Because it determines if it is isolated or not. This means if other substances can enter or not. This usually relies on the membrane with it´s pores. The pores help the membrane be selectively permeable in what it lets in and out.
These are macrophytes that grow entire in the freshwater and become fossilized by the calcification oozes, going back to the Paleozoic era about 250 million years ago.
And they are unicellular to multi-cellular organisms also found in moist terrestrial habitats.
