LIPIDS To be able to Identify the elements which make up lipids Explain that glycerol and fatty acids combine by condensation to produce triglycerides. Demonstrate why the R group of a fatty acid may be saturated or unsaturated. Explain how phospholipids are formed

LIPIDS What are lipids? Why are they useful? What is their biological use? What elements make up lipids? What are lipids? Fats, oils and waxes Why are they useful? Lot of energy per gram useful medium and long term energy stores BUT can’t be broken down quickly so organisms use carbs as short term energy store What is their biological use? Insulation Physical protection (cushioning) Waterproofing – waxy cuticle and exoskeletons Waxes, birds' feathers and mammals' fur. What elements make up lipids? Carbon hydrogen and oxygen

TRIGLYCERIDES The most common type of lipids are the triglycerides also known as fats and oils, or TRIGLYCERIDES Triglycerides are made from a combination of three fatty acid molecules and one glycerol molecule by a condensation reaction. Triglycerides are insoluble in water. They are used for storage They yield more energy per unit mass than other compounds so are good for energy storage. Carbohydrates can be mobilised more quickly, and glycogen is stored in muscles and liver for immediate energy requirements., insulation and protection in fatty tissue (or adipose tissue) found under the skin (sub-cutaneous) or surrounding organs.

Glycerol Fatty Acid Fatty Acid Fatty Acid The glycerol molecule in any triglyceride is always the same. It is the fatty acids that vary

Glycerol is a small, 3-carbon molecule with three alcohol groups. hydrocarbon with a hydrogen replaced by "OH". A hydroxyl group H OH OH OH

Fatty acids are long molecules with a polar, hydrophilic end and a non-polar, hydrophobic "tail". Carboxyl group COOH Acid hydrocarbon with a hydrogen replaced by "COOH". A carboxyl group This disassociates in solution to form COOH -> COO- + H+ Because of the lack of an OH group in solution triglycerides are insoluble in water – no polar molecule so hydrogen bonding cannot occur - Because they have a hydrophobic tail that repels water and this causes the molecules to clump together to reduce the surface area in contact with water. This insolubility results from the many nonpolar covalent bonds of hydrogen and carbon in lipids. Lipids aggregate away from water, which is polar, and are attracted to each other via weak, but additive, van der Waals forces. OH H H H H H Hydrocarbon Chain (14-22 carbon atoms) Methyl Group Carboxyl Group

The hydrocarbon chain can be from 14 to 22 CH2 units long, but it is always an even number because of the way fatty acids are made.. The hydrocarbon chain is sometimes called an R group, so the formula of a fatty acid can be written as R-COO-

General formula CH3 – (CH2)n - COOH

Saturated fatty acids Saturated fatty acids only have C-C bonds Each carbon in the chain is joined by a single bond Each carbon is SATURATED with hydrogen These fatty acids form straight chains, and have a high melting points. COO- Triglycerides containing saturated fatty acids have a high melting point and tend to be found in warm-blooded animals. At room temperature they are solids (fats), e.g. butter, lard.

Unsaturated fatty acids Unsaturated fatty acids only at least one C=C double bond Polyunsaturates have more than one double bond These fatty acids form bent chains, and have a low melting point. H H H H H H H H C C C C C C C C COO- H Triglycerides containing unsaturated fatty acids have a low melting point and tend to be found in cold-blooded animals and plants. At room temperature they are liquids (oils), e.g. fish oil, vegetable oils. H H H H H H

Formation of triglycerides One molecule of glycerol joins together with three fatty acid molecules to form a triglyceride molecule, in another condensation polymerisation reaction:

= = = H H H H H H O H2O H C C C C C C C H OH HO H C H H H H H H H H H

H CH3 – (CH2)n - COOH HO H C CH3 – (CH2)n - COOH HO C H = O CH3 – (CH2)n - C O H C ESTER BONDS = O CH3 – (CH2)n - C O C H = O O C H CH3 – (CH2)n - C H

Phospholipids Phospholipids have a similar structure to triglycerides, but with a phosphate group in place of one fatty acid chain. There may also be other groups attached to the phosphate.

Phospholipids have a polar hydrophilic "head" (the negatively-charged phosphate group) and two non-polar hydrophobic "tails" (the fatty acid chains). O- H H O P = O C = O O- CH3 – (CH2)n - C O C H = O C H CH3 – (CH2)n - C O H

This mixture of properties is fundamental to biology, for phospholipids are the main components of cell membranes.

Steroids Steroids are small hydrophobic molecules found mainly in animals. They include: Cholesterol, which is found in animals cell membranes to increase stiffness Bile salts, which help to emulsify dietary fats Steroid hormones such as testosterone, oestrogen, progesterone and cortisol Vitamin D, which aids Ca2+ uptake by bones. LITTLE IN COMMON WITH OTHER LIPIDS SOME SIMILARITIES - INSOLUBLE IN WATER – SOLUBLE IN OTHER SOLVENTS CHEMICALLY THEY HAVE A FOUR RING STRUCTURE WITH VARIOUS SIDE CHAINS HUMAN STEROIDS ARE SYNTHESISED FROM CHOLESTEROL TOO MUCH CHOLESTEROL AND SATURATED FATS (DENSE) FORM ATHEROMAS

Why is it wrong to call lipids ‘fats’? Because saturated lipids are mainly animal fats but unsaturated lipids are called oils What is the name given to the type of bond that joins fatty acids and glycerol in a lipid molecule? Ester bond Why are lipids insoluble in water? Because they have a hydrophobic tail that repels water and this causes the molecules to clump together to reduce the surface area in contact with water. Describe the difference between a triglyceride and a phospholipid. How do these differences affect the properties of the molecule? A phosphate group replaces one of the fatty acid molecules The phosphate group is ionised which makes it attract water molecules. This end is therefore hydrophilic as opposed to the fatty acid group which is hydrophobic