How come large organisms are multicellular instead of just being comprised of one very large cell?

Answer 1

Transportation of oxygen, waste products and nutrients is one reason. Unicellular organisms are small enough that this can all occur by diffusion. Once you get above a certain (relatively small) size it is necessary to have specialised systems to achieve this. This has been achieved by the evolution of multicellular organisms.

Answer 2

Surface area and cell diffusion!
An large cell has a comparatively small surface area for diffusion of oxygen, hormones, chemicals etc and getting rid of waste products.

Some cells even increase surface area by having little attachments like the villi in intestinal cells!!! This increases absorbtion of "food"even better than before.

Things in the middle of a large cell will not be able to get required chemicals ( eg oxygen, ATP ( engergy) etc) and waste products would not be able to get out and cause toxic buildups and cause a multitude of probs,
THe cell size is actually incredibly similar in all organsims eg bacteria, fly, human, whale and even in plants too! This size is thought to be the optimal size, so that a whale and fly have the same SIZED cells, you just use more to "make"the whale.

Answer 3

If large organisms were just one cell, they couldn't have legs to walk around with. They could roll of course, but at that size the risk of popping on a sharp object would be very great. It's just evolution.

Answer 4

Additionally, how would a large single cell organism reproduce? Its nucleus and chromosomes all would have to be very, very large, relatively speaking. No, the laws of physics would not permit such a creature to develop and survive in our realm.

Answer 5

The bigger the cell, the less surface area they have, compared to their volume. In other words, since the more surface area you have, the more area you have for nutrients and all the other good stuff to get to the cell, you're increasing the demand for nutrients (by increasing volume), but since the volume equation has radius CUBED in it, and the surface area only has radius squared in it, you're making the volume so much bigger compared to the surface area, that cells can't get enough of the good stuff across their plasma membrane (which is only as big as the surface area), to satisfy the increased demand (because of the way bigger volume).