Let us make an in-depth study of the breakdown or degradation of fats in plants.
Like many other metabolites fats also exist in dynamic state in plants i.e., at one time they are being synthesized and at other times broken down to meet specific requirement of the cells.
Active breakdown of fats (which are insoluble) takes place:
(1) During the germination of fatty seeds so that the decomposition products may enter into glycolysis and Krebs’ cycle to release energy and also to synthesise soluble sucrose through glyoxylic acid cycle which is then trans located to the growing regions of the young germinating seedling till it develops green leaves to manufacture its own food.
(2) In plants, when carbohydrates reserve declines, the fats (also the proteins), may form the respiratory substrates which are broken down and oxidised to release energy.
The fats are first hydrolysed in the presence of the enzymes lipases to yield fatty acids and glycerol.
The above hydrolysis takes place in 3 steps. The first two steps are reversible in which diglyceride and monoglyceride are produced as intermediates. The third step is irreversible which completes the process of hydrolysis. The hydrolysis of fats (triglycerides) is accelerated by the presence of Ca++ ions.
(iii) Monoglyceride → Glycerol + Fatty acid
The first two steps occur in cytosol while the third step occurs both in cytosol and glyoxysomes.
Oxidation of Glycerol:
The glycerol may react with ATP under the catalytic influence of glycerol kinase to form glycerol-3-phosphate which is then oxidised in the presence of glycerol-3-phosphate dehydrogenase and NAD+ to produce dihydroxyacetone phosphate and enters into glycolysis.
This conversion of glycerol into pyruvic acid which takes place in cytoplasm yields 2ATPs by substrate level phosphorylation and 2 NADH which on re-oxidation by terminal electron transport chain via the external NADH dehydrogenase (located on the outer surface of the inner mitochondrial membrane in plants) further generate 4 ATP molecules (2 mol./NADH oxidised).
If the pyruvic acid also undergoes complete oxidation into CO2 and H2O in Krebs’ cycle (or TCA cycle) it will produce another 15 ATP molecules. Thus a total of 2 + 4 + 15 = 21 ATPs are produced per glycerol molecule oxidised. However, there is consumption of 1 ATP molecule in the glycerol kinase catalysed reaction. Therefore, the net gain is 21 – 1 = 20 ATPs per glycerol molecule oxidised.
The glycerol produced after the hydrolysis of triglycerides in spherosomes or of monoglycerides in glyoxysomes diffuses out into cytosol and may also be utilised in the synthesis of glucose and other carbohydrates first by converting into dihydroxyacetone phosphate (as described earlier) and then by reverse reactions of glycolysis (although with slight modification) as follows: