Fatty acids are synthesized by the condensation of two-carbon units. Furthermore, in  terms  of  the  enzymic  steps  contained,  the  procedure  is  not  the  reverse  of  β- oxidation.  Fatty  acid  synthesis  contained  a  separate  series  of reactions  to build up long-chain  hydrocarbons  from acetyl CoA units. The key differences between fatty acid synthesis and breakdown are:

a) Whereas their degradation occurs in the mitochondria of eukaryotes, fatty acid synthesis happens in the cytosol of both eukaryotes or prokaryotes;

b) Whereas   NADH   is produced in β-oxidation, fatty acid  synthesis   uses  NADPH   as  the  reductant   ;

c)  During  their  synthesis,  fatty  acids  are  covalently  attaches  to  an ACP  (acyl  carrier protein) as opposed to CoA in their degradation;

d) Whereas  in  β-oxidation  the  individual  activities  are  present  on separate enzymes the enzyme activities  of fatty acid synthesis  in higher organisms  are present in a single,  multifunctional  polypeptide  chain (as a dimer)  called as fatty acid synthase, .