📚 4.4.3 Energy Loss in Food Chains | 4.4.3 食物链中的能量损失
In every ecosystem, energy flows from the Sun to producers and then through a series of consumers. However, not all the energy available at one trophic level is passed on to the next. According to Edexcel IGCSE Science specification point 4.4.3, you need to understand why energy transfer along a food chain is inefficient and the main processes that lead to energy loss: respiration, heat, egestion and excretion. This article explains each concept in detail to build your confidence for the exam.
在每个生态系统中，能量从太阳流向生产者，然后通过一系列消费者传递。然而，一个营养级可用的能量并不会全部传递到下一个营养级。根据 Edexcel IGCSE 科学大纲 4.4.3 部分的要求，你需要理解为什么沿着食物链的能量传递效率低下，以及导致能量损失的主要过程：呼吸作用、热能散失、排遗和排泄。本文将详细解释每个概念，助你从容应对考试。
1. Food Chains and Energy Flow | 食物链与能量流动
A food chain shows the feeding relationships between organisms in an ecosystem. It always begins with a producer, which is usually a green plant or alga that can photosynthesise. The producer is followed by a primary consumer, then a secondary consumer, and sometimes a tertiary consumer. Energy enters the food chain when producers absorb light energy from the Sun and convert it into chemical energy stored in glucose. This energy is then transferred along the chain when one organism eats another.
食物链表示生态系统中生物之间的摄食关系。它总是从生产者开始，生产者通常是能够进行光合作用的绿色植物或藻类。生产者之后是初级消费者，然后是次级消费者，有时还有三级消费者。当生产者吸收太阳光能并将其转化为储存在葡萄糖中的化学能时，能量便进入食物链。随后，当一种生物被另一种生物取食时，能量沿食物链传递。
2. The Source of Energy in Most Ecosystems | 大多数生态系统的能量来源
For nearly all food chains on Earth, the ultimate source of energy is the Sun. Producers trap about 1% of the light energy that reaches them during photosynthesis. This small percentage becomes the total energy input at the first trophic level. In deep-sea hydrothermal vents, chemosynthetic bacteria can act as producers using chemical energy instead of sunlight, but for IGCSE Science, you only need to focus on sunlight-driven food chains.
对地球上几乎所有的食物链来说，最根本的能量来源是太阳。生产者通过光合作用，能够截获到达它们的光能中约 1% 的能量。这一小部分能量构成了第一营养级的总能量输入。在深海热泉口，化能合成细菌可以利用化学能而非阳光作为生产者，但在 IGCSE 科学范围内，你只需关注以阳光为驱动力的食物链。
3. Energy Transfer Along a Food Chain | 沿食物链的能量传递
When a primary consumer eats a producer, the chemical energy stored in the plant's biomass is transferred to the consumer. However, only a fraction of this energy becomes new consumer biomass. Most of the energy is used for the consumer's life processes or is lost to the environment. The same pattern repeats at each trophic level: secondary consumers obtain only a small portion of the energy that was present in primary consumers.
当初级消费者取食生产者时，储存在植物生物量中的化学能传递给消费者。然而，这些能量中只有一小部分会转化成消费者新的生物量。大部分能量被消费者用于生命活动，或者散失到环境中。同样的模式在每个营养级都会重复：次级消费者只能获取初级消费者所含能量的一小部分。
4. Why Energy Transfer is Not 100% Efficient | 为何能量传递并非 100% 高效
Energy transfer from one trophic level to the next is never 100% efficient. This is because living organisms must use energy to stay alive. Energy is lost through respiration, movement, maintaining body temperature, and other metabolic activities. Moreover, not all parts of an organism are eaten or digested. Exam questions often ask you to state the main reasons for energy loss, so it is essential to learn the four key categories: respiration, heat, egestion, and excretion.
能量从一个营养级传递到下一个营养级永远不可能 100% 高效。这是因为生物体必须消耗能量来维持生命。能量会通过呼吸作用、运动、维持体温等代谢活动而散失。此外，生物体的所有部分并非都会被吃掉或消化。考题常要求你说明能量损失的主要原因，因此需要牢记四个关键类别：呼吸作用、热能散失、排遗和排泄。
5. Energy Lost Through Respiration | 通过呼吸作用损失的能量
All living cells respire to release energy for processes such as growth, repair, and active transport. Respiration breaks down glucose and releases energy, some of which is used to make ATP, while much of it is eventually lost as heat. This chemical process is the single biggest reason why less energy is available at the next trophic level. Even producers respire, using a portion of the glucose they produce themselves.
所有活细胞都进行呼吸作用，以释放能量供生长、修复和主动运输等过程使用。呼吸作用分解葡萄糖并释放能量，其中一部分用于合成 ATP，而大部分最终以热能形式散失。这一化学过程是下一个营养级可用能量变少的最重要原因。即使是生产者也会呼吸，消耗掉它们自己制造的一部分葡萄糖。
6. Energy Lost as Heat | 以热能形式散失的能量
Heat is a by‑product of metabolism, particularly from respiration and muscle contraction. In mammals and birds, a great deal of energy is used to maintain a constant body temperature, and this energy is radiated away as heat into the surroundings. Once heat is lost, it cannot be recycled by living organisms. This is a key reason why energy flow in an ecosystem is linear, not cyclic.
热量是新陈代谢的副产品，尤其来自呼吸作用和肌肉收缩。哺乳动物和鸟类会消耗大量能量来维持恒定的体温，这部分能量以热的形式辐射到周围环境中。热量一旦散失，就无法被生物体再次利用。这是生态系统中能量流动呈单向而非循环的关键原因。
7. Energy Lost Through Egestion and Excretion | 通过排遗和排泄损失的能量
Egestion is the removal of undigested food materials from the body as faeces. Even though this material has been eaten, its chemical energy is not absorbed into the consumer's cells and is instead lost to the environment. Excretion is the removal of metabolic waste products such as urea, which still contain some energy. Both egestion and excretion represent energy that was taken in but not assimilated, reducing the amount available for the next trophic level.
排遗是指将未消化的食物以粪便形式排出体外。尽管这些物质被吃进体内，但其化学能并未被消费者细胞吸收，而是散失到环境中。排泄则是排出尿素等代谢废物，这些废物仍含有一些能量。排遗和排泄都意味着摄入但未被同化的能量，从而减少了传递给下一营养级的能量。
8. Calculating Energy Transfer Efficiency | 计算能量传递效率
You may be asked to calculate the efficiency of energy transfer between trophic levels using the formula:
Efficiency (%) = (Energy in higher trophic level ÷ Energy in lower trophic level) × 100
ed on it store only 2 000 kJ in their biomass, the efficiency is (2000 ÷ 20000) × 100 = 10%. This 10% lower. Always show your working in calculations, and make sure the units of energy match.
你可能会被要求使用以下公式计算营养级之间的能量传递效率：
效率 (%) = (较高营养级的能量 ÷ 较低营养级的能量) × 100
例如，若一片草地含有 20 000 kJ 能量，而以其为食的兔子只在体内储存 2 000 kJ 能量，则效率为 (2000 ÷ 20000) × 100 = 10%。这一定律可作为有用的近似值，尽管实际效率常常更低。计算时务必写出过程，并确保能量单位一致。
9. Trophic Levels and the 10% Rule | 营养级与十分之一定律
Ecologists often observe that only about 10% of the energy at one trophic level is transferred to the next. The remaining 90% is used for respiration, lost as heat, or egested and excreted. This sharp drop in available energy explains why the number of trophic levels in a food chain is usually limited to four or five. With so little energy left at higher levels, there is simply not enough to support another level of consumers.
生态学家经常观察到，一个营养级的能量只有大约 10% 传递到下一个营养级。其余 90% 被用于呼吸作用、以热的形式散失，或者通过排遗和排泄损失。可用能量的急剧减少解释了为什么食物链的营养级数量通常限制在四到五个。较高营养级剩余的能量太少，根本不足以支撑更多层次的消费者。
10. Pyramids of Energy | 能量金字塔
A pyramid of energy shows the total energy present at each trophic level over a given area and time. The pyramid is always upright because energy decreases at each successive level. Drawing a pyramid of energy helps visualise the large losses between trophic levels. The base, representing producers, is wide, and the top is narrow. Unlike pyramids of numbers or biomass, pyramids of energy cannot be inverted, making them the most reliable way to represent energy flow.
能量金字塔表示在一定面积和时间尺度内，每个营养级拥有的总能量。这种金字塔总是呈正立的，因为能量在每一级都逐级减少。绘制能量金字塔有助于直观显示营养级之间的大量能量损失。基底代表生产者，较宽，而顶端较窄。与数量金字塔或生物量金字塔不同，能量金字塔永远不会倒置，因此是表示能量流动最可靠的方式。
11. Why Food Chains Are Relatively Short | 为什么食物链相对较短
The inefficiency of energy transfer limits the length of food chains. After three or four transfers, there is too little energy to sustain a breeding population of top consumers. This is why most food chains have only three to five links. Understanding this concept also helps you explain why top carnivores, such as eagles or tigers, are rare and require large territories to obtain enough energy.
能量传递的低效限制了食物链的长度。经过三到四次传递后，剩余的能量太少，不足以维持顶级消费者的繁殖种群。这就是为什么大多数食物链只有三到五个环节。理解这一概念，也有助于你解释为什么像鹰或老虎这样的顶级食肉动物数量稀少，且需要大片领地才能获取足够能量。
12. Linking Energy Loss to the Carbon Cycle and Exam Tips | 能量损失与碳循环的关联及考试技巧
When organisms respire, they release carbon dioxide, which links energy loss directly to the carbon cycle. In exam questions, be prepared to explain that respiration returns CO₂ to the atmosphere while releasing energy as heat. Also remember that energy is not recycled — it flows through the ecosystem in one direction only. Use precise terms such as 'egestion' and 'excretion' rather than vague phrases like 'waste'. Practice calculating efficiency with different numerical examples, and be ready to interpret pyramids of energy.
生物体呼吸时会释放二氧化碳，这就把能量损失与碳循环直接联系在一起。在考试题目中，要准备好解释呼吸作用将 CO₂ 释放回大气，同时以热的形式释放能量。还要记住，能量是不能循环利用的——它只在生态系统中单向流动。请使用'排遗'和'排泄'这样精确的术语，而不要使用'废物'这类模糊的说法。多练习不同数值的效率计算，并准备好解读能量金字塔。
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