Why are the tips of some airplane wings bent upward?

Those are called winglets. To be more specific, that plane (a Boeing 737) uses sharklets, which are a type of winglet that bend up at a pretty extreme angle.

Airplane wings work by creating an area of low pressure above the wing with an area of high pressure below the wing. This “sucks” the wing upward, as well as the aircraft it is attached to. The thing is that the higher pressure air below the wing will curl up and over the wing at the tip in order to try and balance the pressures (fluids of different pressures will always try and balance their pressures). This curling action generates wake vortices, which are vortexes that come off of the tip of the wing.

The problem with this is these vortices create drag, which pulls the plane backwards. That means the engines have to work harder to maintain the plane’s speed, burining more fuel and creating more pollution.

Winglets stop this by “blocking” the flow of high pressure air over the tip, reducing the amount of air that curls over the wing tip. This dramatically reduces the size of these wake vortices, which let’s the plane burn less fuel. It also makes flying safer, as flying into another plane’s vortices can cause a loss of control. By making the vortices smaller, they can dissipate quicker, so they pose less of a risk to other aircraft.

Wingtip fence on an Airbus A320.

“Raked” winglet on a Boeing 787. It doesn’t bend upwards, but it instead tapers off at a greater angle than the rest of the wing, achieving the same results.

Split winglet on Boeing’s new 737MAX.