Describe how lichen are used to indicate air quality

Lichens are composite organisms in which a single species of fungus (mycobiont) lives symbiotically with one or more algal species (phycobionts), some of which may be nitrogen (N) fixing (cyanolichens), e.g. members of the blue green algae. The fungus provides structure and protection for the algae which reciprocates, providing energy and assimilates, via photosynthesis. Many secondary metabolites are synthesised by the fungus, which are unique to lichen symbioses.

In Europe lichens have been used as sensitive bioindicators of air quality for more than a century. As with most vegetation, lichens show a range of sensitivities to pollutants. Not all lichens are sensitive to a particular pollutant, some can be remarkably tolerant. Among lichens are species that are sensitive to sulphur, nitrogen, acidity, halogens (e.g. fluoride), heavy metals and ozone.

youre right there are

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!