Question

what in cells are closely stacked,flattend sacs (plants only) ?

Answer 1

this would be the thylakoid membrane stacks or granum, found inside the chloroplasts.

for next time: "plants only" ALWAYS referes to chloroplast or cell wall, since those are the two differences between plant and animal cells. cell wall doesn't have membrane structures. so: google "chloroplast" and this will be your first hit: http://en.wikipedia.org/wiki/Chloroplast
now, look at the first picture, where you will see - tadaa - membrane stacks called granum :) recherche 101