📚 Respiratory Substrates in Cellular Respiration | 呼吸底物在细胞呼吸中的作用
In cellular respiration, a respiratory substrate is any organic molecule that is oxidised to transfer energy into ATP. Carbohydrates, lipids and proteins can all serve this role, but their pathways, energy yields and respiratory quotients differ in ways that examiners often test.
在细胞呼吸中，呼吸底物指通过氧化分解将能量转化到 ATP 中的有机分子。碳水化合物、脂质和蛋白质都能担任这一角色，但它们的代谢途径、能量产量和呼吸商都存在差异，这些差异正是考官常考的重点。
1. Definition and Classification of Respiratory Substrates | 呼吸底物的定义与分类
Respiratory substrates are the energy-rich carbon compounds that are broken down during cellular respiration. The most common substrates are carbohydrates, especially glucose, followed by lipids and, under certain conditions, proteins.
呼吸底物是细胞呼吸过程中被分解的富能碳化合物。最常见的底物是碳水化合物，尤其是葡萄糖，其次是脂质，以及在特定条件下才被利用的蛋白质。
They can be classified as primary substrates, secondary substrates and reserve substrates depending on the speed at which they can be mobilised. Glucose is immediately available to glycolysis, while stored glycogen or starch must first be hydrolysed.
根据动员速度，呼吸底物可分为主要底物、次要底物和储备底物。葡萄糖可直接进入糖酵解，而储存的糖原或淀粉则必须先水解为单糖。
2. Carbohydrates: The Principal Respiratory Substrate | 碳水化合物：主要的呼吸底物
Carbohydrates, particularly glucose, are the most widely used respiratory substrate in cells. Glucose is broken down in glycolysis, which takes place in the cytoplasm and produces pyruvate, reduced NAD and a net yield of 2 ATP per molecule of glucose.
碳水化合物尤其是葡萄糖，是细胞中最广泛利用的呼吸底物。葡萄糖在细胞质中经糖酵解分解，产生丙酮酸、还原型 NAD，并净生成 2 分子 ATP。
Other carbohydrates enter respiration at different points. Fructose and galactose can enter glycolysis; glycogen and starch are hydrolysed to glucose before oxidation. The overall aerobic oxidation of glucose is:
其他碳水化合物可在不同节点进入呼吸途径。果糖和半乳糖可进入糖酵解；糖原和淀粉需先水解为葡萄糖再被氧化。葡萄糖有氧氧化的总方程式为：
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
3. Lipids: High-Energy Respiratory Substrates | 脂质：高能量密度的呼吸底物
Lipids are stored mainly as triglycerides in adipose tissue. Before they can be used in respiration, triglycerides are hydrolysed into glycerol and fatty acids. Glycerol is converted into triose phosphate and enters glycolysis, while fatty acids are degraded by β-oxidation to form acetyl CoA.
脂质主要以甘油三酯形式储存于脂肪组织中。使用时，甘油三酯先被水解为甘油和脂肪酸。甘油转化为三碳糖磷酸并进入糖酵解，脂肪酸则通过β-氧化被分解为乙酰 CoA。
Fatty acids yield more energy per gram than carbohydrates because they contain many C–H bonds and therefore more hydrogen atoms. This gives a larger supply of electrons and protons for the electron transport chain.
脂肪酸每克释放的能量高于碳水化合物，原因是脂肪酸含有大量 C–H 键和更多的氢原子，从而为电子传递链提供更多电子和质子。
4. Proteins: Conditional Respiratory Substrates | 蛋白质：条件性呼吸底物
Proteins are not normally used as respiratory substrates in a healthy, well-fed organism. However, during prolonged starvation or severe metabolic stress, amino acids can be respired. This process begins with deamination in the liver, where the amino group is removed as ammonia, which is converted into urea.
在健康且营养充足的生物体内，蛋白质通常不作为呼吸底物使用。但在长期饥饿或严重代谢应激时，氨基酸可被呼吸利用。这一过程始于肝脏中的脱氨基作用：氨基以氨的形式被除去，随后转化为尿素。
The remaining carbon skeleton, an α-keto acid, can enter respiration at different stages, such as pyruvate, acetyl CoA or intermediates of the Krebs cycle. Protein respiration has a greater metabolic cost and produces less usable energy per gram than pure fat oxidation.
剩余的碳骨架即 α-酮酸，可在不同阶段进入呼吸途径，例如丙酮酸、乙酰 CoA 或 Krebs 循环的中间产物。蛋白质呼吸的代谢成本更高，每克产生的可用能量也低于纯脂肪氧化。
5. Respiratory Quotient: Definition and Formula | 呼吸商：定义与计算式
The respiratory quotient, RQ, is defined as the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed during respiration. The same ratio is obtained if moles are used instead of volumes.
呼吸商 RQ 的定义是：呼吸过程中产生的二氧化碳体积与消耗的氧气体积之比。若改用摩尔数，所得比值相同。
RQ = volume of CO₂ produced ÷ volume of O₂ consumed
Because different respiratory substrates require different amounts of oxygen for complete oxidation, RQ can be used to identify which substrate is being respired. It is only reliable under aerobic conditions.
由于不同呼吸底物完全氧化所需的氧气量不同，RQ 可用于判断生物正在利用哪种底物。但只有在有氧条件下才可靠。
6. Calculating RQ for Different Substrates | 不同底物的呼吸商计算
For glucose, the balanced equation for aerobic respiration shows that 6 moles of CO₂ are produced for every 6 moles of O₂ consumed, so the RQ is exactly 1.0.
对于葡萄糖，有氧呼吸的平衡方程显示每消耗 6 mol O₂ 便产生 6 mol CO₂，因此 RQ 正好为 1.0。
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O | RQ = 6 ÷ 6 = 1.00
For a fatty acid such as palmitic acid, the oxidation equation is:
对于棕榈酸这类脂肪酸，其氧化方程如下：
C₁₆H₃₂O₂ + 23O₂ → 16CO₂ + 16H₂O | RQ = 16 ÷ 23 ≈ 0.70
Proteins have an approximate RQ of 0.9, because their carbon skeleton is less reduced than that of fatty acids but more reduced than that of glucose.
蛋白质的 RQ 约为 0.9，原因是其碳骨架的还原程度低于脂肪酸，但高于葡萄糖。
| Substrate 底物 | RQ value RQ值 |
| Carbohydrates 碳水化合物 | 1.0 |
| Lipids 脂质 | ≈ 0.70 |
| Proteins 蛋白质 | ≈ 0.90 |
7. Factors Affecting Respiratory Quotient | 影响呼吸商的因素
The most important factor is the type of substrate being respired. A diet rich in carbohydrate gives an RQ close to 1.0, whereas a high-fat diet gives an RQ that falls toward 0.7.
最重要的因素是被呼吸利用的底物类型。富含碳水化合物的饮食使 RQ 接近 1.0，高脂饮食则使 RQ 降至 0.7 左右。
Physiological state also changes RQ. During intense exercise, oxygen may become limiting; anaerobic respiration in muscle produces lactate, and the bicarbonate buffering system can release extra CO₂, making the measured RQ rise above 1.0. In long-term starvation, fat metabolism dominates, so RQ tends toward 0.7.
生理状态也会改变 RQ。剧烈运动时氧气可能不足，肌肉的无氧呼吸产生乳酸，碳酸氢盐缓冲系统会额外释放 CO₂，使测得的 RQ 升高至 1.0 以上。长期饥饿时以脂肪代谢为主，RQ 趋于 0.7。
In anaerobic respiration, such as alcoholic fermentation in yeast, no O₂ is consumed but CO₂ is still released. The RQ is therefore not defined or is considered extremely large.
在无氧呼吸中，例如酵母的酒精发酵，不消耗 O₂ 但仍释放 CO₂，因此 RQ 无法定义，或可视为极大值。
8. Using RQ to Interpret Metabolic State | 利用呼吸商判断代谢状态
RQ is a useful clinical and ecological tool. In a healthy human on a mixed diet, the RQ is typically around 0.85, indicating that carbohydrates and fats are being oxidised in combination.
RQ 是临床上与生态学中很有用的指标。健康人摄入混合膳食时，RQ 通常约为 0.85，提示碳水化合物和脂肪在同时被氧化。
In untreated diabetes mellitus, glucose uptake is impaired, so cells switch to fatty acid oxidation; the RQ moves toward 0.7. Conversely, after a carbohydrate-rich meal, insulin promotes glucose oxidation and the RQ rises toward 1.0.
在未治疗的糖尿病中，细胞摄取葡萄糖受阻，转而氧化脂肪酸，RQ 向 0.7 移动。反之，摄食富含碳水化合物的餐后，胰岛素促进葡萄糖氧化，RQ 向 1.0 上升。
For germinating seeds, RQ can reveal the nature of stored food. Fatty seeds such as castor oil seeds show an initial RQ of about 0.7, which rises as carbohydrate becomes available from gluconeogenesis.
对于萌发种子，RQ 可以揭示储存物质的性质。蓖麻籽等油料种子初期 RQ 约 0.7，随着糖异生产生碳水化合物，RQ 逐渐上升。
9. Order of Substrate Utilisation | 呼吸底物的利用顺序
The order in which respiratory substrates are used is mainly carbohydrate first, then lipid, and finally protein. Carbohydrate is preferred because it is easily transported, rapidly converted into pyruvate, and can generate ATP quickly without producing toxic by-products.
呼吸底物的利用顺序通常为：首先碳水化合物，其次脂质，最后蛋白质。碳水化合物之所以优先，是因为它易于运输、能快速转化为丙酮酸，并可在不产生有毒副产物的情况下迅速生成 ATP。
Lipids are used when carbohydrate supply is exhausted, especially during prolonged exercise, fasting or high-energy demand. Proteins are spared because they perform structural and enzymatic functions; their breakdown would compromise essential body processes. A key exam point is that red blood cells have no mitochondria and can only use glucose as their respiratory substrate.
当碳水化合物储备耗尽时，机体转向脂质，尤其是在长时间运动、禁食或高能量需求下。蛋白质通常被保留，因为它们承担结构和酶功能，分解蛋白质会损害重要生理过程。一个关键考点是：红细胞没有线粒体，只能利用葡萄糖作为呼吸底物。
10. Energy Yield: Comparing Substrates | 能量产率：各底物比较
Complete oxidation of one molecule of glucose generates a net total of 30–32 ATP depending on the efficiency of shuttle systems. The energy yield of carbohydrate is about 16 kJ per gram.
一分子葡萄糖完全氧化最终净产生 30–32 分子 ATP，具体数值取决于穿梭系统的效率。碳水化合物的能量产量约为每克 16 kJ。
One molecule of palmitic acid yields about 106 ATP after activation, because it produces many reduced NAD and FAD molecules during β-oxidation. Fat yields about 38–39 kJ per gram, more than twice that of carbohydrate or protein.
一分子的棕榈酸在活化后约产生 106 分子 ATP，因为β-氧化过程中产生了大量还原型 NAD 和 FAD。脂肪每克产能约 38–39 kJ，是碳水化合物或蛋白质的两倍以上。
However, fat carries the disadvantage of slower oxygen demand and requires more oxygen per mole of CO₂ produced. This explains why RQ of fat is below 1 and why very intense exercise cannot rely mainly on fat as a substrate.
不过，脂肪也有其缺点：耗氧速率较慢，且每产生一分子 CO
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