GCSE Biology: Proteins Revision Guide | GCSE 生物：蛋白质考点精讲

📚 GCSE Biology: Proteins Revision Guide | GCSE 生物：蛋白质考点精讲

Proteins are large, complex molecules that play countless critical roles in living organisms. They are polymers made up of amino acid monomers and are essential for the structure, function, and regulation of the body’s tissues and organs. Understanding proteins is a must for GCSE Biology, as they link directly to topics like enzymes, digestion, DNA, and cell processes.

蛋白质是大型复杂的分子，在生物体内承担着无数关键角色。它们是由氨基酸单体组成的聚合物，对于身体组织和器官的结构、功能以及调节都不可或缺。理解蛋白质是 GCSE 生物学的必修内容，因为它们与酶、消化、DNA 以及细胞过程等主题直接相关。

1. What Are Proteins? | 什么是蛋白质？

Proteins are organic molecules containing carbon, hydrogen, oxygen, nitrogen, and often sulfur. They account for more than half of the dry mass of most cells and are found in every living cell. Proteins are built from one or more chains of amino acids, folded into specific three-dimensional shapes that determine their function.

蛋白质是含有碳、氢、氧、氮，通常还有硫的有机分子。它们占大多数细胞干重的一半以上，存在于每一个活细胞中。蛋白质由一条或多条氨基酸链构建而成，折叠成决定其功能的特定三维形状。

Examples of proteins include enzymes (such as amylase and catalase), structural proteins (like collagen in skin and tendons), transport proteins (such as haemoglobin), and hormones (like insulin). Their variety of shapes allows them to perform a wide array of jobs.

蛋白质的例子包括酶（如淀粉酶和过氧化氢酶）、结构蛋白（如皮肤和肌腱中的胶原蛋白）、运输蛋白（如血红蛋白）以及激素（如胰岛素）。它们形状的多样性使其能够执行各式各样的任务。

Proteins are often described as the ‘workhorses’ of the cell because they carry out most cellular activities. Each protein has a unique sequence of amino acids that is encoded by a gene in the DNA.

蛋白质常被描述为细胞的“主力”，因为它们执行着大多数细胞活动。每种蛋白质都有独特的氨基酸序列，该序列由 DNA 中的基因编码。

2. Amino Acids: The Monomers | 氨基酸：单体

All amino acids share a common basic structure: a central carbon atom (the alpha carbon) bonded to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a variable R group (side chain). The R group gives each amino acid its distinctive chemical properties.

所有氨基酸都有一个共同的基本结构：一个中心碳原子（α-碳）连接着一个氨基(–NH₂)、一个羧基(–COOH)、一个氢原子和一个可变的 R 基团（侧链）。R 基团赋予每种氨基酸独特的化学性质。

H₂N–CH(R)–COOH

There are about 20 standard amino acids used by living organisms to build proteins. While plants can synthesise all of them, animals (including humans) must obtain some from their diet. These are called essential amino acids.

生物体用于构建蛋白质的标准氨基酸约有 20 种。植物能够合成所有种类，但动物（包括人类）必须从饮食中获取一部分，这些被称为必需氨基酸。

The sequence and number of amino acids in a protein are determined by the sequence of nucleotide bases in the gene that codes for it. A single change in one amino acid can dramatically alter a protein’s shape and function.

蛋白质中氨基酸的顺序和数量由编码该蛋白质的基因中的核苷酸碱基序列决定。单个氨基酸的改变就可能极大地改变蛋白质的形状和功能。

3. Peptide Bonds and Polypeptide Chains | 肽键与多肽链

During protein synthesis, amino acids are linked together by peptide bonds through a condensation reaction. In this reaction, the carboxyl group of one amino acid reacts with the amino group of another, releasing a water molecule (H₂O).

在蛋白质合成过程中，氨基酸通过缩合反应由肽键连接在一起。在此反应中，一个氨基酸的羧基与另一个氨基酸的氨基发生反应，释放出一个水分子(H₂O)。

Amino acid + Amino acid → Dipeptide + H₂O

氨基酸 + 氨基酸 → 二肽 + H₂O

A chain of many amino acids joined by peptide bonds is called a polypeptide. A protein may consist of a single polypeptide chain or several polypeptides folded and sometimes chemically modified.

由许多氨基酸通过肽键连接而成的链称为多肽。一个蛋白质可以由一条多肽链组成，也可以由几条折叠在一起且有时经过化学修饰的多肽链组成。

The backbone of a polypeptide consists of the repeating sequence –N–C–C– (from the amino nitrogen, alpha carbon, and carboxyl carbon). Peptide bonds are strong covalent bonds, but the overall shape of a protein is also determined by many weaker interactions.

多肽的主链由重复的 –N–C–C– 序列构成（来自氨基氮、α-碳和羧基碳）。肽键是强共价键，但蛋白质的整体形状也由许多较弱的相互作用决定。

4. Primary Structure: The Sequence | 一级结构：序列

The primary structure of a protein is the linear sequence of amino acids in its polypeptide chain. This sequence is unique to each protein and is directly coded for by the order of DNA bases. It is like the letters of a word—changing even one letter can change the meaning.

蛋白质的一级结构是指其多肽链中氨基酸的线性序列。这一序列对每种蛋白质都是独一无二的，并由 DNA 碱基的顺序直接编码。它就像单词的字母——哪怕改变一个字母也可能改变含义。

For example, the genetic disorder sickle cell anaemia results from a single substitution in the haemoglobin gene: the amino acid valine replaces glutamic acid at position six of the beta chain. This tiny change causes the protein to form abnormal, sickle-shaped red blood cells.

例如，镰刀型细胞贫血症这种遗传病就是由血红蛋白基因中的单个碱基替换引起的：β 链第六位的谷氨酸被缬氨酸取代。这一微小的变化导致蛋白质形成异常的镰状红细胞。

Primary structure determines all higher levels of protein folding. Even though the peptide bonds are the same throughout, the sequence of R groups dictates how the chain will coil, bend, and interact with itself.

一级结构决定了蛋白质所有更高层次的折叠。尽管整条链中的肽键相同，但 R 基团的序列决定了链条将如何卷曲、弯曲以及与其自身发生相互作用。

5. Secondary Structure: Local Folding | 二级结构：局部折叠

Secondary structure refers to regular, localised folding patterns within a polypeptide chain. The two most common types are the alpha-helix (α-helix) and the beta-pleated sheet (β-pleated sheet). Both are stabilised by hydrogen bonds between the backbone –NH and –C=O groups.

二级结构是指多肽链中规则的局部折叠模式。最常见的两种类型是 α-螺旋和 β-折叠片。两者都由主链上的 –NH 和 –C=O 基团之间形成的氢键来稳定。

In an α-helix, the polypeptide backbone coils like a spring, with hydrogen bonds holding each turn at the correct distance. In a β-pleated sheet, segments of the chain line up side by side, connected by hydrogen bonds, giving a sheet-like structure that often folds back on itself.

在 α-螺旋中，多肽主链像弹簧一样盘绕，氢键使每一圈保持在正确的距离上。在 β-折叠片中，多肽链的各个片段并排排列，通过氢键连接，形成通常自我回折的片层结构。

These secondary structures are the first level of folding and are seen in many proteins. For example, keratin in hair and nails contains a high proportion of α-helices, while silk fibres are rich in β-pleated sheets.

这些二级结构是蛋白质折叠的第一个层次，存在于许多蛋白质中。例如，毛发和指甲中的角蛋白含有很高比例的 α-螺旋，而蚕丝纤维富含 β-折叠片。

6. Tertiary Structure: The Overall 3D Shape | 三级结构：整体三维形状

Tertiary structure is the overall three-dimensional folding of a single polypeptide chain. It is formed by interactions between the R groups of amino acids, including hydrophobic interactions, hydrogen bonds, ionic bonds, and disulfide bridges (–S–S–) between cysteine residues.

三级结构是指单条多肽链整体的三维折叠。它由氨基酸 R 基团之间的多种相互作用形成，包括疏水相互作用、氢键、离子键以及半胱氨酸残基之间形成的二硫键(–S–S–)。

The precise tertiary structure gives a protein its specific biological function. For example, the globular shape of haemoglobin allows it to carry oxygen, while the fibrous structure of collagen provides exceptional tensile strength.

精确的三级结构赋予蛋白质特定的生物学功能。例如，血红蛋白的球状外形使其能够携带氧气，而胶原蛋白的纤维状结构提供了出色的抗拉强度。

If the tertiary structure is disrupted, the protein loses its shape and can no longer function. This process is known as denaturation, which we will discuss later. The active site of an enzyme is a direct result of its tertiary structure.

如果三级结构遭到破坏，蛋白质就会丢失其形状，不再具有功能。这一过程称为变性，我们将在后文讨论。酶的活性位点正是其三级结构的直接产物。

7. Quaternary Structure: Multiple Subunits | 四级结构：多亚基组合

Quaternary structure applies to proteins that consist of more than one polypeptide chain, or subunits, assembled into a functional complex. Not all proteins have quaternary structure; some function as single polypeptide chains.

四级结构适用于由不止一条多肽链（或称亚基）组装成功能性复合体的蛋白质。并非所有蛋白质都具有四级结构，有些蛋白质以单条多肽链的形式发挥作用。

Haemoglobin is a classic example: it contains four polypeptide subunits (two alpha and two beta), each carrying a haem group that binds oxygen. The cooperation between these subunits enables efficient oxygen pickup in the lungs and release in tissues.

血红蛋白是一个经典例子：它含有四个多肽亚基（两个 α 亚基和两个 β 亚基），每个亚基携带一个结合氧气的血红素基团。这些亚基之间的协同作用使得它能够在肺部高效地结合氧气，并在组织中释放氧气。

Other proteins with quaternary structure include antibodies (composed of four polypeptides) and the enzyme DNA polymerase, which may use multiple subunits to clamp onto DNA. The same types of bonds that stabilise tertiary structure also hold subunits together.

其他具有四级结构的蛋白质包括抗体（由四条多肽链组成）以及酶 DNA 聚合酶，后者可能利用多个亚基夹住 DNA。稳定三级结构的各类化学键同样将亚基结合在一起。

8. Functions of Proteins | 蛋白质的功能

Proteins are the most versatile macromolecules in living systems. Their diverse functions arise from the variation in their shapes, which are determined by the sequence of amino acids. Here are the major functional categories relevant to GCSE Biology.

蛋白质是生命系统中用途最广的大分子。其多样化的功能源于它们形状的差异，而形状又由氨基酸序列决定。以下是 GCSE 生物学中涉及的主要功能类别。

Enzymatic function: Many proteins act as enzymes, which are biological catalysts that speed up chemical reactions without being consumed. For instance, amylase breaks down starch into maltose.

酶促功能：许多蛋白质作为酶——生物催化剂，在自身不被消耗的情况下加速化学反应。例如，淀粉酶将淀粉分解为麦芽糖。

Structural function: Structural proteins provide mechanical support. Collagen gives strength to skin, tendons and bone matrix; keratin forms hair, nails, and the outer layer of skin.

结构功能：结构蛋白提供机械支撑。胶原蛋白赋予皮肤、肌腱和骨骼基质强度；角蛋白构成毛发、指甲和皮肤外层。

Transport function: Some proteins move substances around the body. Haemoglobin transports oxygen in red blood cells; membrane carrier proteins help move molecules across cell membranes.

运输功能：有些蛋白质在体内运输物质。血红蛋白在红细胞中运输氧气；膜载体蛋白帮助分子穿过细胞膜。

Hormonal function: Certain proteins act as chemical messengers. Insulin is a protein hormone that regulates blood glucose concentration. Growth hormone is another example.

激素功能：某些蛋白质充当化学信使。胰岛素是一种调节血糖浓度的蛋白质激素。生长激素是另一个例子。

Defensive function: Antibodies (immunoglobulins) are proteins produced by white blood cells that recognise and neutralise pathogens such as bacteria and viruses.

防御功能：抗体（免疫球蛋白）是由白细胞产生的蛋白质，能识别并中和病原体，如细菌和病毒。

9. Enzymes: Specificity and Activity | 酶：特异性与活性

Enzymes are a special group of proteins with a depression or cleft called the active site. This is where the substrate molecule(s) bind. The shape of the active site is complementary to the shape of the substrate, giving enzymes their high specificity.

酶是一类特殊的蛋白质，具有一个凹陷或裂隙，称为活性位点。这是底物分子结合的位置。活性位点的形状与底物的形状互补，使得酶具有高度的特异性。

The classic ‘lock and key’ model describes how the substrate fits exactly into the active site like a key into a lock. A more refined model is the ‘induced fit’ model, where the active site slightly changes shape to encompass the substrate more tightly.

经典的“锁钥模型”描述了底物如何像钥匙插入锁孔一样精确地嵌入活性位点。一个更精细的模型是“诱导契合模型”，即活性位点会略微改变形状，以更紧密地包裹底物。

Once the enzyme-substrate complex forms, the reaction takes place, and products are released, leaving the enzyme unchanged and ready to catalyse another reaction. Each enzyme works best at a specific optimum temperature and pH.

一旦酶-底物复合物形成，反应就会发生，产物被释放出来，酶本身不发生改变，并可继续催化下一个反应。每种酶都有其特定的最适温度和 pH 值，在该条件下活性最高。

For example, human amylase in the mouth has an optimum pH around 7, whereas pepsin, which works in the stomach, has an optimum pH of about 2, showing how enzymes are adapted to their environment.

例如，人类口腔中的淀粉酶最适 pH 值约为 7，而在胃中工作的胃蛋白酶最适 pH 值约为 2，这表明酶如何适应其所在的环境。

10. Denaturation: When Proteins Lose Shape | 变性：当蛋白质失去形状

Denaturation is a structural change in a protein that results in the loss of its biological function. It occurs when the weak bonds (hydrogen bonds, ionic interactions, and sometimes disulfide bridges) that maintain the protein’s 3D shape are broken.

变性是蛋白质的结构改变，导致其生物功能丧失。当维持蛋白质三维形状的弱键（氢键、离子键，有时还有二硫键）被破坏时，就会发生变性。

High temperatures and extreme pH values are common causes. In enzymes, denaturation alters the active site so that the substrate can no longer bind, and the enzyme loses its catalytic ability. Prolonged exposure or very high temperatures make this change permanent.

高温和极端 pH 值是常见的原因。在酶中，变性改变了活性位点，使底物无法再结合，酶也就失去了催化能力。长时间暴露或非常高的温度会使这种改变不可逆转。

A familiar example is the cooking of an egg white. The clear, liquid egg white (rich in the protein albumin) becomes opaque and solid upon heating because the albumin molecules denature and unfold, then aggregate with each other.

一个熟悉的例子是煮蛋清。透明且液态的蛋清（富含卵清蛋白）在加热后变得不透明且凝固，因为卵清蛋白分子变性展开，随后相互聚集。

It is worth noting that denaturation does not break the peptide bonds of the primary structure; the amino acid sequence stays intact. Yet, the protein can no longer carry out its intended role.

值得注意的是，变性并不会打断一级结构中的肽键，氨基酸序列保持完整。然而，该蛋白质已无法再执行其原本的功能。

11. Dietary Proteins and Digestion | 膳食蛋白质与消化

Humans obtain amino acids by eating protein-rich foods such as meat, fish, eggs, dairy products, legumes, and nuts. The body breaks down dietary proteins into amino acids, which are then used to synthesise its own proteins. This process is called protein digestion and assimilation.

人类通过食用富含蛋白质的食物（如肉类、鱼类、蛋类、奶制品、豆类和坚果）获取氨基酸。身体将膳食蛋白质分解为氨基酸，而后利用这些氨基酸来合成自身的蛋白质。这一过程称为蛋白质的消化和同化。

Digestion begins in the stomach, where the acidic environment (provided by hydrochloric acid) denatures proteins and activates the enzyme pepsin. Pepsin starts hydrolysing proteins into smaller polypeptides.

消化始于胃，酸性环境（由盐酸提供）使蛋白质变性并激活胃蛋白酶。胃蛋白酶开始将蛋白质水解为较小的多肽。

In the small intestine, pancreatic enzymes like trypsin and chymotrypsin continue breaking polypeptides into even shorter oligopeptides. Finally, enzymes on the lining of the small intestine (peptidases) digest these into single amino acids, which are absorbed into the bloodstream.

在小肠中，胰蛋白酶和糜蛋白酶等胰腺酶继续将多肽分解成更短的寡肽。最后，小肠内壁上的酶（肽酶）将这些寡肽消化成单个氨基酸，氨基酸被吸收进入血液。

Once inside the cells, amino acids are used to build proteins needed for growth and repair. Excess amino acids cannot be stored; instead, they are broken down in the liver through deamination. The amino group is removed, converted into urea, and excreted by the kidneys.

进入细胞后，氨基酸被用来构建生长和修复所需的蛋白质。多余的氨基酸不能被储存，而是在肝脏中通过脱氨作用被分解。氨基被移除后转化为尿素，随后由肾脏排出体外。

12. Testing for Proteins: The Biuret Test | 蛋白质的检验：双缩脲试验

The Biuret test is a simple biochemical test used to detect the presence of protein in a sample. It specifically identifies peptide bonds, so it will give a positive result for proteins and long polypeptide chains but not for individual amino acids.

双缩脲试验是一种简单的生化试验，用于检测样品中是否存在蛋白质。它专门鉴定肽键，因此对于蛋白质和长多肽链会呈现阳性结果，但对单个氨基酸则不会。

To perform the test, add an equal volume of sodium hydroxide (NaOH) solution to the sample, followed by a few drops of dilute copper(II) sulfate (CuSO₄) solution. If protein is present, the mixture changes from blue to violet or purple.

进行该试验时，向样品中加入等体积的氢氧化钠(NaOH)溶液，然后滴加几滴稀硫酸铜(CuSO₄)溶液。如果存在蛋白质，混合物会从蓝色变为紫色或紫罗兰色。

The colour change occurs because the copper ions (Cu²⁺) form a coloured complex with the nitrogen atoms in the peptide bonds. The more peptide bonds present, the more intense the purple colour. This test is often demonstrated in schools with egg white or milk.

颜色变化是因为铜离子(Cu²⁺)与肽键中的氮原子形成了有色配合物。存在的肽键越多，紫色就越深。这一试验常以蛋清或牛奶为材料在学校进行演示。

A negative Biuret test (remaining blue) indicates the absence of protein or the presence of very short peptides. This test, along with the Benedict’s test for sugars and the iodine test for starch, is an essential practical skill for GCSE Biology.

双缩脲试验阴性（保持蓝色）表明不存在蛋白质或只有极短的多肽。该试验与用于检测糖的本尼迪克特试验和用于检测淀粉的碘液试验一起，是 GCSE 生物学的必备实验技能。

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GCSE Biology: Proteins Revision Guide | GCSE 生物：蛋白质 考点精讲