( ( To ( understand ( how ( ( the ( plasma membrane ) ) ( controls ( what ( crosses ( ( ( into or ) out ) ( of ( the cell ) ) ) ) ) ) ) ) ) ) ( , ( you ( ( need ( to ( know ( its composition ) ) ) ) . ) ) ) )
(ROOT (S (S (VP (TO To) (VP (VB understand) (SBAR (WHADVP (WRB how)) (S (NP (DT the) (NN plasma) (NN membrane)) (VP (VBZ controls) (SBAR (WHNP (WP what)) (S (VP (VBZ crosses) (PP (IN into) (CC or) (IN out) (PP (IN of) (NP (DT the) (NN cell))))))))))))) (, ,) (NP (PRP you)) (VP (VBP need) (S (VP (TO to) (VP (VB know) (NP (PRP$ its) (NN composition)))))) (. .)))
To  understand how the plasma membrane controls what crosses into or out of the cell, you need to know its  composition.
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The plasma membrane controls what goes in and out of a cell.