DIRECTIONS: As a living thing grows, its surface cannot keep up with its insides. Read, then answer.
Diffusion is quick across a hundredth of an inch and hopeless across an
inch. So a living thing can stay tiny, stay thin, or build something to carry supplies to the middle.
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Living thing
Stay tiny, stay thin, or build plumbing? And how?
Done for you
an amoeba, one cell across
Stay tiny. Every part of it is already at the surface, so diffusion is enough on its own.
1.
a flatworm, wide but paper-thin
2.
a jellyfish, big but mostly water with thin walls
3.
an insect, with air tubes running to every part inside
4.
you
A flatworm can be an inch wide and is never thick. Say why being flat solves the same problem that your blood solves.
Five Words, In Order
Grade 7 · Building a body · Page 2 of 3
DIRECTIONS: Put the five words in order. Each step can do something the one below cannot.
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This thing
Which level is it?
6.
one muscle cell
7.
a sheet of muscle cells all pulling the same way
8.
the heart
9.
the heart, the blood and every vessel, working together
10.
you
A heart is made of muscle tissue, which is made of muscle cells. Say what the heart can do that a bag of loose muscle cells could not — and what that says about the word "organ".
Four Problems, Four Systems
Grade 7 · Building a body · Page 3 of 3
DIRECTIONS: Being big creates problems a single cell never had. Name the system that solves each.
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The problem a big body has
Which system solves it, and roughly how
12.
food arrives in lumps far too big to cross any membrane
13.
the cells in the middle are nowhere near the outside world
14.
oxygen has to come in and carbon dioxide has to go out, constantly
15.
waste builds up everywhere at once
Every one of those systems ends by bringing something close to a cell and letting go. Say why "close" is the word that matters, and what happens in the last fraction of an inch.