Edexcel A-Level Biology B Topic 1.2

1. Lipids
EDEXCEL A-LEVEL BIOLOGY B
TOPIC 1.2

2. Facts
 A group of organic molecules
 Have a vital role within organisms
 Form an integral part of all cell membrane
 Also used as an energy store
 Many plants and animals contain lipids to provide energy for the
seedling when it starts to grow, this is why seeds are important food
source for many animals

3. Fats and Oils
 Important group of lipids
 Chemically similar, however fats (e.g. butter) are solid
at room temperature, whereas oils (e.g. olive oil) are
liquid at room temperature
 All contain carbon, oxygen and hydrogen atoms
 Contains a lower proportion of oxygen than
carbohydrates
 Made up of fatty acids and glycerol (propane-1,2,,3-
triol)
 Combined using ester bonds
 Glycerol's chemical formula is C3H8O3

4. Fats and Oils Structure
 All have a long hydrocarbon chain
 Pleated backbone of carbon atoms and hydrogen atoms attached
 Have a carboxyl group of –COOH at one end
 Living tissues have 70+ different kinds of fatty acids they vary in 2
ways
1. Length of the chain can differ
2. Fatty acid can be saturated or unsaturated

5. Saturated Vs Unsaturated
Saturated
 Carbon atoms joined together by a single
covalent bond
 e.g. stearic acid, found in both plant and animal
fats
Unsaturated
 Carbon chains have 1 or more double covalent
bonds
 Monounsaturated fatty acid = 1 double bond
 Polyunsaturated fatty acids more than 1 double
bond e.g. linoleic acid, is essential in our diet as
we cannot make it from other chemicals

6. Forming Ester Bonds
 Fat or oil is produced, when glycerol combines with 1, 2 or 3 fatty
acids to form a monoglyceride, a diglyceride or a triglyceride
 During a condensation reaction a bond is formed between the
carboxyl group (-COOH) of a fatty acid, and one of the hydroxyl
groups of (-OH) of glycerol. A molecule of water is removed and
the resulting bond in known as an ester bond.
 This type of condensation reaction is called Esterification
 The nature of the lipid formed depends on which fatty acids are
present. Lipids containing saturated fatty acids are more likely to
be solid at room temperature than those containing unsaturated
fatty acids

7. The Nature of Lipids
 Contain many carbon-hydrogen bonds and little oxygen
 When lipids are oxidised during respiration, bonds are broken and the products are carbon dioxide and
water. This reaction can be can be used to drive the production of a lot of ATP
 Lipids (especially triglyceride) stores 3x as much energy as the same mass of carbohydrates
 Lipids are hydrophilic – waterproofing organisms, oils are important in waterproofing the fur and
feathers of mammals and birds. Insects and plants use waxes for waterproofing their outer surfaces
 Good insulators, a fatty sheath insulates your nerves so the electrical travel faster
 Insulate animals against heat loss e.g. whales have a thick layer of blubber
 Very low density – body fat of water mammals helps them to float easily
 All dissolve in organic solvents, but are insoluble in water, so don’t interfere with the many water based
reactions that go on in the cytoplasm of a cell

8. Phospholipids
 Inorganic phosphate ions ( -PO4^3-) are present in the
cytoplasm of every cell
 Sometimes 1 of the hydroxyl groups of glycerol undergoes
esterification with a phosphate group instead of a fatty
acid, & a simple phospholipid is formed
 Phospholipids are important because the lipid and the
phosphate parts of the molecule give it very different
properties
 Phospholipids
1. Fatty acid chains are neutral and insoluble in water
2. Phosphate head (- negative charge), soluble in water
3. When in contact in water the 2 parts of the molecule behave
differently
4. Polar phosphate = hydrophilic, dissolves readily in water
5. Lipid tails = hydrophobic, don’t dissolve in water
6. If they are tightly packed in water they form a monolayer,
(heads in water, tails in air), or clusters called micelles
7. Micelle = all hydrophilic heads point outwards and all
hydrophobic tails are inside

9. Phospholipid Diagrams
Phospholipid Monolayer Micelle

10. Phospholipids
 Rarely, a phospholipid monolayer may form, at a
surface between air and water in living cells where
there are water based solution on either side of the
membranes. With water on each side the phospholipid
molecules form a bilayer with the hydrophilic heads
pointing into the water, protecting the hydrophobic
tails in the tails in the middle. This structure, the unit
membrane, is the basis of all membranes.