Cells use a phospholipid bilayer as a membrane to bind together the contents of the cell. In order to transport substances into and out of the cell, proteins serve as doors in the cell membrane. Biologists call this the cell wall transport system.
In each of the following puzzles, the goal is to maneuver all the blue balls (objects) into the blue circles (receptors) by moving the black ball.
In order to move around this cellular maze, you must use the cell wall transport proteins (arrows). Like their real-life counterparts, these transporters facilitate movement of objects according to certain rules:
Uniport proteins allow only one-way movement of an object. In the puzzle, these will serve as one-way doors for the black ball.
Symport proteins allow an object to pass only when it is accompanied by a partner. You can move the black ball through a symporter only if a blue ball moves with you.
Antiport proteins allow one object to enter the cell only if another leaves. You can move the black ball through an antiporter only if there is a blue ball on the other side to switch places with you.
Remember, the blue balls can only move in conjunction with the black ball. They cannot move alone. The receptors (the blue circles) cannot move. The puzzle is complete only once all of the receptors are filled by one blue ball each.
Here is the above example puzzle again.
We can initially move the black ball through one of the two uniport proteins.
However, if we move up, we cannot return through the same uniport protein, and we cannot move to the right through the symport protein, as we do not have a blue ball to accompany the black ball. We're stuck.
Instead, we can move to the right though the uniport protein, then up through the antiport protein, switching places with the blue ball in the top middle cell.
From here, we can move right and down through consecutive uniport proteins, then left through another antiport protein, once again trading places with the blue ball.
This completes the puzzle, as the sole receptor is now filled with the blue ball.
You are not exclusively trying to carry the blue balls around with you to fill the receptors. Sometimes you will move a blue ball with you through a symport protein, but sometimes you will switch places with a blue ball through an antiport protein. There are many ways to sequence symport and antiport proteins to move the blue balls through the space of a puzzle.
Because there is no set finish line, it may be difficult to determine where to go while solving the puzzle. The goal is to move all of the blue balls to the receptors, and while it may seem that there could be many ways to do this, each puzzle is designed so there is only one sequence of moves that can accomplish this.
One way to think about each puzzle is to consider which blue balls could be moved to each receptor. With a specific pairing of blue balls to receptors, you can then figure out how to order the required moves to move the blue balls to their assigned receptor.
Another way to think about each puzzle is to consider how the black ball can access each part of the puzzle, and how that affects the arrangement of the blue balls. Sequencing of moves is especially important, as some moves may not make sense in and of themselves, but those moves reposition the blue balls for future moves.