Ch 1 · Energy Flow · O Level Biology 5090
Topic 1 of 4 in Organisms and their Environment — O Level Biology 5090 — 38 lessons.
Introduction
Every mouthful you eat is sunlight in disguise. Light energy captured by plants becomes chemical energy in glucose, passes along food chains from one organism to the next, and at every step some escapes to the surroundings as heat. This one-way flow of energy explains why ecosystems need a constant input from the Sun and why food chains are short.
Key ideas — The Sun and the flow of energy
The Sun is the principal source of energy input to biological systems. Almost all life depends on it, because producers can only make food by trapping light. Most forms of life are completely dependent on photosynthesis: it makes the food at the start of every food chain, and it releases the oxygen that most organisms need for aerobic respiration.
🧑🏫 Teacher's explanation
Think of energy like a flowing river. The Sun is the massive reservoir at the top of the mountain, constantly pouring water (energy) into the system. As the water flows down through different pools (trophic levels), some of it leaks into the ground or evaporates as steam (heat). By the time the water reaches the bottom, there is very little left to power anything else.
📖 Definitions
🔍 Worked example — Easy
A food chain consists of: Grass arrow Grasshopper arrow Frog arrow Snake. If the grass contains 10,000 kJ of energy, calculate the energy available to the snake, assuming 10\% efficiency at each transfer.
🔍 Worked example — Standard
Explain why energy flow is considered a 'one-way' process, unlike the carbon cycle.
🔍 Worked example — Exam-style
A farmer wants to maximize the amount of energy available for human consumption. Compare the efficiency of eating crops directly versus feeding crops to livestock and then eating the livestock. Use the concept of trophic levels in your answer.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Why food chains are short
Only a small fraction of the energy in one trophic level ends up in the next, typically around 10 %. The rest is lost because:
🧑🏫 Teacher's explanation
Think of energy like a bucket of water being passed down a line of people. Each person takes a big gulp to stay hydrated (respiration) and might spill some on the ground (excretion/egestion), and they might not even drink the whole bucket (parts not eaten). By the time the bucket reaches the fifth person, there is barely a drop left.
📖 Definitions
🔍 Worked example — Easy
A field of grass contains 50,000 ext kJ of energy. If the energy transfer efficiency to the next trophic level is 10\%, calculate the energy available to the primary consumers.
🔍 Worked example — Standard
Calculate the percentage of energy transferred from a population of rabbits to a population of foxes if the rabbits contain 1,200 ext kJ and the foxes contain 150 ext kJ.
🔍 Worked example — Exam-style
An ecosystem has a producer level providing 20,000 ext kJ of energy. The food chain is: Producer arrow Herbivore arrow Carnivore arrow Tertiary Consumer. Calculate the energy available to the Tertiary Consumer, assuming 10\% efficiency at every transfer.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Crops or livestock?
Feeding people on plants is more energy-efficient. If humans eat crop plants directly there is only one transfer of energy: producer to human. If the crop is fed to cattle and humans eat the beef there are two transfers: producer to cattle, then cattle to human. Energy is lost at each transfer, so with the extra trophic level far less of the energy in the crop reaches people.
🧑🏫 Teacher's explanation
Think of energy like money in a business. If you give a worker \100 to produce a product, they don't give you back \100. They spend some on rent, some on electricity, and some on food. By the time the finished product reaches the customer, there is much less money left. In biology, energy works the same way. Every time an organism uses energy, it 'spends' it on staying alive.
📖 Definitions
🔍 Worked example — Easy
A field of maize provides 50,000 ext kJ of energy. If 10\% of this energy is transferred to a cow, how much energy is available to the cow?
🔍 Worked example — Standard
Explain why energy is lost as heat during the transfer between trophic levels.
🔍 Worked example — Exam-style
A farmer uses land to grow crops for human consumption. Another farmer uses the same land to grow crops for cattle, which are then eaten by humans. Explain why the first farmer can feed more people than the second farmer.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Question. In a grassland ecosystem the energy in the grass is 20 000 kJ per m² per year. The energy in the grasshoppers that eat the grass is 1 800 kJ per m² per year, and in the birds that eat the grasshoppers 150 kJ per m² per year.
🔍 Worked example — Easy
In a food chain, the producers contain 50,000 kJ/m^2/ extyear of energy. The primary consumers contain 5,000 kJ/m^2/ extyear. Calculate the efficiency of energy transfer between these two levels.
🔍 Worked example — Standard
A food chain consists of: Grass arrow Rabbit arrow Fox. If the grass provides 4,000 kJ/m^2/ extyear and the rabbit provides 400 kJ/m^2/ extyear, calculate the energy available to the fox if the transfer efficiency from rabbit to fox is 10\%.
🔍 Worked example — Exam-style
A student is asked to explain why energy transfer between trophic levels is inefficient. Using the following data: Producer (10,000 kJ), Primary Consumer (1,000 kJ), Secondary Consumer (100 kJ), Tertiary Consumer (10 kJ), explain why a fifth trophic level is unlikely to exist.
⚠️ Common errors
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Practice
Answers