Chapter 2 : Lipids

Glycerol + Fatty Acids

• Glycerol: 3 carbons, each C is bonded to 1 -OH group. Each -OH group can form Hydrogen bonds with water making glycerol polar, hydrophilic and soluble in water

• Fatty Acids: Long hydrocarbon chain which is non-polar and hydrophobic in nature and insoluble as unable to form H bonds with H2O.

• Saturated Fatty Acids have no kinks and have single covalent bonds.

• Unsaturated Fatty Acids have kinks due to presence of CC double Bond. as a result of kinks in fatty acids tails, unsaturated fatty acids cannot pack close together hence weaker hydrophobic interactions between fatty acid tails/molecules. Lesser heat needed to disrupt intercations and bring about change in state from solid to liquid hence M.P. of unsaturated Fatty acid < Saturated.

• Unsaturated Fats: Lower melting point than saturated fats because less energy is required to disrupt the weak hydrophobic interactions between fatty acids due to the presence of kinks. Each molecule is an ester formed by a fatty acid with glycerol. Each molecule has fatty acids with kinks due to the presence of carbon-carbon double covalent bonds. Each molecule has some carbon atoms which do not have covalent bonds linked with hydrogen atoms due to the formation of carbon-carbon double bonds.

Triglycerides

• Contains ester bonds which are formed between the hydroxyl group (-OH) of glycerol and the carboxyl group (-COOH) of an unsaturated fatty acid in a condensation/esterification reaction, which involves the removal of one water molecule, and is catalyzed by enzymes.

• Triglycerides can be hydrolysed into glycerol and 3 fatty acids through hydrolysis/saponification (add either acid or base)

• Good energy storage: More energy per gram than either carbohydrates or protiens, most weight efficicent means for storage in animals and plans.

• Good energy source: High proportionof hydrogen to insignificant proportion of oxygen, high ratio of energy storing C-H bonds. Higher amount of ATP released from oxidation of given mass as comapred to same mass of carbs

• Metabolic water: High proportion of hydrogen allows for metabolic water to be produced when triglyceride is oxidised during aerobic respiration. 

• Animals: useful for locomotion, long term food storage during cold climate for hibernating animals, metabolic water source for camels, thermal insulation reducing heat loss due to slow heat conducitvity, contributes to buoyancy as less dense than water.

• Plants: useful for plants that dispearse sees by wind or insects as seeds are light.

Phospholipids

• 2 -OH groups in glycerol condense with 2 -COOH (carboxyl) Groups of fatty acids forming ester bonds and last -Oh group of glycerol condenses with phosphate group of phosphoric acid to form phosphodiester bond.

• Hydrophilic -ve charged Phosphate group: Repelling charged molecules, soluble in water.

• 2 non polar fatty acid tails are insoluble in water.

• Amphipathic nature (both polar and non-polar): Forming a bilayer in aqueous environments.

• Unsaturated fatty acids: Increase membrane fluidity.

• Saturated fatty acids: Decrease membrane fluidity.

• In aqueous environments, the polar hydrophilic phosphate heads of phospholipids align, face outwards and interact with aquesous medium such that their non-polar hydrophobic fatty acid tails are facing inwards and shielded from aqueous medium and are burried between the two layers formed by the polar phosphate heads. This produces a sheet composed of two parallel (molecular) layers known as the phospholipid bilayer of cell membranes. → allows for comparmentalisation of cell separating cytoplasm from extracellular fluid and separate comparments to be formed inside cell allowing specialised metabolic pathways to take place. 

• Selective barrier: Hydrophobic fatty acid tails facing inwards form the hydrophobic core of cell membrane acting as a selective barrier restricting movement of charged ions and polar molecules across membrane, hence impermeable to charged ions and polar molecules.

• Mmembrane fluidity: Weak hydrophobic interactions hold non polar fatty acid tails together. Phospholipids can move rapidly by diffusion in own layers in membrane contributing to fluidity and rearranging lipid molecules forming transient pores which allow small non polar molecules to diffuse through. Unsaturated fatty acid tails have kinks that keep phospholipids from packing close together enhancing membrane fluidity.

Common QS

• (b) Complete the table below for each molecule, describing their differing roles in cells in relation to their differences in molecular structure. [3]

  • Ensure that each statement which describes structure is correctly matched to its appropriate role/function.

  • Triglyceride

    • A triglyceride molecule consists of 1 glycerol and 3 fatty acids. Serves as a good energy source, i.e. can be used as respiratory substrate / Serves as a good energy storage molecule / Source of metabolic water / Thermal insulation / Buoyancy

  • Phospholipid

    • A phospholipid molecule consists of 1 glycerol, 2 fatty acid tails and a phosphate group. Main structural component of biological membranes responsible for compartmentalisation of the cell / for membranes acting as a selective barrier

  • Glycolipid

    • A glycolipid molecule consists of 2 hydrocarbon chains and a sugar residue. Cell-cell recognition / Cell-cell adhesion

• (c) State the role of cholesterol in cell membranes. [2]

  • Maintains membrane fluidity / increases the stability and flexibility of membranes, by restraining the movement of phospholipids at relatively warm temperatures to decrease fluidity / hindering the close packing of phospholipids at low temperatures to increase fluidity

  • Acts like a plug, reducing the escape or entry of charged ions and small polar molecules through the membrane.

• a) Suggest the advantages of storing lipids over carbohydrates in living organisms. [8]

• There is higher proportion of hydrogen and an almost insignificant proportion of oxygen in lipids (triglycerides) compared to carbohydrates / Lipids (triglycerides) have a higher ratio of energy-storing C-H bonds to carbon atoms than carbohydrates

• Therefore, more than twice the amount of energy in the form of ATP is released from the oxidation of a given mass of lipids compared to an equal mass of carbohydrates, hence lipids are a good energy source

• Lipids (triglycerides) contain more energy per gram than carbohydrates which provide a more weight-efficient means for animals to store energy, thus lipids are good energy storage molecules

• This is useful for animals where locomotion requires mass to be kept to a minimum (e.g. in birds or insects in which small mass is a necessity)

• There is higher proportion of hydrogen in lipids (triglycerides) compared to carbohydrates

• During aerobic respiration, the completion oxidation of lipids (triglycerides)produces a larger amount of metabolic water per gram of lipid compared to the oxidation of an equal mass of carbohydrates

• The metabolic water is retained within the bodies of desert animals, helping them to survive when there is no liquid water for drinking

• Lipids (triglycerides) are good thermal insulators as they conduct heat slowly, thus reducing heat loss from the body. This is important for animals living in cold climates, as the thick subcutaneous layer of fats (thick layer of blubber stored under the skin) reduces the loss of body heat from the deeper regions of the body to the surroundings

• Lipids (triglycerides) are less dense than water, and this is important for providing aquatic mammals with buoyancy (e.g. layer of blubber in whales)

• (a) Explain the differences in molecular structure between triglycerides and phospholipids.[4]

Distinguish between lipids and carbohydrates.