The proton in the nucleus of an atom consists of three quarks; two up quarks with a charge of 2/3 and a down quark with a charge of -1/3, together they form a proton of a positive charge of 1.
Quarks have so strong bonds (the virtual gluon particles), that they only exist together as triplets in baryons (protons and neutrons) or in more unstable doublets of a quark and an anti-quark (of a different type than the other quark, otherwise they would annihilate each other) called mesons.
A proton can decay into a neutron under certain conditions, where one up quark is changed into a down quark in a beta decay which is due to the weak fundamental force. But more about that in the next comic.
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