Lipids

Question 1

Describe the structure and properties of a triglyceride and explain how it relates to its role as an energy store in living organisms.

Answer 1

Structure:

Each trigylceride consists of
1. 3 long non-polar hydrophobic hydrocarbon chains
2. Joined to a glycerol backbone via ester linkages

The long non-polar hydrophobic hydrocarbon chains of the triglyceride
1. cannot form H bonds with water –> triglycerides are insoluble in water (property) and do not affect the water potential of the cell
2. Have a high proportion of C-H bonds from which energy in form of ATP and metabolic water can be released during oxidation –> suitable energy store.

Question 2

What are other roles of triglycerides?

Answer 2

1. Poor conductors of heat, subcutaneous fat provide thermal insulation to mammals.
2. Less dense than water hence improve buoyancy in mammals.
3. Form a protective layer around delicate internal organs of mammals. Act as shock absorbers and protect organs from mechanical damage.
4. Function as a reservoir for storage of fat soluble vitamins e.g. vitamin A, D, K

Question 3

Describe structure of phospholipids and hence explain how it forms bilayers.

Answer 3

Each phospholipid consists of
1. 2 long non-polar, hydrophobic hydrocarbon tails
2. joined to a glycerol backbone via ester linkages
3. with the third OH group joined to a hydrophilic, negatively charged (not polar) phosphate group via a phosphoester linkage

Hence phospholipids are amphipatic, having both hydrophilic and hydrophobic regions

Phosholipids can form bilayers as
1. the non-polar, hydrophobic hydrocarbon tails face inwards, away from aqueous environment and
2. the hydrophilic phosphate group face outwards and interact with aqueous environment by forming H bonds with water

Question 4

The phospholipid bilayer of membranes has a hydrophobic core and is selectively permeable. Describe its significance.

Answer 4

1. It regulates movement of substances by acting as a barrier to charged ions, and polar and large molecules.
2. It acts as a boundary between the intracellular and extracellular aqueous environment
3. It allows compartmentalisation within a cell
4. It is permeable to small hydrophobic molecules and slightly permeable to some molecules e.g. water that can pass through its transient pores.

Question 5

How do properties of phospholipids relate to its function in cell membranes?

Answer 5

1. Unsaturated hydrocarbon chains with kinks (due to cis C-C double bonds) prevent close packing of phospholipids resulting in greater fluidity of the membrane as there are fewer hydrophobic interactions between the hydrocarbon chains of the phospholipids. Greater fluidity allows movement of proteins within the bilayer.
2. Membranes with long hydrocarbon chains have higher melting points than membranes with short hydrocarbon tails as there are more hydrophobic interactions between the long hydrocarbon chains. When fewer hydrophobic interactions are present, larger transient pores form in the bilayer
3. Membranes with more kinks have a lower melting point than membranes with fewer kinks as the greater space between the phospholipids makes it harder to freeze.
4. Additional small molecules, usually charged or polar can be linked to the phosphate group to form a variety of phospholipids.