Why are single celled organisms are microscopic in size?

I am not sure how to answer this question and would be very thankful for any help!

I am also looking for good websites on cells on the internet to revise and to do some activities. (Generally for Y11 Biology)
It would be really great if you could name me some websites that would help me to increase my understanding of Biology.

Thank you so much!

1 Answer

Cells need to be small otherwise they would not be conducive to life. Nutrients and wastes need to be able to enter and exit quickly (if a cell is large then things take longer to move from one side to the other).