📚 Plant Biology for IGCSE OCR Science | IGCSE OCR 科学：植物 考点精讲
Plants are the foundation of most ecosystems and a central topic in IGCSE OCR Science. This article unpacks all essential plant biology concepts you must master, from photosynthesis to transport and reproduction. Every section is paired in English and Chinese to support bilingual learners, ensuring you can explain each idea in both languages and ace your exam.
植物是大多数生态系统的基础，也是 IGCSE OCR 科学的核心主题。本文梳理了你必须掌握的所有植物生物学重要概念，从光合作用到物质运输再到生殖。每个部分都提供中英双语解析，帮助双语学习者既能理解又能用两种语言清晰表达，为考试做好充分准备。
1. Photosynthesis Equation and Requirements | 光合作用的方程式与条件
gy absorbed by chlorophyll. The balanced chemical equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
光合作用是植物利用光能，将二氧化碳和水转化为葡萄糖和氧气的过程。文字方程式为：二氧化碳 + 水 → 葡萄糖 + 氧气，光能被叶绿素吸收。配平的化学方程式是 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂。
To keep photosynthesis working, plants need carbon dioxide (from the atmosphere through stomata), water (absorbed from the soil by roots), light energy (usually sunlight), and chlorophyll (the green pigment in chloroplasts). Limiting any one of these factors reduces the rate.
维持光合作用，植物需要二氧化碳（经由气孔从大气中获得）、水（由根部从土壤中吸收）、光能（通常是阳光）以及叶绿素（叶绿体中的绿色色素）。这些因素中的任何一个受限，都会降低光合作用的速率。
2. Structure of a Leaf and Adaptations for Photosynthesis | 叶片结构及其光合作用适应性
A typical dicotyledonous leaf is adapted to absorb maximum light and carbon dioxide. The broad, flat lamina provides a large surface area. The upper epidermis is transparent to allow light to pass through to the palisade mesophyll, where most photosynthesis happens because cells are packed with chloroplasts.
典型的双子叶植物叶片适应于最大限度地吸收光能和二氧化碳。宽阔而扁平的叶片提供了巨大的表面积。上表皮透明，让光线可以到达栅栏组织，这里是光合作用的主要场所，因为细胞富含叶绿体。
Spongy mesophyll has air spaces for gas exchange, and the lower epidermis contains many stomata. Stomata are surrounded by guard cells that open to let carbon dioxide in and oxygen out, and can close to reduce water loss.
海绵组织含有气隙以利于气体交换，下表皮有许多气孔。气孔周围有保卫细胞，它们打开时让二氧化碳进入、氧气排出；关闭时可以减少水分蒸腾。
3. Mineral Requirements: Nitrate and Magnesium Ions | 矿质元素需求：硝酸根离子和镁离子
Plants need mineral ions for healthy growth. Nitrate ions (NO₃⁻) are essential for making amino acids and proteins, which in turn are required for cell growth. A shortage of nitrate causes stunted growth and yellowing of older leaves.
植物需要矿质离子来维持健康生长。硝酸根离子 (NO₃⁻) 是制造氨基酸和蛋白质所必需的，而氨基酸和蛋白质是细胞生长的基础。缺氮会导致植株矮小、老叶发黄。
Magnesium ions (Mg²⁺) are needed to make chlorophyll. Without enough magnesium, plants cannot produce sufficient chlorophyll, leading to chlorosis—yellowing between the veins of leaves. Because magnesium is mobile in plants, symptoms appear first on older leaves.
镁离子 (Mg²⁺) 是合成叶绿素所必需的。缺镁会导致叶绿素不足，引发失绿症——叶片叶脉间变黄。由于镁在植物体内可以移动，症状首先出现在老叶上。
4. Transpiration: the Loss of Water Vapour | 蒸腾作用：水蒸气的散失
Transpiration is the evaporation of water from the surface of mesophyll cells followed by diffusion of water vapour out of the stomata. It creates a transpiration pull that helps draw water and dissolved minerals up from the roots through the xylem. Transpiration is a consequence of stomata opening for photosynthesis.
蒸腾作用是水分从叶肉细胞表面蒸发，随后水蒸气从气孔扩散出去的过程。它产生蒸腾拉力，帮助将水分和溶解的矿物质从根部通过木质部向上抽提。蒸腾作用是气孔为光合作用打开所带来的必然结果。
Factors that increase transpiration rate include higher temperature (increases evaporation), lower humidity (steeper concentration gradient), faster air movement (removes humid air), and higher light intensity (stomata open wider).
加快蒸腾速率的因素有：温度升高（加快蒸发）、湿度降低（浓度梯度增大）、空气流动加快（带走湿气）、以及光照增强（气孔开得更大）。
5. Xylem and Phloem: Vascular Bundles | 木质部与韧皮部：维管束
In roots, stems and leaves, xylem and phloem are grouped into vascular bundles. Xylem transports water and mineral ions from roots to shoots in a unidirectional flow. Its vessels are dead, hollow tubes with lignin-reinforced walls that provide structural support.
在根、茎、叶中，木质部和韧皮部组成维管束。木质部负责将水和矿质离子从根部单向运输到地上部分。其导管是由死细胞构成的中空管道，壁内含有木质素加固，兼具支持功能。
Phloem transports sucrose and amino acids from sources (e.g., leaves) to sinks (e.g., roots, fruits) in a bidirectional process called translocation. Phloem is made of living sieve tube elements and companion cells that provide energy for active transport.
韧皮部将蔗糖和氨基酸从源头（如叶片）双向运输到库（如根、果实），该过程称为输导作用。韧皮部由活细胞——筛管分子和伴胞组成，伴胞为主动运输提供能量。
6. Transpiration Stream and Cohesion-Tension Theory | 蒸腾流与内聚力-张力学说
The movement of water through xylem from roots to leaves is called the transpiration stream. It is explained by the cohesion-tension theory. Water molecules stick together (cohesion) and adhere to xylem walls (adhesion). As water evaporates from mesophyll cells, tension is created in the leaf, pulling more water up the narrow xylem tubes in a continuous column.
水分从根部经木质部到叶片的运动称为蒸腾流。这可用内聚力-张力学说解释。水分子之间相互吸引（内聚力），并附着在木质部壁上（附着力）。当叶肉细胞失水蒸发时，在叶片中产生张力，把水柱沿着狭窄的木质部导管持续向上拖拽。
This unbroken water column is maintained because of the strong hydrogen bonds between water molecules. If air enters the xylem vessel (cavitation), the column can break, reducing transport efficiency. The continuous uptake of water from the soil keeps the stream flowing.
由于水分子间强大的氢键，这一连续水柱得以维持。如果空气进入木质部导管（空穴现象），水柱可能会断开，降低运输效率。持续从土壤中吸收水分则保证了蒸腾流的流动。
7. Plant Mineral Deficiency: Practical Investigations | 植物矿质缺乏症：实验探究
To investigate the effect of specific mineral deficiencies, you can use water culture experiments. Seedlings are grown in solutions lacking one mineral (e.g., nitrogen, phosphorus, potassium, magnesium) while all other factors are kept the same. Observations of plant height, leaf colour, and root development reveal deficiency symptoms.
为探究特定矿质缺乏的影响，可采用水培实验。将幼苗培养在缺乏某种矿质元素（如氮、磷、钾、镁）的营养液中，其他所有条件保持一致。观察植株高度、叶片颜色和根系发育，即可发现缺乏症状。
Controls are essential: one set of plants in full nutrient solution (positive control), and sometimes one in distilled water (negative control). You must control variables such as light intensity, temperature, volume of solution, and pH. Repeating the experiment increases reliability.
对照组至关重要：一组置于全营养液中（阳性对照），有时还设蒸馏水组（阴性对照）。必须控制光照强度、温度、溶液体积和 pH 等变量。重复实验能提高可靠性。
8. Gas Exchange in Plants: Day and Night | 植物气体交换：白天与夜晚
During daylight, plants carry out both photosynthesis and respiration. Overall, they take in carbon dioxide and release oxygen because the rate of photosynthesis exceeds respiration. At night, photosynthesis stops; plants take in oxygen and release carbon dioxide due to respiration alone. This cycle is vital for understanding net gas exchange.
白天，植物同时进行光合作用和呼吸作用。总体上，植物吸收二氧化碳、释放氧气，因为光合作用速率高于呼吸作用。夜晚光合作用停止，植物仅进行呼吸作用，吸收氧气、释放二氧化碳。这一循环对于理解净气体交换至关重要。
Stomata are the main route for gas exchange. Guard cells become turgid and open stomata in light, allowing CO₂ in. In darkness, guard cells lose turgor, closing stomata to conserve water while restricting gas exchange.
气孔是气体交换的主要通道。保卫细胞在光下膨大，气孔打开，让二氧化碳进入。黑暗中，保卫细胞失水萎蔫，气孔关闭以保存水分，但同时限制了气体交换。
9. Practical: Testing a Leaf for Starch | 实验：检测叶片中的淀粉
Starch is a product of photosynthesis stored in leaves. To test whether photosynthesis has occurred, a leaf is boiled in water to kill it, then boiled in ethanol to remove chlorophyll (decolourise). After rinsing in hot water to soften, iodine solution is added. A blue-black colour indicates starch is present.
淀粉是光合作用产物，储存于叶片中。为检验光合作用是否发生，先将叶片在沸水中煮沸杀死，接着在乙醇中煮沸以脱去叶绿素（脱色）。用热水冲洗软化后，滴加碘液。蓝黑色表示存在淀粉。
In a typical exam investigation, a plant may be destarched by leaving it in darkness for 24 hours. Part of a leaf is covered with aluminium foil, then exposed to light. Only the illuminated part turns blue-black, proving light is necessary for photosynthesis.
在典型的考试探究中，会将植物放在暗处 24 小时以耗尽淀粉。用铝箔遮挡叶片一部分，然后照光。只有照光的部分变为蓝黑色，证明光合作用需要光。
10. Sexual Reproduction in Flowering Plants | 开花植物的有性生殖
Flowers are reproductive structures containing male stamens (anther and filament) and female carpels (stigma, style, ovary). Pollination is the transfer of pollen from anther to stigma. Fertilisation occurs when a pollen tube grows down the style, delivering male gametes to the ovule, fusing with egg cells to form a zygote.
花是生殖器官，包含雄蕊（花药和花丝）和雌蕊（柱头、花柱、子房）。传粉是花粉从花药传到柱头的过程。受精发生时，花粉管沿花柱向下生长，将雄性配子送至胚珠，与卵细胞融合形成合子。
After fertilisation, the ovule becomes a seed, and the ovary develops into a fruit. Seeds contain an embryo, a food store (cotyledons), and a protective seed coat. Dispersal mechanisms (wind, animal, water, self-dispersal) reduce competition and allow colonisation.
受精后，胚珠发育成种子，子房发育成果实。种子包含胚、营养储存（子叶）和保护性种皮。传播机制（风、动物、水、自播）能减少竞争、实现定居。
11. Germination: Conditions and Early Growth | 萌发：条件与早期生长
Germination is the process by which a seed begins to grow. The required conditions are water (to activate enzymes and transport nutrients), oxygen (for aerobic respiration), and a suitable temperature (for enzyme action). Light is usually not needed for the initial stages—a fact demonstrated by growing seeds in a dark cupboard.
萌发是种子开始生长的过程。所需条件有：水分（激活酶、运输养分）、氧气（用于有氧呼吸）和适宜的温度（保证酶活性）。光照对于初期通常不是必需的——在黑暗橱柜中培植种子即可证明。
The radicle (embryonic root) emerges first, anchoring the plant and absorbing water. Then the plumule (embryonic shoot) grows upward. Energy is provided by the breakdown of food reserves in the cotyledon or endosperm. Once leaves develop, the seedling begins photosynthesis and becomes autotrophic.
首先伸出的是胚根（胚根），用于固定植株并吸收水分。然后胚芽向上生长。能量来自子叶或胚乳中储存物质的分解。叶片长出后，幼苗开始光合作用，变为自养。
12. Asexual Reproduction and Cloning in Plants | 植物的无性生殖与克隆
Many plants reproduce asexually through natural methods such as runners (strawberry), rhizomes (ginger), tubers (potato), and bulbs (onion). These produce genetically identical offspring (clones). Artificial methods include taking cuttings and tissue culture, used extensively in horticulture to preserve desirable traits.
许多植物通过自然方法进行无性生殖，如匍匐茎（草莓）、根状茎（姜）、块茎（马铃薯）和鳞茎（洋葱）。这些方法产生遗传上完全相同的后代（克隆）。人工方法包括扦插和组织培养，广泛用于园艺以保留优良性状。
Tissue culture, or micropropagation, involves growing small pieces of plant tissue on a sterile agar medium containing nutrients and plant hormones. This can produce thousands of identical plants rapidly and is used to propagate virus-free stock or rare species.
组织培养（微繁）是在含有营养和植物激素的无菌琼脂培养基上培养微小植物组织块。这能快速产生数千株相同植株，用于繁殖无毒苗或稀有物种。
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