Chapter 10 LIPIDS
Biological lipids are a chemically diverse group of compounds, the common and defining feature of which is their insolubility in water. The biological functions of the lipids are as diverse as their chemistry. Fats and oils are the principal stored forms of energy in many organisms. Phospholipids and sterols are major structural elements of biological membranes. Other lipids, although present in relatively small quantities, play crucial roles as enzyme cofactors, electron carriers, light-absorbing pigments, hydrophobic anchors for proteins, “chaperones” to help membrane proteins fold, emulsifying agents in the digestive tract, hormones, and intracellular messengers. This chapter introduces representative lipids of each type, organized according to their functional roles, with emphasis on their chemical structure and physical properties. Although we follow a functional organization for our discussion, the thousands of different lipids can also be organized into eight general categories of chemical structure (listed in Table 10-2, at the end of this chapter). We discuss the energy-yielding oxidation of lipids in Chapter 17 and their synthesis in Chapter 21. Here, we will focus on four principles of cellular lipid function:
Fatty acids are water-insoluble hydrocarbons used for cellular energy storage. Fatty acids are highly reduced and thus provide a rich source of stored chemical energy for cells. Storage of hydrophobic fats as triacylglycerols is also highly efficient because water is not needed to hydrate the stored fats.
Membrane lipids are composed of hydrophobic tails attached to polar head groups. Cellular membranes are composed of a variety of lipids, including glycerophospholipids and sterols. These lipids are used for structuring membranes as well as for displaying molecules on the membrane surfaces for signaling and molecular recognition.
Lipids have uses in the cell beyond energy storage and construction of membranes. Many lipids are present in the cell at smaller amounts than those making up membranes or being stored as fat. These lipids can function as cellular messengers, hormones, electron carriers, or pigments.
The chemical properties of lipids are related to their structure and composition. As in studies of other biomolecules, methods such as enzymatic, chromatographic, and mass spectrometry can all be used to identify lipids and determine their atomic structure.
Fatty acids are water-insoluble hydrocarbons used for cellular energy storage. Fatty acids are highly reduced and thus provide a rich source of stored chemical energy for cells. Storage of hydrophobic fats as triacylglycerols is also highly efficient because water is not needed to hydrate the stored fats.
Membrane lipids are composed of hydrophobic tails attached to polar head groups. Cellular membranes are composed of a variety of lipids, including glycerophospholipids and sterols. These lipids are used for structuring membranes as well as for displaying molecules on the membrane surfaces for signaling and molecular recognition.
Lipids have uses in the cell beyond energy storage and construction of membranes. Many lipids are present in the cell at smaller amounts than those making up membranes or being stored as fat. These lipids can function as cellular messengers, hormones, electron carriers, or pigments.
The chemical properties of lipids are related to their structure and composition. As in studies of other biomolecules, methods such as enzymatic, chromatographic, and mass spectrometry can all be used to identify lipids and determine their atomic structure.