The CCD typically uses reflective light that bounces off the image and thought the lens into the CCD scan head. The CCD scan head converts the different light intensities to varying voltages, which are sent to the analog to digital converter. Different voltages are converted into 1's and 0's which the computer uses to produce the different shades. The more diodes in the CCD array the more information captured. In other words you only need 300 dpi for a printing press utilizing 150 lpi screening.
Higher resolutions are only necessary when scaling up or blowing up images.
A 35 mm slide is about 1' x 1' in size. If scanned at 2000 dpi at the 1' x 1' size, blowing it up to 2' x 2' would result in 1000 dpi, scaling it to 4' x 4' results in 500 dpi and a 8' x 8' final size would result in 250 dpi. And since printing at 133 lpi requires about 266 dpi, you would have enough information. Looking at input resolution can be misleading too, because there is true resolution and interpolated resolution and most companies quote interpolated. True resolution is the actual or optical resolution captured.