The Sun — the principal source of energy
Statement B18.1.1 wants the exact answer the Sun. The Sun releases light energy, which travels to Earth and is absorbed by producers (green plants and algae). A producer is an organism that captures that energy, not a source that supplies it; light is the form the energy takes, not the object emitting it. Almost every food chain on Earth traces back to this single input.
Light energy and chemical energy — the two forms
Statement B18.1.2 names two forms of energy that must both appear in a full answer. Light energy arrives from the Sun and is absorbed by chlorophyll in producers; during photosynthesis chlorophyll transfers it into chemical energy, stored in the carbohydrates, proteins and fats of the producer's biomass. Feeding passes that chemical energy into the consumer's biomass, and respiration in every living cell releases it to power life processes.
Heat loss — why energy flow is one-way
At every stage some chemical energy is converted to heat (thermal energy) and transferred to the surroundings, mainly by respiration but also by movement and other life processes. Organisms cannot recapture that heat, so energy does not cycle the way matter does: carbon atoms are recycled, but an ecosystem needs a continuous input of light energy from the Sun to keep its energy flow going.
Drawn from real examiner reports.
The Sun, not "the producer"
Asked to state the principal source of energy, candidates name the producer — confusing the organism that captures the energy with the source that supplies it. A producer is simply the first organism in a food chain to receive the Sun's energy, converting light energy into chemical energy by photosynthesis; it generates none of that energy itself. The answer is the Sun.
Flagged Jun 2023 P11 Q12, P42 Q7iii · Nov 2023 P32 Q1c, P41 Q10c
"Light" alone is not the source
Writing just "light" when asked for the source of energy is too vague to credit: light must come from somewhere, and the mark scheme wants the object that emits it. Light energy is the form the energy takes on its journey to Earth, not its origin. Name the Sun first, then describe the light energy it supplies if the question asks how that energy arrives.
Flagged Jun 2023 P11 Q12, P42 Q7iii · Nov 2023 P32 Q1c, P41 Q10c
Food-chain arrows point at the eater
An arrow in a food chain always points the way energy flows — from the organism being eaten toward the organism eating it. In grass grasshopper frog, the first arrow means energy passes from the grass into the grasshopper, not that the grass eats the grasshopper. Reversing the arrows inverts the meaning of the whole diagram.
Flagged Nov 2022 P31 Q10ai
"Plants make food from light" is vague
A full description of how energy enters living organisms must name the pigment and the conversion: chlorophyll transfers light energy into chemical energy, which is used to make carbohydrates during photosynthesis. Answers such as "plants make food using light" or "plants get energy from the Sun" omit both of those marking points, which are credited separately.
Flagged Jun 2022 P43 Q4d · Nov 2022 P41 Q4cii (B6 — same energy conversion)
Energy does not cycle like carbon
Carbon atoms are recycled — the same atoms pass repeatedly between the atmosphere and organisms by photosynthesis, respiration, feeding and decomposition. Energy is not: once chemical energy has become heat in the environment, no living organism can turn it back into chemical or light energy. Hence an ecosystem needs constant sunlight but no new carbon.
Heat is not the only energy loss
Not all the energy missing at each stage was lost as heat. Energy also stays in parts of the organism that are never eaten, and in material that is eaten but not digested and leaves the body in faeces; excretory products carry energy away too. Give both routes — respiration/heat and uneaten or undigested material — when explaining why transfer is inefficient.
Flagged Jun 2023 P42 Q7a · Nov 2023 P11 Q12
Name the form of energy at each step
Say which form is present at every stage — light energy, chemical energy, heat (thermal energy). Writing just "energy" throughout is marked as vague and loses the linked marks, even when the sequence of events described is entirely correct.
Write the flow as one ordered chain
Describe-the-flow questions are marked step by step, so answer as a continuous chain: Sun, chlorophyll absorbs the light, photosynthesis stores chemical energy, feeding passes it on, respiration releases it, heat goes to the surroundings. Match the number of steps to the tariff.
Percentage transfer: part over whole
An energy-transfer percentage is always the energy in the later organism divided by the energy in the earlier one, times 100. Dividing the wrong way round gives an answer above 100%, which is an immediate signal that the fraction has been inverted.
State the principal source of energy for biological systems.
State the principal source of energy for biological systems, and explain why "the producer" is not an acceptable answer to this question. (2 marks)
