Why are cells small and why not just have one big cell?

ancestors have the reasons why he created the microscopic cells....

The reason for many cells instead of one large one: Consider this, you get sick and the virus attacks the cell. It dies, and so do you. If you have many cells and many types of cells they can attack the infection before it spreads. Thus, you have an immune system.

I dont have the patience to type everything but the wiki page has a good general overview.

Basically as a cell get bigger it gets weaker, each cell is made up of components that work together to get things done, each having their own job to do. They in turn work with other cells living together to create a common goal. They are the masters of procreation in its truest form.

Without each cell doing its part and being the literal brick it is to build us we couldn't exist. Also, all life is form from the evolution of single celled organisms, so actually, we all were once just cells.

why must a building be comprised of smaller elements? why must a large company consist of hundreds of employees and not just one?

well every cell is controlled by a nucleus... where the DNA is stored, if the cell were too big then it would be too hard to direct that information around.... and to transport proteins around the cell, etc. By have billions of tiny cells, we are able to have specialized cells with special functions, such as liver cells or blood cells, etc and when these cells get too old (which doesn't take too long) they can be replaced without causing great damage. If we had only a few big cells, the loss of a cell when it is too old and needs to be replaced would create too much damage. Also, given the nature of cells, if humans were to be say just one big cell... we wouldn't be stable, our cytoplasm would keep spilling out all the time, our outer-coat or plasma membrane would rupture all the time.. etc.

hope this kind of answers your question basically it's just not biologically possible lol

imagine your self as one cell only. then there will come a day wherein a virus/bacteria or any other pathogen attacks you. eventually you'll die if you don't have enough defense, and imagine if you are just a single-celled organism, you don't have any antibodies to defend you, and you work just all by your self.

being multicellular is a really big benefit. billions or more cells work in your body. and being multicellular lessens the work load of any other cell in your body. and each of the cells in our body is specialized to do specific metabolic activities. for example, our blood is made of cells and these cells in our blood aids in transporting materials throughout our body. if you are single-celled, imagine doing this work alone. but cells do require some materials for them to work like nutrients from food, O2 we breathe, then the combination of these materials will produce an energy called ATP (adenosine triphosphate) in our body that the cells use in order to do their work.

hope you understand my explanation.

Smaller cells allow the organism to be able to do more functions in the human body. The more cells there are, the more chemical activity can take place. Cells also must maintain a large surface area to volume ratio, which allows more substances to leave in and out of cell. Cells must also have a small volume in order to maintain this larger surface area, which is why our cells are so tiny. If not, then the many essential functions of the human body could not be performed.

Hey, So vaporeon gave a pretty good synopsis of as to the why. Specifically, cells must be incredibly small because some very important functions within the cells, like  ion transport at least in part depend upon diffusion. As was pointed out the larger the cell becomes the volume to surface area ratio becomes more and more unfavorable. Making diffusion difficult. This is why we must breathe as humans. Were to thick to diffuse the oxygen to satisfy the trillions of catalytic reactions in the body.

Additionally, cells depend upon a lipid bilayer which is only a few micrometers thick to make up the cell membrane. The larger the cell, the more unstable it would become.

As far as requirements are concerned. In short they need ions, proteins, and nucleic acids.

I gotta go do Biochem homework, but if you want a more in depth answer just PM me.