📚 Lipids: Types and Functions | 脂质的种类与作用
1. Introduction | 引言
Lipids are a diverse group of organic molecules that share one key physical property: they are insoluble in water but soluble in non-polar organic solvents. In CIE A-Level Biology, lipids are particularly important because of their roles in energy storage, cell membrane formation, insulation and hormone signalling. This article provides a focused overview of the major lipid types, their building blocks, and how their molecular structures relate to their biological functions.
脂质是一类多样的有机分子，它们共同的关键物理性质是：不溶于水，但溶于非极性有机溶剂。在 CIE A-Level 生物中，脂质尤为重要，因为它参与能量储存、细胞膜构成、保温以及激素信号传递。本文聚焦介绍主要脂质种类、其组成单位，以及分子结构如何与其生物学功能相联系。
2. General Properties of Lipids | 脂质的通用性质
Lipids are defined mainly by their solubility. They dissolve readily in solvents such as ethanol, chloroform and acetone, but do not dissolve in water because they are largely non-polar and hydrophobic. The principal elements in lipids are carbon, hydrogen and oxygen, although some lipids also contain phosphorus and nitrogen.
脂质主要依据溶解性来定义。它们易溶于乙醇、氯仿和丙酮等溶剂，但不溶于水，因为脂质大多是非极性、疏水性的。脂质的主要元素是碳、氢和氧，但有些脂质还含有磷和氮。
The major classes of lipids include triglycerides, phospholipids, cholesterol and other steroids, and waxes. Each class has a distinct structure that equips it for a particular biological role. For example, phospholipids form the core of all cell membranes, while triglycerides are the body’s main energy reserve.
脂质的主要类别包括甘油三酯、磷脂、胆固醇及其它类固醇，以及蜡。每个类别都有独特的结构，使其适合特定的生物学作用。例如，磷脂构成所有细胞膜的核心，而甘油三酯是机体主要能量储备。
3. Fatty Acids | 脂肪酸
Fatty acids are the building blocks of many lipids. Each fatty acid molecule consists of a long hydrocarbon chain with a carboxyl group (−COOH) at one end. The chain is usually unbranched and may contain between 4 and 24 carbon atoms. The properties of a fatty acid depend on the length of the chain and the presence of carbon–carbon double bonds.
脂肪酸是许多脂质的组成单位。每个脂肪酸分子包含一条长烃链，一端有一个羧基（−COOH）。碳链通常无分支，长度一般为4到24个碳原子。脂肪酸的性质取决于碳链长度以及是否存在碳碳双键。
Saturated fatty acids contain no double bonds between carbon atoms; the hydrocarbon chain is straight, allowing molecules to pack tightly together. Unsaturated fatty acids contain one or more double bonds. A cis double bond introduces a kink or bend in the chain, preventing close packing and lowering the melting point. In contrast, trans double bonds produce a straighter chain, similar to saturated fats.
饱和脂肪酸的碳原子之间没有双键，碳链平直，分子可以紧密堆积。不饱和脂肪酸含有一个或多个双键。顺式双键会使碳链产生弯折，阻碍紧密堆积并降低熔点；而反式双键则使碳链较平直，性质类似饱和脂肪。
| Property | Saturated fatty acids | Unsaturated fatty acids |
|---|---|---|
| Double bonds | Absent | One or more present |
| Shape of chain | Straight | Bent (cis form) |
| Packing | Close | Loose |
| Typical state at room temperature | Solid (e.g. butter) | Liquid (e.g. olive oil) |
4. Triglycerides | 甘油三酯
Triglycerides, also called triacylglycerols, are formed when one molecule of glycerol reacts with three fatty acids. This is a condensation reaction: each fatty acid joins to glycerol through an ester bond, and three water molecules are released. The overall reaction can be summarised as:
甘油三酯又称三酰甘油，由一分子甘油与三分子脂肪酸反应形成。该反应为缩合反应：每个脂肪酸通过酯键与甘油连接，同时释放三分子水。总反应可表示为：
Glycerol + 3 fatty acids → triglyceride + 3H₂O
In a hydrolysis reaction, triglycerides are broken back down into glycerol and fatty acids by adding water, usually catalysed by enzymes called lipases. This reaction is important during digestion and when stored fats are mobilised for energy.
在水解反应中，甘油三酯通过加水分解回甘油和脂肪酸，通常由脂酶催化。这一反应在消化过程中以及储存脂肪被动员供能时至关重要。
Triglycerides are almost completely hydrophobic because the long fatty acid chains are non-polar. Their primary functions are energy storage and insulation. One gram of fat stores more than twice the chemical energy of one gram of carbohydrate, and the body stores fat as an almost anhydrous mass, so it is a compact energy reserve.
甘油三酯几乎完全疏水，因为长脂肪酸链是非极性的。其主要功能是能量储存和保温。1克脂肪储存的化学能是1克糖类的两倍以上，而且机体以近无水形式储存脂肪，因此它是一种紧凑的能量储备。
5. Phospholipids | 磷脂
A phospholipid resembles a triglyceride, but only two fatty acids are attached to the glycerol backbone. The third hydroxyl group of glycerol is linked to a phosphate group, which is often also attached to a nitrogen-containing alcohol such as choline. This creates a molecule with a hydrophilic phosphate ‘head’ and two hydrophobic fatty acid ‘tails’. Such molecules are described as amphipathic.
磷脂与甘油三酯相似，但甘油骨架上只连接两条脂肪酸。甘油的第三个羟基连接一个磷酸基团，磷酸基团又常连接胆碱等含氮醇。这样就形成了具有亲水性磷酸“头部”和两条疏水性脂肪酸“尾部”的分子，称为两亲性分子。
When phospholipids are placed in water, they spontaneously arrange themselves into a bilayer. The hydrophilic heads face the aqueous environment on either side, while the hydrophobic tails are shielded from water in the middle. This bilayer is the fundamental basis of all cell membranes.
将磷脂置于水中时，它们会自发排列成双层结构。亲水性头部朝向两侧水环境，疏水性尾部则被包裹在中间避开水分。这种双层结构是所有细胞膜的基础。
The phosphate head also gives the membrane its selective permeability: small non-polar molecules such as oxygen and carbon dioxide can diffuse freely through the bilayer, while charged ions and large polar molecules cannot cross without transport proteins.
磷酸头部还赋予膜选择透过性：氧气、二氧化碳等小非极性分子可自由通过双层，而带电离子和较大极性分子必须借助转运蛋白才能跨越。
6. Cholesterol and Steroids | 胆固醇与类固醇
Cholesterol is a steroid lipid with a characteristic structure of four fused carbon rings. Unlike triglycerides and phospholipids, cholesterol is not a fatty acid ester; it is a small, largely hydrophobic molecule with a single hydroxyl group. This hydroxyl group gives cholesterol a weakly amphipathic character, allowing it to sit within the phospholipid bilayer.
胆固醇是一种类固醇脂质，具有四个稠合碳环的典型结构。与甘油三酯和磷脂不同，胆固醇不是脂肪酸酯，而是一个主要疏水的小分子，仅含一个羟基。这个羟基使胆固醇具有微弱的两亲性，使其能嵌在磷脂双层中。
Cholesterol has essential functions in animal cell membranes. It increases membrane fluidity at low temperatures by preventing phospholipids from packing too closely, and it reduces fluidity at high temperatures by restricting movement. Cholesterol is also the precursor for steroid hormones such as testosterone and oestrogen, as well as vitamin D and bile salts.
胆固醇在动物细胞膜中具有重要功能。在低温下，它通过阻止磷脂紧密堆积来增加膜的流动性；在高温下，它又通过限制分子运动来降低膜的流动性。胆固醇还是睾酮、雌激素等类固醇激素，以及维生素D和胆汁盐的前体。
7. Waxes and Other Lipid-Related Compounds | 蜡与其它脂质相关化合物
Waxes are simple lipids formed by an ester bond between a long-chain fatty acid and a long-chain alcohol. They are highly hydrophobic and much harder than fats. In plants, a waxy cuticle covers the surface of leaves and helps reduce water loss. In animals, waxes protect feathers, fur and skin, and are found in the ear canal of humans.
蜡是由长链脂肪酸与长链醇通过酯键形成的简单脂质。它们高度疏水，比脂肪更硬。在植物中，蜡质角质层覆盖叶面，有助于减少水分散失。在动物中，蜡能保护羽毛、皮毛和皮肤，也存在于人类的外耳道。
Other lipid-related compounds include the fat-soluble vitamins A, D, E and K. Vitamin A is important for vision, vitamin D for calcium absorption, vitamin E as an antioxidant, and vitamin K for blood clotting. Because these vitamins are lipids, they are absorbed with dietary fat and can be stored in body fat.
其它脂质相关化合物包括脂溶性维生素A、D、E和K。维生素A对视觉重要，维生素D促进钙吸收，维生素E作为抗氧化剂，维生素K参与凝血。由于这些维生素是脂质，它们随膳食脂肪一起被吸收，并可储存在体脂中。
8. Energy Storage and Metabolic Roles | 能量储存与代谢作用
Triglycerides are the most efficient energy reserves in animals and plants. Fatty acids are highly reduced molecules because they contain many carbon–hydrogen bonds, and oxidation of these bonds releases a large amount of energy. One gram of triglycerides yields around 37 kJ, compared with about 17 kJ for the same mass of carbohydrate or protein.
甘油三酯是动植物中最有效的能量储备。脂肪酸因富含碳氢键而高度还原，氧化这些键会释放大量能量。1克甘油三酯约释放37 kJ，而同样质量的糖类或蛋白质只释放约17 kJ。
Beyond energy storage, lipids provide thermal insulation. Subcutaneous fat (adipose tissue) reduces heat loss through the skin and also acts as a shock absorber around vital organs such as the kidneys and heart. Lipids in the myelin sheath surround and electrically insulate axons, allowing rapid saltatory conduction of nerve impulses.
除能量储存外，脂质还提供隔热作用。皮下脂肪（脂肪组织）减少通过皮肤的热散失，并作为肾脏、心脏等重要器官的减震保护层。髓鞘中的脂质包裹轴突并形成电绝缘层，使神经冲动可以快速跳跃式传导。
Lipids also produce metabolic water when oxidised. For example, the complete oxidation of a fatty acid generates water molecules. This is particularly important for desert animals that have limited access to drinking water.
脂质氧化时还会产生代谢水。例如，脂肪酸完全氧化会生成水分子。这对饮水有限沙漠动物尤为重要。
| Function | Example of lipid |
|---|---|
| Long-term energy storage | Triglycerides in adipose tissue |
| Thermal insulation | Subcutaneous fat |
| Electrical insulation | Myelin sheath lipids |
| Waterproofing | Waxes on leaves and feathers |
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