The element that relates to triglycerides and phospholipids as their building blocks is carbon (C). Both triglycerides and phospholipids are types of lipids, which are composed primarily of carbon, hydrogen, and oxygen.
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Triglycerides consist of one glycerol molecule combined with three fatty acid molecules. Their structure primarily involves long carbon chains in the fatty acids.
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Phospholipids consist of one glycerol molecule, two fatty acid tails, and a phosphate group. Again, carbon is a significant component of the fatty acid chains.
In summary, carbon is a key element in the structure of both triglycerides and phospholipids, which can be considered their "polymer" or fundamental building block in terms of composition.