Subject: Digital Photos: how many bits in a pixel and how many pixels in a byte?

I have been reading about digital cameras and it is not clear what a general 
rule might be about how many Megabytes it takes to store a Megapixel (or how 
many bits there are in a pixel). In other words, If I have a 16 MB storage card 
on my digital camera and  the manufacturer says I can store four pictures that 
are taken at 2000X2000 Pixels (4MP), am I correct in assuming that
each picture consumed about 32 bits of information? (8 bits times 4 = 32 bits).
Can I also then assume that a pixel contains 32 bits of information? I f this 
is unanswerable because of my understanding of how the camera actually works, 
how does it work and is there an absolute way to convert Pixels to Bytes?
Thanks

Re: Digital Photos: how many bits in a pixel and how many pixels in a byte?