📚 A-Level Biology: Last-Minute Revision Notes | A-Level 生物：考前冲刺笔记
These concise yet comprehensive notes cover the key topics you are most likely to encounter in your A-Level Biology exam. Use them to identify knowledge gaps, reinforce core principles, and recall essential terminology under time pressure. Each section pairs an explanation in English with its Chinese equivalent, helping you to internalise concepts in both languages and avoid common pitfalls.
这些精简而全面的笔记涵盖了你最可能在 A-Level 生物考试中遇到的关键话题。可以用它们来查漏补缺、巩固核心原理，并在时间紧迫时回忆重要术语。每一节都将英文解释与中文释义配对，帮助你用双语内化概念，避开常见误区。
1. Cell Structure and Function | 细胞结构与功能
Eukaryotic cells, such as those of animals and plants, possess a true nucleus and membrane-bound organelles including mitochondria, the endoplasmic reticulum, and the Golgi apparatus. These compartments allow compartmentalisation of metabolic reactions, increasing efficiency.
真核细胞，比如动物和植物细胞，具有真正的细胞核以及线粒体、内质网和高尔基体等有膜细胞器。这些区室使代谢反应得以分隔，从而提高了效率。
Prokaryotic cells (bacteria) lack a nucleus and membrane-bound organelles; their DNA is a single circular chromosome found in a region called the nucleoid. They also often carry plasmids — small, circular DNA molecules that replicate independently and can confer antibiotic resistance.
原核细胞（细菌）没有细胞核和膜结合细胞器；它们的 DNA 是一个位于拟核区域的环状染色体。它们通常还携带质粒——一种能独立复制并赋予抗生素抗性的小环状 DNA 分子。
Viruses are acellular and non-living. They consist of a nucleic acid core (DNA or RNA) surrounded by a protein coat called a capsid. They can only replicate by hijacking the machinery of a host cell.
病毒是非细胞结构、无生命的个体。它们由核酸核心（DNA 或 RNA）和称为衣壳的蛋白质外壳组成，只能通过劫持宿主细胞的装置来复制。
When examining specimens, you must distinguish between magnification (how many times larger the image appears) and resolution (the ability to distinguish two separate points). Electron microscopes have far higher resolution than light microscopes because electrons have a much shorter wavelength than visible light.
在观察标本时，你必须区分放大倍率（图像被放大的倍数）和分辨率（区分两个独立点的能力）。由于电子波长远小于可见光，电子显微镜的分辨率远高于光学显微镜。
Cell fractionation involves breaking open cells and spinning the homogenate in a centrifuge. At low speeds, the heaviest organelles such as nuclei pellet first; progressively higher speeds sediment mitochondria, lysosomes, and finally ribosomes. This process exploits differences in size and density.
细胞分级分离涉及打碎细胞并将匀浆在离心机中旋转。低速时，最重的细胞器如细胞核首先沉淀；逐渐增加转速可依次沉降线粒体、溶酶体，最后是核糖体。这一过程利用了大小和密度的差异。
2. Biological Molecules | 生物分子
Monosaccharides such as glucose (C₆H₁₂O₆) are the simplest carbohydrates. Glucose is a hexose sugar and the main respiratory substrate. Two monosaccharides join by a glycosidic bond in a condensation reaction to form a disaccharide, e.g. maltose (glucose + glucose).
单糖（如葡萄糖 C₆H₁₂O₆）是最简单的碳水化合物。葡萄糖是一种己糖，也是主要的呼吸底物。两个单糖通过缩合反应形成糖苷键，连接成二糖，例如麦芽糖（葡萄糖 + 葡萄糖）。
Polysaccharides are polymers of monosaccharides. Starch (in plants) consists of amylose (helical, compact) and amylopectin (branched), both serving as energy stores. Glycogen is the animal equivalent, more highly branched. Cellulose, also a glucose polymer, has β-1,4 glycosidic bonds that make it straight and strong, forming plant cell walls.
多糖是单糖的聚合物。淀粉（存在于植物中）由直链淀粉（螺旋状，紧凑）和支链淀粉（分支状）组成，两者都是储能物质。糖原是动物体内的对应物，分支度更高。纤维素同样是葡萄糖聚合物，但具有 β-1,4 糖苷键，使其笔直而坚固，构成植物细胞壁。
Lipids include triglycerides, which contain one glycerol molecule esterified to three fatty acid tails. Triglycerides are hydrophobic and energy-rich. Phospholipids have two fatty acids and a phosphate group, making them amphipathic and ideally suited for bilayer membranes.
脂类包括甘油三酯，它由一个甘油分子与三个脂肪酸尾巴酯化而成。甘油三酯疏水且富含能量。磷脂则含有两个脂肪酸和一个磷酸基团，具有两亲性，非常适合形成双层膜。
Proteins are made of amino acids linked by peptide bonds. A protein’s primary structure is the sequence of amino acids; secondary structure involves α-helices and β-pleated sheets held by hydrogen bonds; tertiary structure is the overall 3D folding stabilised by hydrophobic interactions, ionic bonds, hydrogen bonds, and disulfide bridges; quaternary structure applies to proteins with more than one polypeptide chain.
蛋白质由氨基酸通过肽键连接而成。蛋白质的一级结构是氨基酸序列；二级结构包括由氢键维持的 α-螺旋和 β-折叠；三级结构是疏水作用、离子键、氢键和二硫键共同维持的立体折叠；四级结构适用于含有多条多肽链的蛋白质。
Benedict’s test (for reducing sugars) yields a brick-red precipitate upon heating. Non-reducing sugars must first be hydrolysed. Iodine tests for starch (blue-black), biuret turns purple in the presence of peptide bonds, and the emulsion test identifies lipids as a milky white layer.
本尼迪克特试验（检测还原糖）加热后产生砖红色沉淀。非还原糖必须先水解。碘液检测淀粉呈蓝黑色，双缩脲试剂遇到肽键变为紫色，乳剂试验检测脂类形成乳白色层。
3. Enzymes | 酶
Enzymes are globular proteins that act as biological catalysts, lowering activation energy without being consumed. The induced fit model states that the active site changes shape slightly as the substrate binds, straining bonds and catalysing the reaction more effectively than the earlier lock-and-key model suggested.
酶是球状蛋白质，充当生物催化剂，可降低活化能而不被消耗。诱导契合模型指出，当底物结合时，活性位点形状微调，从而对化学键施加应力，比早期的锁钥模型更有效地催化反应。
The rate of an enzyme-controlled reaction increases with temperature up to an optimum, where kinetic energy is maximised; beyond this point, denaturation occurs as hydrogen and ionic bonds break, permanently altering the active site.
酶促反应速率随温度升高而增加，直至最适温度，此时动能最大；超过这一温度，氢键和离子键断裂导致变性，活性位点永久改变。
Extreme pH also denatures enzymes by disrupting ionic bonds. Each enzyme has a specific optimum pH. Substrate concentration increases rate until the active sites become saturated; at Vₘₐₓ, all active sites are occupied.
极端 pH 值通过破坏离子键使酶变性。每种酶都有特定的最适 pH。底物浓度增加会使速率上升，直到活性位点饱和；在 Vₘₐₓ 时，所有活性位点都被占据。
Competitive inhibitors resemble the substrate and bind to the active site, reducing the proportion of productive collisions. Their effect can be overcome by increasing substrate concentration. Non-competitive inhibitors bind to an allosteric site, altering the active site’s shape and preventing catalysis irreversibly (or non-reversibly) regardless of substrate concentration.
竞争性抑制剂与底物相似，占据活性位点，降低了有效碰撞的比例；增加底物浓度可减弱其效应。非竞争性抑制剂结合到别构位点，改变活性位点形状，从而阻止催化，无论底物浓度如何都难以逆转。
Immobilised enzymes are trapped in a matrix (e.g. alginate beads) and can be reused repeatedly. They are employed in industrial processes such as lactose-free milk production, where lactase hydrolyses lactose into glucose and galactose.
固定化酶被包埋在基质（如海藻酸盐珠）中，可反复使用。它们用于工业生产，例如无乳糖牛奶的制造中，乳糖酶将乳糖水解为葡萄糖和半乳糖。
4. Membranes and Transport | 膜与运输
The fluid mosaic model describes the plasma membrane as a phospholipid bilayer with proteins embedded like a mosaic. The bilayer is fluid because phospholipids and proteins can move laterally. Cholesterol molecules reduce fluidity and increase stability.
流动镶嵌模型将质膜描述为磷脂双层，蛋白质镶嵌其中，如同马赛克。双层具有流动性，因为磷脂和蛋白质可侧向移动。胆固醇分子降低流动性并增加稳定性。
Passive transport does not require metabolic energy. Simple diffusion moves small, non-polar molecules (O₂, CO₂) directly through the bilayer down a concentration gradient. Facilitated diffusion uses carrier or channel proteins, still down the gradient but enabling polar or charged substances to cross.
被动运输不需要代谢能量。简单扩散使小的非极性分子（O₂, CO₂）顺浓度梯度直接穿过双层。易化扩散利用载体或通道蛋白，同样顺梯度，但允许极性或带电荷物质通过。
Active transport moves ions or molecules against a concentration gradient, requiring ATP hydrolysis. The sodium–potassium pump expels 3 Na⁺ and imports 2 K⁺ per ATP, establishing electrochemical gradients essential for nerve impulses.
主动运输逆浓度梯度移动离子或分子，需要 ATP 水解。钠钾泵每水解一个 ATP 可排出 3 个 Na⁺、泵入 2 个 K⁺，建立神经冲动所必需的电化学梯度。
Co-transport couples the movement of one solute down its gradient to drive another solute against its gradient. In the ileum, Na⁺ moving into epithelial cells via facilitated diffusion provides energy for glucose to be co-transported alongside Na⁺, even against its own concentration gradient.
协同转运将一种溶质顺梯度的运动与另一种溶质逆梯度的运动偶联起来。在回肠中，Na⁺ 经易化扩散进入小肠上皮细胞，为葡萄糖随 Na⁺ 一同转运提供能量，即使葡萄糖逆自身浓度梯度也能进入。
Osmosis is the net movement of water from a region of higher water potential to a region of lower water potential across a partially permeable membrane. Water potential is determined by solute and pressure potentials; pure water has a water potential of zero.
渗透作用是水通过部分透性膜从水势较高的区域向水势较低的区域净移动。水势由溶质势和压力势决定，纯水的水势为零。
5. The Immune System | 免疫系统
Non-specific defences include physical barriers like skin and mucus, as well as phagocytosis. Phagocytes recognise non-self antigens, engulf pathogens into a phagosome, fuse it with lysosomes, and destroy the pathogen using hydrolytic enzymes.
非特异性防御包括皮肤、黏液等物理屏障，以及吞噬作用。吞噬细胞识别非己抗原，将病原体吞入吞噬体，与溶酶体融合，利用水解酶消灭病原体。
Specific immunity involves lymphocytes. T helper cells (Tₕ) bind to antigen-presenting cells and release cytokines that stimulate B cells and cytotoxic T cells. Cytotoxic T cells destroy infected body cells by releasing perforin, causing cell lysis.
特异性免疫涉及淋巴细胞。辅助性 T 细胞（Tₕ）与抗原呈递细胞结合，释放细胞因子刺激 B 细胞和细胞毒性 T 细胞。细胞毒性 T 细胞通过释放穿孔素裂解受感染的体细胞。
B cells, when activated, differentiate into plasma cells that secrete large quantities of antibodies. Antibodies are Y-shaped proteins with variable regions specific to antigens; they agglutinate pathogens, neutralise toxins, and mark invaders for phagocytosis.
B 细胞被激活后分化为浆细胞，分泌大量抗体。抗体是 Y 形蛋白质，其可变区与抗原特异性结合；它们可凝集病原体、中和毒素，并标记入侵者以便吞噬。
Vaccination introduces antigens (often weakened or inactivated pathogens) to stimulate the production of memory cells without causing severe disease. This confers active artificial immunity. When re-exposed, the secondary response is faster and stronger, often preventing illness.
疫苗接种通过引入抗原（通常是灭活或减毒的病原体），在不引起严重疾病的情况下刺激记忆细胞产生，从而赋予人工主动免疫。再次接触时，二次应答更快、更强，通常能阻止发病。
HIV attacks T helper cells, crippling the immune system and leading to AIDS. It is a retrovirus that uses reverse transcriptase to integrate its RNA into the host DNA, making treatment difficult.
HIV 攻击辅助性 T 细胞，摧毁免疫系统，导致艾滋病。它是一种逆转录病毒，利用逆转录酶将其 RNA 整合进宿主 DNA，使治疗变得困难。
6. Gas Exchange | 气体交换
Mammalian lungs contain millions of alveoli, tiny air sacs surrounded by a dense capillary network. The alveolar wall is one cell thick, providing a short diffusion distance. Ventilation maintains steep concentration gradients for O₂ and CO₂.
哺乳动物的肺包含数百万个肺泡——密布毛细血管的微小气囊。肺泡壁只有一层细胞厚，扩散距离短。通气作用维持了 O₂ 和 CO₂ 的陡峭浓度梯度。
In fish, gills extract oxygen from water using a countercurrent exchange system: blood flows in the opposite direction to water across the gill lamellae. This ensures a constant diffusion gradient along the entire length, achieving over 80% extraction efficiency.
在鱼类中，鳃利用逆流交换系统从水中获取氧气：血液流过鳃丝的方向与水流方向相反。这沿整个交换面保持了恒定的扩散梯度，萃取效率可超过 80%。
Insects breathe through a tracheal system — a network of tubes delivering air directly to tissues. Spiracles on the body surface can close to reduce water loss. Muscle contraction ventilates the trachea, and larger insects may pump the abdomen to aid gas exchange.
昆虫通过气管系统呼吸——管网直接将空气输送到组织。体表的气孔可关闭以减少水分流失。肌肉收缩可通气气管，较大昆虫还可能泵动腹部帮助气体交换。
In a dicotyledonous leaf, stomata are usually more abundant on the lower epidermis. Each stoma is bordered by two guard cells that change shape in response to light and water availability, balancing gas exchange with water loss.
在双子叶植物叶片中，气孔通常在下表皮更为丰富。每个气孔由两个保卫细胞围绕，根据光照和水分供应改变形状，以平衡气体交换与水分流失。
7. Transport in Plants | 植物运输
Xylem vessels transport water and dissolved minerals from roots to shoots. Cohesion-tension theory explains that water evaporates from mesophyll cells, creating negative pressure (tension) that pulls a continuous column of water upward, aided by cohesion between water molecules and adhesion to xylem walls.
木质部导管将水和溶解的矿物质从根系运输到地上部分。内聚力-张力假说认为，水分从叶肉细胞蒸发产生负压（张力），拉动连续水柱向上，水分子间的内聚力以及与木质部壁的附着力共同协助这一过程。
Transpiration — the evaporation of water vapour from stomata — is a passive process driven by the water potential gradient. Factors that increase transpiration include higher temperature, lower humidity, stronger wind, and greater light intensity.
蒸腾作用即水蒸气从气孔蒸发，是一个由水势梯度驱动的被动过程。升高的温度、降低的湿度、增强的风速和更强的光照都会增加蒸腾速率。
Phloem translocates organic solutes, primarily sucrose, from sources (e.g. leaves) to sinks (e.g. roots, fruits). The mass flow hypothesis describes active loading of sucrose into sieve tubes at the source, which lowers water potential and draws water in by osmosis, creating a hydrostatic pressure gradient that pushes sap towards sinks.
韧皮部将有机物（主要是蔗糖）从源（如叶片）转运到库（如根、果实）。质量流假说认为，源端主动将蔗糖装载进筛管，降低水势并渗透吸水，产生静水压力梯度，推动汁液流向库。
Companion cells, with their dense cytoplasm and numerous mitochondria, provide the ATP required for active loading of sucrose. Plasmodesmata connect companion cells to sieve tube elements, facilitating the movement of solutes.
伴胞具有浓密的细胞质和大量线粒体，为蔗糖的主动装载提供 ATP。胞间连丝将伴胞与筛管分子相连，促进溶质运输。
8. Nervous Coordination | 神经协调
The resting potential of a neurone (approx. –70 mV) is maintained by the Na⁺/K⁺ pump, which actively transports 3 Na⁺ out and 2 K⁺ in, and by K⁺ leak channels that allow K⁺ to diffuse out, making the inside more negative relative to the outside.
神经元的静息电位（约 –70 mV）由钠钾泵（主动泵出 3 Na⁺、泵入 2 K⁺）和钾离子泄漏通道（允许 K⁺ 外流）共同维持，使膜内相对膜外为负。
An action potential occurs if the membrane is depolarised to the threshold (≈ –55 mV). Voltage-gated Na⁺ channels open, causing a rapid influx of Na⁺ and further depolarisation. As these channels inactivate, voltage-gated K⁺ channels open, allowing K⁺ to efflux and repolarise the membrane, often leading to a brief hyperpolarisation.
如果膜去极化达到阈电位（约 –55 mV），就会产生动作电位。电压门控 Na⁺ 通道开放，Na⁺ 迅速内流，进一步去极化；随后 Na⁺ 通道失活，电压门控 K⁺ 通道开放，K⁺ 外流使膜复极化，通常出现短暂的超极化。
Myelinated axons conduct impulses faster through saltatory conduction: action potentials jump from one node of Ranvier to the next, because the myelin sheath prevents ion leakage and capacitors the membrane, so that depolarisation only occurs at the nodes.
有髓轴突通过跳跃传导加快冲动传导：动作电位从一个郎飞结跳到下一个结，因为髓鞘防止离子泄漏，并使膜具有电容性，只有结区发生去极化。
At a synapse, an arriving action potential opens voltage-gated Ca²⁺ channels, triggering vesicles of neurotransmitter to fuse with the presynaptic membrane. The transmitter diffuses across the cleft and binds to receptors on the postsynaptic membrane, opening ligand-gated ion channels and potentially generating an excitatory postsynaptic potential.
在突触处，到达的动作电位打开电压门控 Ca²⁺ 通道，触发含神经递质的囊泡与突触前膜融合。递质扩散跨过间隙，与突触后膜受体结合，打开配体门控离子通道，可能产生兴奋性突触后电位。
9. Homeostasis and the Kidney | 稳态与肾脏
Homeostasis involves maintaining a stable internal environment through negative feedback. In blood glucose regulation, high glucose stimulates β cells in the pancreas to secrete insulin, promoting cell uptake and glycogenesis; low glucose triggers α cells to release glucagon, stimulating glycogenolysis and gluconeogenesis.
稳态通过负反馈维持
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