Cell Structure and Function | 细胞结构与功能

📚 Cell Structure and Function | 细胞结构与功能

Cells are the basic units of life. Understanding their structure and function is essential for IGCSE Biology, as it forms the foundation for topics such as transport, enzymes, respiration, and reproduction.

细胞是生命的基本单位。理解它们的结构和功能是IGCSE生物学的核心基础，也是学习物质运输、酶、呼吸作用和生殖等章节的前提。

1. Cell Theory | 细胞学说

The cell theory states that all living organisms are composed of one or more cells; the cell is the basic unit of structure and organisation in organisms; and all cells arise from pre-existing cells.

细胞学说指出：所有生物体由一个或多个细胞组成；细胞是生物体结构和组织的基本单位；所有细胞都来自已有的细胞。

Prokaryotic cells lack a true nucleus, while eukaryotic cells possess a membrane-bound nucleus.
原核细胞没有真正的细胞核，而真核细胞具有被膜包围的细胞核。

2. Animal Cells vs Plant Cells | 动物细胞与植物细胞

Animal cells and plant cells share common features but have key differences. The table below summarises these for IGCSE revision.

动物细胞和植物细胞既有共同点，也有重要差异。下表总结了IGCSE考试中需要掌握的区别。

Feature	Animal cell	Plant cell
Cell wall	Absent	Present (cellulose)
Chloroplasts	Absent	Present (in photosynthetic cells)
Vacuole	Small/temporary	Large permanent (tonoplast)
Shape	Irregular	Regular (cubical/rectangular)

3. Organelles and Their Functions | 细胞器及其功能

Each organelle has a specific role. You need to know the names and functions of the main organelles studied in Edexcel IGCSE Biology.

每种细胞器都有特定的功能。你必须掌握Edexcel IGCSE生物中主要细胞器的名称和作用。

Nucleus – contains DNA; controls cell activities.
细胞核 – 含有DNA，控制细胞活动。
Mitochondria – site of aerobic respiration; produces ATP.
线粒体 – 有氧呼吸的场所，产生ATP。
Ribosomes – synthesise proteins.
核糖体 – 合成蛋白质。
Cytoplasm – jelly-like fluid where many metabolic reactions occur.
细胞质 – 胶状液体，许多代谢反应在此进行。
Cell membrane – controls what enters and leaves the cell.
细胞膜 – 控制物质进出细胞。

4. Specialised Cells | 特化细胞

Multicellular organisms have cells that are adapted to their functions. In exams, you may be asked to explain how structure relates to function.

多细胞生物体的细胞都与其功能相适应。考试中常要求你解释结构如何与功能相关联。

Red blood cells – biconcave shape, no nucleus → increase surface area for oxygen transport and more space for haemoglobin.
红细胞 – 双凹盘形、无细胞核 → 增加气体交换的表面积，并为血红蛋白提供更多空间。
Root hair cells – long projection → large surface area for water absorption.
根毛细胞 – 长突起 → 增大吸收水分的表面积。
Neurones – long fibres → transmit electrical impulses rapidly.
神经元 – 长纤维 → 快速传导神经冲动。
Xylem vessels – no cytoplasm; thick lignified walls → transport water and provide support.
木质部导管 – 无细胞质，壁木质化加厚 → 运输水分并提供支撑。

5. The Cell Membrane | 细胞膜

The cell membrane is partially permeable. It is made of phospholipids and proteins, and it controls the movement of substances.

细胞膜具有选择性通透性。它由磷脂和蛋白质组成，控制物质的进出。

Structure: phospholipid bilayer with embedded proteins

结构：磷脂双分子层，其中镶嵌蛋白质

Lipid-soluble molecules pass through easily.
脂溶性分子容易通过。
Water-soluble molecules (ions, sugars) need transport proteins.
水溶性分子（离子、糖类）需要转运蛋白的帮助。

6. Diffusion | 扩散

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is a passive process.

扩散是粒子从高浓度区域向低浓度区域（沿浓度梯度）的净运动。这是一个被动过程。

Example: oxygen diffusing from alveoli into blood.
例子：氧气从肺泡扩散进入血液。
Rate increases with greater concentration gradient, higher temperature, and larger surface area.
浓度梯度越大、温度越高、表面积越大，扩散速度越快。

7. Osmosis | 渗透作用

Osmosis is the diffusion of water molecules from a dilute solution (high water potential) to a concentrated solution (low water potential) through a partially permeable membrane.

渗透作用是指水分子通过半透膜从稀溶液（高水势）向浓溶液（低水势）扩散的过程。

Water moves: dilute → concentrated (through membrane)

水移动方向：稀溶液 → 浓溶液（通过半透膜）

In plant cells, water enters the vacuole → turgid (firm).
在植物细胞中，水进入液泡 → 细胞膨胀（坚挺）。
If water leaves the cell, it becomes plasmolysed.
如果水分流失，细胞会发生质壁分离。

8. Active Transport | 主动运输

Active transport is the movement of particles against a concentration gradient (from low to high), using energy from respiration and carrier proteins in the membrane.

主动运输是粒子逆浓度梯度（从低到高）的移动，需要呼吸作用产生的能量和膜上的载体蛋白。

Occurs in root hair cells for absorbing mineral ions from soil.
根毛细胞吸收土壤中的矿物离子时发生。
Also in the ileum for absorbing glucose after digestion.
回肠消化后吸收葡萄糖时也发生。

9. Enzymes | 酶

Enzymes are biological catalysts that speed up reactions without being used up. They are proteins with an active site that binds to a specific substrate.

酶是生物催化剂，能加速化学反应而不被消耗。它们是蛋白质，具有与特定底物结合的活性位点。

Substrate → (enzyme) → Product

底物 →（酶）→ 产物

Optimum temperature for most human enzymes is around 37 °C.
大多数人体酶的最适温度约为37 °C。
High temperatures denature enzymes (active site changes shape).
高温会使酶变性（活性位点形状改变）。
pH also affects enzyme activity; e.g. pepsin works best at pH 2.
pH也影响酶活性，例如胃蛋白酶在pH 2时活性最高。

10. Respiration | 呼吸作用

Respiration is the process by which living cells release energy from glucose. Aerobic respiration uses oxygen, while anaerobic respiration does not.

呼吸作用是活细胞从葡萄糖中释放能量的过程。有氧呼吸需氧气，无氧呼吸则不需要。

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ 36–38 ATP)

葡萄糖 + 氧气 → 二氧化碳 + 水（+ 约36–38个ATP）

Aerobic respiration in mitochondria.
有氧呼吸在线粒体中进行。
In humans, anaerobic respiration in muscles produces lactic acid.
人体肌肉无氧呼吸产生乳酸。
In yeast, anaerobic respiration produces ethanol and CO₂.
酵母菌无氧呼吸产生乙醇和二氧化碳。

11. Photosynthesis | 光合作用

Photosynthesis is the process by which green plants make glucose using carbon dioxide and water, with light energy absorbed by chlorophyll.

光合作用是绿色植物利用二氧化碳和水，通过叶绿素吸收光能制造葡萄糖的过程。

6CO₂ + 6H₂O →(light, chlorophyll) C₆H₁₂O₆ + 6O₂

二氧化碳 + 水 →（光，叶绿素）葡萄糖 + 氧气

Light intensity, CO₂ concentration, and temperature are limiting factors.
光照强度、二氧化碳浓度和温度是限制因素。
Chlorophyll is located in chloroplasts, mainly in leaf mesophyll cells.
叶绿素位于叶肉细胞的叶绿体中。

12. Cell Division | 细胞分裂

Cell division is essential for growth, repair, and reproduction. Mitosis produces two genetically identical daughter cells, while meiosis produces four genetically different gametes.

细胞分裂对生长、修复和繁殖至关重要。有丝分裂产生两个遗传相同的子细胞，减数分裂产生四个遗传不同的配子。

Mitosis: used for growth and asexual reproduction; maintains chromosome number (2n → 2n).
有丝分裂：用于生长和无性繁殖；保持染色体数目不变（2n → 2n）。
Meiosis: used to produce gametes; halves chromosome number (2n → n).
减数分裂：用于产生配子；染色体数目减半（2n → n）。

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Cell Structure and Function | 细胞结构与功能