Lipids

Most lipids are insoluble in water.

• True

• False

Answer explanation

Explanation:

Lipids e.g triglyceride has 3 long hydrocarbon chains which are non-polar and cannot form hydrogen bonds with water, hence insoluble in water.

Synthesis of the phospholipid components of membranes does not involve the formation of ester bonds.

• True

• False

Answer explanation

There are 2 ester bonds  + 1 phospho-ester bond in phospholipids.

In the ethanol-emulsion test for lipids, a cloudy emulsion is formed after ethanol is added.

• True

• False

Answer explanation

Lipids are soluble in organic solvent eg ethanol and result in clear solution.  Cloudy emulsion is formed after water is added into lipid & ethanol mixture, due to lipids being insoluble in water.

Triglycerides are both hydrophobic and hydrophilic as it is made up of glycerol and fatty acids.

• True

• False

Answer explanation

Triglyceride is formed when a glycerol covalently bonded to 3 fatty acid chains via ester bonds.  The OH of glycerol is involved in ester bond formation with COOH of the fatty acid. 

 

The hydrocarbon chains in the triglycerides are non-polar, hence are hydrophobic.

 

The ester bond (-COO) itself is hydrophilic, however as hydrocarbon chains make up the bulk of the triglyceride, hence overall effect, triglycerides are hydrophobic.

Triglycerides yield more energy than carbohydrates per gram due to a larger number of carbon-hydrogen (C-H) bonds.

• True

• False

Phospholipids are both hydrophobic and hydrophilic (amphipathic).

• True

• False

Answer explanation

Phospholipid’s phosphate head is hydrophilic, while the fatty acid tails are hydrophobic.

Note:  triglyceride (hydrophobic) vs phospholipid (amphipathic).  Comparatively, the phosphate head (with P-O) has greater electronegativity difference than ester bond (C-O). Hence phosphate head in phospholipid exert a greater effect compared to ester bond in triglyceride. Phosphate head is hydrophilic and fatty acid tails in phospholipid is hydrophobic.

When hydrolysed, phospholipids form products that contain carboxyl group.

• True

• False

Answer explanation

Phospholipids will be hydrolysed to form 2 fatty acids [R-COOH], 1 glycerol and 1 phosphate-containing group. Fatty acids (products) contain COOH group.

A liposome (a vesicle made up of a phospholipid bilayer) can be used to transport both polar and non-polar substances from one part of the body to another.

• True

• False

Answer explanation

Hydrophilic (polar) substances are carried inside the aqueous solution in the liposome while hydrophobic (non-polar) substances are carried within the hydrophobic core of the phospholipid bilayer.

Tags

NGSS.HS-ETS1-3

Triglycerides are usually characterised by four interconnected rings.

• True

• False

Answer explanation

Steroids are usually characterised by four interconnected rings.

Fluidity of membranes is regulated by phospholipids.

• True

• False

Answer explanation

Length of the fatty acids tails in phospholipids and the presence of kinks in the fatty acid tails can regulate the fluidity of the membrane.

Longer the fatty acid tails -> more hydrophobic interactions -> less fluid membrane (and vice versa)

More C=C / kinks in the fatty acid tails -> less regular packing -> more fluid membrane (and vice versa)

  

Beside phospholipids, cholesterol also regulate the fluidity of membranes. Cholesterol is known to act as a temperature buffer.

Cholesterol affects the behaviour of phospholipids in the membrane.  Cholesterol disrupting the close and orderly packing of phospholipid molecules at low temperature.  But at higher temperature, cholesterol reduces the phospholipids molecules movement.