B4 Lipids

Assessment Objectives B.4.1 Compare the composition of the three types of lipids found in the human body. (3) B.4.2 Outline the difference between HDL and LDL cholesterol and outline its importance. (2) B.4.3 Describe the difference in structure between saturated and unsaturated fatty acids. (2)

Textbook Chemistry 3 rd Ed John Green, Sandru Damji pp On Moodle

Key Terms Fats Fatty acid Hydrophilic Hydrophobic Iodine number Lipids Oils Phospholipid Polyunsaturated oils Saponification Soap Triglyceride

What are lipids? Lipids are a group of substances that contain the elements carbon, hydrogen and oxygen (just as proteins and carbohydrates), but the proportion of oxygen is less than carbohydrates. Lipids are insoluble in water BUT dissolve in organic solvents Occur in living organisms as Triglycerides (fats and oils), Phospholipids Steroids

Triglycerides (Fats and Oils)

Lipids Fats and oils are triesters formed from the condensation reaction of glycerol (1,2,3,propanetriol) with long chain fatty acids

Fats and Oils Fats  Solid triglycerides i.e. butter, lard. Oils  Liquid in room temperature i.e. olive oil, linseed oil. The main chemical difference between oils and fat is that fats contain SATURATED carboxylic groups (no double bonds). Oils contain at least one double bond and are said to be UNSATURATED. note: most oils contain many double bonds and are named POLYUNSATURATED.

Saturated and Unsaturated

PHOSPHOLIPIDS Phospholipids form the integral part of all the cell membranes. They are made by four components. A back bone such as propane-1,2,3-triol (glycerol) is linked by esterification to two fatty acids and a phosphate group. The phosphate group it is condensed to a nitrogen containing alcohol.

How do you think these molecules would behave in water? In oil?

Below is Lecithin, or phosphatidycholine, a major component of biological membranes and can be isolated from egg yolk or soybeans Focus on the “tails” and “heads” which form a membrane

Micelles Micelles can form from fatty acids or soaps They are spherically shaped masses with the polar head groups facing the water and the non-polar tails facing inward

Steroids Steroids do NOT contain fatty acids but are still considered to be lipids, this is because they have similar properties to triglycerides and are synthesized using common intermediates. Steroids all contain a 17-carbon atom skeleton that varies due to the functional groups and their oxidation states

Steroid Uses The sex hormones are all steroid-based hormones Progesterone: female menstrual cycle Estrogen: female sex hormones Testosterone: male sex hormones Aldosesterone, which is secreted by the adrenal gland (secrete hormones from stress), is also steroid-based and aids in controlling the concentration of sodium and potassium. Vitamin D is a steroid-based vitamin Used to give cell membranes rigidity Growth hormones In bile Acids to help solubulise fats in the digestion system.

Activity Construct a table Identifying the Similarities and differences between triglycerides and phospholipids. DifferencesSimilarities Triglycerides Phospholipids Include information about composition, function, structure, properties…

Steroids and phospholipids are both classes of lipid found in the body. Cholesterol is a steroid. A structure of lecithin, a phospholipid, is shown below. (ii)Compare the composition of cholesterol with a phospholipid such as lecithin. (1) (iii) Determine whether cholesterol or lecithin is more soluble in water. (1)

Markscheme (ii)cholesterol is composed of C, H and O only and phospholipids contains C, H, O, P and N;1 (iii)lecithin;1