Gas exchange, buoyancy and depth

Lesson page: echoesof.science/school/fish/ages-15-16/ · Pupil worksheet: echoesof.science/school/fish/ages-15-16/print/

Learning aims

Relate gill structure to diffusion of scarce dissolved oxygen.
Apply Archimedes' principle to floating and sinking.
Use p V = constant and the pressure-depth rule in calculations.
Plot and evaluate data for Boyle's law.

Lesson plan

Time	Activity
5 min	Starter: why do divers' ears hurt as they go deeper?
10 min	Read the text; quiz questions 1–3 as worked examples.
25 min	Syringe investigation and graph.
5 min	Breathing, floating or sensing? game.
5 min	Link the graph back to a fish at depth.

Materials for the experiment

50 ml plastic syringe with a sealed tip (cap or blocked with silicone)
Clamp stand, boss and clamp
Slotted masses (100 g to 1 kg) and a mass hanger or platform
Vernier calipers or ruler
Graph paper or spreadsheet

Safety: Clamp the stand firmly and keep feet clear of the masses; add masses gently. Do not point the syringe at anyone. Use only air in the syringe.

Common misconceptions

Pupils may think…	Actually…
Fish split water molecules to get oxygen.	They take in oxygen gas dissolved in the water.
Pressure doubles every 10 m.	Pressure rises by about 1 atm per 10 m: 2, 3, 4 atm…, so the volume falls to 1/2, 1/3, 1/4.
Buoyant force depends on the weight of the object.	It depends on the volume of water displaced.

Worksheet answers

Many filaments with thousands of lamellae give a very large surface area; thin membranes give a short diffusion distance; a constant flow of water over the gills keeps oxygen arriving.
At 20 m p ≈ 3 atm, so V = 9.0 ÷ 3 = 3.0 cm³. The fish displaces less water, so the buoyant force falls and it tends to sink unless it adds gas.
Displaced water 0.300 kg, so F_b = 0.300 × 9.8 ≈ 2.94 N; weight = 0.306 × 9.8 ≈ 3.00 N. Weight > buoyant force, so it sinks slowly.
A straight line through (or close to) the origin, so V ∝ 1/p. Uncertainties: friction of the plunger, leaks, reading the scale, temperature changes, the dead volume in the tip.

Quiz answers

A swim bladder holds 6.0 cm³ of gas at the surface (1 atm). If no gas is added, what is its volume at about 20 m depth? About 2.0 cm³. At 20 m p ≈ 3 atm, so V = 6.0 ÷ 3 = 2.0 cm³.[10][11]
A fish displaces 500 cm³ of fresh water. What is the buoyant force on it? (g = 9.8 N/kg) About 4.9 N. Mass of water = 0.50 kg, so F = 0.50 × 9.8 ≈ 4.9 N.[9]
What is the total pressure at 30 m depth in the sea? About 4 atm. 1 atm at the surface plus about 1 atm per 10 m.[11]
Below which dissolved oxygen level is water called hypoxic? 2 mg/L. Summer surface water typically has more than 8 mg/L.[6]
Why are gill lamellae good at gas exchange? Large surface area and thin membranes. Oxygen diffuses across the thin lamellae into the blood.[4]
Which sense organ lets sharks detect electromagnetic fields? Ampullae of Lorenzini. The lateral line detects movement and vibration.[1]

Going further

Find out how deep-sea fish and sharks cope without a gas-filled swim bladder, and compare with how scuba divers adjust their buoyancy.

Sources used in this lesson

[1] Biology 2e, 29.2 Fishes (OpenStax (Rice University)): reliability High
[4] Life in water: vertebrates, breathing (American Museum of Natural History): reliability High
[6] Dissolved oxygen and water (U.S. Geological Survey, Water Science School): reliability High
[9] Buoyancy: Archimedes' principle (NASA Glenn Research Center): reliability High
[10] Boyle's law (NASA Glenn Research Center (Beginner's Guide to Aeronautics)): reliability High
[11] How does pressure change with ocean depth? (NOAA National Ocean Service): reliability High

Full list with links and a note on each source's reliability: echoesof.science/school/fish/sources/

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