IGCSE Biology: Proteins – Key Points | IGCSE 生物：蛋白质考点精讲

📚 IGCSE Biology: Proteins – Key Points | IGCSE 生物：蛋白质考点精讲

Proteins are large, complex biomolecules that play critical roles in virtually all biological processes. From catalysing reactions as enzymes to defending the body as antibodies, proteins are indispensable. In IGCSE Biology, you need to understand their molecular structure, how they are formed, their diverse functions, and how they can be tested and affected by their environment. Let’s break down these key points.

蛋白质是大型复杂的生物分子，几乎参与了所有的生命活动。从作为酶催化反应，到作为抗体保护身体，蛋白质不可或缺。在IGCSE生物学中，你需要掌握它们的分子结构、形成方式、多种功能，以及如何检测和环境影响。让我们一一剖析这些考点。

1. What Are Proteins? | 蛋白质是什么？

Proteins are polymers made up of monomers called amino acids. They contain the elements carbon, hydrogen, oxygen, nitrogen and often sulfur. The long chains of amino acids are called polypeptides. The specific sequence of amino acids and the way the chain folds determine the unique shape and function of each protein.

蛋白质是由称为氨基酸的单体组成的多聚体。它们含有碳、氢、氧、氮，并常含有硫元素。氨基酸长链被称为多肽。氨基酸的特定序列以及链的折叠方式决定了每种蛋白质独特的形状和功能。

2. Amino Acids – The Building Blocks | 氨基酸——基本单位

There are about 20 different amino acids that occur naturally in proteins. Each amino acid shares a common structure: a central carbon atom bonded to a hydrogen atom, an amino group (–NH₂), a carboxyl group (–COOH) and a variable R group (or side chain). The R group differs in each amino acid and gives it distinct properties.

蛋白质中大约有20种不同的氨基酸。每个氨基酸都有一个共同的结构：一个中心碳原子连接着一个氢原子、一个氨基 (–NH₂)、一个羧基 (–COOH) 和一个可变的R基团(侧链)。不同氨基酸的R基团各异，赋予其独特的性质。

H₂N–CH(R)–COOH

The general formula above shows how the components are arranged around the central carbon. Notice that the amino group and the carboxyl group are the reactive parts that link amino acids together.

上面的通式展示了这些组分如何围绕中心碳原子排列。请注意，氨基和羧基是将氨基酸连接在一起的反应部位。

3. Formation of Polypeptides | 多肽的形成

Amino acids join together through condensation reactions. The carboxyl group (–COOH) of one amino acid reacts with the amino group (–NH₂) of another, releasing a molecule of water (H₂O) and forming a peptide bond (–CO–NH–). A chain of two amino acids is a dipeptide; many amino acids linked together form a polypeptide. The sequence is directed by genetic information.

氨基酸通过缩合反应连接在一起。一个氨基酸的羧基 (–COOH) 与另一个氨基酸的氨基 (–NH₂)反应，释放一分子水 (H₂O) 并形成肽键 (–CO–NH–)。两个氨基酸相连形成二肽；许多氨基酸相连则形成多肽。这一序列由遗传信息指导。

Amino acid (1) + Amino acid (2) → Dipeptide + H₂O

During protein digestion, these peptide bonds are broken by hydrolysis, the reverse process that uses water. Understanding this reversible reaction is important for both synthesis and digestion.

在蛋白质消化过程中，这些肽键通过水解作用断裂，即加入了水分的逆反应。理解这个可逆反应对合成和消化都很重要。

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

Proteins have up to four levels of structure, which together create the precise shape needed for their function. The primary structure is the linear sequence of amino acids. The secondary structure arises when the chain folds into local patterns such as alpha-helix or beta-pleated sheet, held by hydrogen bonds. The tertiary structure is the overall three-dimensional folding, stabilised by interactions including hydrogen bonds, ionic bonds, hydrophobic interactions and disulfide bridges. Some proteins have a quaternary structure, where two or more polypeptide chains (subunits) assemble together.

蛋白质具有多达四级结构，它们共同创造了功能所需的精确形状。一级结构是氨基酸的线性序列。二级结构由氢键维持，多肽链折叠成α-螺旋或β-折叠等局部模式。三级结构是整体的三维折叠，依靠氢键、离子键、疏水作用以及二硫键等稳定。有些蛋白质还具有四级结构，即两条或多条多肽链(亚基)组装在一起。

In IGCSE Biology, you are expected to state that the shape of a protein is essential for its function, and that the sequence of amino acids determines this shape. For example, the active site of an enzyme is a result of precise tertiary folding.

在IGCSE生物学中，你应能陈述蛋白质的形状对其功能至关重要，且氨基酸序列决定了这个形状。比如，酶的活性位点就是精确三级折叠的结果。

5. Functions of Proteins | 蛋白质的功能

Proteins perform an incredibly wide range of functions in living organisms. Here are some major categories that regularly appear in IGCSE exams:

蛋白质在生物体内执行极其多样的功能。以下是一些在IGCSE考试中经常出现的主要类别：

1. Structural proteins: Collagen provides strength in skin, tendons and bones; keratin is found in hair and nails.

1. 结构蛋白：胶原蛋白为皮肤、肌腱和骨骼提供强度；角蛋白存在于头发和指甲中。

2. Transport proteins: Haemoglobin in red blood cells carries oxygen from the lungs to respiring tissues.

2. 运输蛋白：红细胞中的血红蛋白将氧气从肺部运送至进行呼吸的组织。

3. Enzymes: Almost all enzymes are proteins that catalyse biochemical reactions, such as amylase breaking down starch.

3. 酶：几乎所有酶都是蛋白质，它们催化生化反应，例如淀粉酶分解淀粉。

4. Hormones: Some hormones are protein-based, like insulin, which regulates blood glucose concentration.

4. 激素：有些激素是蛋白质类的，如调节血糖浓度的胰岛素。

5. Defence: Antibodies (immunoglobulins) are Y-shaped proteins that recognise and neutralise pathogens.

5. 防御：抗体(免疫球蛋白)是Y形蛋白质，可以识别并中和病原体。

6. Movement: Actin and myosin are proteins that interact to cause muscle contraction.

6. 运动：肌动蛋白和肌球蛋白相互作用引起肌肉收缩。

Each function relies on the protein’s specific shape, which is why any alteration to the structure can cause loss of function.

每一项功能都依赖于蛋白质的特定形状，因此任何结构改变都可能导致功能丧失。

6. Enzymes as Proteins | 酶是蛋白质

Most biological catalysts, or enzymes, are globular proteins. Their active site has a complementary shape to the substrate, often described by the ‘lock and key’ model. The specificity of enzymes is directly linked to their tertiary structure. If the protein structure is altered, the active site loses its complementary shape and the enzyme cannot function.

大多数生物催化剂(即酶)是球状蛋白质。它们的活性位点与底物形状互补，常用”锁钥”模型来描述。酶的特异性直接与其三级结构有关。如果蛋白质结构被改变，活性位点就会失去互补形状，酶便无法起作用。

Example: Digestive enzymes like pepsin (in the stomach) and trypsin (in the small intestine) are proteins that break down other proteins into peptides and amino acids.

例如：胃蛋白酶(胃中)和胰蛋白酶(小肠中)等消化酶都是蛋白质，可以将其他蛋白质分解为肽和氨基酸。

7. Denaturation of Proteins | 蛋白质的变性

Denaturation is the permanent change in a protein’s shape caused by extreme conditions, such as high temperature or very low/high pH. The weak bonds holding the secondary and tertiary structures (hydrogen bonds, ionic bonds) break, but the strong peptide bonds of the primary sequence remain intact. Denatured proteins can no longer perform their biological function. A common example is the cooking of egg white: clear liquid becomes opaque and solid due to albumin denaturation.

变性是指蛋白质在极端条件下(如高温或极端pH)发生永久性的形状改变。维持二级和三级结构的弱键(氢键、离子键)断裂，但一级序列的强肽键保持完整。变性后的蛋白质不再具有生物功能。一个常见的例子是蛋清受热：透明的液体因白蛋白变性而变得不透明且凝固。

In the IGCSE exam, you may be asked to explain why enzymes stop working at very high temperatures or extreme pH, and the answer involves protein denaturation and the active site changing shape.

在IGCSE考试中，你可能会被问到为什么酶在极高温度或极端pH下会停止工作，答案涉及蛋白质变性和活性位点形状改变。

8. Food Test: Biuret Test | 食物检测：双缩脲检测

The Biuret test is the standard laboratory method for detecting proteins. A small sample of the test material is mixed with Biuret reagent, which contains sodium hydroxide and copper(II) sulfate. If proteins are present, the solution turns from blue to purple. This colour change occurs because the copper ions react with peptide bonds.

双缩脲检测是检测蛋白质的标准实验室方法。将少量待测样品与双缩脲试剂(含氢氧化钠和硫酸铜)混合。如果存在蛋白质，溶液会由蓝色变为紫色。这种颜色变化是由于铜离子与肽键发生了反应。

Steps: 1) Add an equal volume of Biuret reagent to the food sample. 2) Shake gently and observe the colour. A purple or lilac colour indicates protein; blue indicates no protein.

步骤：1) 往食物样品中加入等体积的双缩脲试剂。2) 轻轻摇动并观察颜色。紫色或淡紫色表明含有蛋白质；蓝色则表明无蛋白质。

It is important to note that the Biuret test detects peptide bonds, so it works for both long polypeptide chains and shorter peptides.

需要注意的是，双缩脲检测检测的是肽键，因此对长多肽链和短肽都有效。

9. Proteins in the Diet | 饮食中的蛋白质

Humans need a regular intake of protein for growth, repair of tissues and synthesis of enzymes and hormones. Good dietary sources include meat, fish, eggs, dairy products, legumes, nuts and seeds. During digestion, protease enzymes hydrolyse proteins into amino acids, which are absorbed in the small intestine and transported via the blood to cells.

人类需要定期摄入蛋白质，以用于生长、组织修复以及合成酶和激素。良好的膳食来源包括肉类、鱼类、蛋类、奶制品、豆类、坚果和种子。在消化过程中，蛋白酶将蛋白质水解为氨基酸，氨基酸在小肠被吸收，并通过血液运送到细胞。

Some amino acids are termed ‘essential’ because the human body cannot synthesise them; they must come from the diet. A varied diet ensures all essential amino acids are obtained.

有些氨基酸

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IGCSE Biology: Proteins – Key Points | IGCSE 生物：蛋白质 考点精讲