I.up [am. angl. əp, brit. angl. ʌp]PRISL.up often appears as the second element of certain verb structures in English (back up, pick up, sit up, etc). For translations, see the relevant verb entry (back, pick, sit, etc).
I.against [am. angl. əˈɡɛnst, brit. angl. əˈɡɛnst, əˈɡeɪnst]PREDL.against often appears as the second element of certain verb structures in English (come up against, guard against, turn against, etc). For translations, see the relevant verb entry (come, guard, turn, etc).
If you look at evolution, biological evolution, most of the interesting things happen where two systems come up against each other and adaptation takes place.