Lipids and cholesterol consist of hydrophilic and hydrophobic particles (see Fig. 1).

There are three major classes of membrane lipid molecules—phospholipids, cholesterol, and glycolipids.

In this bilayer, cholesterol is relatively often found in a “flipped” configuration with the hydroxyl group oriented towards the membrane middle plane. This bilayer has also the highest (least negative) binding free energy among liquid phase bilayers, and the lowest reorientation barrier.

THe hydrophilic hydroxyl group of cholesterol interacts with aqueous environment, whereas the large hydrophobic domain fits between C-tails of lipids.

Cholesterol is a fatty substance essential to many metabolic processes. Your body needs cholesterol, but it can make its own – you don’t need to consume cholesterol in your diet. High levels of LDL cholesterol in the blood have been linked to coronary heart disease.

Cholesterol and other fats are carried in your bloodstream as spherical particles called lipoproteins. The two most commonly known lipoproteins are low-density lipoproteins (LDL) and high-density lipoproteins (HDL).

Cholesterol molecules insert into the bilayer with their polar hydroxyl groups close to the hydrophilic head groups of the phospholipids (Figure 2.47). The rigid hydrocarbon rings of cholesterol therefore interact with the regions of the fatty acid chains that are adjacent to the phospholipid head groups.

Dietary phospholipids mainly derived from soybeans, egg yolk, milk, or marine organisms (fish, roe or krill) are supposed to be highly effective in delivering their fatty acid residues for cell membrane incorporation.

Estrogens are small hydrophobic lipid hormones that promote cell division and the development of reproductive structures in mammals. Estrogens passively diffuse across the plasma membrane and bind to their receptor proteins in the cytoplasm of target cells.

One way to increase membrane fluidity is to heat up the membrane. Lipids acquire thermal energy when they are heated up; energetic lipids move around more, arranging and rearranging randomly, making the membrane more fluid.

The effects of pH on the membrane fluidity of intact human erythrocytes, ghosts, and their lipid vesicles were studied by spin label techniques in the range of pH 3.0 to 9.1. The effects of cholesterol demonstrated that the membrane fluidity was significantly mediated by cholesterol at low pH, but not at high pH.

At low temperatures, cholesterol increases membrane fluidity by preventing membrane lipids from packing close together. At high temperatures, cholesterol decreases membrane fluidity.

Fluidity is important for many reasons: 1. it allows membrane proteins rapidly in the plane of bilayer. 2. It permits membrane lipids and proteins to diffuse from sites where they are inserted into bilayer after their synthesis.

The ratio of saturated and unsaturated fatty acids determines the fluidity in the membrane at cold temperatures. Cholesterol functions as a buffer, preventing lower temperatures from inhibiting fluidity and preventing higher temperatures from increasing fluidity.

While too much cholesterol can harm the body, regulated amounts of cholesterol in cell membranes are absolutely necessary for the normal function of cells. Anomalies in cholesterol amounts are often associated with various disease conditions.

Glycerophospholipids

Phosphoglycerides (also known as glycerophospholipids) are the most abundant phospholipids in cell membranes.

Glycerophospholipids are by far the most abundant lipids in cell membranes. Like all lipids, they are insoluble in water, but their unique geometry causes them to aggregate into bilayers without any energy input.