Describe the phospholipid structure and how its arrangement in the plasma membrane allows for the separation of internal and external environments as well as its selectively permeable nature.

Question

this is for my anatomy and physiology class, answer must be at least 10 sentences long, please.

Answer 1

The key to understanding this concept is knowing the difference between polar and non-polar and how this affects behavior in solution. A polar molecule has a separation of electric charge so that part of the molecule is positive and part is negative. A non-polar molecule is electrically neutral.

Polar molecules are attracted to water (hydrophilic) which itself is polar - the charges in a polar molecule are attracted to the positive and negative charges in a water molecule. Non-polar molecules are repelled by water (hydrophobic).

A phospholipid is a molecule with a polar region and a non-polar region. Think of it like a lollipop - the head is polar because it contains a charged phosphate group. The stick is non-polar, essentially a long hydrocarbon chain. When lots of phospholipids enter solution the heads stick out to face the water while the sticks arrange themselves inwards away from the watery solution and shielded by the heads.

So imagine you have one lollipop facing up and another next to it but facing down. Now alternate this pattern over and over. This is the structure of the phospholipid bilayer of the plasma membrane. In order for a substance to cross this membrane it needs to pass through the "stick" part which is non-polar. Likes dissolve likes, so only non-polar substances can dissolve in order to pass through the membrane. Hence the membrane is permeable to (small) non-polar substances but impermeable to polar substances. These need to cross the membrane through specially dedicated channels which are like little tunnels through the membrane made out of protein.

Answer 2

Well, I won't write the assignment for you, but I'm glad to try and provide some insight into the question.

A phospholipid contains a highly charged phosphate group at one end, with a long neutrally-charged lipid tail (or tails). Because the charged particles do not "like" to be randomly distributed in an largely uncharged solvent (for the same reason that water doesn't mix with oil) the structure of phospholipids causes them to spontaneously arrange themselves so that the charged phosphate groups are on the outside, and the long neutral tails are on the inside where they don't get exposed to charged solvent. When you have a large enough amount of phospholipids, and they are created to surround another aqueous solution (the inside of a cell) then the self-arranging property of phospholipids causes them to form what is known as a phospholipid bilater, where the membrane enclosing the inside of the cell is made up of 2 lays of phospholipids, one with its charged groups pointing inward, the other outward, with the lipid portions sandwiched between. This structure separates the internal and external aqueous compartments because water, a polar particle, does not easily traverse the hydrophobic lipid portion of the phospholipid bilayer (though it can do so better than charged particles like most electrolytes). This means that the cell can selectively allow or prevent these molecules from crossing the membrane by means of special proteins embedded in the bilayer. This is what allows for the "selective permeability" of the membrane.

Incidentally, the precise nature of the lipid portion of a phospholipid determines some of its physical properties. If the fatty acid tail is saturated with all of the possible hydrogen atoms, then it will not develop double-bonds which kink the structure. This means the straight tails can lie together easily, whereas unsaturated fatty acids have a hard time fitting together. This is why saturated fatty acids tend to be solid at relatively high temperatures (like butter, which is soft but solid at room temp) whereas unsaturated fatty acids, like oils, tend to be liquid unless chilled. Soap has a similar structure (polar at one end with most of the molecule hydrophobic) which is important to allowing it to remove oils or other hydrophobic substances when it is used with water to wash dirty items.