📚 IB Edexcel Science: Plant Biology Key Points Revision | IB Edexcel 科学：植物生物学考点精讲
Plants are essential autotrophic organisms that convert light energy into chemical energy through photosynthesis, forming the base of most food chains. In IB and Edexcel Science curricula, plant biology covers key topics such as photosynthesis, transport systems, reproduction, and responses to stimuli. This revision guide presents the core concepts, common experimental setups, and frequent exam pitfalls in a bilingual format, helping you master plant biology with confidence.
植物是基础的自养生物，通过光合作用将光能转化为化学能，构成了大多数食物链的起点。在IB和Edexcel科学课程中，植物生物学涵盖了光合作用、运输系统、繁殖以及对刺激的响应等关键主题。这份双语考点精讲梳理了核心概念、常见实验装置和常考易错点，帮助你自信掌握植物生物学。
1. Photosynthesis Equation and Limiting Factors | 光合作用方程与限制因素
Photosynthesis is the process by which green plants synthesise glucose from carbon dioxide and water using light energy captured by chlorophyll. The overall word equation is carbon dioxide + water → glucose + oxygen, and the balanced chemical equation is shown below.
光合作用是绿色植物利用叶绿素捕获的光能，将二氧化碳和水合成葡萄糖的过程。总文字方程式为二氧化碳 + 水 → 葡萄糖 + 氧气，其化学方程式如下。
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Light intensity, carbon dioxide concentration, and temperature are the three main limiting factors. As one factor increases, the rate of photosynthesis rises until another factor becomes the limiting agent. At low light intensities, the rate is directly proportional to light intensity. Beyond the light saturation point, further increases in light have no effect unless CO₂ or temperature is also raised.
光照强度、二氧化碳浓度和温度是三个主要的限制因素。随着某一因素增强，光合作用速率上升，直至另一因素成为限制因子。在弱光下，速率与光照强度成正比；超过光饱和点后，再增加光照也无济于事，除非同时提高二氧化碳浓度或温度。
Graphs of photosynthesis rate against these factors show plateaus. For temperature, the rate increases up to an optimum (around 25–30 °C for many plants), after which enzymes denature and the rate drops sharply. Examiners often ask to interpret graphs with multiple lines, such as showing rate vs light at different CO₂ levels.
光合速率随这些因素变化的曲线会出现平台期。对温度而言，速率上升到最适温度（许多植物约25–30 °C）后，酶变性导致速率急剧下降。考官常要求分析多条曲线图，如不同二氧化碳浓度下光合速率随光照的变化。
2. Leaf Structure and Adaptations | 叶片结构与适应性
The leaf is the primary organ of photosynthesis, and its internal structure is highly adapted for efficient light absorption, gas exchange, and water transport. The main tissues include the upper and lower epidermis, palisade mesophyll, spongy mesophyll, and vascular bundles (xylem and phloem).
叶片是光合作用的主要器官，其内部结构高度适应高效吸光、气体交换和水分运输。主要组织包括上、下表皮、栅栏状叶肉、海绵状叶肉和维管束（木质部与韧皮部）。
The palisade mesophyll cells are elongated, tightly packed, and contain abundant chloroplasts near the upper surface to maximise light capture. The spongy mesophyll has irregularly shaped cells with large air spaces to facilitate diffusion of CO₂ and O₂. Vascular bundles are located in the midrib and veins, with xylem transporting water upward and phloem carrying sucrose away.
栅栏叶肉细胞长柱形、排列紧密，且富含叶绿体，靠近上表面，以最大限度地捕获光能。海绵叶肉细胞形状不规则，细胞间有大气隙，便于二氧化碳和氧气扩散。维管束位于中脉和叶脉中，木质部向上运输水分，韧皮部运出蔗糖。
Edexcel IGCSE often requires labelling a cross-section diagram of a leaf. IB may ask for structural adaptations in relation to photosynthesis, such as waxy cuticle preventing water loss, stomata on the lower epidermis for gas exchange, and the transparent epidermis allowing light penetration.
Edexcel IGCSE 常要求标注叶片横切图。IB 可能问及与光合作用相关的结构适应，如蜡质角质层防止水分散失、下表皮的气孔用于气体交换，以及透明的表皮层允许光线透过。
3. Stomata and Gas Exchange | 气孔与气体交换
Stomata are microscopic pores mainly found on the lower leaf epidermis. Each stoma is surrounded by two guard cells that change shape to open or close the pore. During the day, guard cells take up potassium ions and water by active transport and osmosis, becoming turgid and causing the stoma to open. This allows carbon dioxide to enter for photosynthesis, but also results in water vapour loss through transpiration.
气孔是主要分布在下表皮的微小孔隙。每个气孔由两个保卫细胞包围，保卫细胞改变形状以开启或关闭气孔。白天，保卫细胞通过主动运输和渗透作用吸收钾离子和水分，变得膨大，使气孔张开。这使二氧化碳进入进行光合作用，但也导致水蒸气通过蒸腾作用散失。
At night, photosynthesis stops; guard cells lose ions and water, become flaccid, and the stoma closes, reducing water loss. Stomatal opening is influenced by light, CO₂ concentration, and water availability. Excessive water loss causes stomata to close even during the day.
夜间光合作用停止，保卫细胞丧失离子和水分，变得松弛，气孔关闭，减少水分损失。气孔的开启受光照、二氧化碳浓度和水分供应影响。过度失水会导致白天也关闭气孔。
4. Transpiration and Factors Affecting It | 蒸腾作用及其影响因素
Transpiration is the evaporation of water vapour from the surface of mesophyll cells followed by diffusion out through stomata. It provides the driving force for water uptake and mineral transport from the roots to the leaves, and also cools the plant.
蒸腾作用是水分从叶肉细胞表面蒸发，然后通过气孔扩散出去的过程。它为水分吸收和矿物质从根部向上运输提供驱动力，同时起到冷却植物的作用。
Factors that increase transpiration rate include higher temperature (increases kinetic energy of water molecules), lower humidity (steeper diffusion gradient), greater air movement (removes water vapour quickly), and more intense light (stomata open wider). A potometer is the apparatus used to measure the rate of water uptake, which serves as an estimate of transpiration rate.
提高蒸腾速率的因素包括较高温度（增加水分子动能）、较低湿度（扩大扩散梯度）、较强空气流动（快速带走水蒸气）以及较强光照（气孔开度更大）。蒸腾计是用来测量吸水速率的装置，以此估算蒸腾速率。
In experiments, potometer readings must be interpreted with care: the water uptake is slightly greater than the transpiration rate because some water is used in photosynthesis and turgidity. Common exam questions ask how to set up a potometer and how to apply a bubble or measure distance moved per time.
实验中，需谨慎解读蒸腾计读数：吸水速率略大于蒸腾速率，因为部分水用于光合作用和维持膨压。常见考题问及如何组装蒸腾计、如何使用气泡或测量单位时间移动距离。
5. Xylem and Phloem: Structure and Function | 木质部与韧皮部：结构与功能
Xylem vessels are dead, hollow tubes formed from cells laid end to end with no end walls. Their walls are thickened with lignin, providing strength and waterproofing. Xylem transports water and dissolved mineral ions upwards from the roots to the leaves. The movement is passive and driven by transpiration pull, cohesion of water molecules, and adhesion to xylem walls (cohesion-tension theory).
木质部导管是由死细胞首尾相连形成的中空管，无端壁。其管壁由木质素加厚，提供强度并防水。木质部将水和溶解的矿物质离子从根部向上运输到叶片。这种运输是被动的，由蒸腾拉力、水分子间的内聚力以及与管壁的附着力驱动（内聚力-张力理论）。
Phloem is composed of living cells: sieve tube elements and companion cells. Sieve tubes have perforated end walls called sieve plates and very little cytoplasm, allowing sugars to flow. Companion cells possess numerous mitochondria and provide energy for active loading of sucrose into the phloem. Phloem transports assimilates, mainly sucrose and amino acids, both upward and downward (translocation).
韧皮部由活细胞组成：筛管分子和伴胞。筛管具有带筛孔的端壁（筛板），细胞质极少，便于糖分流动。伴胞含有大量线粒体，为将蔗糖主动装载到韧皮部提供能量。韧皮部运输同化物（主要是蔗糖和氨基酸），既可向上也可向下（转运）。
IB students need to understand the Cohesion–Tension theory and the Mass Flow hypothesis for translocation, including the roles of source and sink. Edexcel focuses on functional comparisons between xylem and phloem and their locations in vascular bundles.
IB 学生需要理解内聚力-张力理论和转运的压力流动假说，包括“源”和“库”的作用。Edexcel 侧重于木质部和韧皮部的功能比较以及它们在维管束中的位置。
6. Transport of Water and Minerals | 水分和矿物质的运输
Water enters root hair cells by osmosis because the soil water has a higher water potential than the cell sap. Mineral ions, such as nitrates and magnesium, are taken up by active transport against a concentration gradient, requiring energy from respiration. Once inside the root, water crosses the cortex mainly via the apoplast pathway (through cell walls) until it reaches the endodermis, where the Casparian strip forces water into the symplast pathway (through cytoplasm).
水通过渗透作用进入根毛细胞，因为土壤水的水势高于细胞液。硝酸盐和镁等矿物质离子通过逆浓度梯度的主动运输吸收，需要呼吸作用供能。进入根部后，水主要通过质外体途径（经由细胞壁）穿过皮层，直到抵达内皮层，凯氏带迫使水转入共质体途径（穿过细胞质）。
This forced entry ensures selective mineral uptake. Water then moves into the xylem and is pulled up by transpiration. Deficiencies of essential minerals lead to deficiency symptoms, such as chlorosis (yellowing) due to lack of magnesium (needed for chlorophyll) or poor growth due to lack of nitrates (needed for amino acids).
这种强制进入确保了矿质的选择性吸收。随后水进入木质部，由蒸腾作用向上拉拽。必需矿物元素缺乏会导致缺素症状，如缺镁（叶绿素所需）引起的失绿症，或缺氮（氨基酸所需）导致的生长不良。
7. Translocation of Sucrose | 蔗糖的转运
Translocation is the movement of sucrose and other organic solutes through the phloem from sources (e.g., leaves) to sinks (e.g., roots, developing fruits, and meristems). The process is explained by the pressure flow or mass flow hypothesis. At the source, sucrose is actively loaded into companion cells, which increases solute concentration, lowering water potential.
转运是指蔗糖和其他有机溶质通过韧皮部从“源”（如叶片）运输到“库”（如根、发育中的果实和分生组织）。这一过程由压力流动假说解释。在源端，蔗糖被主动装载到伴胞中，导致溶质浓度升高，水势降低。
Water enters the phloem from adjacent xylem by osmosis, generating high hydrostatic pressure. At the sink, sucrose is unloaded (actively or passively) and used or stored, reducing solute concentration. Water then leaves the phloem, lowering pressure. The resulting pressure difference drives a bulk flow of phloem sap from source to sink.
水分通过渗透从邻近木质部进入韧皮部，产生高静水压力。在库端，蔗糖被卸出（主动或被动）并利用或储存，溶质浓度下降。水分随后离开韧皮部，压力降低。由此产生的压力差驱动韧皮部汁液从源向库的整体流动。
This model explains why phloem transport can occur simultaneously in opposite directions in different vascular bundles. It is a key topic for IB Biology, often examined through diagrams and application to experimental data using aphid stylets or radioactive tracers.
该模型解释了为何韧皮部运输可在不同维管束内同时向相反方向进行。这是 IB 生物学的重点，常通过示意图及蚜虫口针或放射性示踪剂的实验数据来考查。
8. Plant Mineral Nutrition | 植物矿质营养
Plants require macronutrients like nitrogen (N), phosphorus (P), and potassium (K), as well as micronutrients like iron and magnesium. Nitrogen is a key component of amino acids, proteins, and nucleic acids. Phosphorus is essential for ATP, cell membranes, and DNA. Potassium regulates stomatal opening and enzyme activation. Magnesium is the central atom in chlorophyll.
植物需要氮、磷、钾等大量元素，以及铁、镁等微量元素。氮是氨基酸、蛋白质和核酸的关键成分。磷对ATP、细胞膜和DNA必不可少。钾调节气孔开闭和酶活化。镁是叶绿素的中心原子。
Deficiencies manifest as specific symptoms. Nitrate deficiency causes stunted growth and yellowing of older leaves. Magnesium deficiency results in interveinal chlorosis on older leaves because magnesium is mobile and moves to younger tissues. Potted plant experiments using water cultures with omitted nutrients are classic demonstrations of essential elements.
缺素会表现出特定症状。缺氮导致生长迟缓、老叶黄化。缺镁使老叶出现脉间失绿，因为镁是可移动元素，会转移到新组织。用缺少某种营养元素的水培法所做的盆栽实验，是证明必需元素的经典方法。
9. Plant Hormones: Auxin and Tropisms | 植物激素：生长素与向性
Auxin (indole-3-acetic acid, IAA) is a plant hormone that controls cell elongation and is responsible for phototropism and gravitropism. In phototropism, unilateral light causes auxin to redistribute to the shaded side of the shoot tip. Higher auxin concentration on the shaded side promotes greater cell elongation, causing the shoot to bend towards the light.
生长素（吲哚-3-乙酸，IAA）是一种控制细胞伸长并负责向光性和向地性的植物激素。在向光性中，单侧光照使生长素重新分布到芽尖背光侧。背光侧较高的生长素浓度促进细胞更快伸长，导致芽向光弯曲。
In gravitropism, auxin accumulates on the lower side of a horizontally placed root or shoot. In shoots, this stimulates elongation and causes upward bending. In roots, a higher auxin concentration inhibits cell elongation, leading the root to bend downwards. Experiments using agar blocks with auxin or decapitated coleoptiles provide evidence for these mechanisms.
在向地性中，生长素积聚在水平放置的根或芽的下侧。在芽中，这促进伸长导致向上弯曲；在根中，高浓度生长素抑制细胞伸长，导致根向下弯曲。用含有生长素的琼脂块或去顶胚芽鞘所做的实验为这些机制提供了证据。
Other plant hormones include gibberellins for stem elongation and seed germination, and abscisic acid for stomatal closure under water stress. Edexcel may ask about commercial uses of auxin as weedkillers and rooting powders.
其他植物激素包括促进茎伸长和种子萌发的赤霉素，以及水分胁迫下促进气孔关闭的脱落酸。Edexcel 可能问及生长素作为除草剂和生根粉的商业用途。
10. Reproduction in Flowering Plants: Flower Structure | 被子植物的繁殖：花的结构
The flower is the reproductive organ of angiosperms. The male part is the stamen, consisting of anther and filament. The anther produces pollen grains containing the male gametes. The female part is the carpel (or pistil), made up of stigma, style, and ovary. The ovary contains ovules, each with an embryo sac housing the female gamete.
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。花药产生含有雄配子的花粉粒。雌性部分是心皮（或雌蕊），由柱头、花柱和子房组成。子房内含胚珠，每个胚珠具有一个包含雌配子的胚囊。
Many flowers have both male and female parts (bisexual), while some are unisexual. Petals are often brightly coloured to attract pollinators; sepals protect the flower bud. Nectaries secrete sugary nectar as a reward for pollinators. Pollination can be by wind, insects, birds, or other agents, and floral structures are adapted accordingly.
许多花同时具有雄性和雌性部分（两性花），有些则是单性花。花瓣通常色彩鲜艳以吸引传粉者；萼片保护花蕾。蜜腺分泌含糖花蜜作为对传粉者的奖励。传粉可借助风、昆虫、鸟类或其他媒介，花的结构也相应适应。
11. Pollination, Fertilisation and Seed Formation | 传粉、受精与种子形成
Pollination is the transfer of pollen from an anther to a stigma. Self-pollination occurs within the same flower or plant, whereas cross-pollination involves different individuals of the same species. Cross-pollination increases genetic variation and is promoted by mechanisms such as dioecy, dichogamy, and self-incompatibility.
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一植株内，而异花传粉则涉及同种的不同个体。异花传粉增加遗传变异，并通过雌雄异株、雌雄蕊异熟和自交不亲和等机制得到促进。
After pollination, a pollen tube grows down the style, carrying two male gametes. One fertilises the egg cell to form a diploid zygote, the other fuses with the polar nuclei to form the triploid endosperm. This is called double fertilisation, a unique feature of angiosperms.
传粉后，花粉管沿花柱向下生长，携带两个雄配子。一个与卵细胞融合形成二倍体的合子，另一个与极核融合形成三倍体的胚乳。这称为双受精，是被子植物的独特特征。
The ovule develops into a seed; the integuments become the seed coat (testa), and the fertilised egg becomes the embryo. The ovary wall develops into the fruit, aiding in seed dispersal. Conditions for germination include water, oxygen, and suitable temperature. Some seeds also require light or scarification.
胚珠发育为种子；珠被变为种皮，受精卵发育为胚。子房壁发育成果实，有助于种子传播。种子萌发的条件包括水分、氧气和适宜温度。有些种子还需要光照或破皮处理。
12. Seed Germination and the Role of Enzymes | 种子萌发与酶的作用
Germination begins with water uptake (imbibition), activating enzymes such as amylase that break down stored starch into sugars. These sugars are respired to provide energy for growth. In cereal grains, gibberellins promote the synthesis of amylase, which hydrolyses endosperm starch. Oxygen is required for aerobic respiration, which supplies ATP for active cell division and growth.
种子萌发始于吸水，激活淀粉酶等酶，将储存的淀粉分解为糖。这些糖通过呼吸作用为生长提供能量。在谷物中，赤霉素促进淀粉酶的合成，后者水解胚乳中的淀粉。氧气是有氧呼吸所必需的，为主动的细胞分裂和生长提供 ATP。
If oxygen is absent, anaerobic respiration occurs, but it yields toxic ethanol and very little ATP, stunting or preventing germination. Temperature must be optimal for enzyme activity. Dry mass of the seedling initially decreases because stored nutrients are used in respiration before photosynthesis begins.
如果缺氧，会发生无氧呼吸，但产生有毒的乙醇和极少的ATP，导致萌发受阻或停滞。温度必须适合酶的活动。幼苗的干重最初下降，因为在光合作用开始前，储存的营养物质被用于呼吸消耗。
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