Using a deep skillet, bring quart of salted water to boil.
While the waiting for the water to boil, stir together ricotta, parmesan, salt, pepper, and red pepper flakes in a bowl large enough to toss the finished pasta. Zest the lemon and add to the ricotta mixture. Cut the lemon in half. Add the juice of half the lemon (no seeds!) to the sauce, and cut the other half into wedges for serving. Put the bowl with the cheese mixture next to the stove.
When the water is boiling, add the pasta. Cook according to package directions. As soon as the pasta is done, use tongs to transfer the cooked pasta directly into the ricotta mixture. Some pasta water with cling to the noodles, and that will emulsify the sauce. Immediately toss the noodles with the ricotta mixture until the sauce is creamy and the pasta is completely coated. Add ¼-½ a cup of pasta water and toss again. If the sauce still seems dry, add a little more pasta water.
Serve with lemon wedges.