Both plant cells and animal cells contain mitochondria and yet they were not visible in the cells you viewed. does this mean that these organelles are not found in cheek and onion cells? explain

This is false and is not a very accurate analogy. While it is true that plant and animal cells both contain mitochondria, visualizing the organelles through a light microscope is almost impossible. The only organelles that can be appreciated through a light microscope are the nucleus, and the plasma and nuclear membranes.