Differentiation produces specialised cells with structures suited to particular functions. Most nucleated body cells inherit the same genome, but express different combinations of genes. Different proteins therefore give cells different properties. A gene being switched off does not normally mean it has been removed from the DNA. Environmental signals can also influence expression.
Similar cells working together form tissues; several tissues form an organ; interacting organs form a system. Systems are interdependent. For example, digestive absorption provides nutrients, lungs provide oxygen, circulation transports both, and excretory organs remove metabolic wastes. A treatment that restores one organ function may improve the whole organism, but it does not automatically replace all functions of that organ.
Example 1
Two cells contain the same genes, but only one produces a muscle contraction protein. What differs?
Expression of the relevant gene differs; different expressed proteins support different functions.
Example 2
Classify a heart containing muscle, connective and nervous tissues.
The heart is an organ because different tissues cooperate in a specific function.
Watch out: Most nucleated body cells share the genome; different patterns of expression produce specialisation.