Out here in the stroma, a bunch of different enzymes use the carbon dioxide molecules and hydrogen ions made during the light dependent phase to assemble sugar fragments which are half of a glucose molecule (just 3 carbon atoms, instead of the 6 required to make a complete glucose molecule).
An alkane, by definition, only has single bonds. Even if your second diagram made sense (it doesn't for the reason above) it would not be an alkane at all because there is a double bond. Alkanes are a functional group. It's called alkil. As a functional group they have the generic formula R(CH2)nH.
A functional group is a portion of a molecule that is a recognizable/classified group of bound. atoms. In organic chemistry it is very common to see molecules comprised mainly of a carbon. backbone with functional groups attached to the chain. The functional group gives the molecule. its properties, regardless of what molecule contains it*; they are centers of chemical reactivity. The functional groups within a molecule need to be identified when naming. *Definition taken from Lecture Supplement, First Edition by Steven Hardinger, Copyright 2008.