IGCSE CIE Biology: Protein – Key Points Review | 蛋白质 考点精讲

📚 IGCSE CIE Biology: Protein – Key Points Review | 蛋白质 考点精讲

Proteins are large, complex molecules that play many critical roles in the body. They are made up of amino acids and are essential for the structure, function, and regulation of tissues and organs. In IGCSE CIE Biology, you need to understand the structure of proteins, how they are formed, their functions, and what happens when they are denatured or tested in the lab.

蛋白质是由氨基酸构成的大型复杂分子，在体内承担着结构、功能和调控等多重关键任务。在 IGCSE CIE 生物学考试中，你需要掌握蛋白质的结构、形成方式、功能，以及变性现象和实验室检测方法。

1. What Are Proteins? | 什么是蛋白质？

Proteins are polymers made of amino acid monomers. They contain carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur. Proteins are one of the four major groups of biological macromolecules, alongside carbohydrates, lipids, and nucleic acids.

蛋白质是由氨基酸单体组成的聚合物。它们含有碳、氢、氧、氮，有时还含硫。蛋白质是生物四类大分子之一，与碳水化合物、脂质和核酸并列。

2. Amino Acids: The Building Blocks | 氨基酸：构建单元

All amino acids share a common structure: a central carbon atom bonded to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a variable R group (side chain). The R group determines the identity and properties of each amino acid. There are about 20 different amino acids commonly found in proteins.

所有氨基酸具有共同结构：一个中心碳原子连接着一个氨基（–NH₂）、一个羧基（–COOH）、一个氢原子以及一个可变的 R 基团（侧链）。R 基团决定了每种氨基酸的种类和特性。蛋白质中常见约 20 种氨基酸。

Essential amino acids cannot be made by the human body and must be obtained from the diet. Non‑essential amino acids can be synthesised by the body.

必需氨基酸是人体无法合成的，必须从饮食中获取。非必需氨基酸则可由人体自行合成。

3. Peptide Bonds and Polypeptides | 肽键与多肽

Amino acids are linked together by peptide bonds formed during condensation reactions. In this reaction, the carboxyl group of one amino acid reacts with the amino group of another, releasing a water molecule. The resulting bond –CO–NH– is a peptide bond.

氨基酸通过缩合反应形成肽键而连接在一起。反应中一个氨基酸的羧基与另一个氨基酸的氨基反应，释放一分子水。形成的 –CO–NH– 键即为肽键。

A dipeptide is formed from two amino acids. A polypeptide is a long chain of many amino acids joined by peptide bonds. Proteins consist of one or more polypeptide chains folded into specific shapes.

两个氨基酸形成二肽。多肽是由许多氨基酸通过肽键连接而成的长链。蛋白质由一条或多条多肽链折叠成特定形状而成。

4. Levels of Protein Structure | 蛋白质的结构层次

Protein structure is described at four levels: primary, secondary, tertiary, and quaternary. Each level of folding is essential for the protein’s final function. IGCSE students need to recall the main features of these levels.

蛋白质的结构可分为四个层次：一级、二级、三级和四级。每一级的折叠都对蛋白质的最终功能至关重要。IGCSE 学生需要记住这些层次的主要特征。

5. Primary Structure | 一级结构

The primary structure is the unique sequence of amino acids in a polypeptide chain. This sequence is determined by the gene coding for that protein. Even a single change in the amino acid sequence can alter the protein’s shape and function, as seen in sickle‑cell anaemia.

一级结构是多肽链中氨基酸的独特序列。该序列由编码该蛋白质的基因决定。即使只有一个氨基酸改变，也可能改变蛋白质的形状和功能，如镰状细胞贫血所示。

6. Secondary Structure | 二级结构

The polypeptide chain can coil or fold into regular, repeating patterns. The two most common secondary structures are the alpha‑helix (α‑helix) and the beta‑pleated sheet (β‑pleated sheet). These shapes are held together by hydrogen bonds between the –CO and –NH groups of the peptide backbone.

多肽链可盘旋或折叠成规则重复的模式。最常见的两种二级结构是 α‑螺旋和 β‑折叠片。这些形状由肽链骨架中 –CO 和 –NH 基团之间的氢键维持。

7. Tertiary Structure | 三级结构

The tertiary structure is the overall three‑dimensional folding of a single polypeptide chain. It is stabilised by interactions between the R groups: hydrogen bonds, ionic bonds, disulfide bridges (covalent bonds between cysteine residues), and hydrophobic interactions. This specific shape determines the protein’s function, e.g. the active site of an enzyme.

三级结构是单条多肽链的整体三维折叠形状。它由 R 基团之间的相互作用所稳定：氢键、离子键、二硫键（半胱氨酸残基间的共价键）以及疏水相互作用。这种特定的形状决定了蛋白质的功能，例如酶的活性位点。

8. Quaternary Structure | 四级结构

Some proteins are made of more than one polypeptide chain. The quaternary structure describes how these polypeptide subunits assemble together. Haemoglobin is a classic example: it consists of four polypeptide chains (two α‑globin and two β‑globin) each with a haem group. Collagen is another example with three helical polypeptides twisted together.

有些蛋白质由不止一条多肽链组成。四级结构描述了这些多肽亚基如何组装在一起。血红蛋白是一个典型例子：它由四条多肽链（两条 α‑珠蛋白和两条 β‑珠蛋白）组成，每条链带有一个血红素基团。胶原蛋白则是由三条螺旋多肽相互缠绕而成。

9. Functions of Proteins | 蛋白质的功能

Proteins perform a vast array of functions in living organisms. You should be able to give examples relevant to the IGCSE syllabus.

蛋白质在生物体内执行大量功能。你需要能给出与 IGCSE 大纲相关的例子。

• Enzymes: Biological catalysts that speed up metabolic reactions (e.g. amylase breaks down starch).
  酶：加速代谢反应的生物催化剂（如淀粉酶分解淀粉）。
• Structural proteins: Provide support and strength (e.g. collagen in tendons and bones, keratin in hair and nails).
  结构蛋白：提供支撑和强度（如肌腱和骨骼中的胶原蛋白，头发和指甲中的角蛋白）。
• Transport proteins: Carry molecules around the body (e.g. haemoglobin transports oxygen).
  运输蛋白：在体内运输分子（如血红蛋白运输氧气）。
• Hormones: Chemical messengers (e.g. insulin regulates blood glucose).
  激素：化学信使（如胰岛素调节血糖）。
• Antibodies: Part of the immune system; they bind to antigens on pathogens.
  抗体：免疫系统的一部分，能与病原体上的抗原结合。
• Receptors: On cell membranes, bind to signalling molecules like hormones.
  受体：位于细胞膜上，与激素等信号分子结合。

10. Denaturation | 变性

Denaturation is the permanent change in the shape of a protein due to the breaking of bonds that maintain its secondary and tertiary structures. The primary structure (amino acid sequence) remains intact, but the protein loses its biological function because its specific shape is destroyed.

变性是指蛋白质由于维持其二级和三级结构的化学键断裂而发生的永久性形状改变。一级结构（氨基酸序列）保持完整，但蛋白质因特定形状被破坏而失去生物功能。

Denaturation can be caused by:

变性可由以下因素引起：

• High temperature (above the optimum) — breaks hydrogen and ionic bonds.
  高温（超出最适温度）—— 破坏氢键和离子键。
• Extreme pH — alters the charges on R groups, disrupting ionic bonds and hydrogen bonds.
  极端 pH —— 改变 R 基团的电荷，破坏离子键和氢键。

A common IGCSE example is the denaturation of egg white (albumin) when cooked; it changes from clear to white and solid.

常见的 IGCSE 例子是煮蛋时蛋清（白蛋白）变性；它从透明变为白色固体。

11. Biuret Test for Proteins | 双缩脲检测蛋白质

The Biuret test is a simple qualitative test used to detect the presence of proteins in a sample. It detects peptide bonds.

双缩脲试验是一种用于检测样品中蛋白质存在的简单定性测试，它检测的是肽键。

Procedure for the Biuret test:

双缩脲试验步骤：

1. Add an equal volume of dilute sodium hydroxide (NaOH) solution to the test sample.
   将等体积的稀氢氧化钠（NaOH）溶液加入待测样品。
2. Add a few drops of dilute copper(II) sulfate (CuSO₄) solution.
   加入几滴稀硫酸铜（CuSO₄）溶液。
3. Mix and observe any colour change.
   混合并观察颜色变化。

A positive result: the solution turns from blue to purple/lilac. A negative result remains blue.

阳性结果：溶液由蓝色变为紫色／淡紫色。阴性结果仍为蓝色。

12. Exam Tips for IGCSE Biology | IGCSE 生物考试技巧

When answering questions about proteins, always use precise scientific vocabulary such as ‘peptide bond’, ‘denaturation’, ‘active site’, and ‘condensation reaction’. Link structure to function clearly. For the Biuret test, state the starting colour (blue) and the positive colour (purple), and name both reagents. For denaturation, stress that it is permanent and that the primary structure is unchanged.

在回答关于蛋白质的问题时，一定要使用精确的科学词汇，如“肽键”、“变性”、“活性位点”和“缩合反应”。要清晰地将结构与功能联系起来。关于双缩脲试验，需说明起始颜色（蓝色）和阳性颜色（紫色），并说出两种试剂的名称。对于变性，要强调它是永久性的，且一级结构不变。

Published by TutorHao | IGCSE Biology Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）