Edexcel IAL 生物 Unit 2 备考全攻略：发育、植物与环境 | Edexcel IAL Biology Unit 2: Development, Plants & Environment — Complete Exam Guide

引言 | Introduction

Edexcel IAL Biology Unit 2（WBI02/01）是 International Advanced Level 生物考试中的核心单元之一，主题为“发育、植物与环境”。该单元涵盖细胞结构与分裂、有性繁殖、植物运输与繁殖、以及生物多样性与保护等关键领域。本文将从五大核心知识点出发，以中英双语形式为你梳理考试重点，并提供高效备考策略。

Edexcel IAL Biology Unit 2 (WBI02/01) is one of the core units in the International Advanced Level Biology examination, themed “Development, Plants and the Environment.” This unit covers cell structure and division, sexual reproduction, plant transport and reproduction, as well as biodiversity and conservation. This article walks through five core knowledge areas in a bilingual format, along with effective exam preparation strategies.

知识点一：细胞结构与细胞器 | Core Concept 1: Cell Structure & Organelles

中文：Unit 2 的第一个重点是真核细胞与原核细胞的结构差异。你需要熟练掌握真核细胞中细胞核、线粒体、内质网、高尔基体、核糖体和溶酶体等细胞器的结构与功能。尤其是线粒体的嵴（cristae）增加表面积以利于有氧呼吸，以及粗面内质网上附着的核糖体负责蛋白质合成。原核细胞（如细菌）则缺乏膜结合的细胞器，其 DNA 以环状形式存在于细胞质中。考试中常要求你比较两者结构，并用显微镜校准（graticule calibration）计算实际细胞大小。

English: The first major focus of Unit 2 is the structural differences between eukaryotic and prokaryotic cells. You must master the structure and function of organelles such as the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, ribosomes, and lysosomes in eukaryotic cells. Pay special attention to mitochondrial cristae, which increase surface area for aerobic respiration, and ribosomes attached to rough ER, which are responsible for protein synthesis. Prokaryotic cells (e.g., bacteria) lack membrane-bound organelles, and their DNA exists as circular molecules in the cytoplasm. Exam questions frequently ask you to compare the two cell types and to calculate actual cell sizes using graticule calibration.

知识点二：细胞分裂与有性繁殖 | Core Concept 2: Cell Division & Sexual Reproduction

中文：有丝分裂（mitosis）和减数分裂（meiosis）的区别是必考内容。有丝分裂产生两个基因完全相同的子细胞，用于生长和修复；减数分裂则产生四个基因不同的单倍体配子，是遗传多样性的来源。你需要能够识别分裂各阶段的显微照片，并解释同源染色体交叉互换（crossing over）和独立分配（independent assortment）如何增加变异。在哺乳动物繁殖部分，掌握精子发生（spermatogenesis）和卵子发生（oogenesis）的过程，以及受精时顶体反应（acrosome reaction）和皮质反应（cortical reaction）的机制。

English: The differences between mitosis and meiosis are essential exam topics. Mitosis produces two genetically identical daughter cells for growth and repair, while meiosis produces four genetically distinct haploid gametes — the source of genetic diversity. You need to identify micrographs of each division stage and explain how homologous chromosome crossing over and independent assortment increase variation. In the mammalian reproduction section, master the processes of spermatogenesis and oogenesis, as well as the mechanisms of the acrosome reaction and cortical reaction during fertilization.

知识点三：植物运输系统 | Core Concept 3: Transport in Plants

中文：植物的木质部（xylem）和韧皮部（phloem）运输系统是 Unit 2 的重中之重。木质部负责水和矿物质从根部向上的单向运输，其导管细胞是死细胞，木质化加厚（lignification）提供结构支撑。蒸腾拉力（transpiration pull）是水上升的主要驱动力，由内聚力-张力理论（cohesion-tension theory）解释。韧皮部则负责有机物（主要是蔗糖）的双向运输，由压力流动假说（mass flow hypothesis）解释，涉及源（source）和库（sink）之间的渗透压梯度。考试中常要求设计实验测量蒸腾速率，如使用光合作用计（potometer）。

English: The xylem and phloem transport systems in plants are among the most critical topics in Unit 2. Xylem is responsible for the unidirectional upward transport of water and minerals from roots; its vessel elements are dead cells with lignification providing structural support. Transpiration pull, driven by the cohesion-tension theory, is the main mechanism for water ascent. Phloem handles the bidirectional transport of organic substances (mainly sucrose), explained by the mass flow hypothesis involving osmotic pressure gradients between source and sink. Exam questions frequently ask you to design experiments measuring transpiration rate, such as using a potometer.

知识点四：植物繁殖 | Core Concept 4: Plant Reproduction

中文：植物的有性繁殖涉及花的结构、授粉（pollination）、受精和种子传播。你需要区分风媒花（wind-pollinated）和虫媒花（insect-pollinated）的结构适应特征：虫媒花通常有鲜艳花瓣、香气和花蜜，花粉粒较大且表面粗糙；风媒花则有轻质花粉、羽毛状柱头和退化的花瓣。双受精（double fertilization）是开花植物独有的过程，一个精子与卵细胞结合形成合子，另一个与极核结合形成胚乳。种子休眠与萌发受赤霉素（gibberellin）和脱落酸（ABA）的拮抗调控。

English: Sexual reproduction in plants involves flower structure, pollination, fertilization, and seed dispersal. You need to distinguish between the structural adaptations of wind-pollinated and insect-pollinated flowers: insect-pollinated flowers typically have bright petals, scent, and nectar with larger, rough-surfaced pollen grains, while wind-pollinated flowers have lightweight pollen, feathery stigmas, and reduced petals. Double fertilization is a process unique to flowering plants — one sperm fuses with the egg cell to form a zygote, and another fuses with polar nuclei to form endosperm. Seed dormancy and germination are antagonistically regulated by gibberellin and abscisic acid (ABA).

知识点五：生物多样性与保护 | Core Concept 5: Biodiversity & Conservation

中文：生物多样性包括物种多样性、遗传多样性和生态系统多样性三个层次。Simpson 多样性指数（D = 1 – Σ(n/N)²）是量化物种多样性的常用工具，数值越接近 1 代表多样性越高。你需要掌握随机抽样和系统抽样（如样带 transect）等野外调查方法。分类学部分，理解 Carl Woese 的三域系统（细菌、古菌、真核生物）及其基于 rRNA 序列的分子证据。保护生物学中，了解就地保护（in-situ，如自然保护区）和迁地保护（ex-situ，如种子库、动物园）的区别，以及 CITES 和 Rio Convention 等国际保护协议的作用。

English: Biodiversity encompasses three levels: species diversity, genetic diversity, and ecosystem diversity. Simpson’s Diversity Index (D = 1 – Σ(n/N)²) is a common tool for quantifying species diversity, with values closer to 1 indicating higher diversity. You need to master field survey methods such as random sampling and systematic sampling (e.g., transects). In taxonomy, understand Carl Woese’s three-domain system (Bacteria, Archaea, Eukaryota) and its molecular evidence based on rRNA sequences. In conservation biology, understand the difference between in-situ conservation (e.g., nature reserves) and ex-situ conservation (e.g., seed banks, zoos), as well as the role of international conservation agreements such as CITES and the Rio Convention.

备考策略与学习建议 | Study Tips & Exam Strategy

中文：

1. 真题训练：Unit 2 考试时间仅 1 小时 30 分，共 80 分。时间压力大，务必通过反复练习历年真题来提升速度。注意带星号 (*) 的题目会评估书面表达质量，需特别注意拼写和语法。
2. 实验设计：Unit 2 强调实验技能，熟悉常见实验（如测定维生素 C 含量、酶活性、蒸腾速率、有丝分裂指数）的原理、步骤和潜在误差来源。
3. 图表分析：考试中包含大量数据分析题，练习从图表、显微照片中提取信息并得出科学结论。
4. 专业术语：生物学中的关键词（如 osmosis、diffusion、active transport）必须拼写正确，定义准确。
5. 计算器准备：考试允许使用计算器，务必熟练进行比率、百分比和统计计算。

English:

1. Past Paper Practice: Unit 2 is only 1 hour 30 minutes for 80 marks. Time pressure is real — improve speed through repeated practice with past papers. Note that questions marked with an asterisk (*) assess the quality of written communication; pay special attention to spelling and grammar.
2. Experimental Design: Unit 2 emphasizes practical skills. Familiarize yourself with the principles, procedures, and potential sources of error in common experiments (e.g., vitamin C assay, enzyme activity, transpiration rate, mitotic index).
3. Graph & Data Analysis: The exam contains substantial data-analysis questions. Practice extracting information from graphs, micrographs, and tables to draw scientific conclusions.
4. Scientific Terminology: Keywords such as osmosis, diffusion, and active transport must be spelled correctly and defined precisely.
5. Calculator Readiness: A calculator is permitted in the exam — be proficient in ratio, percentage, and statistical calculations.

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