📚 Respiratory Substrates | 呼吸底物
All living cells need a continuous supply of ATP to power processes such as active transport, synthesis of macromolecules, muscle contraction and cell division. In aerobic respiration, organic molecules are oxidised to release energy, and the molecules that enter the respiratory pathway to be broken down are called respiratory substrates. Although glucose is the best-known substrate, cells can also use lipids and proteins, and the choice depends on availability, metabolic state and tissue type.
所有活细胞都需要持续供应ATP来驱动主动运输、大分子合成、肌肉收缩和细胞分裂等过程。在有氧呼吸中，有机分子被氧化释放能量，进入呼吸途径被分解的分子称为呼吸底物。虽然葡萄糖是最常见的底物，但细胞也可以利用脂质和蛋白质，具体选择取决于可用性、代谢状态和组织类型。
1. What Are Respiratory Substrates? | 什么是呼吸底物？
A respiratory substrate is any biological molecule that can be oxidised by respiration to release energy for ATP synthesis. Common substrates are carbohydrates, lipids and, less commonly, proteins. The main feature that makes a good substrate is the presence of many C-H bonds: when these bonds are oxidised, energy is transferred to electron carriers and ultimately used in oxidative phosphorylation.
呼吸底物是任何可以被呼吸作用氧化、释放能量用于合成ATP的生物分子。常见底物包括碳水化合物、脂质以及较少使用的蛋白质。优质底物的主要特征是具有大量C-H键：当这些键被氧化时，能量传递给电子载体，最终用于氧化磷酸化。
In A Level Biology, you need to know why different substrates give different amounts of energy and why their respiratory quotient (RQ) values differ. This helps explain how cells regulate metabolism and how experimental data can reveal which substrate is being used.
在A Level生物中，你需要了解为什么不同底物释放的能量不同，以及为什么它们的呼吸商(RQ)不同。这有助于解释细胞如何调控代谢，以及实验数据如何揭示正在使用哪种底物。
2. Carbohydrates as the Main Substrate | 碳水化合物作为主要底物
Carbohydrates, especially glucose, are the preferred respiratory substrate for most cells. Glucose is soluble, relatively stable, and can be rapidly mobilised from glycogen stores in animals or starch in plants. It enters glycolysis by phosphorylation to glucose-6-phosphate, after which it is split into two molecules of pyruvate.
碳水化合物，尤其是葡萄糖，是大多数细胞首选的呼吸底物。葡萄糖可溶、相对稳定，并且能迅速从动物的糖原储存或植物的淀粉中动员出来。它通过磷酸化生成葡萄糖-6-磷酸进入糖酵解，之后裂解为两分子丙酮酸。
The complete oxidation of one molecule of glucose can be summarised by:
一分子葡萄糖的完全氧化可概括为：
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (≈ 2870 kJ mol⁻¹)
Because the number of CO₂ molecules produced equals the number of O₂ molecules consumed, the respiratory quotient for carbohydrate is 1.0.
由于产生的CO₂分子数等于消耗的O₂分子数，碳水化合物的呼吸商为1.0。
Per gram, carbohydrate yields about 15.8 kJ of energy, which is less than lipid but sufficient for tissues such as the brain that rely heavily on glucose.
每克碳水化合物约产生15.8 kJ能量，低于脂质，但对于大脑等高度依赖葡萄糖的组织已经足够。
Stored polysaccharides are hydrolysed to monosaccharides before respiration: starch is broken down to glucose in plants, while glycogen is converted to glucose-6-phosphate in animals.
储存的多糖在呼吸前会水解为单糖：植物中淀粉分解为葡萄糖，而动物中糖原转化为葡萄糖-6-磷酸。
3. Lipids as Energy-Dense Substrates | 脂质作为高能量底物
Lipids, mainly triglycerides, are the most energy-dense respiratory substrates. They are stored as fat droplets in adipose tissue and are used during prolonged exercise, fasting, or low-carbohydrate conditions. A triglyceride is first hydrolysed by lipase into glycerol and three fatty acids.
脂质，主要是甘油三酯，是能量密度最高的呼吸底物。它们以脂肪滴形式储存在脂肪组织中，在长时间运动、禁食或低碳水化合物条件下被使用。甘油三酯首先被脂肪酶水解为甘油和三分子脂肪酸。
Glycerol is converted to triose phosphate and can enter glycolysis, while fatty acids are broken down by β-oxidation in the mitochondrial matrix into acetyl CoA, which enters the Krebs cycle. This yields a large number of reduced NAD and reduced FAD.
甘油转化为磷酸丙糖并进入糖酵解，而脂肪酸在线粒体基质中通过β-氧化分解为乙酰辅酶A，后者进入克雷布斯循环。这会生成大量还原型NAD和还原型FAD。
Lipids contain a higher proportion of hydrogen atoms relative to oxygen than carbohydrates, so more oxygen is needed for their complete oxidation. The RQ of a typical fatty acid such as oleic acid is about 0.70.
脂质中氢原子相对于氧的比例高于碳水化合物，因此完全氧化需要更多氧气。典型脂肪酸如油酸的RQ约为0.70。
Per gram, lipids release about 39.4 kJ of energy. They also release more metabolic water per gram, which is important for desert animals.
每克脂质释放约39.4 kJ能量。它们每克还产生更多代谢水，这对沙漠动物很重要。
4. Proteins as a Last-Resort Substrate | 蛋白质作为最后手段的底物
Proteins are not normally stored as respiratory substrates, but they can be oxidised when carbohydrate and lipid reserves are very low, such as during starvation. Before entering respiration, amino acids are deaminated in the liver: the amino group is removed and converted to urea via the ornithine cycle.
蛋白质通常不作为储存的呼吸底物，但在碳水化合物和脂质储备极低（如饥饿）时可以被氧化。进入呼吸前，氨基酸在肝脏中脱氨：氨基被移除并通过鸟氨酸循环转化为尿素。
The remaining keto acid can be converted into pyruvate, acetyl CoA, or an intermediate of the Krebs cycle depending on the amino acid. The urea is excreted in urine, which requires energy and water.
剩下的酮酸可以根据氨基酸类型转化为丙酮酸、乙酰辅酶A或克雷布斯循环的中间产物。尿素随尿液排出，这需要能量和水分。
Because some energy is lost in urea production and excretion, proteins yield slightly less useful energy in the body than in a bomb calorimeter. Their approximate energy value is 17.0 kJ g⁻¹ and their RQ is about 0.9.
由于尿素生成和排泄会损失部分能量，蛋白质在体内产生的有效能量略低于弹式量热计测得的值。其近似能量值为17.0 kJ g⁻¹，RQ约为0.9。
5. Comparing Energy Values of Substrates | 比较底物的能量值
The main reason lipids provide more energy per gram than carbohydrates is that fatty acids are highly reduced: they have many C-H bonds and few oxygen atoms. Oxidation of these C-H bonds releases more energy than the oxidation of the partially oxidised -OH groups found in glucose.
脂质每克提供比碳水化合物更多能量的主要原因是脂肪酸高度还原：它们含有大量C-H键而氧原子很少。氧化这些C-H键释放的能量多于氧化葡萄糖中部分氧化的-OH基团所释放的能量。
Lipids are also stored without associated water, whereas glycogen is stored with about 2 g of water per gram of glycogen. Therefore, fat stores are lighter and more compact, making them better for long-term energy storage in animals.
脂质的储存不伴随大量水分，而糖原每储存1克约伴随2克水。因此脂肪储存更轻、更紧凑，更适合动物长期能量储存。
| Substrate | Energy yield (kJ g⁻¹) | RQ | Main fate in respiration |
|---|---|---|---|
| Carbohydrate | 15.8 | 1.0 | Converted to glucose → pyruvate → acetyl CoA |
| Lipid | 39.4 | 更多咨询请联系16621398022（同微信）
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