontinuous variation. Natural selection: directional, stabilising, disruptive. Speciation: allopatric (geographic isolation) vs sympatric (reproductive isolation). Hardy-Weinberg principle: p² + 2pq + q² = 1, p + q = 1. 考试经典题：用等位基因频率数据判断一个种群是否处于哈代温伯格平衡。Worked example: if 1 in 10,000 individuals has a recessive condition (q²=0.0001), then q=0.01, p=0.99, carrier frequency 2pq=0.0198 or roughly 1 in 50 -- this exact pattern has appeared in WJEC Unit 4 papers across multiple exam series.
Applications of Genetics 基因应用: PCR (denaturation, annealing, extension -- Taq polymerase). Gel electrophoresis. DNA profiling (VNTRs, STRs). Genetic engineering: restriction enzymes, ligase, vectors (plasmids), transformation. Gene therapy: somatic vs germline. GM crops (Bt toxin, Golden Rice). Ethical issues including gene patenting, designer babies, and informed consent for genetic testing. 实验重点：凝胶电泳条带的解读与亲子鉴定或法医学应用。Practical tip: when interpreting gel electrophoresis results, always check the molecular weight marker lane first -- smaller fragments travel further, so the bands closest to the bottom of the gel are the lightest DNA fragments.
Optional Topic: Immunology 免疫学: This is the most popular WJEC option. Non-specific defences: skin, mucous membranes, phagocytosis (neutrophils, macrophages). Specific immune response: T-lymphocytes (cell-mediated, helper T cells activate B cells), B-lymphocytes (humoral, clonal selection, plasma cells, memory cells). Antibody structure: variable and constant regions, antigen-binding sites. Active vs passive immunity, natural vs artificial. Vaccination: herd immunity, ethical considerations. Autoimmune diseases (rheumatoid arthritis, type 1 diabetes, multiple sclerosis), HIV/AIDS mechanism (reverse transcriptase, integration into host DNA, destruction of helper T cells leading to opportunistic infections). 必背：初级免疫应答与次级免疫应答在潜伏期、抗体浓度和抗体类型（IgM vs IgG）上的三点差异。
Revision Strategy 复习策略
Phase 1: Foundation (Weeks 1-3) 基础阶段: Build comprehensive notes for each unit using the WJEC specification as a checklist. 每完成一个单元，立即做一套对应的past paper选择题来检验理解。重点关注Unit 1和Unit 2，因为它们构成A2知识的基础。Make flashcards for all key definitions -- WJEC mark schemes are notorious for requiring exact phrasing of terms like "a sequence of three nucleotide bases on mRNA that codes for a specific amino acid" for the definition of a codon. One word off and you lose the mark.
Phase 2: Application (Weeks 4-6) 应用阶段: Full past papers under timed conditions. WJEC papers are 2 hours each, worth 100 marks -- that's 72 seconds per mark, so pace yourself ruthlessly. 每套卷子做完后，用mark scheme逐题分析失分原因 -- 是知识漏洞、审题错误还是表达不清。保留错题本记录反复出现的错误模式。Focus particularly on QER (Quality of Extended Response) questions in Units 3 and 4, where WJEC awards up to 3 additional marks for logical structure, coherent argument, and use of specialist terminology.
Phase 3: Intensive (Weeks 7-8) 冲刺阶段: Target weak areas identified in Phase 2. Redo all practical-based questions. 每天默写2-3条核心代谢途径（呼吸作用、光合作用、氮循环），确保能在考场上闭卷准确复述。Use the WJEC command word guide to refine your answering technique: "describe" means state facts without explanation, "explain" requires reasons and mechanisms, "suggest" expects you to apply knowledge to an unfamiliar context.
Practical Endorsement 实验技能: Keep a well-organised lab book. WJEC requires evidence of competency in 12 core practicals including microscopy, enzyme kinetics (effect of temperature on trypsin activity using milk powder), chromatography (TLC of plant pigments with Rf value calculations), dissection (heart or fish gills), and ecological sampling (transects, quadrats, Simpson's Diversity Index fieldwork). 实验考试中最常见的扣分点：未标注控制变量、未说明重复实验、单位转换错误。Top tip: when designing an investigation, always state your independent variable, dependent variable, at least three control variables, and justify your choice of statistical test (Student's t-test for comparing two means, chi-squared for categorical data, correlation coefficient for associations).
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