📚 Biology Unit 2 Core Revision: Cells, Development and Biodiversity (057-101) | 生物第二单元核心复习：细胞、发育与生物多样性（057-101）
This bilingual revision guide addresses the main specification areas in Biology Unit 2, including cell ultrastructure, membrane transport, enzymes, DNA replication, mitosis, stem cells, biodiversity and conservation. Each section pairs English and Chinese explanations so you can review key terms, processes and common exam traps quickly.
本双语复习指南涵盖生物第二单元的主要考点，包括细胞超微结构、膜运输、酶、DNA复制、有丝分裂、干细胞、生物多样性与保护。每节英文与中文对照，帮助你快速复习关键术语、过程和常见考试陷阱。
1. Cell Ultrastructure | 细胞超微结构
Eukaryotic cells contain membrane-bound organelles, each with a specific function. The nucleus stores genetic material and controls protein synthesis; mitochondria produce ATP through aerobic respiration; the rough endoplasmic reticulum (RER) processes proteins, while the smooth ER synthesises lipids; the Golgi apparatus modifies and packages proteins; lysosomes digest worn-out organelles; ribosomes are the site of translation.
真核细胞含有具膜细胞器，每种细胞器都有特定功能。细胞核储存遗传物质并控制蛋白质合成；线粒体通过有氧呼吸产生ATP；粗面内质网加工蛋白质，光面内质网合成脂质；高尔基体修饰和包装蛋白质；溶酶体消化衰老的细胞器；核糖体是翻译的场所。
Prokaryotic cells, such as bacteria, lack a true nucleus and membrane-bound organelles. They have a circular DNA molecule, 70S ribosomes, a cell wall made of peptidoglycan, and sometimes plasmids and flagella. Remember that ribosomes are not membrane-bound, so both eukaryotes and prokaryotes possess them.
原核细胞（如细菌）没有真正的细胞核和具膜细胞器。它们具有环状DNA分子、70S核糖体、由肽聚糖组成的细胞壁，有时还有质粒和鞭毛。记住核糖体没有膜，因此真核生物和原核生物都含有核糖体。
2. Cell Membranes and Transport | 细胞膜与运输
The fluid mosaic model describes the cell membrane as a phospholipid bilayer with embedded proteins, cholesterol and glycoproteins. Phospholipids form a selectively permeable barrier; intrinsic proteins allow facilitated diffusion and active transport; cholesterol regulates membrane fluidity and stability.
流动镶嵌模型将细胞膜描述为磷脂双分子层，其中嵌入蛋白质、胆固醇和糖蛋白。磷脂形成选择性通透屏障；内在蛋白负责协助扩散和主动运输；胆固醇调节膜的流动性和稳定性。
Diffusion is the net movement of particles from a higher concentration to a lower concentration down a concentration gradient; facilitated diffusion uses channel or carrier proteins but still requires no metabolic energy. Active transport moves ions or molecules against the concentration gradient using ATP. Osmosis is the diffusion of water across a partially permeable membrane from a higher water potential to a lower water potential.
扩散是粒子沿浓度梯度从高浓度向低浓度的净移动；协助扩散利用通道蛋白或载体蛋白，但仍不需要代谢能量。主动运输利用ATP逆浓度梯度转运离子或分子。渗透是水通过部分透性膜从较高水势向较低水势的扩散。
ψ = ψₛ + ψₚ
水势 = 溶质势 + 压力势
In plant cells, the cell wall creates pressure potential; as water enters the vacuole, the cell becomes turgid. In animal cells, the absence of a cell wall means cells may burst in hypotonic solutions or shrink in hypertonic solutions.
在植物细胞中，细胞壁产生压力势；当水进入液泡时，细胞
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