Lipids
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Lipids are a diverse group of biological molecules that do not mix with water. The key groups of lipids encompass fats, phospholipids, steroids and waxes. Lipids serve energy storage functions, are the major components of biological membranes, and act as precursors for hormones.
Many lipids can be synthesised by human cells. However, some essential lipids such as omega-3 and omega-6 fatty acids have to be obtained from diet.
A typical fat molecule is a triglyceride consisting of a glycerol part and three fatty acid tails. The long fatty acid tails provide fats with their hydrophobic properties.
In animals, fats serve as long-term energy storage molecules. Fats are mainly deposited in adipose tissue cells that provide thermal insulation and cushion the body's organs.
Phospholipids are the key components of cell membranes. Phospholipids are structurally similar to fats but only have two fatty acid tails. Instead of the third tail, phospholipids have a negatively charged phosphate group. This arrangement results in a dual nature: a strongly hydrophobic part ("tails") that repels water and a hydrophilic part ("head") that attracts water.
In an aqueous environment, phospholipids tend to form a bilayer: the hydrophilic heads face outward, interacting with water, while the hydrophobic tails are concealed within. Phospholipid bilayers comprise the plasma membrane and the internal membranes of eukaryotic cells.
Steroids are lipid molecules that have four fused hydrocarbon rings. A typical steroid is cholesterol – the component of animal cell membranes and a precursor of such hormones as estradiol and testosterone.
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