A cell is wrapped in a membrane that lets some things through and stops others. Working out which is which, and when the cell has to spend energy to do it.
Not started yet.
Students will learn that the cell membrane is selectively permeable, and that materials cross it in two different ways: passive transport, which costs the cell nothing, and active transport, which costs it energy.
Diffusion is the movement of molecules from an area where they are crowded to an area where they are less crowded. It keeps going until equilibrium is reached, which means the molecules are spread evenly. Osmosis is the same process, but specifically the diffusion of water.
Passive transport needs no energy from the cell because the molecules are already moving on their own. Active transport does need energy, because the cell is moving material the opposite way, from less crowded to more crowded. Very large particles are taken in by endocytosis and pushed out by exocytosis.
The book's mesh bag is worth doing for real. A colander, some marbles or large beads, and a cup of sand or rice will show the whole idea in about ten seconds: the big things stay, the small things fall through, and nobody had to push.
The hardest idea here is that passive transport is free. Students expect movement to cost something. Point out that the molecules were already moving before the cell got involved - the cell is not pushing them, it is simply not stopping them.
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Before You Start
The words this lesson uses. Tap a card to see what it means, then come back to them whenever a question uses one.
Part One
Questions about the story you just read. Pick your answer straight off, or press Find it in the story if you would rather hunt for the sentence first.
Part Two
The word cards are at the top of this page. Look back at them as often as you need, then answer.