📚 IB Biology: Plant Biology Revision Highlights | IB 生物学：植物考点精讲
This revision guide covers key plant biology concepts for the IB Biology syllabus, including tissue types, transport systems, photosynthesis, and reproduction. Use it to consolidate your understanding and prepare effectively for examinations.
本复习指南涵盖了 IB 生物学大纲中植物生物学的核心概念，包括组织类型、运输系统、光合作用和繁殖。用它来巩固理解并有效备考。
1. Plant Tissue Systems and Meristems | 植物组织系统与分生组织
Plants possess three main tissue systems: dermal tissue (epidermis) for protection, vascular tissue (xylem and phloem) for transport, and ground tissue (parenchyma, collenchyma, sclerenchyma) for storage, photosynthesis and support. Meristematic tissues are regions of undifferentiated cells where active mitosis occurs, giving rise to primary growth (apical meristems) and secondary growth (lateral meristems such as vascular cambium and cork cambium).
植物具有三大组织系统：保护性的皮组织（表皮）、运输性的维管组织（木质部和韧皮部）以及负责储存、光合作用和支撑的基本组织（薄壁组织、厚角组织、厚壁组织）。分生组织是未分化细胞所在的区域，这里进行活跃的有丝分裂，导致初生生长（顶端分生组织）和次生生长（侧生分生组织，如维管形成层和木栓形成层）。
In dicotyledonous plants, the vascular cambium produces secondary xylem inward and secondary phloem outward, leading to increased girth. Cork cambium generates periderm that replaces the epidermis in woody stems and roots, providing a protective bark layer.
在双子叶植物中，维管形成层向内产生次生木质部，向外产生次生韧皮部，使得茎秆加粗。木栓形成层产生周皮，在木本茎和根中代替表皮，形成具有保护作用的树皮层。
2. Root Structure and Absorption | 根的结构与吸收
The root system anchors the plant and absorbs water and mineral ions from the soil. The epidermis of young roots produces root hairs, which significantly increase the surface area for uptake. Water and dissolved minerals move from the soil into the root via two main pathways: the apoplast pathway (through cell walls and intercellular spaces) and the symplast pathway (through the cytoplasm and plasmodesmata).
根系固定植物并从土壤中吸收水分和矿质离子。幼根的表皮产生根毛，大大增加了吸收的表面积。水和溶解的矿物质通过两条主要途径从土壤进入根内：质外体途径（经过细胞壁和细胞间隙）和共质体途径（经过细胞质和胞间连丝）。
The endodermis, the innermost layer of the cortex, features a Casparian strip – a waxy, suberin-rich band that blocks the apoplast pathway. This forces water and ions to pass through the plasma membrane of endodermal cells, allowing the plant to control the entry of minerals selectively using membrane transport proteins.
内皮层是皮层最内层，具有凯氏带——一条富含木栓质的蜡质带，可阻断质外体途径。这就迫使水和离子必须穿过内皮层细胞的质膜，使植物能够借助膜转运蛋白选择性地控制矿物质的进入。
3. Stem Anatomy and Vascular Arrangement | 茎的解剖与维管排列
Stems provide structural support and house the vascular bundles that connect roots and leaves. The arrangement of vascular tissues differs between monocots and dicots, a key feature used in plant classification.
茎提供结构支撑，并容纳连接根和叶的维管束。单子叶植物和双子叶植物维管组织的排列方式不同，这是植物分类的一个关键特征。
| Feature | Dicot Stem | Monocot Stem |
|---|---|---|
| Vascular bundle arrangement | Arranged in a ring near the periphery | Scattered throughout the ground tissue |
| Vascular cambium | Present between xylem and phloem, allowing secondary growth | Absent; thus no secondary growth |
| Pith and cortex | Well-developed pith (centre) and cortex | No distinct pith; ground tissue is uniform or cortex not clearly defined |
4. Xylem and Water Transport | 木质部与水分运输
Xylem is a complex tissue composed mainly of tracheids and vessel elements, both dead at maturity. Its primary function is to transport water and dissolved minerals from roots to shoots. The cohesion-tension theory explains the mechanism: transpiration at the leaf surface generates negative pressure (tension) that pulls water up the xylem in a continuous column, held together by cohesive forces between water molecules and adhesive forces to xylem walls.
木质部是一种复合组织，主要由管胞和导管分子构成，两者在成熟时都是死细胞。其主要功能是将水和溶解的矿物质从根运送到地上部分。内聚力-张力机制解释这一过程：叶片表面的蒸腾作用产生负压（张力），将水以连续水柱的形式拉上木质部，水分子之间的内聚力以及水与木质部壁的附着力维持了水柱的连贯性。
The transpiration stream can be measured using a potometer, which estimates water uptake by a cut shoot. Environmental factors such as light intensity, temperature, wind speed and humidity affect the rate of transpiration, thereby influencing water movement through the plant.
可用蒸腾计测量蒸腾速率，通过测定切枝的吸水量来估算。光强、温度、风速和湿度等环境因素影响蒸腾速率，从而影响水分在植物体内的运动。
5. Phloem and Translocation | 韧皮部与有机物运输
Phloem transports organic compounds, primarily sucrose and amino acids, from sources (e.g. mature leaves producing photosynthates) to sinks (e.g. roots, developing fruits, growing meristems). Phloem tissue consists of sieve tube elements and companion cells; the latter are metabolically active and support the adjoining sieve elements.
韧皮部运输有机物，主要是蔗糖和氨基酸，从源（例如进行光合作用的成熟叶片）运到库（例如根、发育中的果实、生长中的分生组织）。韧皮部由筛管分子和伴胞组成；伴胞代谢活跃，为相邻的筛分子提供支持。
The pressure-flow hypothesis describes translocation: active loading of sucrose at the source decreases water potential, causing water to enter the phloem from adjacent xylem, generating a high hydrostatic pressure. At the sink, unloading of sucrose removes solutes, lowering pressure. This pressure gradient drives mass flow of phloem sap from source to sink.
压力流动假说描述了运输过程：在源端主动装载蔗糖使水势降低，水分从邻近的木质部进入韧皮部，产生高静水压力。在库端，蔗糖的卸载降低了溶质浓度，压力下降。这种压力梯度推动韧皮部汁液从源向库整体流动。
6. Leaf Structure and Gas Exchange | 叶片结构与气体交换
The leaf is the primary photosynthetic organ. A typical dicot leaf consists of an upper and lower epidermis covered by a waxy cuticle, with stomata mostly on the lower surface for gaseous exchange. The mesophyll differentiates into palisade mesophyll (densely packed, chloroplast-rich cells for light capture) and spongy mesophyll (loosely arranged cells with large air spaces for gas diffusion).
叶片是主要的光合器官。典型的双子叶叶片由上表皮和下表皮组成，覆盖着蜡质角质层，气孔多分布于下表面以进行气体交换。叶肉分化为栅栏组织（细胞紧密排列、富含叶绿体，用于捕获光能）和海绵组织（细胞排列疏松，有大的气隙用于气体扩散）。
Stomatal opening is regulated by guard cells. When guard cells gain K⁺, water follows by osmosis; guard cells become turgid and the stoma opens, allowing CO₂ uptake and O₂ and water vapour release. In darkness or under water stress, guard cells lose K⁺, become flaccid and the stoma closes, reducing water loss.
气孔的开闭由保卫细胞调控。当保卫细胞积累 K⁺，水通过渗透作用进入，保卫细胞膨胀，气孔张开，允许 CO₂ 的吸收和 O₂ 及水蒸气的释放。在黑暗或水分胁迫下，保卫细胞失去 K⁺，变得松弛，气孔关闭，减少水分流失。
7. Photosynthesis: Light-dependent Reactions | 光合作用：光反应
Light-dependent reactions take place in the thylakoid membranes of chloroplasts. Light energy absorbed by chlorophyll and accessory pigments drives the photolysis of water, producing electrons, protons (H⁺) and oxygen. The overall equation for photolysis is:
光反应发生在叶绿体的类囊体膜上。叶绿素和辅助色素吸收的光能驱动水的光解，产生电子、质子 (H⁺) 和氧气。水光解的总方程式为：
2 H₂O → 4 H⁺ + 4 e⁻ + O₂
Excited electrons move through the electron transport chain involving photosystem II (PSII) and photosystem I (PSI). As electrons flow non-cyclically, they reduce NADP⁺ to NADPH via ferredoxin-NADP⁺ reductase. Concurrently, a proton gradient is built up across the thylakoid membrane, driving ATP synthesis by chemiosmosis through ATP synthase.
受激发的电子通过涉及光系统 II (PSII) 和光系统 I (PSI) 的电子传递链传递。当电子进行非循环流动时，它们通过铁氧还蛋白-NADP⁺ 还原酶将 NADP⁺ 还原为 NADPH。同时，在类囊体膜两侧建立起质子梯度，通过化学渗透作用驱动 ATP 合酶合成 ATP。
Cyclic photophosphorylation, using PSI only, produces extra ATP without generating NADPH or O₂. It helps satisfy the higher ATP demand of the Calvin cycle when NADPH levels are sufficient.
环式光合磷酸化仅使用 PSI，可额外产生 ATP 而不生成 NADPH 或 O₂。当 NADPH 水平充足时，这有助于满足卡尔文循环对 ATP 更高的需求。
8. Photosynthesis: The Calvin Cycle | 光合作用：卡尔文循环
The Calvin cycle (light-independent reactions) occurs in the stroma of chloroplasts and uses ATP and NADPH from the light-dependent reactions to fix CO₂ into carbohydrate. The cycle comprises three main phases: carbon fixation, reduction, and regeneration of ribulose bisphosphate (RuBP).
卡尔文循环（暗反应）发生在叶绿体的基质中，利用光反应提供的 ATP 和 NADPH 将 CO₂ 固定为碳水化合物。该循环包括三个主要阶段：碳固定、还原和核酮糖二磷酸 (RuBP) 的再生。
In the carbon fixation step, CO₂ combines with RuBP (a 5‑carbon sugar), catalysed by the enzyme RuBisCO (ribulose‑1,5‑bisphosphate carboxylase/oxygenase). The unstable 6‑carbon intermediate immediately splits into two molecules of 3‑phosphoglycerate (3‑PG), a 3‑carbon compound.
在碳固定步骤中，CO₂ 与 RuBP（一种五碳糖）结合，由 RuBisCO 酶（核酮糖‑1,5‑二磷酸羧化酶/加氧酶）催化。不稳定的六碳中间物立即分裂成两分子 3‑磷酸甘油酸 (3‑PG)，一种三碳化合物。
Reduction follows: 3‑PG is phosphorylated by ATP and reduced by NADPH to form glyceraldehyde‑3‑phosphate (G3P, also called triose phosphate). For every three CO₂ fixed, six G3P molecules are produced; one exits the cycle to contribute to glucose and other organic molecules, while the remaining five are used to regenerate three RuBP molecules, requiring additional ATP.
随后是还原阶段：3‑PG 被 ATP 磷酸化，并被 NADPH 还原，生成甘油醛‑3‑磷酸（G3P，也称磷酸丙糖）。每固定三分子 CO₂，产生六分子 G3P；其中一分子离开循环用于合成葡萄糖和其他有机物，其余五分子用于再生三分子的 RuBP，该过程需要额外的 ATP。
The overall balanced equation for photosynthesis is:
光合作用的总平衡方程式为：
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
9. Factors Affecting Photosynthesis | 影响光合作用的因素
The rate of photosynthesis is influenced by light intensity, carbon dioxide concentration, and temperature. At any given time, the factor that is in shortest supply or at a limiting level sets the overall reaction rate – this is the principle of limiting factors.
光合作用速率受光照强度、二氧化碳浓度和温度的影响。在任何给定时刻，供应最不足或处于限制水平的那个因素决定了整体反应速率——这就是限制因子原理。
At low light intensities, the production of ATP and NADPH is limited; increasing light intensity raises the rate until another factor, such as CO₂ concentration, becomes limiting. Similarly, raising CO₂ concentration around the leaf accelerates carbon fixation until the plant’s enzymatic capacity or light availability restricts further increase. Temperature affects enzyme activity, particularly RuBisCO; while a moderate rise accelerates the Calvin cycle, excessively high temperatures can denature enzymes and cause stomatal closure, reducing CO₂ intake and thus photosynthesis.
在低光照强度下，ATP 和 NADPH 的产生受限；增加光强可提高速率，直到另一个因子（如 CO₂ 浓度）成为限制因子。同样，提高叶片周围的 CO₂ 浓度会加速碳固定，直到植物酶促能力或光照供给制约了进一步提高。温度影响酶活性，尤其是 RuBisCO；温和升温可加速卡尔文循环，但过高的温度会使酶变性，并引起气孔关闭，从而减少 CO₂ 摄入，抑制光合作用。
10. Flower Structure and Sexual Reproduction | 花的结构与有性生殖
Flowers are the reproductive structures of angiosperms. A typical complete flower contains four whorls: the outermost sepals (calyx) that protect the bud, petals (corolla) that attract pollinators, stamens (the male reproductive parts consisting of anther and filament), and carpels (the female reproductive parts comprising stigma, style and ovary).
花是被子植物的生殖结构。一朵典型的完全花包含四轮结构：最外层的花萼（花萼片）保护花蕾，花瓣（花冠）吸引传粉者，雄蕊（雄性生殖器官，包括花药和花丝），以及心皮（雌性生殖器官，由柱头、花柱和子房组成）。
The anther produces pollen grains that contain the male gametophyte; each pollen grain typically has a tube cell and a generative cell that will divide to form two sperm cells. Within the ovule, the embryo sac (female gametophyte) contains the egg cell, synergids, antipodal cells and the central cell with two polar nuclei.
花药产生含有雄配子体的花粉粒；每粒花粉通常有一个管细胞和一个生殖细胞，生殖细胞将分裂形成两个精细胞。在胚珠内部，胚囊（雌配子体）含有卵细胞、助细胞、反足细胞以及含有两个极核的中央细胞。
11. Pollination, Fertilisation and Double Fertilisation | 传粉、受精与双受精
Pollination is the transfer of pollen from the anther to a stigma. It may be by wind, insects, birds or other agents. After landing on a compatible stigma, the pollen grain germinates, forming a pollen tube that grows down the style towards the ovary, guided by chemical signals from the synergids.
传粉是指花粉从花药转移到柱头。可以依靠风、昆虫、鸟类或其他媒介。落到亲和性柱头上后，花粉粒萌发，形成花粉管，在助细胞的化学信号引导下，沿花柱向下生长，朝向子房。
Angiosperms undergo double fertilisation. One sperm nucleus fuses with the egg cell to form the diploid (2n) zygote, which develops into the embryo. The second sperm nucleus fuses with the two polar nuclei in the central cell, producing a triploid (3n) endosperm that serves as nutritive tissue for the developing embryo.
被子植物经历双受精过程。一个精核与卵细胞融合，形成二倍体 (2n) 合子，发育成胚。另一个精核与中央细胞的两个极核融合，形成三倍体 (3n) 胚乳，为发育中的胚提供营养组织。
Self-pollination occurs within the same flower or plant, whereas cross-pollination involves different plants of the same species. Cross-pollination promotes genetic variation, which is enhanced by mechanisms such as self-incompatibility and dioecy.
自花传粉发生在同一朵花或同一植株内，而异花传粉则涉及同一物种的不同植株。异花传粉促进遗传变异，像自交不亲和性和雌雄异株等机制可以增强这一效果。
12. Seed Structure, Dispersal and Germination | 种子结构、传播与萌发
Following fertilisation, the ovule develops into a seed. A typical seed comprises the embryo (a radicle and plumule, one or two cotyledons), a seed coat (testa) derived from the integuments, and in many cases a triploid endosperm. Monocots such as maize retain endosperm, while dicots like beans store nutrients mainly in the cotyledons.
受精后，胚珠发育成种子。一粒典型的种子包含胚（胚根和胚芽，一枚或两片子叶）、由珠被发育而来的种皮（外种皮），以及在许多情况下存在的三倍体胚乳。单子叶植物如玉米保留胚乳，而双子叶植物如菜豆则主要在子叶中储存养分。
Seed dispersal mechanisms (wind, water, animals, explosive dehiscence) reduce competition between offspring and parent and enable colonisation of new habitats. Germination begins when water uptake triggers metabolic activity. Gibberellin hormones are synthesised in the embryo, stimulating the production of amylase to hydrolyse stored starch into sugars, providing energy for cell division and growth until the seedling can photosynthesise.
种子传播机制（风、水、动物、弹裂）降低了后代与亲本之间的竞争，并使开拓新栖息地成为可能。萌发始于吸胀引发的代谢活动。在胚中合成的赤霉素激素刺激淀粉酶的产生，将储存的淀粉水解为糖，为细胞分裂和生长提供能量，直到幼苗能进行光合作用。
Environmental conditions required for germination include water, oxygen (for aerobic respiration) and a suitable temperature. Some seeds also require light or a period of cold stratification to overcome dormancy, ensuring germination occurs under favourable seasonal conditions.
萌发所需的环境条件包括水分、氧气（用于有氧呼吸）和适宜的温度。有些种子还需要光照或一段冷分层来打破休眠，确保萌发发生在有利的季节条件下。
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