A-Level OCR Science: Plant Key Points | A-Level OCR 科学：植物 考点精讲

📚 A-Level OCR Science: Plant Key Points | A-Level OCR 科学：植物 考点精讲

Plants form a fundamental component of the A-Level OCR Biology specification, covering topics from photosynthesis and transport systems to hormonal control and reproduction. This article distills the essential concepts and common examination pitfalls into a clear revision guide, equipping you with the knowledge needed to secure top marks.

植物是 A-Level OCR 生物学考纲中的核心组成部分，涵盖光合作用、运输系统、激素调控和生殖等重要主题。本文凝练关键概念与常见考试易错点，形成一份清晰的复习指南，帮助你获取高分所需的知识。

1. Photosynthesis: Light-Dependent Reactions | 光合作用：光反应

The light-dependent reactions occur on the thylakoid membranes of chloroplasts. Photosystem II absorbs light energy, exciting electrons that pass along an electron transport chain, generating ATP via chemiosmosis. Photolysis of water releases electrons, protons, and oxygen. Meanwhile, Photosystem I absorbs light to reduce NADP⁺ to NADPH.

光反应发生在叶绿体的类囊体膜上。光系统 II 吸收光能，激发电子沿电子传递链传递，通过化学渗透作用生成 ATP。水的光解释放电子、质子和氧气。同时，光系统 I 吸收光能将 NADP⁺ 还原为 NADPH。

Key products of the light-dependent stage are ATP and reduced NADP, which are essential for the Calvin cycle. Be prepared to describe how the proton gradient is established across the thylakoid membrane and the role of ATP synthase.

光反应阶段的关键产物是 ATP 和还原型 NADP，这是卡尔文循环所必需的。要能够描述类囊体膜两侧质子梯度是如何建立的，以及 ATP 合酶的作用。

2. Photosynthesis: Calvin Cycle | 光合作用：卡尔文循环

The Calvin cycle takes place in the stroma and uses ATP and reduced NADP from the light-dependent reactions to fix carbon dioxide. RuBP (ribulose bisphosphate) combines with CO₂ in a reaction catalysed by RuBisCO, forming an unstable 6C compound that immediately splits into two molecules of GP (glycerate 3-phosphate).

卡尔文循环在叶绿体基质中进行，利用光反应提供的 ATP 和还原型 NADP 固定二氧化碳。RuBP（核酮糖二磷酸）与 CO₂ 在 RuBisCO 酶催化下结合，形成不稳定的 6C 化合物，随即分解为两分子 GP（甘油酸-3-磷酸）。

GP is then reduced to TP (triose phosphate) using ATP and reduced NADP. Most TP is used to regenerate RuBP, while some leaves the cycle to form glucose, starch, amino acids, and lipids. Limiting factors such as light intensity, CO₂ concentration, and temperature directly influence the cycle’s rate.

随后 GP 被 ATP 和还原型 NADP 还原为 TP（磷酸丙糖）。大部分 TP 用于再生 RuBP，少数离开循环合成葡萄糖、淀粉、氨基酸和脂质。光照强度、二氧化碳浓度和温度等限制因素直接影响循环速率。

3. Limiting Factors and Agricultural Applications | 限制因素与农业应用

The rate of photosynthesis is controlled by the factor in shortest supply—light intensity, carbon dioxide concentration, or temperature. At low light, the light-dependent reactions limit the production of ATP and reduced NADP. As light increases, CO₂ availability or temperature may become limiting.

光合速率受供应最少的因素控制——光照强度、二氧化碳浓度或温度。低光照时，光反应限制 ATP 和还原型 NADP 的生成。随着光照增强，CO₂ 供应或温度可能成为限制因素。

Growers manipulate these factors in greenhouses: supplementary lighting, CO₂ enrichment, and controlled heating maximise crop yield. You should be able to interpret graphs showing how the rate levels off when a different factor becomes limiting.

种植者在温室中调控这些因素：补充光照、增施 CO₂ 和控温加热以最大化作物产量。要能够解读图表，说明当另一个因素成为限制因子时，光合速率如何趋于平缓。

4. Structure and Function of Xylem | 木质部的结构与功能

Xylem vessels transport water and dissolved mineral ions from roots to shoots. Mature xylem consists of dead, hollow cells arranged end-to-end with no end walls, forming continuous tubes. Their walls are thickened with lignin, which provides strength and waterproofing, preventing collapse under tension.

木质部导管将水分和溶解的矿质离子从根部运送到地上部分。成熟的木质部由死去的、中空的细胞端对端连接而成，无端壁，形成连续的管道。细胞壁因木质素而加厚，提供强度并防水，防止在张力下塌陷。

Water movement in xylem is explained by the cohesion-tension theory. Water molecules cohere to each other and adhere to xylem walls, creating a continuous column. Transpiration at the leaves generates tension, pulling the water column upward.

木质部中的水分运输可用内聚力-张力理论解释。水分子彼此内聚并附着于木质部壁，形成连续水柱。叶片蒸腾作用产生张力，将水柱向上拉。

5. Structure and Function of Phloem | 韧皮部的结构与功能

Phloem transports organic solutes, primarily sucrose, from sources (e.g., leaves) to sinks (e.g., roots, fruits). It is composed of sieve tube elements, which are living cells lacking a nucleus and most organelles, arranged end-to-end with sieve plates. Companion cells alongside them provide metabolic support via numerous mitochondria and plasmodesmata.

韧皮部运输有机溶质，主要是蔗糖，从源（如叶片）到库（如根、果实）。它由筛管分子组成，这些活细胞没有细胞核和大部分细胞器，端对端连接，具筛板。与其相邻的伴胞通过大量线粒体和胞间连丝提供代谢支持。

The mass flow hypothesis describes phloem loading. Sucrose is actively loaded into the phloem at the source, lowering the water potential. Water enters by osmosis, raising hydrostatic pressure. At the sink, sucrose is unloaded, water leaves, and pressure drops, creating a pressure gradient that drives bulk flow.

质流假说描述了韧皮部装载。蔗糖在源端被主动装载到韧皮部，降低水势。水分通过渗透进入，使静水压升高。在库端，蔗糖被卸载，水分流出，压力下降，形成压力梯度驱动物质整体流动。

6. Transpiration and Stomatal Control | 蒸腾作用与气孔调控

Transpiration is the loss of water vapour from leaves, mainly through stomata. It creates the tension necessary for water uptake and mineral transport from roots. Factors increasing transpiration rate include higher temperature, lower humidity, increased air movement, and greater light intensity (which causes stomata to open).

蒸腾作用是叶片散失水蒸气的过程，主要通过气孔进行。它产生必要的张力，促进根部吸水与矿质运输。增加蒸腾速率的因素包括较高温度、较低湿度、空气流动增强和光照强度增大（导致气孔张开）。

Guard cells control stomatal opening. During light, potassium ions are actively pumped into guard cells, lowering water potential; water follows by osmosis, causing the cells to swell and curve due to unevenly thickened cell walls, opening the pore. The hormone abscisic acid (ABA) triggers stomatal closure under water stress.

保卫细胞控制气孔开闭。光下钾离子被主动泵入保卫细胞，降低水势；水分通过渗透进入，细胞因壁厚不均而膨胀弯曲，打开气孔。在水分胁迫下，激素脱落酸 (ABA) 触发气孔关闭。

7. Plant Hormones: Auxin and Tropisms | 植物激素：生长素与向性

Auxins, such as IAA (indole-3-acetic acid), are synthesised in shoot and root tips and regulate directional growth responses. In phototropism, auxin redistributes to the shaded side of a shoot, promoting cell elongation and causing bending toward light. In roots, high auxin concentration inhibits elongation, causing bending away from light.

生长素，如 IAA（吲哚-3-乙酸），在茎尖和根尖合成，调控向性生长反应。在向光性中，生长素重新分布到茎的背光侧，促进细胞伸长，导致向光弯曲。在根中，高浓度生长素抑制伸长，导致背离光源弯曲。

Geotropism (gravitropism) involves auxin moving to the lower side of a root or shoot. In shoots, this stimulates growth upward; in roots, it inhibits growth on the lower side, causing downward bending. Understanding the acid growth hypothesis aids in explaining auxin action on cell walls.

向地性涉及生长素移动到根或茎的下侧。在茎中，这刺激向上生长；在根中，它抑制下侧生长，导致向下弯曲。理解酸生长假说有助于解释生长素对细胞壁的作用。

8. Other Plant Hormones and Commercial Uses | 其他植物激素及商业应用

Gibberellins promote stem elongation, seed germination, and fruit development. They stimulate production of amylase enzymes that break down starch into sugars in germinating seeds. Commercially, gibberellins delay fruit senescence and produce seedless grapes.

赤霉素促进茎伸长、种子萌发和果实发育。它们刺激淀粉酶生成，在萌发种子中将淀粉分解为糖。商业上赤霉素用于延缓果实衰老和生产无籽葡萄。

Ethene is a gaseous hormone that triggers fruit ripening and leaf abscission. It is used to synchronise ripening in harvested fruit. Auxins and cytokinins are also used in tissue culture to induce root and shoot formation.

乙烯是一种气体激素，触发果实成熟和叶片脱落。它用于使采收后果实同步成熟。生长素和细胞分裂素也用于组织培养，诱导根和芽的形成。

9. Plant Reproduction: Flowering and Pollination | 植物生殖：开花与传粉

Flowering is controlled by photoperiodism, where plants detect the length of day and night using the pigment phytochrome. In short-day plants, flowering is triggered when the night length exceeds a critical duration, while in long-day plants, flowering occurs when nights are shorter than a critical period.

开花受光周期现象控制，植物利用光敏色素检测昼夜长度。短日照植物在夜长超过临界时长时诱导开花，而长日照植物在夜长短于临界期时开花。

Pollination can be wind- or insect-mediated. Wind-pollinated plants often possess small, inconspicuous flowers with large amounts of lightweight pollen and feathery stigmas. Insect-pollinated flowers are brightly coloured, produce nectar, and have sticky pollen. Fertilisation involves a pollen tube growing down the style to deliver male gametes to the ovule.

传粉可以是风媒或虫媒。风媒植物通常具有小而不显眼的花，产生大量轻质花粉和羽毛状柱头。虫媒花颜色鲜艳，产生花蜜，花粉具黏性。受精涉及花粉管沿花柱生长，将雄配子送达胚珠。

10. Seed Formation and Germination | 种子形成与萌发

In angiosperms, double fertilisation occurs: one male gamete fuses with the egg cell to form a diploid zygote, while the other fuses with two polar nuclei to form the triploid endosperm, a nutrient-rich tissue. The ovule develops into a seed, and the ovary becomes the fruit.

在被子植物中发生双受精：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，这是一种富含营养的组织。胚珠发育为种子，子房发育为果实。

For germination, seeds require water, oxygen, and a suitable temperature. Water activates enzymes, including gibberellin-induced amylase, which hydrolyses stored starch into glucose for respiration. Oxygen is needed for aerobic respiration, and temperature affects enzyme activity.

种子萌发需要水分、氧气和适宜的温度。水分激活酶，包括赤霉素诱导的淀粉酶，将贮存的淀粉水解为葡萄糖用于呼吸作用。氧气是有氧呼吸所必需的，温度影响酶活性。

11. Nuclear Division in Plants: Mitosis and Meiosis | 植物细胞核分裂：有丝分裂与减数分裂

Mitosis occurs in meristems (apical and lateral) for growth and repair. Unlike animal cells, plant cells do not possess centrioles; a spindle apparatus still forms. Cytokinesis occurs by formation of a cell plate from Golgi-derived vesicles, which fuses to create a new cell wall.

有丝分裂发生于分生组织（顶端和侧生），用于生长和修复。与动物细胞不同，植物细胞没有中心粒，但仍形成纺锤体。胞质分裂通过高尔基体来源的囊泡形成细胞板，融合产生新细胞壁。

Meiosis produces haploid spores in the anthers and ovules. This leads to the formation of pollen grains and embryo sacs. Independent assortment and crossing over during meiosis increase genetic variation, crucial for adaptation and evolution in plant populations.

减数分裂在花药和胚珠中产生单倍体孢子，进而形成花粉粒和胚囊。减数分裂中的独立分配和交叉互换增加遗传变异，对植物种群的适应和进化至关重要。

12. Plant Cloning and Biotechnology | 植物克隆与生物技术

Plants can be cloned naturally through runners, bulbs, or tubers. Artificial methods include taking cuttings, grafting, and micropropagation (tissue culture). Micropropagation involves growing explants on sterile agar containing nutrients and plant hormones, enabling mass production of disease-free plants.

植物可通过匍匐茎、鳞茎或块茎自然克隆。人工方法包括插条、嫁接和微繁殖（组织培养）。微繁殖涉及在含营养和植物激素的无菌琼脂上培养外植体，可大规模生产无病植株。

Tissue culture relies on the totipotency of plant cells and the balance of auxins and cytokinins to direct organogenesis. It is invaluable for conserving rare species and producing genetically uniform crops. Similar hormone manipulation is used in producing transgenic plants.

组织培养依赖植物细胞的全能性以及生长素和细胞分裂素的平衡来定向器官发生。它在保护稀有物种和生产遗传一致的作物方面具有极高价值。类似的激素调控也用于转基因植物的生产。

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