Core Biology Cell Biology 详解

## IB生物 细胞膜 细胞器 有丝分裂 细胞呼吸 IB Core Biology的Cell Biology单元是所有IB生物学生的第一道门槛。细胞是生命的基本单位，从细胞膜的选择透过性到线粒体的呼吸作用，从核糖体的翻译功能到有丝分裂的精确调控，这个单元要求你不仅记住结构名称，更要理解每个细胞器如何协同工作。今天我们用中英双语拆解四大核心模块：细胞膜结构与运输、细胞器功能、细胞分裂、以及细胞呼吸。 IB Core Biology's Cell Biology unit is every IB Biology student's first major challenge. The cell is the basic unit of life — from the selective permeability of the cell membrane to mitochondrial respiration, from ribosomal translation to the precise regulation of mitosis, this unit demands more than memorizing organelle names. You must understand how each organelle works in concert. Today we break down four core modules in both Chinese and English: membrane structure and transport, organelle function, cell division, and cellular respiration. **一、细胞膜结构与跨膜运输 Membrane Structure and Transport** 细胞膜的基本骨架是磷脂双分子层。每个磷脂分子有一个亲水的磷酸头和一个疏水的脂肪酸尾，这种亲水疏水双重特性驱动细胞膜在水中自发组装成双层结构。根据Singer-Nicolson的流动镶嵌模型，蛋白质分子镶嵌在磷脂双分子层中，有的贯穿整个膜（整合蛋白），有的只附着在一侧（外周蛋白）。胆固醇分子插在磷脂之间，调节膜的流动性——温度高时限制磷脂运动，温度低时防止膜凝固。这个考点在IB Paper 1中反复出现，你需要区分integral proteins和peripheral proteins的位置和功能差异。 The fundamental framework of the cell membrane is the phospholipid bilayer. Each phospholipid molecule has a hydrophilic phosphate head and a hydrophobic fatty acid tail. This amphipathic property drives spontaneous bilayer assembly in aqueous environments. According to the Singer-Nicolson fluid mosaic model, protein molecules are embedded within the phospholipid bilayer — some span the entire membrane (integral proteins) while others attach to only one surface (peripheral proteins). Cholesterol molecules sit between phospholipids, modulating membrane fluidity: at high temperatures they restrict phospholipid movement, and at low temperatures they prevent solidification. This concept appears repeatedly in IB Paper 1; you need to distinguish the positions and functional differences between integral and peripheral proteins. 物质通过细胞膜有六大机制。第一种是简单扩散：小分子如氧气、二氧化碳、酒精沿着浓度梯度直接穿过磷脂双分子层，不消耗能量。第二种是协助扩散：较大的分子或带电离子如葡萄糖、钠离子需要通道蛋白或载体蛋白的帮助，依然顺着浓度梯度，不消耗ATP。水分子通过水通道蛋白的协助扩散称为渗透。第三种是主动运输：钠钾泵每消耗一个ATP，泵出三个钠离子并泵入两个钾离子，逆浓度梯度运输。钠钾泵是IB考试必考的主动运输实例，你需要能完整描述其构象变化循环。第四种和第五种分别是内吞作用和外排作用——细胞膜包裹大分子或颗粒形成囊泡进行运输，这需要ATP。第六种是共转运，例如小肠上皮细胞利用钠离子浓度梯度驱动葡萄糖的协同转运。 There are six mechanisms for substances to cross the cell membrane. First is simple diffusion: small molecules like oxygen, carbon dioxide, and alcohol pass directly through the phospholipid bilayer along the concentration gradient without energy expenditure. Second is facilitated diffusion: larger molecules or charged ions like glucose and sodium ions require channel proteins or carrier proteins, still moving along the gradient without ATP. Water molecules passing through aquaporins via facilitated diffusion is called osmosis. Third is active transport: the sodium-potassium pump expels three sodium ions and imports two potassium ions per ATP consumed, moving against the concentration gradient. The sodium-potassium pump is a mandatory active transport example for IB exams — you must be able to describe its full conformational change cycle. The fourth and fifth are endocytosis and exocytosis, where the cell membrane engulfs macromolecules or particles by forming vesicles, requiring ATP. The sixth is cotransport, such as intestinal epithelial cells using the sodium ion gradient to drive glucose symport. **二、细胞器功能图谱 Organelle Function Atlas** 真核细胞的内膜系统包含一系列膜包裹的细胞器，每个都有独特的功能。核糖体是蛋白质合成的场所，由大亚基和小亚基组成。注意：核糖体本身没有膜包裹！80S核糖体存在于真核细胞的细胞质中，70S核糖体存在于线粒体、叶绿体和原核细胞中——这个"70S vs 80S"的区分在Paper 1选择题中经常出现。粗面内质网上附着核糖体，负责合成分泌蛋白和膜蛋白；滑面内质网没有核糖体，负责脂质合成和解毒。高尔基体接收来自内质网的囊泡，对蛋白质进行修饰（如糖基化）、分拣和包装，然后运送到细胞膜或其他目的地。溶酶体含有水解酶，负责细胞内消化和自噬。 The endomembrane system of eukaryotic cells comprises a series of membrane-bound organelles, each with distinct functions. Ribosomes are the sites of protein synthesis, composed of a large subunit and a small subunit. Note: ribosomes are not membrane-bound! 80S ribosomes exist in the cytoplasm of eukaryotic cells, while 70S ribosomes are found in mitochondria, chloroplasts, and prokaryotic cells — this "70S vs 80S" distinction frequently appears in Paper 1 multiple-choice questions. The rough endoplasmic reticulum has ribosomes attached and synthesizes secretory and membrane proteins; the smooth endoplasmic reticulum lacks ribosomes and handles lipid synthesis and detoxification. The Golgi apparatus receives vesicles from the ER, modifies proteins (e.g., glycosylation), sorts and packages them, then dispatches them to the cell membrane or other destinations. Lysosomes contain hydrolytic enzymes and are responsible for intracellular digestion and autophagy. 线粒体是细胞的能量工厂，具有双层膜结构。内膜向内折叠形成嵴，大大增加了呼吸作用电子传递链的表面积。线粒体基质中含有自身的环状DNA和70S核糖体，支持内共生假说。内共生假说认为线粒体起源于被早期真核细胞吞噬的需氧原核生物——支持证据包括线粒体的双层膜、环状DNA、70S核糖体、以及自主分裂方式类似于细菌的二分裂。叶绿体同样支持内共生假说，具有三层膜结构（外膜、内膜和类囊体膜），类囊体堆叠成基粒。光合作用的光反应发生在类囊体膜上，暗反应（卡尔文循环）发生在基质中。液泡在植物细胞中占据大部分体积，储存水分、离子和色素，维持细胞膨压。细胞骨架由微丝、微管和中间纤维组成，提供结构支持、细胞运动和有丝分裂中染色体的分离。 Mitochondria are the powerhouses of the cell, featuring a double-membrane structure. The inner membrane folds inward to form cristae, dramatically increasing the surface area for the electron transport chain of respiration. The mitochondrial matrix contains its own circular DNA and 70S ribosomes, supporting the endosymbiotic theory. This theory proposes that mitochondria originated from aerobic prokaryotes engulfed by early eukaryotic cells — supporting evidence includes the double membrane, circular DNA, 70S ribosomes, and autonomous division resembling bacterial binary fission. Chloroplasts similarly support endosymbiotic theory, with a three-membrane structure (outer, inner, and thylakoid membranes), where thylakoids stack into grana. The light-dependent reactions of photosynthesis occur on thylakoid membranes, while the light-independent reactions (Calvin cycle) occur in the stroma. Vacuoles occupy most of the volume in plant cells, storing water, ions, and pigments, and maintaining turgor pressure. The cytoskeleton — composed of microfilaments, microtubules, and intermediate filaments — provides structural support, cell motility, and chromosome segregation during mitosis. **三、有丝分裂与细胞周期 Mitosis and the Cell Cycle** 细胞周期分为间期和有丝分裂期。间期包含G1期（细胞生长和蛋白质合成）、S期（DNA复制）和G2期（继续生长并合成分裂所需蛋白质）。DNA复制遵循半保留复制原则，由DNA解旋酶解开双链，DNA聚合酶在两条母链上分别合成新的互补链。注意：DNA聚合酶只能沿5'到3'方向添加核苷酸，这导致前导链连续合成而滞后链以冈崎片段形式不连续合成。IB经常考DNA复制的方向性问题，务必牢记。 The cell cycle is divided into interphase and the mitotic phase. Interphase includes G1 (cell growth and protein synthesis), S phase (DNA replication), and G2 (continued growth and synthesis of proteins needed for division). DNA replication follows the semiconservative principle: DNA helicase unwinds the double helix, and DNA polymerase synthesizes new complementary strands on both template strands. Note: DNA polymerase can only add nucleotides in the 5' to 3' direction, which causes the leading strand to be synthesized continuously while the lagging strand is synthesized discontinuously as Okazaki fragments. IB frequently tests the directionality of DNA replication — commit this to memory. 有丝分裂分为四个阶段。前期：染色质凝缩成可见的染色体（每条染色体由两条姐妹染色单体通过着丝粒连接），核膜解体，中心体（动物细胞）移向两极并开始形成纺锤体。中期：染色体排列在赤道板上，纺锤丝附着在着丝粒上。这个阶段最适合观察染色体核型。后期：着丝粒分裂，姐妹染色单体分开，纺锤丝缩短将染色体拉向两极。末期：染色体解旋回到染色质状态，核膜重新形成，纺锤体解体。胞质分裂紧随其后：动物细胞通过收缩环形成分裂沟，植物细胞通过高尔基体囊泡在赤道板处形成细胞板。IB考试常让你比较动物细胞和植物细胞胞质分裂的机制差异，这是经典的Paper 2简答题考点。 Mitosis is divided into four stages. Prophase: chromatin condenses into visible chromosomes (each consisting of two sister chromatids joined at the centromere), the nuclear membrane breaks down, and centrosomes (in animal cells) migrate to opposite poles and begin forming the spindle apparatus. Metaphase: chromosomes align at the e quatorial plate, and spindle fibers attach to centromeres. This stage is optimal for observing karyotypes. Anaphase: centromeres split, sister chromatids separate, and spindle fibers shorten to pull chromosomes toward opposite poles. Telophase: chromosomes decondense back to chromatin, nuclear membranes reform, and the spindle apparatus disassembles. Cytokinesis follows: animal cells form a cleavage furrow via a contractile ring, while plant cells form a cell plate at the equatorial plate via Golgi-derived vesicles. IB exams frequently ask you to compare the cytokinesis mechanisms between animal and plant cells — this is a classic Paper 2 short-answer question. **四、细胞呼吸 Cellular Respiration** 细胞呼吸分为三个主要阶段：糖酵解、克雷布斯循环（柠檬酸循环）和氧化磷酸化。糖酵解发生在细胞质中，不需要氧气，将一分子葡萄糖分解为两分子丙酮酸，净产两分子ATP和两分子NADH。IB特别强调糖酵解的场所和净产量。丙酮酸随后进入线粒体基质，在有氧条件下经过连接反应转化为乙酰辅酶A，释放一分子二氧化碳并产生一分子NADH。 Cellular respiration occurs in three main stages: glycolysis, the Krebs cycle (citric acid cycle), and oxidative phosphorylation. Glycolysis takes place in the cytoplasm and does not require oxygen, breaking down one glucose molecule into two pyruvate molecules with a net yield of two ATP and two NADH. IB particularly emphasizes the location and net yield of glycolysis. Pyruvate then enters the mitochondrial matrix and, under aerobic conditions, undergoes the link reaction to become acetyl-CoA, releasing one carbon dioxide molecule and producing one NADH. 克雷布斯循环发生在线粒体基质中。乙酰辅酶A与四碳化合物草酰乙酸结合形成六碳的柠檬酸，经过一系列脱羧和脱氢反应，最终再生草酰乙酸。每轮循环产生两分子二氧化碳、三分子NADH、一分子FADH2和一分子ATP（通过底物水平磷酸化）。由于每分子葡萄糖产生两分子乙酰辅酶A，克雷布斯循环需要运行两轮。IB不会要求你记住循环中的每一个中间产物，但你必须知道碳原子数变化、NADH和FADH2的产量，以及二氧化碳的释放位置。 The Krebs cycle occurs in the mitochondrial matrix. Acetyl-CoA combines with the four-carbon oxaloacetate to form six-carbon citrate, which undergoes a series of decarboxylation and dehydrogenation reactions to regenerate oxaloacetate. Each cycle turn produces two carbon dioxide molecules, three NADH, one FADH2, and one ATP (via substrate-level phosphorylation). Since each glucose molecule yields two acetyl-CoA, the Krebs cycle runs twice. IB does not require you to memorize every intermediate in the cycle, but you must know the carbon number changes, the yields of NADH and FADH2, and where carbon dioxide is released. 氧化磷酸化由电子传递链和化学渗透两部分组成。电子传递链位于线粒体内膜上，NADH和FADH2将高能电子传递给一系列电子载体（从复合体I到复合体IV），电子在传递过程中释放的能量用于将氢离子从线粒体基质泵入膜间隙，建立质子浓度梯度。氧气是最终的电子受体，接受电子和氢离子形成水——这就是为什么我们需要呼吸氧气。化学渗透是指氢离子通过ATP合酶从膜间隙流回基质，这个流动过程驱动ATP合酶旋转，催化ADP与磷酸根合成ATP。这一过程称为氧化磷酸化，因为ATP的合成依赖于电子传递（氧化）和ADP磷酸化的耦合。理论上一分子葡萄糖经过完整的细胞呼吸可以产生约38分子ATP（真核细胞实际约为30-32个），其中绝大多数来自氧化磷酸化。IB经常要求你解释质子梯度的建立和ATP合酶的作用机制，务必画图练习。 Oxidative phosphorylation consists of two components: the electron transport chain and chemiosmosis. The electron transport chain is located on the inner mitochondrial membrane. NADH and FADH2 donate high-energy electrons to a series of electron carriers (from Complex I to Complex IV), and the energy released during electron transfer is used to pump hydrogen ions from the mitochondrial matrix into the intermembrane space, establishing a proton concentration gradient. Oxygen is the final electron acceptor, combining with electrons and hydrogen ions to form water — this is why we need to breathe oxygen. Chemiosmosis refers to hydrogen ions flowing back from the intermembrane space into the matrix through ATP synthase, and this flow drives ATP synthase to rotate, catalyzing the synthesis of ATP from ADP and phosphate. This process is called oxidative phosphorylation because ATP synthesis depends on the coupling of electron transport (oxidation) and ADP phosphorylation. Theoretically, one glucose molecule can yield about 38 ATP through complete cellular respiration (approximately 30-32 in eukaryotic cells), with the vast majority coming from oxidative phosphorylation. IB frequently asks you to explain the establishment of the proton gradient and the mechanism of ATP synthase — practice drawing diagrams for this. 细胞膜 Cell Membrane
磷脂双分子层 + 流动镶嵌模型
Phospholipid bilayer + fluid mosaic 运输机制 Transport
简单扩散 / 协助扩散 / 主动运输 / 内吞外排
Simple / Facilitated / Active / Endo-Exocytosis 细胞器 Organelles
核糖体 / 内质网 / 高尔基体 / 线粒体 / 叶绿体
Ribosome / ER / Golgi / Mitochondrion / Chloroplast 有丝分裂 Mitosis
前期 → 中期 → 后期 → 末期 → 胞质分裂
Prophase → Metaphase → Anaphase → Telophase → Cytokinesis 细胞呼吸 Respiration
糖酵解 → 克雷布斯循环 → 氧化磷酸化
Glycolysis → Krebs → Oxidative phosphorylation **IB考试高分技巧 IB Exam High-Scoring Tips** 第一，掌握命令词。IB生物Paper 2和Paper 3中，不同的命令词对应不同的答题深度。"State"只需给出名称或短语，"Describe"需要提供细节描述，"Explain"必须给出原因和机制，"Compare"需要同时指出相同点和不同点。很多同学丢分不是因为不知道答案，而是因为对命令词的理解不到位导致回答不完整。 First, master the command terms. In IB Biology Paper 2 and Paper 3, different command terms require different response depths. "State" only needs a name or phrase; "Describe" requires detailed description; "Explain" must provide reasons and mechanisms; "Compare" requires both similarities and differences. Many students lose marks not because they don't know the answer, but because they misunderstand command terms and provide incomplete responses. 第二，标注结构图。绘图题是IB生物的独特考查方式。你需要能绘制并标注细胞膜的流动镶嵌模型、线粒体结构、叶绿体结构、有丝分裂各阶段。绘图要求：清晰铅笔线条、标注线用直尺、标注名称写在标注线末端。最重要的是：不要画箭头，只用直线标注！ Second, annotate diagrams. Drawing questions are a distinctive feature of IB Biology assessment. You need to be able to draw and annotate the fluid mosaic model of the cell membrane, mitochondrial structure, chloroplast structure, and each stage of mitosis. Drawing requirements: clear pencil lines, labels drawn with a ruler, and label names written at the ends of label lines. Most importantly: do not draw arrows — use only straight label lines! 第三，实验设计思维。IB生物强调科学探究能力。Cell Biology单元中的经典实验包括：用透析管模拟细胞膜的选择透过性、用光学显微镜观察细胞并计算实际大小（放大倍数公式：实际大小 = 图像大小 ÷ 放大倍数）、以及用台盼蓝染色区分活细胞和死细胞。实验题中你需要能提出研究问题、确定自变量和因变量、控制其他变量、并评估实验的局限性和改进方向。 Third, experimental design thinking. IB Biology emphasizes scientific inquiry skills. Classic experiments in the Cell Biology unit include: using dialysis tubing to model selective permeability of cell membranes, using light microscopes to observe cells and calculate actual size (magnification formula: actual size = image size ÷ magnification), and using trypan blue staining to distinguish live cells from dead cells. In experimental questions, you need to formulate research questions, identify independent and dependent variables, control other variables, and evaluate experimental limitations and improvements. **常见误区 Common Misconceptions** 误区一：细胞壁存在于所有植物细胞中，但不存在于动物细胞中。没错。但真菌和细菌也有细胞壁，成分不同——植物细胞壁的主要成分是纤维素，真菌是几丁质，细菌是肽聚糖。IB特别爱考细胞壁成分的比较。误区二：溶酶体只存在于动物细胞中，植物细胞的类似功能由液泡承担。误区三：线粒体和叶绿体都是双层膜细胞器，但叶绿体还有第三层——类囊体膜。误区四：有丝分裂产生两个基因完全相同的子细胞，这是生长和修复的基础。减数分裂产生四个基因不同的子细胞，用于产生配子。不要把两者的目的搞混！ Misconception 1: Cell walls exist in all plant cells but not animal cells. Correct. But fungi and bacteria also have cell walls with different compositions — plant cell walls are primarily cellulose, fungi have chitin, and bacteria have peptidoglycan. IB particularly enjoys testing cell wall composition comparisons. Misconception 2: Lysosomes exist only in animal cells; plant cells use vacuoles for similar functions. Misconception 3: Both mitochondria and chloroplasts are double-membrane organelles, but chloroplasts have a third layer — the thylakoid membrane. Misconception 4: Mitosis produces two genetically identical daughter cells, which is the basis of growth and repair. Meiosis produces four genetically different daughter cells for gamete production. Don't confuse their purposes! 🎓 需要IB生物一对一辅导？ 专注IB / A-Level / AP生物课程辅导，中英双语授课，Paper 1选择题技巧 + Paper 2结构化答题训练 + IA内部评估全程指导。添加微信获取免费试听课。 📱 16621398022 同微信

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