I don't know if my experience in the 80's is relevant but my UG college courses in the pre-req sciences (chem, o-chem, biochem, physics, calculus, biology, etc.) were way more scientifically rigorous (and academically difficult) than my MS 1&2 basic sciences, with the exception of anatomy & pathology, which are like nothing else in the UG curriculum. The med school science curriculum assumes you know the college subjects and goes on from there to bombard you with all sorts of "clinically relevant" details. When you take med school biochem, for example, you won't have to sit through exams and lab experiments where you have to show that you can work out difficult problems and compete with a hundred or so undergrads for that "A" you need to get into med school. You're already there! But there will be a ton of stuff to remember and it won't always be clear how it all fits together, unlike your college courses where learning tends to progress logically step by step, as when you study physics or chemistry. So it does require a slightly different approach.
The good news is that by the third year you get into the clinics and wards, and that's when all the basic science starts to come together and make good sense. All of this is to prepare you for your internship and residency, which is just a continuation of med school, designed to make you a practicing physician.
Everything learned along the way, from college, through med school, through post-graduate medical training is essential to producing competent physicians.
Some folks (Lewis Thomas for one) have criticized the emphasis on undergraduate science education in selecting medical students. You don't need to be a science major to be a good physician. You can major in Music or Literature. But there is a good reason for med students to demonstrate UG competence in the BCPM AMCAS "prerequisites." Med school basic science courses are not meant to be a mere summary rehash of UG science courses, and a med student without any experience in these subjects will find it extremely difficult to get through them.