📚 A-Level CIE Biology: Key Revision Points on Plants | A-Level CIE 生物：植物考点精讲
Plants form a fundamental part of the A-Level CIE Biology syllabus, covering topics from cell ultrastructure and transport systems to photosynthesis, reproduction, and responses to the environment. This revision guide condenses the essential concepts and exam-ready details, highlighting the key facts, definitions, and processes you must master. Whether you are preparing for Paper 2 or Paper 4, a strong grasp of plant biology will help you secure high marks in both structured and essay questions.
植物是 A-Level CIE 生物教学大纲的核心内容之一，涵盖从细胞超微结构、运输系统到光合作用、生殖以及对环境响应等多个主题。这份复习指南浓缩了基本概念和应试关键细节，突出了你必须掌握的重要事实、定义和过程。无论你是在准备 Paper 2 还是 Paper 4，扎实的植物生物学知识都能帮助你在结构化问题和论文题中获得高分。
1. Plant Tissues and Cell Types | 植物组织与细胞类型
Plants consist of several distinct tissue types. Epidermal cells form the outermost protective layer, often covered by a waxy cuticle to reduce water loss. Ground tissue includes parenchyma, collenchyma, and sclerenchyma cells. Parenchyma cells are the most abundant, thin-walled, and involved in photosynthesis, storage, and secretion. Collenchyma provides flexible support in growing regions, while sclerenchyma has thick lignified walls for rigid strength. Vascular tissue contains xylem and phloem, responsible for transport.
植物由几种不同的组织类型构成。表皮细胞形成最外层的保护层，通常覆盖着蜡质角质层以减少水分流失。基本组织包括薄壁组织、厚角组织和厚壁组织细胞。薄壁细胞数量最多，细胞壁薄，参与光合作用、储存和分泌。厚角组织在生长区域提供可弯曲的支撑，而厚壁组织具有加厚且木质化的壁，提供刚性支撑。维管组织包含木质部和韧皮部，负责运输。
Xylem vessels are dead at maturity, with lignified walls and hollow lumens, allowing efficient water and mineral transport. Phloem consists of living sieve tube elements and companion cells. Sieve tubes lack nuclei and have perforated sieve plates; companion cells possess nuclei and regulate the metabolic activity of sieve tubes.
木质部导管在成熟时是死的，具有木质化的壁和中空的腔，能高效运输水分和矿物质。韧皮部由活的筛管分子和伴胞组成。筛管没有细胞核，具有穿孔的筛板；伴胞拥有细胞核，调控筛管的代谢活动。
2. The Transport System: Xylem and Phloem Structure | 运输系统：木质部与韧皮部的结构
The xylem transports water and dissolved minerals from roots to shoots. Lignin reinforces the walls in spiral, annular, or reticulate patterns, preventing collapse under tension. Transpiration pull generates the main driving force, and water moves through the apoplast, symplast, and vacuolar pathways in the root cortex before entering the xylem. The Casparian strip in endodermal cells blocks the apoplastic route, forcing water into the symplast at the endodermis, enabling selective ion uptake.
木质部将水分和溶解的矿物质从根部运送到地上部分。木质素以螺纹、环纹或网纹形式加固细胞壁，防止在张力下塌陷。蒸腾拉力产生主要的驱动力，水分通过质外体、共质体和液泡途径在根皮层中移动，然后进入木质部。内皮层细胞中的凯氏带阻断质外体途径，迫使水分在内皮层进入共质体，从而实现选择性的离子吸收。
Phloem transports organic solutes, mainly sucrose and amino acids, from sources (e.g., leaves) to sinks (e.g., roots, fruits). Sieve tube elements form continuous tubes. Companion cells, linked by numerous plasmodesmata, load sucrose into the sieve tubes through active transport. The pressure-flow (mass flow) hypothesis explains phloem translocation.
韧皮部运输有机溶质，主要是蔗糖和氨基酸，从源（如叶片）到库（如根、果实）。筛管分子形成连续的管道。伴胞通过大量胞间连丝相连，利用主动运输将蔗糖装载到筛管中。压力流（集流）假说解释了韧皮部运输。
3. Transpiration and Its Regulation | 蒸腾作用及其调节
Transpiration is the loss of water vapour from aerial plant parts, mainly through stomata. It creates the transpiration pull that draws water up the xylem. Factors affecting transpiration rate include light intensity, temperature, humidity, air movement, and soil water availability. Stomata open when guard cells become turgid due to the influx of potassium ions (K⁺) and water, stimulated by light and low CO₂ concentrations. Abscisic acid (ABA) triggers stomatal closure under water stress by causing loss of K⁺ from guard cells.
蒸腾作用是水分从植物地上部分散失水蒸气的过程，主要通过气孔进行。它产生蒸腾拉力，将水分向上拉入木质部。影响蒸腾速率的因素包括光照强度、温度、湿度、空气流动和土壤水分供应。当保卫细胞由于钾离子（K⁺）和水分流入而变得膨压时，气孔张开，这一过程受光照和低二氧化碳浓度刺激。水分胁迫下，脱落酸（ABA）通过促使保卫细胞失去K⁺而引发气孔关闭。
A potometer can measure water uptake by a leafy shoot, but it does not directly measure transpiration because some water is used in photosynthesis and growth. The apparatus must be airtight, and the shoot cut under water to prevent air embolism.
蒸腾计可以测量带叶枝条的水分吸收量，但不能直接测量蒸腾作用，因为部分水分用于光合作用和生长。装置必须气密，且枝条需在水中剪切以防止空气栓塞。
4. Phloem Loading and the Mass Flow Hypothesis | 韧皮部装载与集流假说
At the source, companion cells actively transport sucrose into sieve tubes, lowering the water potential. Water enters from xylem by osmosis, generating a high hydrostatic pressure. At the sink, sucrose is unloaded, either by diffusion or active transport, increasing water potential and causing water to leave the phloem back to xylem. The pressure gradient drives the mass flow of phloem sap from source to sink. Evidence for mass flow includes the movement of aphid stylets and radioactive tracers; however, the model does not fully explain bidirectional movement in some situations or the precise control of solute distribution.
在源端，伴胞主动运输蔗糖进入筛管，降低水势。水分通过渗透从木质部进入，产生高静水压。在库端，蔗糖被卸载，通过扩散或主动运输，水势升高，水分离开韧皮部回到木质部。压力梯度驱动韧皮部汁液从源到库的质量流动。支持集流假说的证据包括蚜虫口针的移动和放射性示踪剂的使用；然而该模型并不能完全解释某些情况下的双向运输或溶质分配的精确调控。
5. Photosynthesis: Light-Dependent Reactions | 光合作用：光反应
Photosynthesis occurs in chloroplasts. The light-dependent reactions take place in the thylakoid membranes. Chlorophyll a and accessory pigments in photosystems II (PSII) and I (PSI) absorb light energy. PSII absorbs light, exciting electrons that are passed to an electron transport chain; water is photolysed to replace electrons, releasing O₂ and H⁺. The electron transport chain generates a proton gradient across the thylakoid membrane, driving ATP synthesis by chemiosmosis (photophosphorylation). Electrons reach PSI, where light excites them again, and they reduce NADP⁺ to NADPH. Products are ATP, NADPH, and O₂.
光合作用在叶绿体中进行。光反应发生在类囊体膜上。光系统 II（PSII）和光系统 I（PSI）中的叶绿素 a 及辅助色素吸收光能。PSII 吸收光，激发电子并传递到电子传递链；水发生光解以提供电子，释放 O₂ 和 H⁺。电子传递链在类囊体膜两侧形成质子梯度，通过化学渗透驱动 ATP 合成（光合磷酸化）。电子到达 PSI，再次被光激发，将 NADP⁺ 还原为 NADPH。产物是 ATP、NADPH 和 O₂。
Cyclic photophosphorylation involves only PSI and produces ATP without NADPH or O₂. It may provide extra ATP to meet the demands of the Calvin cycle.
环式光合磷酸化只涉及 PSI，产生 ATP 但不生成 NADPH 或 O₂。它可以提供额外的 ATP 以满足卡尔文循环的需求。
6. Photosynthesis: The Calvin Cycle (Light-Independent Reactions) | 光合作用：卡尔文循环（暗反应）
The Calvin cycle occurs in the stroma. It uses ATP and NADPH from the light reactions to fix CO₂. The key steps are: (1) Carbon fixation – CO₂ combines with ribulose bisphosphate (RuBP), catalysed by RuBisCO, forming an unstable 6C compound that splits into two molecules of glycerate-3-phosphate (GP). (2) Reduction – GP is reduced to triose phosphate (TP) using ATP and NADPH. (3) Regeneration – most TP is used to regenerate RuBP, consuming ATP. One-sixth of TP molecules are used to synthesise glucose, sucrose, starch, amino acids, and lipids.
卡尔文循环在基质中进行，利用光反应产生的 ATP 和 NADPH 固定 CO₂。关键步骤：（1）碳固定 – CO₂ 与核酮糖二磷酸（RuBP）结合，由 RuBisCO 催化，形成不稳定的 6C 化合物，分解为两分子甘油酸-3-磷酸（GP）。（2）还原 – GP 被还原为磷酸丙糖（TP），消耗 ATP 和 NADPH。（3）再生 – 大多数 TP 用于再生 RuBP，消耗 ATP。六分之一的 TP 分子用于合成葡萄糖、蔗糖、淀粉、氨基酸和脂类。
Limiting factors of photosynthesis are light intensity, CO₂ concentration, and temperature. At low light, electron excitation is insufficient; at low CO₂, RuBP carboxylation slows; at low or very high temperatures, enzyme activity (especially RuBisCO) is affected, and photorespiration may increase.
光合作用的限制因素是光照强度、CO₂ 浓度和温度。弱光下电子激发不足；CO₂ 浓度低时 RuBP 羧化速率减慢；低温或高温下酶活性（尤其是 RuBisCO）受影响，光呼吸可能增强。
7. Mineral Nutrition and Deficiency Symptoms | 矿物质营养与缺乏症状
Plants require essential mineral elements absorbed from the soil as ions. Nitrate (NO₃⁻) is needed for amino acid and protein synthesis; magnesium (Mg²⁺) is the central atom of chlorophyll. Deficiency in nitrate causes stunted growth and yellowing (chlorosis) of older leaves. Magnesium deficiency leads to interveinal chlorosis because chlorophyll cannot be synthesised. Calcium (Ca²⁺) is important for cell wall formation and membrane stability. Phosphate (PO₄³⁻) is a component of ATP, nucleic acids, and phospholipids.
植物需要从土壤中以离子形式吸收必需矿质元素。硝酸盐（NO₃⁻）用于氨基酸和蛋白质合成；镁（Mg²⁺）是叶绿素的中心原子。缺氮导致生长受阻和老叶黄化。缺镁导致叶脉间失绿，因为无法合成叶绿素。钙（Ca²⁺）对细胞壁形成和膜稳定性很重要。磷酸根（PO₄³⁻）是 ATP、核酸和磷脂的成分。
Root uptake of minerals occurs mainly by active transport, often using proton pumps to create electrochemical gradients that drive ion symport. Mycorrhizal fungi enhance mineral uptake by increasing surface area.
根部对矿物质的吸收主要通过主动运输进行，通常利用质子泵建立电化学梯度，驱动离子协同转运。菌根真菌通过增大表面积增强矿物质吸收。
8. Flower Structure and Pollination | 花的结构与传粉
The flower is the reproductive organ of angiosperms. The male part is the stamen, consisting of anther and filament; the anther contains pollen sacs where pollen grains develop. The female part is the carpel, comprising stigma, style, and ovary; the ovary contains ovules. Some plants have unisexual flowers; others have bisexual flowers. Pollination is the transfer of pollen from anther to stigma. Self-pollination occurs within the same flower or plant; cross-pollination between different plants increases genetic variation.
花是被子植物的生殖器官。雄性部分是雄蕊，由花药和花丝组成；花药含有花粉囊，花粉粒在其中发育。雌性部分是心皮，包括柱头、花柱和子房；子房含有胚珠。有些植物有单性花，另一些有两性花。传粉是花粉从花药转移到柱头的过程。自花传粉发生在同一朵花或同株植物之间；异花传粉在不同植株间进行，增加遗传变异。
Insect-pollinated flowers have large, brightly coloured petals, scent, and nectar to attract insects; pollen is relatively heavy and sticky. Wind-pollinated flowers have small, dull petals, no nectar or scent, produce large amounts of light, smooth pollen, and have feathery stigmas to catch airborne pollen. These adaptations ensure effective transfer of male gametes.
虫媒花具有大而鲜艳的花瓣、气味和花蜜来吸引昆虫；花粉相对较重且粘。风媒花的花瓣小而不显眼，无花蜜或气味，产生大量轻而光滑的花粉，并具羽毛状柱头以捕捉空气中的花粉。这些适应性确保雄配子的有效传递。
9. Fertilisation, Seed and Fruit Development | 受精、种子与果实发育
Following pollination, the pollen grain germinates on the stigma, producing a pollen tube that grows down the style into the ovary and enters the ovule via the micropyle. Double fertilisation occurs: one male gamete fuses with the egg cell to form the diploid zygote; the other male gamete fuses with the two polar nuclei to form the triploid endosperm nucleus. This endosperm acts as nutritive tissue. The zygote develops into the embryo, consisting of a radicle (future root), plumule (future shoot), and one or two cotyledons. The ovule becomes the seed; the ovary wall develops into the fruit, which aids seed dispersal.
传粉后，花粉粒在柱头上萌发，产生花粉管，沿花柱向下生长进入子房，并通过珠孔进入胚珠。发生双受精作用：一个雄配子与卵细胞融合形成二倍体合子；另一个雄配子与两个极核融合形成三倍体胚乳核。胚乳作为营养组织。合子发育为胚，包括胚根（未来的根）、胚芽（未来的芽）和一枚或两枚子叶。胚珠发育为种子；子房壁发育为果实，有助于种子传播。
Seed dormancy and germination are controlled by environmental factors such as water, oxygen, temperature, and light, as well as plant growth regulators like gibberellins. Gibberellins stimulate the production of amylase, which hydrolyses starch into sugars for embryo growth.
种子休眠和萌发受水分、氧气、温度和光照等环境因子以及植物生长调节剂（如赤霉素）的控制。赤霉素刺激淀粉酶产生，将淀粉水解为糖，供胚生长。
10. Plant Hormones: Auxins and Gibberellins | 植物激素：生长素与赤霉素
Auxins, such as indole-3-acetic acid (IAA), regulate cell elongation, apical dominance, and tropisms. IAA is synthetised in shoot tips and young leaves. Uneven distribution of auxin causes differential growth, leading to phototropism and gravitropism. In shoots, higher IAA concentration on the shaded side promotes cell elongation, bending the shoot toward light. In roots, high IAA concentration inhibits elongation, causing the root to bend away from light or towards gravity. Auxins also suppress lateral bud growth, maintaining apical dominance.
生长素，如吲哚-3-乙酸（IAA），调节细胞伸长、顶端优势和向性。IAA 在茎尖和幼叶中合成。生长素的不均匀分布导致差异性生长，引起向光性和向地性。在茎中，背光侧较高的 IAA 浓度促进细胞伸长，使茎向光弯曲。在根中，高 IAA 浓度抑制伸长，使根背光弯曲或向地弯曲。生长素还抑制侧芽生长，维持顶端优势。
Gibberellins promote stem elongation, seed germination, and fruit development. They break seed dormancy in some species by inducing enzyme production. Synthetic auxins and gibberellins are used agriculturally for rooting cuttings, fruit setting, and preventing premature fruit drop.
赤霉素促进茎的伸长、种子萌发和果实发育。它们通过诱导酶的产生打破某些物种的种子休眠。合成的生长素和赤霉素在农业上用于插条生根、座果和防止采前落果。
11. Plant Responses to Environment: Tropisms and Photoperiodism | 植物对环境响应：向性与光周期
Plants respond to directional stimuli through tropisms. Phototropism is growth towards (positive) or away from (negative) light, mediated by auxin redistribution. Gravitropism (geotropism) is growth in response to gravity; roots are positively gravitropic, shoots negatively gravitropic. Statoliths (dense starch grains) in root cap cells detect gravity, causing auxin redistribution that inhibits elongation on the lower side, so roots bend downward.
植物通过向性对方向性刺激作出反应。向光性是朝向（正）或背离（负）光源的生长，由生长素重新分配介导。向地性是响应重力的生长；根为正地性，茎为负地性。根冠细胞中的平衡石（致密淀粉粒）感知重力，引起生长素重新分布，从而抑制下侧的伸长，使根向下弯曲。
Photoperiodism is the response to the relative length of day and night, controlling flowering. Phytochrome pigments exist in two interconvertible forms: Pr and Pfr. Short-day plants flower when night length exceeds a critical value, whereas long-day plants flower when night length is shorter than a critical duration. Phytochrome conversion signals seasonal changes, activating flowering genes.
光周期现象是对昼夜相对长度的响应，控制开花。光敏色素以两种可相互转化的形式存在：Pr 和 Pfr。短日照植物在夜长超过临界值时开花，而长日照植物在夜长短于临界值时开花。光敏色素的转化传递季节变化信号，激活开花基因。
12. Genetically Modified Plants and Agricultural Applications | 转基因植物与农业应用
Genetic engineering has produced crop plants with desirable traits, such as insect resistance (Bt crops), herbicide tolerance, improved nutritional content (Golden Rice with β-carotene), and drought tolerance. The Ti plasmid from Agrobacterium tumefaciens is often used as a vector to introduce foreign genes into plant cells. Transformed cells are selected using marker genes and regenerated into whole plants by tissue culture. This technology raises biosafety and ethical discussions, including gene flow to wild relatives and long-term ecological impact.
基因工程已培育出具有理想性状的作物，例如抗虫性（Bt 作物）、耐除草剂性、改善营养价值（富含 β-胡萝卜素的金大米）和耐旱性。根癌农杆菌的 Ti 质粒常用作载体将外源基因导入植物细胞。通过标记基因筛选转化细胞，并利用组织培养将其再生为完整植株。这项技术引发了生物安全和伦理讨论，包括基因流向野生近缘种以及长期的生态影响。
In the CIE exam, you may be asked to evaluate the advantages and risks of GM crops, linking your answer to specific biological principles such as natural selection, ecosystem dynamics, and food web stability.
在 CIE 考试中，你可能需要评价转基因作物的优势与风险，并将答案与具体的生物学原理相联系，例如自然选择、生态系统动态和食物网稳定性。
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