J1 Bio: Lipids

What are the properties of lipids

• Contain C, H, O

• Contains large proportion of hydrocarbons, smaller proportion of oxygen than in carbohydrates

• Due to hydrocarbon chains, lipids are NON-POLAR

  • INSOLUBLE in water (polar solvent)

  • SOLUBLE in organic solvents (non-polar)

organic solvents: contains carbon, covalently bonded, non-polar

Classification of lipids

• Triglycerides

• Phospholipids

• Steroids (e.g. cholesterol)

Structure of Triglyceride

One glycerol molecule

Three fatty acids

Ester bonds

What is the structure of fatty acid?

Hydrocarbon tail (non polar)

Terminal carboxyl group (polar)

They are bonded together by covalent bonds (C-C single bonds between C of carboxyl, and C of hydrocarbon tail)

What affects melting point of fatty acids?

• Length of hydrocarbon tail

  • The longer the hydrocarbon tail, the higher the melting point is

• Number of C=C double bonds

  • The FEWER the number of C=C double bonds, the HIGHER the melting point is

Explain how LENGTH OF HYDROCARBON CHAIN affects melting point of fatty acid

• The longer hydrocarbon tail results in MORE HYDROPHOBIC INTERACTIONS between fatty acids

• MORE HEAT ENERGY is needed to disrupt the hydrophobic interactions

• hence melting point increases

Explain how does DECREASE IN C=C double bonds affect melting point of fatty acid

• A decrease in C=C double bonds causes less kinks in the hydrocarbon tail

• The fatty acids are MORE CLOSELY PACKED

• MORE Heat energy is required to disrupt the HYDROPHOBIC INTERACTIONS

• Hence melting point increases

Are fatty acids soluble in water?

• YES: When fatty acids are short

  • The carboxyl group can ionise to form a charged COO- group that can interact with water

• NO: When LENGTH of hydrocarbon tail INCREASES

  • Water solubility decreases due to the non-polar hydrophobic nature of hydrocarbon tails

Structure of glycerol

• It is an ALCOHOL

• 3 carbons, each bearing 3 hydroxyl groups

  • 3 hydroxyl groups make it polar and allows it to form hydrogen bonds with water, hence it is insoluble in water

• All single bonds

What are the properties of triglyceride?

• Non polar and hydrophobic

• Insoluble in water, but soluble in organic solvents

• LESS DENSE than water, and therefore float

How would the difference in saturation of fatty acids, affect the state (Solid/Liquid/Gas) of the triglyceride at room temperature?

• SATURATED FATS (e.g. lard and butter)

• Solid at room temperature

  • Saturated fatty acids: need more heat energy to disrupt greater hydrophobic interactions —> HIGHER MELTING POINT

• UNSATURATED FATS (e.g. oil)

• Liquid at room temperature

  • Unsaturated fatty acids: more kinks, less heat energy required to disrupt the weaker hydrophobic interactions —> LOWER MELTING POINT

Property: Triglycerides are non-polar and insoluble in water

What is the FUNCTION of this?

• Triglycerides serve as a GOOD ENERGY STORAGE as they DO NOT affect the WATER POTENTIAL of cells

• Oils on surfaces of skin, fur, feathers repels water and prevents excessive evaporation of water from terrestrial (land) animals

Property: Triglycerides have lower mass per unit volume, due to high ratio of hydrogen to oxygen atoms, comparted to carbohydrates

What is the FUNCTION of this?

• Triglycerides are BETTER ENERGY STORAGE MOLECULES.

  • Because more energy can be stored in a small volume.

  • Triglycerides release twice as much energy in the form of ATP as compared to an equivalent mass of carbohydrate

  • This is especially beneficial to animals, as it reduces the mass they have to carry as they move around

• Triglycerides are less dense than water

  • Improves buoyancy in mammals

• Triglycerides also release metabolic water when oxidised

  • Provide an important source of water, esp for animals living in dry deserts

Property: Triglycerides are highly reduced, with a high ratio of carbon-hydrogen bonds, to carbon atoms

What is the FUNCTION of this?

• Triglycerides are good SOURCES of energy

• as they supply many hydrogens for the reduction of NAD

Property: Unlike ionic and metallic bonds, hydrophobic interactions impede the transmission of heat energy, hence triglycerides are poor heat conductors

• Blubber slows down the rate of heat transfer

• Helping to maintain a stable internal temperature

• protecting the body from heat loss in cold conditions