Phospholipids are used to construct plasma membranes. While there are many types of phospholipids in a plasma membrane, we are going to represent all of them in Petri Dish with a single generic phospholipid, phosphatidic acid. Phosphatidic acid can be used as a precursor to build other types of phospholipids, if necessary.

Phospholipid synthesis begins by diverting acetyl and glyceraldehyde-3-phosphate from the cellular respiration pathway. Acetyl is used to build malonyl in step 1, which is then used in steps 2-8 (known as the fatty acid cycle) to build palmitic acid with a 16-carbon chain. Glyceraldehyde-3-phosphate is converted into glycerol-3-phosphate in step 9, and two palmitic acids are attached to glycerol-3-phosphate to form a phospholipid in step 10.

Step Reactants Products
1 C2H3O- (acetyl) + CO2 C3H3O3- (malonyl)
2 C2H3O- (acetyl) + C3H3O3- (malonyl) C4H7O- + CO2 + H2O
3 C4H7O- + C3H3O3- (malonyl) C6H11O- + CO2 + H2O
4 C6H11O- + C3H3O3- (malonyl) C8H15O- + CO2 + H2O
5 C8H15O- + C3H3O3- (malonyl) C10H19O- + CO2 + H2O
6 C10H19O- + C3H3O3- (malonyl) C12H23O- + CO2 + H2O
7 C12H23O- + C3H3O3- (malonyl) C14H27O- + CO2 + H2O
8 C14H27O- + C3H3O3- (malonyl) C16H31O- (palmitic acid) + CO2 + H2O
9 C3H5O6P2- (glyceraldehyde-3-phosphate) C3H7O6P2- (glycerol-3-phosphate)
10 2 C16H31O- (palmitic acid) + C3H7O6P2- (glycerol-3-phosphate) phospholipid (phosphatidic acid)

The metabolic pathway for building phosphatidic acid in Petri Dish has been simplified. Here is a more detailed version of the actual pathway:

Step Reactants Products
1 acetyl-CoA + bicarbonate malonyl-CoA
2 acetyl-CoA + ACP acetyl-ACP + CoA
3 malonyl-CoA + ACP malonyl-ACP + CoA
4 acetyl-ACP + malonyl-ACP C4H5O2S-ACP + HS-ACP + CO2
5 C4H5O2S-ACP + NADPH + H+ C4H7O2S-ACP + NADP+
6 C4H7O2S-ACP C4H5OS-ACP + H2O
7 C4H5OS-ACP + NADPH + H+ C4H7OS-ACP + NADP+
Steps 4-7 are repeated to build the 16-carbon chain…
28 C14H27OS-ACP + malonyl-ACP C16H29O2S-ACP + HS-ACP + CO2
29 C16H29O2S-ACP + NADPH + H+ C16H31O2S-ACP + NADP+
30 C16H31O2S-ACP C16H29OS-ACP + H2O
31 C16H29OS-ACP + NADPH + H+ C16H31OS-ACP + NADP+
32 C16H31OS-ACP + CoA palmitoyl-CoA + ACP
33 glyceraldehyde-3-phosphate dihydroxyacetone phosphate
34 dihydroxyacetone phosphate + NADH + H+ glycerol-3-phosphate + NAD+
35 glycerol-3-phosphate + palmitoyl-CoA lysophosphatidic acid + CoASH
36 lysophosphatidic acid + palmitoyl-CoA phosphatidic acid + CoASH