AQA GCSE生物 稳态 反射弧 血糖 激素调节
稳态与反应（Homeostasis and response）是 AQA GCSE 生物话题 5 的全部内容，也是 Paper 2 的第一个主题。它把神经调节与激素调节放在一起考，既有反射弧与眼的结构题，也有血糖曲线和实验设计的长答。
Topic 5 of AQA GCSE Biology covers homeostasis and response and opens Paper 2, mixing fast electrical signalling in the nervous system with slower hormonal control. Answers here need structural detail as well as control-system logic, and the wording examiners expect is quite specific.
4.5 在 Paper 2 里的位置与分值
AQA 官方 spec（8461，2016 年 9 月起施教，2018 年 5 月与 6 月首次考试）把整门课拆成两张卷，Paper 1 考话题 1 到 4，Paper 2 考话题 5 到 7，稳态与反应排在 Paper 2 的第一位。两张卷各 1 小时 45 分钟、各 100 分、各占整门 GCSE 的 50%，题型包含选择题、结构题、封闭短答与开放作答题。评分目标按 Ofqual 口径固定，AO1 占 40%、AO2 占 40%、AO3 占 20%，单张卷内部允许在 37% 到 43% 之间浮动。数学技能至少占全卷分值的 10%，必做实验相关的题目至少占 15%，血糖曲线读数与反应时间的数据处理都算在内。基础卷成绩区间是 1 到 5 级，高级卷是 4 到 9 级。
Topic 5 of the 8461 specification was first taught from September 2016 and examined from May and June 2018; Paper 2 covers topics 5 to 7, so homeostasis opens the second paper. Both papers run for 1 hour 45 minutes, carry 100 marks and are worth 50 per cent each, and the assessment objectives are fixed at 40 per cent AO1, 40 per cent AO2 and 20 per cent AO3 across the qualification. Mathematical skills take at least 10 per cent of the marks and practical-based questions at least 15 per cent, which explains why glucose graphs and reaction time data appear so often; Foundation tier awards grades 1 to 5 and Higher tier grades 4 to 9.
稳态的定义与三个被调量
稳态的官方定义是把细胞或生物体的内部条件维持在适合功能运作的最优水平，以应对内部与外部的变化。这句话在阅卷时会拆成三个得分点，调节、内部条件、功能的最优条件，只写维持稳定通常只给一分。人体要控制的量有三个，血糖浓度、体温与水水平，答案里漏掉任何一项都要扣分。稳态的根本原因是酶，酶需要合适的温度与 pH 才能保持活性部位形状，条件偏离过大酶就变性，代谢通路随之断开。常见的错误是把稳态写成内部环境完全不变，其实它是围绕设定值不断修正的动态过程。
Homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function in response to internal and external changes. Markers look for three elements in that sentence, namely regulation, internal conditions and optimum conditions for function, so a vague answer about keeping things stable earns one mark; the three regulated quantities are blood glucose concentration, body temperature and water levels. The underlying reason is enzyme action, since enzymes need a suitable temperature and pH to hold their active site shape. Many students describe homeostasis as a fixed internal state, when it is in fact a series of corrections around a set point.
控制系统的三个部件
所有控制系统的结构相同，受体检测刺激，协调中心接收并处理来自受体的信息，效应器带来反应并把水平恢复到最优值。协调中心的例子有三个，脑、脊髓与胰腺，胰腺在血糖调节里同时充当检测点与协调点。效应器只有两类，肌肉收缩或腺体分泌激素，把器官名称当作效应器写通常不给分。整条链的顺序是刺激、受体、协调中心、效应器、反应，AQA 在 spec 里直接印出了这条链。学生最常把胰腺说成效应器，而它在血糖调节里承担的是协调。
Every control system has the same three parts, receptors that detect stimuli, coordination centres that receive and process information, and effectors that restore optimum levels. The named coordination centres are the brain, the spinal cord and the pancreas, and the pancreas both detects glucose changes and coordinates the hormonal response; effectors are only a muscle contracting or a gland secreting a hormone. The chain of events runs stimulus, receptor, coordinator, effector and response, and specification questions expect each link in that order. Students most often label the pancreas as an effector, when it acts as the coordinator for blood glucose.
三种神经元与突触
受体收到的信息沿神经元以电脉冲形式传到中枢神经系统，中枢神经系统就是脑与脊髓，它再协调效应器的反应。神经元按位置分三种，感觉神经元把脉冲从受体带到中枢神经系统，中间神经元只在中枢神经系统内部传递，运动神经元把脉冲送到效应器。结构适配要写具体，带分支的树突连接多个神经元，一条长轴突把脉冲一次送到远处，末端分支形成突触与下一个细胞相接。两种神经元之间并不直接连通，突触处先由电脉冲促使化学物质释放，化学物质扩散穿过间隙再触发下一个神经元的电脉冲。这一步最容易写错，很多学生以为信号在突触处仍是电信号直接过去。
Information from receptors travels along neurones as electrical impulses to the central nervous system, which is the brain and spinal cord, and the CNS then coordinates the response of effectors. Sensory neurones carry impulses from receptors towards the CNS, relay neurones pass them on inside the CNS, and motor neurones deliver them to effectors; a long axon carries one impulse to a distant cell, and branching ends form synapses. Two neurones are never joined directly, because at a synapse the impulse causes a chemical to be released that diffuses across the gap before triggering a new impulse. Many students lose the mark here by calling the synapse an electrical junction.
反射弧与脑研究的难点
反射弧要按顺序写全，受体、感觉神经元、突触、中间神经元、突触、运动神经元、效应器，中间的突触出现两次，漏一个环节通常丢一分。反射动作是自动且迅速的，不涉及大脑的意识部分，答案里必须有 automatic 与 rapid 两层意思。缩手发生在脑处理疼痛之前，这个时间差就是反射存在的理由，写信息先送到大脑再返回的答案会被判成随意动作。脑的三个区域各有固定描述，大脑皮层负责意识、记忆、语言、感觉与运动，小脑负责平衡与肌肉协调，延髓控制心跳与呼吸速率。脑研究之所以困难有三条可写，结构复杂且连接尚未完全弄清，电刺激只能说明相关而非因果，健康人的脑组织无法为实验获取，只写研究脑很难而不给理由的答案一般不给分。
A reflex arc must be written in full order, receptor, sensory neurone, synapse, relay neurone, synapse, motor neurone and effector, with the synapse appearing twice; reflex actions are automatic and rapid and do not involve the conscious part of the brain. The value of a reflex is that it protects the body before the brain has processed the pain, and an answer that sends the impulse up to the brain and back turns a reflex into a voluntary action. Brain research is difficult because the connections between areas are not fully mapped, electrical stimulation shows correlation rather than causation, and healthy human brain tissue cannot be obtained; an answer that only says the brain is hard to study, without a reason, is ignored because the question asks why.
眼的结构、调节与两类视力缺陷
眼球结构要按图辨认并说明功能，视网膜含光受体，感受光强与颜色；视神经把电脉冲送到脑；巩膜是白色外层，保护眼球并维持形状；角膜是透明外层，光在这里第一次折射；虹膜由肌肉组成，改变瞳孔大小控制进光量；睫状肌与悬韧带共同改变晶状体形状。调节（accommodation）是改变晶状体形状以便看清近处或远处物体的过程。看近物时睫状肌收缩，悬韧带松弛，晶状体变厚并折射更多；看远物时睫状肌放松，悬韧带被拉紧，晶状体被拉薄。两类常见缺陷是近视与远视，前者光线聚焦在视网膜之前，后者聚焦在视网膜之后，矫正手段除眼镜镜片外还有隐形眼镜、改变角膜形状的激光手术与置换晶状体。这一步最容易写反的是睫状肌状态与晶状体厚度的对应关系。
Eye structures must be recognised on a diagram and matched to functions: the retina holds light receptors for intensity and colour, the optic nerve carries impulses to the brain, the sclera is the tough white outer layer, the cornea is the transparent front surface where light first refracts, and the iris controls how much light enters. Accommodation changes the lens shape to focus on near or distant objects; for a near object the ciliary muscles contract, the suspensory ligaments slacken and the lens becomes thicker, while for a distant object the muscles relax and the lens is pulled thin. Myopia and hyperopia are corrected with spectacle lenses, contact lenses, laser surgery or a replacement lens, and the pairing of ciliary muscle state with lens thickness is the detail most often reversed.
体温调节的两条通路
体温由脑中的体温调节中枢监控，这个中枢含有对血液温度敏感的受体，皮肤里的温度受体把神经冲动送到同一个中枢。体温过高时皮肤血管扩张，更多血液流近皮肤表面，汗腺同时分泌汗液，两种机制都让能量从皮肤转移到环境。体温过低时皮肤血管收缩，出汗停止，骨骼肌不自主收缩形成颤抖，颤抖通过肌肉活动释放热量。高级卷要求结合情境解释，例如描述在冷水里停留时先后发生的反应，只背术语不写能量转移方向通常拿不到满分。学生最常把血管扩张写成散热变慢，其实扩张增加的是皮肤表面的辐射、传导与对流散热。
Body temperature is monitored by the thermoregulatory centre in the brain, which contains receptors sensitive to blood temperature, while skin receptors send impulses to the same centre. When the body is too hot, blood vessels near the surface dilate and sweat glands produce sweat, and both mechanisms transfer energy from the skin to the environment; when it is too cold those vessels constrict, sweating stops and skeletal muscles shiver. Higher tier answers must explain the mechanism in context, such as the sequence of responses during a period in cold water. Students often claim that vasodilation slows heat loss, when it actually raises loss by radiation, conduction and convection.
血糖调节与负反馈
血糖浓度由胰腺监控，血糖过高时胰腺分泌胰岛素，胰岛素让葡萄糖从血液进入细胞，肝细胞与肌肉细胞把多余葡萄糖转成糖原储存，血糖随之下降。高级卷加考反方向，血糖过低时胰腺分泌胰高血糖素，把糖原转回葡萄糖释放进血液。两者构成负反馈循环，偏离设定值的方向决定哪种激素占优势，这一对相反作用就是本节的中心。图表题常给出糖尿病受试者与正常人的血糖曲线，要求比较下降速度与恢复时间，读数顺序是起点、峰值、回到基线的用时。常见的错误是把胰岛素说成把糖原变成葡萄糖，方向写反就整段失分。
Blood glucose concentration is monitored and controlled by the pancreas, which releases insulin when the level is too high so that glucose moves from the blood into cells and liver and muscle cells store the excess as glycogen. Higher tier adds glucagon, released when glucose falls, which converts glycogen back into glucose and returns it to the blood; the two hormones form a negative feedback cycle in which the direction of the deviation decides which one dominates. Graph questions compare a person with diabetes against a person without and ask about the rate of fall and the time to recover, so read the start value, the peak and the return to baseline in that order. A common mistake is describing insulin as turning glycogen into glucose, which reverses the mechanism for the whole answer.
1 型与 2 型糖尿病的对比
1 型糖尿病是胰腺不能产生足够的胰岛素，特征是不受控的高血糖，通常用胰岛素注射治疗。2 型糖尿病则是身体细胞对胰腺产生的胰岛素不再正常响应，常见处理方式是控制碳水化合物的饮食加上运动方案，肥胖是 2 型糖尿病的风险因素。对比题的分值一般落在三点上，病因不同、治疗不同、风险因素不同，写成两栏逐条对应最省时间。评估类问题会要求结合社会与伦理因素给建议，例如讨论肥胖筛查或学校饮食政策，这类题需要给出理由与让步条件。阅卷时看到只写 2 型更常见而不写细胞响应下降的答案，一般只给一分。
Type 1 diabetes is a disorder in which the pancreas fails to produce sufficient insulin, producing uncontrolled high blood glucose levels that are normally treated with insulin injections. In Type 2 diabetes the body cells no longer respond properly to the insulin produced, so a carbohydrate controlled diet and an exercise regime are the usual treatments, with obesity acting as a risk factor. Comparison questions award marks for the cause, the treatment and the risk factor, so a two-column answer saves time under exam pressure. Evaluation questions ask for recommendations weighing social and ethical issues, such as obesity screening, and an answer that only says Type 2 is more common earns one mark at most.
肾、ADH 与透析和移植
水、离子与尿素通过不同途径离开身体，肺在呼气时带走水，皮肤在汗液里带走水、离子与尿素，这两条途径不受控制，多余的成分由肾以尿液形式排出。尿素在肝里由氨基酸分解产生，肾功能下降时尿素在血液中积累。肾过滤血液后会选择性重吸收有用物质，葡萄糖、部分离子与水被收回，剩下的形成尿液。ADH 由脑垂体释放，血液变浓时释放量增多，肾小管重吸收更多水，尿量减少、尿变浓；血液过稀时释放量减少，尿量增多，这个释放量由负反馈控制。肾功能衰竭的处理方式有透析与肾移植，移植后需要长期服用免疫抑制剂防止排异，代价是对感染的抵抗力下降。学生在 ADH 题里最常把控制对象搞错，写成肾控制 ADH 分泌。
Water, ions and urea leave the body by several routes, with the lungs losing water during exhalation and the skin losing water, ions and urea in sweat, neither of which is controlled; the kidneys remove the excess in urine after filtering the blood and reabsorbing useful substances. Urea is produced in the liver from the breakdown of amino acids, so failing kidneys allow it to accumulate in the blood. ADH is released by the pituitary gland, and a more concentrated blood triggers a larger release so the kidneys reabsorb more water and produce less, more concentrated urine, with negative feedback controlling that release. Kidney failure is managed by dialysis or a transplant, and transplant patients need long-term immunosuppressant drugs that leave them less able to fight infection; students frequently reverse the loop by writing that the kidney controls ADH release.
月经周期、避孕与生育治疗
月经周期约 28 天，四种激素按顺序介入。促卵泡激素（FSH）由脑垂体分泌，使卵成熟并刺激卵巢分泌雌激素；雌激素使子宫内膜增厚，同时抑制 FSH 的进一步释放；促黄体生成素（LH）同样来自脑垂体，在周期约第 14 天触发排卵；孕激素维持子宫内膜。雌激素抑制 FSH、促进 LH 这条通路是负反馈在生殖激素上的落点。避孕分激素类与非激素类，口服避孕药抑制 FSH 使卵不能成熟，注射剂、植入剂与贴片缓慢释放孕激素，避孕套与子宫帽阻止精子与卵相遇，宫内节育器阻止着床或释放激素，绝育手术属永久方法。生育治疗方向相反，促排卵药让 FSH 与 LH 水平升高，体外受精先促多枚卵成熟，取出后在体外受精再把胚胎放回子宫，可能产生多胎并涉及伦理问题。学生最常把 FSH 与 LH 的来源写反，两者都来自脑垂体。
The menstrual cycle runs for roughly 28 days, with FSH from the pituitary gland maturing an egg and stimulating oestrogen production, oestrogen thickening the uterus lining while inhibiting further FSH release, LH triggering ovulation at about day 14, and progesterone maintaining the lining. Oral contraceptives inhibit FSH so no eggs mature, injections, implants and patches release progesterone slowly, barrier methods stop sperm reaching an egg, intrauterine devices prevent implantation, and sterilisation is permanent. Fertility treatment works the other way, since fertility drugs raise FSH and LH so several eggs mature, and IVF collects those eggs, fertilises them outside the body and returns embryos to the uterus, which can produce multiple births and raises ethical questions. Students most often attribute FSH and LH to the wrong source, although both come from the pituitary gland.
植物激素与两个必做实验
植物用激素协调生长与对环境的反应，对光的反应叫向光性，对重力的反应叫向重力性。生长素分布不均造成根与茎的不均匀生长速率，单侧光下背光一侧生长素更多，细胞伸长更明显，茎向光弯曲；根对生长素更敏感，浓度偏高时反而抑制伸长，所以根向下长、茎向上长。高级卷还要求知道赤霉素参与种子萌发，乙烯控制细胞分裂与果实成熟。农业上的应用有几类，生长素可作除草剂、生根粉并用于组织培养，乙烯控制储运中的果实成熟，赤霉素打破种子休眠、促进开花与增大果实，除草剂的使用会影响生物多样性。两个必做实验要记牢：必做实验 7 是计划并实施一项关于人类反应时间影响因素的调查，用尺子下落距离换算反应时间，重复测量取平均是必写的改进项；必做实验 8 只考生物单科，用新萌发的幼苗研究光或重力对生长的影响，控制温度、水量与种子种类是得分点。
Plants coordinate growth and responses with hormones, where phototropism answers light and gravitropism answers gravity. Unequal distribution of auxin produces unequal growth rates, so the shaded side of a shoot accumulates more auxin, its cells elongate more and the shoot bends towards the light, while roots are more sensitive and stop elongating at higher auxin concentrations. Higher tier adds gibberellins for initiating seed germination and ethene for controlling cell division and fruit ripening, and the agricultural uses include auxins as weed killers and rooting powders, ethene for controlling ripening in storage and gibberellins for ending dormancy and increasing fruit size. Required practical 7 investigates a factor affecting human reaction time using the ruler drop test with distance converted to time, and required practical 8, biology only, uses newly germinated seedlings, where temperature, water volume and seed type are the control variables worth naming.
四周复习排期与阅卷口径
这一主题的复习可以按四周推进，第一周过神经系统的结构与突触，把反射弧的顺序默写到条件反射；第二周攻眼与体温调节的图形题，重点练睫状肌状态与血管直径的对应；第三周做血糖、ADH 与糖尿病的数据题，练从曲线读起点、峰值与恢复时间；第四周做植物激素与两个必做实验的变量设计，并回头重做错题。每周留一次限时训练，按 Paper 2 的 1 小时 45 分钟与 100 分换算节奏，稳态部分通常值得分配 25 到 30 分钟。阅卷时最常丢分的三类答案是机制方向写反，例如胰岛素与胰高血糖素互换、血管扩张与收缩互换；术语混用，例如把协调中心与效应器并列写；实验题只写结论不写控制变量与重复次数。
A four-week plan works well here, with nervous system structure and synapses in week one, eye and temperature graph questions in week two, blood glucose, ADH and diabetes data in week three, and plant hormones plus practical variable design in week four. Keep one timed session each week at the Paper 2 rhythm of 1 hour 45 minutes for 100 marks, which usually justifies 25 to 30 minutes on homeostasis questions. The answers that lose marks most often are reversed mechanisms, such as swapping insulin with glucagon or vasodilation with vasoconstriction, mixed terminology that lists coordination centres beside effectors, and practical answers without control variables or repeats. Writing every response as change, detection and response keeps the direction in place.
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