1.5 Lipids Flashcards
What are the characteristic of lipids?
- they contain carbon, hydrogen and oxygen
- the proportion of oxygen to carbon and hydrogen is smaller than in carbohydrates
- they are insoluble in water
- they are soluble in organic solvents such as alcohols and acetone
What three elements do lipids contain?
Carbon, hydrogen and oxygen
What are the main groups in a lipid?
Triglycerides (fats and oils) and phospholipids
What are the roles of lipids?
- source of energy → when oxidised, lipids provide more than double the energy as the same mass of carbohydrate
- waterproofing → lipids are insoluble in water, both plants and insects have waxy, lipid cuticles that conserve water, while mammals produce an oily secretion from sebaceous glands in the skin
- insulation → fats are slow conductors of heat and when stored beneath the body surface help to retain body heat, they also act as electrical insulators in the myelin sheath around nerve axons
- protection → fat is often stored around delicate organs, such as the kidney
What is a triglyceride comprised of?
- three fatty acids
* glycerol
What bond is present in lipids?
Ester bonds
How is an ester bond formed?
Condensation reaction
How do triglycerides vary?
Variations in the fatty acids, all glycerol molecules are the same.
mono-unsaturated
a molecule with one carbon-carbon double bond
poly-unsaturated
a molecule with more than one carbon-carbon double bond
How does the structure of triglycerides relate to its properties?
- high ratio of energy-storing carbon-hydrogen bonds to carbon atoms → so excellent store of energy
- low mass to energy ratio → good storage molecules, lots of energy can be stored in a small volume, beneficial to animals as less mass to carry around
- large, non-polar molecules, insoluble → don’t affect water potential or osmosis
- high ratio of hydrogen to oxygen atoms → triglycerides release water when oxidised and so provide an important source of water
How does a phospholipid differ from a lipid?
One fatty acid molecule is replaced by a phosphate molecule
hydrophobic
repel water
hydrophilic
attract water
Is a phosphate molecule hydrophobic or hydrophilic?
Hydrophilic
What are the two parts that make a phospholipid?
- hydrophilic ‘head’ → phosphate, interacts with water but not with fat
- hydrophilic ‘tail’ → fatty acid, orients itself away from water but mixes readily with fat
How is the structure of phospholipids related to its properties?
- polar molecules → hydrophilic phosphate head and hydrophobic tail of two fatty acids, in an aqueous environment, phospholipid molecules form a bilayer within cell-surface membranes
- hydrophilic phosphate heads → help to hold at the surface of the cell-surface membrane
- phospholipid structure allows them to form glycolipids → by combining with carbohydrates within cell-surface membrane, these glycolipids are important for cell recognition
Test for lipids
Emulsion test
Emulsion test
- Take a dry and grease-free test tube
- Add 5cm³ of ethanol to 2cm³ of the sample being tested
- Shake the tube thoroughly to dissolve any lipid in the sample
- Add 5cm³ of water and shake gently
- A cloudy-white colour indicates the presence of a lipid
- As a control, repeat the procedures using water instead of the sample, final solution should remain clear
Why will a lipid turn cloudy in the emulsion test?
Cloudy colour is due to any lipid in the sample being finely dispersed in the water to form an emulsion.
