Checkerboard background is transparent but doesn't have alpha channel?

Question

I've had a hard time to understand what is the relationship between transparent and alpha channel in Photoshop for a PNG image, I have this screen shot you can see I have a checkerboard background which doesn't have RGB value,my question is

1.When I save this as a PNG image, the checkerboard is transparent, but you can see I do not have alpha channel, why I can have transparent when there is no alpha channel?

2.Is this checkerboard area has the same effect being as a transparent area as having alpha channel?

3.Will this checkerboard area takes up the storage space on my hard drive if no RGB information is stored?

So can you explain to me what the checkerboard area is if there is no color information on this area, I really do not understand what this area really is.

Answer 1

Png isn't capable of storing true alpha channels. It uses the 4th channel as part of the same layer, with 'transparency' instead of a real alpha layer.

From Adobe KB

PNG does not support arbitrary alpha channels like other formats such as TIFF. PNG specifies that the fourth channel in a file is transparency, and only transparency. When you open a PNG file with transparency in Photoshop, it is considered a single layer image. It is not a flat background image. Alpha channels can contain anything, while transparency is a specific channel relationship. You can have multiple alpha channels per document, but only one transparency channel. Photoshop handles transparency and alpha channels separately. Transparency from an existing PNG can be edited in Photoshop by creating a layer mask from the transparency data.

Photoshop will actually not let you save an image with only a transparency channel, as it considers it to be 'empty'. You can get around this if you really need to by using Save As… or the Legacy Save for Web exporter, which do allow it.