📚 Two Fundamentally Different Types of Cell | 两种根本不同的细胞类型
All living organisms are built from cells, but these cells do not all share the same internal organisation. In A-Level Biology, the most important distinction is between prokaryotic and eukaryotic cells. Understanding this difference helps explain how bacteria, archaea, protists, fungi, plants and animals carry out life processes in fundamentally different ways.
所有生物都由细胞构成，但这些细胞的内部结构并不相同。在 A-Level 生物中，最重要的区别是原核细胞与真核细胞的区别。理解这一差异有助于解释细菌、古菌、原生生物、真菌、植物和动物如何以根本不同的方式进行生命活动。
1. The Cell Theory and the Great Divide | 细胞学说与根本分野
The cell theory states that all living things are made of cells, that the cell is the basic unit of life, and that all cells arise from pre-existing cells. Within this unity, biologists recognise two fundamentally different types of cell: prokaryotic and eukaryotic. The terms come from Greek: ‘pro’ means before, ‘karyon’ means nucleus; ‘eu’ means true. Therefore prokaryotic cells lack a true nucleus, while eukaryotic cells possess a membrane-bound nucleus.
细胞学说指出所有生物都由细胞组成，细胞是生命的基本单位，所有细胞均来自已有的细胞。在这种统一性中，生物学家识别出两种根本不同的细胞类型：原核细胞和真核细胞。术语源自希腊语：“pro”意为“前”，“karyon”意为“核”；“eu”意为“真”。因此原核细胞没有真正的细胞核，而真核细胞具有由膜包裹的细胞核。
This distinction is not merely about the presence or absence of a nucleus. It reflects a deeper divide in cell architecture, including how DNA is organised, how ribosomes are built, how energy is released, and how cells divide. Examiners expect you to compare the two types using specific structural evidence rather than vague labels.
这一区别不仅仅是细胞核的有无。它反映了细胞结构的更深层分野，包括 DNA 的组织方式、核糖体的构成、能量释放方式以及细胞分裂方式。考官期望你使用具体的结构证据来比较两种类型，而不是使用模糊的标签。
2. Prokaryotic Cells: Bacteria and Archaea | 原核细胞：细菌与古菌
Prokaryotic cells are the simplest and smallest cells, typically 0.1-5.0 µm in diameter. They include two domains: Bacteria and Archaea. Despite their structural simplicity, prokaryotes are extremely successful and occupy nearly every habitat on Earth, from hot springs and salt lakes to the human gut and deep-sea vents.
原核细胞是最简单、最小的细胞，直径通常为 0.1-5.0 µm。它们包括两个域：细菌和古菌。尽管结构简单，原核生物极为成功，几乎占据地球上所有生境，从热泉、盐湖到人类肠道和深海热液口。
A typical prokaryotic cell has no nucleus; its genetic material is a single circular DNA molecule located in a region called the nucleoid. Many prokaryotes also contain small extra-chromosomal DNA rings called plasmids, which often carry genes for antibiotic resistance or metabolic advantages.
典型原核细胞没有细胞核；其遗传物质是位于拟核区的单个环状 DNA 分子。许多原核生物还含有小的染色体外 DNA 环，称为质粒，通常携带抗生素抗性基因或代谢优势基因。
3. Eukaryotic Cells: Protists, Fungi, Plants, Animals | 真核细胞：原生生物、真菌、植物、动物
Eukaryotic cells are generally larger, typically 10-100 µm in diameter, and much more structurally complex. They make up the bodies of protists, fungi, plants and animals. The defining feature is the presence of a true nucleus enclosed by a nuclear envelope, in which linear DNA is packaged with histone proteins into chromosomes.
真核细胞通常较大，直径一般为 10-100 µm，结构也复杂得多。它们构成原生生物、真菌、植物和动物的身体。决定性特征是存在由核膜包裹的真正细胞核，其中线性 DNA 与组蛋白包装成染色体。
Eukaryotic cells also contain a variety of membrane-bound organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes and, in plants, chloroplasts and a large central vacuole. These compartments allow different metabolic reactions to be separated and locally regulated, increasing efficiency.
真核细胞还含有多种有膜细胞器，如线粒体、内质网、高尔基体、溶酶体，植物细胞还有叶绿体和中央大液泡。这些区室使不同代谢反应得以分隔和局部调控，从而提高效率。
4. Size and Surface Area to Volume Ratio | 大小与表面积体积比
Prokaryotic cells are usually much smaller than eukaryotic cells. Small size gives a high surface area to volume ratio, which allows rapid uptake of nutrients and removal of waste by diffusion across the cell surface. This is why prokaryotes do not need internal transport systems or complex compartmentalisation.
原核细胞通常比真核细胞小得多。体积小意味着表面积与体积之比较高，使营养物质和废物能够通过细胞表面快速扩散。这就是原核生物不需要内部运输系统或复杂区室化的原因。
Eukaryotic cells, being much larger, have a lower surface area to volume ratio. They overcome this limitation by having extensive internal membranes that increase the total area for reactions, such as the cristae of mitochondria and the thylakoid membranes of chloroplasts.
真核细胞体积大得多，表面积与体积之比较低。它们通过拥有广泛的内膜来增加反应总面积，例如线粒体的嵴和叶绿体的类囊体膜，从而克服这一限制。
5. DNA Organisation: Circular vs Linear | DNA 组织：环状与线性
In prokaryotic cells, most genetic information is stored as a single circular DNA molecule in the nucleoid. This DNA is not associated with histone proteins, although some archaea have histone-like proteins. Plasmids provide additional genes and can be transferred between cells by conjugation, contributing to rapid genetic variation.
在原核细胞中，大部分遗传信息以单个环状 DNA 分子储存在拟核中。该 DNA 不与组蛋白结合，尽管一些古菌具有类组蛋白。质粒提供额外基因，并可通过接合在细胞间转移，有助于快速产生遗传变异。
In eukaryotic cells, DNA is organised into multiple linear chromosomes located inside the nucleus. These chromosomes are wrapped around histone proteins, forming chromatin. During cell division, chromatin condenses further into visible chromosomes. Mitochondria and chloroplasts also contain small circular DNA molecules, reflecting their evolutionary origins.
在真核细胞中，DNA 组织成位于细胞核内的多条线性染色体。这些染色体绕在组蛋白上，形成染色质。在细胞分裂期间，染色质进一步凝缩成可见染色体。线粒体和叶绿体也含有小的环状 DNA 分子，反映了它们的进化起源。
6. Membrane-Bound Organelles: Absence vs Presence | 有膜细胞器：无与有
Prokaryotic cells lack membrane-bound organelles. Their metabolic reactions occur either in the cytoplasm or on the inner surface of the cell membrane. For example, in aerobic bacteria, respiratory enzymes are associated with infoldings of the plasma membrane, although the traditional mesosome model is debated.
原核细胞缺乏有膜细胞器。它们的代谢反应发生在细胞质或细胞膜内表面。例如在需氧细菌中，呼吸酶与质膜的内折有关，尽管传统的中膜体模型尚存争议。
Eukaryotic cells possess many membrane-bound organelles, each with a specific function. Mitochondria carry out aerobic respiration, chloroplasts carry out photosynthesis, the rough endoplasmic reticulum synthesises proteins, and the Golgi apparatus modifies and packages them. This compartmentalisation allows incompatible reactions to occur simultaneously.
真核细胞具有许多有膜细胞器，各有特定功能。线粒体进行有氧呼吸，叶绿体进行光合作用，粗面内质网合成蛋白质，高尔基体修饰和包装蛋白质。这种区室化使不相容的反应能够同时进行。
| Feature (特征) | Prokaryotic cell (原核细胞) | Eukaryotic cell (真核细胞) |
|---|---|---|
| Nucleus (细胞核) | Absent; nucleoid region (无；拟核区) | Present; membrane-bound (有；有膜包被) |
| DNA form (DNA 形式) | Circular, naked (环状，裸露) | Linear, histone-associated (线性，与组蛋白结合) |
| Membrane-bound organelles (有膜细胞器) | Absent (无) | Present (有) |
| Ribosomes (核糖体) | 70S (70S) | 80S in cytoplasm (细胞质中为 80S) |
7. Ribosomes: 70S vs 80S | 核糖体：70S 与 80S
Both cell types have ribosomes for protein synthesis, but the ribosomes differ in size. Prokaryotic cells contain 70S ribosomes, made of a 50S and a 30S subunit. Eukaryotic cells contain larger 80S ribosomes, made of a 60S and a 40S subunit. Note that ‘S’ refers to Svedberg units
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