生物一直都是最受欢迎的A-level科目之一，A-level生物适用于申请医学、生物学、生物医学、生物化学等一系列相关的大学课程。今天给大家带来的笔记是：生物分子和酶 Biological Molecules and Enzymes——Lipids 脂质
笔记回顾：「干货整理」A-level生物A*知识点笔记：生物分子——碳水化合物
Lipids are made up of the elements carbon, hydrogen and oxygen but in different proportions to carbohydrates. The most common type of lipid is the triglyceride.
Lipids can exist as fats, oils and waxes. Fats and oils are very similar in structure (triglycerides).
At room temperature, fats are solids and oils are liquids. Fats are of animal origin, while oils tend to be found in plants.
Waxes have a different structure (esters of fatty acids with long chain alcohols) and can be found in both animals and plants.
脂质是由碳、氢和氧元素组成的，但与碳水化合物的比例不同。最常见的脂质类型是甘油三酯。
脂类可以以脂肪、油和蜡的形式存在。脂肪和油的结构非常相似（甘油三酯）。
在室温下，脂肪是固体，油是液体。脂肪来自于动物，而油类往往在植物中发现。
蜡具有不同的结构（脂肪酸与长链醇的酯类），在动物和植物中都可以找到。
These are made up of 3 fatty acid chains attached to a glycerol molecule.
Fatty acids are chains of carbon atoms, the terminal one having an OOH group attached making a carboxylic group (COOH). The length of the chain is usually between 14 and 22 carbons long (most commonly 16-18).
Three of these chains become attached to a glycerol molecule which has 3 OH groups attached to its 3 carbons. This is called a condensation reaction because 3 water molecules are formed from 3 OH groups from the fatty acids chains and 3 H atoms from the glycerol. The bond between the fatty acid chain and the glycerol is called an ester linkage.
这是由3条脂肪酸链连接到一个甘油分子上组成的。
脂肪酸是由碳原子组成的链，末端有一个OOH基团连接成一个羧基（COOH）。链的长度通常在14到22个碳原子之间（最常见的是16-18个）。
这些链中的三条连接到甘油分子上，而甘油分子的三个碳上有三个OH基。这被称为缩合反应，因为3个水分子是由脂肪酸链的3个OH基和甘油的3个H原子形成的。脂肪酸链和甘油之间的结合被称为酯连接。
The 3 fatty acids may be identical or they may have different structures.
In the fatty acid chains the carbon atoms may have single bonds between them making the lipid saturated. These are usually solid at room temperature and are called fats.
If one or more bonds between the carbon atoms are double bonds, the lipid is unsaturated. These are usually liquid at room temperature and are called oils.
这3个脂肪酸可能是相同的，也可能有不同的结构。
在脂肪酸链中，碳原子之间可能有单键，使脂质饱和。这些物质在室温下通常是固体，被称为脂肪。
如果碳原子之间的一个或多个键是双键，则该脂质是不饱和的。这些通常在室温下是液体，被称为油。
- Storage - lipids are non-polar and so are insoluble in water.
- High-energy store - they have a high proportion of H atoms relative to O atoms and so yield more energy than the same mass of carbohydrate.
- Production of metabolic water - some water is produced as a final result of respiration.
- Thermal insulation - fat conducts heat very slowly so having a layer under the skin keeps metabolic heat in.
- Electrical insulation - the myelin sheath around axons prevents ion leakage.
- Waterproofing - waxy cuticles are useful, for example, to prevent excess evaporation from the surface of a leaf.
- Hormone production - steroid hormones. Oestrogen requires lipids for its formation, as do other substances such as plant growth hormones.
- Buoyancy - as lipids float on water, they can have a role in maintaining buoyancy in organisms.
储存 - 脂类是非极性的，所以不溶于水。
高能量储存--它们的H原子相对于O原子的比例很高，因此比相同质量的碳水化合物产生更多能量。
代谢水的产生--一些水是作为呼吸作用的最终结果产生的。
热绝缘 - 脂肪导热非常缓慢，所以在皮肤下有一层可以保持代谢热。
电绝缘--轴突周围的髓鞘可防止离子泄漏。
防水 - 蜡质角质层是有用的，例如，防止叶子表面的过度蒸发。
激素分泌--类固醇激素。雌性激素的形成需要脂质，其他物质如植物生长激素也是如此。
浮力--由于脂类漂浮在水面上，它们可以在维持生物体的浮力方面发挥作用。
A phosphate-base group replaces one fatty acid chain. It makes this part of the molecule (the head) soluble in water whilst the fatty acid chains remain insoluble in water.
Due to this arrangement, phospholipids form bilayers (the main component of cell and organelle membranes).
一个磷酸盐基团取代了一条脂肪酸链。它使分子的这一部分（头部）可溶于水，而脂肪酸链仍不溶于水。
由于这种安排，磷脂形成了双分子层（细胞和细胞器膜的主要组成部分）。
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