Molecular biology can be confusing because it is a subject that focuses on things so small you can’t even see them so let’s begin with big organisms and work down to the molecular level organisms such as a little girl are made above organ systems such as the digestive system pictured here each of these organ systems are comprised of individual organ as shown in the diagram organs such as the liver the stomach and the small and large intestines are part of the digestive system all of these organs are made up of tissues