Why was 0.5g of substrate added to all of the tubes instead of varying amounts of substrate

answer: land bridge theory. land bridge theory held that shifting portions of dry land explained the finding of identical species on different continents. plate tectonic theory eventually eliminated the land bridge theory.

equal distribution i think.

many other transport proteins, often called pumps, move materials against a concentration gradient. active transport drives molecules across a membrane from a region of lower concentration to a region of higher concentration. this process, shown in figure 5.1, uses transport proteins powered by chemi- cal energy.

cell/ tissue/ organ/ organ system/ body