Why animals cannot carry out the net conversion of CO2 into glucose?

Animals do not have a proper machinery to harvest energy from some other sources so that the process of making glucose from carbon di oxide can be completed. Autotrophs have special adapatations to use some other energy source to facilitate this conversion. Photoautotrophs have special pigments called chloroplasts that get excited after absorbing energy from light particles or photons and lose an electron. This lose electron serves as reaction driving force. Chemoautotrophs on the other hand have mechanisms to obtain this electron from chemical sources available in their environment like hydrogen sulphide gas in case of sulphur bacteria.
 
Due to absence of such methods animals are unable to make glucose in their body.