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.
Rigorously speaking the functional group is not the whole molecule but only that collection of atoms. that provides a specific chemical function. For example, the chemical family of alcohols is. characterized by the function of the hydroxyl (OH) group, and aldehydes, ketones are characterized. by carbonyl groups (C=O). There are various combinations of hydroxyl groups in molecules along.