Lipids include fatty acids (making up fats and oils), steroids (including cholesterol), phospholipids and waxes. One function of lipids in the body is to serve as an energy reserve, others serve as components of cell structure and yet other lipids act as hormones and signaling molecules. Thus, the role lipids play in the human body depends on their structure and chemical composition.
Though the human body more readily and efficiently derives energy from carbohydrates, lipids provide more potential energy per gram, allowing for greater storage capacity. Lipids are stored in the adipose tissue, which humans like to refer to as “fat.” Adipose tissue also serves as a protective cushion for the organs and an insulating layer against heat loss.
When lipids are ingested, they are metabolized in the intestines into chylomicrons. These protein-lipid molecules are transported into cells by other lipoproteins for utilization or storage depending on the body’s needs. The liver regulates the concentration of lipids in the blood, with excess levels resulting in deposition in adipose tissue. Lipids are stored as triglycerides, which are chemically composed of three fatty acid chains. The process of metabolism, breakdown and recycling is presented in this tutorial by the University of Vermont.
Lipids involved in the transportation of other lipids are lipoproteins, best known as LDL, HDL and VLDL. These lipid-protein molecules contain cholesterol, which is probably most famously known in conjunction with the level of LDL (“bad cholesterol”) and triglycerides in the blood as a risk factor for heart disease. The lipoproteins are named based on how compact they are – low density (LDL), high density (HDL) and very low density (VLDL). To counteract the accumulation of the other lipoproteins, HDL acts like a sponge, absorbing excess lipids and cholesterol from physiological processes.
Phospholipids form the bilayer of cell membranes, making them essential to human, mammalian and even eukaryotic life. A tutorial from Davidson College shows the structure of the plasma membrane in detail. The chemical characteristics of the phospholipids allow them to create a semipermeable membrane that allows only particular molecules through to the interior of the cell. This regulation applies even to water, allowing for compartmentalization of the cells and the control of transport across their membranes.
Research has even found that the lipids in the cell membranes serve important functions in cell signaling and enzyme activities in cellular processes. A new avenue of investigation are lipid rafts – concentrated areas of membrane lipids that appear to play important roles in gene regulation and other cell signaling events.
Sex hormones and vitamins
Cholesterol is a steroid and serves as a precursor for androgens – better known as sex hormones, as well as vitamin D and cortisol, a stress hormone. According to the University of California, only about 15 percent of the cholesterol in the human body is ingested. Rensselaer Polytechnic offers a visual of the full chemical pathway for cholesterol synthesis.
Lipids in the brain
The membranes of the brain and nervous system tissue are made of lipids. Though the brain lacks triglycerides, lipids play important roles in signaling transduction and anchoring proteins, as discussed in Basic Neurochemistry. Because of the prevalence of membranes in the nervous system, a high concentration of lipids is found in that system. In recent years, research has found that many neurological disorders may actually have some basis in lipid imbalances.
Lipids in the human body
The various forms of lipids play many roles in the function of the human cell. As understanding of physiological processes involving lipids increases, the more evident their importance in the human body becomes.