📚 IGCSE Science: Animals Revision | IGCSE 科学：动物考点精讲
Animals are multicellular, eukaryotic organisms that obtain nutrients by ingestion. In IGCSE Science, the study of animals covers cell structure, classification, and major physiological systems such as digestion, circulation, gas exchange, nervous coordination, hormonal control, reproduction, and homeostasis. A solid understanding of these topics is essential for success in the Biology or Coordinated Sciences paper.
动物是多细胞真核生物，通过摄食获取营养。在 IGCSE 科学中，动物的学习涵盖细胞结构、分类以及消化、循环、气体交换、神经调控、激素控制、生殖和稳态等主要生理系统。扎实掌握这些考点是生物学或综合科学考试取得好成绩的关键。
1. Characteristics of Animals | 动物的基本特征
Animals are heterotrophic, meaning they cannot produce their own food and must consume other organisms. Their cells lack cell walls and usually contain centrioles. Most animals have nervous and muscle tissues that allow rapid responses to stimuli. They also store carbohydrate as glycogen and reproduce mainly by sexual reproduction.
动物是异养的，意味着它们不能自己制造食物，必须摄取其他生物体。动物细胞没有细胞壁，通常含有中心粒。大多数动物具有神经和肌肉组织，能对刺激做出快速反应。它们以糖原的形式储存碳水化合物，主要通过有性生殖繁殖后代。
Multicellularity allows cell specialization, with tissues organized into organs and organ systems. Animals undergo a stage of embryonic development; many have larval stages. They are motile at some point in their life cycle, a key adaptation for finding food and mates.
多细胞性使细胞特化成为可能，组织进一步形成器官和器官系统。动物经历胚胎发育阶段；许多动物有幼体阶段。它们在生活史的某个阶段能够运动，这是寻找食物和配偶的关键适应特征。
2. Animal Cells | 动物细胞
A typical animal cell contains a nucleus, cytoplasm, cell membrane, mitochondria, ribosomes, and sometimes small temporary vacuoles. Unlike plant cells, there are no chloroplasts, no large permanent vacuole, and no cellulose cell wall. The cell membrane controls the movement of substances in and out of the cell.
典型的动物细胞含有细胞核、细胞质、细胞膜、线粒体、核糖体，有时还有小型临时液泡。与植物细胞不同，动物细胞没有叶绿体、没有大型中央液泡，也没有纤维素细胞壁。细胞膜控制物质进出细胞。
Mitochondria are the sites of aerobic respiration, releasing energy for cellular activities. The nucleus contains genetic material (DNA) and controls cell functions. Ribosomes are responsible for protein synthesis. Animal cells may also have lysosomes containing digestive enzymes.
线粒体是有氧呼吸的场所，为细胞活动释放能量。细胞核含有遗传物质 (DNA)，控制细胞功能。核糖体负责蛋白质合成。动物细胞还可能含有溶酶体，内含消化酶。
3. Classification of Animals | 动物分类
The animal kingdom is divided into two major groups: vertebrates (with a backbone) and invertebrates (without a backbone). Vertebrates are further classified into five classes: fish, amphibians, reptiles, birds, and mammals. Each class has distinctive features related to skin coverings, methods of reproduction, and thermoregulation.
动物界分为两大类：脊椎动物（有脊柱）和无脊椎动物（无脊柱）。脊椎动物再细分为五个纲：鱼类、两栖类、爬行类、鸟类和哺乳类。每一纲在皮肤覆盖物、繁殖方式和体温调节方面都有独特特征。
Mammals have hair or fur, give birth to live young (except monotremes), and feed milk from mammary glands. Birds possess feathers, lay hard-shelled eggs, and maintain a constant body temperature. Fish use gills for gas exchange and have scales; amphibians have moist skin and undergo metamorphosis; reptiles have dry scaly skin and lay soft-shelled eggs.
哺乳动物有毛发或毛皮，胎生（单孔目除外），并由乳腺分泌乳汁。鸟类有羽毛，产硬壳卵，维持恒定体温。鱼类用鳃进行气体交换并有鳞片；两栖类皮肤湿润，经历变态发育；爬行类皮肤干燥带鳞片，产软壳卵。
Important invertebrate groups include arthropods (insects, crustaceans, arachnids), molluscs, annelids, and nematodes. Arthropods have an exoskeleton, jointed legs, and a segmented body. Insects have three body parts, six legs, and usually wings; arachnids have two body parts and eight legs.
重要的无脊椎动物类群包括节肢动物（昆虫、甲壳类、蛛形类）、软体动物、环节动物和线虫。节肢动物具有外骨骼、分节的腿和分节的身体。昆虫有三个体节、六条腿，通常有翅；蛛形类有两个体节和八条腿。
4. Nutrition: Human Digestive System | 营养：人类消化系统
Animals require a balanced diet containing carbohydrates, proteins, lipids, vitamins, minerals, water, and dietary fibre. The human digestive system breaks down large insoluble molecules into small soluble ones that can be absorbed into the blood. Digestion involves mechanical breakdown (chewing, churning) and chemical digestion by enzymes.
动物需要均衡的饮食，包括碳水化合物、蛋白质、脂质、维生素、矿物质、水和膳食纤维。人类消化系统将大分子不溶性物质分解成可被血液吸收的小分子可溶物。消化过程包括机械分解（咀嚼、搅拌）和酶的化学消化。
Carbohydrates are broken down by amylase into simple sugars; proteins by proteases into amino acids; lipids by lipases into fatty acids and glycerol. The stomach produces hydrochloric acid and pepsin. Bile, produced by the liver and stored in the gall bladder, emulsifies fats to increase surface area for lipase action. Absorption of nutrients occurs mainly in the small intestine, where villi increase the surface area.
碳水化合物被淀粉酶分解为单糖；蛋白质被蛋白酶分解为氨基酸；脂质被脂肪酶分解为脂肪酸和甘油。胃分泌盐酸和胃蛋白酶。肝脏产生、胆囊储存的胆汁能乳化脂肪，增加脂肪酶作用面积。营养物质的吸收主要在小肠进行，小肠绒毛增大了吸收面积。
5. Gas Exchange | 气体交换
Animals need to obtain oxygen for aerobic respiration and remove carbon dioxide. The human gas exchange system includes the trachea, bronchi, bronchioles, and alveoli. The alveoli are adapted for efficient gas exchange: they have a large surface area, thin walls (one cell thick), a moist lining, and a dense capillary network.
动物需要获取氧气进行有氧呼吸并排出二氧化碳。人类气体交换系统包括气管、支气管、细支气管和肺泡。肺泡适应高效气体交换：具有大的表面积、薄壁（仅一层细胞厚）、湿润的内衬和密集的毛细血管网。
Ventilation involves the intercostal muscles and diaphragm. During inhalation, the diaphragm contracts and flattens, and the external intercostal muscles raise the ribcage, increasing the volume of the thorax and decreasing pressure, drawing air in. Exhalation is largely passive but can be aided by internal intercostals. Other animals use gills, tracheae, or skin for gas exchange.
通气过程涉及肋间肌和膈肌。吸气时膈肌收缩变平，外肋间肌上提胸廓，胸腔容积增大，压力减小，空气进入。呼气以被动为主，内肋间肌可辅助呼气。其他动物利用鳃、气管或皮肤进行气体交换。
6. Transport: Circulatory System | 运输：循环系统
The human circulatory system is a double circulation consisting of the heart, blood vessels, and blood. 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). The heart has four chambers: two atria and two ventricles.
人体循环系统是双循环，由心脏、血管和血液组成。右心将缺氧血泵入肺部（肺循环），左心将富氧血泵入全身（体循环）。心脏有四个腔室：两个心房和两个心室。
Arteries carry blood away from the heart under high pressure; they have thick muscular and elastic walls. Veins carry blood back to the heart and contain valves to prevent backflow. Capillaries have walls only one cell thick to allow exchange of materials. Blood consists of plasma, red blood cells (transport oxygen using haemoglobin), white blood cells (defence), and platelets (clotting).
动脉在高压下将血液运离心脏，管壁厚而富有肌肉和弹性纤维。静脉将血液送回心脏，内有瓣膜防止倒流。毛细血管壁仅一层细胞厚，便于物质交换。血液由血浆、红细胞（利用血红蛋白运输氧气）、白细胞（防御）和血小板（凝血）组成。
7. Nervous System and Coordination | 神经系统与协调
The nervous system enables rapid, short-lived responses to stimuli. It consists of the central nervous system (brain and spinal cord) and peripheral nerves. A reflex arc is the simplest pathway: stimulus → receptor → sensory neurone → relay neurone in spinal cord → motor neurone → effector (muscle or gland).
神经系统能对刺激做出快速、短暂的反应。它由中枢神经系统（脑和脊髓）和周围神经组成。反射弧是最简单的通路：刺激 → 感受器 → 感觉神经元 → 脊髓中的中间神经元 → 运动神经元 → 效应器（肌肉或腺体）。
Synapses are junctions between neurones where neurotransmitters diffuse across the gap, ensuring impulses travel in one direction. The eye is a sense organ; its structures include the cornea (refracts light), iris (controls light entry), lens (focuses), and retina (contains light-sensitive rods and cones). Accommodation is the lens changing shape to focus on near or distant objects.
突触是神经元之间的连接点，神经递质在此扩散通过间隙，确保冲动单向传递。眼睛是感觉器官；其结构包括角膜（折射光线）、虹膜（控制进光量）、晶状体（聚焦）和视网膜（含有感光的视杆细胞和视锥细胞）。调节是指晶状体改变形状以聚焦近处或远处物体。
8. Hormonal Control | 激素控制
Hormones are chemical messengers produced by endocrine glands, transported in the blood, and act on target organs. Hormonal responses are slower but longer-lasting than nervous responses. Key hormones include insulin (lowers blood glucose), glucagon (raises blood glucose), adrenaline (prepares for ‘fight or flight’), and sex hormones.
激素是由内分泌腺产生的化学信使，由血液运输并作用于靶器官。激素反应比神经反应慢但持久。关键激素包括胰岛素（降低血糖）、胰高血糖素（升高血糖）、肾上腺素（应对“战斗或逃跑”反应）和性激素。
Blood glucose regulation is a negative feedback mechanism: after a meal, glucose rises, insulin is secreted, and liver cells convert glucose to glycogen. When glucose falls, glucagon promotes glycogen breakdown to glucose. Adrenaline increases heart rate, breathing rate, and blood flow to muscles, redirecting energy for rapid action.
血糖调节是负反馈机制：餐后血糖升高，胰岛素分泌，肝细胞将葡萄糖转化为糖原。血糖降低时，胰高血糖素促进糖原分解为葡萄糖。肾上腺素增加心率、呼吸频率和肌肉血流量，为快速行动重新分配能量。
9. Reproduction | 生殖
Sexual reproduction involves the fusion of male and female gametes (sperm and egg) to form a zygote, giving genetic variation. The male reproductive system produces sperm in the testes; the female system releases eggs from the ovaries, and fertilization normally occurs in the oviduct. The uterus supports the developing embryo, and the placenta provides nutrients and oxygen while removing wastes.
有性生殖涉及雄性配子和雌性配子（精子和卵子）融合形成受精卵，产生遗传变异。男性生殖系统在睾丸中产生精子；女性系统从卵巢释放卵子，受精通常发生在输卵管。子宫支持胚胎发育，胎盘提供营养和氧气，同时排出废物。
The menstrual cycle is controlled by hormones: FSH stimulates egg development, LH triggers ovulation, and oestrogen and progesterone regulate the uterine lining. If fertilization occurs, the embryo secretes hCG to maintain the corpus luteum. Contraception methods include barriers, hormonal pills, and sterilization. Some animals reproduce asexually, but most vertebrates rely on sexual reproduction.
月经周期受激素调控：FSH 促进卵子发育，LH 触发排卵，雌激素和孕酮调节子宫内膜。若受精，胚胎分泌 hCG 来维持黄体。避孕方法包括屏障法、激素药物和绝育。有些动物进行无性生殖，但大多数脊椎动物依赖有性生殖。
10. Homeostasis and Excretion | 稳态与排泄
Homeostasis is the maintenance of a constant internal environment. In animals, this includes regulating body temperature, blood glucose, and water balance. Mammals and birds are endotherms and maintain a steady body temperature using insulation, sweating, shivering, and changes in blood flow to the skin (vasodilation and vasoconstriction).
稳态是维持恒定内环境的过程。在动物中，这包括调节体温、血糖和水分平衡。哺乳动物和鸟类是内温动物，通过保温、出汗、颤抖和改变皮肤血流量（血管舒张和血管收缩）来维持稳定体温。
Excretion removes metabolic wastes. The kidneys play a central role by filtering blood in nephrons, reabsorbing useful substances, and forming urine. Urine contains urea, excess salts, and water. The process of filtration is driven by high pressure in the glomerulus, and reabsorption of glucose and water occurs in the tubules. ADH from the pituitary gland controls water reabsorption based on body hydration levels.
排泄将代谢废物排出体外。肾脏通过肾单位过滤血液、重吸收有用物质并形成尿液，发挥核心作用。尿液含有尿素、多余的盐和水。过滤过程由肾小球内的高压驱动，葡萄糖和水的重吸收在肾小管进行。垂体分泌的 ADH 根据体内水分状况控制水的重吸收。
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