Three dials that set the speed of crossing

Science · Organisms & Life Processes · ages 16-18

Name ______________________ Date ____________

Why is the wall of an alveolus (a tiny air sac in the lung) only one cell thick?
- To make the diffusion distance as short as possible
- To make the lung heavier
- To reduce the surface area
- To slow down gas exchange
Which change would make molecules diffuse faster across a surface?
- Make the surface area bigger
- Make the diffusion distance longer
- Make the concentration difference smaller
A bigger concentration difference between the two sides of a membrane makes the diffusion rate go up.
Circle one: True False
The distance molecules must cross gets longer and nothing else changes. What happens to the rate?
- It speeds up
- It slows down
- It stays the same
Human lungs have about 300 million tiny alveoli instead of one giant air sac. How does this help gas exchange?
- It gives a huge total surface area, which speeds up diffusion
- It makes the diffusion distance longer
- It lowers the concentration difference
- It has no effect on the rate
A gas diffuses across a membrane at 12 units per second. The membrane's thickness doubles and nothing else changes. What is the new rate, in units per second?
Answer: ______________
A gas diffuses across a membrane at 12 units per second. The membrane thickness doubles and nothing else changes. What is the new rate, in units per second?
Answer: ______________
Halving the diffusion distance would halve the rate of diffusion.
Circle one: True False
The diffusion distance across a barrier triples. The original rate was 9 units per second. What is the new rate, in units per second? Give your answer as a decimal.
Answer: ______________
The diffusion distance across a barrier triples. The original rate was 9 units per second. What is the new rate, in units per second?
Answer: ______________

Answer key

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

Three dials that set the speed of crossing W1-mt_6JreZAFeb--s1

To make the diffusion distance as short as possible · A thin wall keeps the distance gases must cross as small as possible, which keeps the rate high.
Make the surface area bigger · A bigger surface gives molecules more room to cross at once, so the rate goes up.
True · Concentration difference is one of the two quantities that boosts the rate when it grows.
It slows down · Distance is the odd one out, pulling the rate down as it grows.
It gives a huge total surface area, which speeds up diffusion · Millions of tiny sacs add up to a huge surface area, and a bigger surface area means a faster rate.
6 · Doubling the distance halves the rate, so 12 becomes 6.
6 · Doubling the distance halves the rate, so 12 becomes 6.
False · Distance and rate move opposite ways. Halving the distance doubles the rate instead.
3 · Tripling the distance divides the rate by 3, so 9 becomes 3.
3 · Tripling the distance divides the rate by 3, so 9 becomes 3.