Sample Response: Bacteria are helpful because they produce oxygen, which our bodies need to breathe, and
they help us to digest the food we eat. Bacteria are also
helpful because they are used in medicine to help us
overcome disease. Bacteria are harmful because they
can cause tooth decay and illnesses that can be either
common or quite serious.
What did you include in your response? Check all that
apply.
Bacteria produce oxygen.
O Bacteria help us digest food.
Bacteria can be used to produce medicine.
O Bacteria can cause tooth decay.
O Bacteria can cause illness.

yes? sounds right, but you didn’t ask a question.

cells use diffusion to get substances in and out of themselves. that diffusion will occur across the cell membrane or the outer covering of the cell. because we are talking about the surface of the cell, surface area and volume are  important factors here. as a cell increases in size, its surface area and volume increase as well.  we call that the surface area to volume ratio (sa: vol).  but the catch is that  volume increases at a greater rate than surface  area. in other words, as a cell gets bigger, the sa: vol gets smaller. so a really small cell like bacteria is mostly cell membrane (surface) with very little contents (volume). a really big cell like an egg, is mostly insides with a super thin membrane. so what ends up happening is that the cell cannot efficiently diffuse all of the stuff it needs in/out to survive. therefore cell size is limited to about 100mm. so how do we make organisms bigger than 100mm? cells divide! by dividing and becoming multicellular, organisms are able to grow!   check out the diagrams i attached to see the effect of sa: vol. i hope this ! good luck!