📚 IGCSE WJEC Science: Plants – Key Concepts Review | IGCSE WJEC 科学：植物考点精讲
Plants are fundamental to life on Earth, and in the WJEC IGCSE Science curriculum, a solid understanding of plant biology is essential. This article breaks down the key concepts you need to master – from photosynthesis and mineral nutrition to transport systems and hormonal responses. Each section pairs essential explanations in English with a Chinese translation, helping you to reinforce terminology and grasp the scientific principles doubly well.
植物是地球生命的基础，在 WJEC IGCSE 科学课程中，扎实掌握植物生物学至关重要。本文分解你需要掌握的关键概念——从光合作用和矿质营养，到运输系统和激素反应。每一部分都提供中英对照的关键解释，帮助你巩固术语并深入理解科学原理。
1. Photosynthesis Equation and Basics | 光合作用方程式及基础
chloroplasts of plant cells, where the pigment chlorophyll captures sunlight. The overall word equation can be written as: carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll.
光合作用是绿色植物利用光能、二氧化碳和水制造自身养分的过程。它发生在植物细胞的叶绿体中，色素叶绿素捕获阳光。总文字方程式可写作：二氧化碳 + 水 → 葡萄糖 + 氧气，需要光和叶绿素。
The balanced chemical equation is as follows, showing the conversion of six molecules of carbon dioxide and six molecules of water into one molecule of glucose and six molecules of oxygen.
如下配平的化学方程式显示六分子二氧化碳和六分子水转化为一分子葡萄糖和六分子氧气。
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
In this reaction, light energy is transformed into chemical energy stored in glucose. The oxygen released comes from the splitting of water molecules during the light-dependent stage, not from carbon dioxide. The glucose produced is used immediately for respiration, converted into starch for storage, or used to build other organic compounds such as cellulose, proteins, and lipids.
在这个反应中，光能转化为储存在葡萄糖中的化学能。释放的氧气来自光反应阶段水分子的分解，而不是来自二氧化碳。产生的葡萄糖可立即用于呼吸作用，转化为淀粉储存，或用于构建其他有机化合物，如纤维素、蛋白质和脂质。
2. Limiting Factors of Photosynthesis | 光合作用的限制因素
The rate of photosynthesis is determined by several environmental factors. A limiting factor is the variable that is in shortest supply and therefore restricts the overall rate, even if other factors are abundant. In a graph of photosynthesis rate against a single factor, the curve rises until another factor becomes limiting.
光合作用速率由几个环境因素决定。限制因素是指供应最短缺的变量，因此即使其他因素充足也会限制总体速率。在光合速率对单一因素的曲线中，曲线上升直到另一个因素变成限制因素。
The three principal limiting factors are light intensity, carbon dioxide concentration, and temperature. At low light intensity, photosynthesis increases linearly with more light until the rate plateaus. Similarly, adding CO₂ boosts the rate until the enzymes responsible for carbon fixation are saturated. Temperature affects the kinetic energy of molecules and the activity of enzymes; as temperature rises, the rate increases up to an optimum, beyond which enzymes denature and the rate declines sharply.
三个主要的限制因素是光照强度、二氧化碳浓度和温度。在低光照强度下，光合速率随光强增加而线性上升，直到达到平台期。同样，增加 CO₂ 会提高速率，直到固碳酶饱和。温度影响分子动能和酶活性；温度升高，速率增加直至最适点，超过该点酶变性，速率急剧下降。
In practical contexts, greenhouse growers often manipulate these factors using artificial lights, CO₂ enrichment, and heaters to maximise crop yield. When investigating limiting factors, remember to control all variables except the one being tested, and use aquatic plants like Elodea to count oxygen bubbles as a measure of the rate.
在实际应用中，温室种植者常通过人工补光、补充 CO₂ 和加热来控制这些因素，以最大化作物产量。在研究限制因素时，请记住控制除了被测变量外的所有变量，并利用伊乐藻等水生植物计算氧气泡数量来衡量速率。
3. Leaf Structure and Adaptations | 叶片结构及适应性
A leaf is a highly specialised organ optimised for photosynthesis and gas exchange. From top to bottom, the main tissues are: the waxy cuticle, upper epidermis, palisade mesophyll, spongy mesophyll, lower epidermis with guard cells, and stomata. Each layer plays a distinctive role.
叶片是高度特化的器官，经过优化以进行光合作用和气体交换。从上到下，主要组织依次为：蜡质角质层、上表皮、栅栏叶肉、海绵叶肉、带有保卫细胞的下表皮和气孔。每一层都发挥独特的作用。
- Waxy cuticle – a waterproof layer that reduces water loss by evaporation without blocking light.
- 蜡质角质层 – 防水的透明层，减少蒸发失水而不阻挡光线。
- Upper epidermis – a single layer of transparent cells allowing light to penetrate.
- 上表皮 – 一层透明的细胞，让光线穿透。
- Palisade mesophyll – tightly packed columnar cells near the upper surface, dense with chloroplasts to capture maximum light energy.
- 栅栏叶肉 – 靠近上表面的紧密排列柱状细胞，富含叶绿体以捕获最多光能。
- Spongy mesophyll – loosely arranged cells with air spaces that allow rapid diffusion of CO₂, O₂, and water vapour.
- 海绵叶肉 – 排列疏松的细胞，具有气腔，便于 CO₂、O₂ 和水蒸气的快速扩散。
- Vascular bundles (veins) – contain xylem for water and mineral transport, and phloem for sugar transport.
- 维管束（叶脉）– 含有运输水和矿物质的木质部，以及运输糖类的韧皮部。
- Guard cells and stomata – two guard cells surround each stoma; they change shape to open and close the pore, regulating gas exchange and water loss.
- 保卫细胞与气孔 – 每个气孔由两个保卫细胞包围；它们改变形状以开闭孔隙，调节气体交换和水分流失。
Most stomata are located on the lower epidermis in terrestrial plants, which reduces direct exposure to sunlight and lowers water loss through transpiration. The large surface area of the leaf, thin shape, and transparent epidermis all contribute to efficient light absorption and diffusion.
大部分气孔位于陆生植物的下表皮，这减少了阳光直射，降低了蒸腾失水。叶片的大表面积、薄形和透明表皮都有助于高效的光吸收和扩散。
4. Mineral Requirements for Plants | 植物的矿物质需求
For healthy growth, plants require mineral ions absorbed from the soil through the roots. These minerals are essential for building proteins, chlorophyll, nucleic acids, and for enzyme activation. Deficiencies lead to characteristic symptoms that are often assessed in exams.
为了健康生长，植物需要通过根部从土壤中吸收矿质离子。这些矿物质对构建蛋白质、叶绿素、核酸以及酶激活至关重要。缺乏会导致常见于考题的典型症状。
| Mineral ion / 矿质离子 | Main function(s) / 主要功能 | Deficiency symptom / 缺乏症状 |
|---|---|---|
| Nitrate (NO₃⁻) / 硝酸盐 | Synthesis of amino acids, proteins, and DNA / 合成氨基酸、蛋白质和 DNA | Stunted growth; older leaves turn yellow (chlorosis) / 生长受阻；老叶发黄（黄化） |
| Phosphate (PO₄³⁻) / 磷酸盐 | Component of ATP, cell membranes, DNA; vital for energy transfer / 是 ATP、细胞膜、DNA 的成分；对能量传递至关重要 | Poor root development; purple discolouration of older leaves / 根系发育不良；老叶呈紫色 |
| Potassium (K⁺) / 钾离子 | Regulates stomatal opening and enzyme activation / 调节气孔开闭和酶活化 | Yellowing of leaf margins; weak stems / 叶缘黄化；茎秆脆弱 |
| Magnesium (Mg²⁺) / 镁离子 | Central atom in chlorophyll molecule / 叶绿素分子的中心原子 | Interveinal chlorosis (yellowing between veins) / 叶脉间黄化（脉间失绿） |
Hydroponics experiments, where plants are grown in nutrient solutions lacking one specific mineral, are used to identify the effects of each deficiency. It is important to keep light, temperature, and oxygen constant for valid comparisons.
水培实验将植物种植在缺乏某种特定矿物质的营养液中，用于确定每种缺乏的影响。为了有效比较，必须保持光照、温度和氧气恒定。
5. Transport in Plants: Xylem and Phloem | 植物中的运输：木质部和韧皮部
Plants possess two distinct vascular tissues for transport: xylem and phloem. These are often found together in vascular bundles, with xylem located towards the inside and phloem towards the outside of the stem. In roots, they are arranged centrally, while in leaves they form the veins.
植物拥有两种不同的输导组织：木质部和韧皮部。它们通常共同位于维管束中，在茎中木质部位于内侧，韧皮部位于外侧。在根中它们呈中央排列，而在叶片中则构成叶脉。
Xylem transports water and dissolved mineral ions upwards from the roots to the shoots and leaves. The water moves in a continuous column driven by transpiration pull, cohesion, and adhesion. Xylem vessels are dead, hollow tubes formed from cells aligned end-to-end, with their end walls broken down. Their walls are thickened with lignin, which provides structural strength and makes them waterproof.
木质部将水和溶解的矿质离子从根部向上运输到茎叶。水分在蒸腾拉力、内聚力和附着力的共同作用下形成连续水柱而移动。木质部导管是死细胞、中空的管状结构，由端壁消失的细胞首尾相接而成。其细胞壁沉积了木质素，提供结构强度并使其防水。
Phloem transports organic substances, mainly sucrose and amino acids, from sources (e.g. leaves) to sinks (e.g. roots, fruits, growing tips). This process is called translocation and can occur in both upward and downward directions. Phloem consists of living cells: sieve tube elements, which have no nucleus but have sieve plates, and companion cells, which provide metabolic support and control loading of solutes.
韧皮部将有机物（主要是蔗糖和氨基酸）从源（如叶片）运输到库（如根、果实、生长点）。这一过程称为转运，并可双向进行。韧皮部由活细胞组成：筛管分子无细胞核但有筛板，伴胞提供代谢支持并控制溶质的装载。
6. Transpiration and Factors Affecting It | 蒸腾作用及影响因素
Transpiration is the evaporation of water vapour from the internal surfaces of leaves into the atmosphere, primarily through the stomata. It creates a tension (negative pressure) that pulls water up the xylem, making it a passive but vital process for water movement and mineral transport, as well as for cooling the leaf.
蒸腾作用是水蒸气从叶片内部表面通过气孔蒸发到大气中的过程。它产生张力（负压）拉动水分沿木质部上升，这虽然是被动过程，但对水分运动、矿物质运输以及叶片冷却至关重要。
The rate of transpiration is influenced by four main environmental factors:
蒸腾速率受四个主要环境因素影响：
- Light intensity – increases the rate because stomata open to admit CO₂ for photosynthesis, allowing more water vapour to escape.
- 光照强度 – 升高会使气孔张开以吸收 CO₂ 进行光合作用，导致更多水蒸气逸出，从而提高速率。
- Temperature – higher temperatures increase the kinetic energy of water molecules and the water-holding capacity of air, accelerating evaporation.
- 温度 – 较高温度增加水分子的动能以及空气的持水能力，加速蒸发。
- Humidity – high humidity reduces the concentration gradient of water vapour between the leaf and the atmosphere, slowing transpiration.
- 湿度 – 高湿度降低叶片与大气之间的水蒸气浓度梯度，从而减慢蒸腾作用。
- Wind speed – wind removes the layer of moist air around the leaf (boundary layer), steepening the diffusion gradient and increasing the rate. Still air has the opposite effect.
- 风速 – 风带走叶片周围湿润空气层（边界层），使扩散梯度变陡，提高速率；静止空气则相反。
A potometer is used to measure the rate of water uptake by a cut shoot, which is an estimate of the transpiration rate under the assumption that most water is lost through transpiration. By altering conditions systematically, one can demonstrate the effect of each factor.
蒸腾计用于测量切枝吸水的速率，这可以估算蒸腾速率，前提是大部分水分通过蒸腾流失。通过系统改变条件，可以展示每个因素的影响。
7. Translocation of Sugars | 糖类的转运
Translocation is the movement of assimilates, primarily sucrose and amino acids, through the phloem. Sucrose is loaded into the sieve tubes from companion cells at the source (usually mature leaves) using active transport. This reduces the water potential inside the sieve tube, causing water to enter by osmosis from the adjacent xylem, raising the hydrostatic pressure.
转运是同化物（主要是蔗糖和氨基酸）在韧皮部中的移动。蔗糖在源（通常是成熟叶片）由伴胞通过主动运输装载到筛管中。这降低了筛管内的水势，导致水分通过渗透从邻近木质部进入，从而升高静水压力。
At the sink (e.g. growing roots, developing fruits), sucrose is unloaded and used or stored. Water potential in the sink phloem increases, so water leaves by osmosis, lowering the pressure. This creates a pressure gradient from source to sink, driving a mass flow of phloem sap. The process requires energy for the active loading and unloading steps.
在库（如生长中的根、发育中的果实）处，蔗糖被卸载并利用或储存。库端韧皮部的水势升高，水分通过渗透离开，压力降低。这样就形成了从源到库的压力梯度，驱动韧皮部汁液的集流。该过程需要在装载和卸载阶段消耗能量。
8. Plant Hormones: Auxin and Tropisms | 植物激素：生长素和向性
Plants respond to environmental stimuli by growing towards or away from them; these directional growth responses are called tropisms. The plant hormone indoleacetic acid (IAA), a type of auxin, coordinates cell elongation and is central to phototropism and gravitropism (geotropism).
植物通过朝向或背离环境刺激生长来作出反应；这些定向生长反应称为向性。植物激素吲哚乙酸（IAA，一种生长素）协调细胞伸长，并对向光性和向地性起核心作用。
In phototropism, when a shoot is exposed to one-sided light, IAA accumulates on the shaded side. Higher concentrations of auxin promote cell elongation, so the shaded side grows faster, causing the shoot to bend towards the light. This is a positive phototropic response that maximises light capture for photosynthesis.
在向光性中，当芽受单侧光照时，IAA 在背光侧积累。较高浓度的生长素促进细胞伸长，因此背光侧生长更快，使芽朝向光源弯曲。这是一种正向光性反应，可最大化光合作用的光能捕获。
In gravitropism, roots are positively gravitropic (grow downwards) and shoots are negatively gravitropic. When a root is placed horizontally, IAA accumulates on the lower side due to gravity. In roots, high auxin concentration inhibits cell elongation, so the upper side cells elongate more, causing the root to bend downwards. In shoots, auxin promotes elongation on the lower side, so the shoot bends upwards.
在向地性中，根表现为正向地性（向下生长），芽表现为负向地性。当根水平放置时，由于重力，IAA 在下侧积累。在根中，高浓度生长素抑制细胞伸长，因此上侧细胞伸长更多，导致根向下弯曲。在芽中，生长素促进下侧伸长，因此芽向上弯曲。
9. Plant Responses to Light and Gravity – Experiments | 植物对光和重力的反应——实验
Several classical experiments demonstrate the role of auxin in tropisms. Coleoptiles of oat or maize seedlings are often used: If the tip is removed, the coleoptile stops growing towards light, indicating that the tip produces auxin or an auxin-like signal. If the tip is placed on an agar block and then placed back to one side, asymmetric elongation occurs, proving a diffusible chemical controls growth.
几个经典实验证明了生长素在向性中的作用。通常使用燕麦或玉米胚芽鞘：如果切除尖端，胚芽鞘停止向光生长，表明尖端产生生长素或类似信号。如果将尖端放在琼脂块上，然后放回一侧，就会出现不对称伸长，证明可扩散的化学物质控制生长。
A clinostat is a mechanical device that slowly rotates a potted plant, cancelling out the asymmetric distribution of IAA induced by gravity or directional light. A plant rotated horizontally on a clinostat grows straight, whereas a stationary horizontal plant bends. This demonstrates that graviperception requires a sustained unilateral stimulus to generate unequal auxin distribution.
回转器是一种缓慢旋转盆栽植物的机械装置，可抵消由重力或定向光引起的 IAA 不对称分布。在回转器上水平旋转的植物直立生长，而静止的水平植物发生弯曲。这证明重力感知需要持续的单侧刺激才能产生生长素的不均匀分布。
Understanding these experiments helps to explain how plants coordinate behaviour without a nervous system, using hormones that move through cells and phloem.
理解这些实验有助于解释植物如何在没有神经系统的情况下，利用在细胞和韧皮部中移动的激素来协调行为。
10. Investigating Photosynthesis and Transpiration | 实验探究光合作用和蒸腾作用
For photosynthesis, the standard laboratory investigation uses an aquatic plant such as Elodea, placed in a beaker of water with sodium hydrogencarbonate as a dissolved CO₂ source. By counting the number of bubbles released per minute at different light intensities (varying the distance of a lamp) or CO₂ concentrations, the rate of photosynthesis can be measured. A key safety and validity point is to allow the plant to equilibrate and to use a thermometer to monitor water temperature, preventing heat confounding.
对于光合作用，标准实验室探究使用伊乐藻等水生植物，放在盛有水和碳酸氢钠（作为溶解 CO₂ 来源）的烧杯中。通过计数在不同光照强度（改变灯的距离）或 CO₂ 浓度下每分钟释放的气泡数，可以测定光合速率。关键的安全和有效性要点是让植物平衡，并使用温度计监测水温，防止热量干扰。
A starch test on a variegated leaf can prove that chlorophyll is needed for photosynthesis: after destarching the plant, expose the leaf to light, then test with iodine solution. Only the green parts containing chlorophyll turn blue-black, while the white parts remain yellow-brown. A leaf with a partially covered area (using aluminium foil) shows that light is necessary; the covered area remains negative for starch.
用碘液对斑叶进行淀粉检测可以证明光合作用需要叶绿素：将植物去淀粉后，让叶片见光，然后用碘液测试。只有含叶绿素的绿色部分变成蓝黑色，而白色部分保持黄褐色。用铝箔部分遮盖叶片，被盖住的部分淀粉检测为阴性，表明需要光。
For transpiration, a bubble potometer is used to track the movement of an air bubble in a capillary tube connected to a cut shoot. By measuring the time taken for the bubble to travel a set distance under different conditions (e.g. with a fan, humid atmosphere, or higher temperature), students can calculate and compare transpiration rates. It is vital to cut the stem under water to prevent air locks and to leave the setup to acclimatise.
对于蒸腾作用，使用气泡蒸腾计追踪与切枝相连的毛细管中气泡的移动。通过测量在不同条件（如用风扇、潮湿环境或较高温度）下气泡移动固定距离所需的时间，学生可以计算并比较蒸腾速率。关键是在水下切断茎部以防止空气栓塞，并让装置适应环境。
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