Biology Lecture No. 7: Cell Membranes
Monday January 30 , 2012
-Although it is still unclear as to how cells evolved, it is well known that the composition of the cell
membrane can be up to 50% protein and that its foundational structure is lipid-based.
-The cell membrane follows the fluid mosaic model in that its structure never remains static, but
constantly induces motion.
-The cell membrane is composed of a phospholipid bi-layer whereby hydrophilic heads interact with the
polar, aqueous environment and hydrophobic tails interact with themselves (by the hydrophobic effect).
-The synthesis of this lipid bi-layer is a spontaneous reaction. The hydrophobic tails are of a lower energy
-The cell membrane can be classified as amphipathic due to polar/non-polar capabilities. Detergents are
similar in this respect as they can solvate both aqueous and lipid residues.
Fatty Acid Structure:
-Part of the functional lipid portion of the membrane, this encompasses the hydrophobic/hydrocarbon
tails. In such molecules, each carbon is saturated with hydrogen.
-It is the introduction of the double bond between carbons that discourages carbon-hydrogen bonding
and renders the molecule unsaturated. Every double-bond denotes a bend in the molecular structure.
-Two examples of unsaturated fatty acids are trans-unsaturated and cis-unsaturated. The “trans” version
involves hydrogen atoms located across from each other, while in the “cis” variation, they are proximal.
-There is a greater bend in the double bond for cis-unsaturated molecules and this type is common in
nature. Trans-unsaturated molecules are more related to saturated fats and quite rare in nature (more
prevalent for industrial purposes).
Importance Of The Fatty Acid Structure:
-The structure of fatty acids is extremely diverse in the amount of carbons in a chain and in the number
and position of double bonds. Saturated hydrophobic tails are closer together, while unsaturated tails
are much further apart.
-The cell membrane consists of many unsaturated molecules as they promote great fluidity in the lipid
bi-layer. Membrane fluidity affects function and maintaining correct fluidity is critical to life in many
systems. Physiological Importance Of Unsaturation:
-Fatty acids are synthesized by biological reactions occurring in the cell. When biosynthesized, fatty acids
are always produced as fully saturated molecules.
-Unsaturated fatty acids arise from desaturase enzymes which regulate the abundance of unsaturated
molecules. Different kinds of desaturases introduce double bonds and control the diversity of fatty
-Ectotherms like bacteria and plants are extremely susceptible to the temperature of their environment.
Such organisms rely on these enzymes to keep their cells alive, with desaturase transcript being very
high in lower temperatures where membrane fluidity is often less.
-Humans however, have the ability to regulate the temperature of their environment in a way that
maintains optimal fluidity of the cell membrane. It is important to maintain fluidity within a certain
range (not too rigid, not too fluid).
-Enzymes have a high temperature profile (are most active) in high temperatures (where they face risk
of denaturation) and in low temperatures (where not enough enzyme is being produced to counteract
-If the cell membrane was too fluid, it would not be able to retain essential molecules. However, if the
cell membrane was too viscous, transport of molecules into the cell would be increasingly difficult.
-Hydrophobic molecules (O , CO , and N ) and small, uncharged, polar molecules (H O and glycerol) can
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diffuse (easily navigate) through the cell membrane.
-Large, uncharged, polar molecules (glucose, sucrose) and ions (Cl, K and Na ) cannot easily pass
through the cell membrane.
-Membrane proteins are defined as proteins that interact with the cell membrane and produce a
channel or pore. It is through these protein channels that charged molecules may pass the cell
-The channel or protein surface is hydrophilic except for the surface that interacts with the fatty acid
tails, which is hydrophobic. These are known as integral proteins or trans-membrane proteins.
Trans-membrane Protein Prediction:
-Such proteins are capable of interacting with the hydrophobic core of the membrane as they have
strong alpha-helical structures (which minimize cha