Fuel is stored in 3 tanks, however, contrary to popular belief, the 737–800 has 5 tanks. They are:
Fuel is stored in the wings for primarily 3 reasons
1. Fuel acts as a counter stress for the wings shortly after takeoff when the great stress of the aircraft’s mass acts on them. This prevents a large change in the wingdihedral angle. This effect is so great on the Boeing 747, that if only the centre tank was filled (leaving the wing tanks empty) and the plane would take off, the wings would simply snap.
Due to this reason, fuel is first consumed from the center tank and then the wing tanks. Conversely, during refueling, the wing tanks are filled initially and then the centre tanks.
2. Keeps the centre of gravity more or less in the desired position. If the tanks are at the nose or tail of the aircraft, there will be a large change of momentum as fuel is filled or consumed.
Longitudinal center of gravity is vital for an aircraft’s stability, and any large change in its position is not conducive for flying.
3. The weight of the fuel provides rigidity to the wing, thereby reducing wing flutter. Flutter is the vibration of the wings due to the airflow.
Large flutter is so hazardous that it can even result in total collapse of the wings. The following video explains how flutter acts on aircraft and other structures.