Figure 1.5 Open chain and ring structures of glucose (A) and fructose (B). Notice the two stereoisomers formed during cyclization of the open chain structure. A disaccharide (sucrose) formed from condensation of glucose and fructose (C).

Lipids

Lipids are amphipathic molecules with polar head groups and non-polar hydrocarbon region. Like carbohydrates, lipids also have both structural and storage roles in living systems. Apart from these, lipids play important roles in signal transduction pathways inside the cells. In vertebrates, for example, triacylglycerols are stored as fuels in specialized cells called adipocytes. Phosphoinositides, phosphorylated forms of phosphatidylinositol , are involved in cell signaling and membrane trafficking. Structural lipids perhaps constitute the most important class of the lipids because it is the structural lipids that define a cell. Figure 1.6A shows the structure of a glycerophospholipid, the most common structural lipid present in biomembranes.