Energy Flow in Ecosystems | 生态系统能量流动考点精讲

📚 Energy Flow in Ecosystems | 生态系统能量流动考点精讲

Energy flow is a core concept in IGCSE AQA Biology that describes how energy moves through living organisms in an ecosystem. Understanding how energy is captured from the Sun, transferred between trophic levels, and eventually lost as heat is essential for explaining why food chains are short and how food production can be made more efficient.

能量流动是 IGCSE AQA 生物的核心概念，描述能量如何在生态系统的生物之间移动。理解能量如何从太阳中被捕获、在营养级之间传递并最终以热的形式散失，对于解释为什么食物链通常较短以及如何提高食物生产效率至关重要。

1. What is Energy Flow? | 什么是能量流动？

Energy flow refers to the unidirectional transfer of energy through an ecosystem, starting from sunlight and moving through producers to consumers and decomposers. Unlike matter, which is recycled, energy cannot be reused by living organisms once it has been converted to heat during respiration.

能量流动指能量在生态系统中单向传递的过程，从太阳光开始，经过生产者到消费者和分解者。与物质循环不同，能量一旦在呼吸作用中转变成热能，生物就无法再次利用。

All life depends on a continuous input of energy. In most ecosystems, the primary source is the Sun. Energy enters the biological world through photosynthesis and leaves as heat, meaning ecosystems need a constant supply of light energy to function.

所有生命都依赖持续的能量输入。在大多数生态系统中，主要能量来源是太阳。能量通过光合作用进入生物界，并以热的形式离开，这意味着生态系统需要持续的光能供应才能运转。

2. Food Chains and Food Webs | 食物链和食物网

A food chain is a simple linear diagram that shows the transfer of energy from one organism to another. Each arrow in a food chain represents the direction of energy flow, not ‘who eats whom’. For example: grass → rabbit → fox. The energy originally stored in the grass is passed to the rabbit and then to the fox.

食物链是一个简单的线性图示，显示能量从一个生物体传递到另一个生物体。食物链中的每个箭头表示能量流动的方向，而不是“谁吃谁”。例如：草 → 兔 → 狐狸。草中储存的能量传递给兔，再传递给狐狸。

A food web is a more realistic representation that links many food chains together. It shows that most organisms eat more than one type of food and may be eaten by several different predators. Food webs help us understand how energy flows through a whole community and how organisms depend on each other.

食物网更接近现实情况，它将多条食物链联系起来。它显示大多数生物不只吃一种食物，也可能被多种捕食者捕食。食物网帮助我们理解能量如何在整个群落中流动，以及生物间的相互依赖关系。

When drawing food chains for exams, always start with a producer (a green plant or alga) and point the arrow towards the consumer. Remember: ‘arrow points to the eater’ gives the wrong idea; instead, think ‘arrow shows energy movement to the next trophic level’.

考试中绘制食物链时，一定要从生产者（绿色植物或藻类）开始，箭头指向消费者。记住：“箭头指向捕食者”容易产生误解；正确的理解是“箭头指示能量向下一个营养级转移”。

3. Trophic Levels | 营养级

A trophic level is the position an organism occupies in a food chain. Producers (plants, algae) are at trophic level 1. Primary consumers that eat producers are at trophic level 2. Secondary consumers that eat primary consumers are at trophic level 3, and so on. Tertiary consumers occupy level 4.

营养级是生物在食物链中所处的位置。生产者（植物、藻类）位于营养级1。以生产者为食的初级消费者位于营养级2。以初级消费者为食的次级消费者位于营养级3，依此类推。三级消费者占据营养级4。

It is important to remember that energy decreases at each successive trophic level. This is why most food chains rarely exceed four or five trophic levels – there is simply not enough energy remaining to support another level of consumers.

重要的是要记住，能量在每一个更高的营养级都会减少。这就是为什么大多数食物链很少超过四到五个营养级——剩下的能量根本不足以支持更高一层级的消费者。

4. The Source of Energy: The Sun | 能量来源：太阳

Nearly all energy in ecosystems originates from the Sun. Solar energy travels to Earth as light and is absorbed by photosynthetic organisms. A tiny percentage (around 1–2%) of the light energy reaching a plant is actually captured and converted into chemical energy during photosynthesis.

生态系统中几乎所有的能量都来源于太阳。太阳能以光的形式到达地球，被进行光合作用的生物吸收。照射到植物上的光能只有极小一部分（约1–2%）真正被捕获并在光合作用中转变成化学能。

Some deep-sea ecosystems rely on chemical energy from hydrothermal vents rather than sunlight, but for the IGCSE AQA syllabus, the focus is on sunlight as the main energy source driving terrestrial and aquatic food chains. Energy from the Sun is stored in glucose and then made available to the rest of the ecosystem.

一些深海生态系统依赖热液喷口的化学能而非阳光，但就 IGCSE AQA 大纲而言，主要关注太阳光作为驱动陆地和水生食物链的能量来源。来自太阳的能量被储存在葡萄糖中，然后被生态系统的其他成员利用。

5. Producers and Photosynthesis | 生产者与光合作用

Producers are organisms that can make their own food using energy from sunlight or inorganic chemicals. In most ecosystems, producers are green plants and algae that carry out photosynthesis. The overall equation for photosynthesis is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Light energy is trapped by chlorophyll and converted into chemical energy stored in glucose.

生产者是能够利用太阳光或无机化学物质的能量制造自己食物的生物。在大多数生态系统中，生产者是进行光合作用的绿色植物和藻类。光合作用的总方程式为：6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂。光能被叶绿素捕获并转化为储存在葡萄糖中的化学能。

Glucose produced during photosynthesis is used by the plant for respiration to release energy, or it is converted into other organic molecules such as starch, cellulose, proteins and lipids. Only the energy stored in these organic compounds can be transferred to the next trophic level when consumers eat the plant.

光合作用产生的葡萄糖被植物用于呼吸作用以释放能量，或者转化为其他有机分子，如淀粉、纤维素、蛋白质和脂质。只有储存在这些有机化合物中的能量，才能在消费者取食植物时传递到下一个营养级。

6. Consumers and Energy Transfer | 消费者与能量传递

Consumers obtain energy by feeding on other organisms. Herbivores (primary consumers) eat plants, carnivores eat other animals, and omnivores eat both. When a consumer eats, the chemical energy stored in the prey’s biomass is taken in. This energy is then used for movement, growth, cell repair and maintaining body temperature (in endotherms).

消费者通过取食其他生物来获取能量。食草动物（初级消费者）吃植物，食肉动物吃其他动物，杂食动物两者都吃。消费者进食时，猎物生物量中储存的化学能被摄入。这些能量随后被用于运动、生长、细胞修复以及维持体温（恒温动物）。

During respiration in the consumer’s cells, glucose and other respiratory substrates are broken down to release energy for ATP production. The equation for aerobic respiration is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. This process releases heat, which is lost from the body and cannot be used again by the organism or passed along the food chain.

在消费者细胞的呼吸作用中，葡萄糖和其他呼吸底物被分解，释放能量用于生成ATP。有氧呼吸的方程式为：C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 能量。这一过程释放热量，热量从体内散失，不能被生物体再次利用，也不能沿食物链传递。

7. Energy Loss Between Trophic Levels | 营养级间的能量损失

As energy moves from one trophic level to the next, a large amount is lost. Typically, only about 10% of the energy in one trophic level is incorporated into the biomass of the next level. The remaining 90% does not end up in the body of the consumer; instead it is lost through several key processes.

当能量从一个营养级传递到下一个时，大部分能量会损失掉。通常，一个营养级中大约只有10%的能量被结合到下一级的生物量中。其余90%并没有进入消费者的身体，而是通过几个关键过程损失了。

Understanding these losses is crucial for explaining pyramid shapes and calculating ecological efficiency. Energy ‘loss’ does not mean it disappears from the universe — it is still present, mostly as heat dissipated into the environment, but it is no longer available to the organisms in that food chain.

理解这些损失对于解释能量金字塔的形状和计算生态效率至关重要。能量“损失”并不意味着从宇宙中消失——它仍然存在，主要以热的形式散失到环境中，但食物链中的生物已经无法再利用它。

8. The 10% Rule and Energy Pyramids | 10%法则与能量金字塔

The ‘10% rule’ is a useful guideline stating that approximately 10% of the energy available at one trophic level is transferred to the next. This is not a rigid law — in some systems the efficiency might be slightly higher or lower — but it helps explain why biomass and energy decrease so sharply up a food chain.

“10%法则”是一个很好用的经验规律，即一个营养级中大约10%的能量会传递到下一个营养级。这并不是一条严格定律——在某些系统中效率可能稍高或稍低——但它有助于解释为什么生物量和能量在食物链上急剧减少。

An energy pyramid is a graphical representation of the energy present at each trophic level in a food chain. It is always pyramid-shaped because energy decreases at each successive level. Unlike pyramids of numbers, energy pyramids can never be inverted, making them the most reliable way to represent trophic structure.

能量金字塔是食物链各营养级所含能量的图示。它总是金字塔形状，因为能量在每一个连续的营养级中都减少。与数量金字塔不同，能量金字塔绝不可能倒置，这使其成为表示营养结构最可靠的方法。

Energy pyramids are usually drawn to scale with horizontal bars of equal thickness. The length of each bar is proportional to the energy stored per unit area per unit time (e.g., kJ m⁻² year⁻¹). The wide base represents the large energy captured by producers, and each bar above is roughly one-tenth the length of the one below.

能量金字塔通常按比例绘制，水平柱条厚度相等。每条柱的长度与单位面积单位时间内储存的能量成比例（如 kJ m⁻² year⁻¹）。宽阔的底部代表生产者捕获的大量能量，上方每一层柱长大约是下方一层的十分之一。

9. Detailed Reasons for Energy Loss | 能量损失的详细原因

Why is so much energy lost at each trophic level? The main reasons are:

为什么每个营养级会损失这么多能量？主要原因有：

Uneaten parts: Not all of an organism is consumed. For example, bones, roots, bark, and hair may be left behind. Energy stored in these parts never enters the consumer.
未被取食的部分：生物体并非所有部分都被取食。例如，骨头、根、树皮和毛发可能被留下。这些部分储存的能量从未进入消费者体内。
Excretion and egestion: Some of the food eaten is not digested and is egested as faeces. Also, waste products such as urea contain energy that the body cannot use and are excreted in urine.
排泄与排遗：吃下的一部分食物未经消化，以粪便形式排遗。此外，尿素等废物含有身体无法利用的能量，通过尿液排出。
Respiration: Most of the assimilated energy is used in respiration to provide energy for movement, growth and maintaining body temperature. Respiration releases large quantities of heat, which is lost to the surroundings.
呼吸作用：大部分被同化的能量用于呼吸作用，为运动、生长和维持体温提供能量。呼吸作用释放大量热量散失到周围环境中。
Movement and heat loss: Mammals and birds use a significant proportion of their energy intake just to keep their bodies warm. Constant movement in search of food or mates also uses energy that is ultimately lost as heat.
运动与热量散失：哺乳动物和鸟类使用摄入能量的相当一部分仅仅为了维持体温。持续觅食或求偶的运动也消耗能量，最终以热的形式散失。

In summary, only a small fraction of the energy consumed ends up becoming new biomass that can be passed to the next trophic level. This explains why energy pyramids always narrow towards the top.

总之，被消耗的能量中只有一小部分最终成为能够传递到下一个营养级的新生物量。这就解释了为什么能量金字塔总是向上变窄。

10. Decomposers and Nutrient Cycling | 分解者与物质循环

Decomposers, such as bacteria and fungi, play a vital role in breaking down dead organisms and waste materials. They secrete enzymes to digest organic matter externally and absorb the simpler molecules. This process releases inorganic nutrients back into the soil, which can then be taken up by plants again.

分解者，如细菌和真菌，在分解死亡生物和废物方面起着至关重要的作用。它们分泌酶在体外消化有机物，并吸收简单分子。这一过程将无机营养物释放回土壤，植物可以再次吸收利用。

However, decomposers do not recycle energy. All the energy contained in the dead remains and waste is eventually respired by decomposers and released as heat. Thus, energy flow is linear and non-cyclic, while nutrients are cycled.

然而，分解者并不循环能量。死亡残骸和废物中含有的所有能量最终都会被分解者呼吸并以热的形式释放。因此，能量流动是线性的、不可循环的，而营养物质则可以循环。

It is a common exam mistake to say that energy is recycled. Always remember: ‘Energy flows, nutrients cycle.’ The ecosystem requires a constant input of energy from the Sun because energy lost as heat cannot be recaptured by living organisms.

考试中常见的错误是说能量可以循环。务必牢记：“能量流动，物质循环”。生态系统需要太阳持续输入能量，因为以热散失的能量无法被生物重新捕获。

11. Efficiency of Energy Transfer | 能量传递效率

The efficiency of energy transfer between trophic levels can be calculated using a simple formula:

营养级间能量传递效率可以用一个简单公式计算：

Efficiency (%) = (Energy available to next level ÷ Energy available in previous level) × 100

效率 (%) = (下一营养级可用的能量 ÷ 上一营养级可用的能量) × 100

For example, if a field of grass captures 100,000 kJ of light energy and produces 1,000 kJ of biomass energy, the efficiency of photosynthesis is 1%. If a rabbit eats that grass and stores 100 kJ in its own biomass, the ecological efficiency from producer to primary consumer is (100 ÷ 1,000) × 100 = 10%. Such calculations are routine in AQA IGCSE papers.

例如，如果一片草地捕获100,000 kJ光能并生产出1,000 kJ的生物量能量，光合作用效率为1%。如果一只兔子吃了这些草并在自身生物量中储存了100 kJ，那么从生产者到初级消费者的生态效率为 (100 ÷ 1,000) × 100 = 10%。这类计算在 AQA IGCSE 试卷中很常见。

When calculating efficiency, always check the units and make sure you are comparing the correct trophic levels. The question might provide data in a table showing energy available per square metre per year, and you must identify the relevant figures. Leave your answer to one or two decimal places unless told otherwise.

计算效率时，务必检查单位，确保比较的是正确的营养级。题目可能会以表格形式给出每年每平方米可用能量的数据，你需要识别出相关数字。除非另有说明，答案保留一到两位小数。

12. Improving Food Production Efficiency | 提高食物生产效率

Understanding energy loss helps us design more efficient food production systems. The shorter the food chain, the less energy is lost overall. This is why eating plants directly (being a primary consumer) provides far more energy than eating a carnivore that has fed on another animal. A vegetarian diet is more energy-efficient than a meat-based diet.

理解能量损失有助于我们设计更高效的食物生产系统。食物链越短，总体能量损失越少。这就是为什么直接食用植物（作为初级消费者）比吃捕食其他动物的肉食动物能提供多得多的能量。素食比肉食的能量效率更高。

In agriculture, farmers can reduce energy losses from livestock by:

在农业中，农民可以通过以下方式减少家畜的能量损失：

Restricting movement: Keeping animals in smaller enclosures or pens so they use less energy for movement. More of their food energy goes towards growth.
限制运动：将动物饲养在较小的围栏或圈中，这样它们用于运动的能量更少，更多的食物能量用于生长。
Controlling temperature: Providing warm housing for pigs and chickens reduces the energy they need to maintain body temperature.
控制温度：为猪和鸡提供温暖的圈舍，减少它们维持体温所需的能量。
High-quality feed: Using easily digestible feed decreases the proportion of energy lost in faeces. Antibiotics (though now restricted) were once used to alter gut bacteria and improve absorption.
高质量饲料：使用易消化的饲料可降低粪便中损失的能量比例。抗生素（尽管现在受限）曾用于改变肠道菌群以改善吸收。

While these methods increase efficiency and yield, they also raise ethical concerns about animal welfare. In IGCSE exam questions, you may be asked to evaluate the advantages and disadvantages of intensive farming, balancing food supply demands against the quality of life for livestock.

这些方法提高效率和产量的同时，也引发了有关动物福利的伦理问题。在 IGCSE 考试中，你可能会被要求评估集约化农业的优缺点，在粮食供给需求与家畜生活质量之间取得平衡。

Published by TutorHao | Biology Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Energy Flow in Ecosystems | 生态系统能量流动考点精讲