The gain on the sensor is increased making it more light sensitive and thus you are able to achieve different ISO speeds. The photocell size cannot be changed because it is hardware which means that noise will be the same but the frequency at which it occurs does change according to your ISO setting.
I read an article on the net where a photographer had taken pictures of the sky with traditional film at various ISO's and then in photoshop placed that scan over the photo, changed the mode to 'overlay' or something like that in order to emulate the grain of traditional film. I can't find the link to the website but I do have the PDF document that I downloaded from there. If you want it let me know and I'll email it to you.
THE PHOTOGRAPHER'S PRAYER; If I should die, O Lord, please don't let my wife sell the camera equipment for what I told her it cost us!!