Lipids, driven by their hydrophobic and hydrophilic portions, form the biological membranes found in all living creatures. These include the cell and nuclear membranes as well as membranes that are part of organelles such as mitochondria and the endoplasmic reticulum (ER). Membranes provide separation of different environments to permit a variety of biological functions. Membranes are not static structures: rather they are dynamic and fluid and allow selective movement of ions, energy sources, vitamins and cofactors, and waste. Complex lipids such as cholesterol and sphingolipids both affect the structure of membranes where they are found and are also involved in specific functions.
The variety of lipid and protein molecules, which make up membranes, are responsible for essential functions such as channels and transport across the membrane as well as signaling. Membrane receptors, along with their cytoplasmic partners, transmit signals. An important type of membrane receptors are G-proteins that produce secondary messenger molecules. Second messengers amplify and transmit this signal to various parts of the cytoplasm and nucleus. This ability to transmit a message across a membrane is paramount to not only the normal functions of cells but also their ability to perform specialized functions, which define the human body.
Not surprisingly, lipids and membrane structure and function are important in disease processes as well as treatments. Any deficiencies or problems with lipid synthesis or breakdown lead to a serious disease state and/or death. Modulation of membrane fluidity is, itself, important in membrane functions and, as a result, a variety of diseases. For example, multiple bacteria and viruses are infective and several medications work simply because their hydrophobic nature affects and directly targets lipids and membranes.
Membranes are composed of lipids arranged in a lipid bilayer, with the hydrophilic phosphate “head” groups of the lipid molecules forming the two outside layers and the hydrophobic “tail” groups arranged inside (Figure 8-1).
Simplified Representation of Lipid Bilayer/Membrane. The lipid bilayer is composed of lipid molecules, with hydrophilic head groups (e.g., phosphate) forming the outer surfaces and hydrophobic tails grouped together in the hydrophobic center. See Chapter 3 for more detailed discussion of lipid molecule structures. [Adapted with permission from Naik P: Biochemistry, 3rd edition, Jaypee Brothers Medical Publishers (P) Ltd., 2009.]
A majority of membrane lipids are phospholipids, usually with 16- or 18-carbon tails, some saturated and some unsaturated. Other types of lipids, including cholesterol, modulate the structure and, therefore, the fluidity of the membrane and, in turn, affect some membrane functions.
Particular lipids are found in specific membranes and are responsible for unique functions as summarized below: