📚 A-Level WJEC Science: Plants Essential Revision Notes | 植物考点精讲
Plants form the foundation of most ecosystems and are central to many topics in the WJEC Science specification. This revision guide covers the key concepts you need to master: from cellular ultrastructure and photosynthesis to transport systems, hormones, reproduction, and responses to the environment. Each section is broken down to align with the demands of A-Level examination questions, with clear bilingual explanations to reinforce understanding.
植物是大多数生态系统的基础，也是 WJEC 科学考试大纲中的核心主题。本复习指南涵盖你需要掌握的关键概念：从细胞超微结构和光合作用，到运输系统、激素、繁殖以及对环境的响应。每个部分都按照 A-Level 考试要求进行拆解，并以清晰的双语解释来加深理解。
1. Plant Cell Ultrastructure | 植物细胞超微结构
Plant cells possess all the typical eukaryotic organelles, but three structures are particularly distinctive: a rigid cellulose cell wall, a large permanent vacuole, and chloroplasts. The cell wall provides structural support and prevents osmotic lysis. It is composed of cellulose microfibrils embedded in a matrix of hemicellulose and pectin, and neighbouring cells are connected via plasmodesmata – thin cytoplasmic channels that allow symplastic transport.
植物细胞具有所有典型的真核细胞器，但有三种结构特别突出：坚硬的纤维素细胞壁、大型永久液泡和叶绿体。细胞壁提供结构支撑并防止渗透裂解。它由嵌在半纤维素和果胶基质中的纤维素微纤维组成，相邻细胞通过胞间连丝——允许共质体运输的薄细胞质通道——相连。
The vacuole, enclosed by a tonoplast, stores water, ions, sugars, and pigments, and generates turgor pressure when the cell is fully hydrated. Chloroplasts are the site of photosynthesis; their internal membrane system consists of thylakoids stacked into grana, surrounded by the stroma. The thylakoid membrane houses chlorophyll and the protein complexes of the light-dependent reactions.
液泡由液泡膜包围，储存水分、离子、糖分和色素，当细胞充分吸水时产生膨压。叶绿体是光合作用的场所；其内部膜系统由堆叠成基粒的类囊体组成，周围是基质。类囊体膜上含有叶绿素和光反应相关的蛋白质复合物。
2. Photosynthesis: Light-Dependent Stage | 光合作用：光反应阶段
Photosynthesis occurs in two stages. The light-dependent stage takes place on the thylakoid membranes and converts light energy into chemical energy in the form of ATP and reduced NADP (NADPH). Two photosystems – Photosystem II (PSII) and Photosystem I (PSI) – work in series. Photons excite electrons in PSII, which are then passed along an electron transport chain (ETC) to PSI. The energy released pumps H⁺ ions from the stroma into the thylakoid lumen, creating a proton gradient.
光合作用分为两个阶段。光反应阶段发生在类囊体膜上，将光能转化为 ATP 和还原型 NADP（NADPH）形式的化学能。两个光系统——光系统 II（PSII）和光系统 I（PSI）——串联工作。光子激发 PSII 中的电子，电子随后沿电子传递链传递到 PSI。释放的能量将 H⁺ 离子从基质泵入类囊体腔，形成质子梯度。
The splitting of water (photolysis) at PSII replaces the lost electrons and produces oxygen and protons:
2H₂O → 4H⁺ + 4e⁻ + O₂
The proton gradient drives ATP synthase to produce ATP (chemiosmosis). At PSI, re-excited electrons reduce NADP⁺ to NADPH. Thus the light-dependent stage yields ATP, NADPH, and O₂ as a by-product.
PSII 处的水裂解（光解）补充丢失的电子，产生氧气和质子：反应式如上。质子梯度驱动 ATP 合酶生成 ATP（化学渗透）。在 PSI，再次激发的电子将 NADP⁺ 还原为 NADPH。因此光反应产生 ATP、NADPH 和副产物 O₂。
3. Photosynthesis: Light-Independent Stage (Calvin Cycle) | 光合作用：暗反应（卡尔文循环）
The light-independent reactions take place in the stroma and use ATP and NADPH to fix CO₂ into organic molecules. The Calvin cycle has three phases: carbon fixation, reduction, and regeneration of the CO₂ acceptor ribulose bisphosphate (RuBP). The enzyme RuBisCO catalyses the reaction between CO₂ and RuBP (a 5‑carbon compound) to form two molecules of 3‑phosphoglycerate (3‑PGA), a 3‑carbon compound.
暗反应发生在基质中，利用 ATP 和 NADPH 将 CO₂ 固定为有机分子。卡尔文循环有三个阶段：碳固定、还原以及 CO₂ 受体核酮糖二磷酸（RuBP）的再生。RuBisCO 酶催化 CO₂ 与 RuBP（5 碳化合物）反应，生成两分子 3‑磷酸甘油酸（3‑PGA），一种 3 碳化合物。
3‑PGA is then phosphorylated by ATP and reduced by NADPH to form glyceraldehyde-3‑phosphate (G3P). For every three CO₂ molecules fixed, six G3P molecules are produced; one exits the cycle to form glucose and other carbohydrates, while the remaining five are used to regenerate three molecules of RuBP, consuming further ATP. The overall balanced equation for photosynthesis is:
3‑PGA 随后被 ATP 磷酸化并被 NADPH 还原，形成甘油醛‑3‑磷酸（G3P）。每固定三个 CO₂ 分子，产生六个 G3P 分子；其中一个离开循环用于形成葡萄糖和其他碳水化合物，其余五个用于再生三个 RuBP 分子，并消耗更多 ATP。光合作用总方程式为：
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
4. Factors Affecting Photosynthesis and Limiting Factors | 影响光合作用的因素及限制因子
The rate of photosynthesis is influenced by light intensity, carbon dioxide concentration, and temperature. In WJEC examinations, you must interpret graphs showing the effect of these factors and identify the limiting factor at any given point. At low light intensity, light is the limiting factor; as intensity increases, the rate rises until another factor, often CO₂ concentration or temperature, becomes limiting.
光合作用速率受光照强度、二氧化碳浓度和温度的影响。在 WJEC 考试中，你必须解读这些因素的作用图，并确定任意给定点处的限制因子。在低光照强度下，光是限制因子；随着强度增加，速率上升，直到另一个因子（通常是 CO₂ 浓度或温度）成为限制。
Temperature affects enzyme activity, particularly RuBisCO. At low temperatures, kinetic energy is low; at optimum temperatures (around 25 °C in C3 plants), the rate is maximal; above the optimum, RuBisCO denatures and the rate falls sharply. CO₂ concentration directly limits the Calvin cycle because CO₂ is the substrate for RuBisCO. Growers often enrich greenhouses with CO₂ to maximise yield. Water availability is seldom directly limiting because stomatal closure to conserve water also restricts CO₂ entry.
温度影响酶活性，特别是 RuBisCO。低温时动能低；在最适温度（C3 植物约 25 °C）下速率最大；超过最适温度，RuBisCO 变性，速率急剧下降。CO₂ 浓度直接限制卡尔文循环，因为 CO₂ 是 RuBisCO 的底物。种植者常给温室增施 CO₂ 以最大化产量。水分供应通常不是直接限制因子，因为关闭气孔保水的同时也限制了 CO₂ 进入。
5. Transpiration and Water Transport | 蒸腾作用与水分运输
Water is transported from roots to leaves through the xylem. The cohesion-tension theory explains this movement. Transpiration at the leaf surface – evaporation of water from mesophyll cell walls into air spaces and out through stomata – generates a negative pressure (tension) at the top of the water column. Water molecules exhibit strong cohesion due to hydrogen bonding, allowing the tension to pull a continuous column of water upwards. Adhesion of water to xylem vessel walls also aids capillary rise.
水分通过木质部从根部运输到叶片。内聚力-张力理论解释了这一运动。叶片表面的蒸腾作用——水分从叶肉细胞壁蒸发到气室并经由气孔散出——在水柱顶部产生负压（张力）。水分子因氢键具有很强的内聚力，使张力能向上拉动连续的水柱。水对木质部导管壁的附着力也有助于毛细上升。
The rate of transpiration is measured using a potometer and depends on light intensity, temperature, humidity, and air movement. Stomatal opening is regulated by guard cells through changes in turgor pressure driven by K⁺ ion fluxes. In WJEC tasks, you may be asked to design investigations into stomatal distribution or transpiration rate under varying conditions.
蒸腾速率用蒸腾计测量，取决于光照强度、温度、湿度和空气流动。气孔开闭由保卫细胞通过 K⁺ 离子流驱动的膨压变化调节。在 WJEC 任务中，你可能会被要求设计调查气孔分布或不同条件下蒸腾速率的实验。
6. Translocation and the Phloem | 运输作用与韧皮部
Translocation is the movement of assimilates, mainly sucrose and amino acids, from sources (e.g. mature leaves) to sinks (e.g. roots, developing fruits) through the phloem. The mass flow hypothesis explains this bidirectional transport. At the source, sucrose is actively loaded into companion cells and then into sieve tube elements, lowering the water potential. Water enters from adjacent xylem, increasing hydrostatic pressure.
运输作用是指同化物（主要是蔗糖和氨基酸）通过韧皮部从源（如成熟叶片）向库（如根部、发育中的果实）的移动。集流假说解释了这种双向运输。在源端，蔗糖被主动装载到伴胞，然后进入筛管分子，降低了水势。水从相邻木质部进入，增加静水压力。
At the sink, sucrose is unloaded and used in respiration or stored as starch, causing water to leave the phloem and reducing hydrostatic pressure. The resulting pressure gradient drives the mass flow of phloem sap from source to sink. Evidence for this model includes aphid stylectomy studies showing positive pressure and the presence of sucrose gradients.
在库端，蔗糖被卸载并用于呼吸或被储存为淀粉，导致水离开韧皮部并降低静水压力。由此产生的压力梯度驱动韧皮部汁液从源到库的集流。支持该模型的证据包括蚜虫口针切断研究显示的静压力和蔗糖梯度的存在。
7. Plant Hormones and Tropisms | 植物激素与向性
Plants coordinate growth and responses to stimuli using chemical signals. Indole-3-acetic acid (IAA) is the most important auxin, controlling cell elongation, apical dominance, and tropisms. Phototropism in shoot tips results from the lateral redistribution of IAA to the shaded side, where it promotes elongation, causing the shoot to bend towards the light. Gravitropism in roots involves IAA accumulation on the lower side, which in roots inhibits elongation, causing downward bending.
植物利用化学信号协调生长和对刺激的响应。吲哚‑3‑乙酸（IAA）是最重要的生长素，控制细胞伸长、顶端优势和向性。茎尖的向光性源于 IAA 向背光一侧的横向再分配，在那里促进伸长，导致茎向光弯曲。根的向地性涉及 IAA 在下侧的积累，在根中抑制伸长，导致向下弯曲。
Gibberellins promote stem elongation, seed germination by stimulating amylase production, and fruit development. Ethene is a gaseous hormone that triggers fruit ripening and leaf abscission. WJEC questions often ask you to explain experimental evidence for hormone action, such as decapitation and agar block experiments that demonstrated the role of diffusible auxin in phototropism.
赤霉素促进茎的伸长、通过刺激淀粉酶产生促进种子萌发以及果实发育。乙烯是一种气体激素，引发果实成熟和叶片脱落。WJEC 考题常要求解释激素作用的实验证据，如去顶和琼脂块实验，证明了可扩散的生长素在向光性中的作用。
8. Plant Reproduction: Structures and Double Fertilisation | 植物繁殖：结构与双受精
Flowering plants reproduce sexually. The anther produces pollen grains containing the male gametophyte; the ovule contains the embryo sac with the female gametophyte. Pollination, the transfer of pollen to the stigma, may be self- or cross-pollination. Following compatible pollination, the pollen grain germinates, and a pollen tube grows down the style into the ovary, navigating via chemotaxis.
开花植物进行有性繁殖。花药产生含有雄配子体的花粉粒；胚珠含有带雌配子体的胚囊。传粉是花粉传到柱头的过程，可以是自花传粉或异花传粉。亲和传粉后，花粉粒萌发，花粉管沿着花柱向下生长进入子房，通过化学趋向性导航。
Angiosperms exhibit double fertilisation. One sperm nucleus fuses with the egg cell to form a diploid zygote (2n); the other sperm nucleus fuses with two polar nuclei to form a triploid endosperm (3n), which provides nourishment for the developing embryo. The ovule matures into a seed, and the ovary develops into the fruit. You should be able to label a diagram of a typical flower and describe the fate of each floral part after fertilisation.
被子植物表现双受精。一个精子核与卵细胞融合形成二倍体合子（2n）；另一个精子核与两个极核融合形成三倍体胚乳（3n），为发育中的胚胎提供营养。胚珠成熟为种子，子房发育成果实。你应该能够标注典型花的图示并描述受精后各花部的命运。
9. Seed Germination and Dormancy | 种子萌发与休眠
A seed is a dormant plant embryo enclosed in a protective seed coat (testa). Germination resumes growth when internal and external conditions are favourable. Essential external factors include water for rehydration and mobilisation of enzymes, oxygen for aerobic respiration, and a suitable temperature. Water uptake activates gibberellins, which diffuse to the aleurone layer and trigger synthesis of amylase. Amylase hydrolyses starch in the endosperm into maltose and then glucose, which is transported to the embryo for respiration and growth.
种子是包覆在保护性种皮内的休眠植物胚胎。当内外条件适宜时，萌发恢复生长。必需的外部因素包括：水分用于再水化并调动酶，氧气用于有氧呼吸，以及适宜的温度。吸水激活赤霉素，后者扩散到糊粉层并触发淀粉酶的合成。淀粉酶将胚乳中的淀粉水解为麦芽糖，进而转化为葡萄糖，运输到胚供呼吸和生长。
Dormancy can be imposed by the seed coat (physical dormancy) or by chemical inhibitors such as abscisic acid. Scarification, stratification (cold treatment), and removal of inhibitors can break dormancy. WJEC practical investigations often involve testing the effect of temperature, light, or scarification on germination rates.
休眠可由种皮（物理休眠）或化学抑制剂如脱落酸引起。破皮、层积（低温处理）和去除抑制剂可以打破休眠。WJEC 的实验调查常涉及测试温度、光照或破皮对萌发率的影响。
10. Mineral Nutrition and the Nitrogen Cycle | 矿物质营养与氮循环
Plants require several mineral ions obtained from the soil. Nitrate ions (NO₃⁻) are essential for amino acid, protein, and nucleic acid synthesis. Magnesium ions (Mg²⁺) are the central atom of the chlorophyll molecule. Deficiency symptoms include chlorosis (yellowing) for nitrogen or magnesium deficiency, and stunted growth for phosphate or potassium deficiency. The nitrogen cycle describes how nitrogen is converted between its various chemical forms.
植物需要从土壤中获取多种矿质离子。硝酸根离子（NO₃⁻）对氨基酸、蛋白质和核酸的合成至关重要。镁离子（Mg²⁺）是叶绿素分子的中心原子。缺素症状包括缺氮或缺镁时的黄化，以及缺磷或缺钾时的生长迟缓。氮循环描述了氮在不同化学形态之间的转化。
Key processes include nitrogen fixation (N₂ → NH₃/NH₄⁺) by free-living bacteria (e.g. Azotobacter) or symbiotic bacteria (Rhizobium in root nodules of legumes); nitrification (NH₄⁺ → NO₂⁻ → NO₃⁻) by nitrifying bacteria; assimilation of nitrates by plants; ammonification of organic nitrogen by decomposers; and denitrification (NO₃⁻ → N₂) under anaerobic conditions. Legumes provide root nodules with carbohydrates, and in return receive fixed nitrogen – a mutualistic relationship.
关键过程包括：固氮作用（N₂ → NH₃/NH₄⁺），由自由生活的细菌（如固氮菌）或共生细菌（豆科植物根瘤中的根瘤菌）完成；硝化作用（NH₄⁺ → NO₂⁻ → NO₃⁻）由硝化细菌进行；植物对硝酸盐的同化；分解者对有机氮的氨化作用；以及在厌氧条件下的反硝化作用（NO₃⁻ → N₂）。豆科植物为根瘤提供碳水化合物，作为回报获得固定氮——一种互利共生关系。
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