Curls are over rated... you'll waste time trying to fit them into your workout when you should be concentrating on compound movements...

Biceps and triceps both are biarticulate muscles... (which means basically that they cross 2 joints)... In this case the elbow and the shoulder!

In more precision...

The Biceps muscle Originates on the coracoid process (outer & upper most bone you can feel on your shoulder) of the scapula & Inserts on the radius (bone on the thumb side of the forearm). It's actions are to flex (front raise) and abduct (lateral raise) the shoulder joint, flex (curl) the elbow joint, and supinate (twist from palm down to palm up) the forearm.

The Triceps Muscle Originates on the scapula and humerus (upper arm) & Inserts on the olecranon process (the pointy bone of your elbow). It's Actions are to extend (press down) the elbow joint & adduct (opposite of lateral raise like - decline flies) and extend (opposite of front raise - like straight arm press down) the shoulder joint

So in conclusion... doing bicep curls or tricep press downs isn't working the full muscles potential... only compound movements work biarticulate muscles to their full potential because that's what a compound movement is - an exercise which involves more than one joint.... again as I said you should be concentrating on compound movements.