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.
Here are two example monomers that contain this functional group: Two other functional groups you may see in polymerization reactions are the amine group, , and the alcohol group, . Below are examples of monomers containing these functional groups: Notice that each of these monomers contains two functional groups. That is, they are difunctional.