📚 IB CCEA Science: Animals – Key Exam Points | IB CCEA 科学：动物考点精讲
Animals are a central theme in IB and CCEA combined science syllabuses, linking cell biology, whole-body systems, evolution and homeostasis. A firm grasp of animal structure and function is essential for high marks in both written exams and practical assessments.
动物是 IB 与 CCEA 综合科学课程的核心主题，串联起细胞生物学、全身系统、进化及稳态调节。扎实掌握动物的结构与功能，是在笔试和实验评估中取得高分的关键。
1. Animal Cells | 动物细胞
Animal cells are eukaryotic, meaning they contain a membrane-bound nucleus and other organelles. Unlike plant cells, they have no cell wall, no large central vacuole and typically no chloroplasts. Centrioles are present and help organise spindle fibres during mitosis.
动物细胞是真核细胞，具有膜包被的细胞核和其他细胞器。与植物细胞不同，它们没有细胞壁、没大的中央液泡，通常也不含叶绿体。中心粒存在于动物细胞中，在有丝分裂时帮助组装纺锤丝。
The plasma membrane, made of a phospholipid bilayer with embedded proteins, controls the passage of substances. Temporary small vacuoles may form for endocytosis or exocytosis.
质膜由磷脂双分子层和镶嵌蛋白构成，控制物质的进出。临时形成的小液泡用于胞吞或胞吐。
| Feature | Animal Cell | Plant Cell |
| Cell wall | Absent | Present (cellulose) |
| Vacuole | Small, temporary | Large, permanent |
| Chloroplasts | Absent | Present |
| Centrioles | Present | Absent |
Exam tip: be able to label a typical animal cell diagram and state the function of nucleus, cytoplasm, cell membrane, mitochondria and ribosomes.
应考提示：需要能标注典型动物细胞示意图，并说出细胞核、细胞质、细胞膜、线粒体和核糖体的功能。
2. Animal Tissues | 动物组织
Animal tissues are groups of similar cells working together. The four basic types are epithelial, connective, muscle and nervous. Epithelial tissue covers body surfaces and lines cavities; it offers protection, absorption and secretion. Connective tissue, such as blood, bone and cartilage, supports and binds other tissues. Muscle tissue is specialised for contraction – skeletal muscle is voluntary and striated, while smooth and cardiac muscle are involuntary. Nervous tissue transmits electrical impulses and coordinates responses.
动物组织由执行共同功能的相似细胞构成。四种基本类型是上皮组织、结缔组织、肌肉组织和神经组织。上皮组织覆盖体表并衬在腔体内表面，起保护、吸收和分泌作用。结缔组织（如血液、骨和软骨）起支撑和连接作用。肌肉组织特化用于收缩——骨骼肌受意识控制、有横纹，平滑肌和心肌则不受意识控制。神经组织传递电冲动，协调机体反应。
In exam questions, you may be shown photographs of tissues and asked to identify them by their shape, arrangement of cells and the presence of fibres or striations.
考试中可能会给出组织显微照片，让你通过细胞的形状、排列方式以及纤维或横纹的有无来进行识别。
3. The Digestive System | 消化系统
The digestive system breaks down large, insoluble food molecules into small, soluble ones that can be absorbed into the blood. Mechanical digestion begins in the mouth with chewing; chemical digestion relies on digestive enzymes produced by glands such as the salivary glands, stomach and pancreas. Amylase breaks down starch into maltose, proteases break down proteins into amino acids, and lipases break down fats into fatty acids and glycerol.
消化系统将大的、不可溶的食物分子分解为可吸收的小分子。物理消化从口腔咀嚼开始；化学消化依赖于唾液腺、胃和胰腺等腺体分泌的消化酶。淀粉酶将淀粉分解成麦芽糖，蛋白酶将蛋白质分解为氨基酸，脂肪酶将脂肪分解为脂肪酸和甘油。
The stomach produces hydrochloric acid (HCl), which kills bacteria and provides the acidic pH optimum for pepsin. Bile, produced by the liver and stored in the gall bladder, emulsifies fats, increasing the surface area for lipase action. The small intestine – especially the ileum – is the main site of absorption. Its inner wall is folded into villi, which in turn have microvilli, giving a huge surface area.
胃产生盐酸，可杀灭细菌并为胃蛋白酶提供最适酸性 pH。由肝脏产生、胆囊储存的胆汁能乳化脂肪，增加脂肪酶作用的表面积。小肠（尤其是回肠）是营养物质吸收的主要场所。其内壁折叠成绒毛，绒毛上还有微绒毛，极大扩展了吸收面积。
4. The Circulatory System | 循环系统
Mammals have a double circulatory system: the right side of the heart pumps deoxygenated blood to the lungs (pulmonary circulation), while the left side pumps oxygenated blood to the rest of the body (systemic circulation). This separation maintains high pressure and efficient oxygen delivery.
哺乳动物拥有双循环系统：右心将缺氧血泵送至肺部（肺循环），左心将富氧血泵送至全身（体循环）。这种分隔方式维持了高压，实现了高效的氧气输送。
The heart has four chambers – two atria and two ventricles. Valves prevent backflow: atrioventricular valves (tricuspid and bicuspid) lie between atria and ventricles, and semilunar valves guard the exits to the arteries. Coronary arteries supply the heart muscle itself with oxygen and nutrients.
心脏有四个腔室——两个心房和两个心室。瓣膜防止血液倒流：房室瓣（三尖瓣和二尖瓣）位于心房与心室之间，半月瓣位于心室出口处。冠状动脉为心肌自身提供氧气和营养。
Blood contains red blood cells (carry oxygen using haemoglobin), white blood cells (defence) and platelets (clotting). Arteries carry blood away from the heart under high pressure and have thick, elastic walls; veins return blood at lower pressure and contain valves; capillaries are one-cell thick to allow diffusion.
血液包含红细胞（利用血红蛋白运输氧气）、白细胞（防御）和血小板（凝血）。动脉将血液在高压下带离心脏，管壁厚且有弹性；静脉将血液在低压下导回心脏，具有静脉瓣；毛细血管仅由单层细胞构成，利于物质扩散。
5. Gas Exchange and Respiration | 气体交换与呼吸
Animals need oxygen for aerobic respiration and must remove carbon dioxide, a waste product. The respiratory surface must be thin, moist, well ventilated and have a large surface area. In mammals, gas exchange takes place in the alveoli of the lungs. Alveoli are surrounded by a dense network of capillaries; oxygen diffuses into the blood and carbon dioxide diffuses out.
动物进行有氧呼吸需要氧气，同时必须清除代谢废物二氧化碳。呼吸表面必须薄、湿润、有良好的通气并具有较大的表面积。哺乳动物的气体交换在肺泡中进行。肺泡被密集的毛细血管网包裹；氧气扩散入血，二氧化碳扩散出血液。
During inhalation, the diaphragm contracts and flattens, while the intercostal muscles contract to raise the ribcage, increasing thoracic volume and decreasing pressure, drawing air in. The overall equation for aerobic respiration is:
吸气时，膈肌收缩变平，肋间肌收缩抬起胸廓，胸腔体积增大、气压降低，空气被吸入。有氧呼吸的总方程式为：
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy as ATP)
Anaerobic respiration in animals produces lactic acid (C₃H₆O₃) and much less ATP. The build-up of lactic acid causes muscle fatigue and must be repaid by extra oxygen after exercise (oxygen debt).
动物的无氧呼吸产生乳酸和极少的 ATP。乳酸堆积导致肌肉疲劳，运动后需通过额外吸入氧气偿还氧债。
6. The Nervous System | 神经系统
The nervous system enables rapid, coordinated responses to stimuli. A reflex arc is the simplest functional unit: receptor → sensory neurone → relay neurone (in spinal cord or brain) → motor neurone → effector (muscle or gland). The synapse is a junction between neurones where neurotransmitter chemicals diffuse across the gap to continue the impulse.
神经系统使机体对刺激做出快速、协调的反应。反射弧是最简单的功能单位：感受器 → 感觉神经元 → 中间神经元（在脊髓或脑） → 运动神经元 → 效应器（肌肉或腺体）。突触是神经元之间的连接点，神经递质化学物质通过扩散跨过间隙传递冲动。
The brain and spinal cord form the central nervous system (CNS). A reflex action, such as pulling your hand away from heat, bypasses the brain for speed, involving the spinal cord directly. Conscious actions involve the cerebrum. The cerebellum coordinates balance and movement.
脑和脊髓构成中枢神经系统。反射动作，例如手遇热收回，为追求速度会绕开大脑、直接经由脊髓处理。有意识的行为涉及大脑皮层。小脑协调平衡和运动。
7. The Excretory System | 排泄系统
Excretion removes metabolic wastes, mainly urea (from protein breakdown) and excess water and salts. The kidneys are the main excretory organs. Each kidney contains around a million nephrons that filter the blood. Ultrafiltration occurs in the glomerulus under high pressure, forcing water, glucose, salts and urea into Bowman’s capsule. Large molecules such as proteins and blood cells remain in the blood.
排泄作用清除代谢废物，主要是尿素（来自蛋白质分解）以及多余的水分和盐类。肾脏是主要的排泄器官。每个肾约含一百万个肾单位，负责滤过血液。超滤作用在肾小球的高压下进行，将水、葡萄糖、盐和尿素压入鲍曼囊。大分子如蛋白质和血细胞则留在血液中。
Selective reabsorption then returns all glucose, some water and the required amount of salts back to the blood along the proximal convoluted tubule. Further water regulation occurs in the collecting duct, controlled by the hormone ADH. The remaining filtrate, urine, passes down the ureter to the bladder.
随后，选择性重吸收将近曲小管内的全部葡萄糖、部分水和所需盐分重新吸收回血液。集合管在抗利尿激素调节下进一步调控水分。剩下的滤液即尿液，经输尿管排至膀胱。
8. Reproduction and Development | 生殖与发育
Most animals reproduce sexually, involving the fusion of male (sperm) and female (egg) gametes to form a zygote. This process is called fertilisation and can be external (common in fish and amphibians) or internal (typical of reptiles, birds and mammals). Internal fertilisation often requires copulatory organs. Offspring receive a combination of genes from both parents, increasing genetic variation.
大多数动物进行有性生殖，通过雄性配子（精子）与雌性配子（卵子）融合形成受精卵。此过程称为受精，可以是体外受精（常见于鱼类和两栖动物）或体内受精（爬行类、鸟类和哺乳类典型）。体内受精常需要交接器官。子代获得来自双亲的基因组合，增加了遗传变异。
The human male reproductive system produces sperm in the testes and delivers them via the sperm ducts and penis. The female system releases eggs from the ovaries; fertilisation normally occurs in the oviduct, and the embryo implants in the uterus wall. The placenta allows exchange of nutrients, oxygen and waste between mother and foetus without mixing of blood.
人类男性生殖系统在睾丸产生精子，经由输精管和阴茎输送。女性生殖系统从卵巢排出卵子；受精通常发生在输卵管，胚胎着床于子宫内膜。胎盘在母体血液和胎儿之间交换营养物质、氧气和废物，而双方血液并不混合。
The menstrual cycle is controlled by hormones: oestrogen and progesterone. Understanding these hormonal changes helps you to grasp fertility, contraception and IVF in applied exam questions.
月经周期受雌激素和孕酮调控。理解这些激素变化，有助于在应用型考题中掌握生育、避孕和体外受精等知识。
9. Homeostasis and Thermoregulation | 稳态与体温调节
Homeostasis is the maintenance of a constant internal environment. Core temperature, blood pH, water balance and blood glucose concentration are all kept within narrow limits. Negative feedback loops are the primary control mechanism – a deviation from the set point triggers a corrective response that returns the system to normal.
稳态是指维持稳定的内环境。核心体温、血液 pH、水平衡和血糖浓度都被维持在狭窄范围内。负反馈是主要的调控机制——偏离设定点会触发纠正反应，使系统回到正常水平。
Animals can be endotherms (warm-blooded, e.g. mammals and birds) or ectotherms (cold-blooded, e.g. reptiles and fish). Endotherms generate heat internally and keep a fairly constant body temperature. When too hot, they sweat and increase blood flow to the skin (vasodilation). When cold, they shiver, hair erector muscles contract (producing ‘goose bumps’), and vasoconstriction reduces heat loss. Ectotherms rely on behavioural methods, such as basking in the sun.
动物可以是恒温动物（温血动物，如哺乳类和鸟类）或变温动物（冷血动物，如爬行类和鱼类）。恒温动物在体内产热，维持相对恒定的体温。过热时，会出汗并增加皮肤血流量（血管舒张）。寒冷时，会颤抖，立毛肌收缩（产生鸡皮疙瘩），同时血管收缩减少散热。变温动物依赖行为方式，如晒太阳取暖。
10. Animal Classification and Evolution | 动物分类与进化
Animals are broadly divided into vertebrates and invertebrates. Vertebrates belong to the phylum Chordata and are further split into five main classes: fish (scales, gills, external fertilisation), amphibians (moist skin, jelly-coated eggs), reptiles (dry scaly skin, internal fertilisation, leathery-shelled eggs), birds (feathers, forelimbs modified as wings, hard-shelled eggs, endothermy) and mammals (hair, mammary glands, three middle ear bones, endothermy).
动物大致分为脊椎动物和无脊椎动物。脊椎动物属于脊索动物门，进一步划分为五大类群：鱼类（鳞片、鳃、体外受精）、两栖类（湿润皮肤、胶质卵）、爬行类（干燥鳞片皮肤、体内受精、革质壳卵）、鸟类（羽毛、前肢特化为翼、硬壳卵、恒温）和哺乳类（毛发、乳腺、三块听小骨、恒温）。
Charles Darwin’s theory of evolution by natural selection underpins modern classification. Variation exists within a population; individuals with advantageous traits are more likely to survive and reproduce, passing those alleles to the next generation. Over time, this leads to adaptation and, potentially, speciation.
达尔文以自然选择为基础的进化理论是现代分类学的基石。种群内部存在变异；具有有利性状的个体更可能存活并繁殖，将这些等位基因传递给下一代。历经时间，这会导致适应，并可能形成新物种。
For exams, be ready to interpret phylogenetic trees or cladograms that show evolutionary relationships, and to explain how selective breeding differs from natural selection.
备考时要能解读表示进化关系的系统发育树或支序图，并能说明选择育种与自然选择的区别。
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