Presentation on theme: "Lipids 2 Phospholipids and Steroids. Phospholipids The structure of phospholipids is based on the structure of triglycerides but the third hydroxyl."— Presentation transcript:
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.