Starch biosynthesis lipids

Poly(3HB) synthesis in various subcellular compartments could be used to study how plants adjust their metabolism and gene expression to accommodate the production of a new sink, and how carbon flux through one pathway can affect carbon flux through another. For example, one could study how modifying the flux of carbon to starch or lipid biosynthesis in the plastid affects the flux of carbon to acetyl-CoA and poly(3HB). Alternatively, one could study how plants adjust the activity of genes and proteins involved in isoprenoid and flavonoid biosynthesis to the creation of the poly(3HB) biosynthetic pathway in the cytoplasm, since these three pathways compete for the same building block, i. e., acetyl-CoA. 

It has, thus, been demonstrated that redirecting the poly(3HB) biosynthetic pathway from the cytoplasm to the plastid resulted in an approximate 100-fold increase in poly(3HB) production [24]. However, it must be kept in mind that the rate of poly(3HB) biosynthesis in A thaliana leaves was relatively low, since poly(3HB) accumulated progressively over 40-60 days to reach 10-14% of the dry weight, whereas synthesis of starch can reach 17% dry weight for a 12 h photoperiod and seed storage lipids can reach 8% dry weight per day. 

Besides a source of energy, organisms require a source of materials for biosynthesis of cellular matter and products in cell operation, maintenance and reproduction. These materials must supply all the elements necessary to accomplish this. Some microorganisms utilize elements in the form of simple compounds, others require more complex compounds, usually related to the form in which they ultimately will be incorporated in the cellular material. The four predominant types of polymeric cell compounds are the lipids (fats), the polysaccharides (starch, cellulose, etc.), the information-encoded polydeoxyribonucleic acid and polyribonucleic acids (DNA and RNA), and proteins. Lipids are essentially insoluble in water and can thus be found in the nonaqueous biological phases, especially the plasma and organelle membranes. Lipids also constitute portions ofmore complex molecules, such as lipoproteins and liposaccharides. Lipids also serve as the polymeric biological fuel storage. 

Plants (and other autotrophs) can use CO2 as the sole source of the carbon atoms required for the biosynthesis of cellulose and starch, lipids and proteins, and the many other organic components of plant cells. By contrast, heterotrophs cannot bring about the net reduction of CO2 to achieve a net synthesis of glucose. 

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