📚 Cells: Building Blocks of Life | 细胞：生命的基石
Cells are the smallest units of life. In this chapter, you will learn how to use a microscope, compare plant and animal cells, identify specialised cells, and describe how cells work together to form tissues, organs and systems. These ideas are essential for Cambridge Lower Secondary Science and for understanding how living organisms function.
细胞是生命的最小单位。在本章中，你将学习如何使用显微镜，比较植物和动物细胞，识别特化细胞，并描述细胞如何共同作用形成组织、器官和系统。这些知识是剑桥初中科学课程的重要基础，也是理解生物体如何运作的关键。
1. What Are Cells? | 什么是细胞？
Cells are the basic structural and functional units of all living organisms. Some organisms, such as bacteria and amoeba, are unicellular and consist of only one cell. Others, including plants and animals, are multicellular and may contain billions of cells.
细胞是所有生物体的基本结构和功能单位。有些生物，如细菌和变形虫，是单细胞的，仅由一个细胞组成。其他生物，包括植物和动物，是多细胞的，可能含有数十亿个细胞。
Every cell is surrounded by a cell membrane. The cell membrane controls the movement of substances into and out of the cell, such as nutrients, gases and waste products. Inside the membrane, the cytoplasm carries out many chemical reactions needed for life.
每个细胞都由细胞膜包围。细胞膜控制物质进出细胞，例如营养物质、气体和废物。在细胞膜内部，细胞质进行许多维持生命所需的化学反应。
2. Using a Light Microscope | 使用光学显微镜
A light microscope is used to observe cells because most cells are too small to see with the naked eye. The main parts of a microscope include the eyepiece lens, objective lenses, stage, mirror or light source, and focusing knobs.
光学显微镜用于观察细胞，因为大多数细胞太小，肉眼无法看到。显微镜的主要部件包括目镜、物镜、载物台、反光镜或光源以及调焦旋钮。
To calculate the total magnification of a microscope, multiply the magnification of the eyepiece lens by the magnification of the objective lens being used.
要计算显微镜的总放大倍数，需要将目镜的放大倍数乘以正在使用的物镜的放大倍数。
total magnification = eyepiece lens magnification × objective lens magnification
总放大倍数 = 目镜放大倍数 × 物镜放大倍数
When focusing a microscope, always start with the lowest power objective lens. Use the coarse focusing knob first to bring the image into view, then use the fine focusing knob to make the image sharp. Never allow the objective lens to touch the microscope slide.
对显微镜调焦时，始终从最低倍物镜开始。先使用粗调焦旋钮使图像出现，然后使用细调焦旋钮使图像清晰。切勿让物镜接触载玻片。
3. Animal Cell Structure | 动物细胞结构
A typical animal cell contains three main parts: the nucleus, cytoplasm and cell membrane. Some animal cells also contain small structures called mitochondria, which release energy during respiration.
一个典型的动物细胞包含三个主要部分：细胞核、细胞质和细胞膜。一些动物细胞还含有称为线粒体的小结构，线粒体在呼吸作用中释放能量。
- Nucleus - controls the activities of the cell and contains genetic material. 细胞核——控制细胞的活动并含有遗传物质。
- Cytoplasm - a jelly-like substance where most chemical reactions take place. 细胞质——一种果冻状物质，大多数化学反应在这里进行。
- Cell membrane - a thin layer around the cell that controls what enters and leaves. 细胞膜——细胞周围的一层薄膜，控制物质的进出。
- Mitochondria - small structures where respiration releases energy from food. 线粒体——通过呼吸作用从食物中释放能量的小结构。
4. Plant Cell Structure | 植物细胞结构
Plant cells have all the structures found in animal cells, plus three additional structures: a cell wall, chloroplasts and a large permanent vacuole. These extra parts help plants survive and carry out photosynthesis.
植物细胞具有动物细胞中的所有结构，另外还有三种结构：细胞壁、叶绿体和大的中央液泡。这些额外的部分帮助植物生存并进行光合作用。
- Cell wall - a rigid outer layer made of cellulose that supports the cell and gives it a fixed shape. 细胞壁——由纤维素构成的坚硬外层，支撑细胞并使其形状固定。
- Chloroplasts - contain green chlorophyll, which absorbs light energy for photosynthesis. 叶绿体——含有绿色叶绿素，可吸收光能进行光合作用。
- Large permanent vacuole - filled with cell sap, it helps keep the cell firm and stores water and dissolved substances. 大的中央液泡——充满细胞液，有助于保持细胞坚挺，并储存水和溶解的物质。
5. Comparing Plant and Animal Cells | 植物与动物细胞比较
Plant and animal cells share several features, but they also have important differences. The table below summarises the key structures found in each type of cell.
植物细胞和动物细胞有一些共同特征，但也有重要区别。下表总结了每种细胞中的关键结构。
| Structure | 结构 | Animal cell | 动物细胞 | Plant cell | 植物细胞 |
|---|---|---|
| Nucleus | 细胞核 | Present | 有 | Present | 有 |
| Cytoplasm | 细胞质 | Present | 有 | Present | 有 |
| Cell membrane | 细胞膜 | Present | 有 | Present | 有 |
| Mitochondria | 线粒体 | Present | 有 | Present | 有 |
| Cell wall | 细胞壁 | Absent | 无 | Present | 有 |
| Chloroplasts | 叶绿体 | Absent | 无 | Present | 有 |
| Large permanent vacuole | 大的中央液泡 | Absent | 无 | Present | 有 |
Remember that plant cells also have a cell membrane, even though it is inside the cell wall. The cell wall is an additional layer outside the membrane.
请记住，植物细胞也有细胞膜，即使它位于细胞壁内部。细胞壁是细胞膜外的一层额外结构。
6. Specialised Cells | 特化细胞
In multicellular organisms, cells are often specialised to carry out particular functions. A specialised cell has a shape and structure that match its job. Here are four common examples:
在多细胞生物中，细胞通常特化以执行特定功能。特化细胞的形状和结构与其功能相匹配。以下是四个常见例子：
- Red blood cell - has no nucleus and a biconcave shape to carry more oxygen. 红细胞——没有细胞核，呈双凹形，可携带更多氧气。
- Nerve cell (neurone) - long and thin to transmit electrical signals quickly. 神经细胞（神经元）——又长又细，可快速传递电信号。
- Root hair cell - has a long extension to increase the surface area for absorbing water and minerals. 根毛细胞——有一个长长的突起，可增加吸收水分和矿物质的表面积。
- Palisade mesophyll cell - contains many chloroplasts near the top of the leaf to capture light for photosynthesis. 栅栏叶肉细胞——在叶片上部含有许多叶绿体，以捕获光能进行光合作用。
7. Cells, Tissues, Organs and Systems | 细胞、组织、器官和系统
Cells are organised into higher levels of structure in multicellular organisms. A group of similar cells working together is called a tissue. Different tissues work together to form an organ, and organs work together in an organ system.
在多细胞生物中，细胞组织成更高层次的结构。一组相似的细胞共同工作称为组织。不同的组织共同工作形成器官，器官共同工作在器官系统中。
For example, in the human digestive system, muscle tissue and epithelial tissue work together in the stomach, which is an organ. The stomach, intestines, liver and pancreas form the digestive system, which breaks down food and absorbs nutrients.
例如，在人体消化系统中，肌肉组织和上皮组织在胃这个器官中共同工作。胃、肠、肝和胰腺组成消化系统，该系统分解食物并吸收营养。
In plants, palisade mesophyll cells form palisade tissue, which is part of a leaf. Leaves, stems and roots are organs that together form the shoot and root systems.
在植物中，栅栏叶肉细胞形成栅栏组织，栅栏组织是叶片的一部分。叶、茎和根是器官，它们共同形成地上部分和根系。
8. Life Processes in Cells | 细胞中的生命活动
All living organisms carry out a set of life processes. These processes can be remembered using the letters MRS GREN: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion and Nutrition.
所有生物体都进行一系列生命活动。这些生命活动可以用字母 MRS GREN 来记忆：运动、呼吸、感应、生长、繁殖、排泄和营养。
- Movement - organisms change position or move parts of their bodies. 运动——生物改变位置或移动身体的一部分。
- Respiration - cells release energy from food. 呼吸——细胞从食物中释放能量。
- Sensitivity - organisms detect and respond to changes in their surroundings. 感应——生物探测并响应周围环境的变化。
- Growth - organisms increase in size and cell number. 生长——生物体积和细胞数量增加。
- Reproduction - organisms produce new individuals. 繁殖——生物产生新的个体。
- Excretion - organisms remove waste products made in chemical reactions. 排泄——生物去除化学反应中产生的废物。
- Nutrition - organisms take in and use food for energy, growth and repair. 营养——生物摄入并利用食物以获取能量、生长和修复。
9. Magnification Calculations | 放大倍数计算
You may need to calculate the actual size of a specimen or the magnification used. The formula below links the size of an image, the actual size of the object, and the magnification.
你可能需要计算标本的实际大小或使用的放大倍数。以下公式将图像大小、物体实际大小和放大倍数联系起来。
magnification = size of image ÷ actual size of object
放大倍数 = 图像大小 ÷ 物体实际大小
agnification is 20 ÷ 0.05 = 400 times. The drawing is 400 times larger than the real cell.
例如，如果书中画出的细胞长度为 20 毫米，而其实际长度为 0.05 毫米，则放大倍数为 20 ÷ 0.05 = 400 倍。图画比真实细胞大 400 倍。
10. Making a Microscope Slide | 制作显微镜装片
When preparing a temporary slide of onion cells, follow these steps to get a clear image. Always handle glass slides and cover slips carefully.
制作洋葱细胞临时装片时，请按照以下步骤操作，以获得清晰的图像。操作载玻片和盖玻片时要小心。
- Place a drop of water on a clean microscope slide. 在干净的载玻片上滴一滴水。
- Use forceps to peel a thin layer of onion epidermis and place it flat in the water. 用镊子撕下一层薄薄的洋葱表皮，并将其平放在水滴中。
- Add one drop of iodine solution to stain the cells so the nucleus becomes visible. 加一滴碘液对细胞进行染色，使细胞核可见。
- Lower a cover slip slowly at an angle to avoid trapping air bubbles. 将盖玻片倾斜着慢慢放下，避免产生气泡。
- Place the slide on the stage and focus using the lowest power objective first. 将装片放在载物台上，先用最低倍物镜调焦。
11. Common Misconceptions and Exam Tips | 常见误区与考试技巧
Many students lose marks by confusing cell structures or forgetting units. Here are some common mistakes and how to avoid them.
许多学生因混淆细胞结构或忘记单位而失分。以下是一些常见错误以及避免方法。
- Do not say that animal cells have a cell wall; only plant cells have a cell wall. 不要说动物细胞有细胞壁；只有植物细胞有细胞壁。
- Chloroplasts are not found in all plant cells, but they are found in green parts such as leaves and stems. 并非所有植物细胞都有叶绿体，但叶片和茎等绿色部分有叶绿体。
- Always include units when calculating magnification or size, such as mm or μm. 计算放大倍数或尺寸时，始终要写单位，例如 mm 或 μm。
- The cell membrane is present in plant cells, inside the cell wall. It is not replaced by the cell wall. 植物细胞也有细胞膜，位于细胞壁内侧，它不是被细胞壁取代。
12. Quick Check Summary | 快速检查总结
Cells are the basic units of life and can be observed with a light microscope. Animal cells contain a nucleus, cytoplasm, cell membrane and mitochondria. Plant cells have these structures plus a cell wall, chloroplasts and a large permanent vacuole.
细胞是生命的基本单位，可以用光学显微镜观察。动物细胞含有细胞核、细胞质、细胞膜和线粒体。植物细胞除这些结构外，还有细胞壁、叶绿体和大的中央液泡。
Specialised cells have adaptations that help them perform their functions. Cells are organised into tissues, organs and systems. Always revise the differences between plant and animal cells and practise magnification calculations before your test.
特化细胞具有帮助它们执行功能的适应性特征。细胞组织成组织、器官和系统。考试前务必复习植物细胞和动物细胞的区别，并练习放大倍数计算。
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