📚 Plant Biology for IGCSE CCEA Science | IGCSE CCEA 科学：植物考点精讲
Plants are fundamental to life on Earth, and understanding their structure, function, and life processes is a key part of IGCSE CCEA Science. This article consolidates the essential plant biology topics, from cellular adaptations to reproduction, directly aligned with the CCEA specification.
植物是地球生命的基础，理解植物的结构、功能与生命过程是 IGCSE CCEA 科学的核心部分。本文整合了从细胞适应到繁殖等关键植物生物学专题，紧密贴合 CCEA 考试大纲。
1. Plant Cells and Tissues | 植物细胞与组织
Plant cells share some organelles with animal cells but possess three distinctive features: a rigid cell wall made of cellulose, a large permanent vacuole containing cell sap, and chloroplasts that carry out photosynthesis.
植物细胞与动物细胞共享一些细胞器，但具有三个显著特征：由纤维素构成的坚韧细胞壁、含有细胞液的大中央液泡以及执行光合作用的叶绿体。
The cell wall provides structural support and prevents the cell from bursting when water enters by osmosis. The vacuole helps maintain turgor pressure, keeping the plant firm and upright.
细胞壁提供结构支撑并防止细胞因渗透吸水而破裂。液泡帮助维持膨压，使植物保持坚挺直立。
Palisade mesophyll cells in leaves are tightly packed and contain abundant chloroplasts, making them the primary site of photosynthesis. Root hair cells are elongated to increase surface area for efficient absorption of water and mineral ions.
叶片中的栅栏叶肉细胞排列紧密并富含叶绿体，是光合作用的主要场所。根毛细胞细长，增大了表面积以高效吸收水分和矿质离子。
2. Photosynthesis: The Basics | 光合作用基础
Photosynthesis is the process by which green plants use light energy to convert carbon dioxide and water into glucose and oxygen. The green pigment chlorophyll, housed in chloroplasts, absorbs the light energy required to drive this reaction.
光合作用是绿色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。叶绿体中的绿色色素叶绿素吸收驱动该反应所需的光能。
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
The glucose produced is used for several purposes: it is respired to release energy, stored as starch, converted to cellulose for strengthening cell walls, and combined with nitrate ions to synthesise amino acids and proteins.
产生的葡萄糖有多种用途：用于呼吸作用释放能量，以淀粉形式储存，转化为纤维素以加固细胞壁，以及与硝酸根离子结合合成氨基酸和蛋白质。
To test a leaf for starch, boil the leaf in water to kill the tissue, then heat it in ethanol to decolourise it. After softening in warm water, add iodine solution; a blue-black colour indicates the presence of starch.
测试叶片淀粉时，先将叶片在水中煮沸以杀死组织，然后在乙醇中加热使其脱色。用温水软化后，滴加碘液；呈现蓝黑色表示叶片含有淀粉。
3. Limiting Factors of Photosynthesis | 光合作用的限制因素
The rate of photosynthesis is affected by light intensity, carbon dioxide concentration, and temperature. A limiting factor is the one in shortest supply which restricts the overall rate, even if other factors are optimal.
光合作用速率受光照强度、二氧化碳浓度和温度影响。限制因素是指供应最不足的那个因素，即使其他条件处于最佳状态，它也会制约反应速率。
At low light intensity, the rate of photosynthesis increases proportionally with more light, then plateaus because either CO₂ concentration or temperature becomes limiting. Similarly, raising CO₂ levels only helps up to a point.
在低光照强度下，光合作用速率随光强成比例增加，随后达到平台期，因为二氧化碳浓度或温度转而成为限制因素。同样地，提升二氧化碳浓度也只能在一定范围内起作用。
Temperature affects enzyme-controlled steps. The rate increases as temperature rises to an optimum around 25-30°C, but high temperatures denature the enzymes, causing the rate to drop sharply.
温度影响酶促反应步骤。速率随温度升高而加快，达到约 25–30 °C 的最适温度，但高温会使酶变性，导致速率急剧下降。
4. Leaf Structure and Adaptations | 叶片结构与适应
A cross-section through a leaf reveals its adaptations for photosynthesis. The upper epidermis is transparent to allow light through, and the palisade mesophyll beneath is packed with chloroplasts.
叶片横切面结构展示了其对光合作用的适应。上表皮透明以透光，其下方的栅栏叶肉富含叶绿体。
Spongy mesophyll contains numerous air spaces that facilitate diffusion of CO₂ from the atmosphere to the photosynthesising cells and O₂ out. The vascular bundles (veins) contain xylem and phloem to supply water and remove sugars.
海绵叶肉含有大量气腔，有利于二氧化碳从大气扩散至光合细胞以及氧气向外扩散。维管束（叶脉）中有木质部和韧皮部，输送水分并运走糖类。
Stomata, mainly on the lower epidermis, are pores surrounded by guard cells. Guard cells take up water and become turgid to open the stoma for gas exchange; when they lose water, the stoma closes, reducing water loss.
气孔主要分布在下表皮，是由保卫细胞围成的孔隙。保卫细胞吸水膨胀导致气孔打开进行气体交换；当保卫细胞失水时，气孔关闭，减少水分蒸腾。
5. Xylem and Phloem: Transport Systems | 木质部与韧皮部：运输系统
Xylem tissue transports water and dissolved mineral ions from the roots upward through the plant. It consists of dead, hollow cells arranged end-to-end, strengthened by lignin, which forms continuous tubes and provides structural support.
木质部组织将水和溶解的矿质离子从根部向上运输至植株各处。它由死细胞中空端对端排列而成，由木质素加固，形成连续管道并提供结构支撑。
Phloem transports sucrose and amino acids (assimilates) from sources, such as photosynthesising leaves, to sinks, such as roots, flowers, and developing fruits. Phloem is made of living cells: sieve tube elements and companion cells.
韧皮部将蔗糖和氨基酸（同化物）从“源”（如光合叶片）运输到“库”（如根、花和发育中的果实）。韧皮部由活细胞构成，包括筛管分子和伴胞。
| Xylem | Phloem |
| Dead, hollow tubes; lignified walls | Living sieve tube elements and companion cells |
| Transports water and minerals unidirectionally (upwards) | Transports sucrose and amino acids bidirectionally |
| Flow driven by transpiration pull; passive | Flow requires energy; active translocation |
6. Transpiration and Translocation | 蒸腾作用与输导作用
Transpiration is the evaporation of water vapour from the surfaces of mesophyll cells, followed by diffusion out of stomata. This loss of water creates a tension (transpiration pull) that draws water up the xylem from the roots, forming the transpiration stream.
蒸腾作用是水蒸气从叶肉细胞表面蒸发，随后通过气孔扩散到大气中的过程。这种水分丧失产生张力（蒸腾拉力），将水通过木质部从根部向上拉，形成蒸腾流。
Factors affecting the rate of transpiration include temperature, humidity, wind speed, and light intensity. Higher temperature increases evaporation; high humidity reduces the concentration gradient; wind removes water vapour, steepening the gradient; light stimulates stomatal opening.
影响蒸腾速率的因素包括温度、湿度、风速和光照强度。温度升高加速蒸发；高湿度降低浓度梯度；风带走水蒸气，增大梯度；光照促使气孔开放。
Translocation is the active movement of sucrose and amino acids in the phloem from source to sink. It requires energy from respiration to load assimilates into the sieve tubes and maintain the pressure flow.
输导作用是蔗糖和氨基酸在韧皮部中从源到库的主动运输。此过程需要呼吸作用提供的能量，以将同化物装载进筛管并维持压力流。
7. Mineral Requirements and Deficiencies | 矿质营养与缺乏症
Plants require mineral ions from the soil for healthy growth. Nitrate ions (NO₃⁻) are essential for synthesising amino acids and proteins. Magnesium ions (Mg²⁺) are the central atom of the chlorophyll molecule.
植物需要从土壤中吸收矿质离子以维持健康生长。硝酸根离子 (NO₃⁻) 是合成氨基酸和蛋白质所必需的。镁离子 (Mg²⁺) 是叶绿素分子的中心原子。
A deficiency of nitrates results in stunted growth and chlorosis (yellowing) of older leaves, as nitrogen is mobilised to younger tissues. Magnesium deficiency causes chlorosis between the veins because chlorophyll cannot be synthesised, reducing photosynthetic capacity.
缺氮导致植株矮小和老叶失绿（黄化），因为氮元素被转移到幼嫩组织。缺镁则导致叶脉间黄化，因为叶绿素无法合成，从而降低光合能力。
Roots absorb these mineral ions from the soil by active transport, which requires energy from respiration. This is why waterlogged, poorly aerated soils can lead to nutrient deficiency—lack of oxygen reduces respiration and active uptake.
根通过主动运输从土壤吸收这些矿质离子，该过程需要呼吸作用供能。这就是为何涝渍、通气不良的土壤会导致养分缺乏——缺氧会降低呼吸作用和主动吸收。
8. Plant Tropisms: Phototropism and Gravitropism | 植物向性运动：向光性与向重力性
Tropisms are directional growth responses to external stimuli. Phototropism is growth in response to unidirectional light, while gravitropism (also called geotropism) is growth in response to gravity.
向性运动是对外界刺激的方向性生长反应。向光性是对单侧光照的生长反应，向重力性（又称向地性）是对重力的生长反应。
In shoots, the plant hormone auxin redistributes to the shaded side, stimulating cell elongation and causing the shoot to bend towards the light (positive phototropism). In roots, a higher auxin concentration on the lower side inhibits cell elongation, causing the root to bend downwards (positive gravitropism).
在茎中，植物激素生长素重新分布到背光侧，刺激细胞伸长，使茎向光弯曲（正向光性）。在根中，下侧较高的生长素浓度抑制细胞伸长，使根向下弯曲（正向重力性）。
Gravitropism ensures that roots grow down into the soil for anchorage and water absorption, while shoots exhibit negative gravitropism, growing upwards to access light for photosynthesis.
向重力性确保根向下生长深入土壤，以固着和吸收水分；而茎表现为负向重力性，向上生长以获取光合作用所需的光照。
9. Flower Structure and Pollination | 花的结构与传粉
Flowers are the reproductive organs of angiosperms. Key parts include sepals (protect the bud), petals (attract pollinators), stamens (male, consisting of anther and filament), and a carpel/pistil (female, comprising stigma, style, and ovary containing ovules).
花是被子植物的生殖器官。关键结构包括花萼（保护花蕾）、花瓣（吸引传粉者）、雄蕊（雄性，由花药和花丝组成）和心皮/雌蕊（雌性，包括柱头、花柱和含有胚珠的子房）。
Pollination is the transfer of pollen grains from the anther to the stigma. Self-pollination occurs within the same flower or plant; cross-pollination involves different plants of the same species, enhancing genetic variation.
传粉是花粉粒从花药转移到柱头上的过程。自花传粉发生在同一朵花或同一植株内；异花传粉涉及同一物种的不同植株，提高了遗传变异。
Insect-pollinated flowers are typically large, brightly coloured, scented, and produce nectar. Their pollen is sticky and spiky, and the stigma is enclosed. Wind-pollinated flowers are small, with dull petals, feathery stigmas to catch airborne pollen, and produce large quantities of light, smooth pollen.
虫媒花通常较大、鲜艳、有香味并产生花蜜。其花粉粘滞且带刺，柱头内藏。风媒花较小，花瓣不显眼，具羽毛状柱头以捕捉风中的花粉，并产生大量轻而光滑的花粉。
10. Fertilisation, Seed and Fruit Formation | 受精与种子果实形成
After pollination, a compatible pollen grain germinates on the stigma, producing a pollen tube that grows down the style, carrying two male gametes towards the ovule. This process is controlled by signals from the female tissues.
传粉后，亲和的花粉粒在柱头上萌发，产生花粉管沿花柱向下生长，携带两个雄配子朝向胚珠。这一过程受到雌性组织信号的调控。
Fertilisation occurs when one male gamete fuses with the egg cell to form a diploid zygote, while the other fuses with polar nuclei to form a nutritive endosperm (double fertilisation). The zygote develops into the plant embryo.
一个雄配子与卵细胞融合形成二倍体的合子，另一个与极核融合形成营养性胚乳（双受精）。合子发育为植物胚。
Following fertilisation, the ovule becomes a seed (containing the embryo, food store, and protective seed coat), and the ovary wall develops into a fruit. The fruit protects the seeds and aids in their dispersal by wind, water, or animals.
受精后，胚珠发育为种子（包含胚、营养储存和保护性种皮），子房壁发育为果实。果实保护种子，并借助风力、水流或动物帮助种子传播。
11. Seed Germination Conditions | 种子萌发条件
For successful germination, seeds require water, oxygen, and a suitable warm temperature. Water is absorbed to activate enzymes that break down stored food reserves. Oxygen is essential for aerobic respiration to release energy for growth.
种子成功萌发需要水分、氧气和适宜的温度。吸水可激活分解储存养分的酶。氧气是为生长释放能量所必需的有氧呼吸所必需的。
Germination begins with the seed imbibing
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