Nutrients and energy
Trace how nutrients support organisms, how respiration and photosynthesis transform matter and energy, and how energy moves through marine food webs.
Exam scope and assessment notes
This original lesson maps Cambridge IGCSE Marine Science 0697 syllabus sections 4.1, 4.2, 4.3, 4.4 for examinations in 2024–2029. These are original StudyForA explanations and practice, not Cambridge questions, mark schemes or official grades. Paper 2 assesses practical skills in both familiar and unfamiliar contexts. This course can rehearse planning, measurement, data handling and evaluation, but it cannot provide real laboratory or coastal-field experience, verify equipment use, certify safety, or award a practical mark. Follow teacher supervision, local safety rules and environmental permissions for all practical work.
Before you begin
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Before you start: Which organisms can carry out cellular respiration?
What you will learn
- Identify major nutrients and food-test evidence
- Distinguish respiration from gas exchange
- Explain photosynthesis, producers and marine productivity
- Construct feeding relationships and account for energy transfer and loss
Nutrients and biological roles
Nutrients support growth, repair and energy. Food-test practicals investigate starch, sugars, proteins and lipids; use the correct reagent, sequence, control and safety procedure, then record observations before drawing conclusions. Carbohydrates provide energy; proteins support growth and repair; lipids store energy and insulate; vitamins and mineral salts are micronutrients.
Essential elements have biological roles. In marine systems, dissolved nutrients can limit productivity; upwelling may return nutrient-rich water to surface producers. A nutrient test is evidence for a substance under the tested conditions, not a complete analysis of a food sample.
Respiration and photosynthesis
Respiration is a series of cell reactions that releases usable energy from nutrient molecules; all living cells respire. Aerobic respiration: oxygen + glucose → carbon dioxide + water. Gas exchange is movement of gases between an organism and its surroundings, not the same process as respiration.
Photosynthesis uses light energy to make glucose from carbon dioxide and water, releasing oxygen: carbon dioxide + water → glucose + oxygen. Chlorophyll transfers light energy into chemical energy. Test how light intensity affects an aquatic plant or macroalga while controlling other conditions; productivity is the rate producers transfer energy into carbohydrates.
Feeding relationships and energy
The Sun is the main energy input to most marine biological systems. Food chains start with a producer and show energy transfer; food webs connect chains. A trophic level is an organism’s position. Consumers eat other organisms; decomposers obtain nutrients from dead or waste material.
Energy transfer is incomplete: organisms use energy in respiration and movement, lose heat, excrete or are not eaten. Carbon and nutrients cycle, but energy flows through systems and is dissipated. Explain both matter and energy when interpreting a food web.
Sources for this lesson
Pause and explain
Why is less energy usually available at a higher trophic level?
Worked example
In a marine chain, phytoplankton store 20 000 kJ of energy and small fish store 2 000 kJ. Calculate the transfer percentage and state one reason energy is not fully transferred.
Show the worked solution
- Divide the energy at the next level by energy at the previous level.
- Calculate 2 000 ÷ 20 000 × 100.
- Name a loss pathway such as respiration, heat, excretion or uneaten biomass.
Answer The transfer is 10%. Some energy is used in respiration and dissipated as heat; some biomass is not eaten or is excreted. The values describe the given system and do not establish a universal percentage for every marine chain.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A practical tests an aquatic plant at two light intensities. Name one variable to change, two conditions to keep constant and the result to measure.
Give me a hint
Make the comparison isolate light intensity and connect it to photosynthesis rate.
Compare my reasoning
- Change light intensity in measured steps.
- Keep plant species/size, water volume, temperature and carbon dioxide conditions consistent.
- Measure oxygen production or another suitable indicator over equal time intervals, with repeats.
Change light intensity; control plant material, temperature, water and dissolved carbon dioxide; measure oxygen output per unit time. Repeat each level and use a safe, teacher-approved method. A lamp can also heat water, so temperature must be monitored.
Look for these in your work
- Identifies independent and dependent variables
- Controls relevant conditions
- Plans repeated measurements and a heat check
Construct a food chain from phytoplankton, zooplankton and a small fish, then identify the producer and second consumer.
Give me a hint
Arrows show energy transfer from food to consumer.
Compare my reasoning
- Place phytoplankton first as the producer.
- Add zooplankton that feed on phytoplankton.
- Add the fish if it feeds on zooplankton; count consumer levels from the producer.
Phytoplankton → zooplankton → small fish. Phytoplankton are the producer; zooplankton are the primary consumer; the fish is the secondary consumer. The arrows point toward the organism receiving energy.
Look for these in your work
- Orders organisms correctly
- Points arrows toward consumer
- Names trophic roles accurately
A sample gives a positive starch test and a positive protein test. What can be concluded, and what cannot be concluded from these two tests alone?
Give me a hint
Report the tested evidence without extending it beyond the method.
Compare my reasoning
- State that evidence for starch and protein was detected under the procedures used.
- Do not infer exact concentration unless the method is quantitative and calibrated.
- Mention controls and reagent-specific observations needed to judge reliability.
The sample gave test evidence consistent with starch and protein under the procedures used. These qualitative tests alone do not give their concentrations, prove that other nutrients are absent, or rule out procedural error; controls and correct reagents matter.
Look for these in your work
- States the supported result
- Avoids quantitative overclaim
- Recognises controls and method limits
Common mistakes
- Equating respiration with breathing or gas exchange
- Reversing the photosynthesis equation
- Treating energy as recycled like nutrients
- Claiming every producer is a plant
What can you explain now?
Why is less energy usually available at a higher trophic level?
Compare with the explanation
Energy is used and lost through processes such as respiration and waste
Use the scientific relationship, relevant evidence and correct units. State the limit of any conclusion when the evidence does not support a universal claim.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Why is respiration not the same as gas exchange, even though both involve gases?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.