Gabriel Krotkov
Biology Notes
XXIII. Membranes
10/7/15
A. Fluid Mosaic Model
1. Definition
a. The membrane is a fluid structure with a “mosaic” of proteins in/on the
phospholipids that make up the majority of the membrane.
b. Membrane core made of amphipathic lipids, primarily phospholipids.
i. Amphipathic: w/hydrophobic and hydrophilic regions
2. Progression of Model
a. Phospholipid bilayers
i. In 1915, membranes were isolated from red blood cells and found to be
composed of mainly lipids and proteins
ii. 1925, two Dutch scientists figure out that the cell membranes must be
phospholipid bilayers, because such a material makes the most sense to bridge the
difference between cytosol and cellular environments.
b. Protein location
i. Since the heads of a phospholipid adhere less strongly to water than
does an entire membrane (whole is greater than the sum of its parts), scientists
proposed that proteins made up the difference
ii. Sandwich model (Davson-Denielli model): Protein coating on either side
of the phospholipid bilayer made up the adherence difference.
iii. Strange that they didn’t think to potentially throw out the phospholipid
method after noting the difference in adherence.
iv. Two problems with the model: Membranes differ in
structure/composition; this couldn’t apply across the board. Secondly, membrane
proteins were amphipathic – meaning that they would not particularly love being
straight in the water.
v. Those of you who already know the ending to this story now can
understand why the phospholipids’ amphipathicness is important: the hydrophobic
area, among other things, houses the proteins (or portions of them) that don’t like
water. Also allow the hydrophilic areas of the proteins to maximize contact with
water/fluid. In other words, the fluid mosaic model.
c. Freeze-Fracture
i. A method that demonstrated the fluid mosaic model visually.
ii. Method splits membranes along the bilayer, laying over the layer from
view from the middle.
iii. When membranes treated with freeze-fracture are examined under an
ECM, they show proteins in a cobblestone pattern, interrupting the smooth
continuity of the phospholipids (which, I guess, are so small that they appear nearly
like a wave).
d. Regional developments
i. Recent additions to the fluid mosaic model that demonstrate that
portions of the membrane are specialized
ii. Some long series of proteins exists, as well as lipids forming defined
regions. These may not actually have purposes, but rather be a result of the natures
of the component parts.
3. Membrane Asymmetry
a. Membranes need to by asymmetrical, because different functions need to
be served in the cytoplasm than in the cellular solution.
b. The sidedness is determined by the ER and Golgi as they construct the
membrane.
B. Fluidity
1. Causes and purposes
a. Since a membrane is held together by hydrophobic interactions, which are
effectively like a larger version of Van Der Waals forces, a membrane’s component
parts are not locked in place; lateral motion is quite common.
b. Those same hydrophobic forces, however, prevent molecules from cutting
through the entire membrane – pieces and molecules tend to stay where they are
comfortable water-wise. This means that traversing across to the other
phospholipid layer is rare.
c. Obviously, fluidity in the membrane is required. The fluidity of the
membrane is what allows the membrane to be permeable, and membrane proteins
may require motion to keep them from becoming inert.
d. Too much fluidity will also screw with protein functions – it’s all a balance,
man.
2. Lateral motion
a. Adjacent phospholipids move around constantly, while proteins, being
larger, will move more slowly.
i. Phospholipids can be propelled by the cytoskeleton, motor proteins, or
temperature (similar to Brownian motion).
ii. By the same token, the cytoskeleton can hold membrane components in
place.
b. Temperature, lateral motion, and hydrocarbons
i. As temperature lowers, phospholipids will move more slowly
ii. At a certain point, this means that a membrane will become a solid,
rather than the liquid it was more like beforehand.
iii. The speed of solidification depends on the type of phospholipid that is
the primary component of the membrane: unsaturated hydrocarbons cannot pack
together and remain fluid longer (kinks in the tails due to double bonds), while
saturated hydrocarbons have less room to maneuver in the first place and therefore
are easier to turn into a solid.
iv. Steroids (eg. Cholesterol) can also have effects on membrane fluidity. At
higher temperatures, the large molecule will get in the way and restrict motion of
the phospholipids, while at lower temperatures, the large molecule gets in the way
and prevents phospholipids from packing together. Acts as a resistant for whatever
force wants to change the nature of the phospholipid layers.
3. Evolution for fluidity
a. Evolution develops adaptations to ensure membrane fluidity
i. Colder temperatures will encourage unsaturated hydrocarbons,
increased motor proteins, and steroids to act as a fluidity buffer.
ii. By the same token, warmer temperatures will encourage saturated
hydrocarbons, etc…
iii. The ultimate advantage in this evolutionary arms race is the ability to
change the construction of the membrane seasonally.
C. Proteins
1. Protein types in membrane
a. Integral proteins
i. Located in (at least partly) the hydrophobic region of the bilayer
ii. Consist mainly on “transmembrane proteins” – proteins that extend
across the entire membrane, rather than just one part.
iii. The hydrophobic regions of an integral protein contain at a stretch of
nonpolar amino acid (usually alpha helices)
iv. Can have hydrophilic areas which allow the protein either to create a
hydrophilic channel through the membrane or to simply exist on either side of the
membrane.
b. Peripheral proteins
i. Appendages entirely on the hydrophilic area of the bilayer, which can
interact with areas of an integral protein
ii. Peripheral proteins on the cytoplasmic side can be held by the
cytoskeleton, but are still considered a part of the membrane. Similarly, proteins on
the extracellular side are often held by the ECM.
2. Protein functions
a. Transport
i. Proteins provide the cell with the opportunity to transport material into
and out of the cell, by active transport/facilitated diffusion
ii. The protein allows a hydrophilic material to cross the hydrophobic
region of the membrane without issue.
b. Enzymatic activity
i. As usual, enzymes pop up.
ii. These enzymes will usually have substrates in the surrounding solution.
c. Signaling (via quorum sensing)
i. Membranes will be embedded with receptor proteins that can detect a
specific messenger, the concentration of which will determine behavior of the cell.
d. Identification
i. Identification tags that will be recognized by the membrane proteins of
other cells.
ii. Short-lived
e. Intercellular connections
i. Attachment cell-to-cell, allows cells to create gap junctions and tight
junctions
ii. Long-lasting.
f. Anchoring to ECM & Cytoskeleton
i. Proteins attached to the cytoskeleton and ECM help bind the ECM and
cytoskeleton to the plasma membrane
ii. Done via microfilaments.
iii. Maintains cell shape and stabilizes protein location.
D. Carbohydrates
1. Construction
a. Short, branches chains of <=15 sugars.
b. When bound to other areas of the plasma membrane, become either
“glycolipids” or “glycoproteins”
c. They appear on the outside surface of the cell
2. Purpose
a. Cellular identification, sorting out who is who.
b. Adhesion