One physics problem, four living answers

Science · Organisms & Life Processes · ages 16-17

Name ______________________ Date ____________

Which surface do insects use for gas exchange?
- Insect tracheae
- Lungs with alveoli
- Leaves with stomata
Which surface do fish use for gas exchange?
- Lungs with alveoli
- Gills with lamellae
- Tracheae with spiracles
The insect, fish, leaf and mammal surfaces in this lesson end up thin, large and moist.
Circle one: True False
Why can a large active mammal not rely on diffusion through its skin?
- Skin cannot touch air
- Volume and demand outgrow surface area, leaving the path too long and the area too small
- Mammals never need much oxygen
What does countercurrent flow achieve in fish gills?
- It holds the gradient favourable along the whole plate
- It stops water moving over the gills
- It thickens the gill walls
A carp in normal breathing ventilates its gills with water that washes back and forth like air in lungs.
Circle one: True False
Why do an insect and a fish end up with such different exchange surfaces?
- Insects never need oxygen
- Fish copied the leaves of plants
- Air tubes work in air while huge washed lamellae suit oxygen poor water
A student says gill blood and water flow the same direction in countercurrent exchange. What is wrong?
- Water never touches gills at all
- Opposite flow holds gradient
- Blood never enters the gills

Answer key

For grown-ups. Fold this page away before handing over the rest.

Insect tracheae · Branching air tubes carry air straight to the tissues.
Gills with lamellae · A carp in normal breathing holds feathery gill plates in a one way flow of water.
True · In these four groups the same three traits meet the same physics.
Volume and demand outgrow surface area, leaving the path too long and the area too small · Growth maths forces a large active mammal to fold exchange surfaces inside.
It holds the gradient favourable along the whole plate · Opposite flows mean water always meets blood with less oxygen.
False · For a carp in normal breathing, gill water flows one way only, unlike the in and out pumping of lungs.
Air tubes work in air while huge washed lamellae suit oxygen poor water · The medium decides the build: direct tubes for air, vast washed plates for water.
Opposite flow holds gradient · Same direction flow lets the gradient fall toward equilibrium, while opposite flow keeps uptake going.