OCR Science: Plant Key Points Revision | OCR 科学：植物考点精讲

📚 OCR Science: Plant Key Points Revision | OCR 科学：植物考点精讲

Plants are fundamental to life on Earth and form a core part of the OCR Science specification. Understanding how plants capture energy through photosynthesis, transport water and nutrients, and respond to their environment is essential for exam success. This guide distills the key points you must know, with clear explanations and examples that follow the OCR style.

植物是地球生命的基础，也是 OCR 科学考试大纲中的核心内容。理解植物如何通过光合作用捕获能量、运输水分和养分，以及如何对环境作出响应，是取得好成绩的关键。本指南提炼了你必须掌握的重点，用清晰易懂的解释和符合 OCR 风格的示例助你备考。

1. Photosynthesis Equation and Limiting Factors | 光合作用方程式与限制因素

Photosynthesis is the endothermic reaction in which plants use light energy to convert carbon dioxide and water into glucose and oxygen. It takes place in chloroplasts, which contain chlorophyll that absorbs sunlight.

光合作用是一个吸热反应，植物利用光能将二氧化碳和水转化为葡萄糖和氧气。反应发生在含有叶绿素的叶绿体中，叶绿素吸收阳光。

6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂

This equation summarises the overall change. Glucose is used immediately for respiration, converted into starch for storage, or built into cellulose for cell walls and other organic molecules.

该方程式概括了整个变化过程。葡萄糖可立即用于呼吸作用，转化为淀粉储存，或合成为细胞壁所需的纤维素及其他有机分子。

The rate of photosynthesis is affected by three main limiting factors: light intensity, carbon dioxide concentration, and temperature. At low light, the rate increases linearly with light intensity, until another factor becomes limiting. If CO₂ concentration is too low, the rate plateaus even with bright light. Temperature affects enzyme activity; at low temperatures the rate is slow, and above the optimum, enzymes denature and the rate drops sharply.

光合作用速率受到三个主要限制因素的影响：光强度、二氧化碳浓度和温度。光照不足时，速率随光强线性增加，直到另一个因素成为限制条件。如果 CO₂ 浓度过低，即使光线充足，速率也会趋于平稳。温度影响酶活性；低温时速率缓慢，超过最适温度后酶会变性，速率急剧下降。

In the exam, you need to interpret graphs showing how one factor limits the rate and explain why the curve levels off. Also recall practical setups using pondweed (Elodea) to measure oxygen production as an indicator of photosynthesis rate.

考试中你需要解读呈现某个因素如何限制速率的图表，并解释曲线走平的原因。还要记住使用水蕴草（Elodea）测定氧气产生量以指示光合速率的相关实验设置。

2. Leaf Structure and Function | 叶片结构与功能

Leaves are organs specialised for photosynthesis. Their broad, flat shape provides a large surface area to capture light, while the thin structure minimises diffusion distance for gases.

叶片是专门进行光合作用的器官。它们宽而扁平的形状提供了很大的表面积来捕获光线，同时其薄的结构将气体的扩散距离缩至最短。

Upper epidermis – covered with a transparent waxy cuticle to reduce water loss and allow light penetration.
上表皮 – 表面覆盖一层透明的蜡质角质层，用于减少水分蒸发并让光线透过。
Palisade mesophyll – closely packed, columnar cells near the upper surface, containing many chloroplasts for efficient light absorption.
栅栏组织 – 靠近上表皮、排列紧密的柱状细胞，含有大量叶绿体以便高效吸收光能。
Spongy mesophyll – loosely arranged cells with air spaces that facilitate diffusion of CO₂, O₂, and water vapour.
海绵组织 – 排列疏松的细胞和细胞间的气室，有助于 CO₂、O₂ 和水蒸气的扩散。
Vascular bundle (vein) – contains xylem (for water and minerals) and phloem (for glucose transport).
维管束（叶脉） – 包含木质部（运输水分和矿物质）和韧皮部（运输葡萄糖）。
Lower epidermis with guard cells – paired guard cells control opening and closing of stomata for gas exchange.
下表皮和气孔保卫细胞 – 成对的保卫细胞控制着气孔的开闭以进行气体交换。

Be able to label a cross-section diagram and match each tissue to its function. OCR questions often ask you to explain how leaf structure is adapted for photosynthesis.

你需要能标注叶片横切面示意图，并将每种组织与其功能对应起来。OCR 试题常要求解释叶片结构如何适应光合作用。

3. Stomata and Gas Exchange | 气孔与气体交换

Stomata are tiny pores mainly located on the lower epidermis of leaves. They allow carbon dioxide to enter for photosynthesis and oxygen and water vapour to exit.

气孔是主要位于叶片下表皮上的微小孔隙。它们让二氧化碳进入以进行光合作用，并让氧气和水蒸气排出。

Guard cells control stomatal aperture. During the day, guard cells take in water by osmosis, become turgid, and bend outward, opening the stoma. At night, they lose water, become flaccid, and the stoma closes. This regulation reduces water loss while still permitting gas exchange when photosynthesis is active.

保卫细胞控制着气孔的开度。白天，保卫细胞通过渗透作用吸水，变得膨压并向外弯曲，打开气孔；夜晚，它们失水，变得松弛，气孔关闭。这种调控机制能在保障光合作用所需气体交换的同时减少水分损失。

You must also know that guard cells contain chloroplasts, which produce ATP for active transport of ions, and that potassium ion concentration changes osmotically regulate water movement.

你还应知道保卫细胞含有叶绿体，可为离子主动运输提供 ATP，并且钾离子浓度的变化通过渗透调节水分运动。

4. Transpiration and Transpiration Stream | 蒸腾作用与蒸腾流

Transpiration is the evaporation of water vapour from the mesophyll cells and its diffusion out through stomata. This creates a tension (suction) that pulls water up through the xylem from the roots – the transpiration stream.

蒸腾作用是指叶肉细胞中的水蒸气蒸发并经由气孔扩散出去的过程。这会产生一种拉力（吸力），将水从根部沿木质部向上拉——形成蒸腾流。

The transpiration stream is driven by the cohesion and tension theory: water molecules cohere (stick together) due to hydrogen bonding, and as one molecule evaporates, it pulls the next one up through the narrow xylem vessels. This continuous column of water transports dissolved mineral ions as well.

蒸腾流是由内聚力和张力理论驱动的：水分子因氢键而彼此内聚（相互吸引），当一个水分子蒸发时，它会拉拽下一个分子通过狭窄的木质部导管向上移动。此连续水柱还能运输溶解的矿物质离子。

Factors increasing transpiration rate: higher temperature (more evaporation), higher wind speed (removes humid air), lower humidity (steeper diffusion gradient), and higher light intensity (stomata open wider). You should be able to predict and explain changes using a potometer experiment.

提高蒸腾速率的因素：较高温度（蒸发加快）、较大风速（带走潮湿空气）、较低湿度（扩散梯度更大）和较强光照（气孔开度更大）。你需要能通过蒸腾计实验预测并解释这些变化。

5. Xylem and Phloem | 木质部和韧皮部

Plants possess two types of vascular tissue: xylem and phloem. They form continuous vascular bundles extending from root to shoot.

植物有两种类型的维管组织：木质部和韧皮部。它们构成从根到茎叶的连续维管束。

Feature	Xylem	Phloem
Transport direction	Upwards (root → shoot)	Up and down (source → sink)
Substances transported	Water and dissolved minerals	Sucrose and amino acids (translocation)
Cells involved	Dead, hollow tubes; walls strengthened with lignin	Living sieve tube elements with companion cells
Mechanism	Passive, driven by transpiration pull	Active, requires energy for loading sucrose

特征：木质部向上输送水分和矿物质，由死细胞形成的空心管组成，细胞壁有木质素加固；韧皮部可上下双向运输蔗糖和氨基酸，为活细胞（筛管和伴胞），需要能量主动装载。

Xylem vessels are strengthened with lignin which can form rings, spirals, or solid tubes. This prevents collapse under negative pressure. Phloem sieve plates allow flow of phloem sap between cells, and companion cells provide ATP for active translocation.

木质部导管由木质素加固，可形成环纹、螺旋或实心管状，防止在负压下塌陷。韧皮部筛板允许汁液在细胞间流动，伴胞则为主动转运提供 ATP。

6. Mineral Requirements (Nitrates, Magnesium) | 矿质需求（硝酸盐、镁）

Plants absorb mineral ions from the soil through root hair cells by active transport. Two key minerals are nitrates and magnesium, and deficiencies lead to distinct symptoms.

植物通过根毛细胞主动运输从土壤中吸收矿质离子。两种关键矿物是硝酸盐和镁，缺乏时会出现特定症状。

Nitrates are needed to synthesise amino acids, proteins, and chlorophyll. A nitrate deficiency causes stunted growth and yellowing of older leaves because nitrogen is mobile and re‑located to younger tissues.

硝酸盐是合成氨基酸、蛋白质和叶绿素所必需的。缺氮会导致植株矮小，老叶先变黄，因为氮可在植物体内移动，会被转运至新生组织。

Magnesium is a central component of the chlorophyll molecule. Without enough magnesium, chlorophyll cannot be made, leading to chlorosis (yellowing between leaf veins). This reduces the plant’s ability to photosynthesise.

镁是叶绿素分子的核心成分。缺镁时无法制造叶绿素，导致缺绿病（叶脉间变黄），从而降低光合作用能力。

In exam questions, link mineral functions to visible symptoms and explain why farmers use fertilisers containing NPK (nitrogen, phosphorus, potassium). Active transport requires energy from respiration, so waterlogged or compacted soils limit oxygen and reduce mineral uptake.

在考题中，需将矿物功能与可见症状联系起来，并解释为什么农民施用含 NPK（氮、磷、钾）的化肥。主动运输需要呼吸作用提供能量，因此积水或板结的土壤会限制氧气供应，减少矿质吸收。

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

Plants produce hormones to coordinate growth and responses to environmental stimuli (tropisms). Auxin is the main growth hormone in shoots and roots, promoting cell elongation in shoots and inhibiting it in roots at high concentrations.

植物产生激素来协调生长和对环境刺激的反应（向性）。生长素是茎和根中的主要生长激素，在茎中促进细胞伸长，而在根部高浓度时则抑制伸长。

Phototropism: When light shines from one side, auxin redistributes to the shaded side. In shoots, higher auxin concentration on the shaded side causes cells there to elongate more, bending the shoot towards light (positive phototropism). This maximises light capture.

向光性：当光从一侧照射时，生长素重新分布到背光侧。在茎中，背光侧较高浓度的生长素使该侧细胞伸长更多，从而使茎向光弯曲（正向光性）。这能最大限度地捕获光线。

Gravitropism (geotropism): In roots, auxin accumulates on the lower side due to gravity. High auxin inhibits cell elongation in root cells, so the upper side elongates more, causing the root to bend downwards (positive gravitropism). Shoots exhibit negative gravitropism, bending upwards.

向地性：在根中，重力导致生长素累积在下侧。高浓度生长素抑制根部细胞伸长，因此上侧细胞伸长更快，根向下弯曲（正向地性）。茎则表现出负向地性，向上弯曲。

You should be able to design experiments using clinostats or agar blocks to investigate tropisms, and explain the role of unequal auxin distribution. A common exam question involves predicting shoot/root curvature after removing tips or adding auxin to cut surfaces.

你需要能设计使用回转器或琼脂块研究向性的实验，并解释生长素不均匀分布的作用。常见的考试题会让学生预测去除茎尖或在切口施加生长素后茎和根的弯曲情况。

8. Asexual and Sexual Reproduction in Plants | 植物的无性繁殖与有性繁殖

Plants can reproduce sexually (involving flowers, pollination and seeds) or asexually (e.g., runners, bulbs, tubers). OCR candidates must know examples and the advantages and disadvantages of each method.

植物可以进行有性繁殖（通过花、传粉和种子）或无性繁殖（如匍匐茎、鳞茎、块茎）。OCR 考生需熟悉示例和每种方式的优缺点。

Sexual reproduction produces genetically varied offspring due to meiosis and fusion of gametes. This variation increases the chance of survival in a changing environment. However, it requires pollination vectors (insects, wind) and is slower. The flower structure is adapted to promote pollination: anthers produce pollen, stigma receives it, and ovary contains ovules.

有性繁殖通过减数分裂和配子融合产生遗传变异的后代。这种变异提高了在变化环境中存活的机会。但它需要传粉媒介（昆虫、风），且速度较慢。花的结构适于促进传粉：花药产生花粉，柱头承接花粉，子房包含胚珠。

Asexual reproduction produces clones – offspring genetically identical to the parent. This is rapid and allows successful traits to be preserved, but lacks genetic variation, so all plants are equally vulnerable to disease or environmental change. Examples: strawberry runners, potato tubers, daffodil bulbs, and artificial cuttings.

无性繁殖产生无性系——后代与亲本基因完全相同。这种方式快速且能保留成功性状，但缺乏遗传变异，所有植株同样易受病害或环境变化影响。举例：草莓的匍匐茎、马铃薯的块茎、水仙鳞茎和人工扦插。

9. Seed Dispersal and Germination | 种子传播与萌发

After fertilisation, each ovule develops into a seed containing an embryo, a food store (endosperm or cotyledons), and a protective seed coat. Fruits aid in seed dispersal, preventing competition with the parent plant and colonising new areas.

受精后，每个胚珠发育成种子，包含胚、营养储存（胚乳或子叶）以及保护性的种皮。果实有助于种子传播，避免与亲本植株竞争，并开拓新区域。

Dispersal mechanisms: wind (e.g., dandelion pappus), animal fur (burs), animal ingestion (berries), water (coconut), and mechanical ejection (pea pods). The structure of the fruit is adapted to the dispersal agent, a common OCR question.

传播机制：风力（如蒲公英的冠毛）、动物毛皮（苍耳）、动物摄食（浆果）、水流（椰子）和机械弹射（豌豆荚）。果实的结构适应于传播媒介，这是 OCR 常见的考题。

Seed germination is triggered by water (reactivates metabolism), oxygen (aerobic respiration), and warmth (enzyme activity). The radicle emerges first, anchoring the plant, followed by the plumule (shoot). During early growth, food reserves from cotyledons sustain the seedling until leaves unfold and photosynthesis begins.

种子萌发由水（恢复代谢活动）、氧气（有氧呼吸）和适宜的温度（酶活性）触发。胚根首先伸出，固定植株，随后是胚芽（茎）。早期生长期间，子叶中的养料储备支撑幼苗，直到叶片展开开始光合作用。

10. Practical: Investigating Light Intensity on Photosynthesis | 实验：探究光强对光合作用的影响

A standard OCR practical measures the effect of light intensity on the rate of photosynthesis using pondweed. Canadian pondweed (Elodea) is placed in a boiling tube with sodium hydrogencarbonate solution (CO₂ source), and the number of oxygen bubbles produced per minute is counted at different distances from a lamp.

OCR 的一项标准实验使用水蕴草测量光强度对光合作用速率的影响。将加拿大水蕴草（Elodea）放入装有碳酸氢钠溶液（提供 CO₂）的试管中，在与光源不同距离处计数每分钟产生的氧气泡数量。

If bubbles are counted, keep the lamp distance constant until readings are taken, and allow time for the plant to adjust. A more accurate method uses a gas syringe or capillary tube to measure volume of oxygen evolved. Always identify the independent variable (light intensity, varied by distance), dependent variable (rate of oxygen production), and control variables (temperature, CO₂ concentration, type of plant, time).

若采用气泡计数，需保持灯距恒定后再读数，并留出时间让植物适应。更精确的方法使用气体注射器或毛细管测量放出的氧气体积。始终要明确自变量（通过距离改变光强度）、因变量（氧气产生速率）和控制变量（温度、CO₂ 浓度、植物种类、时间）。

To plot results, calculate the rate as bubbles per minute, and if using distance, remember that light intensity follows the inverse square law: intensity ∝ 1 / distance². A graph of rate versus light intensity will show a curve that eventually plateaus when another factor (often CO₂ or temperature) becomes limiting.

为绘制结果图，计算速率（每分钟气泡数）；若使用距离，记住光强度遵循平方反比定律：强度 ∝ 1 / 距离²。速率对光强度的图形将表现为一条曲线，当其他因素（常为 CO₂ 或温度）成为限制条件时趋于平稳。

Published by TutorHao | Science Revision Series | aleveler.com

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OCR Science: Plant Key Points Revision | OCR 科学：植物 考点精讲