How do the terms polar, nonpolar, hydrophilic, and hydrophobic relate to the phospholipids found in membranes?

Question

how would you draw and label a simple structure of a plasma membrane

Answer 1

Polar molecules are those which have a permanent electric dipole moment. What this means is that although the molecule may have an overall neutral charge, one part of it is more negative than the other.

Using water for example, the oxygen is more electronegative and so this end of the molecule is more negatively charged.

ANSWER:

*to assist with understanding, open this link in another browser for diagrammatic reference.

http://kvhs.nbed.nb.ca/gallant/biology/p...

If you think of the hydrogen bonding in water it become clear why polar molecule are water-loving (or hydrophilic). The polarity this has gives the molecule a partial charge for the molecule allowing it to hydrogen bond with water.

Contrastingly, hydrophobic molecules are non-polar; that is, there is no partial negative or positive charge anywhere on the molecule. Because of this, it repels water.

So in short, those molecule which are polar and have a dipole moment will be hydrophilic and those molecules with no dipole moment (non-polar) are hydrophobic.

Relating this to phospholipids and plasma membranes, the phospholipid can be divided into two sections to examine. The "head" part of the molecule is polar (having a partial charge due to electronegativity) and therefore does not resist interaction with water.

Because of this, the heads of one layer will be in contact with the outside of the cell and the heads of the other layer in contact with the cytoplasm of the cell.

The hydrophobic tails avoid the water and arrange themselves in the middle of the 2 layers.

HYDROPHOBIC = NON-POLAR

HYDROPHILIC = POLAR

Answer 2

You should be able to find a good diagram in your textbook and there are probably literally thousands online.  The Wikipedia entry is full of graphics.

Nonpolar substances are hydrophobic and will not mix with water.  Plasma membranes are composed largely of phospholipids which are composed of a polar, hydrophilic phosphate "head" and a hydrophobic fatty acid "tail."  These molecules spontaneously arrange such that the heads point out toward the extracellular matrix (outside of the cell) and inwards toward the cytoplasm, with the fatty acid tails on the inside.  Polar and charged particles (ions) cannot pass through the fatty acid region and need to be shuttled in through proteinaceous channels.  Some of the most important chemicals used by our body are either polar or ionic (charged), so these are very critical for sustaining life.  These include water, the potassium, sodium and calcium cations and the chloride and phosphate anions.  Nonpolar substances can freely pass through.

That's a very simplified explanation, the Wikipedia entry is much more comprehensive with a long list of references.  If you have a textbook for the class it probably covers it in the detail expected by your instructor.