Marine physiology: cells, transport, gas exchange and water balance
Relate cell structure and exchange mechanisms to organism size, marine habitat, gas exchange surfaces and osmoregulation.
Exam scope and assessment notes
This lesson maps Cambridge International AS & A Level Marine Science 9693 sections 6.1, 6.2, 6.3, 6.4 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 transport process requires metabolic energy to move substances against a concentration gradient?
What you will learn
- Identify organelles and compare cells relevant to marine organisms
- Explain diffusion, osmosis and active transport across membranes
- Relate surface-area-to-volume ratio, ventilation and gill design to gas exchange
- Compare osmoconformers, osmoregulators, euryhaline and stenohaline species
Cells and movement across membranes
The nucleus contains genetic material; the cell membrane controls exchange; cytoplasm is the site of many metabolic reactions; mitochondria carry out aerobic respiration; chloroplasts carry out photosynthesis in photosynthetic cells. Diffusion is net movement down a concentration gradient. Osmosis is water movement across a partially permeable membrane from higher water potential to lower water potential. Active transport uses energy to move substances against a gradient. As organism size increases, surface-area-to-volume ratio tends to fall, so larger animals need specialised exchange surfaces and transport systems.
Gas exchange
Small organisms may exchange gases across the whole surface. Larger or more active organisms use specialised surfaces and ventilation. Coral polyps can exchange by diffusion; fish such as grouper use pumped ventilation, while tuna use ram ventilation as they swim. Gill structures provide a large, thin exchange surface. Relate the method to habitat, activity and oxygen demand rather than describing anatomy in isolation.
Osmoregulation
Osmoconformers have body-fluid water potential close to the surrounding seawater; osmoregulators regulate internal water and ion balance. Marine mussels can serve as osmoconformers; tuna regulate. Stenohaline species tolerate a narrow salinity range; euryhaline species tolerate a wider range. Salmon move between sea and fresh water and adjust ion and water movement across gills and kidneys. Explain direction of water movement from relative water potential and distinguish water balance from ion balance.
Sources for this lesson
Pause and explain
Why does a large active fish need specialised gas-exchange surfaces?
Worked example
A marine cell is placed in a more concentrated solution. Predict net water movement and explain one likely effect on cell volume.
Show the worked solution
- Compare the water potential of the cell and the external solution.
- Water moves by osmosis toward the lower water potential.
- Predict that the cell may lose water and shrink, depending on its regulation and membrane.
Answer Water moves out of the cell toward the more concentrated solution, so the cell may lose volume. The outcome depends on membrane permeability and any active regulation; distinguish osmosis from active transport of ions.
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 fish gill has thin lamellae and water flows across them. Explain two features that support oxygen uptake.
Give me a hint
Consider diffusion distance, surface area and how ventilation maintains a gradient.
Compare my reasoning
- Thin lamellae reduce diffusion distance.
- Many lamellae provide a large exchange surface.
- Water flow maintains a concentration gradient between water and blood.
Thin lamellae shorten the diffusion distance and numerous lamellae provide a large surface area. Ventilation continually brings oxygenated water, helping maintain a gradient for diffusion into blood. The exact efficiency also depends on blood flow and the exchange arrangement.
Look for these in your work
- Links thin surface to diffusion distance
- Links lamellae to area
- Explains gradient maintenance
Compare the expected salinity tolerance of a stenohaline tuna and a euryhaline salmon. What does each term mean?
Give me a hint
Define the range first, then connect it to the species example.
Compare my reasoning
- Define stenohaline as tolerating a narrow salinity range.
- Define euryhaline as tolerating a wider range.
- Relate salmon’s sea/freshwater life to its ability to adjust water and ion balance.
A stenohaline tuna tolerates a relatively narrow salinity range; a euryhaline salmon tolerates a broader range and can move between seawater and freshwater by adjusting osmoregulation. The terms describe tolerance range, not a guarantee that the species can withstand every salinity.
Look for these in your work
- Defines both terms
- Uses the named examples
- Avoids an unlimited-tolerance claim
A small coral polyp exchanges gases by diffusion, while a fast-swimming tuna uses ram ventilation. Explain how size and activity help explain this difference.
Give me a hint
Relate exchange demand and surface-area-to-volume ratio to the method.
Compare my reasoning
- A small polyp has a relatively high surface-area-to-volume ratio.
- A large, active tuna has higher oxygen demand and a lower ratio.
- Swimming forces water across gills, supporting gas exchange by ram ventilation.
A small polyp can exchange gases over much of its surface because its surface-area-to-volume ratio is relatively high and its demand is lower. A larger active tuna needs specialised gills and moves water over them as it swims; this ram ventilation supports greater exchange, but depends on movement and gill flow.
Look for these in your work
- Connects size with surface-area-to-volume ratio
- Connects activity with demand
- Explains ram ventilation
Common mistakes
- Calling all movement across membranes diffusion
- Confusing osmosis with solute movement
- Assuming every fish uses the same ventilation method
- Treating euryhaline as a synonym for osmoregulator
What can you explain now?
Why does a large active fish need specialised gas-exchange surfaces?
Compare with the explanation
Its low surface-area-to-volume ratio cannot meet exchange demands across the outer surface alone
State the mechanism, use relevant evidence and include suitable units. Explain what the evidence cannot establish.
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Make it stick
How do diffusion and active transport differ in direction and energy requirement?
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.