Lipids 2 Phospholipids and Steroids

Phospholipids  The structure of phospholipids is based on the structure of triglycerides but the third hydroxyl group of the glycerol is linked to phosphoric acid which is often linked to a large polar group.

Phospholipids (cont’d.)  The fatty acids which make up phospholipids have a consistent length of between 16 and 18 carbons. This allows them to form neat bilayers.  Phospholipids are said to be amphipathic, having two very different sides to their nature.

Phospholipids (cont’d.)  The ‘head’ containing the polar group and the phosphate group has polar covalent bonds. It is slightly charged and attracts water, i.e. it is hydrophilic.  The ‘tail’ containing the long hydrocarbon group which is non-polar covalent. It is not charged and repels water, i.e. it is hydrophobic. Hydrophilic portion Hydrophobic portion

Phospholipids (cont’d.)  The amphipathic nature of phospholipids is important in the formation of bilayers such as cell membranes.  The hydrophilic groups line up on the outside faces of the membrane. The hydrophobic portions are arranged within the membrane.

Phospholipids (cont’d.)  Phospholipids may have fatty acids which are saturated or unsaturated. This affects the properties of the resulting bilayer/cell membrane: Most membranes have phospholipids derived from unsaturated fatty acids. Unsaturated fatty acids add fluidity to a bilayer since ‘kinked’ tails do not pack tightly together. Phospholipids derived from unsaturated phospholipids allow faster transport of substances across the bilayer.

Phospholipids (cont’d.)  Membranes exposed to the cold have a very high percentage of unsaturates e.g. bacteria grown at low temperature or the membranes of reindeer ears – remember unsaturates are liquid at much lower temperatures.

Phospholipids (cont’d.)  Membranes which are stiffer such as those in nerve cells contain a much higher percentage of phospholipids derived from saturated fatty acids.  They also contain high levels of cholesterol which stiffens membrane structure further.

Steroids  Steroids have a common four ring structure.  Each unit within the four-ring structure is known as an isoprene unit (C 5 H 8 ).

Steroids (cont’d.)  Different steroids vary in the side chains attached to the rings.  Notice that cholesterol and testosterone are almost identical except for the side groups on C3 and C17.

Steroids (cont’d.)  Steroids are classified as lipids since they are soluble in organic compounds but not in water. They have a very powerful effect because of this as they can pass through cell membranes.  Steroids are hormonal in function and have a wide variety of functions.  Other examples of steroids are oestrogen, progesterone, cortisol, cholesterol and aldosterone.