生物5.5 植物与动物响应机制精讲
一、植物的向性反应：向光性与向地性 | Plant Tropisms: Phototropism and Geotropism
植物虽然不能像动物一样自由移动，却能够通过定向生长对光照、重力和接触等环境信号作出精确响应，这种生长方向随刺激方向而定的反应称为向性。向光性是指植物茎向光源方向弯曲生长，让叶片能更大限度地接收光照，从而提高光合作用效率；向地性则表现为根顺着重力方向向下生长、茎逆着重力方向向上生长，使根系牢牢固定于土壤并伸向水分与矿物质。向性反应的本质是生长素等植物激素在器官两侧分布不均，导致两侧细胞伸长速率不同。向性是A-Level生物5.5模块的开篇考点，常以实验设计和结果分析的形式考查。
Although plants cannot move freely like animals, they still make precise directional growth responses to environmental signals such as light, gravity and touch, and a response whose direction depends on the direction of the stimulus is called a tropism. Phototropism is the growth of a shoot towards a light source, allowing leaves to capture more light and so raise the rate of photosynthesis. Geotropism, also called gravitropism, causes roots to grow downwards in the direction of gravity while shoots grow upwards, anchoring the plant and guiding roots towards water and minerals. Tropisms arise because plant hormones such as auxins are distributed unevenly across the two sides of an organ, so the two sides elongate at different rates. Tropisms open module 5.5 in A Level Biology and are often examined through experimental design and interpretation of results.
二、生长素如何引起向光弯曲 | How Auxins Cause Phototropic Bending
生长素主要在茎尖等分生组织中合成，是向性反应中最关键的植物激素。当茎尖受到单侧光照射时，生长素会向背光一侧重新分布并在此积累，使背光侧细胞伸长速率明显高于向光侧，茎因此朝着光源方向弯曲。生长素促进细胞伸长的机制是激活细胞膜上的质子泵，将氢离子泵入细胞壁使其酸化，酸性环境激活酶类使纤维素之间的连接松弛，细胞壁变软，细胞得以吸水伸长。这类题目常给出弯曲方向或激素浓度分布图，要求考生解释现象背后的激素机制，答题时必须紧扣生长素分布与细胞伸长之间的关系。
Auxins are synthesised mainly in meristems such as the shoot tip and are the key plant hormones in tropisms. When a shoot tip receives light from one side, auxin is redistributed so that it accumulates on the shaded side, making the cells there elongate faster than those on the lit side and bending the shoot towards the light. Auxin promotes cell elongation by activating proton pumps in the cell membrane, which pump hydrogen ions into the cell wall and acidify it; the acidic environment activates enzymes that loosen the bonds between cellulose fibres, softening the wall so the cell can absorb water and elongate. Questions often give a diagram of bending direction or hormone concentration and ask you to explain the underlying mechanism, so you must link auxin distribution directly to differential cell elongation.
三、赤霉素与种子萌发和茎伸长 | Gibberellins: Seed Germination and Stem Elongation
赤霉素是一类促进植物生长的重要激素，其两大功能是促进种子萌发和茎的节间伸长。种子吸水后，胚开始合成赤霉素，赤霉素扩散到胚乳外围的糊粉层，诱导糊粉层细胞合成并分泌淀粉酶，将胚乳中储存的淀粉水解为麦芽糖和葡萄糖，为胚的萌发生长提供能量与原料。在茎秆伸长方面，赤霉素既能促进细胞分裂，又能促进细胞伸长，因此矮生品种在施用赤霉素后茎秆会明显长高。这条从胚到糊粉层再到淀粉水解的完整信号通路，是种子萌发大题的高频得分点，考生应能按顺序写出每一步。
Gibberellins are important growth-promoting hormones whose two main roles are seed germination and stem internode elongation. After a seed absorbs water, the embryo begins to produce gibberellin, which diffuses to the aleurone layer surrounding the endosperm and induces those cells to synthesise and secrete amylase. This enzyme hydrolyses the starch stored in the endosperm into maltose and glucose, providing energy and raw materials for the growing embryo. For stem elongation, gibberellins promote both cell division and cell elongation, so dwarf varieties grow noticeably taller after gibberellin treatment. The complete signalling pathway from embryo to aleurone layer to starch hydrolysis is a high-scoring point in long questions on germination, and you should be able to write out each step in order.
四、乙烯与脱落酸：果实成熟与胁迫响应 | Ethene and Abscisic Acid: Ripening and Stress
乙烯是一种在常温下呈气态的特殊植物激素，主要功能是促进果实成熟和叶片、果实的脱落。成熟的果实会释放乙烯并扩散到周围空气中，加速邻近果实的成熟，这就是为什么把一个熟透的香蕉与其他水果同放一个袋子里能让后者更快变熟的原因。脱落酸则主要参与植物的逆境响应：当土壤缺水时，脱落酸促使保卫细胞失水、气孔关闭，从而减少蒸腾失水；在秋冬季节它还会促进叶片脱落，帮助植物度过不良环境。两种激素都体现了植物对内外环境变化的精细调节，也是激素功能配对题的常考对象。
Ethene is a plant hormone that is gaseous at ordinary temperatures, and its main function is to promote fruit ripening and the abscission of leaves and fruit. Ripening fruit releases ethene that diffuses into the surrounding air and speeds up ripening in neighbouring fruit, which is why placing a ripe banana in the same bag as other fruit makes them ripen faster. Abscisic acid is involved mainly in stress responses: when soil water is scarce, it causes guard cells to lose water and the stomata to close, reducing water loss by transpiration; in autumn and winter it also promotes leaf fall to help the plant survive unfavourable conditions. Both hormones illustrate how precisely plants regulate themselves in response to internal and external change, and both are common in questions that ask you to match each hormone to its function.
五、植物激素的商业应用 | Commercial Uses of Plant Hormones
植物激素及其人工合成类似物在农业与园艺生产中应用广泛，是考试中应用类题目的重点。合成生长素被制成生根粉，蘸在插条基部可促进其生根繁殖；高浓度的合成生长素还能作为选择性除草剂，专门杀死双子叶阔叶杂草而不伤害禾本科作物。乙烯用于催熟在运输前采收的青香蕉和番茄，使其到达市场时正好达到可售成熟度。赤霉素能让无籽葡萄的果实增大、提高产量，也用于啤酒酿造中促使大麦快速萌发以生成麦芽；脱落酸类物质则用于延长切花和果蔬的保鲜期。考生应能逐一说出每种激素对应的典型商业用途并解释其原理。
Plant hormones and their synthetic analogues are widely used in farming and horticulture, and they are a focus of application-style exam questions. Synthetic auxins are made into rooting powder that is dipped onto the base of cuttings to encourage root formation, and at high concentrations they act as selective weedkillers that destroy broad-leaved dicot weeds while leaving cereal crops unharmed. Ethene is used to ripen green bananas and tomatoes harvested before transport so they reach the market at the right stage of ripeness. Gibberellins enlarge the fruit of seedless grapes to increase yield and are used in brewing to make barley germinate quickly and produce malt, while abscisic-acid-like compounds extend the shelf life of cut flowers and produce. You should be able to state the typical commercial use of each hormone and explain how it works.
六、光周期现象与光敏色素 | Photoperiodism and Phytochrome
植物能通过感知昼夜相对长短来调节开花时间，这种对光周期的响应称为光周期现象，其感光物质是光敏色素。光敏色素以Pr和Pfr两种可相互转化的形式存在：Pr吸收红光后转变为Pfr，Pfr在黑暗中或吸收远红光后又会变回Pr，因此Pfr的浓度随光照长短而变化，成为判断日长的重要信号。短日照植物在夜长超过临界夜长时开花，长日照植物则在夜长小于临界值时开花。园艺生产常利用这一点，通过人工补光延长日照或遮光缩短日照来调控花期，使花卉在指定节日集中上市，这类应用常结合实验数据考查。
Plants regulate their flowering time by detecting the relative length of day and night, a response to photoperiod called photoperiodism, and the light-sensitive pigment involved is phytochrome. Phytochrome exists in two interconvertible forms, Pr and Pfr: Pr absorbs red light and converts to Pfr, while Pfr converts back to Pr in darkness or after absorbing far-red light. As a result, the concentration of Pfr changes with the length of light exposure and acts as an important signal of day length. Short-day plants flower when the night is longer than a critical length, whereas long-day plants flower when the night is shorter than the critical value. Growers use this by adding artificial light to extend day length or shading plants to shorten it, controlling flowering so that flowers reach the market for specific festivals, and such applications are often examined alongside experimental data.
七、哺乳动物神经系统的组成 | Structure of the Mammalian Nervous System
哺乳动物的神经系统由中枢神经系统和周围神经系统两部分组成。中枢神经系统包括大脑和脊髓，是信息整合与指令发出的中心；周围神经系统则由感觉神经元和运动神经元构成，负责在感受器、中枢与效应器之间传递信号，其中运动神经元又分为控制随意运动的部分和控制内脏活动的自主神经系统。在脑的各区域中，大脑皮层负责意识、语言、记忆和思考等高级功能，小脑负责协调肌肉运动和维持身体平衡，延髓则控制心率、呼吸和血压等自主活动，下丘脑是体温与水平衡的调节中枢。准确区分各区域功能是回答神经系统结构题的基础。
The mammalian nervous system consists of the central nervous system and the peripheral nervous system. The central nervous system, made up of the brain and spinal cord, is the centre for integrating information and issuing commands, while the peripheral nervous system consists of sensory and motor neurons that carry signals between receptors, the CNS and effectors; the motor division is further split into the part controlling voluntary movement and the autonomic system controlling internal organs. Among the brain regions, the cerebral cortex is responsible for consciousness, language, memory and thought, the cerebellum coordinates muscle movement and maintains balance, the medulla oblongata controls autonomic activities such as heart rate, breathing and blood pressure, and the hypothalamus regulates body temperature and water balance. Distinguishing the function of each region accurately is the basis for answering questions on nervous system structure.
八、反射弧：从感受器到效应器 | The Reflex Arc: Receptor to Effector
反射是神经系统最基本的活动方式，其结构基础是反射弧，依次由感受器、感觉神经元、中间神经元、运动神经元和效应器五个环节组成。以膝跳反射为例，用小锤轻敲膝盖下方的髌腱，股四头肌内的感受器受到牵拉而兴奋，产生神经冲动；冲动沿感觉神经元传入脊髓，在脊髓灰质中与运动神经元直接形成突触，运动神经元再将冲动传至股四头肌这一效应器，使其收缩、小腿前踢。由于整个反射弧不经过大脑皮层，反应速度极快，能在身体受到伤害前迅速作出保护性动作，这也是反射比有意识反应更快的原因。
A reflex is the most basic activity of the nervous system, and its structural basis is the reflex arc, which is made up of a receptor, a sensory neuron, a relay neuron, a motor neuron and an effector. In the knee-jerk reflex, a small hammer taps the patellar tendon just below the knee, stretching receptors inside the quadriceps muscle and generating a nerve impulse. The impulse travels along a sensory neuron into the spinal cord, where it synapses directly with a motor neuron in the grey matter, and the motor neuron then carries the impulse to the quadriceps, the effector, causing it to contract and kick the lower leg forward. Because the arc bypasses the cerebral cortex, the response is extremely fast and makes a protective movement before the body can be harmed, which is why reflexes are quicker than conscious responses.
九、肌肉收缩的滑动丝模型 | The Sliding Filament Model of Muscle Contraction
滑动丝模型解释了骨骼肌收缩的分子机制，是本章难度较高、分值较大的考点。骨骼肌由许多肌原纤维组成，每个肌原纤维中的粗肌球蛋白丝与细肌动蛋白丝有规律地重叠排列，构成称为肌节的基本收缩单位。神经冲动到达神经肌肉接头后，促使肌浆网释放钙离子；钙离子与细丝上的肌钙蛋白结合，使原肌球蛋白移位，暴露出肌动蛋白上的结合位点。肌球蛋白头随即与肌动蛋白结合，并利用ATP水解释放的能量拉动细丝向肌节中央滑动，使肌节缩短、肌肉收缩；钙离子随后被主动泵回肌浆网，肌肉恢复舒张。答题时需依次写出兴奋、钙释放、位点暴露、滑动四个关键环节。
The sliding filament model explains the molecular mechanism of skeletal muscle contraction and is one of the harder, higher-scoring topics in this chapter. Skeletal muscle contains many myofibrils, and in each myofibril thick myosin filaments and thin actin filaments overlap in a regular pattern to form the sarcomere, the basic unit of contraction. When a nerve impulse reaches the neuromuscular junction, it causes the sarcoplasmic reticulum to release calcium ions, which bind to troponin on the thin filament and shift tropomyosin aside to expose binding sites on actin. Myosin heads then bind to actin and use the energy released by ATP hydrolysis to pull the thin filaments towards the centre of the sarcomere, shortening it and contracting the muscle; calcium is then actively pumped back into the sarcoplasmic reticulum and the muscle relaxes. In answers you should describe the four key stages in order: excitation, calcium release, site exposure and sliding.
十、战斗或逃跑反应与心率调节 | Fight-or-Flight and Control of Heart Rate
当动物面临威胁时，交感神经系统迅速激活，引发战斗或逃跑反应，帮助身体在短时间内做好剧烈运动的准备。肾上腺髓质释放肾上腺素进入血液，使心率加快、心输出量增大，同时使瞳孔扩大、支气管舒张、血糖升高，并把血流优先分配到骨骼肌。心率由延髓通过自主神经系统持续调节：颈动脉和主动脉体中的化学感受器检测血液中二氧化碳浓度和pH变化，主动脉弓和颈动脉窦中的压力感受器检测血压变化，信息传入延髓后，延髓通过迷走神经的副交感作用降低心率、通过交感神经提高心率，从而维持内环境稳态。这一调节是反馈机制类大题的高频素材。
When an animal faces a threat, the sympathetic nervous system is rapidly activated, triggering the fight-or-flight response that prepares the body for vigorous activity in a short time. The adrenal medulla releases adrenaline into the blood, raising heart rate and cardiac output while dilating pupils and bronchioles, increasing blood glucose and diverting blood flow to skeletal muscle. Heart rate is continuously adjusted by the medulla oblongata through the autonomic nervous system: chemoreceptors in the carotid and aortic bodies detect changes in blood carbon dioxide concentration and pH, baroreceptors in the aortic arch and carotid sinus detect changes in blood pressure, and after this information reaches the medulla it lowers heart rate via the parasympathetic vagus nerve and raises it via sympathetic nerves, maintaining homeostasis. This regulation is a common source of long questions on feedback mechanisms.
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