📚 Year 13 CAIE Biology: Unit Test Mock Exam Analysis | Year 13 CAIE 生物：单元测试模拟卷解析
This mock exam analysis targets key A2 Biology topics in the CAIE 9700 syllabus. Each question has been carefully designed to reflect the depth of understanding required for Year 13 assessments, from energy transfers to genetic control and ecological dynamics. Working through these model answers will help you consolidate content knowledge and sharpen exam technique.
本模拟卷解析针对CAIE 9700生物课程中的关键A2主题。每道题目均精心设计，反映了Year 13考试对理解深度的要求，涵盖能量转移、遗传调控和生态动态等内容。通过这些标准答案的学习，你将能够巩固知识要点并提高应试技巧。
1. Non-cyclic Photophosphorylation | 非循环光合磷酸化
Question: Explain how light energy is converted into chemical energy in the light-dependent reactions of photosynthesis. (8 marks)
Light energy is absorbed by chlorophyll and accessory pigments in photosystem II (PSII), exciting electrons to a higher energy level.
光能被光系统II（PSII）中的叶绿素和辅助色素吸收，使电子跃迁到更高能级。
The excited electrons are captured by an electron acceptor and passed along an electron transport chain (ETC).
激发的电子被电子受体捕获并沿着电子传递链（ETC）传递。
As electrons move along the ETC, energy is released and used to pump protons (H⁺) from the stroma into the thylakoid lumen, establishing a proton gradient.
当电子沿ETC移动时，释放的能量用于将质子（H⁺）从基质泵入类囊体腔，建立质子梯度。
Water molecules are split by photolysis at PSII, yielding electrons that replace those lost from the chlorophyll, alongside oxygen and protons.
水分子在PSII处通过光解作用分解，产生用于替换叶绿素失去电子的电子，同时释放氧气和质子。
Protons within the thylakoid lumen flow back into the stroma through ATP synthase, a process called chemiosmosis, which drives the synthesis of ATP from ADP and inorganic phosphate (Pi).
类囊体腔内的质子通过ATP合酶流回基质，这一化学渗透过程驱动由ADP和无机磷酸（Pi）合成ATP。
In photosystem I (PSI), light energy re-energises electrons, which are then passed to ferredoxin and ultimately used to reduce NADP⁺ to NADPH.
在光系统I（PSI）中，光能重新激发电子，随后电子传递给铁氧还蛋白并最终用于将NADP⁺还原为NADPH。
The products ATP and NADPH provide the energy and reducing power needed for the Calvin cycle in the stroma.
产物ATP和NADPH为基质中发生的卡尔文循环提供能量和还原力。
2. The Krebs Cycle and Oxidative Phosphorylation | 克雷布斯循环与氧化磷酸化
Question: Describe the role of the Krebs cycle in aerobic respiration and explain how the products of the cycle are used in oxidative phosphorylation. (6 marks)
Acetyl CoA (2C) enters the Krebs cycle and combines with oxaloacetate (4C) to form citrate (6C).
乙酰辅酶A（2C）进入克雷布斯循环，与草酰乙酸（4C）结合形成柠檬酸（6C）。
A series of enzyme-controlled reactions regenerate oxaloacetate, releasing two molecules of CO₂ via decarboxylation.
一系列酶控反应重新生成草酰乙酸，通过脱羧作用释放两分子CO₂。
During the cycle, dehydrogenation reactions produce reduced coenzymes: three molecules of reduced NAD and one of reduced FAD.
循环过程中，脱氢反应产生还原型辅酶：三分子还原型NAD和一分子还原型FAD。
Substrate-level phosphorylation directly generates one molecule of GTP, which can be converted to ATP.
底物水平磷酸化直接生成一分子GTP，GTP可转化为ATP。
The reduced NAD and reduced FAD donate electrons to the electron transport chain in the inner mitochondrial membrane.
还原型NAD和还原型FAD将电子提供给线粒体内膜上的电子传递链。
As electrons pass along the chain, protons are pumped into the intermembrane space, creating a chemiosmotic gradient that drives ATP synthesis.
电子沿链传递时，质子被泵入膜间隙，形成化学渗透梯度，驱动ATP合成。
3. Osmoregulation and ADH | 渗透调节与抗利尿激素
Question: Explain how antidiuretic hormone (ADH) regulates the water potential of the blood. (6 marks)
Osmoreceptors in the hypothalamus detect a decrease in blood water potential (more negative).
下丘脑中的渗透压感受器检测到血液水势降低（更负）。
The posterior pituitary gland releases more ADH into the bloodstream.
垂体后叶向血液中释放更多的抗利尿激素（ADH）。
ADH binds to specific receptors on the basolateral membranes of collecting duct cells in the kidney.
ADH与肾脏集合管细胞基底外侧膜上的特异性受体结合。
This receptor binding activates a second messenger cascade involving cyclic AMP (cAMP).
受体结合激活涉及环腺苷酸（cAMP）的第二信使级联反应。
Vesicles containing aquaporin-2 water channels fuse with the luminal membrane, increasing its permeability to water.
含有水通道蛋白-2的囊泡与管腔膜融合，增加了膜对水的通透性。
More water moves out of the collecting duct by osmosis into the hypertonic medullary interstitium, producing a small volume of concentrated urine and raising blood water potential back to normal.
更多水通过渗透作用从集合管进入高渗的髓质间质，产生少量浓缩尿液，使血液水势回升至正常。
4. Propagation of the Action Potential | 动作电位的传播
Question: Describe how an action potential is propagated along a myelinated axon. (5 marks)
An action potential is generated at the axon hillock when the membrane depolarises to threshold.
当膜去极化达到阈值时，动作电位在轴丘处产生。
The influx of Na⁺ at one node creates local circuits that depolarise the adjacent membrane in front of the action potential.
一个郎飞结处Na⁺的内流产生局部电流，使动作电位前方的相邻膜去极化。
The absolute refractory period prevents backward movement of the action potential, ensuring unidirectional propagation.
绝对不应期阻止动作电位向后移动，确保了单向传播。
In myelinated axons, depolarisation and action potentials occur only at the nodes of Ranvier, where voltage-gated Na⁺ channels are concentrated.
在有髓轴突中，去极化和动作电位仅发生在郎飞结处，该处集中有电压门控Na⁺通道。
The action potential ‘jumps’ from node to node in a process called saltatory conduction, which greatly increases the speed of transmission.
动作电位以跳跃式传导的方式从一个结跳至下一个结，大大提高了传递速度。
5. Synaptic Transmission at a Cholinergic Synapse | 胆碱能突触的突触传递
Question: Describe the sequence of events at a cholinergic synapse from the arrival of an action potential to the generation of a postsynaptic potential. (7 marks)
When an action potential reaches the presynaptic knob, voltage-gated Ca²⁺ channels open and Ca²⁺ diffuses into the cytoplasm.
当动作电位到达突触前终扣时，电压门控Ca²⁺通道开放，Ca²⁺扩散进入胞质。
The influx of Ca²⁺ causes synaptic vesicles containing acetylcholine (ACh) to move towards and fuse with the presynaptic membrane.
Ca²⁺的内流促使含有乙酰胆碱（ACh）的突触小泡向突触前膜移动并与之融合。
ACh is released by exocytosis into the synaptic cleft and diffuses across it.
ACh通过胞吐作用释放到突触间隙并扩散穿过间隙。
ACh binds to specific receptor proteins on the postsynaptic membrane, causing ligand-gated Na⁺ channels to open.
ACh与突触后膜上的特异性受体蛋白结合，使配体门控Na⁺通道开放。
Na⁺ enters the postsynaptic neuron, leading to a localised depolarisation called an excitatory postsynaptic potential (EPSP).
Na⁺进入突触后神经元，引起局部去极化，称为兴奋性突触后电位（EPSP）。
If the EPSP reaches threshold, an action potential is triggered in the postsynaptic neuron.
若EPSP达到阈值，则在突触后神经元触发动作电位。
Acetylcholinesterase in the cleft rapidly hydrolyses ACh into acetate and choline, terminating the signal; choline is taken back up into the presynaptic knob for resynthesis of ACh.
间隙中的乙酰胆碱酯酶迅速将ACh水解为乙酸和胆碱，终止信号；胆碱被重新摄取回突触前终扣用于再合成ACh。
6. Hardy-Weinberg Equilibrium | 哈迪-温伯格平衡
Question: In a randomly mating population of snails, 16% of individuals display a recessive shell colour due to the allele b. Calculate the frequency of heterozygous individuals and state two assumptions of the Hardy-Weinberg principle. (4 marks)
The frequency of the homozygous recessive genotype (bb) is 0.16, so q² = 0.16.
隐性纯合基因型（bb）的频率为0.16，因此q² = 0.16。
Taking the square root gives the recessive allele frequency q = 0.4.
开平方得到隐性等位基因频率q = 0.4。
Since p + q = 1, the dominant allele frequency p = 1 − 0.4 = 0.6.
由p + q = 1，得到显性等位基因频率p = 1 − 0.4 = 0.6。
The frequency of heterozygous individuals (Bb) is 2pq = 2 × 0.6 × 0.4 = 0.48 (48%).
杂合子个体（Bb）的频率为2pq = 2 × 0.6 × 0.4 = 0.48（48%）。
Two key assumptions are: the population is large enough for genetic drift to be negligible, and there is no mutation, migration or natural selection altering allele frequencies.
两个关键假设是：种群足够大，以至于遗传漂变可忽略不计；并且没有突变、迁移或自然选择改变等位基因频率。
7. PCR and Gel Electrophoresis | PCR与凝胶电泳
Question: Describe the main stages of the polymerase chain reaction (PCR) and explain how gel electrophoresis separates DNA fragments. (6 marks)
In PCR, the DNA sample is first heated to around 95°C to break hydrogen bonds and separate the double strands (denaturation).
在PCR中，首先将DNA样品加热至约95°C，以破坏氢键并分离双链（变性）。
The temperature is lowered to about 55°C to allow short DNA primers to anneal to complementary sequences on the single-stranded DNA (annealing).
温度降至约55°C，使短的DNA引物与单链DNA上的互补序列退火（退火）。
The temperature is raised to approximately 72°C, the optimum for Taq polymerase, which extends the primers by adding complementary nucleotides (extension).
温度升至约72°C，这是Taq聚合酶的最适温度，该酶通过添加互补核苷酸延伸引物（延伸）。
These three steps are repeated for 25–35 cycles, amplifying the target DNA segment exponentially.
这三个步骤重复25–35个循环，使目标DNA片段呈指数扩增。
In gel electrophoresis, DNA fragments are loaded into wells in an agarose gel and an electric current is applied.
在凝胶电泳中，DNA片段被加入到琼脂糖凝胶的孔中，并施加电流。
Because DNA is negatively charged, fragments migrate towards the positive electrode; smaller fragments move faster and travel farther through the pores of the gel, separating fragments by size.
由于DNA带负电，片段向正极迁移；较小的片段移动更快，通过凝胶孔洞移动得更远，从而按大小分离片段。
8. Microorganisms in the Nitrogen Cycle | 氮循环中的微生物
Question: Outline the roles of named microorganisms in the nitrogen cycle, including nitrification and denitrification. (6 marks)
Saprobiotic bacteria and fungi decompose dead organic matter, releasing ammonium ions (NH₄⁺) in a process called ammonification.
腐生细菌和真菌分解死亡的有机物质，在氨化作用中释放铵离子（NH₄⁺）。
Nitrification is a two-step oxidative process: Nitrosomonas oxidises ammonium ions to nitrite ions (NO₂⁻).
硝化作用是一个两步氧化过程：亚硝化单胞菌将铵离子氧化为亚硝酸根离子（NO₂⁻）。
Nitrobacter further oxidises nitrite ions to nitrate ions (NO₃⁻), which can be absorbed and assimilated by plants.
硝化杆菌将亚硝酸根离子进一步氧化为硝酸根离子（NO₃⁻），后者可被植物吸收和同化。
Nitrogen-fixing bacteria, such as Rhizobium in root nodules, convert atmospheric N₂ into ammonia, which is rapidly converted to ammonium ions.
固氮细菌，例如根瘤中的根瘤菌，将大气中的N₂转化为氨，氨迅速转化为铵离子。
Denitrifying bacteria, such as Pseudomonas, reduce nitrate ions back to N₂ gas under anaerobic conditions, returning nitrogen to the atmosphere.
反硝化细菌，如假单胞菌，在厌氧条件下将硝酸根离子还原为N₂气体，使氮返回大气。
The table below summarises the main conversions:
下表总结了主要的转化过程：
| Process | Microorganisms | Conversion |
|---|---|---|
| Ammonification | Saprobionts | Organic N → NH₄⁺ |
| Nitrification | Nitrosomonas, Nitrobacter | NH₄⁺ → NO₂⁻ → NO₃⁻ |
| Nitrogen fixation | Rhizobium, Azotobacter | N₂ → NH₃ / NH₄⁺ |
| Denitrification | Pseudomonas | NO₃⁻ → N₂ |
9. Natural Selection and Antibiotic Resistance | 自然选择与抗生素耐药性
Question: Using the example of antibiotic resistance in bacteria, explain how natural selection leads to evolutionary change. (6 marks)
Within a bacterial population, random mutations during DNA replication can produce a resistance gene, resulting in genetic variation.
在细菌种群中，DNA复制过程中的随机突变可产生耐药基因，导致遗传变异。
Some individuals possess an allele that confers resistance to a specific antibiotic, while others do not.
某些个体具有赋予其对特定抗生素耐药性的等位基因，而其他个体则没有。
When the population is exposed to the antibiotic, it acts as a strong selection pressure; non-resistant bacteria are killed.
当种群暴露于该抗生素时，抗生素作为一种强大的选择压力；非耐药细菌被杀死。
Resistant bacteria survive and reproduce, passing the advantageous allele to their offspring (differential reproductive success).
耐药细菌存活并繁殖，将有利等位基因传递给后代（差异繁殖成功率）。
Over many generations, the frequency of the resistance allele within the population increases, shifting the population’s genetic composition.
经过多代，种群中耐药等位基因的频率增加，改变了种群的遗传组成。
This is an example of directional selection driven by an environmental change, demonstrating evolution by natural selection.
这是由环境变化驱动的定向选择实例，展示了通过自然选择的进化过程。
10. Hormonal Control of Blood Glucose | 血糖的激素调控
Question: Explain how insulin and glucagon enable the body to maintain a constant blood glucose concentration. (6 marks)
After a meal, when blood glucose concentration rises above the set point, beta cells in the pancreatic islets of Langerhans detect the increase and secrete insulin.
餐后血糖浓度超过设定点时，胰岛中的β细胞检测到升高并分泌胰岛素。
Insulin binds to receptors on the membranes of target cells, especially hepatocytes and muscle cells, increasing their permeability to glucose.
胰岛素与靶细胞（尤其是肝细胞和肌细胞）膜上的受体结合，增加它们对葡萄糖的通透性。
It also activates enzymes that promote the conversion of glucose to glycogen (glycogenesis) and increases the rate of glucose respiration.
它还激活促进葡萄糖转化为糖原的酶（糖原生成），并提高葡萄糖的呼吸速率。
When blood glucose falls below the set point, alpha cells secrete glucagon.
当血糖降至设定点以下时，α细胞分泌胰高血糖素。
Glucagon stimulates glycogenolysis (breakdown of glycogen to glucose) and gluconeogenesis (formation of glucose from amino acids and lipids) in the liver.
胰高血糖素刺激肝脏中的糖原分解（糖原分解为葡萄糖）和糖异生（由氨基酸和脂质形成葡萄糖）。
These antagonistic hormonal actions form a negative feedback loop that returns blood glucose to its normal range.
这些拮抗性激素作用形成负反馈回路，使血糖恢复到正常范围。
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