Energy: photosynthesis, chemosynthesis and respiration
Explain how marine producers capture light or chemical energy and how organisms transfer energy through respiration.
Section 7 · Energy: photosynthesis, chemosynthesis and respiration
Exam scope and assessment notes
This lesson maps Cambridge International AS & A Level Marine Science 9693 sections 7.1, 7.2, 7.3 for examinations in 2025–2030. AS candidates study Topics 1–5; A Level candidates study all nine topics and are expected to know the AS content. These are original StudyForA explanations and practice, not Cambridge questions, mark schemes or grades. The syllabus requires core practical activities and scientific-method experience. Online explanation and simulated data questions can rehearse methods but do not count as completing practical work or demonstrate safe, competent use of equipment. Learners need suitable teacher-supervised laboratory and field opportunities.
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 energy source supports chemosynthesis at a hydrothermal vent?
What you will learn
- Explain wavelength, intensity, penetration and the role of marine pigments
- Describe chloroplast stages, ATP and reduced NADP in photosynthesis
- Compare photosynthesis with chemosynthesis at hydrothermal vents
- Explain aerobic and anaerobic respiration, ATP and mitochondrial structure
Photosynthesis and light
White light has a range of wavelengths. Wavelength, intensity and penetration are different properties; water filters wavelengths with depth. Marine producers use chlorophyll a and accessory pigments such as xanthophylls and phycobilins to absorb light. In chloroplasts, the light-dependent stage at thylakoid membranes captures energy; ATP and reduced NADP support carbon fixation in the light-independent Calvin cycle in the stroma. The syllabus includes chloroplast membranes, thylakoids, thylakoid space, grana and limiting factors. Photosynthesis fixes carbon and transfers energy into organic molecules.
Chemosynthesis and respiration
At hydrothermal vents, chemosynthetic bacteria use energy from oxidising inorganic compounds, such as hydrogen sulfide, to fix carbon without sunlight. These producers support food webs in dark environments. Aerobic respiration releases energy from glucose using oxygen and produces ATP, carbon dioxide and water; mitochondria have outer and inner membranes, cristae and matrix. When oxygen is limited, some organisms use anaerobic respiration, which releases much less ATP per glucose. Respiration is not the same as breathing or gas exchange.
Sources for this lesson
Pause and explain
Where does the light-independent stage of photosynthesis occur?
Worked example
A deep-sea community near a hydrothermal vent receives no sunlight. Explain how a producer can still fix carbon and name an evidence measurement that could test the pathway.
Show the worked solution
- Identify chemosynthetic microorganisms as primary producers.
- Explain that they use chemical energy from inorganic compounds rather than light.
- Measure carbon fixation or biomass production while testing the availability/oxidation of a chemical energy source.
Answer Chemosynthetic bacteria can use energy from oxidising inorganic compounds, for example hydrogen sulfide, to fix carbon. A controlled measurement of carbon fixation or biomass with and without the chemical energy source could test the pathway; dark conditions alone do not prove chemosynthesis.
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 marine producer has access to blue light but little red light at depth. Explain why accessory pigments could affect its photosynthetic activity.
Give me a hint
Connect wavelengths absorbed by pigments with the light available at depth.
Compare my reasoning
- Recognise that wavelength penetration differs in water.
- Explain that pigments absorb particular wavelengths.
- Relate accessory pigments to capturing usable light, while noting other limiting factors.
Water absorbs wavelengths differently, so the available spectrum changes with depth. Accessory pigments can absorb wavelengths that chlorophyll a uses less effectively and transfer the energy into photosynthesis. Nutrients, carbon dioxide and temperature can still limit the rate.
Look for these in your work
- Distinguishes wavelength and penetration
- Explains pigment absorption
- Mentions other limiting factors
A cell produces 36 ATP from aerobic respiration but much less under anaerobic conditions. Explain the difference in energy yield without inventing an exact anaerobic value.
Give me a hint
Compare the role of oxygen and the extent of glucose breakdown.
Compare my reasoning
- State that aerobic respiration uses oxygen.
- Explain that anaerobic respiration yields much less ATP per glucose.
- Avoid giving an exact anaerobic yield unless supplied by the question.
Aerobic respiration uses oxygen to release more usable energy from glucose and produce more ATP. Anaerobic respiration occurs when oxygen is limited or absent and yields far less ATP per glucose; an exact value is not required here.
Look for these in your work
- Uses oxygen availability correctly
- Compares relative yield
- Avoids an unsupported exact value
Compare how a surface phytoplankton community and a hydrothermal-vent community obtain the energy that starts their food webs.
Give me a hint
Contrast light capture with chemical oxidation, then connect both to carbon fixation.
Compare my reasoning
- For phytoplankton, identify photosynthesis using light energy.
- For vent producers, identify chemosynthesis using chemical energy.
- Explain that both pathways can produce organic matter for consumers.
Phytoplankton use pigments to capture sunlight for photosynthesis. Vent bacteria use energy from oxidation of inorganic compounds for chemosynthesis. Both can fix carbon into organic biomass that supports consumers, but their energy sources differ.
Look for these in your work
- Identifies both producer pathways
- States the contrasting energy sources
- Connects production to food webs
Common mistakes
- Assuming every marine food web begins with sunlight
- Confusing light intensity with penetration or wavelength
- Treating ATP as matter that is recycled like nutrients
- Equating respiration with gas exchange
What can you explain now?
Where does the light-independent stage of photosynthesis occur?
Compare with the explanation
In the chloroplast stroma
State the mechanism, use relevant evidence and include suitable units. Explain what the evidence cannot establish.
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Make it stick
Why can chemosynthesis support a food web where photosynthesis cannot?
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.