📚 5.5 Plant and Animal Responses: Key Points Breakthrough | 植物与动物响应考点突破
Organisms must detect and respond to internal and external change to survive. In A-level Biology, topic 5.5 covers the elegant mechanisms of plant tropisms, hormonal control, nervous coordination, and muscle contraction. This revision guide breaks down the essential concepts and highlights common examination pitfalls.
生物体必须感知并响应内外环境的变化才能存活。A-level 生物 5.5 单元涵盖了植物向性、激素调控、神经协调和肌肉收缩等精妙机制。本复习指南将逐一攻克核心要点，并指出常见易错点。
1. Plant Tropisms: Phototropism and Gravitropism | 植物的向性：向光性与向地性
Plants respond to directional stimuli by differential growth called tropisms. Phototropism is growth in response to light – shoots grow towards light (positive phototropism) while roots often grow away (negative phototropism). Gravitropism is a response to gravity; roots grow downwards (positive gravitropism) and shoots grow upwards (negative gravitropism). These responses are regulated by plant hormones, particularly auxins such as indole-3-acetic acid (IAA).
植物通过差异生长对方向性刺激作出响应，称为向性。向光性是针对光的生长反应——茎朝向光源生长（正向光性），根则常背向光源（负向光性）。向地性是对重力的响应；根向下生长（正向地性），茎向上生长（负向地性）。这些反应由植物激素调控，尤其是生长素如吲哚乙酸（IAA）。
In shoots exposed to unilateral light, IAA redistributes to the shaded side, stimulating cell elongation there and causing the shoot to bend towards the light. In roots, high IAA concentration inhibits cell elongation, so when IAA accumulates on the lower side of a horizontal root, growth on that side is suppressed, causing the root to bend downwards.
在单侧光照射下的茎中，IAA 重新分布到背光侧，促进该侧细胞伸长，导致茎向光弯曲。在根中，高浓度 IAA 抑制细胞伸长，因此当水平放置的根下方积累 IAA 时，该侧生长受抑制，导致根向下弯曲。
2. The Role of Auxin (IAA) in Tropisms | 生长素（IAA）在向性中的作用
Auxin is synthesised in the shoot tip and moves down the stem by active transport and diffusion. Auxin binds to receptors on target cells and activates proton pumps, causing the cell wall to become more acidic. This acidification activates expansin proteins that loosen cellulose microfibrils, allowing the cell to take up water and elongate. The chemiosmotic mechanism of polar auxin transport relies on PIN proteins that actively pump IAA out of cells.
生长素在茎尖合成，并通过主动运输和扩散向下运输。生长素与靶细胞上的受体结合，激活质子泵，使细胞壁酸化。这种酸化激活扩展蛋白，使纤维素微纤维松弛，允许细胞吸水并伸长。极性生长素运输的化学渗透机制依赖于 PIN 蛋白，它们主动将 IAA 泵出细胞。
Key exam point: In shoots, auxin promotes elongation; in roots, excess auxin inhibits elongation. The different sensitivity explains opposite tropic responses.
考点提示：在茎中，生长素促进伸长；在根中，过量生长素抑制伸长。这种敏感性差异解释了相反的向性反应。
3. Other Plant Hormones and Their Functions | 其他植物激素及其功能
| Hormone | 激素 | Major Functions | 主要功能 |
|---|---|
| Gibberellins | Stem elongation by stimulating cell division and elongation; seed germination by activating amylase to break down starch. |
| Ethene (ethylene) | Fruit ripening, leaf abscission, and responses to stress. |
| Abscisic acid (ABA) | Stomatal closure during drought; inhibits growth and seed germination; maintains seed dormancy. |
| Cytokinins | Promote cell division and lateral bud growth; delay leaf senescence. |
Understand that these hormones often interact antagonistically (e.g., ABA vs gibberellins in seed germination) to fine-tune plant responses.
请理解这些激素常发生拮抗作用（如 ABA 与赤霉素在种子萌发过程中），以精确调控植物响应。
4. Chemical and Physical Plant Defences | 植物的物理与化学防御
Plants possess a remarkable array of defences against herbivores and pathogens. Physical defences include thorns, spines, tough waxy cuticles, and bark. Chemical defences encompass a vast range of secondary metabolites such as alkaloids (e.g., caffeine, nicotine), tannins, and terpenoids that deter herbivores by tasting bitter or being toxic. Some plants release volatile organic compounds to recruit predators of the herbivores attacking them.
植物拥有一系列出色的防御机制以抵御食草动物和病原体。物理防御包括刺、棘、坚韧的蜡质角质层和树皮。化学防御涵盖大量次生代谢产物，如生物碱（咖啡因、尼古丁）、单宁和萜类化合物，它们通过产生苦味或毒性来驱避食草动物。一些植物还会释放挥发性有机化合物，以吸引食草动物的天敌。
A common exam question asks students to distinguish between constitutive defences (always present) and induced defences (produced in response to attack), such as the hypersensitive response which isolates pathogens by programmed cell death.
常见考题要求学生区分组成型防御（始终存在）与诱导型防御（受攻击后产生），例如通过程序性细胞死亡隔离病原体的过敏反应。
5. Organisation of the Mammalian Nervous System | 哺乳动物神经系统的构成
The mammalian nervous system is divided into the central nervous system (CNS), consisting of the brain and spinal cord, and the peripheral nervous system (PNS), which comprises sensory and motor neurons. The PNS is further split into the somatic nervous system (voluntary control of skeletal muscles) and the autonomic nervous system (involuntary control of internal organs).
哺乳动物的神经系统分为中枢神经系统（CNS，由脑和脊髓组成）和周围神经系统（PNS，包括感觉神经元和运动神经元）。PNS 进一步分为躯体神经系统（对骨骼肌的随意控制）和自主神经系统（对内脏器官的不随意控制）。
The autonomic nervous system has two antagonistic branches: the sympathetic division (fight-or-flight) and the parasympathetic division (rest-and-digest). For instance, sympathetic activity increases heart rate, whereas parasympathetic activity decreases it.
自主神经系统有两个拮抗分支：交感神经（战斗或逃跑）和副交感神经（休息与消化）。例如，交感神经活动增加心率，而副交感神经活动降低心率。
6. The Action Potential and Its Propagation | 动作电位及其传导
The resting potential of a neuron is approximately –70 mV, maintained by the Na⁺/K⁺ pump (active transport) and differential permeability to K⁺. An action potential is an all-or-nothing reversal of membrane potential triggered when a stimulus causes depolarisation to the threshold (around –55 mV).
神经元的静息电位约为 –70 mV，由 Na⁺/K⁺ 泵（主动运输）和膜对 K⁺ 的差异通透性维持。动作电位是一种全或无的膜电位反转，当刺激使去极化达到阈值（约 –55 mV）时触发。
The phases of an action potential:
- Depolarisation: Voltage-gated Na⁺ channels open; Na⁺ rushes in, making the inside positive (~+40 mV).
- Repolarisation: Na⁺ channels inactivate; voltage-gated K⁺ channels open, K⁺ exits, restoring negative internal charge.
- Hyperpolarisation: K⁺ channels close slowly, causing a brief overshoot below resting potential.
- Refractory period: Ensures unidirectional propagation and limits the frequency of impulses.
动作电位的阶段：
- 去极化：电压门控 Na⁺ 通道打开；Na⁺ 涌入，使膜内变正（~+40 mV）。
- 复极化：Na⁺ 通道失活；电压门控 K⁺ 通道打开，K⁺ 外流，恢复膜内负电。
- 超极化：K⁺ 通道关闭较慢，导致电位短暂低于静息水平。
- 不应期：保证单向传导并限制冲动频率。
Saltatory conduction occurs in myelinated axons where the action potential ‘jumps’ between nodes of Ranvier, dramatically increasing speed.
在有髓鞘的轴突中发生跳跃传导，动作电位在郎飞结之间“跳跃”，大大加快传导速度。
7. Synaptic Transmission | 突触传递
When an action potential reaches the presynaptic terminal, it opens voltage-gated Ca²⁺ channels. Ca²⁺ influx triggers synaptic vesicles to fuse with the membrane and release neurotransmitter (e.g., acetylcholine) into the synaptic cleft by exocytosis. The neurotransmitter diffuses across the cleft and binds to ligand-gated ion channels on the postsynaptic membrane, causing an excitatory postsynaptic potential (EPSP) if Na⁺ channels open, or an inhibitory postsynaptic potential (IPSP) if Cl⁻ or K⁺ channels open.
当动作电位到达突触前末梢，电压门控 Ca²⁺ 通道打开。Ca²⁺ 内流促使突触小泡与膜融合，以胞吐方式将神经递质（如乙酰胆碱）释放到突触间隙。神经递质通过间隙扩散，并与突触后膜上的配体门控离子通道结合，若打开 Na⁺ 通道则产生兴奋性突触后电位（EPSP），若打开 Cl⁻ 或 K⁺ 通道则产生抑制性突触后电位（IPSP）。
The enzyme acetylcholinesterase rapidly hydrolyses acetylcholine in the cleft, terminating the signal and allowing the receptor to reset. Summation of EPSPs at the postsynaptic neuron can be temporal or spatial, determining whether an action potential is initiated.
乙酰胆碱酯酶迅速水解突触间隙中的乙酰胆碱，终止信号并使受体复位。突触后神经元的 EPSP 可发生时间性或空间性总和，决定是否触发动作电位。
8. Structure and Function of Skeletal Muscle | 骨骼肌的结构与功能
Skeletal muscle is composed of bundles of muscle fibres (cells) that contain many myofibrils. Myofibrils are made up of repeating contractile units called sarcomeres. Each sarcomere contains thin actin filaments and thick myosin filaments. The characteristic striated appearance arises from the alternating I bands (actin only) and A bands (myosin overlapping actin). The Z line defines the boundary of each sarcomere.
骨骼肌由肌纤维束（细胞）构成，其中包含大量肌原纤维。肌原纤维由重复的收缩单元——肌节组成。每个肌节包含细的肌动蛋白丝和粗的肌球蛋白丝。特征性的横纹外观源于明带（仅肌动蛋白）和暗带（肌球蛋白与肌动蛋白重叠）的交替排列。Z 线界定每个肌节的边界。
Other key structures: the sarcoplasmic reticulum stores Ca²⁺; transverse (T) tubules propagate the action potential deep into the fibre.
其他关键结构：肌质网储存 Ca²⁺；横小管（T 管）将动作电位传导至纤维深处。
9. The Sliding Filament Model of Contraction | 肌肉收缩的滑动丝模型
The sliding filament theory states that contraction occurs when actin and myosin filaments slide past each other, shortening the sarcomere without the filaments themselves shortening. The sequence:
- An action potential arrives at the neuromuscular junction and triggers Ca²⁺ release from the sarcoplasmic reticulum.
- Ca²⁺ binds to troponin, causing tropomyosin to move and expose myosin-binding sites on actin.
- Myosin heads bind to actin forming cross-bridges; ADP and Pi are released, causing the power stroke – the myosin head pivots and pulls the actin filament towards the centre of the sarcomere.
- ATP binds to myosin, causing it to detach from actin; ATP is hydrolysed to ADP and Pi, re-cocking the myosin head.
- The cycle repeats as long as Ca²⁺ and ATP are present.
滑动丝理论指出，收缩是通过肌动蛋白丝与肌球蛋白丝相互滑动实现的，肌节缩短而细丝本身长度不变。过程如下：
- 动作电位到达神经肌肉接头，触发肌质网释放 Ca²⁺。
- Ca²⁺ 与肌钙蛋白结合，使原肌球蛋白移位，暴露出肌动蛋白上的肌球蛋白结合位点。
- 肌球蛋白头与肌动蛋白结合形成横桥；释放 ADP 和 Pi，产生动力冲程——肌球蛋白头摆动，将肌动蛋白丝拉向肌节中心。
- ATP 与肌球蛋白结合，使其与肌动蛋白分离；ATP 水解为 ADP 和 Pi，使肌球蛋白头重新翘起。
- 只要有 Ca²⁺ 和 ATP，循环便持续进行。
Rigor mortis occurs after death because ATP is no longer produced, preventing myosin detachment – thus muscles stay contracted.
尸僵发生在死后，因为不再产生 ATP，阻止了肌球蛋白分离，因此肌肉保持收缩状态。
10. Nervous vs Hormonal Communication | 神经与激素通讯的比较
Both systems enable cell signalling but differ fundamentally. Nervous communication uses electrical impulses and neurotransmitters; it is rapid, short-lived, and targets specific cells via synapses. Hormonal communication uses chemical messengers transported in the blood; it is slower, longer-lasting, and can affect multiple target cells that possess specific receptors. Plants rely largely on hormones, whereas animals use both systems in an integrated manner.
两种系统都能实现细胞通讯，但有根本区别。神经通讯利用电冲动和神经递质；其速度快、持续时间短，通过突触靶向特定细胞。激素通讯利用血液运输的化学信使；速度较慢、持续时间较长，并可影响所有具有特定受体的靶细胞。植物主要依赖激素，而动物整合使用这两种系统。
| Feature | 特征 | Nervous | 神经 | Hormonal | 激素 |
|---|---|---|
| Speed | Very fast (ms) | Slower (seconds to hours) |
| Duration | Short-lived | Long-lasting |
| Specificity | Point-to-point (synapses) | Widespread (blood-borne) |
11. Common Exam Pitfalls and How to Avoid Them | 常见易错点与应对策略
Pitfall 1: Confusing the role of auxin in roots and shoots. Remember, auxin promotes elongation in shoots but inhibits it in roots. Use this to explain gravitropism clearly.
易错点 1：混淆生长素在根和茎中的作用。请牢记，生长素在茎中促进伸长，在根中抑制伸长。用这一点清晰地解释向地性。
Pitfall 2: Describing the action potential as a single, simple depolarisation. You must describe the roles of Na⁺ and K⁺ voltage-gated channels, the refractory period, and all-or-nothing principle.
易错点 2：将动作电位描述为单一、简单的去极化。你必须描述 Na⁺ 和 K⁺ 电压门控通道的作用、不应期以及全或无原则。
Pitfall 3: Forgetting that Ca²⁺ is the critical trigger both for neurotransmitter release at the synapse and for muscle contraction. Be precise: Ca²⁺ enters the presynaptic knob, and it binds to troponin in muscle.
易错点 3：忘记 Ca²⁺ 既是突触处神经递质释放的关键触发因子，也是肌肉收缩的触发因子。表述要准确：Ca²⁺ 进入突触前钮结，以及结合肌肉中的肌钙蛋白。
Pitfall 4: Vagueness about plant defences. Always distinguish between physical and chemical defences, and try to name specific examples (e.g., tannins, alkaloids) to secure top marks.
易错点 4：对植物防御表述模糊。务必区分物理防御和化学防御，并尽量举出具体例子（如单宁、生物碱），以获得高分。
Pitfall 5: Neglecting the role of ATP in muscle contraction. Mention that ATP is needed both for the power stroke (hydrolysis during re-cocking) and for detachment of myosin heads. Without ATP, the cycle cannot continue.
易错点 5：忽略 ATP 在肌肉收缩中的作用。应指出 ATP 既为重新翘起时的动力冲程所需（水解供能），也是肌球蛋白头脱离所需。没有 ATP，循环无法继续。
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