Maybe not impossible mode, but normal mode is. If you're about to die, simply right click and hit play. It'll end the level for you and take you to the upgrade screen. Easy way to rack up a bunch of points and then spend em all at once. Might want to fix this.
Untill then: 6/10. 5 points for quality of the gameplay, smoothness, and overall making of. You lose 2 points however for very little if any replay value, 1 point for bugs, and another point for little originality.
Let's face it, there's a ton of games out there almost exactly like this one, or with the same concept. Something falls from the top and you have to kill it before it reaches the bottom *yawn*. However, animation was well done, I always like this style of camera mode (where it follows you around in a much larger world like somebody's holding it... think Lakitu from Super Mario 64)
Suggestions for upgraded versions/new games: Make collectable one time/limited time use upgrades during levels; items such as shields (for this game maybe a shield would be a wall that the cells get trapped at for 3 seconds), a screen wide bomb, size increasing/decreasing items that last maybe 20 seconds, etc.
Another possibility is adding more npc's, such as a white blood cell that follows you around on some levels and hurts you if it touches you, or cells that take a few seconds to fully 'eat'. You could even throw in a 'boss' cell in one level just for kicks.
Anyhoo, that's just my two cents. If this is exactly the way you want your game to be and don't care for my ideas, all the power to you and it's well done (graphically).