Certainly in the world of living creatures scale does create important differences. In many respects, a mouse's anatomy is a carbon copy of that of an elephant, but, whereas a mouse can climb up a nearly vertical piece of rock without much difficulty (and even if it fell from a height many times its own size, it would not greatly injure itself), an elephant would not be able to perform such a feat. Quite generally, the effects of gravity are less important as we study smaller and smaller objects (be they living or inanimate).

Arriving at unicellular creatures, we see that for them there is no distinction at all between up and down. For them, the surface tension of water is a far more important force than gravity. For example, just observe that surface tension is the force that gives a drop of water its shape. Compared with the size of unicellular creatures, drops of water are very big; evidently, surface tension is very important at this scale.

Surface tension is a consequence of the fact that all molecules and atoms attract each other with a force that we call the Van der Waals force. This Van der Waals force has only a very short range. To be precise, the strength of this force over a distance r is roughly proportional to 1/r7. This means that if you reduce the distance between two atoms by one-half, the Van der Waals force with which they attract each other becomes 2 × 2 × 2 × 2 × 2 × 2 × 2 = 128 times stronger.